GRAVITY AND MAGNETICS JOURNAL by Deji AllenWalker

ABSTRACT

The application of gravity and magnetic methods to geophysical exploration raises some important issues specific to analyses and interpretation. The specific issues presented in this study include the status of magnetic and gravity methods as tools for hydrocarbon exploration, the issue of non-uniqueness in the interpretation of potential field data and remanent magnetization in the Earth’s magnetic field. Other issues are the estimation of the main magnetic field from the outer core of the Earth and the computation of normal gravity field from the geodetic reference systems. Analyses of set of magnetic data shows that remanent magnetization can easily be interpreted from magnetic data without resorting to laboratory analyses of rock samples which is expensive and takes a lot of time. Evaluation of the accuracy of the geodetic reference systems in computing normal gravity field with Nigerian example shows that the choice of the 1967 or 1984 reference Earth models is of minor importance in geophysical applications and the large difference in values of the normal gravity values between the 1930 and 1967 reference Earth models is due to errors inherent in Potsdam absolute gravity value. Our philosophy in applying the potential field methods to hydrocarbon exploration is that interpretation should commence from the basement and proceed to the sedimentary section. Our approach to the issue of non-uniqueness is that interpretation of potential field data is not non-unique if interpretation is supported by plausible geological model. Using a case example from Nigeria we opine that it is more appropriate to exploit global magnetic model for the computation of regional magnetic field from the outer core. This approach eliminates the problems which may arise when adjacent surveys flown with different parameters and specifications are combined.

Gravity and Magnetic Surveys Page 1

CHAPTER ONE

1.1 INTRODUCTION

Gravity and magnetic methods of geophysical exploration are referred to as potential field methods. The operative physical properties in gravity and magnetic exploration are density and susceptibility/remanence respectively. The two methods represent the oldest and in some case the most suitable methods of geophysical exploration. This is probably due to the high cost-benefit ratio which characterizes the application of these methods and by the relatively simple logistic management of gravity and magnetic surveys (Rapolla etal., 2002). Measurements of the magnetic and gravity field of

the Earth are used extensively to explore its structure. Magnetic data are frequently collected by aircraft that can cover substantial amounts of ground in a short time, resulting in large amount of data that require interpretation (Durrheim and Cooper, 1998). The Earth’s gravity field is monopolar. This means that it always points down towards Earth’s centre while the magnetic field is dipolar; which means that direction of the field changes with geographic position. At the geomagnetic poles, the field is vertical or perpendicular to the Earth’s surface. At the geomagnetic equator it is horizontal or parallel to the Earth’s surface. Geomagnetic field strength is generally stronger near its pole and weakest over the magnetic equator. Owing to the dipolar nature of the Earth’s magnetic field and the latitude/longitude dependent nature of the induced magnetic response for a given body and due to the variability of the Earth’s magnetic field over the whole Earth surface, interpretation of magnetic data is theoretically more difficult than that of gravity data. Due to the fact that ambient field in the magnetic equator is horizontal and weak coupled with the fact that structures striking N-S are difficult to identify, the interpretation of magnetic data in equatorial belt is more complex. Magnetic anomalies are as a result of lateral contrast in rock composition and lateral contrast in rock structure (Millegan and Bird, 2008). In the case where there is no lateral contrast, the magnetization is flat. Gravity maps show horizontal variations in crustal and possibly mantle densities. Unlike gravity anomalies, magnetic anomalies are produced by combination of induced and remanent magnetization. Remanent magnetization is acquired when rocks cooled below the temperature of the curie point of the constituent magnetic minerals. In most cases, remanent magnetization could be much larger than the induced magnetization and may have different and opposite direction to the present magnetic field. This may depend on the direction of the Earth’s field at the time the magnetization was acquired and the tilting of the host rock subsequent to deformation (Klasner etal., 1985) . Potential field data generally reflects near surface susceptibility and density variations. The variation may be separated into two wavelength components. Those that have near surface sources (high frequency sources). The low frequency components have deeper sources. In general, low frequency magnetic and gravity data reflects magnetization and density variations respectively that lie within the deep crust or broad scale variation of near-surface basement rocks. The high frequency potential field data generally reflects near surface susceptibility and density variations.


Most objectives of gravity and magnetic interpretation are to gain information about the subsurface structures. Gravity and magnetic data have potential for resolving structural relations and tectonic history of an area, particularly if the tectonic features in an area are poorly known in terms of their origin, subsurface geometry and structural relations with deeper seated and / or adjacent features

(Saheel etal., 2010, Okiwelu etal., 2010, Baag, 1998.) A potential field data can provide considerable insight into the complex structural relations of an area. A potential field data that is well compiled, processed and interpreted over an area should unravel tectonic (lithologic) provinces and determine depth to basement and contour the relief of the basement. The availability of a high resolution aeromagnetic data can provide a precise potential trapping structures. Additional information in respect

of basement study using potential field data may resolve the impact of basement on source rock and reservoir quality and thermal history. Gravity and magnetic field exploration if well planned and executed can give detail information on regional geology and tectonics in conjunction with contoured depth maps of an area. The tectonic interpretation can be used to define details within known productive areas as well as to point out other locations for exploration. Sufficient knowledge of geology/ geologic model of an area are indispensable to successful interpretation. Adequate knowledge of geology on regional and prospect levels using gravity and magnetic data continue to encourage the melding of these fields to produce enhanced interpretations. Regional gravity maps, for example are often used to interpret regional trends and structures of the subsurface geology (Alexander, 2002; Okiwelu, 2002, 2010) .


CHAPTER TWO LITERATURE REVIEW

2.0 GRAVITY METHODS

OVERVIEW

The gravity method is a nondestructive geophysical technique that measures differences in the earth’s gravitational field at specific locations. It has found numerous applications in engineering and environmental studies including locating voids and karst features, buried stream valleys, water table levels and the determination of soil layer thickness. The success of the gravity method depends on the different earth materials having different bulk densities (mass) that produce variations in the measured gravitational field. These variations can then be interpreted by a variety of analytical and computers methods to determine the depth, geometry and density the causes the gravity field variations.

Gravity data in engineering and environmental applications should be collected in a grid or along a profile with stations spacing 5 meters or less. In addition, gravity station elevations must be determined to within 0.2 meters. Using the highly precise locations and elevations plus all other quantifiable disturbing effects, the data are processed to remove all these predictable effects. The most commonly used processed data are known as Bouguer gravity anomalies, measured in mGal.

The interpretation of Bouguer gravity anomalies ranges from just manually inspecting the grid or profiles for variations in the gravitational field to more complex methods that involves separating the gravity anomaly due to an object of interest from some sort of regional gravity field. To perform the later, there are several manual and computer techniques including graphical smoothing and polynomial surface fitting. The interpretation of separated (residual) gravity anomalies commonly involves creating a model of the subsurface density variations to infer a geological cross-section. These models can be determined using a variety of methods ranging from analytical solutions due to simple geometries (e.g., sphere) to complex three-dimensional computer models.The gravity method involves measuring the earth’s gravitational field at specific locations on the earth’s surface to determine the location of subsurface density variations. The gravity method works when buried objects have different masses, which are caused by the object having a greater or lesser density than the surrounding material. However, the earth’s gravitational field measured at the earth’s surface is affected also by topographic changes, the earth’s shape and rotation, and earth tides. These factors must be removed before interpreting gravity data for subsurface features. The final form of the processed gravity data can be used in many types of engineering and environmental problems, including determining the thickness of the surface or near-surface soil layer, changes in water table levels, and the detection of buried tunnels, caves, sinkholes and near-surface faults. Relatively new applications include four-dimensional (4-D) gravity, where temporal variations of the gravitational field can used to determine variations in the water table (Mokkapati, 1995;


Hare et al., 1999) and changing of subsidence levels in sinkholes (Rybakov et al., 2001). Table 1 lists the main uses of the gravity method in engineering and environmental studies.

The gravity method can be a relatively easy geophysical technique to perform and interpret. It requires only simple but precise data processing, and for detailed studies the determination of a station’s elevation is the most difficult and time-consuming aspect. The technique has good depth penetration when compared to ground penetrating radar, high frequency electromagnetics and DC-resistivity techniques and is not affected by the high conductivity values of near-surface clay rich soils. Additionally, lateral boundaries of subsurface features can be easily obtained especially through the measurement of the derivatives of the gravitational field. The main drawback is the ambiguity of the interpretation of the anomalies. This means that a given gravity anomaly can be caused by numerous source bodies. An accurate determination of the source usually requires outside geophysical or geological information.

The use of the gravity data is relatively straightforward as can be seen in the following summary of the fundamentals of the gravity method as applied to engineering and environmental studies including overviews of the theory, data collection, processing, and interpretation. For more detailed information on the gravity technique, numerous papers covering all aspects of the gravity method are available in the following journals: Geophysics, Geophysical Prospecting, Exploration Geophysics, Journal of Environmental and Engineering Geophysics, and the Journal of Applied Geophysics (see the reference list for a partial list of papers related for environmental- and engineering-type gravity investigations). For more detailed investigation on the theoretical background of the gravity method, the reader is referred to books by Grant and West (1965) and Blakely (1995). For overviews of the gravity method, the reader is referred to the books by Telford et al. (1990) and Robinson and Caruh (1988). Books by Burger (1992), Sharma (1997) and Reynolds (1998) contain a chapter on the gravity method with an emphasis on shallow applications, while overview papers by Hinze (1990) and Debeglia and Dupont (2002) specifically focuses on shallow gravity applications.

Gravity Reductions

In gravity work, more than in any other branch of geophysics, large and (in principle) calculable effects are produced by sources which are not of direct geological interest. These effects are removed by reductions involving sequential calculation of a number of recognized quantities. In each case the sign of the reduction is opposite to that of the effect it is designed to remove. A positive effect is one that increases the magnitude of the measured field.

Instrumental Drift

The various factors which may contribute to the instrumental drift of your gravimeter (long term drift, battery supply voltage changes, and vibration, etc.) have been dealt with in the section Instrumental Factors starting on page 22. If your gravimeter is software controlled, it


may be assumed, for a start, that you have determined the mean long-term drift of your instrument by repeat measurements made over a period of at least 48 hours.

Latitude correction

Latitude corrections are usually made by subtracting the normal gravity, calculated from the International Gravity Formula, from the observed or absolute gravity. For surveys not tied to the absolute reference system, local latitude corrections may be made by selecting an arbitrary base and using the theoretical north -south gradient of 8.12 sin 2λ g.u./km.

g = 978.0327(1 + 0.0053024 sin 2θ–0.0000058 sin

22θ) in Gals

Free-air correction

The remainder left after subtracting the normal from the observed gravity will be due in part to the height of the gravity station above the sea-level reference surface. An increase in height implies an increase in distance from the Earth’s centre of mass and the effect is negative for stations above sea level. The free-air correction is thus positive, and for all practical purposes is equal to 3.086 g.u./metre. The quantity obtained after applying both the latitude and free-air corrections is termed the free-air anomaly or free-air gravity.

Bouguer correction

Since topographic masses are irregularly distributed, their effects are difficult to calculate precisely and approximation is necessary. The simplest approach assumes that topography can be represented by a flat plate extending to infinity in all directions, with constant density and a thickness equal to the height of the gravity station above the reference surface. This Bouguer plate produces a gravity field equal to 2πρGh, where h is the plate thickness and ρ the density (1.1119 g.u./metre for the standard 2.67 Mg m−3 density). The Bouguer effect is positive and the correction is therefore negative. Since it is only about one-third of the size of the free-air correction, the net effect of an increase in height is a reduction in field. The combined correction is positive and equal to about 2 g.u. per metre, so elevations must be known to 5 cm to make full use of meter sensitivity. Because


Gravity Geometry

Sphere

applicable to approx. equidimensional bodies (longest dimension << depth) gravity due to sphere

vertical component:

Since, , we get


"Inversion" technique

1. find distance (x1/2) from peak of anomaly where anomaly is half maximum anomaly:

2. depth of body:


3. since

Notes

non-uniqueness:

(can't determine radius AND density contrast) anomaly size is relative to "baseline", or "background" in general, half-width to left and right are unequal

2.1 MAGNETIC METHOD

Compasses and dip needles were used in the Middle Ages to find magnetite ores in Sweden, making the magnetic method the oldest of all applied geo-physical techniques. It is still one of the most widely used, even though significant magnetic effects are produced by only a very small number of minerals. Magnetic field strengths are now usually measured in nanoTesla (nT). The pre-SI unit, the gamma, originally defined as 10−5 gauss but numerically equal to the nT, is still often used. Magnetic Properties Although governed by the same fundamental equations, magnetic and gravity surveys are very different. The magnetic properties of adjacent rock masses may differ by several orders of magnitude rather than a few percent. Poles, dipoles and magnetization An isolated magnetic pole would, if it existed, produce a field obeying the inverse-square law. In reality, the fundamental magnetic source is the dipole (Section 1.1.5) but, since a line of dipoles end-to-end produces the same effect as positive and negative poles isolated at opposite ends of the line (Figure 3.1), the pole concept is often useful. A dipole placed in a magnetic field tends to rotate, and so is said to have a magnetic moment. The moment of the simple magnet of Figure 3.1, which is effectively a positive pole, strength m, at a distance 2L from a negative pole −m, is equal to 2Lm.

Susceptibility

A body placed in a magnetic field acquires a magnetization which, if small, is proportional to the field:

M = kH

The susceptibility, k, is very small for most natural materials, and may be either negative (diamagnetism) or positive (paramagnetism). The fields produced by dia- and paramagnetic materials are usually considered to be too small to affect survey magnetometers, but modern high-sensitivity magnetometers are creating exceptions to this rule. Most observed magnetic


anomalies are due to the small number of ferro- or ferri-magnetic substancesin which the molecular magnets are held parallel by intermolecular exchange forces. Below the Curie temperature, these forces are strong enough to overcome the effects of thermal agitation. Magnetite, pyrrhotite and maghemite, all of which have Curie temperatures of about 600 ◦C, are the only important naturally occurring magnetic minerals and, of the three, magnetite is by far the most common. Hematite, the most abundant iron mineral, has a very small susceptibility and many iron ore deposits do not produce significant magnetic anomalies. The magnetic properties of highly magnetic rocks tend to be extremely variable and their magnetization is not strictly proportional to the applied field. Quoted susceptibilities are for Earth-average field strengths.

RemanenceFerro- and ferri-magnetic materials may have permanent as well as induced magnetic moments, so that their magnetization is not necessarily in the direction of the Earth’s field. The Konigsberger ratio of the permanent moment to the moment that would be induced in an Earth-standard field of 50 000 nT, is generally large in highly magnetic rocks and small in weakly magnetic ones, but is occasionally extraordinarily high (>10 000) in hematite. Magnetic anomalies due entirely to remanence are sometimes produced by hematitic ores.

The Magnetic Field of the EarthThe magnetic fields of geological bodies are superimposed on the background of the Earth’s main field. Variations in magnitude and direction of this field influence both the magnitudes and shapes of local anomalies.In geophysics, the terms north and south used to describe polarity are replaced by positive and negative. The direction of a magnetic field is conventionally defined as the direction in which a unit positive pole would move but, since all things are relative, geophysicists give little thought to whether it is the north or south magnetic pole that is positive.The Earth’s main magnetic field originates in electric currents circulating in the liquid outer core, but can be largely modelled by a dipole source at the Earth’s centre. Distortions in the dipole field extending over regions thousands of kilometres across can be thought of as caused by a relatively small number of subsidiary dipoles at the core–mantle boundary. The variations with latitude of the magnitude and direction of an ideal dipole field aligned along the Earth’s spin axis are shown in the figure below.


Variation in intensity, dip and gradient for an ideal dipole aligned along the Earth’s spin axis and producing a polar field of 60 000 nT.We should note that near the equator the dip angles change almost twice as fast as the latitude angles. To explain the Earth’s actual field, the main dipole would have to be inclined at about 11◦ to the spin axis, and thus neither the magnetic equator, which links points of zero magnetic dip on the Earth’s surface, nor the magnetic poles coincide with their geographic equivalents.The North Magnetic Pole is in northern Canada and the South Magnetic Pole is not even on the Antarctic continent, but in the Southern Ocean at about 65◦S, 138◦E. Differences between the directions of true and magnetic North are known as declinations, presumably because a compass needle ought to point north but declines to do so. In ground surveys where anomalies of many tens of nT are being mapped, regional corrections, which generally amount to only a few nT per km, are often neglected.

The International Geomagnetic Reference Field (IGRF)The variations of the Earth’s main field with latitude, longitude and time are described by experimentally determined International Geomagnetic Reference Field (IGRF) equations, defined by 120 spherical harmonic coefficients, to order N = 10, supplemented by a predictive secular variation model to order N = 8. The shortest wavelength present is about 4000 km. IGRFs provide reasonable representations of the actual regional fields in well-surveyed areas, where they can be used to calculate regional corrections, but discrepancies of as much as 250 nT can occur in areas from which little information was available at the time of formulation.Because the long-term secular changes are not predictable except by extrapolation from past observations, the IGRF is updated every five years on the basis of observations at fixed observatories and is also revised retrospectively to give a definitive model (DGRF). GRF


corrections are vital when airborne or marine surveys carried out months or years apart are being compared or combined but are less important in ground surveys, where base stations can be reoccupied.

Diurnal variationsThe Earth’s magnetic field also varies because of changes in the strength and direction of currents circulating in the ionosphere. In the normal solarquiet (Sq) pattern, the background field is almost constant during the night but decreases between dawn and about 11 a.m., increases again until about 4 p.m. and then slowly declines to the overnight value (Figure 3.4). Peakto- trough amplitudes in mid-latitudes are of the order of a few tens of nanoTesla. Since upper atmosphere ionization is caused by solar radiation, diurnal curves tend to be directly related to local solar time but amplitude differences of more than 20% due to differences in crustal conductivity may be more important than time dependency for points up to a few hundred kilometers apart. Short period, horizontally polarized and roughly sinusoidal micropulsations are significant only in surveys that are to be contoured at less than 5 nT. Within about 5◦ of the magnetic equator the diurnal variation is strongly influenced by the equatorial electrojet, a band of high conductivity in the ionosphere about 600 km (5◦ of latitude) wide. The amplitudes of the diurnal curves in the affected regions may be well in excess of 100 nT and may differ by 10 to 20 nT at points only a few tens of kilometres apart. Many of the magnetic phenomena observed in Polar Regions can be explained by an auroral electrojet subject to severe short-period fluctuations.In both equatorial and polar regions it is particularly important that background variations be monitored continuously. Returning to a base station at intervals of one or two hours may be quite insufficient.

Magnetic stormsShort-term auroral effects are special cases of the irregular disturbances (Ds and Dst) known as magnetic storms. These are produced by sunspot and solar flare activity and, despite the name, are not meteorological, often occurring on clear, cloudless days. There is usually a sudden onset, during which the field may change by hundreds of nT, followed by a slower, erratic return to normality. Time scales vary widely but the effects can persist for hours and sometimes days. Micropulsations are generally at their strongest in the days immediately following a storm, when components with periods of a few tens of seconds can have amplitudes of as much as 5 nT. Ionospheric prediction services in many countries give advance warning of the general probability of storms but not of their detailed patterns, and the field changes in both time and space are too rapid for corrections to be applied. Survey work must stop until a storm is over. Aeromagnetic data are severely affected by quite small irregularities and for contract purposes technical magnetic storms may be defined, sometimes as departures from linearity in the diurnal curve of as little as 2 nT in an hour. Similar criteria may have to be applied in archaeological surveys when only a single sensor is being used (rather than a two-sensor gradiometer).

Geological effects


The Curie points for all geologically important magnetic materials are in the range 500–600 ◦C. Such temperatures are reached in the lower part of normal continental crust but below the Moho under the oceans. The upper mantle is only weakly magnetic, so that the effective base of local magnetic sources is the Curie isotherm beneath continents and the Moho beneath the oceans. Massive magnetite deposits can produce magnetic fields of as much as 200 000 nT, which is several times the magnitude of the Earth’s normal field. Because of the dipolar nature of magnetic sources these, and all other, magnetic anomalies have positive and negative parts and in extreme cases directional magnetometers may even record negative fields. Anomalies of this size are unusual, but basalt dykes and flows and some larger basic intrusions can produce fields of thousands and occasionally tens of thousands of nT. Anomalous fields of more than 1000 nT are otherwise rare, even in areas of outcropping crystalline basement. Sedimentary rocks generally produce changes of less than 10 nT, as do the changes in soil magnetization important in archaeology.In some tropical areas, magnetic fields of tens of nT are produced by maghemite formed as nodular growths in laterites. The nodules may later weather out to form ironstone gravels which give rise to high noise levels in ground surveys. The factors that control the formation of maghemite rather than the commoner, non-magnetic form of hematite are not yet fully understood.

Magnetic Instruments

arly torsion magnetometers used compass needles mounted on horizontal axes (dip needles) to measure vertical fields. These were in use until about 1960, when they began to be replaced by fluxgate, proton precession and alkali vapour magnetometers. Instruments of all these three types are now marketed with built-in data loggers and can often be set to record automatically at fixed time intervals at base stations. All three can be used singly or in tandem as gradiometers, although care must then be taken with precession instruments to ensure that the polarizing field from one sensor does not affect the measurement at the other. Gradient measurements emphasize near surface sources and are particularly useful in archaeological and environmental work.

Proton precession magnetometerThe proton precession magnetometer makes use of the small magnetic moment of the hydrogen nucleus (proton). The sensing element consists of a bottle containing a low freezing-point hydrocarbon fluid about which is wound a coil of copper wire. Although many fluids can be used, the manufacturer’s recommendation, usually for high-purity decane, should always be followed if the bottle has to be topped up. A polarizing current of the order of an amp or more is passed through the coil, creating a strong magnetic field, along which the moments of the protons in the hydrogen atoms will tend to become aligned. When the current is switched off, the protons realign to the direction of the Earth’s field. Quantum theory describes this reorientation as occuring as an abrupt ‘flip’, with the emission of a quantum of electromagnetic energy.


CHAPTER THREEMETHODOLOGY

3.0 Gravity method (Field Procedure):

The group’s major consideration when planning the gravity survey was the location and spacing of the stations. The size, type, depth and density contrast of the feature being defined will determine if the survey is laid out as a grid, a traverse or a series of traverses. These factors will also determine the extent of the grid or traverse and the station spacing. For example, sampling theory dictates that station spacing should be no greater than half the expected width of any bodies being defined. In this case, three traverses were marked out with 10m spacing between them the absolute maximum station spacing which could expect to define the position of the feature was 100m on each traverse; obviously, if more detail is required, a closer station spacing would be used. Another consideration is the horizontal and vertical positional accuracy of the gravity stations. The vertical accuracy of the stations is most important because an error of 0.3µm/s2 will be caused by each decimeter of error in the height estimate. This scale of error may be greater than the size of the anomaly of interest. To obtain the coordinates and elevation, a GPS (Global Positioning System) was used. General field procedures for gravity surveys involve taking readings in loops where the first station is repeated after a few hours to quantify the drift of the gravimeter and allow for correction of this in post processing; However, in our group’s case, readings were taken at the base station every 50 minutes to correct for instrumental drift and tidal effects. These techniques are described in most geophysics texts such as Telford, et al. (1990).

Instrument Type: Scintrex AUTOGRAV CG5 Gravimeter

GPS

Base Plate

Time Recorder (hh:mm:ss)

Station Spacing: 10m

DATA PROCESSING

The acceleration due to gravity depends on latitude, elevation, topography of the surrounding terrain, tides, and density variations in the subsurface (Telford, et al., 1990). Corrections are made during processing to remove the effects of latitude, changes in elevation and topographic effects. The effect of tides can be mathematically modelled and removed during processing, or they can be assumed to occur as a linear drift over a few hours and removed along with the instrumental drift as mentioned previously in the field procedures. The anomalies in the earth’s measured gravity acceleration remaining after these corrections have been applied are due to density variations in the earth’s crust and can provide information about the


regolith. Further descriptions of the standard corrections and how they are applied can be obtained from texts such as Blakely (1996), Telford, et al. (1990).

The fig. above shows the base map of the survey area.

Below was the raw data which was obtained on the field:

TRAVERSE ONE


TRAVERSE TWO

TRAVERSE THREE

The Charts below shows a plot of the ‘g’ observed with the station spacing (unprocessed data)

1. Traverse one:


2. Traverse two:

3. Traverse Three:


3.1 Magnetic method (Field Procedure):

The magnetic survey mapping of the study area was implemented employing Proton precession magnetometers. A magnetic survey mapping of three trasverses was carried out with the proton precession magnetometer. At the start of each transverse, a base station was established at which the base station readings were obtained for diurnal variations. The time, altitude (elevation) and longitude were measured with a GPS (Global positioning System), and a Global positioning system situated at a base station. Two total field readings were subsequently obtained at each successive station along each transverse. The relative spacing was maintained at 10m, 20m, 30m and 40m respectively. The corresponding time, latitude and longitude were also recorded. Upon completing the acquisition of total field readings along each traverse, a reoccupation of the base station was carried out for every 20 minutes. The acquisition of total field readings from the base station at the commencement of the magnetic survey procedure on each transverse and the subsequent reoccupation of the base station at the end of the magnetic survey procedure on each transverse is imperative in order to correct for daily variation in the geomagnetic field due to magnetic effects of the external origin – diurnal variation. This procedure is also implemented in order to ascertain if these variations are erratic and to correct for changes inherent in the measuring instrument influenced by temperature and drift. Materials capable of releasing disruptive effect on magnetic response measured by the proton precession magnetometer were avoided. These materials included chains, belts, wristwatches, keys or any metallic objects.Below was the raw data which was obtained on the field:


TRAVERSE ONE (10m Spacing)

TRAVERSE TWO (10m Spacing):


TRAVERSE THREE (10m Spacing):



CHAPTER FOUR DISCUSSION AND ANALYSIS OF RESULT

4.0 Gravity method (data reduction):

The acceleration due to gravity depends on latitude, elevation, topography of the surrounding terrain, tides, and density variations in the subsurface (Telford, et al., 1990). Corrections are made during processing to remove the effects of latitude, changes in elevation and topographic effects. The effect of tides can be mathematically modeled and removed during processing, or they can be assumed to occur as a linear drift over a few hours and removed along with the instrumental drift as mentioned previously in the field procedures. The anomalies in the earth’s measured gravity acceleration remaining after these corrections have been applied are due to density variations in the earth’s crust and can provide information about the regolith. Further descriptions of the standard corrections and how they are applied can be obtained from texts such as Blakely (1996), Telford, et al. (1990).

Formulae used in computation:

The tables below show the various reduction that has been carried out on the data. The various reductions has been discussed in chapter 2 of this report.


TraverseONE:

Traverse TWO:

Traverse THREE:


Bouguer Anomaly (BA) curves are shown in the figures below:

Traverse ONE:


Traverse TWO:

Traverse THREE:


Residual Anomaly Table of values and curves for the survey areas are shown below:

Traverser ONE:


Traverser TWO:


gmax

Traverser THREE:


gmax

GEOMETRY

The quantitative method used in deriving the depth and lateral extent was Spherical model.

Sphere


g2max

g1max

FOR TRAVERSE ONE:

From the above calculations, is 0.0312mGal the lateral extent of the anomaly is 2R = 2X3.2977= 6.6m, the overburden thickness is 5.86m and the depth to center of anomaly Z is 9.156m for traverse ONE.

FOR TRAVERSE TWO:


From the above calculations, is 0.03mGal the lateral extent of the anomaly is 2R = 2X4.78= 9.56m, the overburden thickness is 11.5m and the depth to center of anomaly Z is 16.3m for traverse TWO.

FOR TRAVERSE THREE:

From the above calculations, is 0.09mGal the lateral extent of the anomaly is 2R = 2X4.46=8.92m, the overburden thickness is 4.02m and the depth to center of anomaly Z is 8.483m for traverse THREE.


From the above calculations, is 0.044mGal the lateral extent of the anomaly is 2R = 2X4.45=8.9m, the overburden thickness is 7.65m and the depth to center of anomaly Z is 12.1m for traverse THREE.

Iso-Anomaly Map for Gravity:


4.1 Magnetic method (data reduction):

Processed data are shown below:

Traverse ONE 10m:

Traverse TWO (10m):


Traverse THREE (10m):


The remaining processed data for the 20m, 30m and 40m spacing will be found in the appendix section of this report. The geometry can simply be calculated from the 10m spacing as it is closer and will give an adequate representation of the causative body.

Remnant Magnetism Table of values and curves for the survey areas are shown below:

Traverser ONE (10m spacing):



Traverse TWO (10m spacing):


Traverse THREE (10m spacing):

V= 8πIR3/3z3

Where V = Vmax/2, I = Intensity of the magnetization, R = Radius,z= Depth to the centre.

On the symmetric profile the coordinate X1/2 of a point at which the anomaly is one half of one of the two extreme values is located. From which η is calculated (Parasnis, 1986)

η= Vmax /2 = z = depth to the centre


For the 10m symmetric profile, z1= 12m, z2 =14m, z3= 16m for Traverse 1, 2, and 3.

I = TOTAL FIELD SGTRENGTH/PERMEABILITY IN FREE SPACE Where I is the INTENSITY OF MAGNETISATION

I = 26m for Traverse 1,2, and 3

R1= 50.5m, R2=59m,R3 = 67.2m

Iso-anomaly Contour Map for the Magnetic data (10M)


CHAPTER FIVE

5.0 CONCLUSION

For Gravity:

Traverse ONE:

From the data analysis section in chapter 4 of this project, is 0.0312mGal the lateral extent of the anomaly is 2R = 2X3.2977= 6.6m, the overburden thickness is 5.86m and the depth to center of anomaly Z is 9.156m for traverse ONE.

Traverse TWO:

From previous chapter, the g maximum is 0.03mGal the lateral extent of the anomaly is 2R = 2 X 4.78= 9.56m, the overburden thickness is 11.5m and the depth to center of anomaly Z is 16.3m for traverse TWO.

Traverse THREE:

Anomaly 1: From the chapter, is 0.09mGal the lateral extent of the anomaly is 2R = 2X4.46=8.92m, the overburden thickness is 4.02m and the depth to center of anomaly Z is 8.483m for traverse THREE.

Anomaly 2: From the above chapter, is 0.044mGal the lateral extent of the anomaly is 2R = 2X4.45=8.9m, the overburden thickness is 7.65m and the depth to center of anomaly Z is 12.1m for traverse THREE.

For Magnetics,

For the 10m symmetric profile, z1= 12m, z2 =14m, z3= 16m for Traverse 1, 2, and 3.

I = 26m for Traverse 1,2, and 3

R1= 50.5m, R2=59m,R3 = 67.2m


APPENDICES


Table of values of filtered data for magnetics:



20M Traverse One Chart (Filtered):

20M Traverse TWO Chart (Filtered):


20M Traverse THREE Chart (Filtered):


30M Traverse ONE Chart (Filtered):




40M Traverse ONE Chart (Filtered):






REFERENCES

[1] Alexander, M., 1999. “Northern Gulf of Mexico basement architecture: Crustal study to prospect leads”. Paper presented at the 1999 Society of Exploration Geophysics sixty-Ninth Annual meeting workshop: The seismic link: Reducing Risk, Houiston, TX.

[2] Alexander, M., 2002. “Importance of the correct map - Choose the proper regional gravity map: Footnotes on Interpretation”. Integrated Geophysics Corporation (IGC footnote series), 4p.

[3] Alexander, M., 2006. “Two dimensional structural modeling with potential field data-Footnotes on Interpretation”. Integrated Geophysics Corporation (IGC footnote series), 4p.

[4] Alexander, M. and Aimadeddine, K., 2008. “Gabon Regional structural framework- Isostatic residual gravity with interpretation: Footnotes on interpretation”. Integrated Geophysics Corporation (IGC footnote series). 4p.

[5] Alexander, M., C. Preito and B. Radovich, 2003. “Basement structural analyses: Key in deep shelf play”. The American oil and Gas Reporter, 4p.

[6] Arafin, S. (2004). “Relative Bouguer anomaly. Leading edge, 23(9), 850-851.”

[7] Baag, C. “Aeromagnetic interpretation of Southwestern continental shelf of Korea: In Geologic


GRAVITY AND MAGNETICS JOURNAL by Deji AllenWalker

Documents

geodetic reference systems

potential field data

water table levels

earths gravitational field

potential field methods

earths magnetic field

subsurface density variations

gravitational field