Interpretations of Aeromagnetic Data from Ilesha Southwest Nigeria.

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
1
DEDICATION
To the glory of God and my parents, Mr. &Mrs. Robert Bassey Umera and
Princess Onne Ijim Agbor
2
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
3
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
4
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.
5
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
1.3
Location of study area
--------------------------------------------- 3
1.4
Geology of Study area
--------------------------------------------- 4
1.5
Objectives of present studies
--------------------------------------------- 5
---------------------- 2
CHAPTER TWO
----------------------------------------------
LITERATURE REVIEW
--------------------------------------------- 6
2.1
Review of previous geophysical surveys in Ilesha
2.2
Basic concepts and definitions -------------------------------------------- 7
6
-------------- 6
2.2.1 Magnetic poles, force and permeability
2.2.2 Magnetic field strength
------------------------------ 8
---------------------------------------------- 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
--------------------------------------------- 16
The Earth’s total field
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
2.7
Total field anomaly
CHAPTER THREE
-------------------------------------- 26
---------------------------------------------- 32
-----------------------------------------------
DATA ACQUISITION AND INTERPRETATION
----------------------- 35
3.1
Magnetic Instruments
---------------------------------------------- 35
3.2
Airborne magnetometers
---------------------------------------------- 38
3.3
Basic aeromagnetic instruments
-------------------------------------- 40
7
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
-------------------------------------- 52
Geophysical modeling software
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
8
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
9
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.
10
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 J i , remanent
J r and resultant magnetization components.
------------------------
15
characteristics of homogeneous space.
---------
19
Fig. 2.5: The elements of the earth’s magnetic components.
---------
21
Fig. 2.4: magnetic field of the earth having
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
----------
28
Fig 2.8: Relationship and notation used to
derive magnetic effect of a dipole.
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
11
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
and amplitude variation.
----------
46
Fig. 3.7: A typical aeromagnetic map magnetic gridded map.
----------
48
(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
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
Fig. 3.6: Example of magnetic anomaly signature
Fig. 3.8: (a) Length of ‘straight slope’ of inflexion tangent.
Fig 4.1: A section of Aeromagnetic map
12
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
13
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
14
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
15
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.
16
• 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
x
x
17
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.
18
1.2. Other minor rocks according to Kayode (2006), Folami (1992) and Rahaman
(1976) are garnet, quartz chlorite bodies and dolorites.
22
11
33
44
66
55
77
15Porphyritic Granite
7
2 Quartz Schist
2
4 Amphibolites Schist
3
5 Migmatite gneiss
3 Granite gneiss
1
6 Quartzite
7 Amphibolites
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.
19
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.
20
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).
21
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.
22
Mathematically, magnetic force is represented by:
Fm =
p1 p2
µr 2
(2.1)
where µ is a constant of proportionality known as magnetic permeability, p1 , p2 ,
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 constant G , 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 µ o
• Instead of m1 , m 2 , as in gravitational force expression, p1 , and p2 , 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 P0 a distance
r away is:
23
H=
(2.2)
F
Po
Substituting equation (2.1) into (2.2), assuming P1 = P0 and P2 = P, we have that:
H=
(2.3)
P
µr 2
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:
r
r
M = Plrr , where P = IA and r is unit vector
(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
24
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
r
r
M M
I=
=
,
LA V
(2.5)
where M is the magnetic moment of a sample of length L and cross sectional area
A . I is expressed in AM −1 . In the cgs. system, the intensity of magnetization is
expressed in emu cm -3 (emu = electromagnetic unit), where 1 emu cm -3 = 1000 AM −1 .
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
r
r
magnetization I to the strength of the inducing magnetic field H through the
expression:
25
r
r
I = kH
(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
r
the inducing field H , 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:
r
r
H ′ = 4πI
(2.7)
r
The magnetic induction A is defined as the total field within the body. It is given
as:
r r r
A = H + H′
(2.8)
By substituting (2.7) into (2.8), we have
r r
r r
r
A = H + 4πl = H + 4πkH
r r
A = H (1 + 4πk ) , where (1 + 4πk ) = µ , so that;
26
(2.9)
r
r
A = µH
We can define magnetic permeability in section. 2.2.1 as the ratio of
r
r
magnetic induction A to magnetic field strength H
r
A
µ= r
H
(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
27
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
28
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 J i
magnetization components. (After Kearey and Brooks, 1991).
29
,
remanent J r and resultant
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.
T = To + ∆Ta , where T is the total field vector in the vicinity of the magnetic rocks,
To is the earth’s undisturbed field vector and ∆Ta 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
30
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:
U=
p
, where U is the potential.
µr
(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:
U =−
I dV
,
ρG di
(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
31
Hs = −
dU
I d  dV
=

ds Gρ ds  di

.

(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:
Hx = −
dU
I d  dV 
=

.
dx Gρ dx  dz 
(2.14)
The vertical component Hz will be
dU
I d  dV 
=


dz Gρ dz  dz 
dU
I d 2V
Hz = −
=
.
dz Gρ dz 2
Hz = −
2.4
(2.15)
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.
32
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
33
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.
34
Meridian
y
Vertical
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:
35
X = F cos I cos D
Z = FSinI
Y = F cos ISinD
tan I =
H = F cos I
F 2 = X 2 +Y 2 + Z 2
Z
H
D = arctan
I = arctan
Y
X
Z
(X
2
+Y 2
)
1/ 2
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
36
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)
37
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
Dolomite
AVERAGE MAGNETIC
SUSCEPTIBILITY (SI).
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
38
Acidic Igneous
0.00817
Table 2.2 magnetic susceptibilities of some selected mineral (Telford et. Al, 1990).
ROCKS
Quartz
Rock salt
Gypsum
Coal
Clay
Chalcopyrite
Cassiterite
Pyrite
Limonite
Harmatite
Chromite
Pyrrhotite
Ilmenite
Magnetite
AVERAGE MAGNETIC
SUSCEPTIBILITY (SI)
-0.00001
-0.00001
-0.00001
0.00002
0.00025
0.00040
0.00113
0.00163
0.00276
0.00691
0.00754
1.57080
1.88500
6.28300
Fig. 2.6: Histogram showing susceptibilities of different rock types. (After Telford et al, 1990)
39
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.
+x
Magnetic
+x
y
-x
θ
x
+p
c
+
z
-
-x
Fig 2.7: Relationship and notation used to derive the magnetic effect of a single pole. (After Burger, 2006).
The potential V of the monopole is;
V=
p
where p is the pole strength given as p = IA , I is the magnetic intensity and
r
A is the cross sectional area.
V=
(2.16)
p IA kFe A
=
=
r
r
r
(2.17)
But r = (x 2 + y 2 + z 2 )
1/ 2
40
V=
kFe A
(x
2
+ y2 + z2
)
1/ 2
(2.18)
.
The magnetic field is determined in a given direction by differentiating V in
that direction, so that
ZA = −
ZA =
KFe A
dV
d
=−
dz
dz x 2 + y 2 + z 2
(
zkFe A
(x
2
+ y2 + z2
)
3/ 2
)
1/ 2
.
(2.19)
(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:
H AX = −
xkFe A
dV
=
2
dx
x + y2 + z2
)
H AY = −
ykFe A
dV
=
2
dy
x + y2 + z2
)
(
(
3/ 2
.
(2.21)
3/ 2
.
(2.22)
The total anomalous field is calculated using the form of equation of total
field anomaly
FAT = Z A sin i + H A cos i
41
(2.23)
Magnetic effect of a dipole
(b)
Consider Figure 2.8 below;
Magnetic North
-x
+x
x
X=0
θ2
P
Φ1
Zn
Φ2
r1
-p
Zp
r2
L
+p
θ
Zn
a
L
θ -90 b
Fig 2.8: Relationship and notation used to derive magnetic effect of a dipole (After Burger, 2006)
a = L cos(180 − φ )
r1 = ( x 2 + z n )1 / 2
b = L sin(180 − φ )
rp = [( x − a ) 2 + z 2 p ]1/ 2
z p = zn + b
sinθ 2 =
zp
rp
sinθ1 =
cos θ 2 =
zn
x
, cos θ1 =
rn
rn
( x − a)
r2p
Lets us assume that the dipole is magnetized along its axis (parallel to its
length).
42
The magnetic field intensity at P due to the negative pole of the dipole is:
R A1 = +
p kFe A
= 2
2
r1
r1
(2.24)
kF A
p
= − e2
2
r2
r2
(2.25)
And that due to the positive pole:
R A2 = −
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
Z A1 = R A1 sin φ1
H A1 = R A1 cos φ1
Z A 2 = R A 2 sin φ 2
H A1 = R A1 cos φ 2
(2.26)
Z A1 = Z A1 + Z A2
(2.27)
H A = H A1 + H A 2
H A1 =
Z A1 =
kFe A
r1
2
sin φ1
kFe A
cos φ1
2
r1
Z A2 = −
kFe A
r2
2
H A2 = −
kFe A
cos φ2
2
r2
(2.28)
sin φ 2
 sin φ sin φ 2 
Z A = kFe A 2 1 −
2 
r2 
 r1
 cos φ cos φ 2 
H A = kFe A 2 1 −
2 
r2 
 r1
The total field is obtained, using equation 2.23
43
(2.29)
(2.30)
FAT = Z A sin i + H A cos i
(c)
Magnetic effect of a sphere
This is somewhat more complex in derivation than that of a dipole.
+
-
Magnetic
x
P
X=
Z
i
FE
R
Fig. 2.9: Notation used for the derivation of magnetic field anomalies over a uniformly magnetized sphere.
(After Burger, 2006).
In deriving an equation for the magnetic effect of a sphere, we shall employ
Poisson relation, given as:
V =−
I dU
.
ρG di
(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
ZA = −
dV
I d 2U
=
.
dz ρG dz 2
44
(2.32)
HA = −
dV
1 d  dU
=

dx ρG dx  dx

.

(2.33)
Recall that the gravitational potential of a sphere is given as:
U=
GM
4
where M = ρV = ρ πR 3
3
r
4
4
Gρ πR 3
Gρ πR 3
3
3
U=
=
1/ 2
2
r
x + z2
(
)
(2.34)
4
Gρ πR 3 ( z − x )
dU
3
=
,
3/ 2
dz
x2 + z2
(
)
4
Gρ πR 3 ( 2 z 2 − x 2 )
d U
3
=
,
5/2
2
dz
x2 + z 2
2
(
)
The vertical component becomes
4 3
πR I (2 z 2 − x 2 )
ZA = 3
.
5/ 2
x2 + z 2
(
(2.35)
)
Similarly,
4

Gρ πR 3 ( z − x )

I d 
3
HA =
ρG dx  x 2 + z 2 3 / 2


(
4 3
πR l (2 x 2 − z 2 )
HA = 3
.
5/ 2
z 2 + x2
(
)
45
)


.



(2.36)
(2.37)
In a more general case, where the sphere will be uniformly magnetized and
the earth’s field is inclined, we have that:
4 3

 πR KF  sin i


 
3z 2
3 xz
3

ZA = 
− 2
cos i  − 1.

2
2 1/ 2
2 1/ 2
2
2 5/ 2
x +z
(x + z )
 
(x + z )
(
)
4 3

 πR KF  cos i

 

3 xz
3x 2
3




.
HA = 
−
1
−
tan
i

3
/
2
2
2
1
/
2
2
2
1
/
2

 (x + z )
 (x + z )
x2 + z2





(
)
(2.38)
(2.39)
Total field anomaly
2.7
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.
HE + HA
FEu =Undisturbed main
field
FAT = 5nT
ZE + Z A
FE = 55000nT
FEU + FAT = FET
(b)
FE = 55005nT
FET = Main field
Plus anomalous field
FA = 12nT
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
46
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
(FE
+ FAT ) = (Z E + Z A ) + (H E + H A )
2
2
2
(2.46)
By expansion, and considering that FE >>> FA , and ignoring FAT 2 , Z A 2 , H A 2 we have
2
2
2
FE + 2 FAT FE = Z E + 2 Z E Z A + H E + 2 H E H A
(2.47)
But :
2
2
FE = Z E + H E
2
Then (2.47) becomes
2 FAT FE = 2Z E Z A + 2 H E H A
FAT FE = Z E Z A + H E H A
FAT =
ZEZA HE H A
+
FE
FE
Z 
H 
FAT = Z A  E  + H A  E 
 FE 
 FE 
(2.48)
By applying the relationship among the geomagnetic elements in sec. 2.3.1,
where
ZE
HE
= sin i ,
= cos i then finally we have
FE
FE
FAT = Z A sin i + H A cos i
47
(2.49)
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:
FAT = Z A sin i + H A cos α cos i
48
(2.50)
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.
49
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
50
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
51
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:
52
•
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.
53
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.
54
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.
55
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
56
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.
57
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.
58
N
E
W
3.6
Interpretation of aeromagnetic data
S
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
59
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).
60
(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.
61
ABAKALIKI
715000
710000
ALEBO
705000
Okpoduma
700000
IDEMBA IZA
695000
MFUMA
MAGNETIC LOW
ABAKALIKI
690000
OGURUDE
685000
CONTOUR LINE
OBUBRA
Ejibafun
680000
CONTOUR INTERVAL
ABBA OMEGA
675000
670000
665000
390000
395000
400000
405000
410000
415000
420000
425000
430000
435000
440000
SCALE, 1: 100,000
0
1
2
3
4Km
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
62
2.5gammas
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
63
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)
64
(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
65
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.
66
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
67
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).
68
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.
69
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:
70
Dyke
Fig. 3.10: Axes of a dyke.
Slab
Fig. 3.11: Axes of a Slab.
71
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.
72
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.
73
Contour maps: This was used in the presentation of the magnetic data of
the area (Fig. 4.1).
N
E
W
S
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 = - 5.90. Declination =
9.00
74
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.
3.1
r = a 0 + a1 ( x − x ref ) + a 2 ( y − y ref ).
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.
75
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
76
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.
Calculated
Observed
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.
77
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 Depth
Dip
Plunge
Strike
Remanent Magnetization
bodies
(m)
(deg)
(deg)
(deg)
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
Calculated
Observed
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
78
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 Depth (m)
Dip
Plunge
Strike
Remanent Magnetization
bodies
(deg)
(deg)
(deg)
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
Calculated
Observed
Fig. 4.4: Observed and Calculated TMI, Profile three.
79
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
bodies
3.0
Dyke
3.0
Dyke
of Depth (m)
Dip
(deg)
Plunge
(deg)
Strike
(deg)
Remanent Magnetization
Rem.H Rem.Az Rem.Ic
0.9
-96.7
21.1
-40.2
58.78
23.9
-43.4
2.2
73.8
10.0
-5.4
26.94
-4.3
-8.0
Profile 4
Calculated
Observed
Fig. 4.5: Observed and Calculated TMI, Profile four.
80
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 Depth (m)
bodies
Dip
Plunge
Strike
Remanent Magnetization
(deg)
(deg)
(deg)
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
Calculated
Observed
Fig. 4.6: Observed and Calculated TMI. Profile five.
81
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 Depth (m)
bodies
Dip
Plunge
Strike
Remanent Magnetization
(deg)
(deg)
(deg)
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
82
Profile 6
Calculated
Observed
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 Depth (m)
bodies
0.3
Dyke
11.5
Dip
Plunge
Strike
Remanent Magnetization
(deg)
(deg)
(deg)
Rem.H Rem.Az Rem. Ic
8.7
-84.2
-107.0
110.06
83
-91.1
57.8
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
84
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
85
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.
86
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