ABSTRACT
This research examines the hydrocarbon and mineralogical potentials of Abia State and environs which covers about 48,400 km2 and lies between latitude 50 0‟N to 700‟N and longitude 700‟E to 800‟E. The study area lies in the Lower Benue Trough consisting of Abia state and parts of Imo, Enugu, Ebonyi and Benue states. Aeromagnetic data sets with sheet numbers; 287 (Nsukka), 288 (Igumale), 301 (Udi), 302 (Nkalagu), 312 (Okigwe), 313 (Afikpo), 321 (Aba) and 322 (Ikot-Ekpene) were obtained from the Nigeria Geological Survey Agency and the Landsat imagery was obtained from the United States Geological Survey database. For the satellite imagery, band 10 of the Enhanced Thematic Mapper Plus (ETM+) of Landsat 8 with a scene size of 220km by 110km and a resolution of 100m was used for vegetation, temperature and drainage studies. The Elevation and linear structures were mapped from data acquired from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). All data sets were analysed using Oasis Montaj 6, Mathlab 17 and ArcGis. Results show that the study area has a surface temperature of 20-34oC with a dendritic drainage pattern. The magnetic anomaly responses range from -102.8nT to 145.6nT and the depth to the magnetic basement as obtained from source parameter imaging and spectral analysis ranges from -102.4km to 7,618km implying that parts of the study area meet the sedimentary thickness for the accumulation of hydrocarbon. The average Curie point depth, geothermal gradient and heat flow were found to be 4.48km, 34.6oCkm-1, 69.2mWm-2 respectively indicating that the study area is geothermaly active. Furthermore, 4,090 lineament features were obtained from the landsat imagery and the lineament density ranges from 22.7 km-1 to 113.7km-1 with a NE-SW structural trend. Areas like: Bende, Umuahia, Arochukwu and Ohafia (Abia State); Okigwe (Imo state); Afikpo, Amagunze, Eha-Amufu (Ebonyi State) have high amplitude anomalies which implies presence of igneous intrusions and as such are not viable for hydrocarbon exploration although their depth to magnetic basement meet the requirement for hydrocarbon generation thus making them viable for mineral prospecting. However, areas like Abi, Obolo-Eke, and Ikem areas in Kogi, Benue and Enugu States have low amplitude anomalies indicative of a sedimentary basin but only Ikem area in Enugu state and Aba in Abia state are likely to have potential for hydrocarbon due to its depth and low lineament density.
TABLE OF CONTENTS
Title Page i
Declaration ii
Certification iii
Dedication iv
Acknowledgement v
Table of Content vii
List of Tables x
List of Figures xi
Abstract xiii
CHAPTER 1: INTRODUCTION
1.1 Background of study 1
1.1.1 Justification 5
1.1.2 Statement of the Problem 5
1.2 Aeromagnetic Principles 6
1.2.1 The Geomagnetic Field, Spherical Harmonics and IGRF 7
1.2.2 Magnetic Properties of Rocks 16
1.2.2.1 Diamagnetic Materials 18
1.2.2.2 Paramagnetic Materials 18
1.2.2.1 Ferromagnetic Materials 19
1.3 Satellite Imagery and Remote Sensing 25
1.3.1 Electromagnetic Radiation and Interactions with The Surface of the Earth 28
1.3.2 Reflectance Characteristics of The Earth‟s Surface 34
1.3.2.1 Vefetation 36
1.3.2.2 Water 36
1.3.2.3 Soil 37
1.4 The Study Area 39
1.4.1 Tectonic Evolution of the Lower Benue Trough 42
1.5 Aim and Objectives 47
1.6 Scope of Study 47
CHAPTER 2: LITERATURE REVIEW
2.1 Historical Background of Aeromagnetic Method and Landsat Imagery 48
2.2 Review of Related Works 53
2.4 Landsat Sensors 60
2.4.1 The ETM+ Sensor 61
CHAPTER 3: MATERIALS AND METHODS
3.1 High Resolution Aeromagnetic and Landsat Data 62
3.2 Aeromagnetic Data Processing 64
3.2.1 Signal Derivatives 65
3.2.2 Upward and Downward continuation 66
3.2.3 Residual and Regional Separation 67
3.2.4 Reduction to Equator 67
3.2.5 Analytical Signal 69
3.2.6 Depth to Basement Determination 69
3.2.7 Determination of Curie Point Depth, Geothermal Gradient and Heat Flow 72
3.3 Landsat Data Processing 74
3.3.1 Calculation of Land Surface Temperature (LST) 75
CHAPTER 4: RESULTS AND DISCUSSION
4.1 Qualitative Results 77
4.1.1 Total Magnetic Intensity 77
4.1.2 Derivative Maps 80
4.1.3 Reductions to Equator Map 84
4.1.4 Upward Continuation Maps 86
4.1.5 Analytical Signal Map 88
4.1.6 Drainage 90
4.1.7 Vegetation 92
4.1.8 Lineament 94
4.1.9 Residual and regional Magnetic Anomalies 100
4.2 Quantitative Analysis 103
4.2.1 Land Surface Temperature 103
4.2.2 Depth to Magnetic Basement 105
4.2.2.1 Source Parameter Imaging (SPI) 105
4.2.2.2 Spectral Analysis 107
4.2.3 Curie Point Depth, Geothermal Gradient and Heat flow Maps 113
4.3 .Discussion 118
CHAPTER 5: CONCLUSION AND RECOMMENDATIONS
5.1 Conclusion 120
5.2 Recommendations. 122
REFERENCES 123
LIST OF TABLES
Table: Page
1.1 Magnetic Susceptibilities of various rocks and minerals 23
1.2 Electromagnetic waves and their description 31
4.1 Orientation of Lineament 99
4.2 Depth Estimates from Spectral Plots 111
4.3. Curie Point Depth, Geothermal Gradient and Heat Flow of the Study Area. 114
LIST OF FIGURES
Figure: Page
1.1 The Vector Total Magnetic Field 9
1.2 Vector Diagram of Ji, Jr and Total Magnetisation 20
1.3 Spectral overview of Electro-magnetic Satellite Imagery 28
1.4: Electromagnetic Waves 79
1.5: Water, vegetation and soil‟s Spectral Reflectance curves 35
1.6. Reflectance Curve of Different Types of Water Bodies 37
1.7. Spectral Reflectance of Mineral Soils 38
1.8. Map of Nigeria Showing the Study Area 40
1.9: The Location and Elevation Map of the Study Area. 41
1.10: Stratigraphic Succession of Lower Benue Trough 45
1.11: Geology Map of the Study Area. 46
3.1 Sheet Map of the Study Area. 62
3.2. Spectral Blocks of the Study Area 70
4.1 Total magnetic Intensity map of the Study area 79
4.2 Horizontal Derivative Map of the Study Area. 81
4.3 Vertical Derivative Map of the Study Area. 82
4.4 Derivative Map of the Study Area with Respect to Depth. 83
4.5 Reduction to Equator Map 85
4.6 Upward Continuation Map of the Study Area 87
4.7 Analytical Signal Map of the Study Area 89
4.8 Drainage Pattern of the Study Area 91
4.9 Vegetation of the Study Area 93
4.10 Lineament Map of the Study Area 96
4.11. Lineament Density map of the Study Area 97
4.12. Rose Plot of the Study Area 98
4.13 Residual Anomaly Map of the Study Area 101
4.14 Regional Anomaly Map of the Study Area 102
4.15. Land Surface Temperature of the Study Area 104
4.16. Source Parameter Imaging of the Study Area 106
4.17. Plot of the Log of Spectral Energy Against Spectral Frequency for the
Study Area. 110
4.18 Depth to Magnetic Basement from Spectral Analysis. 112
4.19 Curie Point Depth Map of the Study Area 115
4.20 Geothermal Gradient Map of the Study Area 116
4.21 Heat Flow Map of the Study Area 117
CHAPTER 1
INTRODUCTION
1.1 BACKGROUND OF STUDY
Airborne magnetic method and satellite imagery methods are used in the study of earth‟s subsurface. The magnetic survey investigates subsurface geology based on the anomalies in the earth„s magnetic field due to the magnetic properties of the underlying rocks. Generally, the magnetic content of rocks is highly variable depending on the types of rocks and the environment it is situated. Magnetic anomalies are commonly caused by dykes, faults and lava flows (Opara et al., 2018).
Strong local magnetic fields indicate rocks with high magnetic susceptibility while rocks with low magnetic susceptibility produce weaker magnetic fields. In a geothermal environment, the susceptibility decreases due to high temperature and when magnetic studies are used with other geophysical methods like the gravity, it can be used to infer heat sources. Magnetic responses can be gotten from sources in depths of kilometers and they are not impeded by high electrical ground conductivities associated with saline groundwater or high level of contamination unlike the electromagnetic method. Increased content of magnetic minerals can be found in cap rocks of oil-filled reservoirs and this with other effects may cause small magnetic anomalies which form indirect hydrocarbon indicators (Mita, 2008).
Hydrocarbon seepage and migration can result in weak magnetic signatures. The hydrocarbons leak in varying quantities to the surface and produce magnetic minerals in the sediments through geochemical interaction (Mita, 2008). The process commonly associated and generated by hydrocarbon microseepage include; the authigenic precipitation of pore filling carbonate cements, which may decrease permeability of sealing cap rock and the diagenetic, largely microbial conversion process of weakly magnetic hematite parent mineral to strongly magnetic magnetite (Stone et al., 2004).
Authigenic magnetite can be produced in two ways. According to Donovan et al., (1984), the two ways are; reduced iron combining with hematite and water to form magnetite and reduced iron which at some depth migrates upwards into an oxidizing zone and the oxidation would directly produce magnetite and maghemite giving rise to authigenic magnetite.
Magnetic enhanced zones have been used in hydrocarbon exploration and they are found when there is a magnetic contrast between sedimentary rocks of normally low magnetic susceptibility and those locally enriched with epigenetic magnetite. Furthermore, seepage induced magnetic anomalies as well as enrichment of magnetic mineralization due to hydrocarbon migration can be used to facilitate the exploration of oil and gas (Mita, 2008).
Subtle but recognizable change in magnetic field profile can be created by authigenic magnetic mineralization in shallow sediments above hydrocarbon deposits and the removal of the magnetic effect of deeper basement rocks produces the sedimentary residual magnetic (SRM) profile. It is only then that the low-level magnetic effects created by hydrocarbon microseepage can be identified as SRM anomalies (Dietmar, 2015).
The relationship between hydrocarbon seepage and formation of authigenic magnetic minerals can identify areas of prospects with greatest petroleum potential. Although the discovery of shallow sedimentary magnetic anomalies does not guarantee the discovery of hydrocarbon accumulation, it does identify areas requiring more detailed evaluation, thereby focusing attention and resources on a relatively small number of high potential sites.
Although small anomalies are caused by chemical changes due to hydrocarbon, there are other many shallow sources that give rise to low magnetic anomalies such as depositional and erosional patterns, sills and dykes, hydrothermal systems (Christine, 2017). The potent way of distinguishing the causative anomaly is to integrate magnetic data with other geophysical data and satellite imagery is quite complimentary.
Satellite based remote sensing of hydrocarbon-induced alteration of soils and sediments holds great promise as a rapid and cost-effective means of detecting areas of elevated hydrocarbon seepage and microseepage. The leakage of hydrocarbon gases creates an oxidation-reduction cell which leads to numerous geochemical and mineralogic changes in soils and near-surface sediments. There are certain changes that occur in chemically reducing environments associated with hydrocarbon seepage. They include; reduction of iron from a ferrous state to a ferric state, conversion of feldspars and micas to clay minerals, and the replacement of mixed-layer clays by kaolinite. These and other changes can be detected by analysis of satellite imagery, as well as by hyperspectral analysis of soils, sediments, and vegetation (Dietmar, 2015).
The successful use of any geophysical technique is dependent on a number of factors apart from a careful survey design. Such factors include; nature and size of target, depth of burial of target, data calibration and the interval of the station where measurements are taken (Yusuf, 2016). Geophysical surveying is sometimes prone to major ambiguities and uncertainties of interpretation (Kearey and Brooks, 2002). These uncertainties can be reduced minimally by combining complementary geophysical methods. Methods that are sensitive to different physical characteristics of the subsurface which are able to complement each other and thereafter provide an integrated approach to a geophysical problem are often used (Reynold, 2011). Combining results of different methods gives a reliable solution to a geological problem but the contrasts and similarities of the methods employed must be well understood (Kamil, 2008).
Remote sensing uses aerial photographs to locate and map surface features. Though photographs are taken from several hundreds of kilometers in space, earth‟s vast area as well as their features is shown (Berger and Anderson, 1992). Slight variations in the moisture of the soil, soil type, distribution of vegetation and mineral can be revealed by satellite imagery. Satellite imagery can also be used to identify geological structures related to mineral and hydrocarbon deposits which may be difficult to identify during ground survey. Satellite radar interferometry can precisely identify faults or slight ground motion connected with hydrocarbon reservoir (Berger and Anderson, 1992).
From the foregoing, satellite imagery and aeromagnetic method play a complementary role. Integrated geophysical and geological data can be used to build both consistent geological models when combined with the geological information gotten from imageries from satellite (Andreas, 2011). Furthermore, geophysical and geological data can calibrate models derived from satellite imagery just as the interpretation of imageries from satellite can give a start models before the commencement of geophysical surveys (Andreas, 2011).
1.1.1 Justification
Nigeria no doubt has been in a state of slow economic growth. This amongst other factors is as a result of the untapped natural resources in various parts of the country (Benyin and Ugochukwu, 2015). Our dependence on crude oil and petroleum products has left us at the fate of oil price which when threatened, our economy staggers. Therefore, there is a pertinent need to ascertain the availability as well as commercial viability of different minerals located across the country while exploring new potentials of petroleum reservoir.
1.1.2 Statement of the Problem
Mineral resources which are all natural resources that can be exploited for the benefit of mankind are abundant in the Lower Benue Trough. Such substances could be organic, metallic or non- metallic. Liquid substances such as petrol are also regarded as mineral resource. Mineral resources based on their usage can be classified to be metallic, non-metallic, fuels and water. Metallic mineral resources include common and abundant metals like iron (Fe), Aluminum (Al), Manganese (Mn), Titanium (Ti), Magnesium (Mg) and scarce metals like copper (Cu), lead (Pb), Gold (Au), and Mercury (Ag).
Non-metallic mineral resources on the other hand include minerals for chemical industries, fertilizers, and others used for special uses like sodium chloride, phosphates, Nitrogen and sulphur. Materials used in building and construction like gravels, gypsum, asbestos and limestone are also forms of non-metallic mineral resources. Fuels as a third category of mineral resources include coal, petroleum, natural gas, oil and shale. Water is the fourth category of mineral resources and it is the most abundant in nature.
The Niger-Delta region has been widely harnessed for petroleum products. Sedimentary basins in Nigeria inclusive of the Lower Benue Trough are found to be endowed with mineral resources. Over a score of mineral resources are found in the Trough as reported by the Nigeria Geological Survey Agency. Some of them are; coal, cassiterite, gypsum, uranium, barytes, fluorspar, limestone, ironstone, clay, silver, glass, sulphur, sand, salt, graphite, manganese, mica, lead-zinc and phosphate (Fatoye and Gideon, 2013). These minerals however are yet to be fully quantified and optimally harnessed. This research therefore seeks to investigate the mineral prospects and petroleum in the region under study for the economic development of Nigeria and Africa at large.
1.2 AEROMAGNETIC PRINCIPLES
Aeromagnetic surveying is the process of using magnetic instrument (magnetometers) that are attached to or suspended from aircraft to carry out large-scale and comparatively fast magnetic surveys. (Reynold, 2011). The principle of aeromagnetic survey is based on the variation in the measured magnetic field of an area which shows how the minerals in earth's crust are distributed (David and Emilio, 2007).
A sensor is towed in a housing known as a „bird‟ or fixed in a stinger in the tail of an aircraft in order to compensate for the aircrafts magnetic field. Aeromagnetic surveys are used for various purposes like geological mapping, environmental and ground water investigations, mineral and oil exploration. (Mark, 2007). During geologic mapping, the magnetic responses of geologic structures and bodies can be noticed. Aeromagnetic study is key in locating magnetic ore bodies that are buried because of the magnetic susceptibilities of the ores. It can detect these bodies at a depth of tens of kilometers within the earth's crust though it is limited by the depth where minerals that are magnetic attain their Curie point thereby ceasing to be ferromagnetic (Grant, 1985). Aeromagnetic survey is of greater advantage when compared to ground survey because magnetic data can be obtained from areas inaccessible to the ground like mountains. It is also rapid, cost effective and covers vast area within a short time (Kearey and Brooks, 2002).
Certain ore may yield magnetic responses that are desirable targets surveys mineral exploration but hydrocarbon reservoirs are not directly detectable by aeromagnetic surveys though data obtained can be useful in locating geologic structures like faults and cracks that provide favorable conditions for hydrocarbon accumulation. Similarly, mapping the magnetic signatures of faults and fractures within water-bearing sedimentary rocks provides valuable constraints on the aquifer's geometry and the framework of groundwater systems (Gibson and Milegan, 1998).
1.2.1. The Geomagnetic Field, Spherical Harmonics and IGRF
The magnetic field of the earth undergoes temporal variations in a range of time scales, which includes reversals of the entire field. Variations of short time span can be detected over time scales spanning from fractions of a second to decades. Some of the short-term variations may occur within the period of a typical survey and ending with those of significance over geological time. These temporal variations may be in the form of diurnal variation, micropulsation and geomagnetic storm, Secular variations are caused by daily changes in currents or geomagnetic storms. It changes the magnetic field of the earth in years. The variations over time scales of one or more years most often show the changes in the earth's interior (Jackson et al., 2000). The main manifestation of secular variation globally is changes in size and position of the departures from a simple dipolar field over years and decades. The effects of these changes at a given locality are predictable with a fair degree of accuracy for periods of five years to a decade. From the point of view of magnetic anomaly mapping, secular variations become important when surveys of adjacent or overlapping areas carried out several years apart are to be compared or merged together (Reeves, 2005).
To define the magnetic field of the earth at any point on the earth's surface as a vector quantity B, three scalar values expressed as three mutually orthogonal components namely the vertical Z, the horizontal north X and the horizontal East component Y are used. However, the scalar magnitude and direction in dip and azimuth of the total field vector F is more convenient to use in aeromagnetic surveys as shown in Figure 1.1. Magnetic surveys only measure the scalar magnitude of the total field vector F.
Figure 1.1: The Vector Total Magnetic Field. It‟s either defined as (A) three orthogonal components (vertical, horizontal north and horizontal east) or (B) as the scalar magnitude of the total field, F and two angles: the declination from true (geographic) north, D and the inclination from the horizontal, I .
The angle the total field vector makes above or below the horizontal plane is called the magnetic inclination, I, which is conventionally positive north of the magnetic equator and negative to the south of it. ( -90°≤ I ≤+90°). The angle between the true (geographic) north and the vertical plane containing F is known as the magnetic declination, D, which is reckoned positive to the east and negative to the west. 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.
The geomagnetic field is as a result of the motions in the fluid, electrically conducting core. The contribution from the crustal field due to rocks which acquired magnetic properties about the magnetic field at the time of their formation from the molten state adds to the core magnetic field. In addition to the crustal and main fields is the external magnetic field which is a relatively small portion or the observed magnetic field that is generated from magnetic sources external to the earth. This field is produced by interaction of the earth's ionosphere with the solar wind (Okiwelu et al., 2010).
During magnetic surveys, external sources of magnetic field are accounted for because the entire broad spectrum of time variations in the geomagnetic field of period shorter than the most rapid secular change has their major cause outside of the earth. These time variations are a nuisance in the study of the internal field and must be eliminated. In addition, every variation in the external field induces electric currents inside the earth. Thus, the changes measured at earth's surface are the resultant of the fields of the external sources and of the internal sources (Okiwelu et al., 2010).
For global magnetic field modeling, spherical coordinates (r, θ, φ) can be exploited (Maus, 2006). The secular magnetic potential fulfils Laplace's equation
D2y = 0 (1.1)
where functions which are solutions of the Laplace's equation are called harmonic. Therefore in polar coordinates, the Laplace's operator according to Maus (2006) can be written as:
where the eigen function D2 is called the surface Laplace operator and it is also the surface spherical harmonics Ym (q ,f) which have the property
l is the degree which specifies the total number of circles in which Ym
(q ,f) is zero while m is called the order. l and m are related to the plane wavenumbers, Kx and Ky
l can be interpreted as the spatial frequency and m can be interpreted as a direction or azimuth. The scalar magnetic potential functions have some relationships with the radius of the earth α as expressed in equations 1.7 and 1.8 which are the internal and external sources respectively.
1.2.2 Magnetic Properties of Rocks
Rocks possess magnetic characteristics due to the level of magnetic minerals they contain. Crustal rocks are generally weakly magnetic. When a magnetic rock is placed in a magnetic field, the material becomes magnetized and the introduced magnetic field is reinforced by the magnetic field induced in the material itself. This process is called magnetization (Kearey and Brooks, 2002).
Some rocks, however, retain their magnetic properties even after the introduced field is withdrawn. Such a material has remanent magnetization and its direction will be fixed with the specimen in a way that it is directed towards the inducing field. Magnetic properties can only exist at temperature below the Curie point. The magnitude of an induced magnetization Ji - acquired by a rock is proportional to that of the earth's field F in their vicinity for a magnetically isotropic substance according to equation 1.23
Ji = kF. (1.23)
where k is the magnetic susceptibility of the rock and it is a dimensionless quantity with a magnitude which in most rocks is much less than unity. Magnetic susceptibility characterizes the ability of a substance to be magnetized when exposed to external magnetic field. Hence the value of a typical susceptibility value may be expressed as (for example) k = 0.0057 SI. In the broad sense, the susceptibility of most rocks therefore reflects the abundance of 'magnetite', the relation between the volume percentage of magnetite, Vm, and magnetic susceptibility, k, may be formulated to a good approximation (Reeves, 2005) as:
k = 33 x Vm x 10-3 SI. (1.24)
The nature of the magnetic susceptibility of the material helps to classify materials into three basic groups namely Diamagnetic, paramagnetic and ferromagnetic materials in addition to other magnetic properties like the various forms of ferri and ferromagnetic properties (Gavrila, 2014). The major three classes are briefly discussed:
1.2.2.1 Diamagnetic Materials.
Diamagnetic substances possess induced magnetization and they have negative magnetic susceptibility and they are characterized by very weak magnetic properties. They have a null atomic magnetic moment and are divided into classical, anomalous, and superconductive substances . Some classical diamagnetic substances are: inert gases, some metals (Cu, Ag, Au, Zn, Ga, In, Sb, and As.), elements such as silicon and phosphorous, many organic compounds like calcites, salts and quarts. For these substances, k has an absolute reduced value of order (0.1 − 10) x 10-6 and it does not depend on temperature. In the second subgroup of diamagnetic anomalous substances are bismuth, antimony, graphite etc. For these substances, k depends on the temperature and it allows absolute values of the order (1 − 100) x 10-6 (Cullity, 2009). Diamagnetic substances are also independent of the magnetizing field (k is negative constant).
1.2.2.2. Paramagnetic Materials.
These materials have positive susceptibility and are generally higher than in diamagnetic materials but still relatively low in absolute value and also independent of the magnetizing magnetic field (k is positive constant). The value of their susceptibility ranges from (10-6 – 10-3). The magnetic moments of paramagnetic materials under the action of a magnetic field tend to orient in the direction of the exterior magnetic field but the resultant magnetization has low values because the effect of the thermal agitation tends to orient them randomly. Magnetic moments are also oriented parallel to the magnetic field and an induced magnetization parallel to the field appears. The susceptibility of paramagnetic materials depends on temperature as it decreases with increase in temperature (Jiles, 1998). Examples of paramagnetic materials include sodium, aluminum, Manganese, Tungsten, Uranium and cobalt oxide.
1.2.2.3. Ferromagnetic Materials.
Materials in this class have ordered magnetic structure and are characterized by having magnetic domains even in the absence of external magnetic field. They also posses remanent magnetization. The ferromagnetic substances are strongly magnetized in the direction of the field when placed in a magnetic field. The resultant magnetic field inside the ferromagnetic material is very large: thousand times greater than the magnetizing field. Then the magnetic susceptibility k is positive and very large: for example about 8000 for soft iron (Gavrila, 2014). A small number of metals are ferromagnetic (Fe, Co, Ni), of rare earths (Gd, Dy) and oxides like CrO2.
The ferromagnetic materials are subdivided into three groups which are; the ferromagnetic materials in which all the atomic magnetic moments are parallel, the ferromagnetic materials in which the atomic magnetic moments create two antiparallel sublattices, one dominating over the other so that the overall material magnetism is strong and the antiferromagnetic materials in which the atomic magnetic moments also create two antiparallel sublattices, but these sublattices are roughly balanced and therefore the magnetism of these materials is relatively weak (Frantisek, 2009)s. The paramagnetic and diamagnetic susceptibilities are field-independent, while the ferromagnetic susceptibility is field-dependent.
Any rock containing magnetic minerals may possess both induced and remanent magnetizations Ji and Jr. The relative intensities of induced and remanent magnetizations are commonly expressed in terms of the Königsberger ratio,
Q = Jr /Ji. (1.25)
where Q ranges from 0.1 to 100 and it‟s dimensionless. The remanent and induced magnetization may be in different directions and may differ significantly in magnitude. The magnetic effects of such a rock arise from the resultant magnetization J of the two magnetization vectors (Figure 1.3). The magnitude of J controls the amplitude of the magnetic anomaly and the orientation of J influences its shape.
:. J =
Ji + Jr = kF + Jr (1.26)
The magnetisation of a rock has the same units as magnetic field (nT), whether it is induced magnetisation, Ji, remanent magnetisation, Jr, or their vector sum.
Figure 1.2: Vector Diagram of Ji, Jr, and Total Magnetization J.
While negative anomalies may form in troughs, sedimentary basins or where faults have dropped down the basement, positive magnetic anomalies can be created by irregularities in the buried basement rocks. Due to high magnetite content of basic igneous rocks, they are usually highly magnetic (O‟ Handley, 2000). By far the most common magnetic mineral is magnetite whose Curie point is 578°C. Although the size, shape and dispersion of the grains of the magnetite within a rock affect its magnetic property, increasing acidity decreases proportionately the magnetite in igneous rocks thus making acidic igneous rocks to be less magnetic than basic igneous rocks although their magnetic behavior changes (Kearey and Brooks, 2002).
There is variability in the magnetic properties of metamorphic rocks. As the grade of metamorphism increases, oxygen‟s partial pressure reduces relatively making the magnetite to be reabsorbed, the iron and oxygen then is incorporated into other mineral phases. However, magnetite can form as an accessory mineral during metamorphic actions due to the relative increase in the partial pressure of oxygen (Clark and Emerson, 1991).
Sedimentary rocks usually have ones of nanoTesla and it is on this basis that magnetic surveying is done because the interpretation of survey data assumes that magnetic sources must be beneath the base of sedimentary sequence. High Resolution Aeromagnetic and High Resolution Gravity data can resolve intra-sedimentary sources and structures within the sedimentary sequences can be subsequently mapped during petroleum exploration.
The sequence of the sedimentary basin and its thickness may be mapped by systematically determining the depths of the magnetic sources (the „magnetic basement‟) over the survey area. Sub basins and their boundaries can be identified with magnetic data even certain sedimentary iron deposits, dykes and sills emplaced in sediments, pyroclastic or volcanic sequences that hitherto appear hidden in a sequence of sediments (Reeves, 2005).
1.3 SATELLITE IMAGERY AND REMOTE SENSING
Satellite imagery investigates and maps in a large scale the geologic properties of remote regions of the earth without any contact with the area on the ground (Laake and Zaghloul, 2009). This is vital especially in monitoring global change in climatic and geological features. It is a source of primary medium spatial resolution and earth observations used in decision-making (Fuller et al., 1994; Townshend et al., 1995; Goward et al., 1997; Goward et al., 2006; Vogelmann et al., 2001; Woodcock et al., 2008; Cohen et al., 2004; Masek et al., 2008).
Geologic information can be deduced from satellite imagery and when combined with other geophysical and geologic data, one can build consistent geological models for the surface as well as the subsurface. Before the beginning of a geophysical survey, satellite imagery can generate start models (Laake and Insley, 2004a, 2004b). Conversely, data from geophysical and geological sources can calibrate models derived from satellite imagery (Imram and Mithas, 2011). Satellite imagery can provide detailed models of the surface and near surface which provide input to data quality estimation before and during acquisition. For data processing, satellite imagery can supply input to processes that correct for noise related to near-surface properties. (Coulson et al., 2009).
Extracting information about objects or areas at the surface of the earth without direct contact or touch with the area or object is called Remote sensing (RS), also called earth observation. Images of the earth surface are taken in various wavelength regions of the electromagnetic spectrum (EMS) and this form the basic principle of remote sensing technique.
The wavelength region represented in the electromagnetic spectrum by an image gotten through remote sensing forms its major characteristics. While some show the quantity of energy radiated by the surface of the earth in the thermal infrared region, others represent reflected solar radiation in the visible and the near infrared regions of the electromagnetic spectrum (Sivakumar et al., 2003).
Remote Sensing can either be active or passive. In active remote sensing, the source of energy is from the remote sensing platform transmitted from the vehicle itself and the energy measured in the microwave region measures the relative return of the transmitted wave from the surface of the earth. Active microwave radar methods use a microwave source onboard of the satellite and measure the back-scatter from the earth. (Sivakumar et al., 2003).
On the other hand, remote sensing is passive if the measurements depend on external sources of energy like the sun. Passive remote sensing uses the sunlight as the source and measures in the infrared and visible spectral band, the reflectance of the surface of the earth (Sivakumar et al., 2003). The Landsat Enhanced Thematic Mapper (ETM) used in this research is an example of such a passive remote sensor. There are five important processes required for a successful operation in remote sensing. They include;
1. Emission of electromagnetic radiation either from the sensor or from the sun
2. Transmission, absorption or scattering of energy from the source to the earth‟s surface
3. Interaction of electromagnetic radiation with the earth‟s surface: emission and reflection
4. Transmission of energy from the surface of the earth to the remote sensor
5. Sensor data output (Sivakumar et al., 2003).
Due to molecular and atomic oscillations, objects above absolute zero temperature radiate electromagnetic energy and these increases with the body's absolute temperature peaking progressively at shorter wavelengths. The distribution of absorbed, emitted and reflected radiation is unique for every body in nature. This spectral properties when exploited is used to distinguish one body from another in the bid to ascertain the shape, size, chemical and physical properties of different bodies. This therefore forms the basis for sensing electromagnetic radiation. As an instance, very near infrared (VNIR) detects specifically vegetation while short wave infrared (SWIR) is the best option for the discrimination of sedimentary rocks. The thermal infrared (TIR) radiation from the surface of the earth shows the property of the surface material. A warm response from dark materials such as non-sedimentary rocks and cool response from ground moisture or voids can be distinguished where evaporation absorbs energy. In general, optical imagery does not penetrate the earth surface (Imram and Mithas, 2011). Figure 1.3 shows a spectral overview of electromagnetic satellite imagery.
Figure: 1.3. Spectral Overview of Electromagnetic Satellite Imagery (Laake and Cutts, 2007)
1.3.1 Electromagnetic Radiations (EMR) And Interactions With The Surface of The Earth
Electromagnetic (EM) radiations are waves of the electromagnetic field propagating through space while carrying radiant electromagnetic energy (Purcell and Morin, 2013). EM radiations synchronizes the electric and magnetic field oscillations and propagates with the speed of light which is equal to 3.0 x 108 m/s in vacuum. In isotropic and homogenous media, the oscillations of the two wave fields are mutually perpendicular and also perpendicular to the direction of energy and wave propagation. This therefore, forms a transverse wave as shown in Figure 1.4 (Browne, 2013).
E = Electric field
M= Magnetic field
Figure 1.4: Electromagnetic Waves.
EMRs have parameters of wave motion such as wavelength (λ), frequency (f) and velocity (c) which are related as expressed in equation 1.27
c = fλ (1.27)
Aside the basic wave theory which explains EM waves, the particle theory gives insight on how EM energy interacts with matter. EMR has many discrete units called photons whose energy (E) is given in equation 1.28. (Sivakumar et al., 2003).
E = hc/λ = hf (1.28)
where h = 6.626x10-34Js (Planck‟s constant), f = frequency of radiation
EM waves have different wavelengths and frequencies and as such have different effects when they interact with matter (Browne, 2013). In order of their increasing wavelengths, they include gamma ray, x-ray, ultraviolet radiation, visible light, infrared radiation, microwave and radio wave. Together, they form the electromagnetic spectrum (EMS) (Bettini, 2016). The various components of the EMS is continuous and does not have clear cut class boundaries. The various sections of the spectrum are of different relevance to earth observation in volume of geospatial data acquisition and in the type of information (Sabins, 1997).
The sun is the major source of EM radiation and it emits 44% of its energy as light and 48% as infrared radiation. Solar radiation consistently replenishes the energy that the earth loses into space (Sabins, 1997). Incident radiation which comes from the sun on the earth surface is transmitted, reflected, emitted or absorbed by the surface. EM radiation on interaction with the surface of the earth not only changes in wavelength, it also changes in magnitude, polarization, phase and direction. A sensor is used to detect these changes and it enables us to obtain both spatial and spectral information of any object of interest (Liliesand and Kiefer, 1993). The majority of the geospatial data acquisition is accomplished from remotely sensed information in the infrared and visible regions (Sabins, 1997).
Visible and infrared wavelengths from 0.3μm to 16μm are divided into three different spectral bands which include the reflective band, the thermal infrared and the intermediate band. The intermediate band lies between the thermal infrared and the reflective band. In the intermediate band, both self-emission and reflection are important factors. The Reflective Band has wavelength from 0.3μm to 3μm. The earth surface reflects radiations due to the sun as recorded by the sensor (Klaus et al., 2009).
Earth's Surface thermal emissions give rise to the energy in thermal infrared band with a wavelength range of 3 -14μm. Surface temperature is needed for studying a variety of environmental problems and it is useful in analyzing; the mineral composition of rocks, the condition of vegetation, etc. (Klaus et al., 2009). The atmospheric windows are situated between 3-5μm and from 8 - 14μm. The latter has narrow absorption band from 9 μm to 10μm caused by ozone and this is omitted by most thermal infrared satellites sensors (Miller, 1994). Infrared radiations with short wavelengths (0.7μm - 3 μm) don't cause the sensation of heat. They include: Near Infrared (NIR) (from 0.7 μm to 1.1 μm) and mid infrared or Short Wave Infrared (SWIR) from (1.1 μm to 3 μm). Thermal emission of the earth's surface at 300K has a peak wavelength of 10μm (Klaus et al., 2009).
1.3.2 Reflectance Characteristics of the Earth’s Surface
In remote sensing applications, surface reflections are revealing and most useful. The reflection intensity depends on the surface refractive index, absorption coefficient, incident angle and angle of reflection. The ratio of the reflected energy to the incident energy is called spectral reflectance ρ(λ) according to equation 1.12.
ρ(λ) = 100ER(λ)/
EI(λ) (1.29)
where EI (λ) = Energy of wavelength that is incident on the object ER(λ) = Energy of wavelength reflected from object
Spectral reflectance is different for different materials on earth‟s surface and it is the primary cause of the colour in an object‟s photographic image. Spectral reflectance depends on wavelength and for a given terrain feature, it has different wavelengths. A plot of ρ(λ) and λ gives a spectral reflectance curve. The spectral response pattern which is characterstic of a terrain feature is called spectral signature. Figure 1.6 shows typical curves for spectral reflectance for some of the earth features (Sivakumar et al., 2003). The three major features of the earth surface and their spectral characteristics are briefly discussed below;
Figure 1.5: Water, Vegetation and Soil‟s Spectral Reflectance Curves (Sivakumar, et al., 2003)
1.3.2.1 Vegetation:
Chlorophyll in the leaves of plants absorbs emissions in wavelengths corresponding to red and blue but reflects green wavelength. Therefore, the behavior of leaves can be likened to diffuse reflectors in the wavelengths of the near infrared. This implies that the information about the type and health condition of plants can be known using remote sensing.
1.3.2.2 Water:
Most of the incident radiation on water is transmitted and not reflected. Therefore, water has low reflectance. The transmittance (Ʈ) of any substance is defined by equation 1.30
Ʈ = Transmitted radiation (1.30)
Incident radiation
Water absorbs more of the near infrared and longer visible wavelengths. Stronger reflectance at shorter wavelengths makes water to look blue or blue-green but at near infrared or red wavelengths, it becomes darker. The materials in the water, depth of a water body, and roughness of the surface of water affect the reflectance variability of a water body. Water reflects EM energy a bit in the NIR range and more in the visible range. Beyond 1.2μm, all energy is absorbed. Spectral reflection curves of different types of water are shown in Figure 1.6 (McCloy 1995).
Figure 1.6: Reflectance Curve of Different Types of Water Bodies: (a) Ocean Water (b) Turbid Water (c) Water with Chlorophyll (McCloy, 1995)
1.3.2.3 Soil:
Most soils have low reflectance. Most radiation incident on the soil surface are either reflected or absorbed with little transmitted. The organic matter and moisture content, texture, structure and iron oxide content are soil characteristics in which soil‟s reflectance properties depend on. The spectral curve shows less peak and valley variations as seen in Figure 1.8 (Klaus et al., 2007).
Figure 1.7: Spectral Reflectance of Mineral Soils. (a) Organic dominated (b) Minimally Altered (c) Iron Altered (d) Organic Affected and (e) Iron Dominated. (Klaus et al., 2007)
By measuring the energy that is reflected by targets on earth‟s surface at different wavelengths, we can build up a spectral signature for that source. And by comparing the response pattern of different features we may be able to distinguish them.
1.4 THE STUDY AREA:
The study area is part of the Lower Benue trough consisting of Abia state and parts of Imo, Enugu, Ebonyi and Benue states. It lies between latitude 50 0‟N to 700‟N and longitude 700‟E to 800‟E covering about 48,400 km2. It has an elevation of about 100m to 600m above the sea level. There is relatively even topography in the Southern parts while the Northern part has a sloppy topography with a highland of 500-600m above the sea level sloping downwards to 100m above sea level. Figure 1.8 shows the map of Nigeria with the study area while Figure 1.9 shows its location and elevation map.
1.4.1 Tectonic Evolution of The Lower Benue Trough
The area of study is part of the sedimentary basin in the Lower Benue Trough (SBT) full of rocks of Cretaceous to Tertiary ages. The stratigraphic history of the SBT is characterized by three sedimentary phases namely; the Abakiliki – Benue phase of the Aptian to the Santonian age, the Anambra – Benin phase of the Campanian to the Mid Eocene age and lastly the Niger Delta Phase of the late Eocene to Pliocene age (Ideozu and Amararu, 2015). The Lower Benue Trough originated during the time the South Atlantic Oceans opened as the African plate and the South American plate separated as a failed arm of an aulacogen (Petters, 1978).
The Cenomanian tectonic activities produced an uplift with a North East-South West trend giving way to tectonic activities which took place in the Santonian times resulting to the folding and uplifting of the Abakaliki Sector of the trough and subsidence of the Anambra platform which formed the Anambra basin (Fatoye and Gideon, 2013). Deltaic sequences and clastic sediments are found in the Anambra basin thus it is their major depocentre. The proto-Niger Delta sequence which consists of Enugu/Nkporo, Mamu, Nsukka and Ajali formations thereafter followed as a result of the erosion of the Abakaliki uplifts and folded belts (Fatoye and Gideon, 2013).
In the SBT, the Asu river group is regarded as the beginning of the sedimentation and are therefore the first transgressive cycle (Olade, 1975) though some pyroclastics of Aptian to early Albian age have been reported scantly by Uzuakpunwa (1974). The sediments are made up of the intrusive and extrusive material of the Abakaliki formation, the Mfamosing limestone of the Calabar flank, the arkosic sandstones, volcanic clastics, marine shales, siltstones and limestone (Akande and Erdtmann, 1998).
Although the Odukpani formation were deposited uncomformably on the basement rocks of the Calabar flank during the late Albian, the marine Cenomanian-Turonian Nkalagu, agala and Agbani formations of the Cross-River group rests comprises of black shales, limestones, siltstones and interfingering regressive sandstones below which lies the Asu river group ; (Fatoye and Gideon 2013).
In the Anambra basin, the sedimentation began with the early Campanian to the early Maastrichtian of the Enugu/Nkporo formations consisting of dark grey shale, bluish shale, thin sandstones, shaly limestone beds, shales, sandy shales, the coal seams of the deltaic inter-bedded Mamu formations, siltstones and mudstone. They together form marine sediments and the third transgressive cycle of the SBT. The lateral grading of the shaly facies to sandstones characterizes the Owelli and Afikpo formations of the Anambra basin (Fatoye and Gideon, 2013). The Nsukka and Imo formations also have marine shales which are deposited in the Paleocene and the tidal Nanka sandstone of the Eocene age overlays them.
Towards the Niger-Delta phase, the Akata shale and the Agbada formation make up the Paleocene equivalents of the Anambra basin while the Benin formation is of the Miocene to recent age. Agbada formation consists of sands and sandstones intercalated with shales (George et al., 2014). Benin formation, the youngest rock stratigraphic unit consists of sand and shale sediments with intercalation of clay beds. The sands are mostly medium to coarse grained, pebbly, moderately sorted with local lenses of poorly cemented sands and clays (Asseez, 1976., Azunna, et al., 2017, Azunna et al., 2018, Chukwu, et al., 2017). Petrographic analysis of Benin Formation indicates that the composition of the rocks is as follows: 95-99% quartz grains, 1- 2.5% of Na+Kmica, 0 -1.0% of feldspar and 2-3% of dark coloured minerals (Azunna and Chukwu, 2018,). Figure 1.10 shows the succession of the stratigraphic sequence of the Lower Benue Trough
Locally, the study area consists of the Cretaceous; shale and limestone of Asu river group, black shale, siltstone and sandstone of the Eze-Aku formation, sandstones, false bedded sandstones, limestone and coal of the Nsukka formation. The Imo group and the Ogwashi-Asaba formations make up the tertiary units with clay, shale and limestone. The quarternary alluvium, sand and clay which are of the Benin formation form the youngest stratigraphic unit of our study area as shown in the geology map of Figure 1.11.
Figure 1.10: Stratigraphic succession of Lower Benue Trough (Fatoye and Gideon, 2013).
Figure 1.11: Geology Map of the Study Area.
1.5 AIM AND OBJECTIVES
This research aims to investigate the hydrocarbon potential of the Lower Benue trough based on airborne magnetic method and satellite imagery with the following objectives:
• Delineation of high frequency anomalies associated with residual and shallow seated bodies.
• Mapping of surface temperature and delineation of low frequency anomalies associated with regional and deep seated bodies.
• Obtaining the lineament associated with the study area which is target point for mineralization and mineral exploration.
• Estimation of the depth to basement using Spectral analysis and Source Parameter Imaging (SPI) as well as delineation of sediment basement contact.
• Obtaining the Curie depth point, heat flow regime and geothermal gradient which are vital parameters for hydrocarbon exploration.
• Using Landsat imagery to obtain drainage pattern, structural features, rose plot, fracture density and land surface temperature which will be correlated with aeromagnetic results to infer mineralogy and petroleum potential.
1.6 SCOPE OF STUDY
This research uses magnetic data (airborne) and landsat imagery to evaluate the mineral and petroleum potentials of some parts of the Lower Benue Trough.
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