Geophysical Investigation of Abakaliki Area using Aeromagnetic Method

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.

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