HEAT FLOW, GEOTHERMAL GRADIENT, SEDIMENTARY THICKNESS AND CHARACTERIZATION OF SUBSURFACE MATERIALS IN LOWER BENUE TROUGH, SOUTH-EASTERN NIGERIA USING HIGH RESOLUTION AEROMAGNETIC DATA

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 J_i,

                        remanent J_r 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)

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;

                                                4πμ_Rr²                                                  …                    

where μ₀ and μ_R 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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