1

Interpretations of Aeromagnetic Data from Ilesha

Southwest Nigeria.

M.Sc. Thesis

By

Umera, Robert Bassey

PG/ M.Sc./09/52098

Presented to the Department of Physics & Astronomy, Faculty

of Physical Sciences, University of Nigeria in partial fulfillment

for the award of M.Sc in Solid Earth Geophysics

Supervisors: Dr. J.U. Chukwudebelu and Dr. P.O. Ezema.

September, 2011

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DEDICATION

To the glory of God and my parents, Mr. &Mrs. Robert Bassey Umera and

Princess Onne Ijim Agbor

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CERTIFICATION

Mr. Robert Bassey Umera a postgraduate student of the Department of

Physics and Astronomy with registration number PG/M.Sc./09/52098 has

satisfactorily fulfilled the requirements for the course and research work for the

award of Master of Science in Solid Earth Geophysics. The work embodied in this

thesis is original and has not been submitted in part or full for any other diploma or

degree of this or any other university.

Head of Department Supervisor (1)

Prof. J.O. Urama Dr. J.U. Chukwudebelu

External Examiner Supervisor (2)

Dr. P.O. Ezema

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ACKNOWLEDGEMENT

Any work of this kind cannot be done without the moral and financial

support of people. This is why I have to thank and be grateful to the following

people.

First to God my father, who in His infinite love, and mercies has seen me

through this tedious task in the lion’s den.

I am especially grateful to my supervisors Dr. J.U. Chukwudebelu, Dr. P. O.

Ezema, my Head of Department Prof. J.O. Urama, and my lecturers Prof Animalu,

Prof Ubachukwu, Prof C.M.I. Okoye and Dr. Asomba.

I am very grateful to my father, Mr. Robert Bassey Umera, my mother

Princess Onne Ijim Agbo, my step mother Mrs. Ama Robert Umera, my brothers

Colins (Coba) and Ayim (Doctor), and my one and only little sister Trillionet.

They all in numerous ways stood by me in the course of schooling in the lion’s

den. Am so grateful! May God in His loving kindness bless and keep you all safe.

Amen.

Special thank you goes to my dear friend, Nwaogu Peace Onyinyechi. You

are indeed so special. I cannot forget my brother and my friend Abuh Sammy

Agim (Zoo Zoo) for his love and care. You are a friend indeed. Am also grateful to

my aunts Mrs. Patricia Eyamba, Mrs. Myrtle Ibokette and Mrs. Virtue Ephraim

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and my uncles Mr. Joseph Bassey, Mr. Eugene Bassey and Mr. Francis Bassey.

God bless you all.

Let me use this opportunity to appreciate the friends I made in the lion’s den.

They are Mrs. Dories, Chisom, Ike, Chimaroke, Igwe, Rita, Chioma, Femi (Ferm

Dirac), Onuk, Adrain, Kelvin, Chucky, Chioma, kelechi, Mr. Ike, Kc, and all staff

and students of GET – HI Tech. God bless you all.

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Title Page ------------------------------------------ i

Dedication ------------------------------------------- ii

Certification ------------------------------------------- iii

Acknowledgement -------------------------------------------- iv

Table of Contents -------------------------------------------- v

Abstract --------------------------------------------- x

List of Figures --------------------------------------------- xi

List of Tables --------------------------------------------- xiv

CHAPTER ONE -----------------------------------------------

GENERAL INTRODUCTION --------------------------------------------- 1

1.1 Introduction --------------------------------------------- 1

1.2 Advantages of Aeromagnetic survey method ---------------------- 2

1.3 Location of study area --------------------------------------------- 3

1.4 Geology of Study area --------------------------------------------- 4

1.5 Objectives of present studies --------------------------------------------- 5

CHAPTER TWO ----------------------------------------------

LITERATURE REVIEW --------------------------------------------- 6

2.1 Review of previous geophysical surveys in Ilesha -------------- 6

2.2 Basic concepts and definitions -------------------------------------------- 7

7

2.2.1 Magnetic poles, force and permeability ------------------------------ 8

2.2.2 Magnetic field strength ---------------------------------------------- 9

2.2.3 Magnetic moment and polarization -------------------------------------- 10

2.2.4 Magnetic susceptibilities --------------------------------------------- 11

2.2.5 Magnetic induction --------------------------------------------- 12

2.2.6 Classification of magnetic materials ------------------------------------- 13

2.2.7 Remanent magnetization --------------------------------------------- 15

2.3 The Earth’s total field --------------------------------------------- 16

2.3.1 The magnetic potential and Poisson relation. ------------------------------ 17

2.4 The Earth’s magnetic field ---------------------------------------------- 18

2.4.1 Magnetic elements and their characteristics ------------------------------ 20

2.4.2 Temporal variation of the earth’s magnetic field ---------------------- 22

2.5 Magnetic susceptibilities of rocks and minerals ---------------------- 24

2.6 Magnetic effects of simple shapes -------------------------------------- 26

2.7 Total field anomaly ---------------------------------------------- 32

CHAPTER THREE -----------------------------------------------

DATA ACQUISITION AND INTERPRETATION ----------------------- 35

3.1 Magnetic Instruments ---------------------------------------------- 35

3.2 Airborne magnetometers ---------------------------------------------- 38

3.3 Basic aeromagnetic instruments -------------------------------------- 40

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3.4 Magnetic Data Processing ------------------------------------- 42

3.5 Data acquisition ------------------------------------- 44

3.6 Interpretation of aeromagnetic data -------------------------------------- 45

3.6.1 Qualitative interpretation: -------------------------------------- 46

3.6.2 Quantitative interpretation -------------------------------------- 48

3.7 Geophysical modeling software -------------------------------------- 52

3.7.1 Main concepts -------------------------------------- 53

3.7.2 Axes used in Potent -------------------------------------- 55

3.7.3 Modeling shapes -------------------------------------- 56

CHAPTER FOUR ---------------------------------------

DATA ANALYSIS AND RESULT -------------------------------------- 58

4.1 Methodologies -------------------------------------- 58

4.2 Interpretation of total field data -------------------------------------- 58

4.2.1 Forward modeling -------------------------------------- 58

4.2.2 Inverse modeling -------------------------------------- 59

4.3 Data presentation -------------------------------------- 59

4.4 Data reduction -------------------------------------- 60

4.5 Data modeling -------------------------------------- 62

4.6 Data interpretation and results -------------------------------------- 62

Profile 1 -------------------------------------- 63

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Profile 2 -------------------------------------- 64

Profile 3 -------------------------------------- 65

Profile 4 -------------------------------------- 66

Profile 5 -------------------------------------- 67

Profile 6 -------------------------------------- 69

CHAPTER FIVE --------------------------------------

CONCLUSIONS AND RECOMMENDATION --------------------- 71

5.1 Conclusions ----------------------------- 71

5.2 Recommendations ----------------------------- 72

References ----------------------------- 73

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ABSTRACT

Results of aeromagnetic data interpretation of Ilesha SW, Nigeria are

presented here. The geology of Ilesha is of the Precambrian type which falls under

the basement complex of Nigeria. Depths to source rocks in this area are expected

to be shallow. The results obtained revealed the presence of rocks such as

Amphibolites, quartz and schist which are the common rock types present in the

study area. An aeromagnetic map of scale 1: 50,000 was hand digitized and

processed using geophysical modeling software (Potent version 4.10.02). Six (6)

profiles were modeled using forward and inverse modeling techniques. The field

data were qualitatively and quantitatively interpreted and results showed NE – SW

trending of the fault zone in the study area and 13 anomalous bodies whose total

magnetic intensity ranged from a minimum negative peak value of -625.5nT to a

maximum positive peak value of 179.43nT. The maximum depth to top of the

magnetic source body obtained is 34.2m and minimum depth is 0.5m. The results

obtained indicate shallow depths to magnetic anomalies, as expected in most areas

of the basement complex of Nigeria.

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LIST OF FIGURES

Fig 1.1: Map of Nigeria showing study area. ------------------------- 3

Fig. 1.2: Geology map of Ilesha area. ------------------------- 5

Fig 2.1: A bar magnet illustrating line of force. ------------------------- 7

Fig. 2.3: Vector diagram illustrating relationship between induced iJ , remanent

rJ and resultant magnetization components. ------------------------ 15

Fig. 2.4: magnetic field of the earth having

characteristics of homogeneous space. --------- 19

Fig. 2.5: The elements of the earth’s magnetic components. --------- 21

Fig. 2.6: Histogram showing susceptibilities of different rock types. ---- 25

Fig 2.7: Relationship and notation used to derive the magnetic effect of a single

pole. ---------- 26

Fig 2.8: Relationship and notation used to

derive magnetic effect of a dipole. ---------- 28

Fig. 2.9: Notation used for the derivation of magnetic field anomalies over a

uniformly magnetized sphere. ---------- 30

Fig. 2.10: Relationships of the total field anomaly. ---------- 32

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Fig. 3.1: Schematic diagram of Fluxgate Magnetometer. ----------- 37

Fig. 3.2: Shows measurement taken with gradiometer. ----------- 48

Fig. 3.3: An aircraft towing magnetometer stinger. ----------- 39

Fig. 3.4 Axis gradiometer system. ----------- 40

Fig 3.5: A section of Aeromagnetic map ----------- 45

Fig. 3.6: Example of magnetic anomaly signature

and amplitude variation. ---------- 46

Fig. 3.7: A typical aeromagnetic map magnetic gridded map. ---------- 48

Fig. 3.8: (a) Length of ‘straight slope’ of inflexion tangent.

(b) length between tangents at ‘half-slope. ---------- 51

Fig. 3.9: Different axes in potent. ---------- 55

Fig. 3.10: Axes of a dyke. ---------- 57

Fig. 3.11: Axes of a Slab. ---------- 57

Fig 4.1: A section of Aeromagnetic map

of Ilesha sheet 243 SW, Nigeria. ---------- 60

Fig. 4.2: Observed and calculated TMI, Profile one. ---------- 63

Fig. 4.3: Observed and Calculated TMI, Profile two. ---------- 64

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Fig. 4.4: Observed and Calculated TMI, Profile three. ---------- 65

Fig. 4.5: Observed and Calculated TMI, Profile four. --------- 66

Fig. 4.6: Observed and Calculated TMI. Profile five. --------- 67

Fig. 4.7: Observed and Calculated TMI, Profile six. --------- 69

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LIST OF TABLES

Table 2.1 magnetic susceptibilities of some mineral. ------------ 24

Table 2.2 magnetic susceptibilities of some selected mineral. ------------ 25

Table 4.1: Results of profile one. ------------ 64

Table 4.2: Results of profile two. ------------ 65

Table 4.3: Results of profile three. ------------ 66

Table 4.4: Results of profile four. ------------ 67

Table 4.5: Results of profile five. ------------ 68

Table 4.6: Results of profile Six. ------------ 69

Table 4.7 Summary of results. ------------ 70

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CHAPTER ONE

GENERAL INTRODUCTION

1.1 Introduction

The concept of geophysics has to do with the application of the laws of

physics to the study of the earth and its surrounding atmosphere. Historically,

Gilbert (1540 – 1603) discovered that the earth behaves as a great and rather

irregular magnet (Telford et. al. 1990). This gave the idea about the characteristics

of the earth’s interior. Gilbert’s discovery and the theory of gravitation by Newton

are said to be the beginning of geophysics. To carry out geophysical investigation

of the earth’s subsurface, signals are sent into the earth and measurements taken.

As the signals propagate through the earth’s interior, they will be influenced by the

internal distribution of the earth’s physical properties. Receiving, measuring and

analysis of these signals can reveal how the physical properties of the earth’s

interior vary vertically and laterally (Kearey and Brooks, 2002).

There are several geophysical methods that have since been employed in the

investigation of the earth’s physical properties and characteristics. Some of them

are seismic, electrical, electromagnetic, magnetic and gravity methods. The method

to be used for a particular investigation or survey may depend strictly on the nature

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or purpose of the study. Sometimes, more than one method may be employed to

carry out a particular survey. For the purpose of this study, we shall employ

magnetic method using aeromagnetic data to investigate the properties of the

subsurface in Ilesha, South West Nigeria.

Aeromagnetic geophysical method has been widely used since its inception.

The most distinguishing feature of this method, compared with other geophysical

schemes, is the rapid rate of coverage and low cost per unit area explored (Reford

and Sumner, 1964). The use of this method makes it possible for geophysicists to

acquire data regardless of ownership or accessibility of remote lands of interest.

This inherent advantage has made it possible for large scale airborne magnetometer

survey to be carried out around the globe.

1.2 Advantages of aeromagnetic survey method

• A speedy survey of large area is carried out

• A survey of several hundred kilometers (km) is achieved per day. So

the cost of one observation point is much less than the ground survey

when a large scale survey is to be carried out.

• It is possible to carry out survey in rocky terrains where there is no

accessible motor road.

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• Due to high speed, drift and diurnal corrections of the earth’s field are

small

• As the air plane flies, high effects due to artificial magnetic materials

such as railroad and buildings, which cause cultural noise is greatly

reduced.

1.3 Location of study area

Ilesha Town is located in Osun State, Southwest Nigeria. It lies within the

tropical climate marked by wet and dry seasons. Its latitude is 7.60 N and longitude

Study area

xx

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Fig 1.1: Map of Nigeria showing study area. (After Rahaman, 1976)

4.70E with an average elevation of 391m above sea level. Temperature in Ilesha is

moderately high during the day and may vary from season to season.

There are two seasons in this study area; wet and dry season. The wet season

occurs from April to September and the dry season occurs from October to March.

The average daily temperature varies between about 200C for a very cold day to

about 350 for a very hot day. The coldest period is in the middle of rainy season

which occurs in July and August (Kayode, 2006). The study area was chosen based

on the anomalies observed on aeromagnetic contour map of Ilesha.

1.4 Geology of Study area

The geology of Ilesha has been discussed in detail by Rahaman (1976);

Kayode (2006, 2009, 2010); Ajayi (2003); Folami (1992); Ajayi (1981); Elueze

(1986, 1988) and Akintorinwa et.al (2010). It consists of Precambrain rocks which

forms the basement complex. The major rocks associated with the area form part

of the proterozoic schist belts in Nigeria as shown in Fig. 1.2. Quartz – schist (2);

quartzite (6); amphibolites (7); granite - gneiss (3); amphibolites schist (4) and

migmatite – gneiss complex (5) are the major rocks in Ilesha as delineated in Fig.

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1.2. Other minor rocks according to Kayode (2006), Folami (1992) and Rahaman

(1976) are garnet, quartz chlorite bodies and dolorites.

Fig. 1.2: Geology map of Ilesha area. (Modified from Kayode et. al, 2010).

1.5 Objectives of present research

The objective of this study is to interpret qualitatively and quantitatively the

aeromagnetic data of Ilesha Southwest Nigeria. This will include:

To determine the susceptibilities of rock types in the area.

1 2

3

4

5

6

7

5

7

1

2

3

1 2

3

4

5

6

7

5 Migmatite gneiss

7 Amphibolites

6 Quartzite

1 Porphyritic Granite

2 Quartz Schist

3 Granite gneiss

4 Amphibolites Schist

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To determine depth of burial of anomalous bodies.

To determine the dip, plunge and type of body causing the magnetic anomalies.

CHAPTER TWO

LITERATURE REVIEW

2.1 Review of previous geophysical surveys in Ilesha

Kayode (2010) interpreted the vertical magnetic components in Ijebu-Jesa

Southwest Nigeria using ground magnetic survey and obtained depth to basement

complex of 38m – 244m.

Momoh et. al. (2008) carried out geophysical investigation of highway

failure, a case study of the basement complex terrain of South west Nigeria (Ilesha

– Owene Highway). They reported that faults, fractures, joints and buried stream

channel were some of the causes of the highway failure. Depths of between 0.3m

and 41.3m were obtained.

Kayode and Adelusi (2010) interpreted the ground magnetic data of Ijebu

Jesa area and obtained depths to basement complex of between 41m and 213m.

Integration of surface electrical prospecting methods for fracture detection in

Precambrian basement rocks of Iwaraja area, Southwest Nigeria, by Adelusi et. al

(2009) showed a NE -SW trending of faults in that area and obtained depths of 10

– 55m.

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Study on the groundwater accumulation of Oke-Ogba area using ground

magnetic survey by Alagbe et.al (2010) revealed depths ranging from 3.0 to 21.0m.

This depth range agrees with the depth range of 2.3 – 21.2m obtained by Adelusi

(2002) using electrical resistivity method.

2.2 Basic concepts and definitions

In this section, we shall look at some of the basic concepts that need to be

defined for proper understanding of the earth’s magnetism and its properties.

2.2.1 Magnetic poles, force and permeability

Magnetic poles:

Consider a bar magnet with two edges labeled A and B as shown below:

Fig 2.1: A bar magnet illustrating line of force. (After Dobrin and Savit, 1988).

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Each of these edges on the magnet is referred to as a “pole” and it is known

as “magnetic poles” on considering both edges. If one spreads tiny particles of iron

on a paper that rests on a bar magnet as in Figure 2.1, one discovers that the iron

particles will align themselves as shown in Figure 2.1. These lines are referred to

as “lines of force”.

It is important to state here that a bar magnet cannot have only one pole. In

order words, monopoles do not exist. For instance if one were to divide the bar

magnet in Figure 2.1 into two, ordinarily one will think that the divided magnet

will have separate poles i. e. A and B in both halves, so that the lines of force will

tend to one edge of the magnet, but this is not the case. The bar magnet when

divided into two will still have two poles in each of the half magnet such that the

iron particles will align to both ends of each of the half magnets. This analogy

shows that monopoles do not exist.

Magnetic force: Magnetic force is similar to the force that exists between

two point charges as stated by Coulomb (1736 – 1806). Coulomb showed that the

force of attraction or repulsion between two electrically charged bodies and

between magnetic poles (dipoles) also obeys an inverse square law like that

derived for gravity by Newton. This led to the invention of torsion balance by

Coulomb.

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Mathematically, magnetic force is represented by:

(2.1)

where µ is a constant of proportionality known as magnetic permeability, ,1p ,2p

are strength of the two magnetic monopoles and r is the distance between the two

poles. Equation (2.1) is identical to the expression of gravitational force but have

two important features:

• Instead of gravitational constantG , permeability µ is used which describes

the magnetic property of the material in which the poles are situated. If they

are in vacuum, then µ becomes permeability of free space oµ

• Instead of21 , mm , as in gravitational force expression, ,1p and ,2p are used.

They may either be positive or negative.

Magnetic permeability µ is a dimensionless constant that describes the

magnetic property of the material in which poles are situated

2.2.2 Magnetic field strength H

This is defined as the force per unit pole strength exerted by a magnetic

monopole P . Thus the field strength H due to a pole of strength 0P a distance

r away is:

2

21

r

ppFm

µ=

24

(2.2)

Substituting equation (2.1) into (2.2), assuming P1 = P0 and P2 = P, we have that:

(2.3)

The magnetic field strength H is often expressed in terms of the density of

lines of force or flux representing the field. It may also be represented in the cgs as

one dyne per unit pole or as one Oersted.

2.2.3 Magnetic moment and polarization

Since a magnet has a pair of poles and are otherwise called dipoles, we can

then define magnetic moment M of a dipole with poles of strength P , a distance

l apart as:

runit vecto is r and where,rr

r IAPPlM r == (2.4)

In equation (2.4), I is the intensity of magnetization and A is the cross sectional area.

The direction of magnetic moments is along the line between the poles and

by convention is from the negative pole towards the positive pole.

Magnetic polarization: When one places a material in a magnetic field, the

material may become magnetized in the direction of the magnetic field. This

oP

FH =

2r

PH

µ=

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magnetization acquired by the material can be lost if the material is removed from

the vicinity of the field. This is known as magnetic polarization or induced

magnetization. It results from alignment of elementary dipoles within the material

in the direction of the field. As a result of this alignment, the material has magnetic

poles distributed over its surface which correspond to the ends of the dipole.

The induced magnetization or polarization is in the direction of the applied

field and its strength is proportional to the strength of that field. The intensity of

induced magnetization I of a material is defined as the dipole moment per unit

volume of material given as

,V

M

LA

MI

rr

== (2.5)

where M is the magnetic moment of a sample of length L and cross sectional area

A . I is expressed in 1−AM . In the cgs. system, the intensity of magnetization is

expressed in -3cm emu (emu = electromagnetic unit), where 1-3 1000cm 1 −= AMemu .

2.2.3 Magnetic susceptibilities

The magnetic susceptibility is a unitless constant that is determined by the

physical properties of the magnetic material. It relates the intensity of

magnetization Ir to the strength of the inducing magnetic field H

rthrough the

expression:

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(2.6)

where k is magnetic susceptibility. k may either be positive or negative. When it is

positive, then it implies that the induced magnetic field is in the same direction as

the inducing field Hr

, while negative value implies that the induced magnetic field

is in opposite direction to the inducing field.

In magnetic prospecting, susceptibility is the fundamental material property

whose spatial distribution we are attempting to determine.

2.2.5 Magnetic induction

The magnetic poles induced in a material by an external field H will

produce a field of their own, H ′ . It is related to the intensity of magnetization I by

the formula:

(2.7)

The magnetic induction Ar

is defined as the total field within the body. It is given

as:

(2.8)

By substituting (2.7) into (2.8), we have

(2.9)

)41( kHA π+=rr

, where µπ =+ )41( k , so that;

HkIrr

=

IHrr

π4=′

HHArrr

′+=

lHArrr

π4+= HkHrr

π4+=

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HArr

µ=

We can define magnetic permeability in section. 2.2.1 as the ratio of

magnetic induction Ar

to magnetic field strength Hr

H

Ar

r

=µ (2.10)

In summary, magnetic induction is a measure of the force exerted on a

moving charge by a magnetic field, whereas magnetic intensity is a measure of the

force exerted on a magnetic pole by a magnetic field, whether the pole is moving

or not.

2.2.6 Classification of magnetic materials

Magnetic materials are classified into three types based on their magnetic

properties. They are:

Diamagnetic material: This type of magnetization was discovered in 1846

by Michael Faraday. It is the fundamental property of all materials and is caused

by alignment of magnetic moments observed with orbital electrons in the presence

of an external magnetic field.

There is no net moment in diamagnetic material since all the electron shells

are full and in the presence of an external field, the net moment opposes the

external field, thus the susceptibilities of diamagnetic materials are usually

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negative and relatively small. There is no interaction of atomic currents (dipoles) in

diamagnetic materials. Examples of diamagnetic materials include graphite,

gypsum, marble, quartz, salt and some other alkali halides.

Paramagnetic materials: Here materials contain unpaired electrons in

incomplete electron shells and the magnetic moment of each atom is uncoupled

from others so they all behave independently. In order words, the magnetic

material has odd numbers of electrons orbiting in their outer shells. Paramagnetic

materials can only be observed at relatively low temperatures. Above this

temperature, paramagnetism will no longer be observed. Such temperature is

referred to as Curie temperature.

It is important to state that paramagnetism results in weakly magnetic

materials and hence small and positive susceptibilities. Hence materials that are not

diamagnetic can said to be paramagnetic.

Ferromagnetism: In metals such as cobalt, nickel and iron, unpaired

electrons are coupled magnetically due to strong interaction between adjacent

atoms and overlap of electron orbits. Groups of atoms that couple together

magnetically are called magnetic domains, about 1 micron in size. Magnetic

domains can be oriented to produce a spontaneous magnetic field in absence of

external field. Magnetic susceptibility is large, but depends on temperature and

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strength of applied field. All domains are oriented in same direction. It has the

following characteristics:

• They are caused by overlapping electron orbits

• They give rise to spontaneous magnetization even in absence of an external

field.

Examples of ferromagnetic materials are cobalt, iron and nickel.

2.2.7 Remanent magnetization

Magnetic field may exist within rock even in absence of external field due to

permanently magnetic particles. This is remanent or permanent magnetization.

Interpretation of magnetic data is complicated as magnetic field due to a

subsurface body results from combined effect of two vector magnetizations that

may have different magnitudes and directions.

Fig. 2.3: Vector diagram illustrating relationship between induced iJ, remanent rJ and resultant

magnetization components. (After Kearey and Brooks, 1991).

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In a simpler way, remanent magnetization is the remaining induced

magnetization in a magnetic material after the induced field (external) has been

removed. If the inducing field is strong, the magnetic material may retain a portion

of its induced magnetization even after the induced field disappears.

Remanent magnetization is the component of the material’s magnetization

that solid earth geophysicists use to map the motion of continents and ocean basins

resulting from plate tectonics. Ferromagnetic materials exhibit this creative

spontaneous magnetization. The direction of remanent magnetization may vary

radically from induced field.

2.3 The Earth’s total field

When a buried object has a magnetic field, such a field will be superimposed

on that of the earth’s magnetic field. The resultant field which will then be

measured is a vector which will have both magnitude and direction.

,ao TTT ∆+= where T is the total field vector in the vicinity of the magnetic rocks,

oT is the earth’s undisturbed field vector and aT∆ is the anomalous magnetic field

vector caused by the magnetized body.

The measurement of the actual field by modern magnetic instruments is

referred to as total field measurement. Generally, interpretation of total field varies

31

in both magnitude and direction. It is more complex than those involving

individual components, such as vertical measurements.

2. 3.1 The magnetic potential and Poisson relation.

Magnetic potential is the work done in bringing a unit magnetic pole from

infinity to a point, say distance r from another source of magnetic polarity of

strength p. Mathematically, it is expressed as:

,r

pU

µ=

where U is the potential. (2.11)

Poisson’s relation can be used to determine the magnetic potential and

magnetic field strength associated with a magnetized body at any point in terms of

gravitational potential. This is important in the prediction of magnetic effect of

buried bodies.

The magnetic potential U according to Poisson can be expressed as:

,di

dV

G

IU

ρ−= (2.12)

where V is the gravitational potential, i is the direction of magnetic polarization, I

is the magnetization or polarization, ρ is the density of causative body and G is the

universal gravitational constant.

The corresponding magnetic field component in any direction s is

32

.

=−=

di

dV

ds

d

G

I

ds

dUH s

ρ (2.13)

If the body is polarized in the z (vertical) direction, and if the horizontal

component Hx of the magnetic field is desired, it can be obtained from the

equation:

.

=−=

dz

dV

dx

d

G

I

dx

dUH x

ρ (2.14)

The vertical component Hz will be

.2

2

dz

Vd

G

I

dz

dUH

dz

dV

dz

d

G

I

dz

dUH

z

z

ρ

ρ

=−=

=−=

(2.15)

2.4 The Earth’s magnetic field

The magnetic field of the earth is a vector, that is, it has both magnitude and

direction. Ninety percent of the earth’s magnetic field looks like a magnetic field

that would be generated from a dipolar magnetic source located at the center of the

earth and aligned with the earth’s rotational axis. The strength of the magnitude is

about 60,000 nT.

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Fig. 2.4: magnetic field of the earth having characteristics of homogeneous space. (After Chapman &

Bartels, 1940).

The magnetic field of the earth can be classified into three separate

components:

Main field: This is said to be the largest component of the magnetic field

and is believed to be caused by electrical current in the earth’s outer core. For

exploration work, this field acts as the inducing magnetic field. It is not constant in

time and varies relatively slowly.

External magnetic field: This is a relatively small portion of the observed

magnetic field that is generated from magnetic sources external to the earth. It is

partly cyclical and partly random. It is believed that this field is produced by

34

interactions of earth’s ionosphere with the solar wind, hence temporal variations

associated with the external magnetic field are correlated to solar activity.

Crustal field: These are basic targets in magnetic prospecting. It is

otherwise a variation of the main field associated with the magnetism of crustal

rocks. It contains both magnetism caused by induction from the earth’s main

magnetic field and from remanent magnetization. The crustal field is usually but

not always smaller than main field and it is relatively constant in time and place.

Basically, it is caused by local magnetic anomalies in the near surface crust of the

earth.

2.4.1 Magnetic elements and their characteristics

Let us consider a thin iron, of about 0.5mm in diameter and 4cm in length

that was not magnetized. This thin iron is hung at its center by a thread so that it

will be free to orient itself in space in any direction, it will be observed that this

thin iron would assume a direction that is neither horizontal nor in line with the

geographic meridian. The orientation sustained by this iron is the direction of the

earth’s total magnetic field at this point.

35

Fig. 2.5: The elements of the earth’s magnetic components. (After Lowrie, 2002).

The magnitude of the field F, the inclination of the thin iron from the

horizontal I and its declination D, the angle it makes with geographic north, all

completely define the magnetic field. The elements as shown in the Figure 2.5 can

be grouped in pairs of three as (H,D,Z), (X,Y,Z), and (H,D,I), where H is the

horizontal component, D is the declination angle ( the angle between the vertical

plane through the axis of the magnetic needle and the geographic north). Z is the

vertical component, X is the north component, Y is the east component, I is the

inclination angle or magnetic dip (angle by which a freely pivoted magnetic needle

dips below the horizontal). It is positive when the north seeking pole of the needle

points downward and negative if it points upwards. The Cartesian (X,Y,Z) and

spherical polar F,D,I components are related as follows:

Meridian

Vertical

y

36

DIFX coscos= FSinIZ = X

YD arctan=

ISinDFY cos= H

ZI =tan

( ) 2/122arctan

YX

ZI

+=

IFH cos= 2222 ZYXF ++=

The vertical plane through F and H is called the magnetic meridian. Lines of

equal declination, inclination, horizontal intensity etc, when plotted on maps are

usually referred to as isomap charts. They show the variation in the geomagnetic

field over the earth’s surface. Oddly enough, the magnetic field reflects little or

nothing of the variation in surface geology and geography such as mountain

ranges, submarine ridges, and earthquake belts. This indicates that the source of the

field lies deep within the earth or far outside it (Telford et. el. 1990).

2.4.2 Temporal variation of the earth’s magnetic field

These are time dependent variations and are resolved in to secular changes,

solar – diurnal changes, lunar diurnal changes and changes resulting from magnetic

storms

Secular variations are slow changes in the earth’s field which take place

progressively over centuries. They are usually noted in all magnetic elements at

magnetic observatories everywhere in the world. The rate of change varies with

37

time. Observations of Earth’s magnetic field made over 400 years show a gradual

change in position of the magnetic pole.

They are also due to slow movement of eddy currents in earth’s core.

Diurnal variations: These are daily changes in field due to changes in

currents of charged particles in ionosphere. They are regularly recorded at

magnetic observatories and are of more direct significance in magnetic

prospecting. They are small but oscillilate more rapidly in the earth’s field with a

periodicity of about a day and amplitude averaging about 25 gammas.

The records of diurnal variations generally show two types of variations: the quiet

day and the disturbed day.

The quiet day variation is smooth regular and low in amplitude. It can be

separated into predictable components having both solar and lunar periodicities.

The disturbed day is less regular and is associated with magnetic storms.

Magnetic Storms: These are short term disturbances in magnetic field

associated with sun spot activity and streams of charged particles from the sun.

They can be up to 1000 nT in magnitude, and make magnetic surveying

impossible. Magnetic survey must generally be discontinued during storms of any

severity (Dobrin and Savit, 1988)

38

2.5 Magnetic susceptibilities of rocks and minerals

Magnetic susceptibility k is the physical parameter of magnetic survey

(equivalent to density in gravity). Rocks with significant concentrations of

ferri/ferro-magnetic minerals have highest susceptibilities:

Table 2.1 magnetic susceptibilities of some mineral. (Telford et. al, 1990).

ROCKS AVERAGE MAGNETIC

SUSCEPTIBILITY (SI).

Dolomite 0.00012

Lime Stone 0.00031

Sands Stone 0.00038

Shale 0.00063

Amphibolite 0.00075

Schist 0.00126

Quartzite 0.00440

Slate 0.00628

Granite 0.00281

Olivine – Diabase 0.02513

Diabase 0.05655

Porphyry 0.06283

Gabro 0.07540

Basalt 0.07540

Diorite 0.08797

Peridotite 0.16336

39

Acidic Igneous 0.00817

Table 2.2 magnetic susceptibilities of some selected mineral (Telford et. Al, 1990).

ROCKS AVERAGE MAGNETIC

SUSCEPTIBILITY (SI)

Quartz -0.00001

Rock salt -0.00001

Gypsum -0.00001

Coal 0.00002

Clay 0.00025

Chalcopyrite 0.00040

Cassiterite 0.00113

Pyrite 0.00163

Limonite 0.00276

Harmatite 0.00691

Chromite 0.00754

Pyrrhotite 1.57080

Ilmenite 1.88500

Magnetite 6.28300

Fig. 2.6: Histogram showing susceptibilities of different rock types. (After Telford et al, 1990)

40

2.6 Magnetic effects of simple shapes

(a) Isolated pole (monopole)

Lets us consider a magnetic field above a single pole. Although such a pole

cannot exist, let us assume the body to be very long and thin oriented vertically,

and magnetized along its length. The top surface has pole strength of –p and the

bottom surface will be +p, and it is sufficiently far removed for its effect to be

negligible.

The potential V of the monopole is;

r

pV = where p is the pole strength given as IAp = , I is the magnetic intensity and

A is the cross sectional area. (2.16)

r

AkF

r

IA

r

pV e=== (2.17)

But ( ) 2/1222 zyxr ++=

Fig 2.7: Relationship and notation used to derive the magnetic effect of a single pole. (After Burger, 2006).

θ

+x

-x

+-

-

z

+p

y

c x

-x

+x Magnetic

41

( )

.2/1222

zyx

AkFV e

++= (2.18)

The magnetic field is determined in a given direction by differentiating V in

that direction, so that

( )

.2/1222

zyx

AKF

dz

d

dz

dVZ e

A

++−=−= (2.19)

( )

.2/3222

zyx

AzkFZ e

A

++= (2.20)

On considering the figure above, we will then determine the horizontal field

due to the monopole. It is convenient to orient our coordinate system so that the +x

axis is oriented towards magnetic north (Fig. 2.5). This orients the horizontal

component of the anomalous field HAX and HAY parallel to X and Y of the earth’s

field, vertical down is the Z axis. This magnetic field component oriented by black

arrows in Fig. 2.7 is considered positive. Using the same approach above, we

determine HAX and HAY as:

( )

.2/3222

zyx

AxkF

dx

dVH e

AX

++=−= (2.21)

( )

.2/3222

zyx

AykF

dy

dVH e

AY

++=−= (2.22)

The total anomalous field is calculated using the form of equation of total

field anomaly

iHiZF AAAT cossin += (2.23)

42

(b) Magnetic effect of a dipole

Consider Figure 2.8 below;

Lets us assume that the dipole is magnetized along its axis (parallel to its

length).

Fig 2.8: Relationship and notation used to derive magnetic effect of a dipole (After Burger, 2006)

θ2 Φ2 Φ1

2r

L

-p

+p

Zp

1r Zn

X=0 P

x

+x -x Magnetic North

θ -90 b L

θ a

Zn

pp

p

nn

nnp

pp

n

r

ax

r

z

r

x

r

zbzz

zaxrLb

zxrLa

222

11

2/122

2/12

1

)(cos sin

cos ,sin

])[( )180sin(

)( )180cos(

−==

==+=

+−=−=

+=−=

θθ

θθ

φ

φ

43

The magnetic field intensity at P due to the negative pole of the dipole is:

2

1

2

1

1r

AkF

r

pR e

A =+= (2.24)

And that due to the positive pole:

2

2

2

2

2r

AkF

r

pR e

A −=−=

(2.25)

Next, we determine the horizontal and vertical component of the magnetic

field at p due to each of the poles (-p and +p). These components are

211222

111111

cos sin

cos sin

φφ

φφ

AAAA

AAAA

RHRZ

RHRZ

==

==

(2.26)

21

211

AAA

AAA

HHH

ZZZ

+=

+=

(2.27)

22A12

1

1 sin Z sin2

2φφ

r

AkF

r

AkFZ ee

A −== (2.28)

sinsin

2

2

2

2

1

1

−=

rrAkFZ eA

φφ

(2.29)

coscos

2

2

2

2

1

1

−=

rrAkFH eA

φφ (2.30)

The total field is obtained, using equation 2.23

22A12

1

1 cosH cos2

2φφ

r

AkF

r

AkFH ee

A −==

44

iHiZF AAAT cossin +=

(c) Magnetic effect of a sphere

This is somewhat more complex in derivation than that of a dipole.

In deriving an equation for the magnetic effect of a sphere, we shall employ

Poisson relation, given as:

.di

dU

G

IV

ρ−= (2.31)

assuming the body susceptibility and density are uniform. The direction here is

vertical, i.e. Z, so that the vertical and horizontal field anomalies ZA and HA will be

defined as

.2

2

dz

Ud

G

I

dz

dVZ A

ρ=−= (2.32)

Fig. 2.9: Notation used for the derivation of magnetic field anomalies over a uniformly magnetized sphere.

(After Burger, 2006).

i

Z

R

x Magnetic

- +

P X=

FE

45

.1

=−=

dx

dU

dx

d

Gdx

dVH A

ρ (2.33)

Recall that the gravitational potential of a sphere is given as:

3

3

4 V M where R

r

GMU πρρ ===

( )

3

4

3

4

2/122

33

zx

RG

r

RG

U+

==πρπρ

(2.34)

The vertical component becomes

( )

.

)2(3

4

2/522

223

zx

xzIR

Z A

+

−=

π (2.35)

Similarly,

( )

.

)(3

4

2/322

3

+

−=

zx

xzRG

dx

d

G

IH A

πρ

ρ (2.36)

( )

.

)2(3

4

2/522

223

xz

zxlR

H A

+

−=

π (2.37)

( ) ,

)(3

4

2/322

3

zx

xzRG

dz

dU

+

−=

πρ

( ),

)2(3

4

2/522

223

2

2

zx

xzRG

dz

Ud

+

−=

πρ

46

In a more general case, where the sphere will be uniformly magnetized and

the earth’s field is inclined, we have that:

( )

.1cos)(

3

)(

3sin

3

4

2/1222/122

2

2/522

3

−

+−

++

= izx

xz

zx

z

zx

iKFR

Z A

π

(2.38)

( )

.tan)(

31

)(

3cos

3

4

2/1222/122

2

2/322

3

+−

−

++

= izx

xz

zx

x

zx

iKFR

H A

π

(2.39)

2.7 Total field anomaly

For simplicity, we shall use ZE, HE and FE as references to the earth’s main

field. If we derived values for ZA and HA, it will become easier to determine FA.

We seek to obtain FAT, where FAT is the total field anomaly, ZA is the vertical field

anomaly component.

Fig. 2.10: Relationships of the total field anomaly. (After Burger, 2006)

(a) Vector of the main field and anomalous field. (b) components of the undisturebed main filed

FET = Main field

Plus anomalous field

nTFE 55005=

nTFE 55000=

ZE + ZA FEU + FAT = FET

HE + HA

(b)

FA = 12nT

FEu =Undisturbed main

field

FAT = 5nT

47

Consider Fig. 2.10 (b), if the anomalous field is oriented such that HA is

directed toward magnetic north i.e. the HA – ZA plane is parallel to a magnetic

meridian.

From Fig. 2.10 (b), using Pythagoras theorem

( ) ( ) ( )222

AEAEATE HHZZFF +++=+ (2.46)

By expansion, and considering that EF >>> AF , and ignoring 222,, AAAT HZF we have

AEEAEEEATE HHHZZZFFF 222222

+++=+ (2.47)

But :

222

EEE HZF +=

Then (2.47) becomes

E

AE

E

AE

AT

AEAEEAT

AEAEEAT

F

HH

F

ZZF

HHZZFF

HHZZFF

+=

+=

+= 222

+

=

E

E

A

E

E

AATF

HH

F

ZZF (2.48)

By applying the relationship among the geomagnetic elements in sec. 2.3.1,

where

iF

Z

E

E sin= , iF

H

E

E cos= then finally we have

iHiZF AAAT cossin += (2.49)

48

If HA does not lie along a magnetic meridian, we use the component of HA

parallel to the meridian, because this is the only effect of HA or the total anomaly.

In such a case:

iHiZF AAAT coscossin α+= (2.50)

49

CHAPTER THREE

DATA ACQUISITION AND INTERPRETATION

3.1 Magnetic instruments

Instruments used in magnetic survey can be classified in to two:

(i) Mechanical instruments and (ii) Magnetometers

(i) Mechanical Instruments: These are instruments that are mechanical in

nature. They usually measure the “altitude” of the magnetic field. The simplest

type of these instruments is the simple compass.

The simple compass consist of nothing more than a testing magnet that is

free to rotate in a horizontal plane. The positive pole of the test magnet is attracted

to the earth’s negative magnetic pole, and the negative pole of the test magnet is

attracted to the earth’s positive magnetic pole. This will enable the test magnet to

align itself along the earth magnetic field. It provides measurement of the

declination of magnetic field.

Mechanical magnetic instruments in recent times are not commonly used.

Other types include:

Dip needle and torsion magnetic instruments.

50

The dip needle is used to measure the declination of the magnetic field. The

torsion is a device that can measure via a mechanical means, the strength of the

vertical component of the magnetic field

(ii) Magnetometers: These are the most common types of magnetic

instruments. They are usually operated non-mechanically and are capable of

measuring the strength or a component of the strength of the magnetic field.

The common types of magnetometers are: Fluxgate magnetometers, Proton

precession magnetometers and Alkali vapor magnetometers (optical pumped

magnetometers).

(a) Fluxgate Magnetometers

They measure components of magnetic field parallel to cores with accuracy

of 1-10 nT. It comprises of two parallel cores of high permeability µ of

ferromagnetic material. Primary coils are wound on two cores in series in opposite

directions. Secondary coils are also wound, but in opposite direction to primary

coils

51

Fig. 3.1: Schematic diagram of Fluxgate Magnetometer. (After Carl Moreland, 1992).

Operation of Fluxgate Magnetometer

• An alternating current at 50-1000 Hz is passed through primary coils,

producing magnetic field that drives each core to saturation through a

magnetization hysteresis loop.

• With no external magnetic field, cores saturate every half cycle.

• Voltages induced in secondary coils have opposite polarity as coils are

wound in opposite directions leading to zero net voltage.

• In Earth's magnetic field, component of field parallel to cores causes one

core to saturate before the other, and voltages in secondary coils do not

cancel.

(b) Proton Precession Magnetometer

This makes use of sensor consisting of bottle of proton-rich liquid, usually

water or kerosene, wrapped with wire coil. Two sensors indicate a gradiometer

52

Fig. 3.2: Shows measurement taken with gradiometer. (After Carl Moreland, 1992).

• Protons have a net magnetic moment, and are oriented by Earth’s magnetic

field or an applied field.

• Measures precession as protons reorient to Earth’s field.

• Precession frequency proportional to total field strength.

• Measures total field strength, so instrument orientation not important, unlike

fluxgate.

• Oberhausen Effect adds electron-rich fluid to enhance polarization effect,

and increases accuracy.

3.2 Airborne Magnetometers

Proton precession magnetometers are used extensively in marine and

airborne surveys:

53

• At sea, sensor bottle is towed in a "fish" 2-3 ship’s length astern to remove it

from magnetic field of the ship

• In air, sensor is towed 30 m behind aircraft or placed in a "stinger" on nose,

tail or wingtip.

Fig. 3.3: An aircraft towing magnetometer stinger. (Telford et al, 1990).

Often active compensation for magnetic effect of aircraft is calculated.

Effectiveness of compensation is called figure of merit (FOM).

• In airborne work, separation is 2-5 m for stinger and up to 30 m for bird.

• In ground work, separations of 0.5 m are common.

54

Example of 3-axis gradiometer system:

Fig. 3.4 Axis gradiometer system. (After Carl Moreland, 1992).

Advantages:

• No correction for diurnal variation is required as measurement is difference

of two magnetic sensors.

• Vertical gradient measurements emphasize shallow anomalies and suppress

long wavelength features.

3.3 Basic aeromagnetic instruments

Dobrin and Savit (1988) suggested the following basic instruments or

equipment for aeromagnetic surveying:

Magnetometer stinger – This is mounted or towed and is called bird sensor

Digital data acquisition system: They are digital magnetometers that record time,

synchronization, navigation and other pertinent survey data.

55

Analog recorder: to record selected parameter. Usually, magnetic and altimeter

data for in-flight quality control and quick review after flight.

Doppler navigation system: To provide spatially based sampling and navigation

support.

Track recovery system: Usually, a vertically mounted video camera or 35mm

film camera system to provide actual visual track information to supplement the

Doppler navigation.

Recording altimeters: Barometric and radar altimeters for vertical position

information.

Magnetic compensation unit (fixed wing only): to compensate for the induced,

(both electrical and plat form motion) and permanent, magnetic fields of the air

craft.

Sometimes, the following additional ancillary instruments may be used:

Other navigational system, electronic or inertial systems.

Other geophysical instruments, Gamma – ray spectrometer, active or passive EM

system, multispectral scanners, etc.

Ground equipment: base station magnetometer and recording unit and field

computer system.

56

3.4 Magnetic data processing

The procedure employed for processing magnetic data obtained from land is

not the same with that carried out in airborne and marine. For the purpose of this

work, we shall consider that of aeromagnetic method.

Usually, data obtained from aeromagnetic survey are often too large to be

processed by hand. This has given rise to use of modern computers for the

processing of the data obtained. Typical aeromagnetic data are made up of three

data sets:

• The magnetic field measurement, which are the primary data.

• The location recovery, generally in the form of station numbers transferred

unto topography maps or set of aerial photographs.

• Base station data.

The following steps may be employed in the processing of these data:

Editing: Here we carry out the first step in processing which is removal of

extraneous data, after which one removes from each line of survey, the spikes in

each data variable.

Locations: It is important to know a particular location data was recorded or

obtained. This is often one of the difficulties encountered in airborne survey. The

methods for determining and plotting the location depend to a greater extent on the

positioning system used. Positioning systems such as GPS, Loranc, VLF often

57

yield absolute location data recorded on digital tape and synchronized with

magnetic data.

Data correction: Aeromagnetic data must be corrected for aircraft motion

and temporary variation of the earth’s magnetic field.

Time variation: They are those that are time dependent. Magnetic variations

experienced during surveys are results of both geology (spatial) and external

influences on the earth’s magnetic field.

Compensation: When we consider the field of a survey vehicle such as

aircraft, it becomes necessary to apply compensation, since those fields are major

source of errors in airborne survey.

IGRF removal: It is a mathematical representation of the earth’s main

magnetic field due to sources in the core. Once this field is removed from the data,

the remaining data becomes residual magnetic anomaly due to subsurface rocks.

Leveling: These are due to the minor flight elevation changes occurring

along the flight lines by the Aircraft.

Interpolation to regular grid: After all the above steps have been

accomplished, the data so far obtained becomes series of profile lines with a high

data density along the lines and a low data density between lines. In order to obtain

contour maps, the data will then be reduced to a regular grid. These processes are

otherwise referred to as interpolation.

58

Data display: The data display may be residual contour maps, offset

profiles and multiparameter profiles.

3.5 Data acquisition

An aeromagnetic map on a scale of 1:50,000, sheet 243 SW was acquired

from the Nigerian Geology Survey Agency (NGSA). The aeromagnetic data was

acquired at a nominal flying altitude of 152m (about 500ft) with flight lines spaced

2km in the direction 60/240 (dip/azimuth)degree and contour interval of 20nT.

Magnetic instruments used are air plane, Magnetometers, Magnetometer Stinger,

digital data acquisition system track recovering system, recording altimeters,

magnetic compensation unit and Doppler navigation system. Regional correction

was based on IGRF (1st January, 1974).

The map (Figure 3.5) was hand digitized along flight lines. Although hand

digitization is the most elementary least efficient method of digitization, its

accuracy when carefully done compares favorably with other more sophisticated

methods (Bath, 1974). Sophisticated method like automated digitized data are cost

effective and does not come with the aeromagnetic contour map. Also, this does

not allow students appreciate and know the manual way of hand digitizing of data.

59

3.6 Interpretation of aeromagnetic data

Fig 3.5: A section of Aeromagnetic map of Ilesha sheet 243 SW, Nigeria. (Nigerian Geological Survey

Agency, 1974).

A magnetic map in itself is of little value for exploration. It becomes useful

only when it has been interpreted and used to discover geological structures.

Various approaches are used to make the interpretations, and these can be divided

into three groups.

Qualitative – inspection of the map

Profile methods – involving the study of profiles

Map methods – involving mathematical processes applied to map data.

For the purpose of this study, emphasis will be laid on qualitative and profile

methods of interpretation

S

N

W E

60

3.6.1 Qualitative interpretation:

This involves the description of the survey results and the explanation of the

major features revealed by a survey in terms of the types of likely geological

formations and structures that gave rise to the evident anomalies. Typically, some

geological information is available from outcrop evidence within the survey area

(or nearby) and very often the role of the geophysicist is to extend this geological

knowledge into areas where there is no outcrop information (i.e. extrapolation from

the known to the unknown) or to extend mapped units into the depth dimension

(i.e. to help add the third dimension to the mapped geology).

General inferences can be made from magnetic anomaly shapes

For example, in Fig. 3.5, anomaly B has the same form as anomaly A, but

has longer wavelength, and so must be deeper. Amplitude of B is greater than that

of A, so that B has greater magnetization.

Fig. 3.6: Example of magnetic anomaly signature and amplitude variation. (After Reeves, 2005).

61

(a) Qualitative profile interpretation

This may involve identifying zones with different magnetic properties.

Zones with low or no susceptibilities are areas of sedimentary rocks while high

variations are typical of basement regions.

(b) Qualitative map interpretation

Magnetic data acquired on grids can be displayed as maps as shown is Fig.

3.6 such as aeromagnetic map of Abakiliki, Nigeria. One can access from the

contour map, areas of sedimentary basin, igneous rocks, faults and fractures.

62

Fig. 3.7: A typical aeromagnetic map magnetic gridded map. (Source, Geology Survey Society of

Nigeria,1974).

3.6.2 Quantitative interpretation

This involves making numerical estimates of the depth and dimensions of

the sources of anomalies and this often takes the form of modeling of sources

which could, in theory, replicate the anomalies recorded in the survey. In other

390000 395000 400000 405000 410000 415000 420000 425000 430000 435000 440000

665000

670000

675000

680000

685000

690000

695000

700000

705000

710000

715000

ABAKALIKI

Okpoduma

Ejibafun

ALEBO

MFUMA

OBUBRA

ABBA OMEGA

IDEMBA IZA

OGURUDE

ABAKALIKI

SCALE, 1: 100,000

MAGNETIC LOW

CONTOUR LINE

CONTOUR INTERVAL 2.5 gammas

0 1 2 3 4Km

63

words, conceptual models of the subsurface are created and their anomalies

calculated in order to see whether the earth-model is consistent with what has been

observed, i.e. given a model that is a suitable physical approximation to the

unknown geology, the theoretical anomaly of the model is calculated (forward

modeling) and compared with the observed anomaly. The model parameters are

then adjusted in order to obtain a better agreement between observed and

calculated anomalies.

Depth Estimation

Often one of the most useful pieces of information to be obtained from

aeromagnetic data is the depth of the magnetic source (or rock body). Since the

source is usually located in the so-called 'magnetic basement' (i.e. the igneous and

metamorphic rocks lying below the - assumed non-magnetic - sediments), this

depth is also an estimate of the thickness of the overlying sediments. This is an

important piece of information in the early phases of petroleum exploration.

Sufficient depth estimates from a large number of magnetic sources allow the

depth of the basement to be contoured and this is then a rough isopach map of the

sediments. For this reason, several methods have evolved in the early days of

magnetic interpretation simply to estimate the depth of sources from their

anomalies without reference to any specific source models. Two simple manual

64

methods are described, together with the most sophisticated method which was

developed before computer based techniques became commonplace. The

‘wavelength’ of anomalies is primarily related to their depth of burial; shallow

bodies give sharp short wavelength anomalies, deep bodies give broad anomalies.

The amplitude of the anomalies, on the other hand, is directly related to the

strength of magnetization of the source.

(a). The ‘Straight-Slope’ Method

The tangent is drawn to the steepest gradient of an individual magnetic

anomaly on a section of profile. The horizontal distance, Ss, over which the

tangent line is coincident with the anomaly profile is measured. A depth estimate is

then obtained by multiplying Ss by a factor which usually falls in the range 1.2 to

1.6. For a vertical dyke-like body with various α values of width to depth-of-burial

(α = w/h). For an approximation which disregards the geometry of the source, it

may be said that: h = 1. 4 Ss ± 20%

The straight-slope method gives ambiguity on account of the indistinct

points where tangent and curve start to diverge. (Figure 3.7)

65

(b) Peter's 'Half-Slope' method

This is the most widely used. Here the same tangent is drawn as in the

straight-slope method but ambiguity is reduced by drawing two more tangents at

half the slope of the first (Fig. 3.7). Now the horizontal distance between these two

new points of tangency is given as S½. The depth estimate is : h = 0.63 S½ in the

case where h = 2 w. Note that S½ ≈ 2.2 Ss

Fig. 3.8: (a) Length of ‘straight slope’ of inflexion tangent;

(b) length between tangents at ‘half-slope. (After Reeves, 2005).

In present-day interpretation practice, these methods can only be considered

as 'rough-and-ready' first indications of depth, but they are still useful for the

66

geophysicist to have in mind when first confronted with an aeromagnetic map of a

new area, or with an anomaly on a field profile.

Profile methods of interpretation

After completing the qualitative study it is important to extract quantities

from the magnetic data. In oil survey, the basement depths are needed. In mineral

surveys, susceptibilities and dips are usually more important. This process of

interpretation has to follow a series of steps. From the location of an anomaly, we

know the approximate location and horizontal extent of the body which causes it.

Next from the form of the anomaly, the other parameters of the body, its shape and

depth, may be calculated. Finally, from the amplitude of the anomaly, the

magnetization may be determined.

3.7 Geophysical modeling software

The usual enormous data obtained in aeromagnetic survey has made it

almost impossible to analyze the data manually. The use of geophysical softwares

becomes paramount. Geophysical softwares such as Potent, Oasis Montaij, and

Saki are among the popular softwares employed in analysis of potential field work.

In our case we made use of Potent version 4.10.02.

67

Potent is a program for modeling the magnetic and gravitational effects of

subsurface structures. It provides a highly interactive 3-dimensional environment

that is well suited for:

• Detailed ore body modeling for mineral exploration. Potent is used by

mining and exploration companies world-wide. One can interpret surface,

airborne and down-hole data; separately or simultaneously.

• Stratigraphic modeling for petroleum exploration. Potent is an economical,

versatile and highly interactive tool for building models of complex layered

structures.

• Education. Educational establishments around the world use Potent for

teaching and research purposes.

• Environmental and ordnance work. Potent is used for industrial

decontamination studies and to help locate unexploded ordnance.

3.7.1 Potent Main concepts

The main concepts in Potent are:

• Observations

• Model

• Calculation

• Visualization

68

The primary function of the program is to bring these together in a coherent

and intuitive way.

Model

A Potent model consists of an assemblage of simple 2-D or 3-D geometrical

bodies such as cylinders and ellipsoids. The main task as an interpreter is to devise

a model that is geologically possible and also is consistent with the observed

physical values

Calculation

A model is consistent with the observed physical values if its calculated field

matches the observed values to some (subjective) degree of precision. One assesses

this by calculating the field (TMI in this case) due to the model and comparing it

with the observed field. The algorithms used in potent for magnetic calculations

for 2D version of Slap, dyke and polygonal prisms are based on well known and

readily derivable formula due to semi infinite slap (Grant and West, 1965). The

magnetic calculation for the sphere uses the fact that the magnetic effect at external

points is equal to that of a point dipole of the same magnetic moment located at its

center. The demagnetization effects are calculated using the correction formula

described in Emerson, et al, (1985). The formula for the magnetic effect of a 3D

rectangular prism was derived along lines similar to those of Bhattacharyya (1964).

69

Visualization

One subjectively assesses the "match" between the observed and calculated

physical values by visualizing them in the most appropriate manner. Visualization

is an inherent part of the modeling process.

Inversion modeling

Inversion modeling is a mathematical process that automatically adjusts

modeling parameters so as to improve the fit between the calculated field and the

observed field.

3.7.2 Axes used in Potent

Fig. 3.9: Different axes in potent.

70

Observations and model are positioned relative to axes (X,Y,Z) where Z, the

elevation of the observation, is directed vertically upwards. The depth (or rather

depth-below-datum) therefore corresponds to -Z.

The X and Y axes define a horizontal reference surface. Generally, it is

convenient to choose coordinates so that true north corresponds to +Y and east to

+X.

A third horizontal axis P is defined in the (X,Y) plane. This is the profile

axis onto which observations are projected in order to display them in profile form.

The origin of the P axis is the projection onto it of the first observation of the

profile. Each profile line that is displayed on a plan is the P axis for that profile.

The field axis F also is directed vertically upwards from the (X,Y) plane. It is used

for plotting observed and calculated field values when they are displayed in profile

form. The shape of a body is defined in its own coordinate system (A,B,C), in

which (0,0,0) is the reference point about which the body is defined. The position

of the body is defined as the (X,Y,Z) coordinates of its reference point.

3.7.3 Modeling Shapes

The following shapes were used in our modeling processes:

71

Dyke

Fig. 3.10: Axes of a dyke.

Slab

Fig. 3.11: Axes of a Slab.

72

CHAPTER FOUR

DATA ANALYSIS AND RESULTS

4.1 Methodologies

This study focused on the interpretation of aeromagnetic data from Ilesha

Southwest Nigeria. It involves the following methods:

4.2 Interpretation of total field data

The end result of a magnetic survey and data processing is usually a set of

magnetic profiles or a magnetic contour map, which may be preferred in digital

form. The duty of the interpreter here is to relate the anomalies to the subsurface

magnetic bodies. There are three basic approaches to interpretation challenges:

forward modeling, inverse method and data enhancement (Dobrin and Savit,

1988). Two of these approaches have been used.

4.2.1 Forward modeling

This is one of the most widely used methods of interpretation. It is the process

of interpreting the geometry of the source or the distribution of magnetization

within the source by trial and error modeling. If the observed and calculated field

does not fit, a further adjustment of the model is done until there is good agreement

between the calculated and the observed magnetic data.

73

4.2.2 Inverse method

Inversion modeling is a mathematical process that automatically adjusts

model parameters so as to improve the fit between the calculated field and the

observed field. An anomaly may be caused by an infinite number of permissible

sources. To minimize these infinite number down to a smaller number, some form

of constrains are placed on the modeling parameters. Generally, two parameter sets

govern the shape of the anomaly. They include; shape of the body and distribution

of magnetic material within the body. In the process of inverse modeling, all

parameters adjust automatically.

4.3 Data presentation

There are several methods of presenting magnetic data (Obot and Wof

1981), but only two of these methods were adopted in this study. These methods

are as summarized below:

Profiles: This is the oldest form of data presentation, but it has the

advantage of being able to show details that cannot be shown in grids based

presentations. The aeromagnetic profiles of the study area were generated from the

aeromagnetic map of Ilesha SW. A section of the map is shown in figure 4.1. Most

of the modeling bodies used were dykes.

74

Contour maps: This was used in the presentation of the magnetic data of

the area (Fig. 4.1).

Fig 4.1: A section of Aeromagnetic map of Ilesha sheet 243 SW, Nigeria. (Nigerian Geological Survey

Agency, 1974).

4.4 Data reduction

i) International Geomagnetic Reference Field (IGRF)

Modeling of our profiles was preceded by IGRF estimation. Here, the

latitude, longitude, flight altitude and the year our data was obtained were input in

to the potent software and the field estimated. This enabled us to work with the

local field of our study area. The values of the IGRF are:

Total field = 32525nT, Inclination = -8.00, Azimuth = - 5.9

0. Declination =

9.00

S

N

W E

75

ii) Removal of regional.

Before modeling the data, it is convenient to remove regional effect. For our

case, a degree one (1) regional effect was extracted from the data. Degree one (1)

was chosen because of the number of our data points and because our study area is

more of an inclined plane surface.

).()( 210 refref yyaxxaar −+−+= 3.1

X-ref, Y-ref are the X and Y coordinates of the geographical centre of the

dataset. They are used as X and Y offsets in the modeling body, a0, a1 and a2 are

coefficients, and r is the regional effect to be removed.

The regional may be defined as the value of the field which would exist if

there were no local disturbance due to the source we are trying to interpret. The

regional is actually unknown and may become quite subjective. It can be treated as

an additional variable in an interpretation, but reasonable limits may be set from

common sense provided by human intervention. (Reeves, 2005)

All anomalies occur as local variations imposed upon:

(a) Other local variations,

(b) Regional variations and

(c) Noise.

76

4.5 Data modeling

The digitized data was loaded into the potent software version 4.10.02. After

regional extraction and IGRF was removed, certain modeling parameters like

susceptibility range, depth, dip, plunge and so on (depending on the type of body

used) was input into the modeling software and the data inverted. This was done

severally by trial and error until there was a close match between the observed and

Calculated TMI (Total magnetic intensity).

4.6 Data interpretation and results

Traditionally in potential field measurements, data are displayed in the form

of contour maps. Joints and faults are normally represented as elongated closed

contours. Faults of regional dimension are characterized by alignments of the

contour features, (Onyedim, 2007).

At the eastern part of the map (Fig 4.1), there is an obvious NE - SW trend

and at the western end there is a strong N – S trend. This clearly shows the Ifewara

fault zone, which is the dominant feature in Ilesha Southwest (Folami, 1992,

Elueze, 1986). Here, most of the lithology boundaries are tectonic (Boesse and

Ocan, 1988). Further confirmation of the N – S trending of the fault is evident in

the work of Onyedim (2007), who applied steerable filters in the enhancement of

77

the Ifewara fault zone. Other trending includes NE – SW, NW – SE as evident in

the aeromagnetic map. (Fig.4.1)

A quantitative data interpretation of the study area is given below.

Profile 1

The total magnetic intensity obtained for this profile has a minimum

negative peak value of – 59.93nT to a positive maximum peak value of 61.12nT.

Two rock units were delineated near Ajibodu and Itagunmodu axis with magnetic

susceptibility values of 0.004 and 0.07.

Fig. 4.2: Observed and calculated TMI, Profile one.

They are:

Quartzite (Metamorphic)

This forms the first rock unit. It has a slab – like shape with depth to top of

magnetic source of 0.5m. This merely depicts an outcrop that may be caused by

tectonic activities over geologic time.

Calculated

Observed

78

Amphibolites

This forms the second rock unit in this profile. It also has same slab – like

shape with depth to top of magnetic anomaly being 16.7m dipping at 10.50.

Table 4.1: Results of profile one.

K value Types of

bodies

Depth

(m)

Dip

(deg)

Plunge

(deg)

Strike

(deg)

Remanent Magnetization

Rem.H Rem.Az Rem. Ic

0.004 Slab 0.5 31.0 -81.0 8.2 -0.67 21.1 -43.4

0.07 Slab 16.7 -10.5 - 71.1 - 1.4 18.91 0.5 2.2

Profile 2

Fig. 4.3: Observed and Calculated TMI, Profile two.

The magnetic signatures along this profile show minimum negative

amplitude of – 45.36nT and maximum amplitude of 62.70nT. The susceptibilities

obtained here are 0.0849, 0.0885 and 0.0205. Three rock units were delineated

Calculated

Observed

79

Amphibolites Schist

This forms the first and second rock unit along this profile. The depth to top

of magnetic anomaly is 13.1m and 34.2m.

Quartz Schist

This forms the third rock unit. It has depth of 7.4m. The susceptibility value

is 0.0205.

Table 4.2: Results of profile two.

K value Types of

bodies

Depth (m) Dip

(deg)

Plunge

(deg)

Strike

(deg)

Remanent Magnetization

Rem.H Rem.Az Rem. Ic

0.0849 Dyke 13.1 26.8 27.8 37.3 -0.61 -15.0 23.7

0.0885 Dyke 34.2 -101.2 22.5 32.3 -2.25 1.23 -3.2

0.0205 Dyke 7.4 -6.9 -4.8 11.7 0.900 0.1 0.0

Profile 3

Fig. 4.4: Observed and Calculated TMI, Profile three.

Calculated

Observed

80

A total magnetic intensity with minimum negative peak value of – 106nT

and maximum positive peak value of 75.3nT were obtained. The modeling bodies

are two dyke-like bodies in nature and their susceptibility value is 3.0, thus one

rock unit was delineated.

Schist

This is the only rock unit delineated in this profile with dept of burial of

about 0.9m and 2.2m. The magnetic signatures obtained here are similar to those of

profile 4.

Table 4.3: Results of profile three.

K value Types of

bodies

Depth (m) Dip

(deg)

Plunge

(deg)

Strike

(deg)

Remanent Magnetization

Rem.H Rem.Az Rem.Ic

3.0 Dyke 0.9 -96.7 21.1 -40.2 58.78 23.9 -43.4

3.0 Dyke 2.2 73.8 10.0 -5.4 26.94 -4.3 -8.0

Profile 4

Fig. 4.5: Observed and Calculated TMI, Profile four.

Calculated

Observed

81

Two bodies were used in modeling this profile; dyke and slab. The magnetic

intensity here shows a minimum negative amplitude of -266.7nT and maximum

positive amplitude of 169.9nT. Susceptibilities of 0.0042 and 0.0035 were

obtained. This shows that the area is characterized by metamorphic rocks. The rock

unit found here is Quartz Schist.

Quartz Schist

The depths to top of magnetic anomaly here are 8.4m and 1.0m.

Table 4.4: Results of profile four

K value Types of

bodies

Depth (m) Dip

(deg)

Plunge

(deg)

Strike

(deg)

Remanent Magnetization

Rem.H Rem.Az Rem.Ic

0.0042 Dyke 8.4 18.0 98.5 56.6 3.001 -125.1 15.5

0.0035 Slab 1.0 -9.0 39.9 15.315.3 7.149 -19.8 -0.6

Profile 5

Fig. 4.6: Observed and Calculated TMI. Profile five.

Calculated

Observed

82

The magnetic signature observed here are similar to those of profile 2. The

major feature delineated here is the Ifewara fault zone. It has a negative minimum

total magnetic intensity of -84.35nT and a positive maximum total magnetic

intensity of 179.43nT. Susceptibilities here are; 0.01 and 0.03. Three dyke-like

bodies were used to model this profile. Two of which have susceptibilities of 0.01

and the third body has a susceptibility of 0.03. Two rock units were delineated in

this area:

Quartz

The depth to top of magnetic source is 2.3m and 23.9m.

Schist

The depth to top of magnetic anomaly here is 12.0m.

Table 4.5: Results of profile five.

K value Types of

bodies

Depth (m) Dip

(deg)

Plunge

(deg)

Strike

(deg)

Remanent Magnetization

Rem.H Rem.Az Rem.Ic

0.01 Dyke 2.3 42.5 -87.2 -110.2 -0.67 -3.7 7.4

0.01 Dyke 23.9 -11.3 84.1 -48.8 -0.85 10.2 30.1

0.03 Dyke 12.0 -35.5 14.0 8.2 19.91 -4.2 -5.9

83

Profile 6

Fig. 4.7: Observed and Calculated TMI, Profile six.

This profile cuts across Ilesha town, Irekete and Iregun areas. It has a minimum

negative total magnetic intensity of -625.5nT and maximum positive peak value of

71.8nT. Susceptibility of 0.3 reveals only one type of rock unit with a dyke like

shape.

Schist

The depth to magnetic source here is 11.5m. The nature of the magnetic signature

shows that this area is characterized by a fault fracture trending NE – SE.

Table 4.6: Results of profile Six.

K value Types of

bodies

Depth (m) Dip

(deg)

Plunge

(deg)

Strike

(deg)

Remanent Magnetization

Rem.H Rem.Az Rem. Ic

0.3 Dyke 11.5 8.7 -84.2 -107.0 110.06 -91.1 57.8

Calculated

Observed

84

Table 4.7 Summary of results

Profiles X(m) Y(m) No. of

Bodies

k Value

(SI)

Types of

Bodies

Depth

(m)

Dip

(deg).

Plunge

(deg).

Strike

(deg).

Remanent Magnetization

Rem.H Rem.Az Rem.Ic

Profile 1 1.6

0.6

0.2

0.2

2 0.004

0.07

Slab

Slab

0.5

16.7

31.0

-10.5

-81.0

-71.1

8.2

-1.4

-0.672

18.91

5

21.1

0.5

-43.4

2.2

Profile 2 16.9

21.4

23.6

20.2

1.2

0.1

3 0.085

0.088

0.021

Dyke

Dyke

Dyke

13.1

34.2

7.4

26.8

-101.2

-6.9

27.8

22.5

-4.8

37.3

32.3

11.7

-0.619

-2.252

0.900

-15.02

1.23

0.1

23.7

-3.2

0.0

Profile 3 1.6

4.9

0.2

-0.3

2 3.0

3.0

Dyke

Dyke

0.9

2.2

-96.7

73.8

21.1

10.0

-40.2

-5.4

58.78

26.94

23.9

-4.3

-43.4

-8.0

Profile 4 4.5

7.1

-0.5

-0.7

2 0.0042

0.0035

Dyke

Slab

8.0

1.0

18.0

-9.0

98.5

39.9

56.6

15.3

3.00

7.14

80.34

-13.5

-125.1

-19.8

Profile 5 2.7

4.4

19.8

1.8

-0.7

0.8

3 0.01

0.01

0.03

Dyke

Dyke

Dyke

2.3

23.9

12.0

42.5

-11.3

-35.5

-87.2

84.1

14.0

-110.2

-48.8

8.2

-0.679

-0.855

19.919

-3.7

10.2

-4.2

7.4

30.1

-5.9

Profile 6 1.2 0.0 1 0.3 Dyke 11.5 8.7 -84.2 -107.0 110.06 -91.1 57.8

85

CHAPTER FIVE

CONCLUSIONS AND RECOMMENDATIONS

5.1 Conclusions

Aeromagnetic data of Ilesha Southwest, Nigeria has been interpreted using

Potent version 4.10.02, geophysical software to detect the presence of anomalous

bodies and their respective depths. Subsurface modeling of these profiles have

revealed 13 anomalous bodies of either slab – like or dyke – like shapes, mostly of

amphibolites, quartzite, schist, and quartz. This is in line with the basic rock units

that are characterized by our study area.

The results obtained have further confirmed the presence of Ifewara fault

zone in the western part of Ilesha, trending NE – SW. This is in line with the

submissions of Onyedim (2007) who delineated a major fault trending NNW –

SSW in Ilesha SW, using steerable filters. The results so far obtained have further

justified the effectiveness of hand digitized data as submitted by Bath (1974).

Quantitatively, results obtained have shown maximum depth to anomalous

source of 34.2m and minimum depth of 0.5m. This confirms the result obtained by

Momoh et al. (2008) and Alagbe et al. (2010). While the former obtained depth

ranges of 0.3m to 41.3m, the later obtained depths ranging from 3.0m to 21.0m.

The depth range agrees with the result obtained by Adelusi (2002) who used

electrical resistivity method and obtained 2.3m – 21.2m. Geologically, it is

86

expected that depths within Ilesha should be shallow since we are dealing with a

basement complex. Hydrocarbon search is ruled out because of shallow depths but

ore minerals have potential on account of high susceptibilities obtained in the

course of this study.

It is important to state that rocks such as quartzite, amphibolites schist, and

quartz schist have economic importance and uses. For instance schist can be used

for flooring ground after building and it can be used for decorating gardens.

Quartz schist can be used for decoration purposes, for carving materials, as an

abrasive in grinding, sand blasting and cutting softer stones. Amphibolites on the

other hand are local host of gold mineralization.

5.2 Recommendations

These results and findings may further be confirmed by carrying out ground

magnetic survey of the study area. It is also important to carry out gravity survey in

the area to confirm the results of magnetic survey.

87

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