MAPPING OF MAGNETIC ANOMALIES IN SOME PARTS OF NIGERIA CHAD BASIN, USING AEROMAGNETIC DATA

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ABSTRACT

Aeromagnetic data over some part of the Chad Basin, Nigeria was analyzed. The aeromagnetic anomaly map, its qualitative and quantitative interpretation helped in identifying the nature and depth of the magnetic sources in the study region. The regional-residual separation was performed with a first order polynomial using oasis montaj software. The residual data obtained was contoured into 3-D maps and this revealed two prominent magnetic anomaly source intensities; the low frequency anomaly and the high frequency anomaly sources. The low frequency anomalies emanates from deep seated bodies in areas with thicker sediments and may be viewed as the magnetic basement depth. The high frequency anomaly emanates from shallow seated geologic bodies in areas of shallower sediments and may be attributed to the volcanic rocks that intruded into the sedimentary formation and this could possibly be responsible for the mineralization found in the area. The maximum magnetic field amplitude is greater than 223 nT mostly above the earth surface while the lowest is less than 96 nT. Varying magnetic highs and lows have been found in the study area which might likely indicate undulating basement surface and likely traps. Depth to the basement of the basin structure ranges from about 0.5 km in the southern part of the study area and gets deeper toward the northern part up to 3.0 km. Lineament and structural complexity map and magnetic depth map values have been utilized to construct the interpretation of the main subsurface structures affecting the studied area, which correlate with the previous studies of the Chad Basin. The lineaments map shows the long regional trends which control the subsurface structure beneath the studied area in an ENE–WSW, WNW–ESE and E–W dominant structural trends directions. Results of the Spectral analysis method enabled delineation of the lithology and mapping the depths of subsurface geologic structures. The study indicated that there is a major source of disturbance at the southern end of the map which trends in an approximately NE-SW direction and is a possible target for feature analysis as it has very high magnetic field anomaly.

Also, the Standard Euler 3D plots in clear terms showed that the shallow depth of about 214 m for the possible causative sources. The depths in the eastern part of the area are not uniform. In the study area, maximum depth is greater than 770m and is found mostly at the southwestern part of the area where the highest magnetic field intensity amplitude from TMI is located. Therefore, this area has the thickest sedimentary rocks and could be the most probable areas for future analysis. The Energy Power Spectrum calculated average depth to the top of regional sources is about 7 km whereas the shallow sources have depth of about 1.5 km as an average depth to their sources. The depth to the top of the intrusive causative targets was calculated from the aeromagnetic map using spectral analysis method. It was found that the depth result generally ranges from 285.6 m (outcropping and shallow magnetic bodies) to 5400.3 m (deep lying magnetic bodies).

Title page

i[D1]

Declaration

Certification

iii

Dedication

Acknowledgement

Abstract

vii

Content

viii-x

List of Figures

List of Tables

xii

List of abbreviations and symbols

xiv-xv

CHAPTER ONE: INTRODUCTION

1.1 Background to Study

1-6

1.2 Statement of Problem

1.3 Aim and Objectives

1.4 The Geography of the Area

8-11

1.4.1 Topography

1.4.2 Drainage

1.4.3 Climate

9-10

1.4.4 Vegetation

10-12

1.4.5 People

12-13

1.4.6 Economy

13-15

CHAPTER TWO: LITERATURE REVIEW

2.0 Literature Review

2.1 Previous work done

15-19

2.2 Geology of the area

19-23

2.2.1 The Chad Formation

23-27

2.3 Basic theory of aeromagnetic method

29-33

CHAPTER THREE: MATERIALS AND METHODOLOGY OF STUDY

3.1 Data Acquisition

34-36

3.2 Data Interpretation

3.2.1 Regional- Residual separation using the Fast Fourier Transform (FFT)

3.2.2 Reduction-to-the-pole

3.2.3 Edge detection methods (Source Edge Detection)

3.2.3.1 The tilt angle derivative (TDR)

3.2.3.2 Standard 3D-Euler Deconvolution (ED)

39-41

3.2.4 Depth to basement estimation

3.2.4.1 Analytic signal (total gradient) method

41-42

3.2.4.3 Source Parameter Imaging (SPI) Technique (local wave number technique)

42-43

CHAPTER FOUR: RESULTS AND DISCUSSION

4.0 Introduction

4.1 Results

4.2.1 Total Magnetic Intensity Data

45-46

4.2.2 Lineament Extraction and Presentation

47-49

4.2.3 Edge detection for depth map presentation

4.2.3.1 The tilt angle derivative (TDR)

50-51

4.2.3.2 Standard 3D-Euler Deconvolution (ED)

52-53

4.2.4 Depth to basement estimation (Analytic signal (total gradient))

54-55

4.2.5 Energy Power Spectrum

56-57

4.2.6 Source Parameter Imaging (SPI)

57-62

CHAPTER FIVE: DATA ANALYSIS

5.1 Introduction

5.2 Data Analysis

5.2.1 Analyses of results of Total Magnetic Intensity

63-64

5.2.2 Analyses of results from Lineament

64-65

5.2.3 Analyses of edge detection results

5.2.3.1 The tilt angle derivative (TDR)

5.2.3.2 Standard 3D-Euler Deconvolution (ED)

65-66

5.2.4 Analyses of depth to basement estimation results

5.2.4.1 Analytic signal (total gradient)

5.2.4.2 Energy Power Spectrum

5.2.4.3 Source Parameter Imaging

67-68

CHAPTER SIX: CONCLUSION AND RECOMMENDATION

6.1 Summary

6.2 Conclusion

70-71

6.3 Recommendation

6.4 Appendix

72-79

Reference

80-84

LIST OF FIGURES

Figure 1: Map of Nigeria showing Bornu-Chad basin (After Obaje et al., 2004)

Figure 2: Map of the study area showing the various LGA’s within

Figure 3: Geologic Map of the Chad basin (modified after NGSA). Source: NGSA (2006). Geological and Mineral Resources Map, Nigerian Geological Survey Agency

Figure 4.1: Total Magnetic Intensity Colour-shaded Grid

Figure 4.2: Lineament map of the basin

Figure. 4.3: Rose plot diagram showing major structural trend

Figure 4.4: Structural complexity map of the study area

Figure 4.5: Tilt derivatives map of the study area

Figure 4.6: Standard 3D-Euler deconvolution of the study area

Figure 4.7: Analytical signal colour shaded relief map of the area

Figure 4.8: Power spectrum of aeromagnetic data showing the corresponding average depths

Figure4.9: Source Parameter Imaging of the study area

Figure 4.9a: Five Profiles across source parameter imaging of the study area

Figure 4.9b: Depth Presentation of profile 1

Figure4.9c: Depth Presentation of profile 2

Figure 4.9d: Depth Presentation of profile 3

Figure 4.9e: Depth Presentation of profile 4

Figure 4.9f: Depth Presentation of profile 5

LIST OF TABLES

Table 1: Generalized stratigraphic column of the study area (modified after Avbovbo et. al)

Table 4.1a: Lineament Orientation Data

72-75

Table 4.1b: Lineament Orientation

76-79

LIST OF ABBREVIATIONS AND SYMBOLS

Tm microtesla

nT nanotelsa

The gradient operator

Degree

m Minute

s Seconds

2D Two-Dimensions

3D Three-Dimensions

E East

e.g. Example

eq. Equation

et al. and others

Fig. Figure

GPS Global Positioning System

km kilometre

m metre

Ma Million years

N North

N – S North-South

NE – SW Northeast-Southwest

NNE North Northeast

NNPC Nigerian National Petroleum Commission

NNW –SSE North Northwest-South Southeast

NW – SE Northwest-Southeast

NYG Nigerian Younger Granite

SSW South Southwest

WSW West South West

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MAPPING OF MAGNETIC ANOMALIES IN SOME PARTS OF NIGERIA CHAD BASIN, USING AEROMAGNETIC DATA

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