ABSTRACT
In order to identify the mineral prospects and potential for geothermal energy in the Lower Benue Trough South-Eastern Nigeria, through the determination of heat flow, geothermal gradient and the sedimentary thickness of the subsurface materials. Nine sheets of high resolution aeromagnetic data were used (Sheet 287 Nsukka, Sheet 288 Igunmale, Sheet 289 Ejekwe, Sheet 301 Udi, sheet 302 Nkalagu, Sheet 303 Abakaliki, sheet 312 Okigwe, sheet 313 Afikpo and sheet 314 Ugep). These sheets were merged into one composite sheet. The total magnetic intensity map, depth to basement maps, gradient maps, heat flow maps were produced through quantitative analysis. The result of the analysis shows that the magnetic intensity value ranges from -47.3 nT to 151 nT, and this is observed in the northern part near Bende with a prominent NE-SW trending Long wavelength anomaly shape. The Total magnetic intensity map of the area was reduced to the magnetic equator, this is because the area of the acquired data is along the equatorial zone. The reduction to equator (RTE) map ranges in value from -34.84 to 135.26 nT, the areas with high observed intensity (pink coloration) are around Okigwe, Isuikwua, Afikpo area (SW part of the composite map) and the areas west of Nsukka, low magnetic intensity was observed in the areas near Udi (western part of the composite map) and Eastern part of Igunmale, Ayamelum and the northern part of Nsukka (Blue coloration), the RTE map comprised of both the regional and the residual anomaly. the regional anomaly map shows variation in magnetic intensity values from 28.3 to 77.7 nT. The regional anomaly map also shows the regional trends of the area as observed trend is in the NE-SW and E-W. The residual anomaly map ranges in intensity value from -13.866 nT to 7.550 nT. This ranges shows the variation of these values within a short ranges. In all, it is confirmed that the occurrence of igneous bodies outcrops covered by thin overburdens within these areas also estimated the depth to basement to range from 2.2 to 4.8 km. The Heat flow calculated over the area of study shows variation from one place to another, these values range from 103.83 mW/m2 to 254.4 mW/m2 with an average value of 167.5 mW/m2. The geothermal gradient value of the area ranges from 42 to 102 OC/km with an average of 73 oC/km. The curie depth value for the area ranges from 5.6 km to a depth of 13.5 km, and average depth of 8.8 km. In conclusion, the use of remotely sensed (Airborne) magnetic data might not provide the 100 percent accuracy required to estimate the exact geothermal gradient and the heat flow, hence a test hydrocarbon well can be sunk and further geophysical and geochemical exploration is highly required.
TABLE OF CONTENTS
Title page i
Declaration ii
Certification iii
Dedication iv
Acknowledgements v
Table of contents vi
List of Tables ix
List of Figures x
Abstract xii
CHAPTER 1:
INTRODUCTION
1.1 Background of Study 1
1.1.1 Heat flow and geothermal
gradient 3
1.1.2 Sedimentary
thickness 8
1.1.3 Aeromagnetic
method 9
1.1.4 The
principle of magnetism 10
1.1.5 Geomagnetic
Elements 12
1.2 Statement
of the Problem 15
1.3 Aim
and Objectives 15
1.4 Justification 16
1.5 Scope 16
1.6 The Study Area 16
CHAPTER
2: LITERATURE REVIEW
2.1 Geology
of the Study Area 18
2.2 Review
of Previous geophysical and aeromagnetic studies in the
Lower Benue Trough 27
2.3 Overview
of Geophysical Methods 34
2.3.1 Gravity
method 35
2.3.2 Electrical
resistivity method 35
2.3.3 Induces polarization method 35
2.3.4 Self potential method 36
2.3.5 Electromagnetic method 36
2.3.6 Radiometric
method 37
2.3.7 Seismic
method 37
2.3.8 Magnetic
method 37
2.4
Magnetic Survey Methods 41
2.4.1 Ground
magnetic survey 41
2.4.2 Marine
(sea-borne) magnetic survey 42
2.4.3 Aeromagnetic
survey 42
2.5 Fundamental and Basic Concept
of Magnetic Prospecting 43
2.5.1 Magnetic pole 43
2.5.2 Magnetic force 44
2.5.3 Magnetic
field strength 44
2.5.4 Magnetic
intensity, or magnetization 45
2.5.5 Magnetic moment 45
2.5.6 Magnetic
induction 46
2.5.7
Relative permeability 46
2.5.8 Magnetic
susceptibility 47
2.6 Units of
Magnetism 49
2.7 Solid
Earth Structure 49
2.8 The
Geomagnetic Field 51
2.8.1 Main
magnetic field 52
2.8.1.1 The
crustal field 52
2.8.1.2 The
core field 53
2.8.2 Magnetic
component 53
2.8.3
International geomagnetic
reference field (IGRF) 55
2.8.4 The Earth’s
total field 56
2.9 Nature
of the geomagnetic field 57
2.9.1 Variations in the geomagnetic
field 57
2.10 Magnetism of Rocks and Minerals 58
2.10.1 Magnetic susceptibility of rocks
and minerals 60
2.10.2 Induced and remanent magnetization
of rock 61
2.10.3 Types of normal remanent magnetization
(N.R.M) 62
2.11 Magnetic
anomalies 62
2.11.1 Total field anomaly 63
2.12 Magnetic
effects of simple shapes 65
2.12.1 Magnetic
effect of an isolated pole (monopole) 65
2.12.2 Magnetic
effect of a dipole 67
2.12.3 Magnetic
effect of a sphere 68
2.13 Interpretation Methodologies 70
2.13.1 Qualitative interpretation 70
2.13.1.1 Reduction to magnetic equator or pole 71
2.13.1.2 Regional-residual filter 72
2.13.1.3 Vertical derivatives 72
2.13.1.4 Analytical
signal 73
2.13.1.5 Upward
continuation 74
2.13.2 Quantitative analysis 74
2.13.2.1 Spectral analysis 75
2.13.2.2 Curie-point depth (CPD) 78
2.13.2.3 Geothermal gradient 79
2.13.2.4 Heat flow 80
CHAPTER
3: MATERIALS AND METHODS
3.1 Data Source 81
3.1.1 The survey pattern and equipment specification 81
3.2 Material Used 83
3.2.1 Interpretation methodologies 83
CHAPTER
4: RESULT AND DISCUSSION
4.1 Qualitative Analysis 84
4.1.1 Total magnetic intensity map
(TMI) 84
4.1.2 Reduction to magnetic equator
(RTE) 87
4.1.3 Regional anomaly map 89
4.1.4 Residual anomaly map 91
4.1.5 Vertical derivatives 93
4.1.6 Analytical signal map 97
4.1.7 Upward continuation 99
4.2 Quantitative Analysis 106
4.2.1 Spectral analysis 106
4.2.2 Currie depth point 112
4.2.3 Geothermal gradient 114
4.2.4 Heat flow 116
CHAPTER
5
CONCLUSION
AND RECOMMENDATION
5.1 Conclusion 120
5.1 Recommendations 123
REFERENCES 124
APPENDIX 135
LIST OF TABLES
1.1: Geophysical
Methods (Kearey and Brooks, 2002). 2
2.1: Geophysical Surveying Methods
(Okwueze et al, 2003) 39
2.2: Magnetic
susceptibility of various rocks and minerals 48
2.3: Layers of the earth 51
4.1: Table
of Calculations 118
LIST OF FIGURES
1.1: Heat
transfer mechanisms within the Earth, along with the
percentage
amount of heat flow in each layer 4
1.2: Heat
flux through a slab 5
1.3: Estimated Temperature Profile
of Earth’s Interior 7
1.4: The
magnetic flux surrounding a bar magnet 10
1.5: Geomagnetic Elements 13
1.6: Geology map of the study area 17
2.1: Location of the Benue Trough in the West
African Rift System
(WARS) and Central African Rift System (CARS) 19
2.2: Location of the Benue Trough of Nigeria modified
from 20
2.3a: Idealized
N–S stratigraphic cross-section across the Chad Basin–
Benue
Trough – Niger Delta (After Obaje, 2009) 24
2.3b: Summary of the Stratigraphic succession of southeastern
Nigerian sedimentary basins 25
2.4:
Stratigraphic and
lithologic section of Lower Benue Trough and Anambra Basin (Modified after
Akaegbobi et al., 2000) 26
2.5: Magnetic pole 43
2.6: The structure of the earth (www.earthonlinemedia.com) 50
2.7: Definition of the geomagnetic
field elements (Parkison, 1983) 54
2.8: Ferromagnetism 59
2.9: Ferrimagnetism 59
2.10: Antiferromagnetism 60
2.11: Vector diagram illustrating
relationship between induced Ji,
remanent
Jr and resultant magnetization components 61
2.12: Relationships of the total field anomaly 64
2.13:
Relationships and notations
used to derive the magnetic effect of
a single pole (Burger,
2006) 66
2.14:
Relationship and notation
used to derive the magnetic field of a
dipole 67
2.15:
Notation
used for the derivation of magnetic field over a
uniformly
magnetized sphere 68
2.16: The Composite Total magnetic
Intensity map over the area, each
of the 1:100,000
sheet demarcated 70
2.17: Example
of magnetic anomaly signature and amplitude variation 73
2.18: Pictorial representation of upward
continuation technique in
Cartesian
coordinate system 74
2.19: The three components of energy
spectrum for interpretation 78
2.20: The Composite Total magnetic Intensity map over the area, 82
4.1: The Total magnetic intensity
map of the area of study (Merged composite sheet) 86
4.2: The total magnetic intensity
map reduced to magnetic equator
(RTE) 88
4.3: The Regional magnetic Anomaly
map of the area 90
4.4: The residual Magnetic anomaly
map of the area (the areas with
short wavelength
remanent magnetism resulting from near
surface magmatic
emplacement in Black Polygon) 92
4.5a: The first vertical derivative map
of the area 94
4.5b: The second vertical derivative map of the area 95
4.5c: The grey scale image of the
first vertical derivative map showing
the defined
orientation 95
4.6: The Analytical signal map of
the area (Colour shaded image) 98
4.7a: The upward continuation map UC
distance of 50 m 100
4.7b: The upward continuation map UC
distance of 500 m 101
4.7c: The upward continuation map UC
distance of 5,000 m 102
4.7d: The upward continuation map UC
distance of 10,000 m 103
4.8: The profile 1 to 3 line for
the upward continuation filter outlined
on the RTE map of the
area 104
4.9:
Upward continuation curve
examination along profile 1 at height
of 50 m, 500 m, 5,000 m
ad 10,000 m respectively 105
4.10: Upward continuation curve
examination along profile 2 at height
of 50 m, 500 m, 5,000 m
ad 10,000 m respectively 105
4.11: Upward continuation curve
examination along profile 3 at height
of 50 m, 500 m, 5,000 m
ad 10,000 m respectively 105
4.12: The
average radial power spectrum was calculated and displayed
in a
semi-log figure of amplitude versus frequency for block 1 108
4.13: The deep magnetic source depth
variation (D1) 109
4.14: The 3D deep magnetic sources (D1)
basement morphological
Model 110
4.15: The shallow magnetic source (D2)
depth variation 111
4.16: The currie depth point map over
the area 113
4.17: The geothermal gradient map over
the area showing the rate of temperature
variation with depth. 115
4.18: The heat flow map over the area 117
CHAPTER 1
INTRODUCTION
1.1 BACKGROUND OF STUDY
Earth’s subsurface is
chiefly made of rocks, minerals, hydrocarbon and water. A knowledge of the
subsurface is essential in exploration, geotechnical, engineering and
geological studies (Anbazhagan, 2018). Subsurface studies in determining the
rock strata, groundwater quality and soil type is quite indispensable in
project design and execution. Such investigation helps in identifying the
geophysical and geological properties of the subsurface earth materials such as
porosity, thickness, stress, weathering condition, tectonic activities, faults
and cracks (Anbazhagan et al., 2017).
This therefore helps engineers in proper financial and logistic planning before
executing a project.
In the energy sector,
the energy resources are successfully utilized based on the fundamental
understanding of the mechanical deformation of rocks in the upper crust that
is; creating and sustaining fracture networks in enhanced geothermal systems
(EGS) and unconventional oil and gas reservoirs. The rock's inherent properties
and their response to natural and applied lithostatic, tectonic and hydraulic
stresses at varying degrees affect to a large extent in estimating and
regulating rock and seal geomechanical stability carbon storage reservoir (Dan,
2014).
Since the controlling static and dynamic
parameters governing the physical and chemical processes in the subsurface are
poorly known especially from direct observation, there is therefore a pertinent
need for earth scientists to carry out surveys in order to estimate the likely
behavior of the earth’s heterogeneous subsurface from the data generated (Caumon,
2018). This prediction can come in the form of forecasts which use any of;
reservoir production decline curves, physically and mathematically based data
processing in the form of seismic wave processing, upward continuation of
potential locations, and traditional ground penetrating radar processing
(Nobakht et al., 2013). It can also
involve Geophysical representations (e,g maps and tomography), resolution of
inverse problems that explicitly use physical models in computing observations
from some earth and physical parameters (Perrouty et al., 2014). There are various geophysical methods used in
studying the subsurface which explicitly incorporate geological knowledge in
subsurface interpretation as seen in Table 1.1. However, an integration of any
two or more complementary methods provides more reliable evidence on the
subsurface's characteristics. This work therefore seeks to investigate the heat
flow, geothermal gradient, sedimentary thickness of the subsurface in portions
of the Lower Benue Trough using airbone magnetic data of high resolution.
Table 1.1: Geophysical
Methods (Kearey and Brooks, 2002)
|
Method
|
Measured parameter
|
Operative physical property
|
|
Seimic
|
Travel times of reflected/refracted seismic
waves
|
Density and elastic moduli, which determine
the propagation velocity seismic waves
|
|
Gravity
|
Spatial variations in the strength of the gravitational
field of the earth
|
Density
|
|
Magnetic
|
Spatial variations in the strength of the
geomagnetic
|
Magnetic susceptibility and remanence
|
|
Electricak
restivity
|
Earth resistance
|
Electrical conductivity
|
|
Induced
polarization
|
Polarization voltages or
frequency-dependent ground resistance
|
Electrical capacitance
|
|
Slef-potential
|
Electrical potentials
|
Electrical conductivity
|
|
Electromagnetic
|
Response to electromagnetic radiation
|
Electrical conductivity and inductance
|
|
Radar
|
Travel times of reflected radara pulses
|
Dielectric constant
|
1.1.1 Heat flow and geothermal gradient
The transfer of heat or thermal energy to the
earth's surface is known as heat flow. (SMU, 2021). The source of the heat from
the earth’s interior comes mostly through the cooling of the earth's center and
generation of radioactive elements like thorium (Th), Uranium (U) and/or
potassium (K). Heat produced through the process of radioactive decay of U235,
U238, Th232 and K40 constitute 80% of the heat
in the earth’s interior while 20% of it comes from a mix of planetary accretion
heat and residual heat (Sanders, 2010; Arndt, 2011). Heat moves from the deep
hot region to the surface because the earth’s interior is much hotter than the
surface. Heat flow anomalies exist in various sections of the world, areas with
high radioactivity or thin crust usually have high rate of heat flow. However, there
are several places that have higher heat flow anomalies above the average
crustal flow of heat without a well-defined radioactive or tectonic explanation
(SMU, 2021).
Heat transfer occurs majorly in three ways;
conduction, convection and radiation. Whereas thermal conduction involves the
transfer of energy through matter, convection involves the energy carried with
moving matter and radiation involves energy carried by electromagnetic waves
especially when matter is absent (Cermark et
al., 1991). For the subsurface,
heat transfer is majorly through conduction and convection which is the usual
mechanism for heat transfer. Figure 1.1 shows the heat transfer mechanisms
within the earth.
Fig 1.1: Heat
transfer mechanisms within the Earth, along with the percentage amount of
heat flow in each layer (Doney et al., 2019).
Heat constantly flows from its sources within
the surface of the earth and there is an estimated mean heat flow of 65mW/m2
and 101mW/m2 over the continental and oceanic crusts
respectively (Pollack et al., 1993). Rock
cores or cutting on a device that tests the amount of energy a rock sample can
pass are used to assess thermal conductivity. Consider a solid body with a slab of
horizontal boundaries z = z1, z = z2 (Figure 1.2). The
heat flux q which is the rate at which energy flow through the area is related
to the temperature distribution dT, thickness of the material dz and the
thermal properties of the conducting body K.
Fig. 1.2: Heat flux through a slab
The heat flux q measured in Wm-2 can
be expressed in equation 1.1 (Aniko and Elemer, 2017);
where K = Thermal conductivity of the
material
dz = z2 – z1
dT = T2 – T1;
T1 and T2 are temperatures
at the lower and upper surfaces respectively.
Equation 1.1 is often
called Fourier’s law of conduction where K
which is never negative and it shows how conductive the material is. The higher
the conductive nature of the material, the higher the value of K measured in Wm-1deg-1.
The negative sing on the RHS explains the fact that heat always flows from hot
region to cold region. If however the temperatures of the upper and lower
surfaces are identical irrespective of their magnitude, then there will be no
heat flux on the material. Nevertheless, the heat flux is smaller if the
material is thick (z2>>z1) than when it is thin. Geothermal
gradient γ can be deduced from 1.1 thus
γ =
dT/dz. … (1.2)
Geothermal gradient therefore
can be defined as the rate at which temperature inceases with increasing depth
beneath the earth’s surface. The magnitude of the geothermal gradient depends
on the rate at which heat is produced at depth, the dynamics of the systems in
addition to conductivity of rocks (Arndt, 2011). The geothermal gradient is
calculated in 0C/m (or K/km) and it has average values ranging from
0.045 – 0.065 0C/m on a continental scale (Aniko and Elemer, 2017).
At the measurement
site, the earth's temperature gradient is calculated from collecting
temperature values in a well specific depths. Because the temperature logs
obtained after drilling are affected by drilling fluid circulation, the
temperature is considered to be at equilibrium if the temperature measurements
are made after the drilling fluid has stopped influencing the well and these
values are considered to be of the highest quality because they include a
number of data points which assist in understanding the variations in earth’s
geology and structure (Cermak et al., 1991).
Temperature values
obtained from oil and gas wells are often called bottom-hole temperatures
(BHT). They are taken at the bottom of the intervals the well was drilled at
that time though the values need corrections added to them to compensate for
the drilling fluid.
As a rule, there is
an increase in temperature as the depth of the crust increases as result of the
flow of heat from the hot mantle. There is an average temperature rise of 25 –
300C per kilometer in most parts of the planet, there is a lot of
depth near the surface aside tectonic plate boaundaries (Fridleifsson et al., 2011). The temperature profile
of the predicted inner earth is as shown in Fig. 1.3.
The geothermal
gradients with the highest values of 40-80 K/km are measured at mid-ocean
ridges (oceanic spreading centres) or in an island arcs where there is proximity
of magma to the surface. On the contrary, the subduction zones is where the
lowest geothermal dradients occur and this is where cold lithosphere descends
into the matle as well as when the upwelling parts of the mantle ascend nearly
adiabatically with a low geothermal gradient of about 0.3K/km(Arndt, 2011).
Fig. 1.3: Estimated Temperature Profile of
Earth’s Interior (Alfe et al., 2002)
However, there may be
a negative or inverse geothermal gradient, a phenomenon where there is a drop in
temperature rise with depth, particularly near the earth's surface. This is
mainly due to the low diffusivity of rocks and the diurnal temperature
variations which hardly affect underground temperatures at shallow depths
(Huang et al., 2010). Negative
geothermal gradients can also occur in areas with deep aquifers or where deep
water transfers heat by advection and convection and it results to shallower
volumes of water adjacent rocks are heated to a higher temperature than rocks
at a deeper level (Ziagos and Blackwell, 1986).
1.1.2 Sedimentary
thickness
Sedimentary thickness
refers to the distance across a packet of sedimentary rock, whether it be a
facies, stratum, bed, seam, lode etc. The thickness is measured at right angles
to the surface of the seam or bed and thus is independent of its spatial
orientation (Onwe et al., 2015). Mineral
or organic particles aggregate and are cemented together to form sedimentary
rocks. These organic particles and minerals which form sedimentary rocks are
collectively called sediments (Wilkinson et al., 2008). Sedimentary rocks are formed
in layers called starta, which form a structure known as bedding. When
sediments are deposited in large structures, they are referred to as
sedimentary basins. The volume of sediment that is determined by the depth,
shape, and size of the basin of the basin whereas tectonics determines the
depth and scale of a basin.. At tectonic uplift, the land finally rises above
the surface of the sea, erosion removes materials and the zone becomes viable
for new sediments to be deposited. Conversely, a basin if formed and sediments
are deposited at tectonic subsidence (Press et
al., 2003).
1.1.3 Aeromagnetic
method
Aeromagnetic survey
is a form of magnetic survey where magnetometers are mounted within or towed
behind a low flying aircraft. Aeromagnetic survey is rapid and it is used when
the magnetic properties of a large area is to be surveyed. It works on the
principle that variations in the measured magnetic field of an area reflect the
distribution of magnetic minerals in the earth’s crust (Gibson, 1998).
Aeromagnetic method is very useful in the detection, location and
characterization of magnetic sources, surface geologic mapping where in areas
outcrop is rare or absent, magnetic effects of geologic bodies and structures
can be observed or where the bedrock is covered by glacial overburden, water
bodies, sand or vegetation (Reeves, 2005).
Magnetic method is
aimed at investigating anomalies in the earth's magnetic field caused by the
magnetic properties of the underlying rocks. It is useful in determining the
magnetic source's depth, possibly sediment thickness and in delineating
subsurface structures. It is appropriate for locating buried magnetic ore
bodies due to their magnetic susceptibilities. Aeromagnetic surveying is rapid
and cost-effective, typically it cost some 40% less per line kilometer when
compared to ground survey. Large areas can be surveyed quickly without
incurring the expense of sending a field party into the survey area, and data
can be collected from areas that are inaccessible to ground survey. (Karey and
Brooks, 2002).
Magnetic surveys can detect magnetic sources
deep inside the earth's crust (tens of kilometers), restricted only by the
depth at which magnetic minerals enter their Curie point and cease to be ferrimagnetic.
Broad correlations can be made between rock type and magnetic properties, but
the relationship is often complicated by temperature, pressure, and chemical changes
that rocks are exposed (Grant, 1985).
Nonetheless, by determining the position, shape,
and attitude of magnetic sources and combining this information with available
geologic data, a meaningful geological understanding of a given area can be
generated. Magnetic anomalies produced by certain types of ore bodies can be
useful targets for mineral exploration surveys. Magnetic data can be used to identify
geologic structures that provide optimal conditions for oil/gas extraction and
accumulation, even though hydrocarbon reserves are not directly observable by
aeromagnetic surveys. (Gibson and Millegan, 1998). In the same way, mapping the
magnetic signatures of faults and fractures within water-bearing sedimentary
rocks offers useful constraints on aquifer geometry and groundwater system
framework.
1.1.4 The principle of magnetism
Fig. 1.4: The magnetic flux surrounding a bar magnet
The basic principle of magnetism can be
explained thus: within the vicinity of a bar magnet, a magnetic flux is
developed which flows from one end of the magnet to the other (Fig. 1.4). This
flux can be mapped from the directions assumed by a small compass needle
suspended within it. The points within the magnet where the flux converges are
referred to as the poles of the magnet. The force F between two magnetic
poles of strengths m1 and m2 separated by a
distance r is expressed as;
4πμRr2
…
where
μ0 and μR are constants corresponding to the magnetic permeability of vacuum and
the relative magnetic permeability of
the medium separating the poles. The force exerted on a unit positive pole at that point is known as the
magnetic field B due to a pole of strength m at a distance r from the pole.
Magnetic fields can be defined in terms of magnetic
potentials in a similar manner to gravitational fields. For a single pole
of strength m, the magnetic potential V at a distance r from
the pole is given by;
The magnetic field component in any direction
is then given by the partial derivative of the potential in that direction. If
we were to measure the magnetic field along the surface of the earth, we would
record magnetization due to both the main and induced fields. We're interested in the induced field because
it relates to our measurement's proximity to rocks with high or low magnetic
susceptibility.
When measurements are
taken near rocks with high magnetic susceptibility, greater magnetic field
strengths are reported than when measurements are taken further away from rocks
with high magnetic susceptibility.
1.1.5 Geomagnetic
elements
From the point of view of geomagnetism, the
earth may be considered as being made up of three major parts which inclide:
core, mantle and crust. Convection processes in the iron core's liquid portion
create a dipolar geomagnetic field that resembles that of a broad bar magnet
aligned roughly along the earth's axis of rotation. Magnetic disturbances are
produced when the geomagnetic field interacts with the rocks of the Earth's
crust, as seen in extensive surveys conducted close to the surface. In SI units, a magnetic field is defined as
the flow of electric current required to generate it in a coil. As a result,
volt-seconds per square meter, Weber/m2, and Teslas are the units of
measurement (T).
Since the earth's magnetic field is just
around 5 x 10-5 T (Reeves, 2005), the nanoTesla (nT = 10-9
T) is a more convenient SI unit of measurement in geophysics.The geomagnetic
field has a value of about 50 000 nT at this stage. Traditional aeromagnetic
surveys can detect magnetic disturbances as small as 0.1 nT, which may be
geologically significant.
Magnetic anomalies with amplitudes of tens,
hundreds, and (less frequently) thousands of nT are usually observed. The gamma
(γ), an old (cgs) unit of magnetic field, is
numerically equivalent to one nT.
The main geomagnetic field is defined as a
vector quantity at any point on the earth's surface by three scalar values
(Fig. 7), which are normally expressed as three orthogonal components
(vertical, horizontal-north, or horizontal-east components) or the scalar
magnitude of the total field vector as well as its orientation in dip and
azimuth. Aeromagnetic surveys have often measured only the scalar magnitude of
F, so the latter system is more realistic for now. The magnetic inclination, I,
is the angle created by the total field vector above or below the horizontal
plane. It is usually positive north of the magnetic equator and negative south
of it, ranging from -90 to 90 degrees.
The magnetic declination, D, is the angle
between the vertical plane containing F and true (geographic) north, which is
positive to the east and negative to the west and ranges from 0 to 360 degrees.
The value of D is commonly displayed on topographic maps to alert the user to
the difference between magnetic north, as registered by a compass, and true
north. D is less than 15° in most places on the Earth, though it reaches values
as large as 180° along lines joining the magnetic and geographic poles.
Fig. 1.5: Geomagnetic elements
The orthogonal components X (northerly
intensity), Y (easterly intensity), and Z (vertical intensity, positive
downwards) characterize the geomagnetic field vector B; Inclination (or dip) I, (the angle between
the horizontal plane and the field vector, determined positive downwards) and
declination (or magnetic variation) D (the horizontal angle between true north
and the field vector, measured positive eastwards).
The orthogonal components can be used to
calculate declination, inclination, and total strength, thus using the
equations 1.6 – 1.9
Geophysicists have been able to develop a
mathematical model for the earth’s magnetic field, i.e., its shape and
intensity across the earth surface, Magnetometer surveys indicate that there
are many unexpected variations in this model, called “magnetic anomalies”. A
magnetic high anomaly is where the measured field strength is higher than the
value predicted by the global model, and a magnetic low is where the measured
field strength is lower than the value predicted by the global model.
Magnetic anomalies on
the earth's surface are caused by induced or remanent magnetism. Secondary
magnetization caused in a ferrous body by the earth's magnetic field induces
induced magnetic anomalies. Magnetic highs could be caused by the presence of
magnetically charged rocks in the subsurface. Magnetic prospecting searches for
changes in the earth's magnetic field induced by subsurface geologic structure
changes or differences in the magnetic properties of near-surface rocks. The
magnetic susceptibility of rocks relates to their natural magnetism. As
compared to igneous or metamorphic rocks, sedimentary rocks have a much lower
magnetic susceptibility due to the presence of magnetite (a common magnetic
mineral). The majority of magnetic surveys are used to map the geologic
structure of the basement rocks (the crystalline rocks that lie beneath the sedimentary
layers) or to detect magnetic minerals directly (Reeves, 2005).
1.2 STATEMENT
OF THE PROBLEM
For economic
sustainability and diversity, there is a need to explore more petroleum and
mineral deposits in Nigeria. Since sedimentary thickness is a diagnostic tool
in hydrocarbon exploration, this research will seek to identify more prospects
of hydrocarbon and mineral exploration.
Nevertheless, fossil
fuels are used as energy sources thus bringing adverse environmental impacts
and climate change (David, 2011). Azunna et
al. (2020) noted that the earth’s temperature has risen to 0.8 0C
due to anthropogenic carbon (IV) oxide emission with much of the increase
taking place in the last thirty-five years. The quest for more renewable usage therefore
becomes more pertinent.
Energy from
geothermal sources requires no fuel while it provides true baseload energy at a
reliability rate of above 90% (William, 2010). As of 2007, approximately 10GW
of geothermal electric power had been installed worldwide (Europian, American
and Asian Countries) generating about 0.3 percent of global electricity demand
while an additional of 28GW of geothermal energy is used for applications such
as district heating, room heating, spas, industrial processes, desalination,
and agriculture (Fridleifsson
et al.,
2011).
This research
therefore seeks to study the Lower Benue Trough's heat flow and geothermal
gradient so as to ascertain the prospects for mineral and hydrocarbon
exploration as well as the viability of the region in generating geothermal
energy.
1.3 AIM AND OBJECTIVES
The aim of this
research is to determine the heat flow, geothermal gradient and the sedimentary
thickness in order to characterize the subsurface material using high
resolution aeromagnetic data over the Lower Benue Trough, South-Eastern Nigeria.
In order to achieve this aim, the following are set objectives:
1.
Generating the total magnetic intensity map, upward continuation map,
derivative map and analytical signal map, this to ascertain areas of high, intermedite
and low magnetic intensities.
2.
Determining the depth to basement, curie depth, geothermal gradient, and
heat flow through quantitative analysis.
1.4 JUSTIFICATION
The Benue trough (lower) has been identified
to contain some minerals like coal, ironstone, clay, sulphur, graphite, mica,
lead, zinc, limestone, barites and phosphate (Fatoye and Gideon, 2013). Sadly,
the potentials of mineral prospecting is yet to be fully identified in order to
identify their abundance and industrial viability. The power need of the
country has grown astronomically and the meager power available need to be
supplemented by other sources. This research therefore is apt in identifying
the mineral prospects and potential for geothermal energy in the study area.
1.5 SCOPE
This research uses high resolution
aeromagnetic data to investigate the heat flow, geothermal gradient and
sedimentary thickness of parts of Lower Benue Trough.
1.6 THE STUDY AREA
The geographical coordinates of the study
area lie between latitudes 05° 00ʹ N and 07° 00ʹ N and longitudes 07° 30ʹ E and
09° 00ʹ E (Osinowo and Taiwo, 2020). The Southern Benue Trough includes the
southernmost part of the Benue Trough, which makes up the major sedimentary
basin in Africa,
with a length of over
1000 kilometers and a width of 150 to 250 kilometers, (Figure 1.6). It is a portion of the Cretaceous West
African Rift System (WARS) that stretches for about 4000 kilometers from
Nigeria to the neighboring Republic of Niger before terminating in Libya (Binks
and Fairhead, 1992).
The states are Abia, Imo, Enugu, Anambra,
Ebonyi, Cross River, and Benue. The major towns are Nsukka, Igumale, Ejekwe,
Udi, Nkalagu, Abakaliki, Okigwe, Afikpo, and Ugep.
Fig.
1.6: Geology map of the study
area (Osinowo and Taiwo, 2020).
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