GEOPHYSICAL INVESTIGATION OF ABAKALIKI AREA USING AEROMAGNETIC METHOD

CONTENTS

TITLE PAGE ——————————————————————————ii

CERTIFICATION ————————————————————————iii

DEDICATION —————————————————————————-iv

ACKNOWLEDGEMENT ————————————————————– v

TABLE OF CONTENTS   ————————————————————- v

LIST OF FIGURES ———————————————————————-ix

LIST OF TABLES————————————————————————-x

ABSTRACT   ————————————————————————— xi

CHAPTER ONE:  INTRODUCTION—————————————————–1

1.1 Background to the study —————————————————————-3

1.2 Geology of the study area ————————————————————–4

1.3 Purpose of study ————————————————————————-6

CHAPTER TWO: LITERATURE REVIEW ——————————————–7

2.1 Review of geophysical work in the upper Benue Trough——————– -7

2.2 Review of geophysical work in the lower Benue Trough—————9

2.3 Review of geophysical work in the middle Benue Trough—————–13

2.4 Review of geophysical work in the Basement Complex (Oban Massif) -14

CHAPTER THREE: THEORY OF MAGNETIC METHOD———————–15

3.1 Introduction—————————————————————————–15

3.2 Magnetic pole——————————————————————-15

3.3 Magnetic force——————————————————————-16

3.4 Relative permeability, susceptibility and magnetization——————17

3.5 Units of magnetism————————————————————-17

3.6 Earth’s magnetic field———————————————————-18

3.6.1 Elements of the earth’s magnetic field————————————-18

3.6.2 Nature of the geomagnetic field——————————————–19

3.7 Variations in the geomagnetic field——————————————20

3.8 Magnetism of rocks and minerals——————————————–20

3.9 Magnetic susceptibility of rocks and minerals——————————22

3.10 Induced and remanent magnetization of rocks—————————-23

CHAPTER FOUR: DATA ANALYSIS AND RESULTS.

4.1 Source of data——————————————————————-25

4.2 Data processing and interpretation——————————————-26

4.3 Processes of analysis———————————————————–26

4.3.1 Gridding and contouring—————————————————-26

4.3.2 Filtering————————————————————————28

4.3.2.1 Reduction to pole (RTP) ————————————————–28

4.3.2.2 Vertical derivative (VD) ————————————————–30

4.3.2.3 First vertical derivative (FVD) ——————————————-30

4.3.2.4 Second vertical derivative (SVD) —————————————-32

4.3.2.5 Horizontal derivative (HD) ————————————————33

4.3.2.6 Upward continuation (UC) ————————————————34

4.4 Interpretation (qualitative and quantitative) ———————————35

4.5 Modeling (forward and inverse) ———————————————–36

4.6 Forward and inverse modeling using potentQ —————————36

4.7 Depth estimation ————————————————————-41

CHAPTER FIVE: DISCUSSION OF RESULTS, CONCLUSION AND RECOMMENDATION————————————————————44

5.1 Discussion of result ————————————————————44

5.2 Conclusion ———————————————————————-45

5.3 Recommendation ————————————————————–45

REFERENCES———————————————————————-46

LIST OF FIGURES

Fig 1.1.Geologic map of the study area                                                                   6

Fig 3.1.Magnetic pole                                                                              16

Fig 3.2 Elements of the Earth’s Magnetic field                                               18

Fig 3.3. Ferromagnetism                                                                      21

Fig 3.4. Ferrimagnetism                                                                                            21

Fig 3.5. Antiferrromagnetism                                                               21

Fig 4.1.Grid map of the total magnetic intensity of the study area          27

Fig 4.2.Grid map of the reduction to pole                                    29

Fig 4.3.Grid map of the first vertical derivative (FVD)                            31

Fig 4.4.Grid map of the second vertical derivative (SVD)                         32

Fig 4.5.Grid map of the horizontal derivative (HD)                          33

Fig 4.6.Grid map of the upward continuation (UC)                               34

Fig 4.7.Grid map showing modeled areas                                        37

Fig4.8a. Model A                                                                         38

Fig4.8b. Model B                                                                 38

Fig 4.8c. Model C                                                                               39

Fig4.8d. Model D                                                                                         39

Fig4.8e.Model E                                                                                       40

Fig4.9. Source parameter imaging (SPI)                                                    42

 LIST OF TABLES

Table 3.1 Magnetic susceptibilities of various rocks and minerals –22-23

Table 4.1 Extract of data from Oasis Montaj ——————————–25

Table 4.2 Summary of model results ——————————————41

ABSTRACT

   Aeromagnetic data over Abakaliki area of the lower Benue trough of Nigeria was interpreted qualitatively and quantitatively using Oasis montaj software (version: 6.4.2 H.J).The interpretation unveiled basic intrusive bodies like dyke, lacolyte and batholyte in the study area. It also showed fault zone which trends North East to south western (NE-SW) part of the study area.

Quantitative interpretation of the area was carried out by source parameter imaging and forward and inverse modeling. Source parameter imaging unveiled predominance of deep seated bodies in the south western part of the area, while shallow bodies are predominant in the south eastern part of the study area. Depth obtained by source parameter imaging ranged from 99.50m to 5930.78m.  Forward and inverse modeling was carried out using potent Q software which is an extension of the Oasis montaj software used in the work. The magnetic anomalies over the area were modeled by bodies in the form of sphere and ellipsoid by varying the total magnetic intensity parameters such as susceptibility, inclination, declination, depth of burial and by varying the length, width and height of the bodies used in the model. The radius of the spherical bodies are 1457m, 17704m and 4883m for models A, B and D representing Obubra, Abakaliki and Ameka respectively. Lengths of the ellipsoids are 6099m and 5341m.Width of the ellipsoids are 411m and 2203m while Heights of the ellipsoids are 6017m and 275m (for models C and E representing Enyigba and Ameri respectively).  Depth obtained by forward and inverse modeling ranged from 477m to 6366m. Depth obtained for model A (Obubra) is 546m with susceptibility of 0.0180SI (signifying limestone).  Height obtained for model B (Abakaliki) is 50m (likely the out crop near college of Agricultural sciences-CAS EBSU) with susceptibility of -0.0017SI (signifying calcite). Depth obtained for model C (Enyigba) is 956m with susceptibility of -0.0134SI (signifying Rock salt). Depth obtained for model D (Ameka) is 6366m with susceptibility of -0.009SI (signifying Quartz). Depth obtained for model E (Ameri) is 477m with susceptibility of -0.006SI (signifying Calcite). These values correlate with some of the works done in the study area. The presence of out crop, intrusive and minerals like calcite, pyrite, rock salt and limestone were delineated in the area.

                                                    CHAPTER ONE

INTRODUCTION

1. Introduction:

The study of geophysics has helped man to access hidden treasures in the sub-surface of the earth. These treasures usually appear as anomalies which could be accessed by different geophysical survey methods such as magnetic method, gravitational method, seismic method, electromagnetic method etc.

In geophysical exploration during the last decade, potential field methods have a renewed interest in the search for solid minerals and hydrocarbons.  In  the  gravity  and  magnetic method data processing, the first and the most crucial step is the removal of the effect of deep-seated  structures  from  the  observed  Bouguer  gravity  or  from  the  observed  total magnetic  fields,  in  order  to  enhance  the  signatures  of  shallow  bodies  (Ndougsa  et  al., 2007). These shallow bodies are associated in solid mining exploration firstly to precious metals  (gold,  diamond)  which  have  a  different  density  with  the  surroundings  (gravity exploration) and secondly to substances such as magnetite, hematite, which have contain an iron ore deposit (magnetic exploration) (Ndougsa et al., 2012).

Magnetic survey investigates the subsurface based on variation in the earth’s magnetic field that results from the magnetic properties of the underlying rocks. Magnetic survey can be carried out on land, sea and in air. However, magnetic survey is in principle similar to gravity survey. Aeromagnetic survey aids in indicating major basement surface structures which reveal encouraging exploration areas that could be studied in broader detail using seismic geophysical survey method. The earth’s magnetic field acts on the magnetic minerals in the crust, inducing a secondary field which reflects the distribution of the minerals. The main magnetic field induces a field which varies slowly from one place to another while the crustal field which is the portion of the magnetic field associated with the magnetism induced by the earth’s main magnetic field varies more rapidly (Reford, 1962).

The traditional role of aeromagnetic studies over continental areas is to establish geologic and tectonic frame works and to explore for minerals. The magnetic method is a relatively inexpensive method of learning about geologic hazards such as seismically active faults, shallow magma chambers and volcanic centers. Induced magnetic method may be the only way to study hazardous structures in places where they are concealed beneath young deposits, water and vegetation. Aeromagnetic data have long been used by the petroleum industry to map structures and to enhance depth to magnetic basement (Steenland, 1965).

 The use of computers in the 1960s for processing and interpreting geophysical data has application in estimation of depth to basement and mapping of basement structures. It has been demonstrated in literature and other forms that high resolution aeromagnetic data provide valuable data that solves problems in petroleum exploration.

Aeromagnetic survey has application also in mapping of potential ground water resources in the arid and semi-arid areas of the world. The fact that most sedimentary rocks and surface cover formations like water are effectively nonmagnetic implies that the observed anomalies are attributable to the underlying igneous and metamorphic rocks.

Aeromagnetic survey is the oldest potential field method used for hydrocarbon exploration. The aim of the aeromagnetic survey is to detect minerals or rocks that have unusual magnetic properties by causing anomalies in the intensity of the earth’s magnetic field. Aeromagnetic survey is the exploration method of the earth’s magnetic field intensity with magnetometers installed in airplanes or helicopters. The process is to operate the magnetometer continuously along equally spaced parallel flight lines covering the survey area. The principle of the geophysical survey is similar to a magnetic survey carried out with a hand held magnetometer but it allows much larger areas of the earth’s surface to be quickly covered. The aircraft flies in a grid –like pattern with height and line spacing determining the resolution of the data. As the aircraft flies, the magnetometer records tiny variation in the intensity of the ambient magnetic field due to the temporal effects of the constantly varying solar wind and spatial variations in the earth’s magnetic field. The spatial variation of the earth’s field is due to the regional magnetic field and the local effect of magnetic minerals in the earth’s crust. Subtraction of the solar and regional effects reveals the spatial distribution and relative abundance of magnetic minerals.

Aeromagnetic survey covers much larger areas of the earth’s surface quickly for regional reconnaissance than hand-held magnetometer.

The increasing demand for metals of all kinds and the use of petroleum products have led to the development of many geophysical techniques of ever increasing sensitivity for detection and mapping of unseen deposits and structures (Telford et al, 1990).

1.2    Background to the study

Magnetic method is the primary tool in the search for minerals. It is used in many areas such as locating intra-sedimentary faults, defining subtle lithological contacts, mapping salt domes in weakly magnetic sediments. These applications have increased the methods utility in all areas of exploration especially in the search for minerals, oil and gas, geothermal resources, natural hazards assessment, mapping impact structures, underground water survey, engineering and environmental studies. Aeromagnetic methods traditionally are used to Map crystalline basement, igneous rocks at depth and for mapping faults that offset Basin fills and for delineating buried igneous bodies in the near surface (Grauch, 2001).

 Aeromagnetic data represents variations in the strength of the Earth’s magnetic field that are produced by changes in magnetization of the crust. Magnetization of rocks is determined by the quantity of magnetic minerals (commonly titanomagnetites) and by the strength and direction of remanent magnetization carried by those magnetic minerals. The quantity of magnetic minerals is measured as magnetic susceptibility and produces an induced magnetization. The remanent magnetization is based on the permanent alignment of magnetic domains within the rock and is measured using paleomagnetic methods (Butler, 1992).

Aeromagnetic surveys respond to the total magnetization of rocks, which is the vector sum of the induced and remanent magnetizations. Igneous and crystalline metamorphic rocks commonly have high total magnetizations compared with other rock types, whereas sedimentary rocks and poorly consolidated sediments have much lower magnetizations (Reynolds et.al, 1990; Hudson et.al, 1999).

The aeromagnetic geophysical method plays a distinguished role in terms of rapid rate coverage of geophysical survey area. The main purpose of aeromagnetic geophysical survey is to detect minerals or rocks that have unusual magnetic properties which reveal themselves by causing anomalies in the intensity of the earth’s magnetic field (USGS, 1997).

Aeromagnetic   data was once presented as contour plots but it is more commonly expressed as colour and shaded computer generated Pseudo-topographic images where in the apparent hills, ridges and valleys are referred to as aeromagnetic anomalies. The Aeromagnetic survey is applied in mapping these anomalies in the earth’s magnetic field and it is subsequently correlated with the underground geological structures.

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