2D 3D Land Seismic Data Acquisition

2D and 3D Land Seismic Data Acquisition and Seismic Data Processing

Kiran Kumar Talagapu

M.Sc.(Tech.) Geophysics Department of Geophysics

College of Science and Technology Andhra University

Visakhaptanam - 530003 Andhra Pradesh, India

CERTIFICATE

This is to certify that Mr. Kiran Kumar Talagapu, a final year student of M. Sc.

(Tech.) Geophysics, Department of Geophysics, Andhra University, Waltair,

Visakhapatnam has participated in the project work 2-Dimensional and 3-

Dimensional Land Seismic Data Acquisition and Seismic Data Processing

from 10th December 2004 to 31st January 2005, at Oil and Natural Gas

Corporation (ONGC) Chennai.

( R.V.S. MURTHY ) CHIEF GEOPHYSICIST (S)

CERTIFICATE

This is to certify that this training project report is bonafide work of

Mr. T. Kiran Kumar, submitted in partial fulfillment of M.Sc. (Tech.) degree in

Geophysics during the final year degree course.

(PROF. A. LAKSHMIPATHI RAJU) HEAD OF THE DEPARTMENT,

DEPARTMENT OF GEOPHYSICS, ANDHRA UNIVERSITY,

VISAKHAPATNAM.

Acknowledgements

Every work we do is linked directly or indirectly to many different aspects, circumstances and people. Aspects which we try to understand, work on and come to a conclusion, circumstances which motivate us and people who help us and guide us to achieve what we are intend to. Recollecting the near past events of my training period I am deeply indebted to the people who were responsible for the successful completion of my work. To begin with I am thankful to our Head of the Department Prof. A. Lakshmipathi Raju for the initiation. He took all the pains of shuffling students and assigning the projects. My equivocal thanks are due to the General Manager Head Geophysical Services, Chennai, Mr. D. Dutta who considered our request and allowed us to go through the training in this organization.

I am thankful to Mr. R. V. S. Murthy who has prepared a flawless schedule. It was him who has insisted us to go through the rigorous field work and gave us an insight of what is called the maximum utilization of resources. I express my special sense of gratitude to the party chiefs of the geophysical parties we have visited. The immense cooperation given by them is unforgettable. Not to forget, the party members who have been equally helpful. I am also grateful to Mr. C.M. Varadarajan ,DGM(GP) who prepared the schedule of the second phase of the project (Seismic Data Processing). My sincere thanks are due to Mr. Kailash Prasad and his group members for guiding me all through the different dimensions of the processing aspects. They are pin point right when they say, Information is Power.

This was the punchline which motivated me towards going through the excellent library facility of ONGC. My heartfelt thanks are due to my other project colleagues also as they had been great all through the period of project work. Last but not the least, I am thankful to my beloved parents and my brother. The fact that I am a part of my family forms firm good reason to be thankful.

Kiran Kumar Talagapu

Preface

This is a project report submitted to the Department of Geophysics, Andhra University in partial fulfillment of the M. Sc. (Tech.) degree in Geophysics. The project work forms a paper which is evaluated for a maximum of 100 marks as a part of the academic curriculum.

Under this programme, the students of M. Sc. (Tech.) Geophysics undergo training at premier organizations like, Oil and Natural Gas Corporation (ONGC), National Geophysical Research Institute (NGRI), National Institute of Oceanography (NIO), Indian Institute of Geo-Magnetism (IIGM), etc., engaged in geophysical activities. Under the above program, I had training at Oil and Natural Gas Corporation (ONGC) Chennai, CMDA Towers, Egmore, Chennai, from 10th December, 2004 to 31st January, 2005. During my training at ONGC Chennai, I had associated with the 2D and 3D Land Seismic Data Acquisition undertaken by three of the geophysical parties (identity not revealed) exploring Oil and Natural Gas in the operational areas of the Krishna-Godavari basin. One of the three field parties GP X is acquiring 2D land seismic data in West Godavari Sub-basin and. The second and third parties (GP Y and GP Z) are deployed in the East-Godavari sub basin of the KG basin, acquiring 3D land seismic data. I have been associated with all these parties on a tentative schedule of 10 days (from 13th to 22nd of December, 2004) with GP X, 10 days (from 23rd to 31st of December, 2004) with GP Y and 6 days (from 1st to 6th January, 2005) with the GP Z. Further I had training in Data Processing at Regional Computer Center, ONGC Chennai, from 10th to 31st of January, 2005 thus completing the full schedule of 7 weeks training. All through these 7 weeks, I was exposed to many aspects of the seismic data acquisition and processing. With first hand information on certain technical terms, I was taken through the deeper aspects of seismic survey designing, parameters considerations based on the previous data, experimental surveys for finalizing the parameters from stages of the production work, uphole survey technique, regular survey and through various steps of processing like De-convolution, Stacking, Migration etc.

This report consists of all these aspects in a brief. Starting from the Introduction to the Seismic Methods in Chapter 1, the Next chapter deals with the Modern Seismic Data Acquisition. In Fundamentals of Seismic Prospecting (Chapter 3), I tried to give a brief introduction about the basics of the different types of waves. Chapter 4 is about the

Reflection Field Equipment where I described about the seismic sources, seismic receivers and the instrumentation. Chapters 5 and 6 deal with the 2D Survey Designing and 3D Survey Designing respectively.

Reflection Field Layouts are discussed in chapter 7 while chapter 8 focuses on the Reflection Field Method for the land survey. In this I have included the field parameters that were assumed in the field with a brief theoretical background of each parameter. The end part of this chapter deals with the starting of the production work with parameters decided based on the experimental studies. The next chapter (chapter 9) deals with some of the basic concepts of Seismic Data Processing. The subsequent chapters (chapter 10, 11, 12 and 13) deal with some of the important aspects of the seismic data Processing in detail. In these I have given a detailed description of the various stages of data processing with the results shown in the form of seismic records. At the end of the report there is an appendix touching some of the important aspects which could not be explained in the due course of the chapters.

Kiran Kumar Talagapu

Contents

Chapter 1 Introduction

1.1 Introduction 1.2 Historical Perspective

1.2.1 Milestones in Seismic Industry

Chapter 2 Modern Seismic Data Acquisition

2.1 Land Data Acquisition 2.2 Marine Data Acquisition 2.3 Transition Zone Recording

Chapter 3 Fundamentals of Seismic Prospecting

3.1 Seismic Wave Fundamentals 3.1.1 Compressional Waves (P-waves) 3.1.2 Shear Waves (S-waves) 3.1.3 Air Wave 3.1.4 Mode-Converted Waves 3.1.5 Rayleigh Waves 3.1.6 Love Waves 3.1.7 Direct and Head Waves 3.1.8 Ground Waves

3.2 Characteristics of Seismic Events 3.2.1 Reflections 3.2.2 Critical Reflection 3.2.3 Refractions 3.2.4 Diffractions 3.2.5 Multiples

3.3 Seismic Noise 3.3.1 Coherent noise

3.3.2 Incoherent noise

Chapter 4 Reflection Field Equipment

4.1 Seismic Sources 4.1.1 Land Energy sources 4.1.2 Explosive Sources

4.1.2.1 Charge Size 4.1.2.2 Charge Depth

4.1.3 Vibrators 4.1.4 Other sources

4.2 Seismic Receivers 4.2.1 Geophones

4.2.1.1 Electrical Characteristics 4.2.2 Hydrophones 4.2.3 Dual Sensors

4.3 Seismic Instrumentation 4.3.1 Basic components 4.3.1.1 Roll-along switch 4.3.1.2 Pre-amplifier

4.3.1.3 Multiplexer 4.3.1.4 Main Amplifier 4.3.1.5 A/D Converter 4.3.1.6 Gain Controller 4.3.1.7 Formatter 4.3.1.8 Tape Drive

4.3.2 Telemetry System 4.3.3 Storage

Chapter 5 Basics of 2D Survey Design

5.1 Basic Concepts in 2D Surveys

Chapter 6 Basics of 3D Survey Design

6.1 Why 3D Seismic Survey? 6.2 Basic Concepts in 3D Surveys

6.2.1 Preliminary Parameters 6.3 3D Survey Design Sequence 6.4 Land 3-D Layouts

Chapter 7 Reflection Field Layouts

7.1 Split-Dip and Common Midpoint Recording 7.2 Spread Types 7.3 Arrays 7.4 Resolution

Chapter 8 Reflection Field Method for Land Survey

8.1 The Seismic Field Party 8.2 Seismic Data Acquisition 8.3 2D Survey Parameters (Before the Experimental Survey) (GP X) 8.4 3D Survey Parameters (Before the Experimental Survey) 8.5 Experimental Survey

8.5.1 Uphole Survey (Depth Optimization)

8.5.2 Noise Experiment (Determination of NTO and Array Length) 8.5.3 Fold Back Experiment (Element Spacing Determination ) 8.5.4 Shot Depth and Charge Size Optimization

8.6 2D Survey Parameters (After the Experimental Survey) (GP X) 8.7 3D Survey Parameters (After the Experimental Survey)

(GP Y and GP Z)

Chapter 9 Seismic Data Processing

9.1 Introduction 9.2 Why Processing? 9.3 Seismic Data Processing 9.4 Objectives of Data Processing 9.5 Basic Data Processing Sequence

Chapter 10 Seismic Data Processing Stage I (Pre-Processing)

10.1 Preprocessing 10.1.1 De-Multiplexing 10.1.2 Reformatting 10.1.3 Re-sampling 10.1.4 Editing 10.1.5 Geometry Merging (Labeling) 10.1.6 Static Corrections 10.1.7 Amplitude Recovery (Geometric Spreading Correction) 10.1.8 Muting

10.2 Sorting 10.3 Filtering

Chapter 11 Seismic Data Processing Stage II (De-convolution)

11.1 Introduction: 11.2 Convolutional Model 11.3 De-convolution 11.4 De-convolution Methods

11.4.1 Deterministic De-convolution 11.4.2 Statistical De-convolution

Chapter 12 Seismic Data Processing Stage III (Velocity Analysis, NMO, DMO

and Residual Static Corrections)

12.1 Velocity Analysis 12.2 Normal Moveout Correctons (NMO)

12.3 Dip Moveout Correctons (DMO) 12.4 Residual Statics Corrections

Chapter 13 Seismic Data Processing Stage IV (Stacking, Time Variant Filtering

and Migration) 13.1 Stacking 13.2 Time Variant Filtering 13.3 Migration

Appendix

Bibliography

List of Figures Figure 1(a) P Wave Motion. Figure 1(b) S Wave Motion. Figure 2(a) Surface Wave Motion. Figure 2(b) The particle motion in the wave front of a Rayleigh wave

consists of a combination of P Wave and SV vibrations in the vertical plane. The particles move in retrograde sense around an ellipse that has its major axis vertical and minor axis in the direction of wave propagation.

Figure 2(c) In a Love wave the particle motion is horizontal and perpendicular to direction of propagation. The amplitude of the wave decreases with depth below the free surface.

Figure 3 Reflection of plane compress ional wave at interface. Figure 4 Refraction of plane compress ional wave across interface. Figure 5 Diffraction from the edge. The source a of diffracted radiation

has been set into oscillation by waves generated on surface. Radial lines with arrows are ray paths; circular arcs are wave-fronts.

Figure 6 Internal structure of a moving magnet geophone. Figure 7(a) Surface Geometry and Sub surface Nature and Behavior in 2D Layout. Figure 7(b) Fresnel Zone. Figure 8(a) 3D Layout. Figure 8(b) 3D Cable Configuration used by GP Y. Figure 8(c) Shows the generalized stratigraphy of KG Basin. Figure 9 Field Geometry and the Structure of the Bore Hole dug for

doing the UPHOLE Survey (at GP X and GP Y). Figure 10 Field Geometry and the Structure of the Bore Hole dug for

doing the UPHOLE Survey (at GP Z). Figure 11 Shows a field record as obtained by the uphole survey team

of GP X for a source (1m of detonating cord) at a depth of 60m into a spread of geophones. The first four geophones are at an offset of 1m, 3m, 5m, 25m, and 50m.

Figure 12 Geometric Correction for the Uphole Data. Figure 13(a) t-d plot drawn for the data obtained by the first three

receivers of uphole A of GP X. Figure 13(b) t-d plot drawn for the data obtained by the last two receivers

of uphole A of GP X. Figure 14 Noise Spread (Transposed Spread), G1, G2, G108

represent the Geophones. Figure 15(a) Noise Section for Shot point 1 at GP X. Figure 15(b) Noise Section for Shot point 2 at GP X.

Figure 15(c) Noise Section for Shot point 3 at GP X. Figure 15(d) Noise Section for Shot point 4 at GP X. Figure 16 Noise Section prepared by GP Y. Figure 17 Layout for the Fold Back Experiment, G1, G2, G216

represent the Geophone strings (12 Geophones per string). Figure 18 Seismic Section obtained by conducting Fold Back

Experiment by GP X. Figure 19(a) Seismic Section obtained by conducting Fold Back

Experiment, without the application of filter, by GP Y. Figure 19(b) Seismic Section obtained by conducting Fold Back

Experiment, after the application of filter, by GP Y. Figure 20(a) 1st Shot gather obtained during the regular production work

by GP X Figure 20(b) 2nd Shot gather obtained during the regular production work

by GP X. Figure 21(a) Seismic data volume in processing coordinates midpoint,

offset and time. Figure 21(b) Conventional processing flowchart. Figure 22(a) Raw Field record in SEG D format. Figure 22(b) Raw Field record in SEG Y format. Figure 23(a) Editing (Raw Field Record). Figure 23(b) Record Obtained after Editing. Figure 24 Seismic Data Merging. Figure 25 Amplitude Decay with time/depth. Figure 26(a) Uncorrected Record. Figure 26(b) Seismic Record obtained after doing the Spherical

Divergence Correction. Figure 26(c) Seismic Record obtained after doing the Amplitude

Correction. Figure 26(d) Seismic Record obtained after doing the Amplitude

Correction and applying filter. Figure 27(a) Time Variant Filtering (Record without the application of

filter). Figure 27(b) Time Variant Filtering (Application of High Pass Filter 8-

16Hz). Figure 28(a) Spiking Deconvolution. Figure 28(b) Specturm of the Raw Data and the Decon Data. Figure 29 Velocity Analysis. Figure 30 Selection of Velocity Function. Figure 31 NMO Stack. Figure 32(a) Surface consistent statics model to establish the travel

time model equation. Figure 32(b) Picking travel time deviations from NMO corrected gathers. Figure 33 Decon and Residual Stack.

Figure 34 Brute Stack. Figure 35 Final Stack. Figure 36 Geometrical representation of Migration. Figure 37 Migration Stack. Figure 38 Aerial Network Principle (Master Slave Relation). Figure 39(a) A Hypothetical stacking chart Each dot represents a single

trace with the time axis perpendicular to the plane of the page.

Figure 39(b) Different types of Gathers.

T

Chapter 1

Introduction

1.1 Introduction

he seismic method has three important/principal applications a. Delineation of near-surface geology for engineering studies, and coal and mineral

exploration within a depth of up to 1km: the seismic method applied to the near surface studies is known as engineering seismology.

b. Hydrocarbon exploration and development within a depth of up to 10 km: seismic method applied to the exploration and development of oil and gas fields is known as exploration seismology.

c. Investigation of the earths crustal structure within a depth of up to 100 km: the seismic method applies to the crustal and earthquake studies is known as earthquake seismology.

Definition by Robert E. Sheriff: Seismic survey is a program for mapping geologic

structure by observation of seismic waves, especially by creating seismic waves with artificial sources and observing the arrival time of the waves reflected from acoustic-impedance contrasts or refracted through high velocity members. 1.2 Historical Perspective

A.D. 100 The earliest known seismic instrument, called the seismoscope, was produced in China to indicate the direction form which the tremor came during an earthquake motion.

1848 In France, Mallet began studying the Earths crust by using Acoustic waves. This science developed into earthquake seismology, solid earth or crustal geophysics.

1914 In Germany, Mintrop devised the first seismograph, it was used for locating enemy artillery during World War I.

1917 In the United States, Fessendon patented a method and apparatus for locating ore bodies.

1920 The introduction of refraction methods for locating salt domes in the Gulf Coast region of the United States began.

1923 A German seismic service company known as Seismos went international (to Mexico and Texas) using the refraction method to locate oil traps.

1.2.1 Milestones in Seismic Industry

As the search for oil moved to deeper targets, the technique of using reflected seismic waves, known as the seismic reflection method, became more popular during World War II, because it aided delineation of other structural features apart from simple salt domes. During 1960s the so-called digital revolution ushered in what some historians now are calling the Information Age. This had a tremendous impact on the seismic exploration industry. The ability to record digitized seismic data on magnetic tape, then process that data in a computer, not only greatly improved the productivity of seismic crews but also greatly improved the fidelity with which the processed data imaged earth structure. Modern Seismic Data Acquisition could not have evolved without the digital computer. The late 1970s saw the development of the 3D seismic survey, in which the data imaged not just a vertical cross-section of earth but an entire volume of earth. The technology improved during the 1980s, leading to more accurate and realistic imaging of earth. In 1990s depth section preparation got focused from the prevailing time section preparation after processing the data. In 2000s data is being acquired with an additional parameter of time as the 4th dimension of the existing 3D data acquisition system. This is called 4D data acquisition.

As the seismic industry made one breakthrough after another during its history, it also created new challenges for itself. Now we record not just p-waves but also converted s-waves for a wide range of objectives. Using the multi-component seismic method, commonly known as the 4-C seismic method, we are now able to see through gas plumes caused by the reservoir below. We are able to sometimes better image the sub-salt and sub-basalt targets with the 4C seismic method. Using the converted s-waves, we are able to detect the oil-water contact, and the top or base of the reservoir unit that we sometimes could not delineate using only p-waves.

S

Chapter 2

Modern Seismic Data Acquisition

ubsurface geologic structures containing hydrocarbons are found beneath either land or sea. So there is a land data-acquisition method and a marine data-acquisition method. The two methods have a common-goal, imaging the earth. But because the environments differ, so each required unique technology and terminology.

2.1 Land Data Acquisition: In land acquisition, a shot is fired (i.e., energy is transmitted) and reflections from the boundaries of various Lithological units within the sub-surface are recorded at a number of fixed receiver stations on the surface. These geophone stations are usually in-line although the shot source may not be. When the source is in-line with the receivers at either end of the receiver line or positioned in the middle of the receiver line a two-dimensional (2D) profile through the earth is generated. If the source moves around the receiver line causing reflections to be recorded form points out of the plane of the in line profile, then a three-dimensional (3D) image is possible (the third dimension being distance, orthogonal to the in-line receiver-line). The majority of land survey effort is expended in moving the line equipment along and / or across farm fields or through populated communities. Hence, land operations often are conducted only during daylight thus making it a slow process.

2.2 Marine Data Acquisition: In a marine operation, a ship tows one or more

energy sources fastened parallel with one or more towed seismic receiver lines. In this case, the receiver lines take the form of cable called Steamer containing a number of hydrophones. The vessel moves along and fires a shot, with reflections recorded by the streamers. If a single streamer and a single source are used, a single seismic profile may be recorded in like manner to the land operation. If a number of parallel sources and/or streamers are towed at the same time, the result is a number of parallel lines recorded at the same time. If many

closely spaced parallel lines are recorded, a 3D data volume is recorded. More than one vessel may be employed to acquire data on 24-hour basis, since there is no need to curtail operations in nights.

2.3 Transition Zone Recording: Because ships are limited by the water depth in which they safely can conduct operations, and because land operations must terminate when the source approaches the water edge, or shore lines, transition-zone recording techniques have been developed to provide a continuous seismic coverage required over the land and then into the sea. Geophones that can be placed on the sea bed or used with both marine and land shots fired into them. Techniques have been developed to use both Geophones and hydrophones in the surface area where the shore line / water edge is likely to migrate towards land and sea depending on the tide of sea a day. The combination of such hydrophone / geophones is called a Dual Sensor. The advantage of why this is to see that either of the receiver of Dual Sensor pickups the surveyed from the slots recorded using a land or marine source and data gaps all along the coast within the area of prospect.

In this report, though the principle of all sorts of seismic operations like land, marine and TZ are discussed, the ultimate emphasis is given on the land acquisition only as the training has been in this regard.

T

Chapter 3

Fundamentals of Seismic Prospecting

3.1 Seismic Wave Fundamentals

he transmission of energy into the earth can be explained by assuming that the Earth has the elastic properties of a solid. The Earths crust is considered as completely elastic (except in the immediate vicinity of the shot), and hence the name given to this type of acoustic wave transmission is elastic wave propagation. Several kinds of wave phenomenon can occur in an elastic solid. They are classified according to how the particles that make up the solid move as the wave travels through the material.

3.1.1 Compress ional Waves (P-waves): On firing an energy source, a compress

ional force causes an initial volume decrease of the medium upon which the force acts. The elastic character of rock then caused an immediate rebound or expansion, followed by a dilation force as shown in figure 1(a). This response of the medium constitutes a primary compress ional wave or P-wave. Particle motion in a P-wave is in the direction of wave propagation. The P-wave velocity is a function of the rigidity and density of the medium. In dense rock, it can vary from 2500 to 7000 m/sec, while in spongy sand, form 300 to 500 m/sec.

3.1.2 Shear Waves (S-waves): Shear strain occurs when a sideways force is exerted on a medium;(figure 1(b)) a shear wave may be generated that travels perpendicularly to the direction of the applied force. Particle motion of a shear wave is at right angles to the direction of propagation. A shear waves velocity is a function of the resistance to shear stress of the material through which the wave is traveling and if often approximately half of the materials compress ional wave velocity. In liquids such as water, there is no shear wave possible because shear stress and strain cannot occur in liquids.

3.1.3 Air Wave: On land the energy source (shot) generates an airwave known as the air blast, which itself can set up an air-coupled wave, a secondary wave-front in the surface layer. This wave generally travels by about 350 m/sec velocity slower, than the compress ional wave, the speed of the airwave depends mainly on temperature and humidity, as shown below: V = 1051 + 1.1F ft/sec where F = Fahrenheit temperature V = 331.5 + 0.607C m/sec where C = Celsius temperature.

3.1.4 Mode-Converted Waves: Each time a wave impinges on a boundary, a portion of the energy is reflected and the remaining transmitted. Depending upon the elastic properties of the boundary, the incident P-wave or S-wave may convert to one or the other or to a proportion of each. Such converted waves sometimes, degrade the signal-to-noise ratio. This degradation causes problems during data processing.

3.1.5 Raleigh Waves: It is a type of seismic surface wave propagated along the free surface of a semi-infinite medium. Figure 2(a) shows surface wave motion.This medium is known as weathering layer or low-velocity layer (LVL). Raleigh waves are of low frequency nature, traveling horizontally with retrograde elliptical motion and away from the energy source (shot). The particle motion of this wave reduces (amplitude) with increase in depth, eventually reversing in direction. This point is in the vicinity of the base of the weathering layer. Because the motion of the ground appears to roll, this wave is commonly known as ground roll (figure 2(b)).

3.1.6 Love Waves: The Love wave (figure 2(c)) is a surface wave borne within the LVL, which has horizontal motion perpendicular to the direction of propagation with, theoretically no vertical motion. Also known as the horizontal SH-wave, Q-wave, Lq-wave or G-wave in crustal studies, such waves often propagate by multiple reflection within the LVL, dependent upon the LVL material. If such waves undergo mode conversion, a number of noise trains appear across the seismic record, obscuring reflected energy content even further.

3.1.7 Direct and Head Waves: The expanding energy wave front that moves along the air-surface interface outward form shot commonly is observed as the direct wave and has

the velocity of the surface layer through which it travels. Head waves are the portions of the initial wavefront that are transmitted down to the base of the weathering layer or the water bottom and are refracted along the weathering base. They return to the surface as refracted energy or refractions. Sometimes the refracted velocity is higher than the velocity of propagation in the surface layer. In that case, refracted head waves appear in the mid-to-far-offset traces before arrival of the direct wave.

3.1.8 Ground Waves: When a layer of the Earth has an extreme density or velocity contrast at both its upper and lower boundaries, a wave traveling along the layer may undergo internal reflection (i.e., stay within the layer, reflecting from upper interface to lower, back up again, and so on). Such waves are called guided waves and exhibit mainly vertical particle motion. They appear as short shingled waves, repeating on the shot record.

3.2 Characteristics of Seismic Events

Seismic wave created by an explosive source emanate outward from the shot point in a 3D sense. Huygens principle is commonly used to explain the response of the wave. Every point on an expanding wave front can be considered as the source point of a secondary wave front. The envelope of the secondary wave fronts produces the primary wave fronts after a small time increment. The trajectory of a point moving outward is known in optics as a ray, and hence in seismics as a raypath.

3.2.1 Reflections: The phenomenon in which the energy or wave from a seismic source

has been returned from an interface having acoustic impedance contrast (reflector) or series of contrasts within the earth is called reflection. This phenomenon is pictorially represented in figure 3. The amplitude and polarity of reflections depend on the acoustic properties of the material on both sides of discontinuity. Acoustic impedance is the product of density and velocity. The relationship among incident amplitude Ai, reflected amplitude Ar, and reflection coefficient Rc, is:

,

where,

Where velocity is constant, a density contrast will cause a reflection and vice versa. In other words, any abrupt change in acoustic impedance causes a reflection to occur. Energy not reflected is transmitted. With a large Rc, less transmission occurs and, hence signal-to-noise ratio reduces below such an interface.

3.2.2 Critical Reflection: When an impinging wave arrives at such an angle of

incidence that energy travels horizontally along the interface at the velocity of the second medium, then critical reflection occurs. The incident angle ic, at which critical reflection occurs can be found using Snells Law.

3.2.3 Refractions: The change in direction of a seismic ray upon passing into a medium with a different velocity, is called refraction. Snells law describes how waves refract. It states that the sine of the incident angle of a ray, (sin i), divided by the initial medium velocity V1 equals the sine of the refracted angle of a ray (sin r), divided by the lower medium velocity V2, that is:

when a wave encounters an abrupt change in elastic properties, part of the energy is reflected, and part is transmitted or refracted (figure 4) with a change in the direction of propagation occurring at the interface.

3.2.4 Diffractions: Diffractions (figure 5) occur at sharp discontinuities, such as at the edge of a bed, fault, or geologic pillow. When the wave front arrives at the edge, a portion of the energy travels through into the higher velocity region, but much of it is reflected. The reflected wave front arrives at the receivers get aligned along the trajectory of a parabola on the seismic record. In conventional in-line recording, diffractions may arrive from out of the plane of the seismic line / profile. Such diffractions are considered as noise and reduce the signal-to-noise ratio. However, in 3D recording, in which specialized data processing techniques are used (i.e.,, the 3D seismic migration), the diffractions are considered as useful scattered energy because the data-processing routines transfer the diffracted energy back to the point from which is generated, thereby enhancing the subsurface image. Hence in 3D surveys, out-of-the plane diffractions events are considered part of the signal. 3.2.5 Multiples: Seismic energy that has been reflected more than once is called multiple while virtually all seismic energy involves some multiples. The important distinction between long-path and short-path multiples is that a long-path multiple arrives as a distinct event whereas a short-path multiple arrives soon after the primary and changes the wave shape. 3.3 Seismic Noise

The reliability of seismic mapping is strongly dependent on the quality of the records / data. We use the term signal to denote any event on the seismic record from which we wish to obtain information. Everything else is noise, including coherent events that interfere with the observation and measurement of signals. The signal-to-noise ratio (S/N), is the ratio of the signal energy in a specified portion of the record to the total noise energy in the same portion. Poor records result whenever the signal-to-noise ratio is small. Seismic noise may be either

a) Coherent or b) Incoherent

Another important distinction is between a) noise that is repeatable and b) noise that is non repeatable.

The properties coherence, travel direction and repeatability form the basis of most methods of improving record quality.

3.3.1 Coherent noise includes surface waves, reflections or reflected refractions from near-surface structures such as fault planes or buried stream channels, refractions carried by high-velocity stringers, noise caused by vehicular traffic or farm tractors, multiples and so forth. All the preceding except multiples travel essentially horizontally and all except vehicular noise are repeatable on successive shots. Coherent noise is sometimes subdivided into:

a) energy that travels essentially horizontally and b) energy that reaches the spread more or less vertically

3.3.2 Incoherent noise is often referred to as random noise (spatially random), which implies not only non-predictability but also certain statistical properties. Incoherent noise is due to scattering from near-surface irregularities and in homogeneities such as boulders, small-scale faulting, and so froth. Non repeatable random noise may be due to wind shaking a geophone or causing the roots of trees to move, which generates seismic waves, stones ejected by the shot and falling back on the earth near a geophone, ocean waves beating on a seashore, distant earthquakes, a person walking near a geophone, and so on.

S

Chapter 4

Reflection Field Equipment

4.1 Seismic Sources

eismic sources can be broadly divided into two categories: land energy sources and marine energy sources.

4.1.1 Land Energy sources: The choice of energy source is critical in land data acquisition because resolution and signal-to-noise ratio quality are limited by the source characteristics. A geophysicist should select a source based on the following five criteria:

Penetration to the required depth: Knowing what the exploration objectives are, the geophysicist should select a source that has adequate energy to illuminate the target horizons. Past experience can help here.

Bandwidth for the require resolution: If high resolution reflections are required to delineate subtle geological features such as a stratigraphic traps, the source must transmit a broad range of frequencies, both high and low. For very shallow targets, a detonator may possess adequate energy and frequency bandwidth. For deeper reflections, the longer travel path to a deep reflector requires the selection of a source that has enough energy at the higher frequencies to maintain a broad reflection bandwidth.

Signal-to-noise- characteristics: Different areas have different noise problems. They may dictate the source selection.

Environment: When working in populated areas, there are special safety requirements to which geophysicists must adhere.

Availability and Cost: The time of arrival of a crew can be extremely important.

Land Energy Sources are of two types: Explosive sources and Non Explosive sources.

4.1.2 Explosive Sources: Explosive sources produce robust P-waves. The selection of explosives as the sources of choice depends primarily on near-surface conditions and the accessibility of other energy sources. If drilling is fast and efficient, single shot hole filled with explosives might be the most economical source option. The explosive source consists of a detonator and an explosive charge. In the seismic industry, the explosive charge is commonly referred to as powder and the detonators are referred to as caps or primers. 4.1.2.1 Charge Size: The choice of charge size depends largely on the depth to the horizon of interest. The best charge size is that which achieved the maximum signal-to-noise ratio (S/N) at the target depth. Deeper targets usually require larger charge sizes. Generally, larger charge sizes cause more ground roll and air blast contamination of the record. Alternatively, smaller charge sizes mean higher frequency content, but less energy going into the ground. De convolution enhances the frequency content such that the bandwidth will be higher and have an improved S/N ratio compared to a record with a smaller charge size. 4.1.2.2 Charge Depth: The charge depth depends on the depth of the weathering layer and the level of noise interference one encounters when testing. Generally, the shallower the source, the stronger the air-blast and the ground-roll. On the other hand, it is usually not economical to go much beyond 50m depth. If the drilling is really tough and expensive, one may have to limit the shot hole depth to as little as 2m or a surface shot may be used instead. 4.1.3 Vibrators: Vertical vibrators produce and asymmetric radiation pattern of P-waves and S-waves. Horizontal vibratos produce weak P-waves and robust S-waves. If multiple dynamite patterns do not pump enough energy into the ground, vibrators may be preferred on technical grounds, regardless of relative cost. Vibrators are designed in two basic groups: Buggy-mounted and truck-mounted units. 4.1.4 Other sources: Although dynamite and Vibroseis are used in majority of surveys, other sources can be and are used in the field 3D surveys, such as:

Airguns and mud guns (used in transition zone surveys) Shotgun (Betsy) Mini-Seis (Thumper) Land air gun

Dinoseis

Elastic wave generator (EWG) Mini-vibes

4.2 Seismic Receivers

4.2.1 Geophones: Conventional geophones are based on Faradays law of electromagnetic induction. This law states that relative motion of a conductor through a magnetic field induces an electromagnetic force (EMF) which causes a current to flow through the conductor, if the conductor is an element of an electrical circuit. The two types of geophones widely used in geophysical surveys are

1. moving coil geophone and 2. moving magnet geophone(figure 6) The essential ingredients to make a geophone are a permanent magnet, a conductor and a spring which positions either the conductor in the magnetic field space (in moving coil geophone) or the permanent magnet in the electric field space (as in moving magnet geophone). The conductor in reality is a length of copper wire wrapped into a cylindrical coil shape. It is often referred to as the coil or element. The conductors or the magnets motion through the magnetic/electrical field, according to Faradays law, causes an EMF to be induced that is proportional to the velocity of the earths motion. Hence, such a geophone is called a velocity phone because its output is proportional to the velocity of the earths motion.

The large amount of subsurface information carried by seismic signal would be fully available for interpretation only if the geophones follow ground movement faithfully with minimum distortion.

4.2.1.1 Electrical Characteristics:

Sensitivity: Geophones are available with a wide range of sensitivities. For example, at one end of the sensitivity scale, a geophone can produce 0.1V output for a 2.5cm/sec velocity, while another geophone can produce as much as 0.4mV output for a tiny movement of 2.5 X 10 m/sec.

Tolerances: Geophones have typical tolerances. That are as follows: o Natural frequency within + 0.5Hz. of the manufacturer stated value o Natural frequency distortion with a maximum 20 tilt, + 0.1Hz. o Sensitivity within + 5% of the manufacturer stated value.

A large variety of modern geophones are available today to meet the specific requirement of the user. Close tolerance digital grade geophones have distortions as low as 0.03%, tolerance of 2-2.5% on frequency, sensitivity and damping, and very high geophone-

to-geophone uniformity. To modern geophones has done away with the shunt resistance, resulting in very low distortion and high spurious response up to 250Hz. These geophones maintain their natural frequency specifications with high tilt angles.

The particular receiver type depends on the characteristics of the data to be recorded and the environment where the data acquired. In normal land operations, geophones have a resonant frequency of 10 or 14Hz., but in some parts of the world it is still normal practice to use 6 to 8Hz phones. However, geophones with resonant frequencies up to 40Hz are being manufactured. Receivers are usually wired in groups of 1, 4, 6, 9, 12 or 24. While the trend is towards higher number of phones (9, 12, 24 on even 72 in the Middle East), however numbers (e.g., 6) are still used in certain areas, e.g., South America. In hilly, terrain, where the height difference between the ends of any receiver group exceeds 2m, geophones may be clustered in a small area. In steep terrain (over 5m. elevation difference) one can spread the phones out parallel to topographic contours to minimize inter-array statics smear. Three-

component 3D recording requires three times the number channels of recording capacity since each component is recorded separately. This increased number of channels may make it difficult to create a patch that creates sufficient fold. Since shear-wave reflections contain a lower frequency bandwidth, phones with lower resonant/natural frequencies are used.

4.2.2 Hydrophones: The hydrophone is an electro acoustic transducer that converts a pressure pulse into an electrical signal by means of the piezoelectric effect. If mechanical stress is applied on tow opposite faces of a piezoelectric crystal, then electrical charges appear on some other pair of faces. If such a crystal is placed in an environment experiencing changes in pressure, it will produce a voltage proportional to that variations in pressure.

4.2.3 Dual Sensors: For ocean bottom cable (OBC) applications, combining the output of geophones and a hydrophone is now widely accepted technique for reducing the ghosting effect caused by the water/air interface. To overcome the disadvantage of using two separate sensors, both geophone and hydrophone are available in a single unit known as dual sensors or the 4-component (4C) receivers consist of a hydrophone, two horizontal geophones and a vertical geophone installed in a single water proof enclosure for recording P, SV and SH waves.

4.3 Seismic Instrumentation

Once a seismic signal is transmitted and received, it must be recorded. The different types signals are as follows:

Source Signal: The pressure field created by the seismic source.

Reflectivity Signal: The earths reflection sequence convolved with the source wavelet.

Seismic Signal: Everything received as a result of the source firing. The seismic signal includes the reflectivity signal as well as ground roll, refractions, diffractions, sidesweep, channel waves etc.

Received Signal: The electrical output of the receiver group. This is the seismic

signal plus all environmental noise.

Recorded Signal: The data, that is the instrument filtered signal plus any addition instrument noise, which goes onto the tape.

The information contained in a signal can be characterized by three quantities: Signal-to-noise ratio, Bandwidth and Duration

In seismic exploration, the recorded signal bandwidth is usually 0-250Hz. or lower. Often, data are processed in a narrower band, say 5-80Hz., even though they may be recorded in a broader band. The duration of recorded signals depends on the nature of the source and target depth. A reflection is a physical event caused by a change in the acoustic impedance of the earth. It is recorded signal, that event is represented by a wavelet that has two components the earth filter and the acquisition wavelet. The wavelet can be described in the time domain or alternately, in the frequency domain. The Fourier transform can be used to move form one representation to the other. If a wavelet has a short extent in a time and appears like a spike, it is likely to be composed of a broad band of frequencies, each separate frequency having its own phase value. The amplitude and phase of a wavelet contain all the spectral information of a wavelet. These spectra are called the frequency-domain representation of the wavelet, whereas the wavelet in time is considered to be in the time domain. When seismic recording first began in a 1920s the recording systems consisted of heavy, metal cased geophones connected by wire cables to a recording truck. The signal was

recorded on a rotating photographic drum. Drums were replaced by analog magnetic tape recorders during the late 1950s but these often failed to operate well. In the early 1960s they were replaced by digital tape recorders, each of which had an analog-to-digital converter at the input stage to the tape drive. The individual analog amplifiers also were unreliable, and by late 1960s, they were being replaced in recording devices by a single multiplexed analog amplifier.

In late 1970s, distributed systems were introduced that performed amplification, filtering, digitization and multiplexing at or near the receiver stations. By the mid 1980s distributed systems were in wide use throughout the industry.

4.3.1 Basic components: The basic components of the land recording systems are: 4.3.1.1 Roll-along switch: It allows the observer to record a selected subset of the geophones connected to the recording truck. It minimizes the need to move the recording truck.

4.3.1.2 Pre-amplifier: This is a fixed gain amplifier that raises the incoming seismic signal above the background instrument noise level. The preamplifier has low noise, high input impedance and low distortion. Its input impedance is equal to or greater than the cable impedance to the farthest station so that no signal amplitude is lost because of mismatching of impedances. The amplifier must be completely linear throughout its operating range.

4.3.1.3 Multiplexer: This is an electronic switch that time shares data form multiple channels. It changes multiple parallel inputs to a serial output relay for amplification,

digitization and recording. The multiplexer cycles through all of the inputs during each digital sampling interval.

4.3.1.4 Main Amplifier: This amplifier receives all analog signals input to it and passes then on to the A/D converter with an amount of gain determined by the gain controller.

4.3.1.5 A/D Converter: Analog signals are converted to digital signals with this device. It allows the analog stream of data to be recorded in digital form. The received incoming signal must be filtered to prevent aliasing prior to conversion to a digital form.

4.3.1.6 Gain Controller: The received signal includes, reflections, refractions, ground roll and environmental noise, all of which may have amplitudes varying in a range from microvolts to volts. A fixed form of amplification with only a relatively small number of data bits cannot handle that range without some dipping at the most significant bit end of the converter. Instead a variable or automatic gain control (AGC) level is determined for application by the main amplifier in the feedback loop with the A/D converter to reduce or amplify incoming signal to keep signal levels within the desired converter range. The controller sets the amount of gain while the amplifier applies it to the incoming signal. The

AGC level set at each sample is recorded on tape as part of the gain word. 4.3.1.7 Formatter: The formatter arranges the data stream (in the form of voltage and gain levels) into a binary code for writing onto magnetic tape. In addition, instrument operational commands are distributed by the formatter to all the other components, making the formatter the brain of the recording operation.

4.3.1.8 Tape Drive: Data finally are recorded on tape in digital form, ready to be passed on to the processing center for further processing. Magnetic tape may be replaced by floppy disks, depending upon the system in use. In land using recording a non-distributed system, an analog seismic signal travels from the geophones along electrical conductors (the cable) to a roll-along switch in the recording truck (or doghouse or dog box), after which it is converted to a digital signal and recorded on tape or disk. In contrast, in a distributed system, the seismic signal passes from the geophone string directly into an amplifier and/or A/D converter, after which it travels in digital form along a cable to the recording truck. Because digital transmission of multiplexed data uses many fewer cables than analog transmission, layout of the large receiver spreads often used for 3D acquisition became considerably simpler.

Today, majority of the acquisition systems provide 24-bit recording technology. A 24-bit technology system offers high fidelity because it records data over a large dynamic range. Peculiarities for each system need to be examined for the task at hand. In land operations, these recording units are usually truck or buggy mounted and can, therefore, travel easily to areas of data acquisition. Lower channel count systems with higher sampling rates, such as the DMT/SUMMIT and the 24-bit OYO DAS, can be used for small, near-surface 3D surveys. In the case of very low channel count systems (e.g. less than 120), it is

normal practice for several recorders to be used together in a master-slave pattern to reach sufficient channel capacity even for small 3D surveys.

If a 3D survey crosses a variety of terrains (e.g., mountain, plain, transition zone), it is desirable to use one type of recorder to cover all the survey areas. Thus shots of different types in the mountains or in the swamp can be recorded by the same instrument. If more than one recorder is used, amplitude and phase matching will be required to compensate for the recorder differences. Seam less receiver coverage from a variety of sources enables application of surface-consistent processes as de convolution, statics, and amplitude correction.

Different Types of Seismic Recorders

Manufacturer System T/D/R Boxes Stations

per Box

Line Units Central

System

Sercel SN388 Distributed System Station Unit (SU)

1-6 Crossing Station Unit

(SU)

Central Control Unit

(SU) Sercel 408UL Telemetry

System/Distributed System/Remote

Seismic Recording

Field Digitizer Unit

(FPU)

1 Line

Acquisition

Unit Cross (LAUX)

Central Module (CMU)

4.3.2 Telemetry System

True telemetry system has no physical connection between the station recording unit and the control system in the recording truck. These systems should be used where access is limited due to rugged terrain, permit problems, or any other reason. Sercel Eagle system is an example of such systems. The SAR (Seismic Acquisition Remote unit) records the signal and sends it via radio frequencies to the CRS (Central Recording Station). Some telemetry systems can receive data in real time. Other telemetry systems have a disadvantage over distributed system in that the radio transmission of the data from the boxes to the recording unit takes longer than real time. For some systems, data transmission time may be on the order of minutes per source point, which may slow down the shooting crew. Tree cover may also cause a problem for the signal transmission, and FM interference may be significant in populated areas. Mixed systems may be used to cross-rivers or roads at select locations.

4.3.3 Storage: The data obtained in the seismic field survey is stored on magnetic tapes or cartridges. While the conventional storage devices are the tape drives the latest equipment uses the cartridges with 10 GB memory capacity for storing the data. The data is stored in SEG D format. Previously it used to get recorded in SEG B format or SEG C formats.

T

Chapter 5

2D Survey Design Basics

5.1 Basic Concepts in 2D Surveys

he guiding principle should be to design a seismic survey that will image the selected target in the most economical way for costs and time. Resolution parameters, such as the frequency required to image the target, are starting design factors. Shallow horizon of interest and deeper horizons may be interpretational needs; thus, the definition of the representative horizons is the beginning of the design. 5.1.1 Near Surface layer: The velocity of the surface layer is used as a factor in computing offsets and determining the effect of ground roll. Usually the weathered layer is very low velocity because of exposure and erosion, but it may be quite complex and have several layers of variant velocity. The velocity and maximum dip of each layer are initial parameters. This information can be obtained in approximate form from existing well logs or seismic data in the area. If the area is frontier area, then noise tests, experience, or geologic theory can be the source of this information. 5.1.2 Shallow Layer: While the target layer is most important for imaging, a shallow

layer may be necessary for processing or interpretation. Good data in the shallow part is needed to use the velocity analysis with confidence. The velocity Vs and the approximate arrival time are the needed parameters. These parameters allow computation of depth of the layer Zs by the familiar time-distance formula

where: t = two-way travel time to the shallow horizon

Vs = average velocity to the layer, and Zs = depth to the shallow layer This formula provides the information needed for the near offset, i.e.,

5.1.3 Target Layer: The layer is the horizon of primary interest for the survey. When

parameters conflicts arise during the design, the requirements for the target layer should prevail. For imaging the target horizon, geologic knowledge of the expected thickness and reflectivity is needed to estimate the frequency range

5.1.4 Group Interval: Group Interval is the basic sampling on the earths surface by the survey. It is the distance on the ground between receiver stations. Group interval represents which largest spatial sampling shall prevent aliasing during migration:

where, Vmin = minimum velocity,

= maximum dip of the target horizon in degrees, and Fmax = maximum frequency expected

5.1.5 Fresnel Zone: Fresnel zone (figure 7 (b)) is the smallest part of the reflector making an unambiguous image of the individual event and is circular at zero offset but elliptical with offset. Fresnel zone is given by:

where, tz = two-way record time of the target horizon

5.1.6 Far Offset: Far offset is a function of the depth modified by the velocity field. The far offset required should be computed first for the target horizon and then for the deep horizon. The velocity of the surface layer is involved because of the initial angular influence on the down going seismic waveform at the depth of the horizon. Once the maximum offset

Maximum group interval =

is computed in combination with the near / offset, the group interval, the ideal parameters can be evaluated within the framework of the available equipment.

where, Z is the depth of horizon, Vs is the velocity of the surface layer, and V the average velocity to the target.

If the horizon is dipping, then the distance, Hmax should be extended by:

where, Z = depth of horizon and = dip This extension is quite important in 3D exploration (Migration aperture). Neglect of this factor can result in underestimating field costs. The well-founded rule of thumb says that the spread length should be equal to or a little greater than the depth of the reflection being imaged. The far-trace distance should preserve full fold on the target horizon. Data processing often requires considerable muting of the shallow data on the greater offsets because of NMO stretch, noise trains, and other factors. The target horizon should be protected by the survey for the mute. The custom is to automatically mute below the 20 to 30 percent stretch factor the formula most used for this step is:

where H = offset distance, V= velocity at time Tz and Tz = arrival time of the event at H = 0. When Tm exceeds 0.3, then data processing will probably form an automatic mute.

Too large on offset range for a given number of receiver stations may result in inadequate fold for the shallow layer or even the target layer. On the other hand, if the offset

range is not long enough, accurate velocity analysis and the suppression of multiples during the processing can be endangered.

5.1.7 Record Length: Part of the survey design is to determine the required sampling rate in time and the record length is a function of depth and velocity of the deepest horizon.

Where Td = two-way arrival time of the deepest horizon of interest at the maximum offset Tr = required record length in time, and L = length in time of the longest processing filter.

Normally 200ms is adequate for the filter length. The extra time in recording is balanced against the possible benefits from data from very deep horizons. Signal length becomes more important when the source is vibratory in nature. Some allowance should also be made for migration.

5.1.8 Sample Rate: The sampling rate in time is more or less standard, ranging from 2 to 4 ms depending on the resolution needed. The rule is

A sample rate of 2ms is used for most seismic surveys.

5.1.9 Group Interval and Field Equipment: The group interval possible with a particular recording equipment given by

where, Hmax = far offset,

Hmin = near offset, and NC = the number of channels available for recording.

5.1.10 Fold Coverage: Each source position yields a certain amount of subsurface coverage. For flat layers, the sampled point is half the distance from source to receiver. The subsurface sampling is half the interval of the surface coverage. Foldage is defined as the number of times a particular sub-surface sampling point (CMP) is covered by different sources receiver locations. The maximum fold of coverage is given by

Where, S is the number of units of group interval in the source spacing and NC the number of channels available.

5.1.11 Source Interval: The source interval in the distance between source positions. The source interval is function of the desired fold coverage and the number of channels available.

Where, NC is the number of channels and F is the fold.

5.1.12 Source Power: There is a decision to be made in some cases on the source power. For dynamite, the charge size in kilograms if the unit. For vibratory sources, the available power (in pounds per square inch) is specified by equipment model. For instance, a large vibrator can generate 50,000 psi. Marine sources such as air guns and water guns are defined in terms of their volume and peak-to-peak strength. The power needed is function of target depth and the environmental noise. As the earth has a natural attenuation, target depth is the primary consideration. Noise is also involved since more power has the potential to generate more noise.

The amount of energy generated in a shot hole is proportional to the quality of dynamite. It is well known that an explosive source in a cylindrical enclosure generates pressure waves and shear waves of both polarizations.

5.1.13 Line Location and Orientation: The geometry of the survey is not

independent of the target. The location, direction, and length of the lines are important

considerations in the survey design. Dip lines for instance, are favored over strike lines. Some of basic concepts generally accepted for lines locations/orientations are:

The lines should, when possible, be perpendicular to fault planes. Since definition of the fault plane is best on the seismogram when the lines are perpendicular to the plane.

Line ties are important to interpretation. When there is existing seismic data nearby, new lines are planned in such a manner that they can be ties with the existing data. One very helpful ties is to a well. The closer the line can approach the well, the more useful the tie of the seismic data to the well log.

When there is no conflict with other needs, lines should be planned to minimize elevation and terrain problems. Sometimes a small shift in the line location can avoid a troublesome obstacle.

Figure 7(a) shows the Surface Geometry and Sub surface Nature and Behavior of 2D Layout.

S

Chapter 6

3D Survey Design Basics

6.1 Why 3D Seismic Survey?

ub-surface geological features of interest in hydrocarbon exploration are 3-dimensional in nature. A 2D seismic section is a cross-section of 3D seismic response. Despite the fact that 2D seismic section contains signal from all directions, including out-of-plane of the profile, 2D migration normally assumes that all the signal comes from the plane of the profile itself.

Although out of plane reflections (side-sweeps) are often recognizable by the experienced seismic interpreter, the out of plane signal sometimes causes 2D migrated section to mistie. These misties are due to inadequate imaging of the subsurface resulting from the use of 2D rather than 3D migration. On the other hand, 3D migration of 3D data provides adequate and detailed 3D image of the subsurface, leading to a more reliable interpretation. When integrated with well logs, core and other petrophysical and production data, 3D data permits reservoir characterization. The integrity of any 3 D data set leans heavily upon the suitability of acquisition geometry.

6.2 Basic Concepts in 3D Surveys

The 2D surveys are as linear as the terrain allows. Source and receiver are normally in-line with each other. Arrays may be multi-dimensional, but most often are also in the line of survey. For 3D surveys, this is seldom the case. The source interval of a 2D survey must be extended to include a definition of the source line. For 3D surveys, source line must be defined, since for most common designs, the source line is orthogonal to the receiver lines. The receiver line becomes the receiver lines. As many receiver lines are laid out as the equipment for acquisition allows. Also, the receiver, layout may not be lines but circles, checkerboards and other patterns developed for 3D surveys. Thus the simple parameters that defined the traditional 2D line now must be extended to include more geometry. The analysis of 2D designs centers on the subsurface coverage in the form of common-depth points (CDPs). For 3D surveys, the CDP becomes two-dimensional and is

termed a bin. These bins may be square or rectangular and define the spatial resolution of the data sampling. Indeed, deciding the bin size will be the first step in designing a 3D template. Subsurface sampling will be, as with the CDP, half the surface size. The accent of 2D lines is on the fold of coverage and the offset range. For 3D survey the fold may less, but the azimuth range is added to the offset range as a parameter. If structure is complex, then good azimuths range becomes more important. Where structure is complex the velocity analysis must include an azimuthal property. The range of azimuths in the bin is also a consideration. Another new factor is the use of computers to do the design. Moreover, interpretation is usually conducted on workstations. The multiple source and receiver lines, the difficulty of computing, fold coverage, azimuthal distribution, and offset ranges in the bins make the use of a computer program to aid in 3D design almost a necessity. The Fresnel zone assumes some new characteristics in three dimensions. Essentially, the theoretical point source expands as it propagates in depth, illuminating a circular area at vertical incidence. In a seismic context, this is the reflecting surface constructively contributing to the reflection. A good approximation to the radius of the zone is which shows that the zone increases in radius with depth but decreases with higher frequency wave fronts. Migration serves to reduce the zone to some minimal size when accurately done and the data fits the assumption. It should be noted that when the reflecting point is offset, the circle becomes elliptical. This angular effect actually reduces the size of the zone along the minor axis of the elliptical response. Dip and structure also are factors in the actual response..

where, V = average velocity to the event,

T = arrival time and t = peak-to-zero crossing of the wavelet.

An important aspect of 3D data and Fresnel zones is the extra dimension of focusing possible with migration.

6.2.1 Preliminary Parameters: There are some parameters that need to be estimated as input when designing the 3D survey. The physics and concepts are somewhat independent of whether the survey is to have two or three dimensions.

6.2.1.1 Offset: The imaging of shallow target and deep horizons still requires certain offset of source and receiver. An approximation to the required offset for a given horizon is very simple and used often when surveys are designed in the field: Offset = depth of the horizon. New factors include the fact that the offset may now be measured at an angle and the depth is now that of a plane rather than a line.

6.2.1.2 Fold: The fold required for noise suppression is a function of the S/N conditions. This translates in 3D to the number of traces in a bin. Because of the extra focusing by migration and the flexibility of binning, fold can be less than required in 2D surveys. Field tests or existing 2D seismic data can yield an estimate of the needed fold for the 3D survey.

6.2.1.3 Frequency: The temporal frequency required is not much different from that of 2D surveys. The rules for the resolution of layer of given thickness are best determined by modeling. The general rule is that the resolution of a thin bed requires it to be sampled twice within a quarter wavelength of the highest frequency. As a field approximation, the maximum frequency expected:

T = Two way time of the horizon

6.2.1.4 Objectives of the Survey: The most important information is defining the objectives of the survey. Although this seems a rather obvious comment, many times the objectives of the survey except for the aerial extent and approximate spatial sampling are not part of the input to design. Requirements of good fold on a shallow reference layer or a deep reflection for survey are clearly stated.

6.2.1.5 Migration Aperture: When the beds are dipping, the extent of the survey must be increased by:

D = Z tan

where, Z = depth, = dip

6.2.1.6 Seismic Data Input: The most useful direct input is existing seismic data. The seismic sections give information abut many of the design parameters such as noise, source power, weathering problems, and general structure. Field records and final stack should be checked for environmental and source generated noise conditions. The array design should be studied for possible use in the 3D survey. There are areas where neither source nor environmental noise is a problem, which greatly simplifies the survey design and makes the whole project less expensive. Type of source, the power used in either surveys, quality of reflections at depth, frequency content of shallow data are some of the key factors in deciding the source power to be used. If extensive static corrections made during processing indicate problems in the near surface, this should be noted on the survey design. The design of the survey can reduce processing problems in many cases.

6.3 3-D Survey Design Sequence

There are many ways to begin and complete a survey design. The specific sequence of steps that follow are general guide lines. Some design templates will dictate a different sequence of other parameters.

A summary of the proposed sequence for developing a design sequence is:

Determine the subsurface bin size. Twice the chosen bin size is the source and receiver station spacings.

Compute the number of source stations per kilometer required to achieve fold with available equipment. The number of stations per square kilometer allows computation of the source line spacing.

Compute the receiver line spacing.

Find the number of receiver lines allowed by the field equipment, constrained by the required offset ranges. The result is the template.

Decide the in-line and cross-line roll alongs.

Allow for obstacles and run analyses of the offset distribution ranges of offsets and azimuthal properties of the bins.

Estimate time and costs of the script and iterate until attributes, costs, and time are satisfied.

6.3.1 Bin size: For 3-D data the bin is the basic building block for the rest of the survey. Bin size depends on target size, spatial resolution needed, and economics. The traces when their subsurface reflection point falls with in the bin, are treated as a CDP, and corrected and summed to represent that bin position by a point. A bin can be any size but rectangles and squares are the popular. The basic sampling theorem applies to the bin.

where, b is the bin size, Fm = Maximum frequency expected,

= maximum dip in degrees and Vmin = minimum velocity.

6.3.2 Source line spacing: The bin size will, however, allow more design calculations if the fold and number of channels on the equipment are known .

where, NS = shots per square kilometer, F = desired fold, R = number of channels

B = subsurface bin size. Determining NS allows for the computation of the next important parameter source line spacing.

6.3.3 Receiver line spacing: The new information required is the minimum offset and the offset ranges needed. The controlling parameter will be the largest minimum offset within a bin. The minimum offset has been previously established with preliminary calculations and modeling. The approximation is that the maximum offset needs to be at least as long as the depth of the most reflection to be imaged. Thus, a2 = (c2-b2)1/2 where a is the receiver line spacing, c is the largest minimum offset and b is the source line spacing. A smaller near offset would require a smaller receiver line spacing and be more expensive. As with the source line spacing, the receiver line spacing from calculation may be reduced.

6.3.4 Number and length of the receiver lines in the template: The problem is to be determine the number of receiver lines possible with the template. The number of lines is constrained by the required maximum offset which sets the length of the lines. The maximum offset found in the preliminary 2-D calculations or 3-D modeling is a function of deepest horizon to be imaged. The field estimate is that the maximum offset should be a little greater than the depth of the deep horizon, but exact formula include dip. The target parameters are the number and length of the receiver lines. The source line shift is an adjustable variable. The second constraint is the number of channels available with the equipment.

6.3.5 Determining the template movement: Usually the field people prefer to roll along the direction of receiver lines. The increment is at the source line spacing. At the end of the coverage in in-line direction the next swath would be done in same manner incremented in the source direction and continued until the coverage was completed.

6.3.6 Estimation of nominal fold: Stacking fold is the number of field traces that contribute one stack trace. Fold controls the signal to noise ratio. Fold should be decided by looking at previous 2-D and 3-D surveys in the area.

6.3.6.1 In-line fold: For an orthogonal straight-line survey, in-line fold is defined similarly to the fold on 2-D data. The formula is as follows:

In line fold = ___( no. of receivers x station interval )__ . 2 x Source interval along the receiver line

(or) In line fold = ( number of receivers x receiver interval )

2 x Shot line interval

6.3.6.2 Cross-line fold: Similar to the calculation of in-line fold, the cross-line fold is: Cross line fold = source line length _.

2 x receiver line interval

6.3.6.3 Total Fold: The total 3D nominal fold is the produce of in-line fold and cross-line fold: Total nominal fold = ( in-line fold ) x ( cross-line fold).

6.4 Land 3D Layouts: Numerous layout strategies have been developed for land 3D surveys. One has to establish which features are important in the area of the survey in order to select the best design option.

6.4.1 Full fold 3-D: A full fold 3D survey is one where source points and receiver stations are distributed on an even two-dimensional grid with station spacings equal to the line spacings. The grids are offset by one bin size. A full fold 3D survey has outstanding offset and azimuth distributions as long as one can afford to record with a large number of channels. All other 3D designs are basically subsets of such full-fold surveys, and the designer has to decide which aspects of a 3D design are absolutely necessary and which can be compromised.

6.4.2 Swath: The swath acquisition method was used in the earliest 3D designs. In this geometry Source and receiver lines are parallel and usually coincident. While source points are taken on one line, receivers are recording not only along the source line but also along neighboring parallel receiver lines, creating swath lines halfway between pairs of source and

receiver lines. The offset distribution in all occupied bin lines is excellent. However inadequate sampling in the cross-line direction makes this design a poor mans 3-D, because many bins are empty. The azimuth mix is very narrow and depends on the number of live receiver lines in the recording patch and the line spacing. Most companies prefer to have the source points at the half-integer positions. Parallel swaths are sometimes considered on land when severe surface restrictions exist, or when costs have to be minimized. The operational advantages are attractive, but are achieved at the cost of a poor azimuth mix and poor cross-line sampling.

6.4.3 Orthogonal: Generally, source and receiver lines are laid out orthogonal to each other. Because the receivers cover a large area, this method is sometimes referred to as the patch method. This geometry is particularly easy for the survey crew and recording crew, and keeping track of station numbering is straightforward. In an orthogonal design, the active receiver lines form a rectangular patch surrounding each source point location creating a series of cross spreads that overlap each other. This technique allows more surface area to be acquired prior to receiver stations moves. This method is easy to lay out in the field and can accommodate the extra equipment and roll along operation. Usually all the source points between adjacent receiver lines are recorded. Then the receiver patch is rolled over one and the process is repeated. The azimuth distribution for the orthogonal method is uniform as long as wide recording patch is used.

Figure 8(a) shows a 3D layout and the subsurface nature while figure 8(b) shows the 3D cable layout used by GP Y.

S

Chapter 7

Reflection Field Layouts

7.1 plit-Dip and Common Midpoint Recording: Virtually all routine

seismic work consists of continuous coverage (profiling), that is, the cables and source points are arranged so that there are no gaps in the data other than those due to the fact that the geophone groups are spaced at intervals rather than continuously spaced. Single coverage implies that each reflecting point is sampled only once, in contrast to common-midpoint, or redundant, coverage where each reflecting point is sampled more than once. Areal or cross coverage indicates that the dip components perpendicular to the seismic line have been measured as well as the dip components along the line. Each of these methods can employ various relationships between sources and geophone groups. 7.2 Spread Types: By spread we mean the relative locations of the source point and the centers of the geophone groups used to record the energy form the source. In split-dip shooting the source point is at the center of a line of regularly spaced geophone group often results in a noisy trace (because of ground roll or truck noise with a surface source, or gases escaping from the shot hole and ejection of tamping material); hence the source may be moved 15 to 50 m. perpendicular to the seismic line. Often the geophone groups nearest the source are not used, which creates a sourcepoint (shotpoint) gap. Often the source is at the end of the spread of active geophone groups to produce an end-on spread, and in areas of exceptionally heavy ground roll the source point is offset by an appreciable distance along the line from the nearest active geophone group to produce an in-line offset spread. Alternatively, the sourcepoint may be offset in the direction normal to the cable, either at one end of the active part to produce a broadside-L or opposite the center to give a broadside-T spread. End-on and in-line offset spreads often employ sources off each end to give continuous coverage and two records for each spread. The in-line and broadside offsets permit recording reflection energy before the ground-roll energy arrives at the spread. Cross spreads, which consist of two lines of geophone groups roughly at right angles to each other, are used to record 3D dip information. 7.3 Arrays: The term array refers either to the pattern of geophones that feeds a single channel or to a distribution of shotholes or surface energy sources that are fired

simultaneously; it also includes the different locations of sources for which the results are combined by vertical stacking. A wave approaching the surface in the vertical direction will affect each geophone or an array simultaneously so that the outputs will affect the various

geophones at different times so that there will be a certain degree of destructive interference. Similarly, waves traveling vertically downward from a source array will add constructively whereas waves traveling horizontally away from the source array will arrive at a geophone

with different phases and will be partially cancelled. Thus, arrays provide a means of discriminating between waves arriving from different directions.

The two popular types of array designs are the linear array and the areal array. Arrays are linear when the elements are spread along the seismic line or areal when the group is distributed over an area. 7.4 Resolution

7.4.1 Vertical Resolution: Resolution refer to the minimum separation between two features such that we can tell that there are two separate features. If seismic wavelets were a

spike, the resolution would not have been a problem. Rayleigh criterion of resolution states that two events can be resolved if their separation is half cycle, since events are recorded in terms of two-way time, therefore real separation of the features must be quarter cycle. Thus resolvable limit is wavelength/4. 7.4.2 Horizontal Resolution: Horizontal resolution depends on the radius of the first Fresnel Zone. A Fresnel Zone is that portion of the reflector, which sends back energy to the receiver within a half cycle delay, so that it will produce constructive interference. The size of the zone depends on frequency, the higher the frequency the smaller the zone. Effective radius of the first Fresnel zone is half of the actual radius. If we consider point source, the effective radius of first Fresnel zone is

where, V = average velocity of the reflector,

t = two way time and f = frequency.

T

Chapter 8

Reflection Field Method for Land

Survey

8.1 he Seismic Field Party: The land seismic data acquisition team is divided into the following groups:

Survey Crew

Fixing the control points for the line based on the GPS points given before hand,

Ranging/Filling team for putting the pickets of the specified intervals along the line based on the control points on the line,

Leveling team for giving the elevations at the shot point location and the receiver point location.

Shot Hole Drilling Crew

For drilling the holes up to the specified depth for putting the charge for blasting.

Uphole Survey Crew

For measuring the velocity and thickness of the weathered layer (Low Velocity Layer) and velocity of the sub-weathered layer (in crude terms for depth optimization)

Recording Unit

Shooting Crew: For filling the drilled holes with the charge of specified quantity and detonating it.

Jug hustlers (the Cable laying crew): For laying the cable and planting the geophones at the specified pickets and for observing them all through the recording time for further corrections.

Recording Crew: For recording the seismic signals received by the geophones after blasting the charge.

8.2 Seismic Data Acquisition

8.2.1 The Program: Usually the seismic crew receives the program in the form of lines

on a map that indicate where data are to be acquired. Before beginning a survey the following

questions should be asked: Is it possible that the proposed lines will provide the required information? Data migration may require that lines be located elsewhere than directly on top of features in order to measure critical aspects of a structure. Crustal areas may be so extensively faulted that lines across them are nondefinitive. The structures being sought may be beyond seismic resolving power. Near surface variations may be so large that the data are difficult to interpret whereas moving the seismic line a short distance may improve data quality. Obstructions along a proposed line may increase difficulties unnecessarily, whereas moving the line slightly may achieve the same objectives at reduced cost. Where the dip is considerable, merely running a seismic line to a wellhead may not extend sufficiently beyond faults and other features to establish the existence of such placements. Lines may cross features such as faults so obliquely that their evidences are not readily interpretable. Lack of cross control may result in features located below the seismic line being confused by features to the side of the line. Objective of the Survey: The objective of the survey done by the GP X, GP Y and GP Z is to map strati-structural features within the specified formation at an area in Krishna-Godavari Basin of Andhra Pradesh which lie within the lower to upper cretaceous section. The seismic equivalent of these geological objectives are as under: For GP X

Area Depth (m) Two Way Travel Time in (ms) Dip Area 1 1500 to 4300 1250 to 3000 10 to 15

Area 2 1700 to 4200 1400 to 2900 10 to 15

For GP Y

Area Depth

(m) Two Way Travel Time in

(ms) Average Velocity

(m/sec) Dip

Area 1 1800 to 3600 1500 to 2500

Area 2 1800 to 3400 1500 to 2500

For GP Z

Area Depth

(m) Two Way Travel Time in

(ms) Average Velocity

(m/sec) Dip

Area 1 1800 to 3400 1500 to 2500 2400 to 2720 100 to 120

Reasons for the Survey: Out of the wells drilled in the area some have proved the presence of gaseous hydrocarbons from the formation and some have been dry. There by the area assumed important for exploration from these targets

Geology of the Area: The Krishna-Godavari basin has been subdivided into three sub-basins Krishna Sub-basin West Godavari Sub-basin and East Godavari Sub-baisn. The area under investigation for the GP X and GP Y lies in West Godavari Sub-basin while that of GP Z lies in the East Godavari Sub-basin. Figure8(c) shows the generalized stratigraphy of KG Basin.

8.2.2 Permitting: Once the seismic program has been decided o n, it is usually necessary to secure permission to enter the land to be traversed. Permission to enter may involve a payment, often a fixed sum per source location, as compensation in advance for damages that may be incurred. Even where the surface owners do not have the right to prevent entry, it is advantageous to explain the nature of the impending operations. Of course, a seismic crew is responsible for damages resulting from their actions whether or not permission is required to carry out the survey.

8.2.3 Layout of Line

The survey crew lays out the lines to be shot, usually by using an Electronic Total Station (refer Appendix), Compass Theodolite, and transit-and-chain survey that determines the positions and elevations of both the source points and the centers of geophone groups. Usually the survey crew is given a few GPS stations beforehand in the area of operation. The survey crew divides themselves into three main groups. The first group fixes the control points (using the Electronic Total Station) which control the direction of the source line or the receiver line. Usually these control points are given at an interval of about 1km. along the line, on either side of the line. The second group does the ranging and the filling (using the compass Theodolite) part on the line along the line at specified interval and placing the pickets (made of flat bamboo sticks with the marking of the picket number on

them) at those stations. The third group does the leveling i.e., gives the elevation values at each picket.

Thus the survey crew lays the grid of source lines and receiver lines with specified picket intervals, receiver line intervals and source line intervals on ground. What they do is to project the details on the given onto ground very precisely.

8.2.4 Shothole Drilling: The next team of people to star their activities unit in the

scene is the drilling crew (when explosives are used as the energy source). Depending on the number and depth of holes required and the case of drilling, a seismic crew deploys the drilling crews. Whenever conditions permit, the drills are truck-mounted. Water trucks are often required to supply the drills with water for drilling. In areas of rough terrain, the drills may be mounted on tractors or portable drilling equipment may be used. Usually the drilling crew places the explosive in the holes before leaving the site.

Seismic survey is divided into two main classes which are interlinked. These are: Experimental Surveys and Regular/Production Survey

8a.3 2D Survey Parameters (Before the Experimental Survey) (GP X)

Instrument 408 UL

Source Type Dynamite Group Interval 20m Field Season 2004-05 Type of Shooting Asymmetrical spread (216 + 40) Channel/Foldage 256/64 Spread Length 4300 + NTO Shot Interval 40m

No. of Geophones per group 12 Geophone Pattern Linear Shot Hole Pattern Single

Record Length 6S Sample Rate 2ms.

Gain Mode 24bit K Gain dB 0, 12 Low Cut Filter (Hz/dB) Out High Cut Filter (Hz/dB) 200/370 Notch (50 Hz) NA

8.4 3D Survey Parameters (Before the Experimental Survey) (GP Y

and GP Z)

Parameters GP Y GP Z Instrument 408UL SN388

Source Type Dynamite

Group Interval 40 m. 40 m.

Field Season 2004-05 2004-05

Type of Shooting Asymmetric Split Spread Asymmetric Split Spread

Channel/Foldage 1008(168 per line)/6 X 6 1008( 168 per line )/6X6 Spread Length (m) 6680 m (each line) 6680 m (each line) Shot Interval (m) 40 m 40 m No. of Geophones per group 12

Geophone Pattern Areal Areal

Shot Hole Pattern Orthogonal Single Orthogonal Single

Record Length (sec.) 6 5 Sample Rate (m sec.) 2 2 Gain Mode 0

K Gain (dB) 12 Low Cut Filter (Hz/dB) Out Out High Cut Filter (Hz) 200 125 Notch (50 Hz) Out Out Receiver Line Interval (m) 280 280 Source Line Interval (m) Bin size (m x m) 20 x 20 20 x 20

8.5 Experimental Survey

8.5.1 Uphole Survey (Depth Optimization)

Sheriff defines an uphole survey as follows

Successive sources at varying depths in a borehole in order to determine the velocities of the near-surface formations, the weathering