Theoretical principles of seismic wave propagationfaculty.kfupm.edu.sa/ES/alghamdi/GEOP415/Notes/GEOP415.doc · Web viewHave you ever heard a big clap of thunder and heard the windows

Theoretical principles of seismic wave propagation

A solid body can be deformed by the application of an external force. If the solid is perfectly elastic, it will return to its original shape once that force is removed. In the context of exploration seismology, the earth can generally be considered as perfectly elastic because the stresses generated by seismic exploration activities are too small to permanently deform subsurface rocks. The elastic limit is the maximum stress that can be applied to a solid without permanently deforming it.

When an impulsive or transitory stress is applied to a finite area on the surface of an elastic solid, a strain is generated in the immediately adjacent subvolume. The strained subvolume then transfers stress to adjacent interior areas within the solid, which generates strains in surrounding subvolumes. In this fashion an impulsive stress propagates through a solid as an elastic wave. Elastic waves that propagate in the earth are known as seismic waves.

The exploration seismologist is primarily interested in seismic body waves that propagate through the earth's interior; however, seismic energy can also propagate as surface waves. Seismic surface waves are generally regarded as a form of noise in seismic exploration because they only contain information about the very near surface. For land reflection surveys, however, surface waves may have amplitudes that are significantly greater than the amplitudes of the sought after body waves. Also, depending on the depth of interest, these surface waves may arrive at the seismic sensors at nearly the same time as the desired signals. In these situations, suppression of surface waves during acquisition and/or their removal during processing become important considerations of the seismic experiment.

Seismic body waves can be subdivided into two distinct wave types on the basis of the direction of the seismic wave propagation relative to the direction that particles in the medium are displaced during propagation. For compressional waves, particles in the medium move parallel to the propagation direction. For shear waves, medium particles move perpendicular to the propagation direction. Within a medium, compressional waves propagate faster than shear waves. For this reason, compressional waves are usually referred to as primary waves, or P-waves, and shear waves are also referred to as secondary waves, or S-waves.

The seismic velocity of a medium is a function of its elasticity and can be expressed in terms of its elastic constants. For a homogeneous, isotropic medium, the seismic P-wave velocity Vp is given by

where is the shear modulus, k is the bulk modulus and is the density of -the medium. Using the same notation, the S-wave velocity Vs is given by

As the names imply, the shear modulus is proportional to the shear strength of the medium, while the bulk modulus describes the incompressibility of the medium. The elastic constants have real values and are never negative, so a comparison of the expressions for Vp and Vs supports the observation that P-waves always travel faster than S-waves. Also, the expression for Vs implies that shear wave propagation can only be supported by solid bodies with shear strength. Fluids such as air and water do not support the propagation of S-waves. In any case, most seismic reflection surveys are exclusively concerned with P-wave acquisition, processing and interpretation.

Figure 1: Huygen's Principle is a kinematic description for calculating the position of a wavefront at time (t + Dt) from the position of the wavefront at time t. Huygen's Principle does not address the amplitude of the wavefront.

Seismic wave behavior can sometimes be understood intuitively in terms of ray path theory. First, consider a wave front at a particular time t inside a medium of velocity V. (A wave front is a constant phase surface inside a moving wave.) According to Huygen's Principle, the location of the wave front at a later time (t + Dt) can be calculated by assuming that the wave front will advance by a distance V*Dt from each point along the current wave front. By using every point on the wave front at time t as the origin of an arc with radius V*Dt, the position of the wavefront at time (t + Dt) can be constructed from the envelope of the arcs. In other words, each point along the wavefront at time t acts as an independent wave source, and the wavefront at time (t + Dt) can be constructed by summing all the individual source contributions (Figure 1). A line segment from a point on the time t wavefront to the position where its arc touches the time (t + Dt) wavefront defines a raypath of the seismic wave. Note that the raypath is perpendicular to both wavefronts. A raypath that extends from a seismic source to a detector is referred to as a travel path.

Figure 2: Refraction and reflection of a ray path at planer impedance boundary.

Two important concepts in seismic propagation are reflection and refraction. Figure 2 shows a two-layer model with a higher velocity layer over a lower velocity layer. In the figure, a down-going ray in the upper layer is partitioned into an up-going reflected ray and a down-going refracted ray at the layer boundary. For this simple homogeneous model, the raypath geometry can be specified by three principles: (1) raypaths in a constant

velocity medium are straight. (2) At an impedance boundary, the reflected raypath is reflected at an angle equal to the angle of incidence. (3) At an impedance boundary, a change in velocity will cause the transmitted ray path to bend or refract. The refraction angle can be calculated using Snell's law, which is given by

where V1 and V2 are velocities in the upper and lower layers, is the

angle of the incident raypath with respect to the vertical, and is the angle of transmission of the refracted raypath with respect to the vertical. Note that Snell's law is also valid for the reflection angle if V2 on the right hand side of the equation is replaced by V1. Figure 2 is incomplete for the general elastic case because it neglects the phenomenon of seismic mode conversions. For the general case, each layer should have a P-wave velocity and an S-wave velocity. Then if the incident ray in the diagram represents a down-going P-wave, it will be partitioned into four waves at the layer boundary: a reflected P-wave, a refracted P-wave, a reflected converted S-wave and a refracted converted S-wave. The angles of reflection and refraction for the converted raypaths can also be calculated using Snell's law.

For the purposes of exploration geology, the local geology of a sedimentary basin can frequently be represented by a simple "layer-cake" model consisting of a stack of homogeneous layers with planar upper and lower surfaces. As was the case in Figure 2, all changes in density and acoustic velocity in a layer-cake model are confined to the layer interfaces. When a seismic wave encounters an interface, it is partitioned into a reflected wave that bounces off the interface and a refracted wave that crosses the interface, but may change its propagation direction.

Seismic velocities vary greatly with the type of rock or medium. P-wave velocities of sedimentary rocks range from 1500 m/s for water, to 4500 m/s for salt, and between 800 and 5000 m/s for sandstones. These materials, together with shales and carbonates, are the main components of the world's sedimentary basins. Oil and gas deposits are almost exclusively located in porous sedimentary rocks (such as sandstones and carbonates) and may be held in place by impermeable sedimentary rocks (such as salt and shale). Using seismic methods, it is usually possible to derive estimates of subsurface seismic velocities. Unfortunately, even perfect knowledge of the seismic velocity does not provide a unique identification of rock type. The seismic velocity of a sedimentary rock may vary depending on its fluid

content, precise mineral composition, degree of compaction, and strength of cementation, among other factors; as a result, the velocity ranges of different rock types overlap.

Seismic DeformationWhen an earthquake fault ruptures, it causes two types of deformation: static; and dynamic. Static deformation is the permanent displacement of the ground due to the event. The earthquake cycle progresses from a fault that is not under stress, to a stressed fault as the plate tectonic motions driving the fault slowly proceed, to rupture during an earthquake and a newly-relaxed but deformed state.

Typically, someone will build a straight reference line such as a road, railroad, pole line, or fence line across the fault while it is in the pre-rupture stressed state. After the earthquake, the formerly stright line is distorted into a shape having increasing displacement near the fault, a process known as elastic rebound.

Seismic WavesThe second type of deformation, dynamic motions, are essentially sound waves radiated from the earthquake as it ruptures. While most of the plate-tectonic energy driving fault ruptures is taken up by static deformation, up to 10% may dissipate immediately in the form of seismic waves.

The mechanical properties of the rocks that seismic waves travel through quickly organize the waves into two types. Compressional waves, also known as primary or P waves, travel fastest, at speeds between 1.5 and 8 kilometers per second in the Earth's crust. Shear waves, also known as secondary or S waves, travel more slowly, usually at 60% to 70% of the speed of P waves.

P waves shake the ground in the direction they are propagating, while S waves shake perpendicularly or transverse to the direction of propagation.

Although wave speeds vary by a factor of ten or more in the Earth, the ratio between the average speeds of a P wave and of its following S wave is quite constant. This fact enables seismologists to simply time the delay between the arrival of the P wave and the arrival of the S wave to get a quick and reasonably accurate estimate of the distance of the earthquake from the observation station. Just multiply the S-minus-P (S-P) time, in seconds, by the factor 8 km/s to get the approximate distance in kilometers.

The dynamic, transient seismic waves from any substantial earthquake will propagate all around and entirely through the Earth. Given a sensitive enough detector, it is possible to record the seismic waves from even minor events occurring anywhere in the world at any other location on the globe. Nuclear test-ban treaties in effect today rely on our ability to detect a nuclear explosion anywhere equivalent to an earthquake as small as Richter Magnitude 3.5.

Seismographs and SeismogramsSensitive seismographs are the principal tool of scientists who study earthquakes. Thousands of seismograph stations are in operation throughout the world, and instruments have been transported to the Moon, Mars, and Venus. Fundamentally, a seismograph is a simple pendulum. When the ground shakes, the base and frame of the

instrument move with it, but intertia keeps the pendulum bob in place. It will then appear to move, relative to the shaking ground. As it moves it records the pendulum displacements as they change with time, tracing out a record called a seismogram. One seismograph station, having three different pendulums sensitive to the north-south, east-west, and vertical motions of the ground, will record seismograms that allow scientists to estimate the distance, direction, Richter Magnitude, and type of faulting of the earthquake. Seismologists use networks of seismograph stations to determine the location of an earthquake, and better estimate its other parameters. It is often revealing to examine seismograms recorded at a range of distances from an earthquake:

On this example it is obvious that seismic waves take more time to arrive at stations that are farther away. The average velocity of the wave is just the slope of the line connecting arrivals, or the change in distance divided by the change in time. Variations in such slopes reveal variations in the seismic velocities of rocks. Note the secondary S-wave arrivals that have larger amplitudes than the first P waves, and connect at a smaller slope.

While the actual frequencies of seismic waves are below the range of human hearing, it is possible to speed up a recorded seismogram to hear it. You can click on this earthquake recording to hear a seismogram from the 1992 Landers earthquake in southern California, recorded near Mammoth Lakes in an active volcanic caldera by the USGS. The original record, 800 seconds long, has been speeded up 80 times so that you hear it all within 10 seconds.

75 kb u-law; 149 kb WAV; 75 kb Quicktime The clicks at the beginning of the recording are the sharp, high-frequency P waves, followed by the rushing sound of the drawn-out, lower-frequency S waves. This

recording is also interesting because of the small, local earthquakes within the Mammoth caldera that sound like gunshots. The passage of the S wave from the magnitude 7.2 Landers event through the caldera actually triggered a sequence of small earthquakes there. The triggered earthquakes are similar to a burst of creaks and pops you hear from your house frame after a strong blast of wind. Landers triggered earthquakes up to magnitude 5.5 throughout eastern California and Nevada, and in calderas as far away as Yellowstone.

Listen to more earthquakes with:

John Louie's The Sound of Seismic (MP3s) Andy Michael and Dennis Ross's Listening to Earthquakes

(WAVs)

Locating EarthquakesThe pricipal use of seismograph networks is to locate earthquakes. Although it is possible to infer a general location for an event from the records of a single station, it is most accurate to use three or more stations. Locating the source of any earthquake is important, of course, in assessing the damage that the event may have caused, and in relating the earthquake to its geologic setting.

Given a single seismic station, the seismogram records will yield a measurement of the S-P time, and thus the distance between the station and the event. Multiply the seconds of S-P time by 8 km/s for the kilometers of distance. Drawing a circle on a map around the station's location, with a radius equal to the distance, shows all possible locations for the event. With the S-P time from a second station, the circle around that station will narrow the possible locations down to two points. It is only with a third station's S-P time that you can draw a third circle that should identify which of the two previous possible points is the real one:

This example uses stations in Boston, Edinborough, and Manaus. With the distances shown, all three circles can intersect only at a single point on the Mid-Atlantic Ridge spreading center.

What Is Seismology?

Seismology is the study of earthquakes and seismic waves that move through and around the earth. A seismologist is a scientist who studies earthquakes and seismic waves.

What Are Seismic Waves?

Seismic waves are the waves of energy caused by the sudden breaking of rock within the earth or an explosion. They are the energy that travels through the earth and is recorded on seismographs.

Types of Seismic WavesThere are several different kinds of seismic waves, and they all move in different ways. The two main types of waves are body waves and surface waves. Body waves can travel through the earth's inner layers, but surface waves can only move along the surface of the planet like ripples on water. Earthquakes radiate seismic energy as both body and surface waves.

Body Waves

P Waves

The first kind of body wave is the P wave or primary wave. This is the fastest kind of seismic wave. The P wave can move through solid rock and fluids, like water or the liquid layers of the earth. It pushes and pulls the rock it moves through just like sound waves push and pull the air. Have you ever heard a big clap of thunder and heard the windows rattle at the same time? The windows rattle because the sound waves were pushing and pulling on the window glass much like P waves push and pull on rock. Sometimes animals can hear the P waves of an earthquake. Usually we only feel the bump and rattle of these waves.

The arrow shows the direction that the wave is moving.

S Waves

The second type of body wave is the S wave or secondary wave, which is the second wave you feel in an earthquake. An S wave is slower than a P wave and can only move through solid rock. This wave moves rock up and down, or side-to-side.

The arrow shows the direction that the wave is moving.

Surface Waves

Love Waves

The first kind of surface wave is called a Love wave, named after A.E.H. Love, a British mathematician who worked out the mathematical model for this kind of wave in 1911. It's the fastest surface wave and moves the ground from side-to-side.

The arrow shows the direction that the wave is moving.

Rayleigh Waves

The other kind of surface wave is the Rayleigh wave, named for John William Strutt, Lord Rayleigh, who mathematically predicted the existence of this kind of wave in 1885. A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean. Because it rolls, it moves the ground up and down, and side-to-side in the same direction that the wave is moving. Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves.

The arrow shows the direction that the wave is moving.

Seismic Body Waves

Seismic waves which travel through the interior of the earth are called body waves. There are two types of body waves:

S-waves (also called secondary or shear waves). The particles making up the medium through which the S-wave is traveling move perpendicular to the direction of propagation of the S-wave (transverse). The S-wave motion can be split into SH (horizontally polarized motion) and SV (vertically polarized motion). S-waves involve shearing and rotation of the material through which the wave passes but does not involve volume change.

P-waves (also calles primary or pressure waves). The particle motion of P-waves is longitudinal, that is, in the same direction in which the P-wave is travelling. The particles in the medium vibrate about an equilibrium position. The medium through which a P-wave travels experiences compression and rarefaction, but not rotation.

The notation for the various seismic ray paths within the earth are as follows:

P, a P-wave in the mantle S, an S-wave in the mantle p, a P-wave reflected from the surface of the earth

close to the earthquake hypocenter s, an S-wave reflected from the surface of the earth

close to the earthquake hypocenter K, a P-wave through the outer core I, a P-wave through the inner core J, an S-wave through the inner core c, a reflection from the mantle-outer core boundary i, a reflection from the outer core-inner core

boundary

The P-wave is always the first wave to arrive at a seismometer, closely followed by its reflection from the surface. The S-waves arrive next and finally the surface waves. Body waves are reflected and transmitted at

interfaces where the seismic velocity and/or density changes, obeying Snell's Law. At such an interface, or discontinuity, some of the energy of an incident body wave is reflected as a P-wave, some as an S-wave, some is transmitted as a P-wave and some as an S-wave.

Examples of the notation of seismic waves are:

PKIKP, started out as a P-wave, passed down through the mantle and outer core, then through the inner core and up through the outer core and mantle.

sSP, travelled as an S-wave to the earth's surface close to the earthquake focus, reflected, then travelled through the mantle as an S-wave, was reflected again at the surface, this time converted to a P-wave and travelled through the mantle.

Seismic Waves and the Slinky: A

Guide For Teachers (v.2.2, December, 2000)

Prof. Lawrence W. Braileã

Department of Earth and Atmospheric Sciences Purdue University

West Lafayette, IN 47907 [email protected]

Objectives: This teaching guide is designed to introduce the concepts of waves and seismic waves that propagate within the Earth, and to provide ideas and suggestions for how to teach about seismic waves. The guide provides information on the types and properties of seismic waves and instructions for using some simple materials – especially the slinky – to effectively demonstrate seismic wave characteristics and wave propagation. Most of the activities described in the guide are useful both as demonstrations for the teacher and as exploratory activities for students. With several regular metal slinkys, and the modified slinky demonstrations described in this teaching guide, one can involve an entire class in observation of the demonstrations and experimenting with the slinkys in small groups. For activities that involve several people, such as the 5-slinky and human wave demonstrations, it is convenient to repeat the demonstrations with different groups of students so that each person will have the opportunity to observe the demonstration and to participate in it. Waves: Waves consist of a disturbance in materials (media), that carries energy and propagates. However, the material that the wave propagates in generally does not move with the wave. The movement of the material is generally confined to small motions, called particle motion, of the material as the wave passes. After the wave has passed, the material usually looks just like it did before the wave, and, is in the same location as before the wave. (Near the source of a strong disturbance, such as a large explosion or earthquake, the wave-generated deformation can be large enough to cause permanent deformation which will be visible as cracks, fault offsets, and displacements of the ground after the disturbance has passed.) A source of energy creates the initial disturbance (or continuously generates a disturbance) and the resulting waves propagate (travel) out from the disturbance. Because there is finite energy in a confined or short-duration disturbance, the waves generated by such a source will spread out during propagation and become smaller (attenuate) with distance away from the source or with time after the initial source, and thus, will eventually die out.

ãCopyright 2000. L. Braile. Permission granted for reproduction for non-commercial uses.

Waves are often represented mathematically and in graphs as sine waves (or combinations or sums of sine waves) as shown in Figure 1. The vertical axis on this plot represents the temporary motion (such as displacement amplitude A) of the propagating wave at a given time or location as the wave passes. The characteristics or properties of the wave – amplitude, wavelength, peaks, etc. – are illustrated in Figure 1.

Wavelength, (or Period, T)

Ampl

itude

, A

Distance, x (or Time, t)

Properties of a sine wave: y = Asin 2ft

Peak

Trough

Frequency, f = 1/T

Figure 1. Properties of a sine wave. The horizontal axis displays time or distance and the vertical axis displays amplitude as a function of time or distance. Amplitude can represent any type or measure of motion for any direction. Seismic wave motion is commonly displayed with a similar plot, and sometimes the wave itself looks very similar to the sine wave shown here. Additional information on the properties of waves can be found in Bolt (1993, p. 29-30) or Rutherford and Bachmeyer (1995). Sometimes the actual wave looks very much like the representation in the graph (Figure 1). Examples are water waves and a type of seismic surface waves called Rayleigh waves. Commonly, propagating waves are of relatively short duration and look similar to truncated sine waves whose amplitudes vary with time. Waves generated by a short duration disturbance in a small area, such as from an earthquake or a quarry blast, spread outward from the source as a single or a series of wavefronts. The wavefronts at successive times, and corresponding raypaths that show the direction of propagation of the waves, are illustrated in Figure 2. A good model for illustrating wave motion of this type is water waves from a pebble dropped in a still pond or pool. The disturbance caused by the pebble hitting the surface of the water generates waves that propagate outward in expanding, circular wavefronts. Because there is more energy from dropping a larger pebble, the resulting waves will be larger (and probably of different wavelength).

W

avefro nt

t0t1

t2t3

Raypath

Compressional (P) motion

Shear (S) motion

Wavefronts and Raypaths in Seismic Wave Propagation

Source

Perpendicular

angle

Waves in Water Experiment: Experiments with water waves in a small wave tank, consisting of a rectangular plastic storage box (about 60 to 120 cm long by 40 cm wide by 15 cm high; the exact size isn’t important); a plastic storage box designed to fit under a bed, available at discount department stores such as K-Mart, Wal-Mart and Target, with about 5 cm of water in it, can easily illustrate the common properties of water waves (Figure 3). Drops of water from an eyedropper or small spherical objects (a table tennis ball, a golf ball, or a small rubber ball works well) dropped onto the surface of the water are convenient sources. With the wave tank one can Figure 2. Wavefronts and raypaths for a seismic wave propagating from a source. Three positions (successive times) of the expanding wavefront are shown. Particle motions for P (compressional) and S (shear) waves are also shown. Raypaths are perpendicular to the wavefronts and indicate the direction of propagation of the wave. P waves travel faster than S waves so there will be separate wavefront representations for the P

and S waves. If the physical properties of the material through which the waves are propagating are constant, the wavefronts will be circular (or spherical in three-dimensions). If the physical properties vary in the model, the wavefronts will be more complex shapes.

Figure 3. Large wave tank using a plastic storage container (75 liter "under bed" container). Distances in cm can be marked on the bottom of the container for convenient measurement of wave velocity from the distance and travel time. Small floating flags (Figure 4) are useful for identifying the time and relative amplitude of propagating water waves. The flags are made from small floats (about 2x2x1 cm rectangular blocks of closed cell foam, Styrofoam or cork) with a toothpick and piece of tape attached. The floating flags are very sensitive to wave motion in the wave tank. In this sequence of photos, taken every one-half second, one can see the waves propagating outward from

the source. Lines drawn on the bottom of the tank are spaced at 10 cm. A. Source (a table tennis ball dropped from about 40 cm height) is just ready to hit the water surface. B. (~ 0.5 s after the source) Water waves have propagated from the source to about 10 cm distance. A circular expanding wavefront is visible and the wave height is larger than in the later photos. C. (~ 1.0 s) The wavefront has propagated to about 20 cm distance and the waves have decreased in amplitude. D. (~ 1.5 s) The wavefront has propagated to about 30 cm distance. E. (~ 2 s) The wavefront has propagated to about 40 cm distance and the wave amplitudes have decreased so much that they are difficult to see in this photo. The small floating flags are sensitive detectors (similar to seismometers) of the waves. Because the waves have traveled about 40 cm in 2 seconds, the velocity of propagation is about 20 cm/s or 0.2 m/s. demonstrate that: the size (amplitude) of the waves is related to the energy of the source (controlled by the mass and drop height); the waves expand outward (propagate) in circular wavefronts; the wave height decreases and eventually dies out with distance away from the source (or with time after the source) because of spreading out of the wave energy over a larger and larger area (or volume); the waves have a speed (velocity) of propagation that can be measured by placing marks every 10 cm on the bottom of the tank and timing the wave with a stopwatch; the waves reflect off the sides of the tank and continue propagating in a different direction after reflection. About 3-4, small floating flags can be used to more effectively see the motion of the water as the wave passes. The flags are particularly useful for noting the relative amplitude of the waves as the degree of shaking of the flags is a visible indication of the size of the wave. Flags can be made from small pieces (~2x2x1 cm) of closed cell foam, styrofoam or cork. Place a small piece of tape on a toothpick and stick the toothpick into the foam or cork to create a floating flag (Figure 4) that is sensitive to waves in the water. By placing the flags in the wavetank at various distances from the source, one can easily observe the time of arrival and the relative amplitudes of the waves. A glass cake pan (Figure 5) can be used as small wave tank on an overhead projector. Waves generated by dropping drops of water from an eyedropper into about 2 cm of water in the tank will be visible on the screen projected from the overhead projector. The velocity of propagation, attenuation of wave energy, and reflection of waves are important concepts for understanding seismic wave propagation, so experimenting with these wave properties in the water tank is a very useful exercise. Additional suggestions for experiments with water waves are contained in Zubrowski (1994).

 Figure 4. Small floating flags used to help detect the motion of water waves in a wave tank. The flags can be "anchored" using a length of thread and a nickel for a weight. The flags are sensitive to the motion caused by relatively small waves. By noting the degree of shaking of the flags, one can identify (approximately) large, medium

Bottom of wave tank

Float (~2 x 2 x 1 cmclosed cell foam orstyrofoam)

Nickel

Toothpick

Flag made with tape

Thread or fishing line inserted through float

and small (barely detectible) wave action. Be sure the water surface is calm (and the flags still) before initiating a water wave experiment.   

 Figure 5. Small wave tank using a clear glass cake or baking dish. The dish with about 2 cm of water in it can be placed on an overhead projector so that wave propagation in the water can be seen on a screen.

 Elasticity: Earth materials are mostly solid rock and are elastic, and thus propagate elastic or seismic waves in which elastic disturbances (deformation or bending or temporary compression of rocks) travel through the Earth. Elastic materials have the properties that the amount of deformation is proportional to the applied force (such as the stretching of a spring as masses are suspended from the spring), and that the material returns to its original shape after the force is removed (such as the spring returning to its original length after the masses are removed).

 Elasticity of a Spring Experiment: An experiment designed to illustrate the elastic characteristics of a spring is shown in Figure 6. Measurements of the stretching of the spring as masses are added and removed are given in Table 1 and graphed in Figure 7. In contrast, some materials like most metals, are ductile. For example, if one bends a copper wire, it stays bent rather than returning to its original shape. One can even make a spring out of copper wire by wrapping the wire tightly around a cylindrical shaped object, such as a cardboard tube. Repeating the elasticity experiment with the copper wire spring will result in an elastic behavior (straight line relationships between the stretching and the amount of mass added) for small mass, and permanent deformation (stretching) for larger masses (the spring will not return to its original length as masses are removed). Data for the copper wire spring experiment can be tabulated, as in Table 1, and graphed, as in Figure 7. Because this experiment produces a very different stretching versus added mass curve, as compared to the regular spring example, it is useful to ask the students before the experiment what they expect the results to look like and then compare the actual results with their predictions. 

Wood

PVC

Pip

e

Mass

SpringLength of Spring

Standard

Measuring Elasticity of a Spring

0 50 100 150 200 250 300 350 4000

2

4

6

8

10

12

14

16

Added Mass (grams)

Stre

tchi

ng (l

engt

h - o

rigin

al le

ngth

, cm

)

Elasticity of a Spring

Adding mass:

Removing mass:

1. Deformation (stretching) isproportional to applied force (mass).

2. Spring returns to its original shape(length) when force is removed.

   Figure 6. Schematic diagram illustrating mass and spring apparatus for measuring the elasticity (stretching of the spring as mass is added) of a spring.     

 Table 1. Observations of extension of a spring upon adding and removing masses.

Added Mass (g)

Spring Extension (cm)*(adding masses)

Spring Extension (cm)*(removing masses)

0 0.0 0.3100 3.7 3.6200 7.7 7.5300 11.4 11.4400 15.3 15.1

*Spring extension (stretching) is the length of the spring with mass attached minus the length of the spring with no mass. Different springs have different spring constants (elasticity) and will display different amounts of stretching for a given amount of added mass. Many springs are tightly-wound and require a small amount of force (suspended mass) to begin extending. For these springs, one should use the difference between this initial suspended mass and the total mass as the "added mass."

Figure 7. Graph showing measurements of elasticity of a spring. Measurements were made as mass was added and removed. The stretching of the spring is defined as the spring length (with added mass) minus the original length.

P- and S- Waves (propagation along raypath)

X

Y

S-wave particle motion -- perpendicular to direction of propagation (usually approximately in SV and SH directions)

Earth’s surface

* P-wave particle motion -- parallel to direction of propagation

Source

Seismograph

Z (down)

SV

SH

Note that the extension versus mass data define a straight line. The slope of this line is related to the spring constant or elasticity of the spring. The two properties that characterize simple elasticity are described in the figure.   

 Seismic Waves: Unlike waves in water which are confined to a region very near to the water's surface, seismic waves also propagate through the Earth's interior. Because of the elastic properties of Earth materials (rocks) and the presence of the Earth's surface, four main types of seismic waves propagate within the Earth. Compressional (P) and Shear (S) waves propagate through the Earth’s interior and are known as body waves (Figure 8). Love and Rayleigh waves propagate primarily at and near the Earth's surface and are called surface waves. Wave propagation and particle motion characteristics for the P, S, Rayleigh and Love waves are illustrated in Figures 9-12. Further information and characteristics of these four kinds of seismic waves are given in Table 2.

Figure 8. Raypath from the source to a particular location on the surface for P- and S- wave propagation in a material with variable velocity. P and S particle motion are shown. Because major boundaries between different rock types within the Earth are normally approximately parallel to the Earth's surface, S-wave particle motion is commonly in the SV (perpendicular to the raypath and vertical) and SH

perpendicular to the raypath and horizontal) directions.  Additional illustrations of P, S, Rayleigh and Love waves are contained in Bolt (1993, p. 27 and 37) and in Shearer (1999, p. 32 and 152). Effective animations of P and S waves are contained in the Nova video "Earthquake" (1990; about 13 minutes into the program) and of P, S, Rayleigh and Love waves in the Discovery Channel video "Living with Violent Earth: We Live on Somewhat Shaky Ground" (1989, about 3 minutes into the program).      

Figure 9. Perspective view of elastic P-wave propagation through a grid representing a volume of material. The directions X and Y are parallel to the Earth's surface and the Z direction is depth. T = 0 through T = 3 indicate successive times. The disturbance that is propagated is a compression (grid lines are closer together) followed by a dilatation or extension (grid lines are farther apart). The particle motion is in the direction of propagation. The material returns to its original shape after the wave has passed.       

Figure 10. Perspective view of S-wave propagation through a grid representing a volume of elastic material. The disturbance that is propagated is an up motion followed by a down motion (the shear motion could also be directed horizontally or any direction that is perpendicular to the direction of propagation). The particle motion is perpendicular to the direction of propagation. The material returns to its original shape after the wave has passed.           

 Figure 11. Perspective view of Rayleigh-wave propagation through a grid representing a volume of elastic material. Rayleigh waves are surface waves. The disturbance that is propagated is, in general, an elliptical motion which consists of both vertical (shear; perpendicular to the direction of propagation but in the plane of the raypath) and horizontal (compression; in the direction of propagation) particle motion. The amplitudes of the Rayleigh wave motion decrease with distance away from the surface. The material returns to its original shape after the wave has passed.  

   Figure 12. Perspective view of Love-wave propagation through a grid representing a volume of elastic material. Love waves are surface waves. The disturbance that is propagated is horizontal and perpendicular to the direction of propagation. The amplitudes of the Love wave motion decrease with distance away from the surface. The material returns to its original shape after the wave has passed.      

Table 2: Seismic WavesType (and

names)Particle Motion Typical Velocity Other Characteristics

P, Compressional, Primary, Longitudinal

Alternating compressions (“pushes”) and dilations (“pulls”) which are directed in the same direction as the wave is propagating (along the raypath); and therefore, perpendicular to the wavefront

VP ~ 5 – 7 km/s in typical Earth’s crust; >~ 8 km/s in Earth’s mantle and core; 1.5 km/s in water; 0.3 km/s in air

P motion travels fastest in materials, so the P-wave is the first-arriving energy on a seismogram. Generally smaller and higher frequency than the S and Surface-waves. P waves in a liquid or gas are pressure waves, including sound waves.

S, Shear, Secondary, Transverse

Alternating transverse motions (perpendicular to the direction of propagation, and the raypath); commonly polarized such that particle motion is in vertical or horizontal planes

VS ~ 3 – 4 km/s in typical Earth’s crust; >~ 4.5 km/s in Earth’s mantle; ~ 2.5-3.0 km/s in (solid) inner core

S-waves do not travel through fluids, so do not exist in Earth’s outer core (inferred to be primarily liquid iron) or in air or water or molten rock (magma). S waves travel slower than P waves in a solid and, therefore, arrive after the P wave.

L, Love, Surface waves, Long waves

Transverse horizontal motion, perpendicular to the direction of propagation and generally parallel to the Earth’s surface

VL ~ 2.0 - 4.5 km/s in the Earth depending on frequency of the propagating wave

Love waves exist because of the Earth’s surface. They are largest at the surface and decrease in amplitude with depth. Love waves are dispersive, that is, the wave velocity is dependent on frequency, with low frequencies normally

propagating at higher velocity. Depth of penetration of the Love waves is also dependent on frequency, with lower frequencies penetrating to greater depth.

R, Rayleigh, Surface waves, Long waves, Ground roll

Motion is both in the direction of propagation and perpendicular (in a vertical plane), and “phased” so that the motion is generally elliptical – either prograde or retrograde

VR ~ 2.0 - 4.5 km/s in the Earth depending on frequency of the propagating wave

Rayleigh waves are also dispersive and the amplitudes generally decrease with depth in the Earth. Appearance and particle motion are similar to water waves.

Slinky Demonstrations of P and S Waves: The P and S waves have distinctive particle motions (Figures 8, 9, and 10, and Table 2) and travel at different speeds. P and S waves can be demonstrated effectively with a slinky. For the P or compressional wave, have two people hold the ends of the slinky about 3-4 meters apart. One person should cup his or her hand over the end (the last 3-4 coils) of the slinky and, when the slinky is nearly at rest, hit that hand with the fist of the other hand. The compressional disturbance that is transmitted to the slinky will propagate along the slinky to the other person. Note that the motion of each coil is either compressional or extensional with the movement parallel to the direction of propagation. Because the other person is holding the slinky firmly, the P wave will reflect at that end and travel back along the slinky. The propagation and reflection will continue until the wave energy dies out. The propagation of the P wave by the slinky is illustrated in Figure 13.

Figure 13. Compressional (P) wave propagation in a slinky. A disturbance at one end results in a compression of the coils followed by dilation (extension), and then another compression. With time (successive times are shown by the diagrams of the slinky at times t1 through t6), the disturbance propagates along the slinky. After the energy passes, the coils of the slinky return to their original, undisturbed position. The direction of particle motion is in the direction of propagation.

Demonstrating the S or Shear wave is performed in a similar fashion except that the person who creates the shear disturbance does so by moving his or her hand quickly up and then down. This motion generates a motion of the coils that is perpendicular to the direction of propagation, which is along the slinky. Note that the particle motion is not only perpendicular to the direction of motion but also in the vertical plane. One can also produce Shear waves with the slinky in which the motion is in the horizontal plane by the person creating the source moving his or her hand quickly left and then right. The propagation of the S wave by the slinky is illustrated in Figure 14. Notice that, although the motion of the disturbance was purely perpendicular to the direction of propagation (no motion in the disturbing source was directed along the slinky), the disturbance still propagates away from the source, along the slinky. The reason for this phenomenon (a good challenge question for students) is because the material is elastic and the individual coils are connected (like the individual particles of a solid) and thus transmit their motion to the adjacent coils. As this process continues, the shear disturbance travels along the entire slinky (elastic medium).

Figure 14. Shear (S) wave propagation in a slinky. A disturbance at one end results in an up motion of the coils followed by a down motion of the coils. With time (successive times are shown by the diagrams of the slinky at times t1 through t6), the disturbance propagates along the slinky. After the energy passes, the coils of the slinky return to their

SlinkyWood block

(~9 x 9 x 2 cm)

Holes (6.3 cm apart)

Screw and washer*

8 c

m

8 cm

8 cm

2 cm

8 c

m

3 cm wide strips cut from manila folder

tape

Cardboard “house” attached to a slinky to illustrate shaking caused by P- and S-waves

* (#6 x 1/2 in. screws; 3/16 in. washers)

original, undisturbed position. The direction of particle motion is perpendicular (for example, up and down or side to side) to the direction of propagation. P and S waves can also be generated in the slinky by an additional method that reinforces the concept of elasticity and the elastic rebound theory which explains the generation of earthquakes by plate tectonic movements (Bolt, 1993, p. 74-77). In this method, for the P wave, one person should slowly gather a few of the end coils of the slinky into his or her hand. This process stores elastic energy in the coils of the slinky that are compressed (as compared to the other coils in the stretched slinky) similar to the storage of elastic energy in rocks adjacent to a fault that are deformed by plate motions prior to slip along a fault plane in the elastic rebound process. When a few coils have been compressed, release them suddenly (holding on to the end coil of the slinky) and a compressional wave disturbance will propagate along the slinky. This method helps illustrate the concept of the elastic properties of the slinky and the storage of energy in the elastic rebound process. However, the compressional wave that it generates is not as simple or visible as the wave produced by using a blow of one’s fist, so it is suggested that this method be demonstrated after the previously-described method using the fist. Similarly, using this "elastic rebound" method for the S waves, one person holding the end of the stretched slinky should use their other hand to grab one of the coils about 10-12 coils away from the end of the slinky. Slowly pull on this coil in a direction perpendicular to the direction defined by the stretched slinky. This process applies a shearing displacement to this end of the slinky and stores elastic energy (strain) in the slinky similar to the storage of strain energy in rocks adjacent to a fault or plate boundary by plate tectonic movements. After the coil has been displaced about 10 cm or so, release it suddenly (similar to the sudden slip along a fault plane in the elastic rebound process) and an S wave disturbance will propagate along the slinky away from the source. Illustration of Energy Carried by the Waves: The fact that the seismic waves (P or S) that propagate along the slinky transmit energy can be illustrated effectively by using a slinky in which one end (the end opposite the source) has a small wood block attached.

The wood block has a cardboard model building attached to it as shown in Figures 15 and 16. As P or S wave

     Figure 15. Diagram showing how to attach a slinky to the edge of a small wood block with screws and washers. A cardboard "building" is attached with tape to the wood block. The model building is made from manila folder (or similar) material and is taped together. The diagram is a side view of the slinky, wood block and model building. When seismic waves are propagated

along the slinky, the vibrations of the cardboard building show that the wave energy is carried by the slinky and is transmitted to the model building.     

   Figure 16. Photograph of slinky attached to a small wood block (Figure 15) and cardboard "house."    energy that propagates along the slinky is transmitted to the wood block, the building vibrates. This model is a good demonstration of what happens when a seismic wave in the Earth reaches the surface and causes vibrations that are transmitted to houses and other buildings. By generating P, S-vertical and S-horizontal waves that transmit vibration to the model building, one can even observe differences in the reaction of the building to the

different directions of motion of the propagating wave.

 Wave Propagation in All Directions: An additional demonstration with P and S waves can be performed with the 5-slinky model. By attaching 5 slinkys to a wood block as shown in Figure 17, 5 people can hold the ends of the 5 slinkys (stretched out in different directions to about 3-4 m each). One person holds the wood block and can generate P or S waves (or even a combination of both) by hitting the wood block with a closed fist or causing the block to move quickly up and then down or left and then right. The purpose of this demonstration is to show that the waves propagate in all directions in the Earth from the source (not just in the direction of a single slinky). Attaching an additional slinky (with small pieces of plastic electrical tape) to one of the five slinkys attached to the wood block makes one slinky into a double length slinky which can be stretched out to 6-8 m. For one of the other four slinkys, have the person holding it collapse about half of the coils and hold them in his or her hands, forming a half slinky, stretched out about 1½ - 2 m. Now when a source is created at the wood block, one can see that the waves

take different amounts of time to travel the different distances to the ends of the various slinkys. An effective way to demonstrate the different arrival times is to have the person holding each slinky call out the word "now" when the wave arrives at their location. The difference in arrival times for the different distances will be obvious from the sequence of the call of "now."This variation in travel time is similar to what is observed for an earthquake whose waves travel to various seismograph stations that are different distances from the source (epicenter). Although these two demonstrations with the five slinky model represent fairly simple concepts, we have found the demonstrations to be very effective with all age groups. In fact, the five slinky demonstrations are often identified as the "favorite" activities of participants.         

Figure 17. Diagram showing how five slinkys can be attached to the edge of a wood block. Photographs of the five slinky model are shown in Figures 18 and 19. When the slinkys are stretched out to different positions (five people hold the end of one slinky each) and a P or S wave is generated at the wood block, the waves propagate out in all directions. The five slinky model can also be used to show that the travel times to different locations (such as seismograph stations) will be different. To demonstrate this effect, wrap a small piece of tape around a coil near the middle of the slinky for one of the slinkys. Have the person holding that slinky compress all of the coils from the outer end to the coil with the tape so that only one half of the slinky is extended. Also, attach an additional slinky, using plastic electrical tape, to the end of one of the slinkys. Have the person holding this double slinky stand farther away from the wood block so that the

Wood block

Slinky

Holes, 6.3 cm apart

Screw and washer

~ 10 cm

~ 25 cm

Five Slinky P and S Wave Propagation Demonstration

double slinky is fully extended. When a P or S wave is generated at the wood block, the waves that travel along the slinky will arrive at the end of the half slinky first, then at approximately the same time at the three regular slinkys, and finally, last at the double length slinky. The difference in travel time will be very noticeable.  

 Figure 18. Photograph of five slinky model.  

Figure 19. Close-up view of five slinky model showing attachment of slinky using screws and washers. Human Wave Demonstration - P and S Waves in Solids and Liquids: This demonstration involves a class or group in simulating seismic wave propagation in solids and liquids. If you have 20 or more people in the group, half can perform the demo while the other half watches; then switch places. The concepts that are involved in this demonstration are very dramatically illustrated to the participants. Once they have done the human wave activity, they should always remember the properties of P and S waves propagating in both solids and liquids. Have about 10 people stand at the front of the room, side by side, with their feet about shoulder width apart. Instruct the group to not be too rigid or too limp when pushed from the side. They should give with the force that they will feel from the person next to them, but not fall over, and then return to their upright position. In other words, they should be "elastic". Have a “spotter” at the end of the line in case the last person begins to fall. (It is important to stress these instructions to the participants so that the demonstration will work effectively and so that participants do not fall over as the wave propagates down the line of people.)  To represent wave propagation in a solid, have each person put their arms over the shoulders of the person next to them (“chorus line style”; the “molecules” or "particles" of the solid are tightly bonded). Push on the person at the end of the line and the deformation (leaning to the side and then straightening up) will propagate down the line of people approximating a P wave (Figure 20). Note that the propagation down the line took some time (there is a velocity for the wave propagation) and that although each person was briefly subjected to a deformation or disturbance, the individuals did not move from their original locations. Also, the motion of each person as the wave passed was in the direction of propagation and that, as the wave passed, the people moved closer together temporarily (compression), and then apart (dilation) to return to their original positions. 

Figure 20. Human wave demonstration for the P wave in a solid. Instructor begins the P wave motion (in this case from left to right) in the line by pushing (compressing) on the first person in the line and then pulling the person back to an upright position.

 For the S wave in a solid, make the first person at the end of the line bend forward at the waist and then stand up straight. The transverse or shear motion will propagate down the line of people (Figure 21). Again, the wave takes some time to propagate and each person ends up in the same location where they started even though a wave has passed. Also, note that the shear motion of each particle is perpendicular to the direction of

propagation. One of the observers can time the P and S wave propagation in the human wave using a stopwatch. Because the shear wave motion is more complicated in the human wave, the S wave will have a slower velocity (greater travel time from source to the end of the line of people), similar to seismic waves in a solid.        

 Figure 21. Human wave demonstration for the S wave in a solid. The teacher starts the S wave motion (in this case propagating from right to left) in the line by causing the first person in the line to bend forward at the waist and then stand up straight.

Next, to represent wave propagation in a liquid, have the people stand shoulder-to-shoulder, without their arms around each other. Push on the shoulder of the end person and a P wave will propagate down the line. The P wave will have the same characteristics in the liquid as described previously for the solid. Now, make the person at the end of the line bend forward at the waist – a transverse or shear disturbance. However, because the “molecules” of the liquid are more loosely bound, the shearing motion will not propagate through the liquid (along the line of people). The disturbance does not propagate to the next person because the liquid does not support the shearing motion. (Compare pressing your hand down on the surface of a solid such as a table top and on the surface of water and moving your hand parallel to the surface. There will be considerable resistance to moving your hand on the solid. One could even push the entire table horizontally by this shearing motion. However, there will be virtually no resistance to moving your hand along the surface of the water.) Only the first person in the line – the one that is bent over at the waist – should move because the people are not connected. If the next person bends, “sympathetically”, not because of the wave propagating, ask that person if he or she felt, rather than just saw, the wave disturbance, then repeat the demonstration for S waves in a liquid.

Velocity of Wave Propagation Experiment: Developing an understanding of seismic wave propagation and of the velocity of propagation of seismic waves in the Earth is aided by making measurements of wave speed and comparing velocities in different materials. For waves that travel an approximately straight line (along a straight raypath), the velocity of propagation is simply the distance traveled (given in meters or kilometers, for example) divided by the time of travel (or "travel time") in seconds. Using a stopwatch for measuring time and a meter stick or metric tape for measuring distance, determine the wave velocity of water waves in the wave tank, the P wave in the slinky, and the P wave in the human wave experiment. Also, determine the velocity of sound in air using the following method. On a playfield, measure out a distance of about 100 meters. Have one person with a stopwatch stand at one end of the 100 meter line. Have another person with a metal garbage can and a stick stand 100 meters away from the person with the stopwatch. Have the person with the stick hit the garbage can so that the instant of contact of the stick with the garbage can is visible from a distance. The person with the stopwatch should start the stopwatch when the stick strikes the can and stop it when the sound generated by the stick hitting the can is heard. The measured speed of sound will be the distance divided by the travel time measured on the stopwatch. (This measurement assumes that the speed of light is infinite – a reasonable approximation as the actual speed of light is about 3 x 108 or 300 million meters/second, much faster than the speed of sound, and that the reaction time for the person operating the stopwatch is about the same for starting the watch when the can is struck and for stopping the watch when the sound is heard. The measurement should be repeated a few times to obtain an estimate of how accurately the measurement can be made. An average of the time measurements can be used to calculate the sound speed. The difference between the arrival of a light wave and associated sound is commonly used to determine how far away a lightning strike is in a thunderstorm. For example, because the speed of sound in air is about 330 m/s, if the difference in time between seeing a lightning strike and hearing the associated thunder is 3 seconds, the lightning is 1 km away; similarly, 6 s for 2 km away, etc.) Make a list of the wave velocities (in m/s) for the water waves, slinky, human wave, and sound wave in air. (Measured wave speeds should be approximately 0.25-0.5 m/s for water waves in a wave tank, 2 m/s for the compressional human wave, 3 m/s for P waves in the slinky, and 330 m/s for sound waves in air.) Compare these wave velocities with the compressional wave velocity in the Earth which varies from about 1000 m/s for unconsolidated materials near the Earth's surface to about 14,000 m/s in the Earth's lowermost mantle (1 to 14 km/s). The seismic velocity in solid rocks in the Earth is controlled by rock composition (chemistry), and pressure and temperature conditions, and is found to be approximately proportional to rock density (density = mass/unit volume; higher density rocks generally have larger elastic constants resulting in higher seismic velocity). Further information on seismic velocities and a diagram showing seismic velocity with depth in the Earth is available in Bolt (1993, p. 143) and Shearer (1999, p. 3).

 Attenuation of Waves: To demonstrate the property of anelasticity, a model with two slinkys can be constructed. Anelasticity is the absorption of energy during propagation

which causes waves to attenuate in addition to the attenuation caused by the energy spreading out (for example, like the spreading of water waves created by dropping a pebble into a pond). This effect is an important concept in evaluating earthquake hazards and comparing the hazards in two locations such as the western United States and the eastern United States. Seismic waves propagate very efficiently in the eastern United States resulting in damage over a wide area from a large earthquake. In contrast, waves propagating in the western United States are attenuated by absorption of energy to a much greater degree. Thus, although earthquakes of a given size occur much more often in the western US, the earthquake hazard in the eastern US is significant because the area of damage (for equivalent earthquakes) is larger in the eastern US. Attach two slinkys to a 30 cm long piece of “one by two” (1” by 2” wood trim) using washers and screws (Figure 22) similar to that shown in Figure 15. For one of the slinkys, place a strip of foam (about a 3 m strip of ½” thick foam 7 cm wide; the 3 m strip can be pieced together from 2-3 shorter pieces; tape or sew together the strips) in the extended coils. The foam should fit snugly within the coils and will absorb wave energy that is propagated along the slinky.    

Figure 22. Schematic diagram illustrating the construction of a model using two slinkys (one with a foam strip inside the coils of the slinky) that can be used to demonstrate the concept of absorption of energy by material during propagation (anelasticity). A photo of the slinky model for demonstrating absorption is shown in Figure 23.  

Two slinkys used to demonstrate varying efficiency of wave propagation due to absorption of energy (anelasticity)

M etal slinky with strip of foam inside coils

Metal slinky

Two slinkys attached to wood block

Wood blocks with cardboard “buildings”

~3 m

 Figure 23. Slinky model used to demonstrate attenuation of waves by absorption of energy. Small wood blocks and model buildings (Figure 15) can be attached to the free ends of both the regular slinky and the foam-lined slinky so that the wave energy that reaches the end of the slinky will be more visible for comparison. Extend each slinky about 3 m and cause P or S waves to be generated simultaneously in both slinkys by hitting the wood block (for P waves) or quickly moving the wood block vertically or horizontally (for S waves). The regular slinky will propagate the waves very efficiently while the slinky with the foam will strongly attenuate the wave energy (by absorbing some of the energy) as it propagates. Reflection of Waves: The reflection of wave energy at a boundary between two types of materials can be demonstrated with slinkys by attaching a regular metal slinky to a plastic slinky (Figure 24). The attachment can be made with small pieces of plastic electrical tape. Generating P or S waves in the metal slinky will result in reflection of some of the wave energy at the boundary between the two stretched slinkys. Additional information about reflection and conversion of energy (S to P and P to S waves) of seismic waves at boundaries is given in Figure 25 and in Bolt (1993, p. 31-33)  

Incident Raypath

Refracted

Reflec

ted

Interface

Reflection and refraction of P or S raypaths (and associated wavefronts which are perpendicular to raypaths)

i i

r

Velocity 1

Velocity 2

 Figure 24. Two slinkys – a metal slinky and a plastic slinky – attached together using small pieces of electrical tape. The two slinkys can be used to illustrate reflection and transmission of wave energy at a boundary between elastic materials with differing elastic properties. 

Figure 25. Reflection and refraction (transmission) of seismic waves (P or S waves) at an interface separating two different materials. Some energy is reflected and some is transmitted. The effect can be illustrated with two slinkys – one metal and one plastic – taped together. Waves traveling along one slinky are partially reflected at the boundary between the two types of slinkys. For seismic waves in the Earth, an incident P or S wave also results in converted S and P energy in both the upper material (reflected) and the lower material (transmitted). If the angle of approach of the wave (measured by the angle of incidence, i, or the associated raypath) is not zero, the resulting transmitted P and S raypaths will be bent or refracted at the boundary. A similar change in angle is also evident for the reflected, converted waves.

 Surface Waves: The Love wave (Figure 12; Table 2) is easy to demonstrate with a slinky or a double length slinky. Stretch the slinky out on the floor or on a tabletop and have one person at each end hold on to the end of the slinky. Generate the Love wave motion by quickly moving one end of the slinky to the left and then to the right. The horizontal shearing motion will propagate along the slinky. Below the surface, the Love wave motion is the same except that the amplitudes decrease with depth (Table 2). Using the slinky for the Rayleigh wave (Figure 11, Table 2) is much more difficult. With a regular slinky suspended between two people, one person can generate the motion of the

Rayleigh wave by rapidly moving his or her hand in a circular or elliptical motion. The motion should be up, back (toward the person generating the motion), down, and then forward (away from the person), coming back to the original location and forming an ellipse or circle with the motion of the hand. This complex pattern will propagate along the slinky but will look very similar to an S wave (compare Figures 10 and 11). Excellent illustrations of the wave motion of Love and Rayleigh waves can also be found in Bolt (1993, p. 37). Rayleigh wave motion also decreases with depth below the surface. Further details on the characteristics and propagation of Love and Rayleigh waves can be found in Bolt (1993, p. 37-41).  Oscillations of the Whole Earth: When a very large earthquake occurs, long period surface wave energy penetrates deep within the Earth and propagates all around the globe. At particular points around the globe, the timing between this wave energy (which just keeps circling the globe for many hours or even days before dying out or attenuating) results in constructive and destructive interference of the wave energy. The resulting oscillations at certain frequencies are called normal modes or free oscillations of the Earth and are vibrations of the whole Earth. Further information about Earth normal modes, the phenomenon of standing waves, and diagrams illustrating the modes of vibration of the whole Earth are available in Bolt (1993, p. 34-37 and 144-145) and Shearer (1999, p. 158-162).

 Seismic Waves in the Earth: Seismic body waves (P and S waves) travel through the interior of the Earth. Because confining pressure increases with depth in the Earth, the velocity of seismic waves generally increases with depth causing raypaths of body waves to be curved (Figure 26). Because the interior structure of the Earth is complex and because there are four types of seismic waves (including dispersive surface waves), seismograms, which record ground motion from seismic waves propagating outward from an earthquake (or other) source, are often complicated and have long (several minutes or more) duration (Figures 27 and 28). An effective computer simulation that illustrates wave propagation in the Earth is the program Seismic Waves (Figure 29) by Alan Jones (see reference list). Using this program, which shows waves propagating through the interior of the Earth in speeded-up-real-time, one can view the spreading out of wavefronts, P, S, and surface waves traveling at different velocities, wave reflection and P-to-S and S-to-P wave conversion. The program also displays actual seismograms that contain arrivals for these wave types and phases. Exploring wave propagation through the Earth with the Seismic Waves program is an excellent follow-up activity to the seismic wave activities presented in this teaching guide.

Rad

i us =

637

1 km

Rad i

us =

633 6

km

Radi

us =

348

6 km

Radi

us =

1216 km

Outer Core

InnerCore

Mantle

Earthquake

Crust (thickness exaggerated)

P and S raypaths

Distance along surface (km)

Angular Distance(degrees) Seismo-

graph

P, SS

PP

Surface Waves

PS

pP

PcP

PKP

  Figure 26. Cross section through the Earth showing important layers and representative raypaths of seismic body waves. Direct P and S raypaths (phases), including a reflection (PP and pP), converted phase (PS), and a phase that travels through both the mantle and the core (PKP). P raypaths are shown by heavy lines. S raypaths are indicated by light lines. Additional information about raypaths for seismic waves in the whole Earth and illustrations of representative raypaths are available in Bolt (1993, p. 128-142) and Shearer (1999, p. 49-60). Surface wave propagation (Rayleigh waves and Love waves) is schematically represented by the heavy wiggly line. Surface waves propagate away from the epicenter, primarily near the surface and the amplitudes of surface wave particle motion decrease with depth.

   Figure 27. Seismograms recorded by a 3-component seismograph at Nana, Peru for an earthquake located near the coast of central Chile on September 3, 1998. The three seismograms record motion in the horizontal (east-west and north-south) and the vertical (Z) directions. P, S, Rayleigh and Love waves are identified on the record. The S wave arrives significantly after the P-wave because S-wave velocity in rocks is lower than P wave velocity. Additional arrivals between the P and the S wave are P and S waves that have traveled more complicated paths (such as the pP and PP phases and P-to-S converted phases, Figure 26) from the earthquake location to the seismograph. The surface waves arrive after the S waves because surface wave velocities in rocks are lower than the shear wave velocity. The surface waves extend over a long time interval because surface wave propagation is dispersive (the velocity of propagation is dependent on the frequency of the wave). This dispersive character can easily be seen in the Rayleigh wave on the vertical (Z) component seismogram in that the earliest Rayleigh wave energy has a longer period (lower frequency; see Figure 1) than the later arriving waves. The seismic data that are displayed here were acquired from the IRIS website (www.iris.edu) using the WILBER program from the IRIS Data Management Center.

 Figure 28. Model of the Earth's interior and selected raypaths for seismic wave propagation from the 1994 Northridge earthquake to seismograph stations around the world. Because of the existence of several types of seismic waves and the complex structure of the Earth's interior, many arrivals (phases) of seismic energy are present and are identified on the seismograms. This figure is part of a poster (Hennet and Braile, 1998) that is available from IRIS. 

 Figure 29. Partial screen views of the Seismic Waves computer program. The upper image shows seismic wavefronts traveling through the Earth's interior five minutes after the earthquake. The lower image shows the wavefronts 7 minutes after the earthquake. P waves are shown in red; S waves are shown in blue; and Surface waves are shown in yellow. Three and four letter labels on the Earth's surface show relative locations of seismograph stations that recorded seismic waves corresponding to the wavefront representations in the Seismic Waves program.

 The Slinky: The slinky was invented in 1943 and over 250 million of them have been sold. The history of the slinky, including its invention and the information about the company that manufactures the slinky, can be found at the discovery and slinkytoys internet addresses in the reference list. Slinkys (Figure 30), including the original metal slinky, the plastic slinky, slinky junior, several "designer" slinkys made from a variety of metals, and slinky toys can be found at the slinkytoys internet address listed in the reference list. Both the original metal slinky and plastic slinkys are usually available (for about $2) at discount department stores such as K-Mart, Wal-Mart and Target. A "long" slinky can be ordered, but can also be made by taping together two regular slinkys with small pieces of plastic electrical tape to make a double-length slinky. The original metal slinky is the most effective type of slinky for most of the wave propagation activities. A

double-length slinky is useful for illustrating Love wave motion (and the concept of standing waves) on the floor. The plastic slinky does not propagate compressional or shear waves as efficiently as the metal slinky, but is useful for illustrating reflection and transmission of wave energy at a boundary between two elastic materials with different properties. For this demonstration, tape a metal slinky and a plastic slinky together by attaching the end coils with small pieces of plastic electrical tape. 

 Figure 30. Slinkys. A. Original metal slinky; B. Plastic slinky; C. Long slinky; D. Slinky junior. Slinky lessons for teaching, including physics and wave activities, can be found at the Newtons (from the Newton's Apple PBS television program), and the teachingtools and eecs.umich (National Engineers Week activities) internet addresses. The National Engineers Week slinky activities are associated with a video ("Slinky Science Shindig") that is available from the eweek.org address. Slinkys are educational and fun! It is useful to have many of them available for the activities described in this teachers guide and for your students to perform the activities, and to experiment and discover.

 Summary: The slinky in various forms provides an excellent model to demonstrate and investigate seismic wave characteristics and propagation. A summary of the slinky models and their uses in demonstrations and activities is given in Table 3. Additional information on seismic waves, wave propagation, earthquakes and the interior of the

Earth can be found in Bolt (1993, 1999). Illustrations of seismic wave propagation through the Earth and seismograms are contained in Bolt (1993, 1999) and Hennet and Braile (1998).  

Table 3: Slinky Types, Models and DemonstrationsNumber Slinky Types/Models Demonstrations Figure

Number 1 Regular metal slinky P and S waves 13, 14

2 Long slinky (or attach 2 regular slinkys together with plastic electrical tape)

Love wave on floor or tabletop ---

3 Slinky with cardboard “building”

Illustrate that P and S energy is propagating and causes the cardboard building to shake as wave arrives. Differences in shaking can be seen for P and S motion

15, 16

4 Two slinkys (plastic and metal) attached with plastic electrical tape

Reflection and transmission of energy at a boundary between materials of different types (elastic properties or seismic velocities)

24, 25

5 Five slinkys attached to wood block

Show that waves propagate in all directions from source; that travel time is different to different distances; and that wave vibration for P and S sources will be different in different directions from the source

17, 18, 19

6 Two slinkys attached to wood block, one with soft foam within the coils; cardboard buildings can be attached to the slinkys to help see the differences in shaking

Illustrate the concept of attenuation due to absorption of energy during propagation and that some materials propagate waves more efficiently than other materials

22, 23

  Notes to the Teacher: The activities described in this teachers guide are designed for both classroom demonstration and inquiry-based activities for students. The elastic properties of a spring and the waves in a water tank activities are appropriate for student experiments. Several of the slinky activities and the human wave demonstration actively

involve students in the class. The slinky demos should also be performed by the students to increase their understanding of the wave propagation characteristics and concepts. All of the activities provide opportunities for developing and practicing observation skills and experience with measuring and timing. These demonstrations and activities are suitable for inclusion in Earth science teaching at a variety of levels. At higher grade levels, more complete treatment, increased emphasis on necessary vocabulary (defining the properties of elasticity, describing the characteristics of motion of the different types of seismic waves, etc.), involving quantitative measurements (measuring and graphing the elasticity of a spring, calculating the velocity of propagation of waves, etc.), and connection to related seismology and other Earth science lessons and activities are desirable.  The depth of investigation and the length of time devoted to the seismic wave activities will depend on the grade level and characteristics of the students, time available, and teacher preference. The teacher will also need to determine the "degree of constructivism" to be employed in the teaching strategy for these demonstrations and activities. For example, several of the activities, such as the P- and S-wave propagation in a slinky, could be performed first as a demonstration and then by the students to develop further understanding by first-hand experience. Alternatively, the teacher could choose to have a brief classroom discussion about P and S waves and then challenge the students to use the slinky to discover P- and S-wave propagation and determine the primary characteristics of the waves (property of propagation, material returns to original condition after the wave has passed, reflection of wave energy, particle motion) and the distinctive differences between P and S waves. This student exploration would then be followed by a "final" demonstration to emphasize the key concepts and clarify any misunderstandings. The seismic wave activities should normally be included in an earthquake unit of an Earth science curriculum that also covers plate tectonics and the causes of earthquakes, Earth's interior structure, seismographs and seismograms, earthquake location methods, and earthquake hazards. Appropriate assessment methods for the activities and science content presented here can include both written and oral responses by students and specific assessment activities. The scope and depth of questioning will depend on class level, time devoted to the seismic wave activities and how much related Earth science material (such as studies of plate tectonics, earthquake statistics and hazards, Earth structure, etc.) has been covered. If student teams have completed the elasticity of a spring, waves in water, and/or velocity of propagation experiments, the teacher will have some written material from the results of the student's experiments that can be assessed. Here are some suggested activities that can be used for authentic assessment: (1) Have students perform the P and S wave slinky demonstrations and describe their observations and the wave characteristics that they observe. (2) Have students repeat the elasticity experiment with a spring that has a different spring constant (a "weaker" or "stronger" spring). Have students predict what the graphed line will look like in comparison with their previous result, then perform the experiment and graph the results. Their predictions should be close to the actual results. (3) Ask students to predict what would

happen if a rubber band was used instead of the spring in the elasticity experiment (they should describe the expected result in terms of the two main properties of elasticity). (4) Provide the students with a graph showing a hypothetical seismic wave. Have the students interpret the graph and identify principal wave characteristics (frequency, amplitude, peaks, etc.). (5) Provide the students with copies of unlabeled diagrams illustrating the four types of seismic waves (lower three grids of the perspective views of P, S, Rayleigh and Love wave propagation in Figures 9-12 or from Bolt (1993, p. 27 and p. 37; remove labels identifying wave type). Have the students identify which diagram corresponds to the four types of seismic waves and what characteristics of the wave motion allow them to identify the wave type. (6) Obtain the Seismic Waves computer program and demonstrate to the students or provide the students with the opportunity to run the program. Next, with the program available on a monitor, start waves propagating (pause or restart as necessary) for one of the earthquakes and have the students watch the wavefront diagram (interior of Earth; it is convenient to set the display to view only the Earth cross section view; use the "Options" menu and "Select View …" to choose the cross section view) and have them answer the following questions. What are the approximate shapes of the initial (in the upper mantle near the source) P and S wavefronts? Why are they shaped that way? From the P and S wavefronts, estimate the relative velocity of the S wave as compared to the P wave (for example, how much longer does it take for the S wave to arrive at a station or to travel from the source to the core-mantle boundary as compared to the P wave)? How long does it take for the P wave to travel directly through the Earth to the opposite side of the Earth from the source? This distance (the diameter of the Earth) is about 12,742 km. From these measurements, what is the approximate average velocity for P waves in the Earth (in km/s)? Explain the new wavefronts that are generated when the P or S wave hits the core-mantle boundary. Why is there no S wave that travels directly through the Earth to the other side? Can there be S waves in the inner core (the program may not be of much help here except to visualize wavefronts that propagate in the Earth's core because no S wave phases from the inner core are displayed; not all seismic phases or raypaths are illustrated by the program)? How could S waves be generated so that they would travel through the inner core? Open the whole Earth view (surface of Earth with oceans and continents is visible). How are the patterns of the wavefronts that you see propagating and expanding from the source similar to the water waves in the wave tank experiment (or generated by a pebble dropped into a mud puddle or a pond)? How are they different? Below are some suggested questions for a written assessment. (1) What is the source of energy for the generation of seismic waves; in the slinky demonstrations? in the Earth? (2) How could we demonstrate that energy is carried by the waves in the slinky demonstrations? (3) How can we determine the velocity of propagation of a wave? (4) Explain the property of elasticity. (5) Explain how the slow motions of the Earth's plates (like slowly deforming the stretched slinky in the P and S demonstrations alternative methods of wave generation) can produce a rapid release of energy (slip along a fault in an earthquake, release of stored elastic energy in the slinky) resulting in seismic waves propagating in the Earth (or the slinky). 

The activities and the science content contained in this teachers guide have significant connections to the National Science Education Standards (NSES; National Research Council, 1996) as detailed in Table 4.                 

Table 4. "Seismic Waves and the Slinky: A Guide for Teachers" and the National Science Education Standards (NSES).

NSES StandardHow standard is addressed by Seismic Waves

and the Slinky demonstrations, lessons and activities*

Science Teaching Standards Many of the activities are inquiry-based (A, B) and provide opportunities for ongoing assessment (C).

Professional Development Standards

The guide for teachers provides opportunities and appropriate resource material for teachers to learn about an Earth science topic that is not likely to have been included in their previous educational experiences (A, C) and includes suggestions for effective teaching strategies (B).

Assessment Standards Authentic assessment activities and questions for assessing achievement in learning key concepts are included (C).

Science Content Standards  

- Unifying Concepts and Processes in Science

Activities provide experience with observation, evidence and explanation, and constancy, change and measurement.

- Science as Inquiry Activities provide opportunities for practice of inquiry and of fundamental science skills (Grades 5-8 and 9-12, A).

- Physical Science Standards Activities explore properties and changes of properties in matter, motion and forces, transfer of energy (Grades 5-8, B).

Activities explore structure and properties of matter, motions and forces, and interactions of energy and matter (Grades 9-12, B).

- Earth and Space Science Activities explore structure of the Earth system (Grades 5-8, D).

Activities relate to energy in the Earth system (Grades 9-12, D).

- Science in Personal and Social Perspectives

Activities explore natural hazards (Grades 5-8, F).

Activities explore natural and human-induced hazards (Grades 9-12, F).

Table 4. (cont.)

Science Education Program Standards

Seismic wave activities are developmentally appropriate, interesting and relevant, and emphasize student understanding through inquiry, and are connected to other school subjects (B).

Seismic wave activities provide practice with mathematics and analysis skills (C).

Activities provide experience with a variety of materials and resources for experimentation and direct investigation of phenomena (D).

Science Education System Standards

Because only relatively simple and inexpensive resources are necessary to perform the seismic wave demonstrations and activities, they are easily accessible to all students.

 *Letters in parentheses identify specific standards within the six areas (Science Teaching, Professional Development, Assessment, Science Content, Science Education Programs, and Science Education System Standards) of the NSES.  

References: Bolt, B.A., Earthquakes and Geological Discovery, Scientific American Library, W.H.

Freeman, New York, 229 pp., 1993.Bolt, B.A., Earthquakes, (4th edition), W.H. Freeman & Company, New York, 364 pp.,

1999.Earthquake, NOVA series videotape, 58 minutes, available from 800-255-9424;

http://www.pbs.org, 1990.Hennet, C., and L.W. Braile, Exploring the Earth Using Seismology – Color Poster, The

IRIS Consortium, Washington, DC., www.iris.edu, 1998.IRIS website, http://www.iris.edu.Jones, Alan, Seismic Waves, Computer program for visualizing seismic wave

propagation through the Earth's interior, download from: http://www.geol.binghamton.edu/faculty/jones.

Living with Violent Earth: We Live on Somewhat Shaky Ground, Assignment Discovery series videotape, Discovery Channel, 25 minutes, http://www.dsc.discovery.com, 1989.

National Research Council, National Science Education Standards, National Academy of Sciences, Washington, D.C., 262 pp., 1996.

Rutherford, B., and S. A. Bachmeyer, Earthquake Engineering – The Epicenter Project Book, Pitsco, Inc., Pittsburg, Kansas, 24 pp., http://www.pitsco.com/, 1995.

Seismology -- Resources for Teachers, http://www.eas.purdue.edu/k-12/seismology_resources.html, a list of seismology-related reference materials for education.

Shearer, P. M., Introduction to Seismology, Cambridge University Press, Cambridge, UK, 260pp, 1999.

 Slinky websites:

http://www.discovery.com/stories/history/toys/SLINKY/shoulda.htmlhttp://www.slinkytoys.com/main.htmhttp://www.tpt.org/newtons/9/slink.htmlhttp://www.teachingtools.com/SlinkyShindig/slinky.htmlhttp://www.eecs.umich.edu/~coalitn/sciedoutreach/funexperiments/quickndirty/

eweek/ slinky.htmlhttp://www.eweek.org/1999/Forms/disce.html#slinky

Zubrowski, B., Making Waves, Beech Tree Books, New York, New York, 96 pp, 1994.   

Chapter 1: INTRODUCTION

Examples of 3-D Data ImprovementSurvey Design

Volume Concept

Slicing the Data Volume

Manipulating the Slices

Dynamic Range and Data Loading

Synergism and Pragmatism in Interpretation

Questions

Related LinksAdditional Readings

 INTRODUCTION   The earth has always been three-dimensional and the petroleum reserves we seek to find or evaluate are contained in three-dimensional traps. The seismic method, however, in its attempt to image the subsurface has traditionally taken a two-dimensional approach. It was 1970 when Walton (1972) presented the concept of three-dimensional seismic surveys.The essence of the 3-D method is areal data collection followed by the processing and interpretation of a closely-spaced data volume. Because a more detailed understanding of the subsurface emerges, 3-D surveys have been able to contribute significantly to the problems of field appraisal, development and production as well as to exploration.The fundamental objective of the 3-D seismic method is increased resolution. The resolving power of seismic data is always measured in terms of the seismic wavelength, which is given by the quotient of velocity and frequency (Figure 1-3). Seismic velocity increases with depth because the rocks are older and more compacted. The predominant frequency decreases with depth because the higher frequencies in the seismic signal more quickly attenuated. The result is that the wavelength increases significantly with depth, making resolution poorer.Figure 1-2 summarizes resolution issues. Vertical resolution has two limits, both resulting from the interaction of the wavelets from adjacent reflecting interfaces. The limit of

separability is equal to one-quarter of a wavelength (or half a period) and is simply the bed thickness corresponding to the closets separation of two wavelets of a given bandwidth (Figure 1-4). For thinner intervals than this, the amplitude is progressively attenuated until the limit of visibility is reached, when the reflection signal becomes obscured by the background noise. The limit of visibility depends on the acoustic contrast of the geologic layer of interest relative to the embedding material, the random and systematic noise in the data, and the phase of the data or the shape of the seismic wavelet.Migration is the principal technique for improving horizontal resolution, and in doing so performs three distinct functions. The migration process (1) repositions reflections out-of-place because of dip, (2) focuses energy spread over a Fresnel zone, and (3) collapses diffraction patterns from points and edges. Seismic wavefronts travel in three in three dimensions and thus it is obvious that all the above are, in general, three dimensional issues. 

Figure 1-4   

Examples of 3-D Data Improvement The interpreter of a 2-D vertical section normally assumes that the data were recorded in one vertical plane below the line traversed by the shots and receivers. The extent to which this is not so depends on the complexity of the structure perpendicular to the line. Figure 1-6 demonstrates that, in the presence of moderate structural complexity, the points at depth from which normal reflections are obtained my lie along an irregular zig-zag track. Only by migrating along and perpendicular to the line direction is it possible to resolve where these reflection points belong in the sub-surface. 

Figure 1-6 Figure 1-9 shows improved continuity of an unconformity reflection. The 2-D migration has collapsed most of the diffraction patterns but some confusion remains. The crossline component of the 3-D migration removes energy not in the plane of this section and clarifies the shape of the unconformity surface in significant detail.Figure 1-13 shows portions of the three lines passing through and close to a salt diapir. Line 180 shows steeply-dipping reflections at the edge of the salt mass, brought into place by the 3-D migration. Line 220 shows an apparent anticline which is caused by reflections dipping up steeply toward the salt face in a plane perpendicular to that of Figure 1-13. In this prospect, 3-D migration imaged reflections underneath a salt face (Blake, Jennings, Curtis, Phillipson, 1982). 

Figure 1-13 When comparing sections before and after 3-D migration to appraise its effectiveness, it is important to bear in mind the way in which reflections have moved around. In the

presence of dip perpendicular to the section under scrutiny, the visible data before and after 3-D migration are different. It is unreasonable to compare detailed character and deduce what 3-D migration did. It is possible to compare a section before 3-D migration with the one from the same location after 3-D migration and find that a good quality reflection has disappeared. The migrated section is not consequently worse; the good reflection has simply moved to its correct location in the subsurface. 

Survey Design The sampling theorem requires that, for preservation of information, a waveform must be sampled such that there are at least two samples per cycle for the highest frequency. For example, 4 ms sampling is theoretically adequate for frequencies up to 125 Hz. In practice normally require at least three samples per cycle for the highest frequency. With this safety margin, 4 ms sampling is adequate for frequencies up to 83 Hz.If the sampling theorem is not satisfied the data are aliased. In the case of a dipping event, the spatial sampling of that event must be such that its principal alignment is obvious; if not, aliases occur and spurious dips result after multichannel processing. Table 1-2 shows the frequencies at which this aliasing occurs for various dips and subsurface spacings. Clearly, a 3-D survey must be designed such that aliasing during processing does not occur. Tables like the one presented can be used to establish the necessary spacing considering the dips and velocities present.Table 1-3 also shows the two formulas needed to calculate the width of the extra strip around the periphery of the prospect over which data must be collected in order to ensure proper imaging in the area of interest. The calculation of migration distance, the extra fringe width needed for structure, should use the local value of dip measured perpendicular to the prospect boundary. The Fresnel zone radius, the extra fringe width needed for stratigraphy, needs to be considered for the proper focusing of amplitudes. The two strip, or fringe, widths thus calculated should be added together in defining the total survey area.A typical 3-D seismic interpreter does not get involved in designing surveys but nevertheless needs to appreciate these issues.Proper design of a 3-D survey is critical to its success, and sufficiently close spacing is vital. The formulas of Table 1-3 are addressing structural design issues. In areas of shallow dip where the survey objectives are stratigraphic, the selected spacing must be such that there are at least two samples within the lateral extent of any expected stratigraphic feature of interest, for example the width of a channel. 

Volume Concept Collection of closely-spaced seismic data over an area permits three-dimensional processing of the data as a volume. With 3-D data, the interpreter is working directly with a volume rather than interpolating a volumetric interpretation from a widely-spaced grid of observations. One property of the volume pervades everything the 3-D interpreter does: The surface seismic wavefield is closely sampled in every direction, so that there is no grid loop around which the interpreter must tie, and no grid cell over which he must guess at the surface structure and stratigraphy.Figure 1-16 shows a view of a 3-D volume through a salt dome. It demonstrate the volume concept well and the interpreter can use a display of this kind to help in appreciation of subsurface three-dimensionality. Figure 1-17 shows another cube, in this case generated interactively, which helps in the three-dimensional appreciation of a much more detailed subsurface objective. Neither of these displays, however, permits the interpreter to look into the volume data.True 3-D display has recently become a reality on computer workstations and Figure 1-18 shows an example. The portion of the volume being displayed is composed of voxels, or volume elements, and these are rendered with differing degrees of transparency so that the interpreter can really see into the volume. In Figure 1-18 there are four interpreted surfaces as well as the semi-transparent data. As with any volumetric display the dynamic range is reduced because of the quantity of data viewed. 

Figure 1-18   

Slicing the Data Volume The vast majority of 3-D interpretation is performed on slices through the data volume. There are no restrictions on the dynamic range for the display of any one slice, and therefore all the benefits of color, dual polarity, etc., can be exploited. The 3-D volume contains a regularly-spaced orthogonal array of data points defined by the acquisition geometry and maybe adjusted during processing. The three principal directions of the array define three sets of orthogonal slices or sections through the data, as shown in Figure 1-19.

The vertical section in the direction of boat movement or cable lay-out is called a line (sometimes an inline). The vertical section perpendicular to this is called a crossline. The horizontal slice is called a horizontal section, time slice, Seiscrop* section, or depth slice. A diagonal line may be extracted to tie two locations of interest, such as wells. A zig-zag sequence of diagonal line segments may be necessary to tie together several wells in a prospect. All these vertical sections are referred to as arbitrary lines.More complicated slices are possible for special applications. A slice along or parallel to a structurally interpreted horizon, and hence along one bedding plane, is a horizon slice, horizon Seiscorp section, or amplitude map. Fault slices generated parallel to a fault face have various applications in structural and reservoir interpretation.  

Manipulating the Slices Because 3-D interpretation is performed with data slices and because there is a very large number of slices for a typical data volume, several innovative approaches for manipulating the data have emerged. The large amount of a regularly-organized data in 3-D volume gives the interactive approach enormous benefits.

(1) (1)   Data Management – The interpreter needs little or no paper; the selected seismic data display is presented on the screen of a color monitor and the progressive results of interpretation are returned to the digital database.

(2) (2)   Color – Flexible color display provides the interpreter with maximum optical dynamic range adapted to the particular problem under study.

(3) (3)   Image composition – Data images can be composed on the screen so that the interpreter views what is needed, no more and no less, for the study of one particular issue. Slices through the data volume are designed by the user in order to customize the respective to the problem.

(4) (4)   Idea flow – the rapid response of the system makes it easy to try new ideas. The interpreter can rapidly generate innovative map or section products in pursuit of a better interpretation.

(5) (5)   Interpretation consistency – The capability to review large quantities of data in different forms means that the resulting interpretation should be more consistent with all available evidence. This is normally considered the best measure of interpretation quality.

(6) (6)   More information – Traditional interpretative tasks performed interactively will save time; however, the extraction of more detailed subsurface information is more persuasive and far-reaching.

Dynamic Range and Data Loading Interactive interpretation must commence with data loading and this is a critical first step. Should the data be loaded at 8, 16, or 32 bits? Is clipping of the highest amplitudes acceptable?.

Data processing has always been performed using 32 bits to describe each amplitude value. This large word size ensures that significance is retained during all computations.Figure 1-23 shows a typical statistical distribution of amplitudes in a data volume. There are large number of a very low amplitudes, a fairly large number of moderate amplitudes but a very small number of high amplitudes. Mainstream structural interpretation tends to work on moderate amplitude horizons. The high amplitude tails of the distribution are localized anomalies which, in tertiary clastic basins, are often the hydrocarbon bright spots. The interpreter avoids the low amplitudes as much as possible causes they are the most subject to noise.If interpretation is to be conducted using 8-bits only, scaling 32-bit amplitude numbers to 8-bit amplitude numbers must be done during data loading. If the maximum amplitude in the volume is set 128, relative amplitudes are preserved within the precision of the 8-bits. Clipping of the highest amplitudes is a common reaction to this problem so that a smaller value is set to 128. More dynamic range is then available for the mainstream structural interpretation but the highest amplitudes are destroyed and hence unavailable for stratigraphic or reservoir analysis.A common and generally desirable solution is to load the data using 16 bits for each amplitude value. In this way clipping is irrelevant and unnecessary as there is plenty of dynamic range for structural interpretation and bright spot studies. 

Figure 1-23 

Synergism and Pragmatism in Interpretation Seismic technology has, over the years, become increasingly complex. Whereas a party chief used to handle data collection, processing, and interpretation, experts are now generally restricted to each discipline. Data processing involves many highly sophisticated operations and is conducted in domains unfamiliar to the nonmathematically-minded interpreter.Seismic interpretation today thus involves a wide range of seismic technologies. If the results of these are studied by the interpreter in concert, significant synergism can result. However, pragmatism retains its place. The interpreter must continue to take a broad view, to integrate geology and geophysics, and, to an increasing degree, engineering, and to make simplifying assumptions is order to get the job done. The progress of seismic interpretation depends on the continued coexistence of technological synergism and creative pragmatism. 

Questions   

1. In each of the four principal branches of exploration geophysics, what particular property is measured?

 2. What kind of measurements were made in the earliest exploration geophysical

surveys?   

3. Name two factors that can cause the path followed by a seismic wave to change direction.

Related Links http://basalt.geol.vt.edu/mgi/4174/node1.htmlhttp://walter.kessinger.com/work/seisx_acquisition.htmlhttp://www.gravmag.com/gmprimr.htmlhttp://www.greatgeophysics.com/logginginfo.htmlhttp://www.technos-inc.com/Surface.html#I9

Chapter 2: STRUCTURAL INTERPRETATION Direct Contouring and the Importance of the Strike PerspectiveFault Recognition and MappingInterpretation in the Vicinity of SaltComposite DisplaysAdvantages and Disadvantages of Different DisplaysInterpretation ProceduresSubtle Structural FeaturesVisualization and Autotracking

Questions Related Links Additional Readings  

Direct Contouring and the Importance of the Strike Perspective The 3-D seismic interpreter works with a volume of data. Normally this is done by studying some of each of the three orthogonal slices through the volume. The interpreter of structure needs to be able to judge when to use horizontal sections and when to use vertical ones in the course of an overall interpretive project.Figure 3-1 demonstrate the conceptual relationship between a volume of subsurface rock and a volume of seismic data. Consider the rectangular solid of Figure 3-1 to be the equivalent volume of seismic data. The gray plane is a dipping reflection and its intersection with the three orthogonal faces of the solid show the two components of dip

and the strike before. Hence the attitude of a reflection on a horizontal section indicates directly the strike of the reflecting surface. This is the fundamental property of the horizontal section from which all its unique interpretive value derives.Contours follow strike and indicate a particular level in time or depth. When an interpreter picks a reflection on a horizontal section, it is directly a contour on some horizon at the time (or depth) at which the horizontal section was sliced through the data volume.Figure 3-2 shows three horizontal sections, four milliseconds apart. By following the semicircular black event (peak) from level to level and drawing contours at an appropriate interval, the structural contour map at the bottom of Figure 3-2 was generated. Note the similarity in shape between the sections and the map for the anticlinal structure and the strike east of the faults. 

Figure 3-2 Figure 3-5 demonstrate a simple exercise in direct contouring from a suite of horizontal sections. The red event (trough) expanding in size from left to right has been progressively circumscribed in the lower part of the figure. The last frame in a raw contour map of this horizon. This first structural representation has been made quickly and efficiently without the traditional intermediate tasks of timing, posting and contouring. When drawing structural contours from horizontal sections in this way, it is wise to visualize the three-dimensionality of the structure and to appreciate where on the seismic waveform the contour is being drawn (Figure 3-4). The latter problem applies particularly to the use of variable area displays as used, for example, in Figure 3-5. The contour is here drawn all the way around the red event only because the dip is down all the way around the structure; this is a consistent point on the seismic waveform, namely its upper edge (Figure 3-4). 

Figure 3-5 An event on a horizontal section is generally broader than on a vertical section as dips are usually less than 45o. Figure 3-7 shows the effect of dip and frequency on the width on an event on a horizontal section. A gently dipping event is very broad and a steeply dipping event is much narrower. Increasing dip and increasing frequency both make horizontal section events narrower. The width of an event on a horizontal section is strictly half the spatial wavelength.Because typical dips are much less than 45o, fewer horizontal sections than vertical ones are needed to study the full extent of a reflection within a given data volume. This gives horizontal sections greater efficiency than vertical sections in structural mapping. Combining this benefit with the fact that horizon tracks (picks) are directly contours, then the value of horizontal sections to structural interpretation is sunstantial.  

Fault Recognition and Mapping

 When an interpreter works with 3-D data after having previously mapped from 2-D data over the same prospect, the most striking difference between maps is commonly the increased fault detail in the 3-D map. Figures 3-8 and 3-9 provide a typical comparison and also demonstrate increased detail in the shape of the structural contours. Comparison of Figures 3-10 and 3-11 also shows a considerable increase in the number of faults and in the structural detail. The three well locations indicated in blue appear structurally quite different on the 2-D and 3-D maps.Figure 3-12 shows a vertical section from the 3-D data which provided the map of Figure 3-9. The event terminations clearly show several faults. The horizontal section of Figure 3-13 is from the same data volume and, in contrast, does not show clear event terminations. Figure 3-14 shows four horizontal sections from a different prospect but one in a similar tertiary clastic environment. Here event terminations clearly indicate the positions of the three major faults on each of the four sections.Why are event terminations visible at the faults in Figure 3-14 but not in Figure 3-13? The answer lies simply in the relationship between structural strike and fault strike. Any horizontal section alignment indicates the strike of the feature. If there is a significant angle between structural strike and fault strike, the events will terminate.If structural strike and fault strike are parallel, or almost so, the events will not terminate but will parallel the faults. Comparison of Figure 3-13 and 3-9 demonstrate that situation.Figure 3-15 shows a variety of structural features: prominent faults, more subtle faults, culminations and various character changes. It is very important that horizontal sections play their proper role in fault interpretation. In the early stages of structural interpretation of a prospect, the major faults will be identified on some widely-spaced vertical sections. The way in which these faults join up into a fault framework should then be established from horizontal sections.Today’s interactive workstations help in the coordinated use of vertical and horizontal sections by providing the capability of cross-posting. When fault is picked on a vertical section, its intersection will appear on an intersecting horizontal section.When faults have been picked on several vertical and horizontal sections, the faults can be displayed as surfaces to check their geological validity. 

Figure 3-8  

Figure 3-10 

Figure 3-12 

Figure 3-15   

Interpretation in the Vicinity of Salt 

The horizontal section of Figure 3-18 shows a rim syncline surrounding a salt diapir. The narrow events around the salt indicate the steep dips near the intrusion. Figures 3-19 and 3-20 show a deeper horizontal section from the same volume without and with interpretation. The horizon of interest, marked in green of Figure 3-20, is intersected twice, once on either side of the rim syncline. The faulting at this level, marked in yellow, is complex but can be seen fairly well on this one horizontal section. From pre-existing 2-D data in the area only one of these faults had been identified (Blake, Jennings, Curtis, Phillipson, 1982).Interpretation of seismic reflection terminations against salt is a very important matter because many hydrocarbon traps are found in this structural position. Depth migration and pre-stack depth migration in 3-D have recently become economically feasible and have been used extensively for imaging under salt (Appendix A). It is the abrupt large velocity contrast that make this more elaborate migration necessary. After such processing the whole data volume is in depth and thus horizontal sections become depth slices. 

Composite Displays The interpreter of 3-D data is not restricted to single slice displays. Because the work is done with a data volume, composite displays can be helpful in appreciating three-dimensionality and also in concentrating attention on the precise pieces of data that provide insight into the problem at hand.Figure 3-24 is a composite of horizontal and vertical sections spliced together along their line of intersection. The vertical section shows that the circular structure is a syncline. The horizontal section pinpoints the position of its lowest point. The fault on the left of this structure can be followed across the horizontal section. Figure 3-25 provides a different view of the structure. The same horizontal section is here spliced to the portion of the vertical section above in the volume. 

Figure 3-24  In Figure 3-29 one horizon has been tracked indicating the interpret correlation across the faults. At the bottom of this figure a portion of the data from each of the four fault blocks is enlarged and again carries the interpreted track. Each block has been adjusted vertically to bring the track segments into continuity so that the correlation between these blocks of data can be assessed easily. 

Figure 3-29 

Interpretation Procedures

 The interpreter of 3-D data has a real opportunity to generate accurate subsurface structure maps but to do so a large amount of data must be studied. Figure 3-32 charts a recommended procedure for 3-D interpretation using an interactive workstation. The interactive capabilities required to follow this procedure include: 

(1) (1)   Automatic and manual tracking of horizons on vertical and horizontal sections(2) (2)   Automatic spatial horizon tracking and editing through a 3-D data volume(3) (3)   Correlation of vertical sections with well data(4) (4)   Extraction, storing and manipulation of seismic amplitudes(5) (5)   Manipulation of maps(6) (6)   Flexible use of color(7) (7)   Extraction and use of seismic attributes

 The procedure of Figure 3-32 also addresses several areas of stratigraphic and reservoir interpretation. Some of the important principles implicit in the procedure of Figure 3-32 are that you 

        Understand the phase of data before embarking on the mainstream interpretation

        Use horizontal sections to full advantage; benefit from the efficiency of strike         Study only as many vertical and horizontal sections as is necessary to provide

initial input control for automatic spatial tracking         Use intermediate horizon products to full advantage for refining the

interpretation         Do not smooth any map or map-style product until degree of smoothing

required can be judged intelligently         Engage in stratigraphic and reservoir studies in order to get the most out of

the data  

Figure 3-32  

Advantages and Disadvantages of Different Displays With increasingly successful amplitude preservation in seismic processing, interpreters are increasingly suffering from the limited optical dynamic range of conventional seismic displays. Too common are the variable area sections where some events of interest are heavily saturated and others have barely enough trace deflection to be visible. This applies to all displays, vertical and horizontal, made with variable area techniques. Horizontal sections, historically, were first made with variable area using one polarity only, normally peaks.

Dual polarity variable area provides five clearly discernible amplitude levels. The highest amplitude peaks are saturated and appear as continuous black areas; the medium amplitude peaks do not coalesce and appear as discontinuous black areas which look gray; the lowest amplitudes are below the variable area bias level and appear white; the medium amplitudes troughs appear pink; and the highest amplitude troughs are continuous red areas. In the detail in the seismic waveform provided by dual polarity variable area is inadequate, then the increased dynamic range of full variable intensity color is required. Gradational blue and red is most useful application; this is illustrated in the middle row of the sections in Figure 3-35 and explained in detail by the diagram of Figure 3-37.A further option available to the structural interpreter is horizontal sections displayed in phase, using instantaneous phase derived from the complex trace (Taner, Koehler and Sheriff, 1979). Phase indicates position on the seismic waveform without regard to amplitude, making a phase section like one with fast AGC (Automatic Gain Control), destroying amplitude variations and enhancing structural continuity. 

Figure 3-37  

Subtle Structural Features Some form of strike view of the data is very helpful in recognizing subtle faults and establishing the spatial patterns of faulting. Figure 3-22 shows many small faults affecting a thin limestone that are much more easily recognized horizontally than vertically.Figure 3-41 is a horizontal section, or time slice, from a data volume in which a subtle, small-throw fault became a significant part of the interpretation at the target level. The interpreter working on the data first noticed these on the horizontal sections and considered the fault real because it preserved its character over many contiguous sections.Figure 3-43 shows several straight lineations, principally through the black structural event, that are caused by subtle faulting and jointing. These are so subtle that they would never recognized on vertical sections. Here they are identified by the linear patterns that appear in the strike view.Horizontal sections are thus undoubtedly valuable in the study of faults, subtle and not so subtle. Coherence applied to seismic data and then viewed in time slice from is an extension of this value for both fault recognition and mapping.  

Visualization and Autotracking 

Once horizons and faults have been interpreted they can be visualized as surfaces (Figure 3-45). The relationship between the surfaces can then be studied to help in validation of the interpretation and in placement of the wells to intersect multiple objectives.The horizon surfaces above will have been produced using automatic spatial tracking starting from seed, or control, points on vertical and horizontal sections. This procedure uses the tracker in a controlled interpolatory manner, that is it is operating between points where the interpretation has been prescribed. With good data the automatic spatial tracker can be used in an extrapolatory manner from minimum seed points. Figure 3-46 is an example where the tracker was seeded in the northeast and moved outwards independently to define several fault blocks and the faults between them. Untracked points are also evident in Figure 3-47 and in several places they line up. These lineations of untracked points indicate places where the tracker had difficult and may indicate subtle faults, sharp changes of dip, facies changes or other boundaries. Thus lineations of untracked points can be used as a source of geologic information.  

Questions   

1. A P-wave traveling at speed of 3000 m/s in one layer refracts across a boundary into another layer where its speed increases to 4000 m/s. If the frequency of vibration is 30 Hz, is the wavelength of the refracted P-wave longer or shorter than the wavelength of the incident p-wave? Explain!

 2. An incident P-wave traveling at speed of 3000 m/s is critically refracted at a

boundary. If the critical angle is 30 degrees, what is the speed of the refracted P-wave?

 3. At a distance of 100 m from a source, the amplitude of a P-wave is 0.1000 mm,

and a distance of 150 m the amplitude diminished to 0.0665 mm. What is the absorption coefficient of the rock through which the wave is traveling?

Related Links http://walter.kessinger.com/work/seisx_theory.htmlhttp://www.seismo.unr.edu/ftp/pub/louie/class/100/seismic-waves.htmlhttp://www.geo.mtu.edu/UPSeis/waves.htmlhttp://www.gps.caltech.edu/~polet/body_waves.htmlhttp://www.eas.purdue.edu/~braile/edumod/slinky/slinky.htmhttp://www.gedling.notts.sch.uk/Science/vol/waves.html  

Chapter 3: RESERVOIR IDENTIFICATION  

Bright Spots As They Used To Be

The Character of Hydrocarbon Reflections

Examples of Bright Spots, Flat Spots, Dim Spots and Phase Changes

Phase Problems and Multiple Contacts

Use of Frequency, Amplitude Variations With Offset and Shear Waves

Philosophy of Reflection Identification

Questions and Interpreter Should Ask in an Attempt to Validate Hydrocarbon Indicators

The Occurrence of Hydrocarbon Indicators 

Questions

Related LinksAdditional Readings 

Bright Spots As They Used To Be With the improvements in seismic processing over two decades, we can now consider polarity and phase as well as amplitude and spatial extent. Frequency, velocity, amplitude / offset and shear wave information can also help in the positive identification of hydrocarbon indicators. Most direct hydrocarbon indication relates to gas rather than oil reservoirs as the effect on acoustic properties of gas in the pore space is significantly

greater than oil. Figure 5-2 (derived from Gardner, Gardner, and Gregory, 1974) summarizes the different effects of gas and oil and shows that the effect of either diminishes with depth.Figure 5-4 is interpreted sufficiently to highlight the various hydrocarbon indicators on the section. The flat spot is easily identified by its flatness, and because it is unconformable with adjacent reflections. Hence it is a good indicator of the hydrocarbon / water contact. Nevertheless there is no high amplitude associated with it.The reflection from the top of the reservoir (Figure 5-4) changes from a peak to a trough across the fluid contact and this again implies a significant change in acoustic properties between the gas sand above the hydrocarbon/water contact and the water sand beneath it.  

Figure 5-4   

The Character of Hydrocarbon Reflections If the seismic data under interpretation have been processed to zero phase, then the detailed character of the bright spots, flat spots and other hydrocarbon indicators can be very diagnostic. Figure 5-5 shows diagrammatically the hydrocarbon indicators which may be associated with different relative acoustic impedances of gas sand, water sand and embedding medium.The top diagram of Figure 5-5 illustrates the most common situation: the water sand has an acoustic impedance lower than the embedding medium and the impedance of the gas sand is further reduced. For this situation the signature of the sand is peak-over-trough and, for the gas-filled portion, the amplitude is greater. This is the classical zero-phase bright spot with high amplitude for the top and base reflections.In the second diagram the situation is reversed; the water sand has a higher acoustic impedance than the embedding medium and hence has a signature of trough-over-peak. When gas replaces some of the water in the pores of the sand, the acoustic impedance is reduced, the contrast is reduced at the upper and lower boundaries, and the reservoir is seen as a dim spot. Again, if the sand is thick enough, a flat spot can be expected at the point where the dimming occurs and this again will be a trough (red).In the third diagram the reduction in acoustic impedance of the sand, because of gas saturation, causes the acoustic impedance to change from a value higher than that of the embedding medium to one lower than that of the embedding medium. Hence the polarities of the reflections for the top and base of the sand switch. The signature changes from trough-over-peak to peak-over-trough across the fluid contact. Again, if the sands is thick enough, a fluid contact reflection should be visible and it will be a trough.Figure 5-6 shows the magnitude of acoustic impedance changes between water sand and hydrocarbon sand and hence the effect on seismic amplitude reflected from the interface between either of them and a uniform embedding medium. For a bright spot (without phase change) the water sand is located just right of the center line and the gas sand is much farther to the right. For a phase change, or polarity reversal, the movement from

water sand to gas sand must be from left to right across the center line. In the last situation illustrated in Figure 5-6, the bright spot is exactly the same in amplitude and phase as the one illustrated at the top of the figure; the difference is that the last one labeled phase change/bright spot combination, came from a water sand with higher acoustic impedance than the embedding shale and was thus located on the left of the center line. 

Figure 5-6  

Examples of Bright Spots, Flat Spots, Dim Spots and Phase Changes Figure 5-7 shows a Gulf of Mexico bright spot known to be a gas reservoir. The reservoir reflections have very high amplitude and hence the interference from other nearby reflections, multiples or noise small. The bright reflections show the zero-phase response of two reservoir sands, each peak-over-trough and located one on top pf the other. The upper sands is fairly thin so there is only a hint of a flat spot reflection at the downdip limit of brightness. The lower sand is much thicker and the flat spot reflection is very clear.Flat spot reflections are highly diagnostic indicators of gas but the interpreter should make several validity checks before drawing a conclusion. In Figure 5-7 the flat spot reflection is flat, bright and shows one symmetrical trough. 

Figure 5-7  Figure 5-21 is a practical example of a dim spot. The discovery well penetrates a gas column of about 400 ft (130 m) but the acoustic contrast of the gas sand with its embedding medium is small. Outside the reservoir the contrast between the sand and the embedding medium is much greater, as the amplitudes indicate. Here the reservoir is cemented, so the increase in acoustic impedance is caused partly by lack of gas and partly by the cementation. 

Figure 5-21 Figures 5-24, 5-25, 5-26 and 5-27 illustrate a phase change; all four figures are exactly the same piece of data displayed with different colors and gains. Figure 5-24 uses the standard blue and red gradational scheme and the amplitude anomaly is clearly visible. Its visibility is perhaps enhanced further by the yellow, green and gray color scheme of Figure 5-25. In order to check for a phase change, or polarity reversal, it is necessary to judge the structural continuity from the bright reflections to their non-bright equivalents downdip. There is a very great difference in amplitude between these, causing a great difference in color intensity. Figures 5-26 and 5-27 use the same colors respectively as

Figures 5-24 and 5-25 but with a higher gain applied to the data. This makes it easier to judge the downdip continuity on the left of the bright spot and hence to observe that red correlates with blue (Figure 5-26) and green correlates with yellow (Figure 5-27). In this way a polarity reversal is established. 

Figure 5-24  Figure 5-30 shows a horizon slice indicating a channel. To the northeast the channel is bright, to the southwest it is not. The structural contours for this horizon have been superimposed and they demonstrate that the bright part of the channel is structurally above the dim part. This combination of structural and stratigraphic information helps validate gas content.Figure 5-33 is an example of gas velocity sag. Here the high amplitudes are still in blue and red but the lower amplitudes are expressed in gradational gray tones. This provides the double benefit of highlighting the bright reflections and also helping establish fault definition by increasing the visibility of low amplitude event terminations. This section also demonstrate another phenomenon: there are bright events within the reservoir which have little expression outside, this illumination of internal layering is fairly common in clastic reservoirs. 

Figure 5-33  

Phase Problems and Multiple Contacts Unfortunately data phase is not always what it is supposed to be. Data processed to zero phase fairly often is close to 90o phase. Figure 2-24 shows a 90o phase flat spot and associated bright spot. It is the red and blue reflections together forming the flat spot that best demonstrates the 90o –phaseness of the data.Figure 5-36 shows a flat spot with 90o phase character, yellow-over-red, at the green arrow. This is further confused, e.g. by comparison with Figure 2-24, by the fact that here the flat spot appears to be broken into four pieces. This is fact caused by interference of strong internal reflections with the fluid contact reflection.When we observe what appears to be two flat spots (e.g. Figures 2-24 and 5-36) the question arises as to whether we could be seeing two fluid contact reflections, for example gas-oil and oil-water. In fact fluid contact is always an increase in acoustic impedance and thus two contacts in the same reservoir will always have the same character; so one red contact and one blue contact is possible.Figure 5-38 and 5-39 each show two flat spots from two separate contacts. The upper one is a gas-oil contact and the lower one an oil-water contact. Note that both fluid contact reflections are blue, as they should be for American polarity zero-phase data. Note the strong structurally dipping reflection separating the upper and lower reservoirs. They have a common oil-water contact, and the gas-oil contact only in the upper reservoir. The

somewhat high amplitude on the top reservoir reflection between the two flat spots demonstrate a bright response of oil. The difference in amplitude of the oil and the gas is clear with the higher dynamic range color scheme of Figure 5-39. 

Figure 5-38   

Use of Frequency, Amplitude Variations With Offset and Shear Waves Gas reservoirs attenuate high frequencies more than rocks without gas saturation, a good example where low instantaneous frequency anomalies indicate gas is shown in Figure 5-40.Interval velocity is reduced if a low velocity gas sand is included in the interval studied. For many years RMS velocities derived from normal moveout have been used to compute interval velocities, and for gross effects and trends this is valuable. The stability of interval velocities gets progressively worse for greater depths and also for thinner beds. This generally means that interval velocities are not sufficiently accurate to play a useful role in bright spot validation.The variation of amplitude with recording offset has recently become a popular subject because of the possibility of extracting a significant amount of lithologic information from this kind of data. However, there are many difficulties both of a theoretical and practical nature (Backus and Goins, 1984). Among the practical issues, the data are prestack and hence have a lower signal-to-noise ratio, and, being multidimensional, there are many possible modes of display.The application of the horizon slice concept has significantly increased the visibility of amplitude / offset effects for one horizon. Consider a volume of one line of prestack seismic data (Figure 5-42). The three dimensions are (1) CDP position along the line, (2) traveltime and (3) recording offset. The shape of one reflection without normal moveout correction is a cylindrical hyperbola as shown. By tracking this horizon and displaying the resultant amplitudes as if it were a slice, a horizon offset section is obtained.A horizon offset section prepared in this way is shown in Figure 5-43. The variation in amplitude with CDP position and with offset (approximately converted to incident angle) is shown for the trough immediately below the black arrow in Figure 5-41. The horizon offset section has been spatially smoothed, as an alternative to partial stacking, for increase of signal-to-noise ratio. The interpreter can observe, on this one section, the variation of amplitude with offset over many depth points for this horizon of interest. The amplitude increases with offset for most of the depth points and is hence consistent with gas content.This method of validation requires that the gas sand and the embedding medium have very different Poisson’s ratios. Because this is not always the case this method lacks

certainty, even on theoretical grounds. On practical grounds poor signal-to-noise ratio is a common problem.The full and proper application of AVO to 3-D data involves both 3-D imaging and 3-D display. It is a complicated and computationally-demanding activity. The variation of amplitude with offset can be expressed as a single gradient factor and horizon slices in AVO gradient can be produced. Up to the present time the demonstrated value of these displays has been small so not very much has been done. Far offset-near offset amplitude difference is similar to AVO gradient.Interpretation of shear wave amplitudes in conjunction with conventional compressional wave amplitudes can provide another method to bright spot validation. On land, S-wave data have generally been collected in a separate operation . S-waves are not transmitted through water so, at sea, it is necessary to use PSSP waves, mode converted at the water bottom.The fundamental underlying principle is that compressional waves are sensitive to the type of pore fluid within rocks, whereas shear waves are only slightly affected. Hence the S-wave response of a reservoir sand will change little from below to above the gas / water contact, while the P-wave response normally changes greatly. Referring to Figure 5-5, it is clear that the P-wave dim spot would correlate on an S-section with a higher amplitude reflection. Where a phase change occurs across the gas / water contact on the P-section, the correlative P-wave and S-wave reflections from the gas sand will have opposite polarity. 

Figure 5-40  

Philosophy of Reflection Identification Traditional approaches to reflection identification involve sliding a synthetic seismogram up and down on the real trace seeking a character match; we generally try to minimize the time mistie. With the all-too-common polarity and phase errors that may exist in our data, a more general and flexible approach will be more reliable. Let us not simply assume that the data phase and polarity are what they are supposed to be. If synthetic seismograms are used, the ones made with many different phases of wavelet should be compared to the real data. Colors is very helpful in detecting detailed character and recognizing phase and polarity errors.The interpreter should be able to understand the complete character of the seismic data in the region of the geologic interface being tied. All the local reflections need to be understood; the more they cannot be understood, the more questions hang over the reflection identification.Some general recommendations for seismic reflection identification then include: 

Don’t necessarily assume the data polarity and phase are what they are supposed to be.

If you are going to use synthetic seismograms, use a suite of seismograms of various phases and of both polarities.

If you have the ability to make comparisons in color, do so because much more detailed character will be visible and you will more readily recognize polarity and phase problems.

Consider all the local character, that is, seek to explain all the reflections in the neighborhood of the objective.

  Questions and Interpreter Should Ask in an Attempt to Validate Hydrocarbon Indicators 

(1) (1)         Is the reflection from the suspected reservoir anomalous in amplitude, probably bright?

(2) (2)         Is the amplitude anomaly structurally consistent?(3) (3)         If bright, is there one reflection from the top of the reservoir and one from

the base?(4) (4)         Do the amplitudes of the top and base reflections vary in unison, dimming

at the same point at the limit of the reservoir?(5) (5)         Are the data zero phase?(6) (6)         Is a flat spot visible?(7) (7)         Is the flat spot flat or dipping consistently with gas velocity sag or tuning?(8) (8)         Is the flat spot unconformable with the structure but consistent whit it?(9) (9)         Does the flat spot have the correct zero-phase character?(10) (10)     Is the flat spot located at the downdip limit of brightness (or dimness)?(11) (11)     Is a phase change (polarity reserval) visible?(12) (12)     Is the phase change structurally consistent and at the same level as the flat

spot?(13) (13)     Do bright spot, dim spot, or phase change show the appropriate zero-

phase character?(14) (14)     Is there a low-frequency shadow below the suspected reservoir?(15) (15)     If the reservoir is thick, are there significant reflections inside?(16) (16)     Do statistical crossplotting techniques indicate a flat spot?(17) (17)     Is there an anomaly in moveout-derived interval velocity?(18) (18)     Is a study of amplitude versus offset on the unstacked data likely to yield

further validation evidence?(19) (19)     Are any shear wave data available for further validation evidence?

 Every hydrocarbon indicator is potentially a reservoir, but any one indication can be spurious. Confident identification of a hydrocarbon necessarily involves the accumulation of evidence.Figure 5-47 contains several suspected hydrocarbon reservoirs. Try asking the above questions for these data. You should find affirmative answers for questions 1 through 15 for many separate reservoirs. How many did you find? While scrutinizing these data, it is useful to bear in mind simple reservoir models, such as those portrayed in Figure 5-5. An effective way to interpret the reservoirs from Figure 5-47 is to draw an overlay for the reservoir reflections. On the assumption of zero-phaseness, interfaces should be drawn

along crestal amplitudes, and it is important to mark the top and bottom of each suspected hydrocarbon reservoir and the top and bottom of the correlative aquifer in each case.Figure 5-49 presents the same section as Figure 5-47, but with the results of a well inserted. 

Figure 5-47  

The Occurrence of Hydrocarbon Indicators The nature of hydrocarbon indication – that is, whether the phenomenon is bright spot, phase change, or dim spot – depends on the relative acoustic impedances of hydrocarbon sand, water sand, and shale (Figure 5-5). Each of these acoustic impedances increases with depth (Figure 5-50) and they also each increase with rock age. It is difficult to be quantitative because they are also dependent on lithology, porosity, and local environment. Figure 5-50 is thus plotted for the qualitative product of depth and age. The effect of compaction on the shale causes its acoustic impedance to increase less rapidly than that of the sand. Below the point where the shale acoustic impedance crosses that of the water sand, phase changes must be occur. Below the point where the shale acoustic impedance crosses that of the hydrocarbon sand, dim spots must be occur. Of course, all the phenomena are reducing in visibility with depth and age, and somewhere there is a cutoff below which no hydrocarbon observations will be possible.Figure 5-51 is an attempt to separate the effects of depth and age. The depth is the depth of maximum burial, and old rocks are unlikely to have been at a shallow depth for all their geologic history. Nevertheless, Figure 5-51 indicates that bright spots will occur at great depths for very young rocks. It also indicates that hydrocarbon phenomena will occur in older rocks that are reasonably shallow. 

Questions  

1. Show that the refracted ray path between the points A and B, as shown in Figure 3-22 and defined by Fermat’s principle, satisfies Snell’s law.

2. Suppose that layer with a velocity of V1 = 1500 m/sec and thickness of 100 m lies above another layer with a velocity of V2 = 3000 m/s. Compute the expected crossover distance and intercept time for the critically refracted waves.

 3.      From a seismic refraction survey, the travel time data given in the accompanying

table were obtained for first arriving waves. 

a. Plot the data and determine velocities. b. How many layers are indicated by these data? c. What are the velocities for the direct and refracted waves? d. Determine the thickness of the layers.

 DISTANCE (meters)                   TIME (second)

 10                                              0.010 20                                              0.020 30                                              0.030 40                                              0.040 50                                              0.045 75                                              0.055100                                             0.065125                                             0.075150                                             0.080175                                             0.085200                                             0.090250                                             0.100300                                                                                          0.110

Related Links http://www-geology.ucdavis.edu/~gel161/sp98_burgmann/cannon/refraction.htmlhttp://www.globalgeo.co.za/refra.htmhttp://www.terraplus.com/papers/dave6.htmhttp://www.seismo.unr.edu/ftp/pub/louie/dome/refraction.htmlhttp://www.geophysical.biz/seisrf1.htm 

Chapter 4:  HORIZON AND FORMATION ATTRIBUTES

Classification of AttributesTime-Derived Horizon Attributes

Coherence and Continuity

Post-Stack Amplitude Attributes

Hybrid Attributes

Frequency-Derived Attributes

Amplitude Variation with Offset

Use of Multiple Attributes VISUALIZATION OF HORIZON ATTRIBUTES

Nature of Visualization

Perception of Three Dimensions

Attribute / Structure Relationships

Attribute / Attribute Relationships

Complex Structural Relationships

Relationships between Structure and Stratigraphy

Integrating Information

Applications of Stereopsis

Use of Motion

Questions  

Related LinksAdditional Readings 

HORIZON AND FORMATION ATTRIBUTES 

Classification of Attributes An attribute is necessarily a derivate of a basic seismic measurement. All the horizon and formation attributes available (Figure 8-1) are not independent of each other but simply different ways of presenting and studying a limited amount of basic information. That basic information is time, amplitude, frequency and attenuation and these form the basis of our attribute classification.As a broad generalization time-derived attributes provide structural information, amplitude-derived attributes provide stratigraphic and reservoir information. Frequency-derived attributes are not yet well understood but there is widespread optimism that they will provide additional useful stratigraphic and reservoir information. Attenuation is not used today but there is a possibility that in the future it will yield information on permeability. Most attributes are derived from the normal stacked and migrated data volume but variations of basic measurements as a function of angle of incidence (and hence source to receiver offset) provides a further source of information. The principal examples of these pre-stack attributes are AVO.Post-stack attributes can be extracted along one horizon or summed over a window (Figure 8-1). The latter provides the concept of a formation attribute. In some cases the window is a constant flat time interval so that the display is effectively a thick time slice, sometimes termed a stat (statistical) slice. The window may be of a constant time interval but hung from one structurally-interpreted horizon so that the window properly follows a reservoir interval. The window may also be the interval between two structurally-interpreted horizons, for example the top and the base reservoir reflections.Attributes are normally calculated and extracted from the data volume following automatic spatial tracking or snapping. Windowed attributes generally use the sample values every 2 or 4 milliseconds. 

Figure 8-1  

Time-Derived Horizon Attributes Residual, or high spatial frequency residual map, is the arithmetic difference between a high-precision automatically-tracked time map and its spatially-smoothed equivalent.Figure 8-4 shows the area under study in the next example; Figure 8-5 is the auto-tracked high-precision time map; Figure 8-6 is the residual. The black features are gaps in the interpreted horizon which were recognized as faults in the mainstream of the interpretation using vertical and horizontal sections. A linear pattern in the residual parallel to the existing faults and towards the east end of the area is a rather clear indication of another fault or similar geologic feature. In contrast, the north-south patterns towards the west end are highly regular and parallel and do not fit the existing fault patterns; they must be identified as data collection lineations or noise.Dip, or dip magnitude is another time-derived horizon attribute addressing issues of structural detail (Dalley et al., 1989). On the high-precision automatically-tracked time surface one time value is considered in relation to its immediate neighbors to form a local plane. The true dip of that local plane is the attribute dip; the direction of that dip is the azimuth, or dip azimuth.Figure 8-13 shows the dip map of a horizon which was autotracked with little vertical and horizontal section control. Although the tracking was successful over much of the area, the tracker jumped onto the wrong event in several places. These tracking busts show as very straight high-dip pseudo-faults primarily in green and blue. This demonstrates that time-derived attribute displays such as dip can be used to quality control the performance of an autotracker as well as to establish further structural detail.Azimuth, or dip azimuth, is used in a similar way to dip. Figure 8-14 shows an example of an azimuth map from Lake Maracaibo, Venezuela. The data is clearly noisy but nonetheless several meaningful patterns can be seen. The red arrows indicate anomalies which conform with existing fault patterns and are thus probable additional faulting. Two of the indicated faults are magenta and blue and thus dip to the northeast; the third is red and yellow and thus dips to the southwest.A dip-azimuth combination map (Figure 8-17) combines the separate dip and azimuth attributes onto the one display and is thus an alternative approach to the fault visibility. In Figure 8-17 he dip is coded to color density and the azimuth is coded to hue according to the circular legend. Many faults associated with salt movement are visible.Curvature and Roughness are derivatives of dip and azimuth and thus serve as second derivatives of structure. Figure 8-18 illustrates these attributes and compares them with azimuth for the same horizon surface.Edge, or edge detection, is a spatial operator usually occupying 9 points which operates like a spatial smoothing filter but has the opposite effect; spatial differences or edges are accentuated regardless of orientation. Figure 8-19 is an edge map on the top Tosca reflection in the Neuquen Basin of Argentina. In the west there is a swarm of north-south faults and several 7 km long arcuate faults. These arcuate faults look impressive on the edge map but have a barely-visible displacement on vertical sections. Similar arcuate patterns on the edge maps from deeper horizons indicate that these faults are in fact conical in shape pointing downwards with their apexes on an igneous intrusion.

Illumination, lighting, shaded relief or sun shading is a display technique already well known from topographic and other kinds of mapping. Portions of the autotracked time surface pointing towards the source are highlighted, those pointing away from the source are in shadow. Figure 8-21 has an illumination direction from the northwest. The fault running north-south down the west side is highlighted. Just east of this is a sequence of en echelon faults in shadow. Just southeast of center is a clearly-visible graben.  

Coherence and Continuity Coherence, continuity, semblance and covariance are all rather similar. they aim to convert a volume of continuity (the normal reflections) into a volume of discontinuity (the faults and other boundaries). These attributes operate within a time window and use a variety of mathematical approaches similar to correlation. Because the attributes are derived direct from the processed data they are free of interpretative bias, in contrast to the horizon attributes discussed in the last section which require an interpreted horizon as their input.Figure 8-30 shows a coherence time slice delineating stratigraphic features. The channel is, for the most part, clear and the lower left point bars are visible where the channel changes direction. Figure 8-31 shows the normal time slice for comparison; here the red and blue structural reflections dominate the section, which is normally the case.Figure 8-36 is a continuity time slice showing striking channels in the vicinity of a salt dome. Figure 8-37 shows again how difficult it is to see these features in the regular data.Figure 8-24 is a coherence time slice covering a large area of the Gulf of Mexico. Many faults are outstandingly visible. Figure 8-25 provides the regular time slice for comparison. Some of the faults are visible here but, as is normal with time slices, the fault visibility depends greatly on relative strike of fault and structure. On the coherence time slice on Figure 8-24 it is clear that the faults are equally visible regardless of their orientation relative to structural strike. This value of a coherence time slice is further exemplified by comparison of Figures 8-26 and 8-27 from the North Sea. Here the faults are very curved but nevertheless are equally visible along their entire length.Figure 8-28 is a continuity time slice from the Gulf of Mexico showing radial faults around a salt dome. Continuity involves a straightforward multiple cross-correlation calculation which also yields related attributes such as the dip and the azimuth of the maximum correlation. 

Post-Stack Amplitude Attributes

 Reflection amplitude measured at the crest of an identified reflection is by far the most widely-used amplitude attribute. Reflection amplitude extracted over one horizon produces a display normally called a horizon slice.Composite amplitude is the absolute value summation of the amplitudes of reflections identified at the top and base of a reservoir, or other, interval.Acoustic impedance derived from amplitude by seismic inversion is another way of combining information from reservoir top and base (with thickness limitations).Reflection strength, or envelope amplitude, is the amplitude derived from the complex trace. Figures 8-38 to 8-41 show an example where reflection strength had a dramatic influence on the interpretation. The dips seen on the reflection strength section of Figure 8-39 are opposite to those seen on the regular section of Figure 8-38. These depositional clinoforms, if that is what they are, make sensible spatial patterns on the horizon slice of Figure 8-41 and can, to some extent, be discerned on the amplitude-enhanced section of Figure 8-40.Figure 8-42 shows a vertical section from the North Sea showing some high-amplitude reflections from channel. The horizontal blue lines indicate a constant time window of 100 ms enclosing the channel reflections. Figure 8-43 displays areally the average absolute amplitude calculated over this time. Window. This stat slice shows clearly the extent of the channel and its boundaries are sharply delineated. This approach has many applications when the objective has high amplitude and is rather discontinuous. The success of these stat slices depends critically on the amplitude of the objective reflections being dominant within the time window used.Root-mean-square amplitude tends to be more effective than absolute amplitude because the high amplitudes are boosted by the squaring.For another North Sea prospect Figure 8-44 maps the total energy over the reservoir interval defined as the time window between two structurally-interpreted horizons. Energy is the square of the seismic amplitude. The geologic environment here is shale-dominated so more reflection energy indicates more sand. The number of zero crossing mapped over the same area (Figure 8-45) indicates layering and thus should also be related to total quantity of sand. Because the top reflection here was a peak and the base a trough, the number of zero crossings between them must be an odd number.Energy half-time (Figure 8-46) for the same reservoir over the same area attempts to map vertical distribution of sand within the reservoir interval. Following the diagrammatic legend in Figure 8-46, energy half-time first sums energy over the interval starting from the top. Then the calculation sums a second time until half the previous total value is reached. If this point occurs above the midpoint of the interval, the sands are located primarily towards the top of the reservoir.Amplitude ratio (of top and base reflections) is a horizon-based amplitude attribute that addresses internal distribution. Relative to energy half-time it relies perhaps less on signal-to-noise but more on the zero-phaseness of the data.  

Hybrid Attributes

 Hybrid attributes combine components of amplitude and frequency, and for this reason have interesting potential. Water shape is a neural network classification of trace character. A window of the seismic data, normally hung from an interpreted horizon, is analyzed, and some selected number of characteristic traces generated (Figure 8-47).Figure 8-49 shows a porosity map generated from the attribute loop area. Loop area is the area under the half-period of the seismic trace and is explained diagrammatically in Figure 8-50. In the project leading to Figure 8-49 (Van de Sande, 1996), loop area was found to correlate with porosity better than reflection amplitude.  

Frequency-Derived Attributes Instantaneous frequency tends to be one of the less reliable approaches to the direct detection of hydrocarbons. In fact all frequency attributes tend to be noisy, and the industry has not yet really harnessed frequency for stratigraphic and reservoir studies.Figure 8-52 shows six different frequency-derived attributes displayed for the same small area. For comparison Figure 8-53 shows five amplitude-derived attributes over the same area and also the vertical section showing the window over which all these attributes were calculated. Three of the attributes in Figure 8-53 address vertical distribution within the interval.Geologic frequency is simply the number of reflecting layers within unit thickness of rock. Higher geologic frequency means more layering. If it is detectable seismically, it must be secondary to the frequency characteristics of the propagating wavelet.First, second and third dominant frequencies involve completely different transform mathematics and notionally force-fit the spectrum to yield three maxima. First dominant frequency should primarily express the properties of the wavelet. Second dominant frequency should primarily express the geologic frequency and thus be the most useful. For the window of data shown in the small insert of Figure 8-53, second dominant frequency, first dominant frequency and composite amplitude were crossplotted to yield Figure 8-58. The white points, well separated in second dominant frequency, are plotted in their proper geographic position in Figure 8-59. The color indicates amplitude and thus the existence of hydrocarbon. The white dots are believed to indicate additional layering and thus sand. In fact a well was drilled into one of the areas of white dots and found some previously-bypassed reserves.  

Amplitude Variation with Offset AVO has become a very popular subject and is covered thoroughly by Castagna and Backus (1993). However, the amount of AVO work performed in 3-D is extremely small

and Castagna and Backus barely mention the subject. Furthermore, the 3-D AVO projects which have been performed show little incremental benefit over the 3-D analysis of the post-stack amplitudes.Figure 8-63 is the arithmetic difference between the amplitudes of Figures 8-61 and 8-62; in other words Figure 8-63 is a display of the AVO attribute far-near amplitude difference. A channel is now clearly visible in yellow, red and green, which indicates increasing amplitude with offset. This channel, meandering across parts of both eastern and western lobes, is not visible on any normal amplitude display and is interpreted as the sand-filled channel facies of a slope fan. Furthermore, it probably indicates the areas of highest porosity and permeability within the reservoir. 

Figure 8-63  

Use of Multiple Attributes Attribute are normally extracted from the relevant level in the seismic data as horizon attributes and / or windowed attributes over at least one half-period. They are then selected on the basis of geophysical and petrophysical reasoning, that is, we use attributes which appear reasonable. Each attribute may be crossplotted against the reservoir property of interest using multiple wells and the one that correlates best selected. The other attributes are then tested in turn to find how much of the remaining variance in the relationship they explain. The statistics then help with the selection based on their contribution to variance reduction. The resultant attributes, one or several together, are then used in geostatistical cokriging to interpolate the reservoir property being mapped between wells.There are many dangers in statistics and much has recently been written about these dangers as applied to reservoir characterization. Hirsche et al., (1997 and 1998) has been particularly vocal with remarks including: Neglecting geology and geophysics reduces geostatistics to a purely statistical process that may give false confidence in spurious results. Schuelke and Quirein (1998) offer a similar caution: The use of statistical methods without the foundation of a physical basis for the correlation between the seismic attribute(s) and the rock property …is very risky. Hart and Balch (1999) remind us that the probability of obtaining a statistically significant but spurious correlation between an attribute and a log-derived property is proportional to the number of attributes tested and inversely proportional to the number of wells used in the calibration.  VISUALIZATION OF HORIZON ATTRIBUTES  

Nature of Visualization Visualization is the graphical presentation of data in an intuitive fashion that exposes the information in the data and provides new insight. Common examples of visualization include pie charts, bar graphs or xy plots, seismic sections, time slices, and contour maps. A structure contour map is an attempt to represent a three-dimensional surface in a two-dimensional display. Horizon attributes are typically visualized as two-dimensional map displays where the variation in the attribute value is calibrated to a particular color range. Three-dimensional visualization attempts to convey significantly more information through the representation of multi-parametric data in three dimensional surfaces and volumes.The most common used to represent a three-dimensional surface has for many years been the contour map. Each color on this type of map represents a range of times. The appearance of a continuous gradation can be achievable by using a larger number of individual color bins. By looking at a contour map, and understanding from the legend the relationship between color and structural highs and lows, it is possible to form a mental image of the three-dimensional surface.This representation of the 3-D surface is limited because it requires mental integration of complex variations of data along the three dimensional structure.These difficulties can be managed by using 3-D visualization to represent the data. In Figure 8-65, for example, the interpreted horizon is shown a three-dimensional surface, with the reflection amplitude shown in color. It now becomes a simply a matter of observation to identify the relationship, if any, between the structure and the horizon attribute. 

Figure 8-65  

 Perception of Three Dimensions There are a number of cues that the mind uses to perceive and understand a three dimensional object. These include: 

- -         Projection- -         Lighting and shading- -         Depth of field- -         Depth cueing- -         Obscuration- -         Transparency- -         Stereopsis- -         Parallax- -         Motion

 Projection refers to a method of graphically depicting three-dimensional objects and spatial relationships on a two-dimensional plane (Foley, et al., 1990). A perspective projection is one in which parallel lines and planes converge to infinitely distant vanishing points. In a parallel projection, parallel lines and planes are made to be truly parallel in the display. This may result in an optical distortion in the perception of three dimensions, but can provide a display (e.g. an isometric orthographic parallel projection) where measured distances along each of the three axes are the same. Both projections have their application and value in the interpretive process.The perception of the overall shape of the surface is significantly improved by lighting, and the shadows provide detailed information not only about major discontinuities (faults and folds) but also about portions of the surface that are relatively smoother or rougher than the surrounding surface. Indoor lighting typically provides a number of sources of light which may shine simultaneously on a single object, so that shadows are cast in multiple directions, or very diffuse light so that shadows are greatly softened. A light source at an infinite distance illuminates the surface with parallel rays. A light source close to the surface illuminates the surface with diverging light rays. This affects the shadows and highlights on the surface being viewed.When the human eye, or the camera, focuses on an object, the focus actually occurs on a plane perpendicular to the direction of viewing at a particular distance from the observer. Objects or portions of the object that are either closer to or further away from the observer are out of focus. This phenomenon is called depth of field (Foley, et al., 1990). If this technique were used, it would require re-rendering the image to clearly view different depths on the surface or in the volume which would be both time consuming and awkward. Typically the entire three-dimensional surface or volume is rendered in focus allowing the user to examine all depths in the image simultaneously with essentially the same clarity.Depth cueing refers to a phenomenon where the more distant objects are rendered with a lower intensity than nearer objects. This can be viewed as a type of “atmospheric” attenuation. It is usually not desirable when rendering complex three-dimensional data for analysis.Obscuration is a process where the nearer portions of a complex three-dimensional surface may obstruct or block the view of portions of the surface that are further away from the viewer. This is done as a matter of course in the three-dimensional rendering of opaque surfaces and solid. By allowing the interpreter control over the transparency of the rendered surfaces, it is possible to see through one horizon to the data that would be hidden behind it if it were opaque. The partial transparency of the nearer surface gives the mind very clear information about the relative position of the surfaces.Stereopsis refers to the ability to view an object simultaneously from two slightly different directions and composite the three-dimensional image. Stereoscopic displays have seen relatively limited use to date in three-dimensional interpretation , but their use will increase over time. The increase in the amount of information that can be perceived moving from a lighted perspective display to a stereoscopic display can be quite remarkable. A stereo pair of images, one image for each eye, can be composited with the aid of a stereo viewer, a device used for a number of years in analysis of aerial photography.

Work is currently underway at several research labs (including Sharp Laboratories and British Telecom) to produce auto stereoscopic displays – displays where a stereo 3-D images are perceived without any need for specialized eye wear. These displays are still a few years away from commercial application.Parallax is the apparent displacement of closer objects with respect to farther objects when observed from two different viewpoints. The effects of parallax are most obvious when combined with motion.Motion or animation is one of the most important cues for three-dimensional perception. The human visual system is very sensitive to small relative changes in what is being viewed. Moving an observer’s viewpoint with respect to an interpreted horizon (distance, orientation, etc) can greatly improve the understanding of the nature of the horizon and the attribute displayed on the horizon. Motion can be used to enhanced the effects of all of the other three-dimensional perception cues previously discussed, maximizing the visual effects of parallax, stereoscopy, obscuration and changing the shadows cast by a light source.  

Attribute / Structure Relationships The interpreter frequently needs to look at the variation of an attribute of the data (for example the trace amplitude) along a horizon surface. A multitude of horizon attributes can be extracted or calculated once a horizon has been picked. Seismic attribute analysis has been used effectively in a number of studies for fault interpretation, estimation of reservoir properties, and reservoir mapping.In particular, the interpreter may be interested in the relationship between the geometry of a three-dimensional interpreted horizon, and the variation of one (or many) data attributes along that horizon. An understanding of this information and its implications usually must be communicated by the interpreter to others. Visualization techniques provide an effective means of accomplishing these goals.The horizon in Figure 8-66 lies slightly below an unconformity. When the unconformity is picked, the data can be flattened on the unconformity. The resulting structure is an approximation of the structure as it existed at the time deposition occurred on the unconformity surface (Figure 8-67). In this image the horizon is being viewed from the northeast. This highlights the correlation between that low amplitude zone and the crest of the Upper Cretaceous paleo-anticline. At the time of the unconformity, the structural trend was northeast-southwest. The low amplitude zone lies along the crest of the paleo –anticline. The fact the north-south faults form sharp-edged boundaries to the low amplitude zone suggests that these faults existed prior to the formation of the unconformity. 

Figure 8-66  

Attribute / Attribute Relationships In Figures 8-66 and 8-67, the time-structure of horizon is used to define the shape of the three-dimensional surface. It is possible to use any attribute (not necessarily time) to define the shape of the three-dimensional surface while a second attribute is represented by the color on the surface. In Figure 8-68, the regional or low spatial frequency component of the horizon has been removed. The color in Figure 8-68 still represents reflection amplitude.  

Complex Structural Relationships In Figure 8-69 the horizon and fault surface are viewed from the side. Trace amplitude is shown on the surfaces in color. From the side view it is clear that the fault surface itself has a very complex structure. The interpreted horizon appears high near the fault, has a large depression and then rises again farther from the fault surface.Figure 8-70 was created by moving the viewpoint around to look up the growth fault from below the horizon. This view highlights the structural relationship between the horizon and the fault surface. The fault surface has a very complex shape, with two major ridges with large recessed regions between and around them. The structure of the horizon shows clear control by the shape of the growth fault, with large bowl shaped depressions and ridges in the horizon corresponding to the depressions and ridges in the fault surface. Over-printed on the large scale structure of the growth fault is a finer structure of fault grooves that likely trend parallel to the direction of movement. 

Figure 8-70  

Relationships between Structure and Stratigraphy In 3-D data the explorationist is provided with a unique opportunity to explore the stratigraphy of the subsurface in detail. Time slices and horizon slices allow the interpreter to see features and detail in the data that would otherwise be missed. Three-dimensional visualization provides a means of integrating the structural and stratigraphic aspects of interpretation in detail.Figure 8-73 shows displays of the reflection amplitude at the interpreted horizon for the pick at constant phase (Figure 8-73a), and the pick across phase (Figure 8-73b). The dip display of the two versions of the picked horizon are shown in Figures 8-74a and b. The stream channel is interpretable in all of the displays. For the pick across phase it is most evident in the amplitude display. For the pick at constant phase, the channel is most

prominent in the dip display. There is a striking difference between the information conveyed by the two interpretations of the horizon when they are displayed in three dimensions. 

Figure 8-73  

Integrating Information The interpreter must often integrate more information than can be represented by a three-dimensional structure, a single mapped attribute, and a light source. For example, there may be effects caused by a shallower event (e.g., amplitude or frequency shadows). In this case a visual correlation between the shallow feature and the target horizon would provide a quick means of evaluating these effects.An example combining two of these cases is shown in Figures 8-76 and 8-77. The Top Reservoir horizon structure is shown in Figure 8-76 as a 3-D surface in perspective, viewed from the north at an elevation of about 45o, with a light source shining down on the surface from above. Reflection amplitude is shown on the surface in color. 

Figure 8-77  A second example of the use of texture mapping comes from the Pickerill Field in thu U.K. waters of the North Sea. A geophysical reservoir characterization study (Dorn, et al., 1996) successfully derived a relationship between corrected reflection amplitude at the Top Rotliegend (top reservoir) reflection and log-derived reservoir porosity. A seismic–guided estimated porosity map was produced (Figure 8-78).Figure 8-79 is a display that integrates all of the above information to aid in planning development well locations in the northwest half of the field. The structure of the surface is the interpreted Top Rotliegend time structure. The estimated porosity is shown in color on the surface. The reservoir boundary is shown in bright green, and several of the exploration well locations are shown in pink. A light source is used to cast shadows at discontinuities of the surface to highlight the location of faults. Finally, the location of the Plattendolomit rafts is highlighted by the brick pattern overlaid on the surface.By using this type of display, potential drainage compartments can be identified. Well locations within a component can be optimized to avoid potential barriers to flow (small throw faults), and to encounter the highest predicted porosity. The well path can also be adjusted to avoid the Plattendolomit rafts, avoiding potential drilling problems, and helping to minimize drilling costs. 

Figure 8-79  

Applications of Stereopsis One of the most important three-dimensional visual cues is stereopsis. The use of stereo displays is increasing over time because they allow the interpreter to see additional 3-D detail in the data.Figure 8-80a is a shaded perspective display of an interpreted reef structure shown in perspective view, with a light source and reflection amplitude displayed on surface in color. This image combines the use of several 3-D cues, but the image still looks like a flat 2-D projection of a 3-D surface. Figure 8-80b is a stereo pair of the same data. Once the two images in Figure 8-80b has been properly fused into a stereo image, the horizon and reef will appear to come partially out of the page toward the viewer – it will appear as a true 3-D surface. Notice, in particular, how much more prominent the breaks in the reef appear, and the associated debris slopes.The additional information content provided to the interpreter by stereo displays is substantially more striking with the larger stereo images that can be displayed on properly equipped workstations. Some information that can be observed in the stereo images could be interpreted from non-stereo displays, but the interpretation requires more time and effort. Other information would simply be missed without the stereo display. 

Figure 8-80  

Use of Motion Animation or motion of 3-D interpretation can also play an important role in interpretation and communication of complex structural and stratigraphic relationships. Three applications of motion are discussed below: changing the observer’s viewpoint, interactive three-dimensional flattening, and interactive movement of the light source.Motion in the form of changing the observer’s viewpoint can greatly aid interpreting and understanding complex 3-D structures. Motion accentuates the effects of many of the visual cues used to perceive 3-D object. The shifting of shadows, highlights, changing perspective, obscuration, etc., all provide very useful visual information.Horizon flattening is a process frequently used to understand subtle geometric relationships between reflections. It can be great aid in detecting and properly interpreting angular relationships. With interpreted 3-D horizons, flattening can be performed interactively in three dimensions (Dorn, et al., 1995). When using interactive flattening it is important to recall that this is just flattening process – a relative vertical shifting of time and amplitude. It is not true reconstruction, and should not be used as such. It can, however, provide some very useful geologic insight.Figure 8-84 is a reflection amplitude map from a top reservoir horizon in 3-D survey. Interpretation of subtle reservoir faulting through the central area of the map is important

for infill development drilling. Due to a low signal-to-noise ratio, direct interpretation of fault traces from horizon attribute maps, such as Figure 8-84, is difficult.

Figure 8-84 

Questions  1.      Using Fermat’s principle, show that in Figure 4-28 the reflected ray between the

source point S and receiver R has a reflection angle 2 equal to incident angle 1.

 2.      Suppose that a reflection survey indicates a depth of 750 m to the first reflector.

The horizontal reflecting interface is the boundary between layers defined by the velocities V1 = 1500 m/s and V2 = 2500 m/s. What will be the two-way reflection time at offsets equal to zero and 1500 m?

 3.      Suppose that a seismic reflection survey was done over the layered sequence shown

in the Figure 4-29 where interval velocities and layer thicknesses are given. Determine the average and root-mean-square velocities as functions of zero-offset reflection time.

 4.      Using the layered model given in Figure 4-29, determine the offset for a ray

path with a departing angle of 15 degrees from the source point S. 

Related Links http://www.searchanddiscovery.net/documents/geophysical/steeples/index.htmhttp://www.searchanddiscovery.net/documents/geophysical/sheriff/index.htmhttp://www.aug.geophys.ethz.ch/teach/seismik1/03_geometry_files/frame.htmhttp://hyperphysics.phy-astr.gsu.edu/hbase/waves/standw.htmlhttp://www.agg.dkrz.de/people/Menyoli/index12.htmlhttp://www.kettering.edu/~drussell/Demos/reflect/reflect.html 

Chapter 5: DEPTH CONVERSION AND DEPTH IMAGING

 Introduction

Sources and Computation of Velocity

General Considerations in Depth Conversion

Depth Conversion Using a Single Velocity Function

Depth Conversion Using Mapped Velocity Function

Depth Conversion Using Layers

Map Migration

Dealing with Conversion Errors

Concept of 3-D Depth Imaging

Why Time Imaging Is Not Depth Imaging

Required Elements of 3-D Depth Imaging3-D Post-stack Depth vs. 3-D Post-stack Time Imaging

Noise Characteristics of Depth-Image Data

Pre-stack Depth Imaging

Questions

Related LinksAdditional Readings  

Introduction Depth conversion concerns the seismic interpreter because seismic measurements are made in time, but the wells based on a seismic interpretation are drilled in depth. The depth conversion can now carried out as part of the data processing, but this depth imaging is only done in special circumstances. The way this has occurred for depth conversion is shown in Figure 10-1. Depth imaging is used when the velocity distribution and structural complexity are such that the time image of the subsurface does not permit the interpreter to understand the geology (Figure 10-2).Depth conversion of a time interpretation, on the other hand, is computationally simple, and can be quickly repeated whenever new information becomes available. The most common procedure for preparing a depth image of the earth from 3-D seismic data is to time-migrate the 3-D data volume, usually after stack, and convert the time interpretation into a depth model of the earth. Accurate depth conversion is particularly important because the 3-D data volume presents the promise of much more reliable interpretation than with 2-D seismic methods, so errors in depth conversion can be the largest errors in the final interpretation. 

Figure 10-1  

Sources and Computation of Velocity Seismic P-wave velocity may be measured directly by recording a conventional seismic energy source (such as a dynamite charge, a vibrator, or an air gun) with a special geophone lowered down an exploration well. This conventional well velocity survey (or checkshot survey) records a small number of shots at large geophone depth intervals (usually 100m or more) from a single source position. The arrival time of the first energy from each shot is assumed to be P-wave arrival, and the relationship of time to depth given by the survey can be used to convert time to depth directly.Sometimes a vertical seismic profile (VSP) is recorded, with much closer geophone depth intervals and perhaps several source positions for each geophone depth (a walk-away VSP). A VSP is intended to image the subsurface in the vicinity of the well bore, but it also provides vertical velocity information in the same way as a checkshot survey does, and usually more accurately.Seismic data themselves provide velocity information through measurement of normal moveout (NMO). If a seismic reflection is recorded from a horizontal reflector, and the earth above the reflector (the overburden) has a uniform P-wave velocity V, the traveltime T(x) for a source-receiver separation of x is given by the hyperbolic equation 

 2

22

)0(2

)( VxTT x

   Given the relationship between T(x) and x for a reflector, the overburden velocity can be computed. C. Hewitt Dix (1955) pointed out that if the overburden is considered to be not uniform but made up of several horizontal layers, the stacking velocity (although he did not use that term) is approximately equal to the “root mean square” of the layer velocities. The velocity V of a uniform layer between two horizontal reflections with zero-offset times of t1 and t2 and stacking velocities of V1 and V2 is then given by 

    With some approximations then, we can compute interval velocities between reflections from the “velocities” used by the processing center to stack the data. These stacking velocity values are readily available from the processing geophysicist, and are typically supplied to the interpreter as a listing of time-velocity pairs.The reflections used by the processing center to compute stacking velocities are not always those mapped by the interpreter, and in any case they are unmigrated reflections. Furthermore, the interval velocities computed are really in part horizontal velocities, not vertical velocities. The earth rarely has the same velocity vertically as horizontally because it is not isotropic. Also the layers used for velocity analysis are rarely uniform. Finally, the reflections are not, in general, horizontal, and the closer they are to horizontal the less interesting they are to the explorationist.Geostatistical cokriging is a statistical technique for using a measured value of one quantity at many points to estimate the value of another quantity at those many points, given a few measurements of the second quantity. Because depth is closely related to time the technique works quite well for direct conversion of time to depth without computing velocity. The results are very similar to using pseudovelocities, with the advantage of giving an estimate of the accuracy of the final depth map, but this requires special software.Velocities may be available from several sources: well velocity surveys, VSPs, stacking velocities, and pseudovelocities. The interpreter typically combines them by using the sparse, accurate data points at wells to calibrate maps generated from numerous but less accurate, stacking velocity measurements. Often the interpreter must reject anomalous values, using the criteria of geological reason. The final velocity field used for depth conversion must represent a reasonable approximation to the geological model represented by the final interpretation. 

12

12

122

22

tttVtV

V

General Considerations in Depth Conversion The velocities available for depth conversion may have varying accuracy. Actual measurements in well, whether a velocity survey or a VSP, are usually very accurate, with errors often as small as 0.1% or better, but velocities computed form moveout are often in error by 5% or more. One of the subtler characteristics of time migrated data is that a vertical line through the time-migrated data volume (along the time axis) does not necessarily represent a vertical line through the earth.If there is an abrupt lateral change in velocity, as at a fault, the approximation that the stacked seismic trace is the same as the zero-offset seismic trace, and that the migrated seismic trace represents data along an image ray, would lead the interpreter to expect a discontinuity in deeper reflections, a “fault shadow”, even when they are not faulted (Figure 10-9).  

Figure 10-9  When using any layer-based depth conversion technique, we must simulate this fault shadow healing by smoothing a layer before building downwards from it. A technique that works well is to remove wavelengths shorter than the layer thickness, from both the time and the depth maps of the top of the layer. The exact tie of a well to a seismic trace should be represented by the synthetic seismogram computed from sonic and density logs in the well, calibrated by a velocity survey or a VSP. In the real life, the synthetic seismogram is rarely a good match to the processed seismic trace. The well tie of a reflection is usually best determined by answering two questions: 

Is there a noticeable change in acoustic impedance as measured by the well logs at about the correct time as defined by velocity survey?

Is the mapped seismic event of the expected polarity for the direction of change of the acoustic impedance, taking into account the polarity conventions assumed by the processing center, and the phase manipulations in the processing sequence?

 If the answer to both of these questions is “yes”, then the mapped event probably represents the acoustic impedance change. If either answer is “no”, the mapped event may not be reliably identified.  

Depth Conversion Using a Single Velocity Function

 Many small 3-D interpretations can be converted from time to depth using a single velocity function, often from a velocity survey in one well, either in the area of the 3-D survey or nearby. Such a function is usually a series of time-depth pairs, with the time recorded from the surface to several widely-spaced points down the well. Depth conversion uses the grid manipulation functions of the mapping system. A non-linear time-depth function may also be used, and could be any mathematical expression which fits the data and which can be evaluated using the grid manipulation functions of the mapping system.The results of such a conversion are shown in Figure 10-6. Because the measured time-depth function is accurate only at one point (the well where it was recorded), the depth map does not usually tie to the depths for the mapped formation measured in other wells. To correct for the mistie, the interpreter can fit a smooth surface to the mistie values, shown in Figure 10-7, and subtract this surface from the depth map to give a corrected depth map, Figure 10-8, which ties exactly at each well.  

Depth Conversion Using Mapped Velocity Function Where several wells have velocity surveys, or where other methods provide velocities, some way of handling the varying velocities must be used. The easiest way to do this is to map the velocity in some way: either the actual average velocity at each point (as was done in Figure 10-4) or constant in the velocity function fitted to the data points at each well. If a straight line time-depth function of the form Z = a +bT is used, where Z is depth and T is time and a and b are constants, the interpreter could map either a or b, and the other constant over the whole area, or could map both of the constants. With both constants mapped, the depth map would be produced by multiplying the time map gryd by the b map grid, and adding the a map grid. This depth map requires residual corrections in the same way as the map prepared using a single velocity function, unless the only velocities used are pseudovelocities.  

Depth Conversion Using Layers Where there are major velocity changes in the overburden which result largely from changes in lithology rather than from depth of burial, interpolation between control points should use a series of layers with different velocities for each layer. The simplest case is where the velocity for each layer is constant. In this case, the interval from the surface to the base of the first layer is converted to depth using one of the methods described above. Then the time interval over the next layer is converted to a depth interval using a constant velocity (or single velocity function) and added to the depth to the base of the first layer

to give the depth to the base of the second layer, and the process repeated for each subsequent layer. Figure 10-10 shows the time map for a shallow reflection marking a major velocity break 

Figure 10-10 Lateral velocity variations in the layers are accommodated in the same way that they are in a single layer case: by mapping the variations, either directly as variations in the velocity over the interval of the layer, or by variations in parameters of a mathematical function. As for a single layer, the velocity may change with depth. However, such functions introduce complications in depth conversion. The time map to be converted to depth must be the pseudotime map that would be recorded if the velocity function for the layer held for the total depth interval from the survey datum to the base of the current layer. If there are abrupt lateral thickness changes in the shallower section, these must be smoothed out to simulate the smoothing inherent in processing. The procedure then for each layer is this: 

1. 1.      Smooth time and depth maps for the top of the layer.2. 2.      Convert the smoothed depth map to the top of the layer to a pseudotime map,

using the (possibly mapped) time-depth function for the layer.3. 3.      Compute the time interval from the smoothed time map at the top of the layer

to the unsmoothed time map of the base of the lauer.4. 4.      Add this time interval to the pseudotime map for the top of the layer to give a

pseudotime map for the base of the layer.5. 5.      Convert this pseudotime map to depth using the (possibly mapped) time-

depth relationship for the layer. As with the single layer case, the final map will not tie to the wells. The constant part of the error is easily removed, as in Figure 10-11, and the residual error can either be left in the final map or removed by subtracting an error grid as in Figure 10-7.  

Map Migration Where the Hubral effect becomes significant, usually where the dip on velocity interfaces exceeds about 15o, the most accurate solution to depth conversion of time map is image-ray migration. This is done with map-migration software designed for the purpose starting with a horizontal map picked on unmigrated data.In principle, an interpreter could produce a more accurate map of a complex area by interpreting unmigrated seismic data and using map migration. In real exploration situations, this is almost always impossible, because crossing reflections become impractical to map. Where this approach might have been the only way of resolving complex structure with 2-D seismic exploration, 3-D seismic surveys allow 3-D depth

migration, which, although expensive, may be the only practical technique where velocity structure is complex.  

Dealing with Conversion Errors All the depth conversion methods described here, with the exception of the pseudovelocity method, fail to tie exactly at wells. The amount of this well mistie can show how accurate the map is likely to be away from the well ties, where a new well is likely to be drilled. The constant component of all well misties should be subtracted from the depth map to give a map such as Figure 10-8 where the average mistie is zero.The estimated errors for a map can be reduced in several ways. Firstly, some of the misties may be incorrect because either the depth in the well is incorrect, or because the interpreter has picked locally on the wrong reflection. The interpreter should check carefully both the seismic interpretation and the well depth at any wells where the mistie is much larger than average. Once this possibility has been eliminated, the remaining errors may be largely due to errors in the velocities, usually due to inadequate control points.Once the interpreter has made a best estimate for corrected depth map, there are still misties at wells. A final presentation map that has no errors at wells is made by gridding a residual error surface from the final misties, and subtracting this from the final depth map.  

Figure 10-8  

Concept of 3-D Depth Imaging Depth imaging has grown significantly in the past decade as a percent of the seismic processing industry. Geologists, engineers and geophysicists are more commonly utilizing depth-imaged seismic data in daily activities to find and understand hydrocarbon reservoirs. And although the tradition of depth imaging grows from interests in structural definition of hydrocarbon traps, the quality of depth imaging in 3-D is high enough to also have significant impact in reservoir definition and stratigraphic imaging. One very important concept that needs to be understood is the sensitivity of seismic imaging to variations in overlying earth velocity, and why depth images can be significantly different from time-imaged data. Another signal-based concept requiring clear understanding is that of incomplete seismic illumination in shadow zones. These must not be mistaken for changes in reflection amplitude variation caused by changes in lithology, fluids or pressure.

Why Time Imaging Is Not Depth Imaging The imaging of seismic data is intended to represent the earth subsurface reflectivity with sufficient accuracy for rendering structural geology, strstigraphy, and reservoir properties. Field data are recorded in the time domain with varying source-receiver offsets to reflect energy from the subsurface at multiple angles.Time imaging of these data attempts to add together the subsurface reflections and position (migrate) them to the appropriate 3-D xyz positions while still retaining time as the z axis. Time imaging generally employs elements of a “flat earth” processing model, and cannot correct for rapid variations in earth velocity. Therefore time imaging is forgiving of small earth-model errors but fails when velocity varies rapidly.Depth imaging uses a velocity model in the depth domain to compensate for propagation effects. Each vertical and horizontal change in the velocity field is honored in a specialized migration algorithm to account for the bending of the energy down to reflection wavelength scales. In this sense, depth imaging is a correcting lens attempting to place the reflected energy in its correct xyz depth position. Although costlier, depth imaging is generally more accurate than time imaging. As a result, depth imaging is fast becoming the process of choice in areas of high velocity complexity.The inset in Figure 10-16 helps illustrate this difference in time and depth imaging. The plot of offset versus time shows the ray arrivals as recorded at the surface. Ideal time imaging requires the arrivals to fall along a hyperbola, but complex ray bending scatters individual arrivals about a hyperbolic trajectory. In this case time imaging is unsatisfactory, precisely because it assumes straight rays, and because it further assumes the reflection comes from the horizontal location of the midpoint at x = 15,000 feet.Correct depth imaging, however, is designed to correct for these distortions and place the events in their appropriate horizontal and vertical position. The effect of depth imaging is to use the source-receiver pairs whose common reflection points are the same, based on good knowledge of the velocity through which the rays travel. 

Figure 10-16 The benefit of the correction process inherent in depth imaging comes at a substantial price. Significant interpreter effort is required build the interval velocity model and to update the velocity estimates while honoring geologic constraints.  

Required Elements of 3-D Depth Imaging 

Effective 3-D depth imaging of surface seismic data can be accomplished using the required elements of (1) appropriate acquisition coverage, (2) a robust and accurate velocity representation of the subsurface, (3) 3-D ray tracing, (4) a depth migration algorithm, and (5) an imaging expert. High-quality imaging results can be obtained when each of these elements is also of high quality.Even with optimal acquisition, depth imaging cannot overcome the shortcomings of blind spots in the subsurface. Although 3-D seismic coverage at the surface may be evenly distributed, substantial ray bending through lenses like rugose salt often prevents parts of lower reflecting surfaces from being touched by penetrating energy. As Figure 10-16 shows for a single surface CMP, there are gaps in the sub-salt illumination. Seismic processing alone cannot heal these gaps. Another effect can occur when energy is not reflected back from the subsurface, but enters a later of high contrast at what is known as the critical angle, where the wave energy is blocked. An example of this is shown in Figure 10-17; shots from outside a salt body travel to the reflector, but only some of them return to the surface. With a valid model of the subsurface velocity, maps can be generated showing where and why this takes place based on counting the hits from ray tracing.Another approach to understanding illumination variations is to model them using full wavefields. This approach offers a volumetric view and understanding of lost data zones, and also calibrates troublesome variations in amplitude that do not represent geology. Figure 10-18 shows slices from 3-D volume of zero-offset-wavefield depth migration amplitudes. Input to the migration consists of the simulated surface wavefield from a grid of point sources of equal strength distributed throughout the earth model. The migration amplitudes have variations solely due to variations of wave propagation in the earth, migration aperture, and migration algorithm, but are independent of reflector lithology and reflectivity. Therefore, the value of such maps for interpretation is to distinguish between incomplete illumination effects and geology. 

Figure 10-18 Because the depth imaging process is employed in areas of high geological complexity (from a velocity perspective), maximum information of the resulting images is gained when the experience of the depth imager is used in the interpretation. Signal, noise, shadow zones, depth uncertainty and position uncertainty can be validly assessed when using the specific knowledge of how the image was formed and how it can be modified.With adequately recorded seismic data and the appropriate velocity model, seismic imaging via high quality migration is then possible.  3-D Post-stack Depth vs. 3-D Post-stack Time Imaging Depth imaging is not only converting the time recordings to depth, but is also positioning the data at the appropriate horizontal and vertical depth location. When comparing the two approaches in a simple velocity area, they should give comparable results. However, comparisons in complex velocity areas will be different. Inadequate time imaging can be

corrected through depth migration. Depth migration affects both the structural and the amplitude information of the image.As an example of post-stack time versus depth 3-D imaging, refer to Figure 10-22 and 10-23. The difference between these sections is the migration velocity and the migration algorithms. In both cases, the input data are stacked in the time domain. The geology of the area is quite interesting. As observed in the time section of Figure 10-22, the shallow central anticline is underlain by a salt diapir. Deeper, the east dip extends under the salt from the center to the west. 

Figure 10-22 

Figure 10-23  The high impact of depth migration is due to its application where the velocity model is complex. The salt velocity is about twice that of the sediments, and waves traveling through salt rapidly deviate from trajectories appropriate to the time processing model of a layered earth. Additional differences are due to horizontally changing velocity in the sedimentary section. Although lateral sedimentary velocity variation is often modest, in some areas it can vary by up to a few thousand feet per second over several thousand feet, as it does below salt in Figure 10-23. Thus the impact of depth imaging is fairly small above salt but quite large in the deeper central section where imaging is greatly affected by the rapid velocity contrast of the geology.  

Noise Characteristics of Depth-Image Data Depth migration is often applied in complex velocity environments where signal is desired in otherwise noisy areas. Reflections of the subsurface that are correctly imaged are the signal we desire. Coherent energy that does not represent the earth reflectivity is a danger in our interpretation efforts. Mispositioned signal does not qualify as noise.Surface-generated coherent noise is common in land data, and especially troublesome where the wavefield is strongly reflected or diffracted by surface objects (dunes, valleys, karts, etc). Marine data also have surface-generated noise caused by sea bottom diffractors and waves trapped in near-surface low velocity zones. Surface generated noise can sometimes be localized but often affects the entire seismic section.Seismic reflection energy converted to refractions is another coherent noise in our imaging, and is caused by energy traveling along high velocity layer boundaries.Some reflections do not fit the migration theory and so are considered coherent noise. Included are multipaths and multiples. Multiphats are waves that reflect from several single interfaces much like hitting a pool ball off three cushions. Another type of unwanted reflection energy is multiples. These reflections travel several times in the layers of the earth bouncing up and down repetitively. Examples of strong multiple generators are the air-water interface, ocean bottom, carbonate layers and salt.

All of the coherent noises described above exist in the time-imaged seismic sections, because they are a result of the seismic acquisition and the wavefield paths over which they travel. The depth imaging does not create the noise, but does distribute it differently than does time imaging.Much of the noise characteristics of depth-imaged areas can be better understood by employing seismic modeling. 3-D ray tracing can predict where the shadow zones are. The position and strength of multiples and mode conversions are also predictable. These predictions should be confirmed on the time-migrated data, and aid in the interpretation of time-imaged data.Figure 10-26 illustrates a subsalt depth migration containing both signal and noise. The velocity model used is one of a simple sedimentary velocity encasing salt. However, the salt velocity layer is made infinitely thick below its top, so that the depth migration is used to image and identify the base of the salt. Seismic ray trace modeling of the multiple generated from the top of the salt plus an extra bounce in the water layer shows complexity in 3-D. To match the depth-migrated wavefield image, the ray traced multiple was also “ray” depth migrated, and posted on the seismic cross-section in yellow. Where its overburden is simple, the multiple is very continuous and can be wrongly identified as base salt. Where the 3-D ray paths are complex, the multiple incoherently migrates into alternately scattered and convergent zones. Having the ray migrated multiple in this case allowed us to avoid a wrong interpretation of the salt base and proceed with completing the salt velocity model and then imaging the subsalt section.  

Figure 10-26  

Pre-stack Depth Imaging Post-stack depth imaging uses the time stack as input to a depth migration. This is a reasonable thing to do when the uncertainties of the velocity model are large, or cost is a major issue. Stacking seismic data is the wrong thing to do in complex velocity areas, and depth migrating this simply moves the wrong data around without constructing the right image. Pre-stack depth migration handles each trace independently to create more precise image points in the subsurface.It is important to note, for example, that the existence of an amplitude in stacked data at a particular trace and time does not guarantee that it will find a place in the post-stack depth image.With a good earth velocity model in areas of complex geometry and velocity, imaging accuracy and precision are improved by pre-stack depth migration, which puts the signal

in the right location for each separate seismic trace and corrects for the lens effects of large velocity contrasts. Because the depth imaging areas are challenging, however, the signal-to-noise ratio can vary greatly. The signal is placed in the correct CRP prior to stacking the traces, and gathers are more important in the interpretation process. It becomes important to recognize noise and improperly positioned signal on the gather data, and relate these back to the 3-D stacked volume to complete an effective interpretation.As an example of using pre-stack gathers in evaluating depth imaging, refer to Figure 10-27 and 10-28. in Figure 10-27, the traces are 3-D pre-stack depth migrated and plotted prior to stacking. Figure 10-28 is a stack of the traces, and represents one of the cross-sections of the 3-D project. 

Figure 10-27 

Figure 10-28 

Questions  

1.      Design a field layout for a reflection survey to produce data with continuous subsurface coverage from refracted waves. If a receiver interval of 50 m is used with a 48-channel recording system, what distance between shot points will give continuous coverage?

 2.      Using the same system and receiver interval, design the field layout for a

reflection survey that will produce continuous singlefold subsurface coverage. Define the distance between shot points.

 3.      Suppose that we need high-resolution reflection data to recognize relatively

thin layers. If the instruments has a minimum of 2 milliseconds sampling time, what is the highest frequency that can be recorded?

 4.      Suppose that vibroseis will be used in a reflection survey. Maximum depth of

investigation is 5000 m. The average velocity down to this depth is about 4000 m/s, and the instrument is capable of recording up to 10 s. What is the maximum sweep length we can use?

Related Links http://www.searchanddiscovery.net/documents/geophysical/steeples/index.htmhttp://www.tenrats.org/geo.shtmlhttp://mullara.met.unimelb.edu.au/ES304/MODULES/SEIS/DESIGN/design.htmlhttp://www.ladwp.com/water/rd/papers/abstract2.htm

http://www.nga.com/geo_down_hole_seismic.htm 

Additional Readings

Seismic data acquisition

Acquisition of seismic exploration data involves the synthetic generation of seismic waves, and their subsequent detection after passing through or reflecting off the region of interest (i.e., the "target"). The most frequently practiced form of seismic acquisition is the reflection seismic survey. A reflection seismic survey typically involves generating hundreds to tens of thousands of seismic source events, or shots, at different locations in the survey area. The seismic energy generated by each shot is detected and recorded at a variety of distances from the source location (Figure 3). The detectors used to transform ground movement into an electrical voltage that can be recorded are geophones and hydrophones, generically referred to as receivers. For every source event, each receiver generates a seismogram or trace, which is a time series representing the earth movement at the receiver location. Each trace has a reference time zero corresponding to the time of its source event. A record of all traces for each shot is usually written to a medium such as magnetic tape for subsequent study, including processing, display and interpretation.

Figure 3: Geophones record the movement of the ground due to the propagation of seismic energy as seismograms or seismic traces. Seismograms are freqently displayed as "wiggle traces." (Click for larger figure.)

Precise relative and absolute positioning information must also be collected and recorded for all source and receiver locations. This task has been greatly simplified since the deployment of the Global Positioning Satellite (GPS) constellation. Absolute source and receiver positions are usually known to within five meters thanks to the availability of differential GPS techniques. The relative distance between the source and receiver for each trace is its

offset.

Seismic reflection surveys are acquired in both land and marine environments. Although the fundamental principles of the two survey types are the same, the acquisition equipment and procedures differ by necessity. Moving-coil electromagnetic geophones that sense vertical velocity are usually used as receivers in land acquisition. The seismic source on land is usually either dynamite planted in a borehole or Vibroseis, a vibrating mechanism mounted on large trucks. Unlike dynamite, the Vibroseis signal is not impulsive, but rather lasts from 7 to 40 seconds. To emit its signal, the Vibroseis source sweeps through a range of frequencies from about 10 Hz to 60 Hz. Because seismic reflectors in the earth are more closely spaced than the length of the Vibroseis signal, the reflections in raw Vibroseis records overlap, making raw Vibroseis data uninterpretable. A Vibroseis trace must be processed to produce a replacement trace with a signal equivalent to that of an impulsive source. This is accomplished by cross-correlating the raw seismogram with the Vibroseis sweep.

Marine acquisition of seismic reflection data is generally accomplished using large ships with multiple airgun arrays for sources. Airguns are deployed behind the seismic vessel and generate a seismic signal by forcing highly pressurized air into the water. Receivers are towed behind the ship in long streamers that are several kilometers in length. Marine receivers are composed of piezoelectric hydrophones, which respond to changes in water pressure. Being pressure sensitive, hydrophones measure the acceleration of the medium as a seismic wave passes through it, unlike geophones, which respond to the velocity of the medium. Because of sensitivity and noise issues, responses from a group of 5 to 50 hydrophones are summed to produce a single seismogram, and the group is considered a single receiver. Note that the equipment just described is generally intended for probing subsurface depths of a few hundred meters to 10 kilometers. Marine seismic acquisition is also performed for shallow hazard surveys using smaller ships, higher frequency sources and much shorter hydrophone streamers, frequently with only a single receiver.

Until the 1980's, reflection seismic acquisition was carried out by arranging the source and receivers in a line for a shot, then advancing the equipment along a linear transit as necessary to complete the survey. Geographic and cultural obstacles on the earth's surface frequently forced some deviation from this idealize acquisition pattern, but the end result was usually the acquisition of a 2-D seismic profile along a nearly linear transit. Since the mid-1980's, improvements in computational power have made the acquisition and processing of 3-D seismic surveys a practical, though expensive, endeavor. Because geological structures are three dimensional in

nature, 3-D acquisition and processing are usually desirable, or even necessary, to produce a proper representation of the subsurface. As a result, most seismic surveys and the vast majority of seismic data are now collected as 3-D surveys. The shift from 2-D acquisition to 3-D acquisition has had operational consequences that have greatly changed the nature of the seismic exploration industry. Because the facilities and equipment necessary for modern acquisition are very expensive, most seismic surveying is now carried out by a handful of large contracting companies. Because terabytes of data are acquired in medium size surveys, most data processing is also performed by contractors, and the client frequently only receives a processed and greatly compressed seismic image volume as a product for use in interpretation.

Recent advances in acquisition technology are improving the quality and economics of 3-D acquisition. In land acquisition, the logistics of deploying receiver stations and cables are a significant expense, severely limiting the number of receivers that can be deployed per shot. Thanks to advances in electronics, the receiver signal is now commonly digitized at or near the receiver, and the data are either recorded near the receiver stations during a survey, or transmitted by radio or fiber-optic to a base station. This improvement in data handling makes it economical to deploy more receivers during a survey. In marine surveys, the streamer mode of acquisition has traditionally allowed only nearly "in-line" source-receiver azimuths and provided only limited angular raypath coverage of the subsurface. Today, seismic vessels can deploy a number of streamers behind the ship in parallel -- as many as 20 -- allowing the acquisition of a wider azimuth of source-receiver offsets and making it possible to acquire more data in a fixed amount of time. Another new development in marine acquisition is the deployment of multiple vertical cables, each anchored at the seafloor bottom and containing multiple receivers. As the source ship shoots around and over the vertical cable deployment, data are acquired with multiple source-receiver offsets and receivers at multiple depths. The variety of receiver depths beneath the water surface makes the elimination of water surface multiples a simple task during subsequent data processing.

An important subset of reflection seismology is multi-component seismic acquisition. Multi-component seismology has recently been transformed by new technological developments that have improved the practicality of marine acquisition. In the solid earth, the complete elastic wavefield is composed of both P-waves and S-waves and is a vector quantity. Multi-component receivers that measure particle displacements in three perpendicular orientations are necessary, therefore, to detect the full elastic wavefield. In a marine environment, however, water cannot transmit shear wave energy, so direct recording of multi-component information is not

possible using towed streamers. Marine multi-component acquisition is feasible using ocean bottom seismograms (OBS's), but placing individual multi-component receivers on the sea floor is time consuming and expensive. These practical constraints on ocean bottom acquisition have recently been addressed by the development of cables containing multi-component arrays that can be deployed on the sea floor more economically. This advance in acquisition technique has been matched by the recent appreciation of the importance of converted mode energy in reflection seismology. Although marine seismic airguns generate only P-wave energy, some portion of the P-wave energy is converted into S-wave energy as the direct wave travels downward into the subsurface and encounters impedance boundaries. The converted mode corresponding to conversion upon reflection from down-going P-wave to up-going S-wave is sometimes referred to as the C-wave, and is now known to have practical applications in exploration. Multi-component seismic data are discussed further in the section on seismic interpretation.

In addition to reflection seismology, there are a number of other important methods in seismic exploration. In refraction seismology, for each source event, only the initial ground movement that arrives at each receiver is significant in later analysis. This results in a data set of time versus source-receiver offset for each shot. These first arrivals or head waves followed travel paths that were refracted at the critical angle upon entering a high velocity layer. These travel paths are predominately horizontal, and these data can be interpreted to reveal the depths and seismic velocities of layers that support critical angle refraction paths. (A refraction experiment can only observe a layer if its seismic velocity is higher than all the velocities above the layer.) Because refraction seismology only uses seismic energy that propagates through critical angle travel paths, relatively large source-receiver offsets are required, and the data analysis usually assumes a layered geology without structural complications. In spite of these limitations, seismic refraction experiments are useful for shallow engineering studies and for large scale crustal studies -- depths that are generally too shallow or too deep for seismic reflection surveys.

Another subset of exploration seismology is downhole seismology. Vertical seismic profile (or VSP) surveying involves recording the responses of geophones in a borehole or well for sources at the earth's surface. Likewise, a reverse VSP is collected using a source in the borehole and receivers at the surface. A crosshole survey measures the seismic responses of geophones in one borehole to a source in another borehole. Downhole methods are especially useful for accurately determining seismic P-wave and S-wave velocities since the distance of equipment down the hole is a known value. (In the crosshole case, this implies analysis using crosshole tomography.)

Additionally, either the sources or receivers, or both, are much closer to the target than in surface reflection acquisition, and the seismic travel paths avoid at least one of two passes through the near surface weathering zone. As a result, downhole data can be used to construct very high-resolution images of the subsurface target region.

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Primer on Gravity and Magnetics

by Dick Gibson

This page has received the Virtual Professor's Seal of Good

Practices!

Basic statement for non-professionalsGravity and Magnetics primer for geoscientists • Quiz

What do we do with the Earth's gravity and magnetic fields?

A basic statement for non-professionals

The primary goal of studying gravity and magnetic data is to provide a better understanding of the subsurface geology of the Earth. Gravity and magnetic measurements are both non-destructive remote sensing methods that are relatively cheap, and are used to determine information about the subsurface that is useful especially in exploration for oil and gas and mineral deposits.

Gravity data provide information about densities of rocks underground. Because there is a wide range in density among rock types, geologists can make inferences about the distribution of strata that may be favorable for trapping oil and gas. For example, limestone, which is mostly the mineral calcite (calcium carbonate), has a relatively high density, sometimes as much as 2.8 grams per cubic centimeter, compared with some porous sandstones, which may have a density of

2.4. Pure rock salt has a very low density, about 2.15. Careful measurements of gravity can permit the interpretation of density (and therefore rock) distribution in the subsurface. Identifying a salt dome (a vertical column of salt that rises from great depth) will help locate many possible oil and gas traps.

The magnetic field of the Earth is probably generated by electrical

currents in the liquid outer core. Effectively, it is reasonable to think of the field as like that of a bar magnet at the earth's core, as shown in the picture on the left (from Steve Borron's Site, which is useful and interesting). It affects magnetic minerals that are distributed in many rocks in the crust, so that the rocks have a component of magnetization. For many purposes this is approximately proportional to the distribution of one mineral, magnetite (iron oxide), which is by far the most common magnetic mineral in the earth's crust. We can therefore determine the distribution of some rock types that are usually magnetite-rich. Most sedimentary rocks contain little magnetite, so generally we are "looking at" igneous and metamorphic rocks. Because of the nature of magnetic fields, geophysicists can determine the distance to the magnetite-rich rocks, which in turn suggests how thick (and how prospective) the overlying sedimentary rocks are. In the case of mineral exploration, such analysis can help pinpoint the locations of ore bodies.

If you have questions about the use of gravity and magnetic data for exploration, please feel free to contact Dick Gibson. He'll try to answer!

Gravity and Magnetics Primer for Geoscientists

For a more thorough treatment, you are referred to any good geophysics textbook, such as Nettleton's Gravity and Magnetics in Oil Prospecting (McGraw-Hill, 1976). You might also consider Dick Gibson's Gravity & Magnetics Short Course, which has been conducted for five oil companies, on three continents. See also the lecture series or 1-day program, Architecture of the Continents.

Magnetic maps in the Former Soviet Union section illustrate some of the basic concepts of magnetic interpretation. In addition, you may be interested in Integrated Geophysics Corporation's Glossary of Terms. Through the courtesy of IGC, this Glossary is being

modified by IGC and others for inclusion in the new AAPG book, Gravity and Magnetics Exploration Case Histories.

This section has benefitted from the comments of David Chapin, who is however in no way responsible for any errors of omission or commission!

Regional tectonic features expressed in the gravity map of the U.S.

Note: The official units for gravity and magnetic measurements are milligals (mGals) and nanoTesla (nT). In some reports and maps you may enounter gammas (1 gamma= 1 nanoTesla) or the abbreviation mgal. Also, some older gravity maps may be contoured in "gravity units." One gravity unit (g.u.) = 0.1 mGal. If you encounter (as I did in the Soviet Union work) milli-Oersteds, I can save you some work by letting you know that 1 milli-Oersted = 100 gamma = 100 nT. After you wade through all these words, try the Grav-Mag Quiz at the bottom of the

page!

Two qualities of potential-field data which are important to bothquantitative and qualitative interpretation are amplitude and frequency.

Amplitude, or intensity, refers to the value of a gravity or magnetic anomaly in milligals or gammas. In hydrocarbon exploration, gravity and magnetic anomalies are typically measured to 0.01 milligal and 0.01 gamma (nT), respectively, or better than one part in 5,000,000 of the earth's magnetic field, and even better than that for gravity. This instrument accuracy is attainable, but for most practical purposes in exploration surveys, realistic accuracies are on the order of 0.1-0.2 milligal and 0.5-1.0 gamma (nT). In general, the value is proportional to the density or magnetic susceptibility contrast in the rocks beneath the sensor. (Susceptibility is a measure of the ease with which a material

can be magnetized. Geologically, it can be thought of as a measure of magnetite content, although there are a few other minerals that contribute to bulk rock magnetization under special circumstances.) Again generally, gravity highs indicate the presence of relatively dense rocks, and magnetic highs are caused by rocks with abundant magnetite in them. Gravity highs might come from dense basalts or dense dolomites, but only the former are also magnetic. Thus specific lithologies may be implied, but statements about lithology in the subsurface are usually interpretations.

Sometimes the data are nearly inescapable in their implication of certain rock types (especially when diverse data are interpreted synergistically), but more often several alternatives are possible and should be constrained as much as possible with other information. Very high intensity anomalies (more than 50 milligals or more than 200 gammas) typify major rock type changes, usually (but not

always) in basement rocks. Thus, the magnetic map to the left, of the Zapolyarnoye area of West Siberia, with a contour interval of 100 nT, shows lithologic changes first and foremost. The intrabasement lithologic boundaries can be further interpreted, in this case as rift faults bounding basalt-filled grabens (magnetic highs) and horsts (magnetic lows) that contain drape anticlinal structures trapping huge gas fields. This interpretation is critically dependent on knowledge of the tectonic and geologic setting of the area.View a model through this horst.

Such changes are commonly clearer in magnetic data, reflecting the fact that magnetite content varies much more in basement rocks than does density. In fact, susceptibilities may vary by several orders of magnitude, while densities in the crust typically span only relatively small percentage changes (e.g., from perhaps 1.8 g/cc for porous diatomites to 3.3 g/cc for basalts and ultramafics. Within the basement, densities between 2.6 and 3.0 g/cc reflect a wide variety of igneous and metamorphic rock types.). Structural features, such as folds and faults, usually cause much smaller anomalies (Fig. 1). The observed anomaly is of course the result of both structural and lithologic effects; if a small anomaly caused by a large structure is superimposed upon a large anomaly caused by a lithologic contrast, the two features may be inseparable. Zones of lithologic contrast are often loci of structural disturbance.

Frequency is a measure of how narrow or broad anomalies are, and is a quantity which is mathematically proportional to the depth to sources. High-frequency, narrow anomalies come from sources that are relatively shallow, and broad, low-frequency anomalies are caused by contrasts at greater depths (Fig. 2). This quality of these data is irrespective of intensity; that is, a magnetic high that is very broad almost certainly comes from great depth. It is a mistake to equate magnetic highs with topographic or structural highs, just as magnetic highs should not necessarily be expected to coincide with gravity highs.

Because magnetic anomalies come from contrasts between relatively few rock types, quantitative analysis such as computing a depth to source is appropriate (if specifications of the data acquisition are known; in some data sets, such necessary parameters as magnetometer altitude may not be known). Gravity anomalies, as mentioned above, are caused by contrasts in all parts of the rock section and can therefore be misleading. It is possible for relatively shallow sources to be distributed in a way that produces a broad gravity anomaly which would be expected to come from deep bodies of rock. Computer modeling is appropriate for both gravity and magnetic data analysis. See Gibson's abstract on West Siberia for an example of a magnetic model.

Regional tectonic features expressed in the gravity map of the U.S.

by Dick Gibson

This discussion is in reference to an image of the complete Bouguer gravity map of the United States, provided by the USGS (annotations by Gibson). Note that some of this information is interpretive, and many details have been ignored in this general statement. And as with any geoscience data set, the gravity map is best used in conjuction with other data, such as magnetics, crustal structure, or regional geology.

Several long linear gravity highs are evident in this map. Although they have similar gravity expression, they arise from significantly different tectonic features. The Mid-Continent and Mid-Michigan Rifts are marked by intense gravity highs because these billion-year-old extensional graben systems are filled with dense basaltic rocks (confirmed by magnetic high character). In addition, parts of the Mid-Continent Rift have been uplifted. In Kansas, the Nemaha Ridge portion of the rift system is a rejuvenated uplift of rift structures, but lithologic contacts (dense, magnetic basalt vs. non-basalt) are present too. For more information, visit the Iowa Geological Survey.

The linear high in southeast Texas is usually interpreted as the Inner Metamorphic Belt of the Ouachita Orogen -- a band of dense, uplifted rocks in a collisional setting. The Wichita Uplift, in southern Oklahoma, is a gravity high primarily because it is a structural high, marking the southern margin of the Anadarko Aulacogen (failed rift of Cambro-Ordovician age). The present expression of the Wichita Uplift probably reflects Pennsylvanian rejuvenation during the Ancestral Rockies orogeny.

The long high offshore the east coast and at the western side of the Florida Shelf coincides in most places with the bathymetric escarpment at the edge of the continental shelf. Although this map is a Complete Bouguer map, which means that topographic and bathymetric effects should have been removed, the coincidence of this

high with the bathymetry suggests that some problems remain. The presence of a not-quite-coincident East Coast Magnetic Anomaly indicates a geotectonic feature that may also contribute to the long gravity high.

In the Gulf Coast of southernmost Alabama and Mississippi, a barely-visible yellow-green area marks the Wiggins Arch (see enlargement at left). This is a gravity low over a high-standing basement terrane, and the Mississippi Salt Basin, to the north, is a gravity high. One explanation for this unexpected situation is to infer that the Wiggins Arch is a block of relatively low-density crust, left behind as Yucatan rotated away to open the Gulf of Mexico. The Mississippi

Salt Basin would be an area of thinned crust, with a high-standing area of relatively dense lower crust or mantle beneath the basin. The crust-mantle density contrast (probably 2.9 to 3.3 vs. 2.67 for crust) is far more important than the relatively thin low-density sediments in the basin (including salt). It is very important in looking at regional gravity data to consider the possibility of crust-mantle effects.

The Llano Uplift, in central Texas, contains outcropping Precambrian rocks. They are overlain on the margins of the uplift by Phanerozoic sedimentary rocks, and the center of the gravity high coincides with the Precambrian outcrop.

The Idaho and Sierra Nevada Batholiths are large gravity lows, indicating that they contain a deep "root" of relatively low-density granitic material, which sticks down into the denser lower crust-upper mantle. Both are much more complicated than that in detail; for example the east side of the Sierra Nevada uplift is a late Tertiary normal fault with great throw. The central Rocky Mountains of Colorado probably have a similar deep low-density mass, to account for the huge low there. However, some of this effect may be due to other variations in crustal thickness. Also, in the San Juan Mountains (SW Colorado), a large part of the gravity low coincides with a huge pile of Tertiary volcanics. They may have depressed the crust, and their relative low density also may contribute to that part of the regional gravity low.

The Grenville Front, along a NNE-SSW line in Ohio and Kentucky, is a 1-billion-year-old collisional contact between two different terranes. Their differences have some expression in the gravity map, but the front is much more distinctive in a magnetic map, as shown by the black dotted line in the magnetic map on the right. (See also, for example, Gibson's Interpreted Magnetic Basement Terrane Map of the U.S.)

Disagreements? Let me know! e-mail . Below is a magnetic map of the US (from USGS).

Grav-Mag QUIZ

What is a reasonable geologic interpretation for the NE-SW blue anomaly (A-B) on this magnetic map of northwesternmost Texas? Contour interval = 100 nT, blues are lows and yellows and reds are highs, and the anomaly is about 70 km (45 miles) long. Click in circle of your choice.

A. A pile of volcanic rocksB. A reversely polarized dikeC. A narrow horst of basement rock

uplifted to shallow depth

©2000 Richard I. Gibson

Back to Training IndexGrav-Mag mall

A. Electric Well Logging B. Gamma Ray Logging C. Caliper Logging D. Overview of Well Logging Applications E.   Electric Logging Equipment F.   Logging and Ground Water Investigations G. Logging of Limestone and Dolomite Formations H. Identifying Lignite Coal Beds I.   Locating Minerals  

A. ELECTRICAL WELL LOGGING

Potential

In electrical well logging, two electrical properties are measured in the borehole: potential and resistivity. It has been observed that in a borehole, the electrical potential varies according to the nature of the beds traversed. For example, salt water sands and brackish water sands are usually more negative than the associated shale or clay. On the other hand, fresh water sands may either be more negative or more positive than the associated formation.

Borehole potentials are caused by electrochemical reactions taking place between the formations and the mud column. Potential measurements are made by recording, in terms of depth, the potential changes between an electrode in the hole and another electrode at the surface, usually in the mud pit. An idealized potential (S.P.) curve is represented on the left side of Figure 1. From a potential curve it is possible to pick up the boundaries of many formations and to obtain information on the nature of some of these formations. Potential and resistivity are simultaneously recorded.

Resistivity

The electrical conductivity of a bed is controlled by the nature, quantity and distribution of the water contained in the bed. Because these factors vary appreciably from one bed to another, conductivity measurements made in a borehole can be used to pick up formation changes and to obtain information on the nature of the formation traversed.

In practice it is not the conductivity but its reciprocal, the resistivity, which is measured. The resistivity curve is obtained by recording either the resistance changes of an electrode placed in the hole (Single Electrode method), or the apparent resistivity given by a multiple-electrode arrangement. The measurements are plotted in terms of depth and

the resulting record is called a resistivity curve. An idealized resistivity curve is shown on the right side of Figure 1. It can be seen that fresh water sands and dense formations have a much greater resistivity than salt water sands, clays and shales.

The equipment required to make single-electrode measurements is much simpler and less expensive than that needed for multiple-electrode measurements, but single-electrode measurements have less lateral penetration than multiple-electrode measurements have, therefore they do not always permit distinguishing oil sands from water sands invaded by mud. On the other hand, a single-electrode curve is as good, if not better, than a multiple-electrode resistivity curve for all other problems, especially for obtaining correlation between wells and for determining the depth and thickness of each bed.

Alternating current of low frequency is used for this measurement. As the logging electrode travels in the hole, changes in formation resistivity cause changes in the electrode resistance, which in turn cause voltage changes in the logging circuit. These changes are rectified and recorded as the resistivity curve.

The lateral penetration of a single-electrode measurement is about ten times the electrode diameter, i.e., 18" with the electrodes having a diameter of 1-1/2" and a height of 8". These dimensions were selected to give approximately the same resistivity values that would be obtained with a short normal resistivity curve.

Electric Log

The combination of a potential curve and of one or several resistivity curves placed side by side constitutes an electric log. Such logs are extremely valuable for geologic studies (correlation between wells, subsurface mapping, research on sedimentation), for seismic problems (determination of the best shooting point in a shot hole), for the location of fresh water bearing beds, for determining the exact thickness and position of sand, clay or shale beds, etc.

Requirements to Obtain Good Logs

It is not possible, with conventional logging instruments, to obtain a good electric log in the section of the hole that does not contain water or water base mud. It is therefore necessary that the hole be filled with water base drilling mud or water in the section where the electric log is needed. If this is not possible, a gamma ray probe may be used to obtain a good log.

Unusual Logging Conditions

If the hole is losing mud to the formation and the mud level has dropped appreciably at the time the electric log is to be made, the hole should be filled before the measurements are started. This is usually done by dumping water in the hole. If the composition of this water is different from that of the mud used for drilling, the potential and resistivity curves will probably exhibit a shift at the interface, and the amplitude of the kicks in the

section having the saltiest water may be less than in the other section. These differences are usually small unless one water is much saltier than the other.

If some of the water or mud enters permeable beds or fractures during logging, the potential curve will be unstable and it will probably not repeat well in the part of the hole above the lowest point taking water. It may also exhibit a considerable drift. Such potential drift and instability are observed no matter the type of logging equipment used, and cannot be suppressed. Drift is generally encountered only in shallow formations, i.e., where the potential curve is generally flat and not very useful, even when the water level does not drop. Because the resistivity curve is generally not affected by this movement of water the usefulness of the log is not impaired. The same instabilities are observed also in artesian wells and they cannot be suppressed either, unless the flow of water is stopped.

Logging Shallow Formations

Even when the hole is well conditioned for electric logging measurements and there is no loss of mud into the formation, it frequently happens that the potential curve drifts appreciably to the left in the upper part of the hole. This is a natural phenomenon and it cannot be corrected.

When logging fresh water sands, it sometimes happens that the potential curve "reverses" i.e., the potential in sands is more positive (i.e., it kicks more to the right) than that in clay. This reversal may happen, in particular, when the drilling mud is saltier than usual. The usefulness of a reversed potential curve is not impaired when the logging operator is aware of this possibility. Nothing can be done about this condition, except replacement of the used mud by fresh mud.

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B. GAMMA RAY LOGGING

Radioactivity

Radioactivity is the emission of rays caused by the spontaneous change of one element into another. Although several types of rays are emitted, only gamma rays have enough penetration to be of practical use in logging the natural radioactivity of rocks.

Radioactivity of Rocks

All natural rocks contain some radioactive material. However, compared to that of uranium or radium ore, even of low grade, the radioactivity of most rocks is very small. The radioactivity of a rock is usually expressed in terms of equivalent amount of radium per gram of rock required to produce the same gamma ray intensity. Although there is no fixed rule regarding the amount of radioactivity a given rock may have, shales, clays and

marls are generally several times more radioactive than clean sands, sandstones, limestones and dolomites.

Because shales, clays and marls have radioactivities that are of the same order, the term "shale" will generally be used here to denote any of these three formations. Similarly, for the sake of simplification, the term "sand" will be used to denote either sands, sandstones, limestones, and dolomites, since these four rocks have radioactivities of the same order.

The radioactivity of clean sands, i.e., sands free of shaly materials (shale, clay, marl) is generally very low. Sands that contain some shaly material have a somewhat higher radioactivity, and the increase is proportional to the amount of shale contained. Therefore, shaly sands and sandy shales generally have a radioactivity that is between that of clean sands and that of shale. In a given area, the radioactivity of shales does not generally vary too much, so that a gamma ray log is an approximate measurement of the quantity of shale contained in a formation.

The radioactivity of shale varies from area to area. In the tertiary and more recent formations, i.e., those usually found in the Gulf Coast and in California, the radioactivity of the sediments, as a whole, is generally several times weaker than in the older rocks found in other petroleum provinces.

Some organic marine shales have a much higher radioactivity than the ordinary shales in the same area. However, they are generally relatively thin and are not found too frequently. When present, marine shales make excellent geologic markers.

Interpretation of Gamma Ray Logs

The interpretation of gamma ray logs can be summarized as follows:

1. In a given area, only the relative radioactivity of the various rocks is of significance.

2. Rocks of low radioactivity include primarily clean sands, sandstones, limestones, and dolomites. Anhydrite, salt, lignite and coal have also a low radioactivity. Their radioactivity increases when they are shaly.

3. Ordinary shales have a much higher radioactivity than the rocks listed above. The radioactivity of sandy shales is less than that of shales. Shales are sufficiently high in radioactivity and can generally be easily distinguished from the other rocks on a gamma ray log.

Cased Holes

Most of the gamma rays emitted by the formation can penetrate casing, so that a gamma ray curve can be obtained in cased holes, although the amplitudes of the curve are somewhat reduced. For example, a 5/16 inch thickness of steel reduces the gamma ray intensity about one fourth.

Effect of Mud

The mud has two effects on the gamma ray curve:

1. It absorbs a small percent of the radiation and therefore reduces the log amplitude; unless the hole diameter is very large (more than 24") this effect is very small and can be ignored.

2. The shale or clay contained in the mud increases the radioactivity background, so that even clean sands show a slight radioactivity on the log. If the mud is uniform, this small increase is constant from top to bottom. However, if the shale has settled at the bottom of the hole there will be an increase in the radioactivity measured in this interval that has to be considered when interpreting the log. The effect of the mud on the gamma ray log is the same whether the mud is fresh or salty. Because this effect is usually very small, a gamma ray log is very useful in wells containing salty mud since, in this case, the electric log is generally poor.

Effect of Hole Size

The larger the hole, the smaller the gamma ray intensity reaching the probe. However, this effect is small and can generally be neglected.

Application of Gamma Ray Logs

Gamma ray logs are used in the following instances.

1. To log cased holes (no electric log can be obtained in cased holes).

2. To log dry holes (no electric log can be obtained in holes that do not contain water or mud).

3. To log holes containing salt water or salty mud (the electric logs obtained in such holes are generally poor).

4. To supplement the information given by the electric log (identification of formations, estimating the amount of clay in sands, etc.)

5. To locate radioactive ores, uranium in particular.

6. To help locate lignite and coal beds.

7. To help locate clay and fresh water sands.

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C. CALIPER LOGGING

The caliper logging system provides a means for a continuous recording of borehole diameter versus depth. These logs are generally run in uncased wells, but for many applications have proven very valuable when used within casing. The caliper tool has three arms that may be motored open or closed by control from the surface. Caliper arms are ordinarily supplied in two lengths to provide a maximum extension of either 15 inches or 30 inches. Caliper logs are used to determine hole and casing diameter, locate caved zones, casing, and the absence of casing; and permit the recognition of mud cake. (i.e., permeable zones). An example of a caliper log is shown in Figure 2.

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D. OVERVIEW OF WELL LOGGING APPLICATIONS

Geophysical logs and, more particularly, electric logs give a detailed and continuous picture of the formation penetrated in the course of drilling and are one of the most useful tools currently available for subsurface investigation. They are used in:

1. Core Holes 2. Shot Holes 3. Water Wells 4. Oil & Gas Wells (conventional and progress logging) 5. Mineral Exploration 6. Soil Mechanics

Core Holes

An electric log gives a detailed picture of all the beds penetrated by the drill. By correlating the logs obtained in the wells of a given area, accurate geological maps are obtained showing structures, faults and changes in lithology and sedimentation.

Seismograph Shot Holes

Electric or gamma ray logs made in shot holes provide the seismologist with invaluable information. The logs permit selecting the best shooting point and they supply qualitative indications of changes in surface velocity. Further, by correlating the logs made in the area of interest, an accurate geologic map can be obtained.

Water Wells

An electric log run in a water well permits determining the exact depth and thickness of each aquifer and estimating the quality of the water.

Oil & Gas Wells (Conventional Logging)

Once an oil or gas field is discovered and the basic reservoir data are obtained in several key wells, all the information that is generally needed in the subsequent wells is an electric log that gives, besides correlations, the exact depth and thickness of the producing zones. Electric loggers are perfectly adapted to this type of logging. Either single-electrode or multiple-electrode equipment can be used. The latter are also useful to decide the depth of the oil-water contact. The logs are also invaluable in secondary recovery work (water flooding) and for underground storage investigations.

Oil & Gas Wells (Progress Logging)

In certain areas several electric logs are run in each well as the drilling progresses. This is done for the sole purpose of ascertaining, by means of correlation with the logs of other wells in the vicinity, the stratigraphic position of the well as the drill goes deeper. For this purpose, a single-electrode electric log or a simplified multiple-electrode log is sufficient and can be obtained to depths of approximately 8000 feet with portable loggers. Skid units and truck mounted equipment are also available for deeper wells. This procedure permits making appreciable savings, not only on the logging costs but also on rig time since the equipment is always available when needed.

Mineral Exploration

Electric logs can be used for locating coal, lignite and certain ores.

Soil Engineering

Electric logs are used frequently for investigating the foundations of dams, bridges, highways, etc.

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E.  ELECTRIC LOGGING EQUIPMENT

There are several types of electric logging equipment and they are generally classified in two groups: Single-electrode equipment and multiple-electrode equipment. Each group can be further divided depending on whether the equipment is portable.

Single-Electrode Equipment

This is the simplest and least expensive. Further, it can generally be operated by any member of the drilling crew. The single-electrode equipment gives a Potential curve (S.P.) and a single-electrode resistivity curve. The single-electrode resistivity curve is equivalent to a very short normal curve and, therefore, it gives good detail under normal conditions, even in very thin beds. Single-electrode equipment is recommended for correlation work (core holes, shot holes, progress logging), water wells, mining applications and soil mechanics studies. It is also used with success in oil and gas wells

when the main purpose of the logs, besides obtaining correlations, is to determine the depth and thickness of the beds penetrated by the drill. For the estimation of petroleum saturation and/or formation porosity, and for logging wells that contain salt water or salty mud, single-electrode equipment is not generally recommended; multiple-electrode equipment is preferable. However, in holes of the same size, drilled with the same type of mud through the same producing horizons, changes in fluid contents and porosity are often reflected on the resistivity curve obtained with single-electrode equipment. Because the latter is relatively inexpensive, many operators have been prompt to take advantage of this empirical method for making quantitative determinations.

Multiple-Electrode Equipment

When it is essential to obtain true resistivity data, Multiple-Electrode equipment should be used. This equipment gives the S.P. and two, or more, resistivity curves. Multiple-Electrode equipment is also recommended for the logging of wells that contain salty mud or salt water.

Gamma Ray Logs

Gamma ray logs are used in the following instances:

1. In air drilled holes and in cable tool holes.

2. In cased holes (instead of electric logs that are meaningless).

3. In holes that contain salty mud where even multiple-electrode logs may not give a usable record.

4. When data on the clay or shale content of certain formations are needed, for example for a better interpretation of electric logs.

5. For locating coal or lignite beds that, sometimes, are not clearly seen on electric logs.

6. For uranium, potash and phosphate exploration (these minerals are generally more radioactive than the other types of rocks).

Gamma ray logs can be obtained whether the well is cased or not. Gamma ray logs can always be obtained no matter the nature of the fluid in the hole (air, water, mud, oil).

Caliper Logs

Caliper logs, either alone or with other types of logs, are useful for solving certain problems. Their main applications are:

1. To compute the amount of cement needed for a cementing operation.

2. To compute the amount of gravel needed for gravel packing.

3. To help identify certain formations subject to caving. For example, shales are generally more subject to caving than sandstones and limestones, therefore they can readily be seen on a caliper log. A somewhat similar condition exists where potash is associated with common salt. The salt saturated brine used for drilling dissolves more potash than salt, consequently, the hole enlargements shown on the caliper log in the salt section generally correspond to the potash layers.  

Advantages of Single-Electrode Resistivity Curve.

An important advantage of single-electrode resistivity curves is that they give considerable detail: under usual conditions beds having a thickness of one foot or more can be located and their boundaries can be accurately picked.

Another advantage of this curve is that its response is dependable: for every increase (or decrease) in formation resistivity there is an increase (or decrease) on the resistivity curve. Conversely, every increase shown by the curve corresponds to an increase in the formation resistivity. On the other hand, the response of multiple-electrode logging systems is not always consistent, for example, increases in formation resistivity are sometimes recorded as decreases. Also there are sometime spurious deflections on multiple-electrode logs several feet from bed boundaries.

The two foregoing advantages of the single-electrode resistivity log result in curves that sharply delineate lithology changes. With this log supplemented by the S.P. curve, it is generally possible to identify the type of formation traversed by the well and, in the case of water bearing beds, to estimate changes in the groundwater salinity. The qualitative interpretation of the data is easy and does not require charts.

Single-electrode equipment gives curves from the bottom of the hole up to the casing shoe or to the mud level, whichever is deeper.

Single-electrode equipment is small (and generally highly portable), rugged, simple, relatively inexpensive, and can be operated by a member of the drilling crew after a few hours' instruction.

Shortcomings of Single-Electrode Resistivity Curve.

There are two cases when single-electrode resistivity measurements may not be as efficient as multiple-electrode ones. One is when the hole diameter is larger and the mud is salty. In this case, the curves lose some of their detail. Thin beds cannot be seen and the boundaries of thick beds cannot accurately be picked.

The other case is when true resistivities are needed, for example, when it is necessary to estimate the oil saturation in petroleum reservoirs. The single-electrode resistivity curve is not well adapted to this problem for the following reasons:

1. As pointed out above, a single-electrode resistivity measurement is basically identical to that obtained with a very short normal device. A glance at conventional departure curves for beds of finite thickness shows that the resolving power of a very short normal device for determining true resistivities is small, more particularly for beds whose resistivities are much greater than the adjacent formation resistivity, and/or those which are invaded by mud. In the case of invaded beds, not only is the resolving power poor, but true resistivity determinations cannot be made, even in theory, unless the extent of invasion and the resistivity of the invaded zone are known.

2. Apparent resistivities obtained from single-electrode measurements are greatly affected by changes in hole diameter. Therefore, it is illusory to try to determine the true resistivity of a formation unless a caliper log is available or unless it is known that the formations of interest do not appreciably cave.

These shortcomings are found in any type of single-electrode equipment, but they can be remedied if a special logging procedure is used and if special features are embodied in the logger.

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F. LOGGING AND GROUNDWATER INVESTIGATIONS

Today's water well drillers operate over larger areas and they sink deeper wells than their predecessors of some forty years ago. They use rotary drilling equipment instead of cable tools. For these reasons yesterday's approach to the drilling of water wells is now quite often inadequate.

In order to take full advantage of present day's opportunities, the progressive water well driller is borrowing techniques that are now standard in oil well drilling. Among those, electric logging is probably the simplest and most effective. This technique is no longer too expensive for water well work because portable loggers have brought its cost to a level permitting its regular use in ground water development work.

Driller's Log

In continuously cored holes the examination of cuttings permits obtaining logs that are reasonably accurate. This is not so when rotary drilling is used, as the samples taken contain cuttings from several feet of hole and debris resulting from the erosion of the exposed formation by the mud stream. Electric logging, because of its accuracy, reliability and simplicity, is the usual answer.

Electric Log

Conventional electric logs consist of a potential curve and of one or more resistivity curves recorded side by side as a graph. Electric logs can only be recorded in open holes.

The potential curve is the continuous recording in terms of depth of the natural electric potential in millivolts. It is sometimes called the S.P. curve (Spontaneous Potential). The potential in clay (or shale) is generally used as reference, for convenience. Potential readings in aquifers vary in direction and amplitude according to the respective salinities of the drilling mud and the formation water.

A resistivity curve is obtained by lowering one or several electrodes and recording - also in terms of depth -appropriate electrical measurements. As the logging tool travels in the hole the recorded variations of formation resistivity constitute a resistivity curve. When only one electrode is in the hole the graph obtained is called "single electrode resistivity curve". The graph obtained when the downhole tool carries several electrodes is called "Normal" or "Lateral" resistivity depending upon the electrode arrangement.

Electric Logs

Basically, an electric logger consists of a downhole tool - sometimes called logging electrode or "sonde" - connected to a reel mounted logging cable, a depth measuring device, a control panel and an automatic recorder. The other end of the logging cable is connected to the surface control panel and the recorder by a means of a collector. The measuring device is mechanically or digitally connected to the chart drive mechanism so that the chart moves in synchronism with the sonde. Recording pens moving back and forth, according to the amplitude of the signals (potential and resistivity) received from the sonde, draw the corresponding curves.

"Two Curve" loggers are the simplest and least expensive. They permit recording a potential and a single electrode resistivity curve and are satisfactory for usual water well drilling applications. "Multiple Electrode" loggers allow recording a potential and one or more resistivity curves. The latter loggers are somewhat more complicated and expensive but they permit obtaining quantitative information regarding the fluid content and other characteristics of the formation penetrated by the drill. Loggers of both types are available.

The more electric logs are made in an area and correlated with pumping tests and production performances, the more efficiently the subsequent wells can be completed. For example, because the composition of the water of a given formation is relatively uniform over a large area, it is generally possible to calibrate, for the area, the readings of the electric log in terms of water salinity. If water samples and an electric log are available in a test hole, it is a simple matter to determine limiting resistivity and potential values beyond which the water can be used for domestic or industrial purposes. This information can then be applied for all the wells drilled in the general area.

Although electric logs cannot determine the yield of an aquifer, they are extremely useful in helping solve this problem since they make it possible to "count" the net sand (or gravel) thickness of the aquifer. This information, combined with the pumping and other tests made in the area, generally permits fairly accurate prediction of the yield of most aquifers.

After several electric logs are available in a given area, cross sections and maps can be prepared, from which it is possible to predict the net sand (or gravel) thickness at any location. From this information, supplemented by the pumping tests and production performances of the wells, the depth at which other wells must be drilled in order to produce at a given rate can be predicted. This is important since the drilling costs can thus be estimated accurately in terms of the quantity of water wanted.

As seen from the foregoing discussion, there are many advantages to making electric logs in all wells drilled for water. These advantages are enhanced if proper consideration is given to the few limitations of this modern technique. If these limitations are kept in mind, the electric log, in conjunction with the driller's log, can be used to great advantage in eliminating guesswork in the development of ground water supplies. It can thus be appreciated how the routine use of electric loggers in water wells will supply a wealth of information rapidly, simply and at very reasonable cost.

Gamma Ray Logs

Gamma ray logging equipment is used to record the variation of natural radioactivity of the formation in open and cased holes. Gamma ray logs can be obtained whether the well is filled with water or any type of drilling fluid.

Clay and shale are moderately radioactive while clean sand, gravel, sandstone are much less radioactive. The gamma ray curve is therefore a "formation log" and permits distinguishing clay and shale beds from sands, gravels, sandstones and limestones. Shaly and clayey sands or gravels are generally mediocre aquifers. They can usually be identified by their intermediate radioactivity and comparison between electric and gamma ray logs of the same well.

In many areas, desirable aquifers are separated from unwanted ones by very thin impervious beds that are impossible to locate on the driller's log and difficult to pick up on the electric log. It is often possible to locate such thin beds on the gamma ray log.

Caliper Logs.

Particularly when drilled with rotary equipment, the size of the hole does not remain constant. Caliper logs give the means for determining the volume of the hole for operations such as gravel packing or casing cementing.

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G. LOGGING OF LIMESTONE AND DOLOMITE FORMATIONS

Limestone and dolomite have essentially the same physical properties. Therefore, the following discussion of limestone logging applies also to dolomite.

Electric Log

As far as electric log interpretation is concerned, limestones can be classified in three types: 1, dense limestone, 2, limestone having intergranular porosity and 3, fractured limestone.

Dense Limestone

Dense limestone has a very high resistivity and generally very little S.P.

Limestone with Intergranular Porosity

This type of limestone gives the same electric log as that obtained in sandstone, i.e.,

1. The resistivity is less than that of dense limestone, but generally greater than that of clay or shale. The resistivity decreases when the porosity increases and when the formation water salinity increases.

2. The S.P., with respect to that of associated shale or clay, is small if the limestone contains fresh water (from minus 50 to plus 50 mv). If the formation water is brackish or salty, the S.P. is generally negative and large (more than minus 50 mv).

Fractured Limestone

The resistivity of fractured limestone is less than that of dense limestone, and it decreases when the porosity increases. Generally, the S.P. is very small, regardless of the type of formation water.

Gamma Ray Log

An electric log does not always permit distinguishing a fractured limestone from clay or shale, but the distinction can be made if a gamma ray log is also available. Limestone, whether porous or dense, generally has a very low radioactivity provided it does not contain shaly material. Its response on the gamma ray log is, therefore, the same as that of sand and sandstone. On the other hand, clay and shale have a high radioactivity so that it is possible to distinguish limestone from clay or shale with a gamma ray log.

Note that the gamma ray log is affected neither by the type of formation water nor by the salinity of the drilling mud.

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H. IDENTIFYING LIGNITE COAL BEDS

Because coal and lignite generally have a very high resistivity, they can be located with an electric log. The single-electrode resistivity curve is usually preferable because it gives a very detailed log. Although dense limestone and similar rocks have also a high resistivity, the differentiation between them and coal or lignite can be made with the driller's log since coal and lignite are soft while dense rocks are hard to drill.

There are a few coals (some cannel coals) and lignites (in certain parts of the Rhine Valley) that have fairly low resistivities (of the same order as those of the associated formations: marl, shale, schist). They cannot usually be accurately located with an electric log, even when it is supplemented by the driller's log, but other measurements help solve the problem: a gamma ray log, a caliper log and/or a temperature log generally brings the additional information required, as explained below.

With very few exceptions, coal and lignite have an extremely low radioactivity while all the associated formations have a higher one, especially clay and shale. Therefore, a gamma ray log is a very useful supplement to the electric log for locating coal and lignite beds.

Since coal is often friable, the hole usually exhibits enlargements opposite coal beds. These enlargements are readily detected on a caliper log.

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I. LOCATING MINERALS

Conductive Minerals.

A number of minerals have a much greater conductivity than the usual rocks and can, therefore, be located from resistivity measurements made in wells which have cut them. The most common conductive minerals are: Graphite, Pyrite, Chalcoprite, Pyrrhotite and Galena. Some of them, in particular the first three, exhibit also spontaneous electrochemical phenomena, and they can also be located by making S.P. measurements.

If the ore body is not cut by the borehole, it can be located only if:

1. There is continuity of conductivity, so that the ore body acts as a single conductive mass of large size,

2. The volume of the ore body is large enough,

3. The ore body is not too far from the borehole in which the measurements are made.

Native copper, although extremely conductive, cannot be directly located from electrical measurements - unless the concentration is extremely high - because the copper nuggets are generally separated by rock that is much less conductive than the copper.

Resistive Minerals

Many minerals (hematite, limonite, blende, coal, etc.) have a high resistivity, generally much higher than that of the formation in which they are found. and they can be located from resistivity measurements made in wells which have cut them. If the mineral is not cut by the borehole, it can be located only if it is close to it and if it is relatively thick, as explained above for conductive minerals.

Radioactive Minerals

Radioactive minerals (most uranium ores, many phosphates) can be located by gamma ray logging, provided they have been cut by the well or are only a few inches from it.

Indirect Location of Minerals.

It is possible sometimes to locate indirectly certain minerals, in particular.

1. If they are associated with a formation that can be identified on the log, or

2. If the mineral has produced, by electrochemical action or by dissolution, a halo around the deposit. This halo may be detected by resistivity and/or S.P. measurement.

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SURFACE GEOPHYSICS

Surface geophysical methods are non-invasive tools used to characterize the subsurface. This webpage provides a brief guide to surface geophysical applications and methods. You may also view a summary of this document. This document is extracted from "Technotes vol. 1" which is available upon request.

Applications of Surface GeophysicsThere are five major areas where surface geophysical methods may be applied:

Assessment of natural geologic and hydrogeologic conditions; Detection and mapping of contaminant plumes, spills, and leaks;

Detection and mapping of landfills, trenches, buried waste, drums, underground structures and utilities;

Evaluation of soil and rock properties and man-made structures;

Mineral exploration and evaluation.

Assessment of Natural Geologic and Hydrogeologic ConditionsProbably the most important task of any site investigation will be characterizing the natural geologic and hydrogeologic conditions. The characterization of geological heterogeneity is one of the most difficult problems we face. Understanding the hydrogeologic conditions can make the difference between success and failure in site characterization, since any anomalous conditions will often control the integrity of the soil and rock mass, ground water flow and contaminant transport.

A variety of surface geophysical methods can be used to characterize natural soil and rock conditions. Soil types along with their spatial extent and thickness can often be assessed by ground penetrating radar and electromagnetic measurements. Depth to the water table and rock can be determined as well as the depth and thickness of soil and rock layers. Larger structural features such as dip, syncline, folds and faults can be located and mapped. Smaller localized features such as, the degree of weathering, existence of sand and clay lenses, fracture zones, buried relic stream channels, cavities, sinkholes and other geologic hazards can be located, mapped and assessed.

Detection and Mapping of Contaminant Plumes, Spills and LeaksAn objective of many site investigations is the location and mapping of inorganic or organic contaminant plumes. Surface geophysical methods can be employed in two ways to solve this problem. In cases where the contaminant cannot be detected directly by the surface geophysical methods, the methods can be used to assess the natural geologic and hydrogeologic conditions which are controlling ground water flow. These controlling features may include fractures, a buried channel or natural geologic traps which may hold contaminants. Once the contaminant flow pathways have been identified, direct sampling

methods can be used to further assess conditions. Direct detection of contaminants by surface geophysical methods can be accomplished in some cases.

Inorganics

Inorganic contaminants from landfills, as well as salt brines, acid spills and natural salt-water intrusion are detectable by electrical methods because of their high values of specific conductance. For example, electromagnetic profiling measurements will often allow the detection and lateral mapping of landfill leachate or inorganic chemical spills. Sounding measurements with resisitivity or time domain electromagnetic can provide the vertical extent of inorganics plumes. Time series measurements can be made to map the changes in lateral and vertical movement of inorganic contaminants over time. Even measurements of subtle changes in soil salinity can rapidly be made.

Organics

Hydrocarbons or other organic chemical contaminants are not generally detectable by geophysical methods at the typical ppm or ppb levels of regulatory concern. Furthermore, highly water soluble organics, such as alcohol, are not detected by geophysical methods even at much higher concentrations.

Some organic DNAPL's such as tetrachloroethene, a common dry cleaning solvent, and toluene, a common industrial solvent and principal component of gasoline, have dielectric properties which may allow them to be mapped as bright spots using ground penetrating radar. Insoluble lenses of organics and hydrocarbons floating on the water table may be detected due to one or a combination of factors (Olhoeft, 1992). For example, floating organics can suppress the capillary fringe (in finer grain soils) and will cause the water table to be mapped by ground penetrating radar. The extent of hydrocarbons can then be inferred from the suppressed capillary fringe. When soil wetting by organics occurs, it can result in a change of electrical conductivity and may be measured by electromagnetic measurements.

Identifying the presence of organic contaminants and their location is a difficult one using geophysics. The best approach to mapping organics is to infer their possible location from the mapping of geological structure and migration pathways including observations of subtle changes in geohydrological heterogeneity. Both surface geophysics and borehole geophysical logging can be used to aid in this process.

In all cases, it is recommended that assessment of natural geologic and hydrologic conditions should be carried out as early as possible in the program. This is particularly important when dealing with organics.

Detection and Mapping of Landfills, Trenches, Buried Wastes, Drums, or Other Underground Structures and UtilitiesAn important objective in many RI/FS projects, as well as real estate transfers, is determining the if there is "something" buried at the site. On sites suspected of containing buried metal; electromagnetics, metal detector and magnetometer measurements can be employed to locate and define the edges of a landfill or trenches based upon the presence of the buried metal.

Some sites may not contain metals; in this case, ground penetrating radar and electromagnetics can often be used to define the boundaries of burial areas based upon the difference in electrical properties between undisturbed soils and trash.

An estimate of the depth of a trench or landfill can sometimes be made by resisitivity, or seismic refraction measurements or as a result of modeling magnetic and gravity measurements.

Magnetic, electromagnetic and ground penetrating radar methods can also be employed to detect buried drums, tanks, and utility lines. In many cases the trenches associated with utilities will be of interest because they form a permeable pathway for contaminant migration. As-built plans of utility locations can be developed using the surface geophysical methods.

A metal detector and magnetometer are also used to located unexploded ordnance (UXOs) at military sites. These methods can also be used to locate abandoned wells.

Evaluation of Soil and Rock Properties and Man-Made StructuresThe surface geophysical methods can be applied to the evaluation of a wide range of measurements related to assessment of soil and rock properties, as well as inspection and non-destructive testing of man-made structures. They are also frequently used for archeological investigation. Both earthen and concrete structures can be evaluated. Physical properties, as well as anomalous conditions can often be evaluated, including:

Non-Destructive Testing for:

Microcavities beneath roads and runways; Concrete for delamination and rebars;

Voids behind tunnels, walls and beneath foundations;

Dynamic response of man-made structures;

Soil piping into sewers and drainage system;

Cavities due to acid spills;

Surface subsidence due to piping and collapse;

Mapping abandoned mines, tunnels, etc.

Verification of foundations and determination of pile lengths;

Soil and Rock Properties:

Corrosion measurements of soil; Elastic properties of soil and rock;

Rippability of rock.

Archeological Sites, Historic Structures and Forensic Investigations:

Inspection of potential historic and archeological sites; Location of burial sites and graves for criminal investigation.

Mineral Exploration and EvaluationSurface geophysical methods are often used for mineral exploration. Measurements can be made by most of the surface geophysical methods to define geologic structural traps which often confine mineral deposits.

Direct detection of minerals require that some physical, electrical or chemical parameter be associated with the ore body to make it different than the surrounding soil and rock. The resistivity, induced polarization/complex resisitivity, electromagnetic, magnetic and radiometric methods are often used for this purpose.

Surface Geophysical MethodsSurface geophysical methods vary widely in the terms of the parameter they measure, including physical, electrical and chemical parameters. Some methods are best used as an effective means of measuring lateral changes, i.e. ground penetrating radar, frequency domain electromagnetic and resistivity profiling, very low frequency, spontaneous

potential, magnetic, gravity, thermal and radiation measurements. Others are best used for measuring depth and thickness of geologic strata, i.e. time domain electromagnetic and resisitivity soundings, seismic refraction and reflection measurements.

In most cases, measurements will be made on a station by station basis, along profile lines. However, some methods, including ground penetrating radar, frequency domain electromagnetics, magnetics, metal detector, thermal infrared, and radiation measurements, can provide continuous measurements along a profile line. These continuous measurements provide high lateral resolution for mapping subtle lateral changes in subsurface conditions.

Many of the surface geophysical methods can be adapted for use in fresh or salt water covered areas. There is also a wide range of borehole geophysical logging techniques which complement the surface geophysical methods. However, this guideline only addresses the methods used on land which include:

Ground Penetrating Radar Frequency Domain Electromagnetics (FDEM)

Time Domain Electromagnetics (TDEM)

Very Low Frequency (VLF)

Resistivity

Spontaneous Potential (SP)

Seismic Refraction

Seismic Reflection

Magnetic

Metal Detector

Gravity

Thermal

Radiometric

Ground Penetrating Radar

Ground penetrating radar is a reflection technique which uses high frequency electromagnetic waves (from 10 MHz to more than 1,000 MHz) to acquire subsurface information. Ground penetrating radar responds to changes in electrical properties (dielectric and conductivity) which are a function of soil and rock material and moisture content;

GPR being towed along surface.

Uses:

Ground penetrating radar is primarily used to obtain high resolution cross-section of natural geologic, hydrogeologic, and anomalous conditions;

Ground penetrating radar is also used for location and evaluation of man-made structures, including location of utilities;

Ground penetrating radar has some applications in detecting and mapping contaminant plumes buried wastes and drums.

Advantages:

Measurements are relatively easy to make; Ground penetrating radar provides the highest resolution of any surface

geophysical method;

Continuous profile measurements are effective for larger surveys and provide high lateral resolution;

Output is a continuous graphic picture-like display;

Does not require intrusive ground contact;

The antenna may be pulled by hand or vehicle at traverse speeds from 0.5 to 5 mph;

Antennas of various frequencies (20 to 1000 MHz) can be selected;

A lower frequency provides greater penetration with less resolution, a higher frequency provides higher resolution with less penetration;

Under some conditions, surveys can be made through asphalt and concrete;

Station profile measurements can be made in more difficult terrain (brush, steep hills, etc.) and may have a slight advantage in terms of depth penetration;

Penetration can be as great as a few 100 feet in materials having low conductivity (a few millimhos/meter or lower), but is commonly less than 30 feet.

Limitations:

Depth of penetration is site specific; Penetration in silts and clays and in materials having conductivities above 15 to

20 millimhos/meter is limited.

Frequency Domain Electromagnetics (EM)Frequency domain electromagnetics measures electrical conductivity of soil and rock by measuring the magnitude and phase of an induced electromagnetic current. Electrical conductivity is a function of the soil and rock matrix, percentage of saturation, and type of pore fluids.

EM31 Ground Conductivity Meter.

Sketch of EM34 conductivity soundings and photo of EM34 in continuous mode.

Uses:

Frequency domain electromagnetic measurements are primarily used for profiling to detect and map lateral changes in natural geologic and hydrogeologic conditions;

Electromagnetics measurements are also applicable to detecting and mapping contaminant plumes;

Electromagnetics measurements can be used for mapping buried wastes, metal drums and tanks, and metal utilities;

Can be used for azimuthal measurements to determine fracture orientation.

Advantages:

Measurements are relatively easy to make; Electromagnetics has excellent lateral resolution for profiling;

Does not require intrusive ground contact;

Provides station measurements with depths ranging from a few feet to 200 feet;

Continuous data may be acquired to depths of 50 feet with equipment hand-carried or vehicle-mounted.

Disadvantages:

Frequency domain electromagnetics has limited applications to soundings due to its vertical resolution;

Susceptible to interference from nearby metal pipes, cables, fences and vehicles and noise from powerlines;

Effectiveness of electromagnetic measurements decreases at very low conductivities (use resisitivity).

Time Domain Electromagnetics (TDEM)Time domain electromagnetics measures the electrical conductivity of soil and rock by inducing pulsating currents in the ground with a large transmitter coil and then monitors their decay over time with a separate receiver coil. Electrical conductivity is a function of the soil and rock matrix, percentage of saturation, and type of pore fluids.

TDEM sounding.

Uses:

Time domain electromagnetic measurements are primarily used for soundings to determine depth and thickness of natural geologic and hydrologic conditions;

These measurements can also be applied to detection and mapping of landfill leachate plumes, seepage from brine pits, salt-water intrusion and to mineral exploration.

Advantages:

Good vertical resolution for sounding;

Station measurements from about 20 to 3,000 feet deep;

Does not require intrusive ground contact;

Measures a smaller lateral volume of soil/rock than resisitivity for the same depth of measurement; therefore provides better lateral resolution.

Disadvantages:

Deeper measurements can require a large transmitter coil up to 1000 by 2,000 feet for which space may not be readily available;

Susceptible to interference from nearby metal pipes, cables, fences, vehicles and noise from powerlines;

Effectiveness of electromagnetic measurements decreases at very low conductivities (use resisitivity).

VLF (Very Low Frequency)VLF measurements are made by measuring the distortions of a VLF wave from a distant transmitter. Distortions of the VLF wave occur due to a local increase in electrical conductivity usually found within fractures. The increase in electrical conductivity is a function of the conductive material, such as water, clay or minerals within the fracture.

Sketch of VLF measurements over fracture.

.

Uses:

VLF measurements are primarily used for location and mapping of near vertical contacts, fractures and faults containing water, clay, or minerals.

Advantages:

Measurements are easily and rapidly made; Does not require intrusive ground contact;

Can provide relatively deep measurements.

Disadvantages:

Station measurements only; Does not function well in horizontal layers of soil and rock with very few

fractures or if soil is electrically conductive (i.e. clayey soils or water covered areas);

Axis of fracture must be oriented approximately toward the VLF transmitter;

Susceptible to interference from nearby metal pipes, cables, fences, vehicles, noise from powerlines, and loss of signal from the VLF transmitter.

ResistivityResisitivity measurements are made by injecting a DC current into the ground through two electrodes and measuring the resulting voltage at the surface at two other electrodes. The depth of measurement is related to electrode spacing. Resisitivity measures bulk electrical resistivity which is a function of the soil and rock matrix, percentage of saturation and type of pore fluids.

Resistivity sounding.

Uses:

Resistivity measurements are primary used for soundings to determine depth and thickness of geologic strata;

Also can be applied to profiling measurements for locating anomalous geologic conditions, detecting and mapping contaminant plumes, locating buried wastes and mineral exploration;

Can be used for azimuthal measurements to determine fracture orientation.

Advantages:

Good vertical resolution (sounding); May also be used for profiling;

Measurements can be easily made to depths of few hundred feet or more;

Various electrode configurations are available for different applications.

Disadvantages:

Requires intrusive contact with the ground; Station measurements only;

Electrode array can be quite long, with outermost electrode spacing from 9 to 18 times the depth of interest;

Susceptible to interference from nearby metal fences, buried pipes, cables, etc.;

Generally, cannot be used over asphalt or concrete;

Effectiveness decreases at very low resisitivity values (use electromagnetic measurements).

SP (Spontaneous Potential)Spontaneous potential (SP) measures the natural voltage on the ground surface between two electrodes. SP measures voltage produced by geochemical differences associated with mineral composition or flowing water.

Sketch of SP measurements next to dam.

Uses:

Primary applications for SP measurements are for assessing seepage from dams, fracture flow, flow within caves and recharge zones (i.e. sinkholes, etc.);

SP can also be used for mineral exploration;

Advantages:

Measurements are relatively easy to make; Can detect flowing water;

Time series measurements can be made to monitor any changes in seepage.

Disadvantages:

Requires intrusive contact with the ground; Station measurements only;

Susceptible to interference from stray currents, cathodic protection, natural earth currents and cultural features;

Measurements also influenced by natural geochemical, biochemical reactions.

Seismic RefractionSeismic refraction measurements are made by measuring the travel time of a refracted seismic wave as it travels from the surface through one layer to another and is refracted back to the surface where it is picked up by geophones. The travel time of a seismic wave is a function of soil and rock density and hardness.

Sketch of refraction survey.

Uses:

Primary application for seismic refraction is for determination of depth and thickness of geologic strata, structure and anomalous conditions;

Depth can be calculated under each geophone to produce a detailed two-dimensional top of rock profile;

Detail is inversely proportional to geophone spacing;

If compressional P-wave and shear S-wave velocities are measured, in situ elastic moduli of soil and rock can be determined;

Can be used for azimuthal measurements to determine fracture orientation;

Also has application for evaluation of man-made structures.

Advantages:

Typical measurements are less than 100 feet but can easily made to greater depths, if necessary;

Can resolve up to 3 to 4 layers;

Can provide depth under each geophone;

Both P and S waves can be determined;

The source of seismic energy can be as simple as 10 pound sledge hammer.

Disadvantages:

The survey line length (source to farthest geophone) may be 4 to 5 times the desired depth of investigation;

Requires intrusive contact with the ground;

Station measurement only;

Sensitive to acoustic noise and vibrations;

Seismic velocity of layers must increase with depth;

Will not detect thin layers or layers with inverted velocities;

Deeper measurements will require explosives as an energy source.

Seismic ReflectionThe seismic reflection technique measures the travel time of seismic waves from the ground surface downward to a geologic contact where part of the seismic energy is reflected back to geophones at the surface while the rest of the energy continues to the next interface. The travel time of the seismic wave is a function of soil and rock density and hardness.

Sketch of seismic reflection survey.

Uses:

Primary application is for determination of depth and thickness of geologic strata, structural and anomalous conditions.

Advantages:

Provides a high resolution cross section (as compared to refraction) of soil/rock along profile line;

The high resolution method uses frequencies of up to a few 100 Hz;

Measurements can be made from about 50 feet to a few 1,000 feet deep;

Measurements to these depths can often be made without explosives, often using a 10 pound sledge hammer as a seismic source;

The survey line length (source to farthest geophone) is usually 1 to 2 times the desired depth of investigation (much less than that required for refraction measurements);

Both P and S waves can be measured.

Disadvantages:

Requires intrusive contact with the ground; Station measurement only;

Sensitive to acoustic noise and vibration;

Can require extensive processing.

MagneticA magnetometer is used to measure the intensity of the earth's magnetic field. Deviations of magnetic intensity are caused by changes in concentrations of natural ferrous minerals or by ferrous metals.

Location and Mapping of Buried Ferrous Metals

Magnetic gradient measurements.

Uses:

Magnetic measurements can be used for location and mapping of buried ferrous metals (i.e. wastes, drums or underground structures and utilities).

Generally, vertical magnetic gradiometer measurements are used for the detection and mapping of buried ferrous metals.

A vertical gradiometer response is a function of the object's mass and depth. Response to a steel drum is proportional to the reciprocal of the depth to the fourth power for gradient measurements and is proportional to the reciprocal of the depth cubed for total field measurements.

Advantages:

Measurements are relatively easy to make; Does not require intrusive ground contact;

Detects ferrous metal (iron or steel) only;

Station or continuous profile measurements;

Carried by hand or vehicle-mounted;

Gradient measurements can detect a single 55-gallon drum to a depth of about 10 feet (total field measurements about 18 feet);

Gradient measurements can detect a dozen drums to depths of about 20 feet (total field measurements about 36 feet);

Vertical gradient measurements are insensitive to natural changes in the earth's magnetic field and much less sensitive to interference from nearby steel objects.

Disadvantages:

Gradient measurements are less sensitive than total field measurements; Gradient measurements are still somewhat susceptible to interference from steel

pipes, fences, vehicles and buildings.

Mapping Geologic Structures

Sketch of magnetic survey over geologic structure.

Uses:

Magnetic measurements can be used for geologic mapping by responding to changes of magnetic susceptibility in soil and rock. Generally total field

measurements used for geologic mapping. Primary application is for characterizing geologic structure, faults and mineral exploration.

Advantages:

Measurements are relatively easy to make; Does not require intrusive ground contact;

Usually carried by hand for station measurements;

Can be vehicle-mounted to provide continuous measurements.

Disadvantages:

Susceptible to interference from steel pipes, fences, vehicles and buildings; Total field measurements susceptible to natural fluctuations in earth's magnetic

field (a base station must be used to remove natural fluctuations in earth's field).

Metal DetectorA metal detector responds to the presence of buried ferrous and non-ferrous metals. A metal detector response is a function of the area of the metal object and its depth. The response to a drum is proportional to the reciprocal of the depth to the sixth power.

Sketch of metal detector survey over drums and photo of EM61 metal detector.

Uses:

Metal detector measurements can be used to detect the presence of buried metal containers, drums and tanks. May also be used to locate and trace buried utilities.

Advantages:

Measurements are relatively easy to make; Does not require intrusive ground contact;

Provides continuous measurements;

Provides better spatial definition than electromagnetic or magnetometer;

Can be hand-carried or vehicle-mounted.

Disadvantages:

Susceptible to interference from nearby fences, pipes, cables, vehicles, buildings, etc.

Response to metal decreases rapidly as depth is increased.

GravityGravity measurements detect changes in the earth's gravitational field caused by local changes in the density of the soil and rock or engineered structures.

Sketch of gravity survey over cavity.

Uses:

Standard gravity measurements are primarily applied to characterizing geologic structure using widely spaced stations (100's to 1,000's of feet apart).

Microgravity measurements can be used to characterize detailed localized geologic conditions (such as bedrock channels, caves, and abandoned tunnels and mines) usually within the upper few 100 feet. Microgravity uses closely spaced stations (a few feet to about 50 feet) and a microgravimeter (capable of reading to a few microgals).

Advantages:

Provides a means to characterize conditions in geologic and cultural environments, where other geophysical methods may fail;

Does not require intrusive ground contact;

Data can be interpreted to provide estimates of depth size and the nature of the anomaly;

Can be used inside buildings and structures.

Disadvantages:

Station measurements only; Instruments carried by hand only;

Requires base station for drift corrections;

Requires accurate elevation measurements;

The process of making microgravity measurements is a relatively slow and tedious in the field and requires extensive processing and corrections;

Susceptible to cultural and natural vibrations.

ThermalSurface temperature may be measured by indirect thermal infrared measurements. Subsurface temperature maybe measured by thermisors on probes driven into the soil.

Sketch of thermal infrared measurments.

Uses:

Primary application of thermal infrared is to locate fractures, caves and seeps and to map contaminants floating on water;

Probes may be used to locate shallow seepage.

Advantages:

Measurements are relatively easy to make;

Thermal infrared instruments provide continuous picture-like display with very high lateral resolution and may be hand-carried or vehicle-mounted with traverse speeds .5 to 5 mph. These measurements do not require any ground contact;

Probes are limited to station measurements and require intrusive contact with the ground. They can be permanently mounted for time series measurements and can also be used to obtain vertical thermal gradient.

Disadvantages:

Thermal measurements are sensitive to diurnal and seasonal changes in temperature.

RadiometricMeasurements of gamma radiation are counted by a sintilometer which responds to natural radioactive radiation from potassium, uranium and thorium or man-made sources.

Sketch of radiometric survey.

Uses:

Primarily application of radiometric measurements are for mineral exploration; Also has application for tracking natural or induced radioactive tracers in ground

water;

Can also be used to locate fractures and permeable zones in soil due to vertical migration of radio-nuclides.

Advantages:

Measurements are relatively easy to make; Does not require intrusive ground contact;

Total count or spectrometer measurements may be made;

Station or continuous measurements;

Can be hand-carried or vehicle-mounted;

Provides relative values if uncalibrated;

Can provide quantitative values if instruments are calibrated.

Disadvantages:

Requires a number of corrections for quantitative use; Susceptible to noise from a wide range of natural sources and possible cultural

sources.

SummaryThis document is intended to provide a brief overview of the surface geophysical methods. The details of each method are omitted and can be found throughout the literature. A few references are provided for further reading.

All of the surface geophysical methods, like any other means of measurement, have advantages and limitations. There is no single, universally applicable surface geophysical method, and some methods are quite site specific in their performance. Thus, the user must select the method or methods carefully and understand how they are applied to specific site conditions and project requirements.

There is not a single and unique geologic model for a given set of geophysical field data. This ambiguity can be resolved only through the use of sufficient ground-truth information along with geologic and geophysical knowledge and experience of the interpreter. Failure to consider such expertise will inevitably lead to poor results from surface geophysical surveys.

The success of any surface geophysical survey is dependent upon many factors. One of the most important is the competency of the person(s) responsible for planning, carrying out the survey and interpreting the data. An understanding of the theory, field procedures and methods for interpretation of data along with an understanding of the site geology is necessary to successfully complete a surface geophysical survey. Personnel not having specialized training and / or experience should be cautious about using the surface geophysical methods and solicit assistance from qualified practitioners.

Technos has more than 30 years of experience in site characterization applying remote sensing, surface geophysics, downhole geophysics, and geotechnical instrumentation. We look forward to helping you with your site characterization projects.

ReferencesBenson, R. C., R. A. Glaccum, M. R. Noel, 1982. Geophysical Techniques for Sensing Buried Wastes and Waste Migration. For the Environmental Protection Agency. Published by the National Water Well Association, 236 p.

Griffith, D. H. and R. F. King, 1981. Applied Geophysics for Engineers and Geologists, Pergamon Press.

Olhoeft, G. R., 1992. Geophysical Detection of Hydrocarbon and Organic Chemical Contamination. In: Proc. Symposium on the Application of Geophysics to Engineering and Environmental Problems, SAGEEP, April 26-29, Chicago, IL.

U.S. Army Corps of Engineers (USACOE), 1979. Geophysical Exploration. Engineering Manual EM 1110-1-1802, Washington, DC.

Zohdy, A. A., G. P. Eaton, and D. R. Mabey, 1974. Application of Surface Geophysics to Ground Water Investigations. U.S. Geological Survey, Techniques of Water Resources Investigation, Book 2, Chapter D1, 116 p.

Also see ASTM Guidelines as they become available.

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