ABSTRACT
Aeromagnetic data obtained from the Nigerian Geological Survey Agency, Abuja were used to carry out this research. The study area is at the coordinates of 5°00′00′′ - 6°02′00′′ N and 7°00′00′′ - 8°01′00′′ E covering a distance of about 12, 100 km2. The study area belongs to two geologic basins which include the Benue Trough and Niger Delta. These basins are known for their economic importance. The study aims at using a computer program to spectrally analyse and compute the sedimentary thickness of the area and correlate the geological and the enhanced aeromagnetic maps for solid minerals deposit. Also, to estimate the hydrocarbon window, Curie point depth, geothermal gradient and heat flow within the study area. The obtained data were processed and enhanced using applicable techniques. Two depth sources were revealed; using Geosoft’s Oasis montaj version 7.1; these are sedimentary thickness of 7.9 km for deeper sources and 0.42 km for shallower sources. The Curie point estimated in the study area ranges from 7.1 km to 12.5 km with an average of 10.1 km. Qualitative analysis of the study area shows that it is structurally controlled, giving room to mineralization; and the shallow source depth of 0.42 km in the study could be attributed to intrusion and mineralization. Shallow depths encourage mineralization, and the study area is endowed with solid minerals (Fig. 2). Geothermal gradient and heat flow estimated in the study averaged at 59.7°C/km and 149.3 mWm-2 respectively. The high geothermal gradient in the area is as a result of the rifting and metamorphic activities in the host basins. The sedimentary thickness (7.9 km deep) and geothermal gradient averaging 59.7°C/km are testaments to possible hydrocarbon accumulation in the area.
TABLE OF CONTENTS
Title page i
Declaration ii
Certification iii
Dedication iv
Acknowledgements v
Table of Contents vi
List of Tables’
List of Figures
Abstract
CHAPTER
1: INTRODUCTION 1
1.1 Overview 1
1.2 Location of the Study Area 2
1.3 Climate of the Study Area 2
1.4 Physiography of the Study Area 3
1.5 Geology of the Study Area 5
1.5.1 General geology 5
1.5.2 Local geology 7
1.6 Statement of the Problem 12
1.7 Aim and Objectives 12
1.8 Scope of the Study 12
1.9 Justification of the Study 13
1.10 Advantages and Disadvantages of Airborne
Magnetic 13
CHAPTER 2: LITERATURE REVIEW 14
2.1 Overview 14
2.2 Basic Theory of the Magnetic Method 15
2.2.1 The nature of the geomagnetic field 15
2.2.1.1
The main field 15
2.2.1.2 The external field 17
2.2.1.3 Variations in the main field 18
2.2.2 Magnetization
of rocks 18
2.2.3 Magnetic
moment 18
2.2.4 Intensity
of magnetization 19
2.2.5 Magnetic
susceptibility 19
2.2.6 Magnetic
induction 20
2.3 Definition
of Techniques 21
2.3.1 Reduction
to magnetic equator 21
2.3.2 Shuttle
Radar Topography Mission (SRTM) elevation data 23
2.3.3 Source
parameter imaging (SPI) 23
2.3.4 Analytic
signal 24
2.3.5 Derivatives 24
2.3.6 Vertical
derivatives 25
2.3.7 Spectral
analysis 26
2.3.8 Curie
point depth 27
2.3.9 Geothermal
gradient 29
2.4 Previous
Aeromagnetic Study of the Area 31
CHAPTER
3: MATERIALS AND METHODS 37
3.1 Data
Acquisition and Description 37
3.2 Instrumentation 50
3.2.1 The
proton magnetometer 50
3.3 Methodology
of the Study 51
CHAPTER
4: RESULTS AND DISCUSSION 56
4.1 Qualitative
Interpretation 56
4.1.1 The TMI map
and results 56
4.1.2 The RTE map
and results 58
4.1.3 The SPI map
and results 59
4.1.4 First vertical
derivative map of depth slice 1 63
4.1.5 The Analytic
signal map of depth slices 1 and 2 and results 64
4.2 Quantitative
Interpretation 73
4.2.1 Estimation of
Sedimentary thickness by means of spectral analysis 73
4.2.2 Estimation of
Curie point depth, geothermal grad. and heat flow 75
4.3 Discussion 82
4.4 Application of these findings to the search for Solid Minerals 84
CHAPTER 5: SUMMARY,CONCLUSION AND RECOMENDATIONS 85
5.1 Summary 85
5.2 Conclusion 86
5.3 Recommendations 88
References 89
LIST
OF TABLES
Table Page
1.1. Summary of
Lithology, Structure and Topography 10
3.1.
Some of the acquired data 43
4.1. Calculated SPI values in the study area 59
4.2. Spectral depth estimation of the study area 74
4.3a. Estimated depth to the top boundary 80
4.3b. Estimated depth to
centroid 81
4.3c. Estimated Curie
point depth
81
4.4. Estimated geothermal gradient and heat flow 82
LIST
OF FIGURES
Figure Page
1.
Location map of the study area (adapted from
Google 2015 map) 2
2.
Geology
map of the study area (After Nigeria’s geology map of mineral resources by
NGSA)
3.
The
Earth’s dipolar magnetic field and the inclination of field at
varying latitudes.
4.
The magnetic elements
16
5.
Geothermal gradient of the Earth’s interior
(after Wikipedia, 2015) 30
6.
The TMI map of Okigwe area 38
7.
The TMI map of Afikpo area 39
8.
The TMI map of Aba area 40
9.
The TMI map of Ikot-Ekpene area 41
10.
The TMI of the study area 42
11.
The Shuttle Radar Topography Mission (SRTM)
map of the study area (measured in metres) 49
12.
A typical flight pattern of the survey 50
13. TMI map of the
study area at interval of 20 nT min = -100.6 nT and max = 207.2 nT 65
14. RTE map of the
study area contoured at interval of 20 nT min = -98.5 and max = 197.7 66
15. Source Parameter
Imaging (SPI) map of the study area 67
16. First
vertical derivative map of depth slice 1 68
17. First
vertical derivative of slice 2 69
18. Lineament
trend of the study area 70
19. Analytic signal map of depth slice 1 71
20. Analytic signal map of depth slice 2 72
21. 2D radially
averaged power spectrum of the study area 74
22a. Chart for block 1 76
22b. Chart for block 2 76
22c. Chart for block 3 77
22d. Chart for block 4 77
22e. Chart for block 5 78
22f. Chart for block 6 78
22g. Chart for block 7 79
22h. Chart for block 8 79
22i. Chart for block 9 80
CHAPTER
1
INTRODUCTION
1.1
OVERVIEW
Geophysical exploration methods represent a
primary tool for investigation of the subsurface and are applicable to a wide
range of problems.
The particular geophysical method, precisely
used in this work is the magnetic method, precisely the aeromagnetic method in
which measurements of the Earth’s magnetic field are made using magnetometers
towed behind an aircraft. The aircraft typically flies in a grid-like pattern
with height and line spacing determining the resolution of the data (Taufiq et al., 2014)
The results of interpretation made on
aeromagnetic data are very useful in oil and mining industries. In oil
exploration they are mainly used to determine the thickness of the non-magnetic
sedimentary section overlaying a magnetic basement. The assumption usually made
is that the structure of the sediments is controlled by the basement topography
and in this case variations in the depth of the basement can suggest possible
location of oil traps. In mining exploration the prime use is as an aid to
geological mapping. It is particularly useful for these two reasons. Firstly,
not all rocks are magnetic and is therefore usually possible to follow the
outlines of magnetic structures over considerable distances without too much
confusion from neighbouring anomalies. Secondly, many of the minerals contain an
appreciable quantity of magnetite.
In this chapter, the geology, climate and
physiography of the study area which lies in southern Nigeria will be
discussed.
Figure 1: Location map of the study area (adapted from Google 2015 map)
1.2
LOCATION
OF THE STUDY AREA
The study area falls within the Lower Benue
Trough and Niger Delta, which are sedimentary basins. The geographical
coordinates of the study area are 5⁰00′N –
6⁰00′N. The study area covers
an area of 12,100 Km2. The major towns are Okigwe, Afikpo, Aba and Ikot
Ekpene (Figure 1).
1.3
CLIMATE
OF THE STUDY AREA
Nigeria may be divided into four climate
regions as follows (Iloeje., 1981).
i.
the subequatorial South
ii.
the tropical hinterland
iii.
the tropical continental north
iv.
the high plateau
The study area falls in the sub – equatorial
south. The subequatorial south extends from the coast to roughly 130 to 160
kilometres inland. The annual rainfall is between 2000 and 4000 mm (Jalon
Consultants., 1980; Amos-Uhegbu et al., 2013). The total
rainfall increases from 150 centimetres in the west to more than 300
centimetres in the Delta area. Relative humidity is normally over 90% in the
early morning, but falls to between 60% and 80% in the afternoon. Mean annual
temperatures register over 29⁰ C (Amos-Uhegbu et al.,
2013).
1.4 PHYSIOGRAPHY OF THE STUDY AREA
The physiography of the study area can broadly
be described in terms of three distinct parts namely; northern part; central
part and southern part. In the northern part is located such major town as
Afikpo. Okigwe is the major town in the central part and Umuahia is located in
the southern part.
The northern part can be described in terms of
two physiographic units (Ahamefula., 2012). These units are:
i.
the north–east and western low lands which
have some isolated ridges, and
ii.
the high lands of NW–SE trend that consists of
long and scattered ridges. These ridges are continuous in some areas and isolated
in other places.
The northern fringes of the area have the
highest elevation of about 120 m above sea level. These ridges are prominent in
villages such as Amankwu, Ndi Uduma and Asaga. Observation shows that in some
places low lands or valleys occur in between ridges.
The central part of the study area, where
Okigwe is located, is dominated by two striking features. Respectively, these
are the Udi – Okigwe – Arochukwu and Awka – Umuchu – Umuduru sedimentary cuestas
(Uma., 1986). These cuestas form regional watersheds standing at about 200m
above the surrounding plains, the Udi – Okigwe – Arochukwu cuesta separates the
west – flowing headstreams of the Imo River and the east flowing headstreams of
the Cross River. The Udi – Okigwe – Arochukwu cuesta is more imposing than the
smaller, less continous Awka – Umuchu – Umuduru cuesta which separates the west
and north – west flowing streams of Marnu and Orashi rivers from those of the
Imo River that flow eastwards.
In the area between these two major topographical
features are undulating terrains comprising dissected ridges and numerous
isolated hills. Springs and effluent seepage can be found at the foot of some
of these hills and ridges. Most of the effluent seepage are emphemeral and some
represent through flow of rainfall events; however a few of them are perennial,
but a much reduced discharge during the dry season (Uma., 1986).
The southern part of the basin, where Umuahia
can be found, is characterized by a gentle slope of the ground towards the
south. The ridges trend generally in the north – south direction and are
separated by valleys mostly those of the Imo River and its major tributaries,
which are 40 to 60 m below the surrounding landscape. Some of the valleys are
dry. Towards the most southern end of the study area, which contains the major
town of Aba, the ridges give way to a nearly flat topography.
1.5 GEOLOGY OF THE STUDY AREA
1.5.1 General geology
The study area lies in two major tectonic
features in southern – eastern Nigerian. These are the Niger Delta complex and
the Lower Benue trough.
The north easterly trending Benue Trough is
located at a zone of weakness existing between the West African and Congo
cratons since the late Precambrian to early Palaeozoic eras. It is one of the
marginal basins that developed during the cretaceous following the opening of
the South Atlantic Ocean in the Mesozoic.
The tectonic activity that gave rise to the
structural trough occurred in Albian times, that is during the early cretaceous
period (Murat., 1972). The stratigraphic sections of the trough are composed
primarily of a hierarchy of sedimentary rocks deposited during major
transgressive – regressive cycles as a result of epeirogenic movements. The
trough was filled in three phases as a result of these movements.
The first transgressive – regressive cycle
extended from mid – Albian to the end of the Cenomanian (Murat., 1972) and
resulted in the disposition of the Asu River Group. The second cycle which
comprises the Lower Turonian (Murat., 1972) resulted in the deposition of the
Cross – River Group. The deposits of the third cycle (upper Turonian – lower Santonian)
have been largely eroded as a result of folding and uplift. The deposits are
mainly marine shale. The tectonic event which resulted in this folding and
uplift took place from the upper Santonian to probably lower campanian (Murat.,
1972) and was characterized by compresional movements along established NE–SW
trend. These movements gave rise to the Abakiliki anticlinorium and the Afikpo
Syncline. Simultaneously, there was subsidence of the Anambra platform to the
West of Abakiliki anticlinorium. The Anambra basin is the result of this
subsidence.
In addition to the sediments the Benue Trough
is characterized by Tertiary to Recent Volcanic rocks of basic and intermediate
composition.
Most of the study area is situated on the
Benue Trough. The remaining part is situated on the Niger Delta Basin. The
Niger Delta Basin is a Tertiary structure formed during the Eocene and filled
with Tertiary sediments of enormous thickness.
1.5.2 Local geology
Figure 2 shows the geology of the study area.
It reveals the various formations in the area which has been summarised in
table 1.1 in terms of lithology, structure, and topography.
Some of the Formations in the study area are
Ajalli Formation, Mamu Formation, Nkporo Formation etc.
Ajalli Formation consists of false bedded
sandstone. The formation is slightly diachronous, ranging from Middle to late
Maastrichtian from south to north. Cross bedding is the dominant sedimentary
structure of the formation. It is associated with reactivation surfaces, mud
drapes, tidal bundles, backflow ripple channels cut and fills, lateral
accretion surfaces, as well as Skolithos and Ophiomorpha ichnogenera (Ladipo et al., 1992). The topography of Ajali
Formation shows plateau in the north, cuesta in the south and escarpment.
Mamu
Formation succeeds the Upper Campanian and is made up of sands, gravels, shales
with some limestones in the south and coal seams in the central to the upper
parts of the basin. The thickness of the Formation varies across the Lower
Benue Trough. The topography shows lower slopes.
The
basal facies of the late Cretaceous sedimentary cycle in the Anambra Basin (of
the Lower Benue Trough) is the Nkporo Shale which indicates a late Campanian
age. In general this Formation is exposed at Leru 72 km Enugu – Port Harcourt
Express way and has been described as a coarsening upward deltaic sequence of
shales and interbedded sands and shales (Ladipo et al., 1992). The lithology is mainly Shales with lenticular sand
bodies. Nkporo Shale is moderately dip to west and south in terms of structure
and its lithology shows lower slopes.
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