UNIVERSITY OF GHANA 

COLLEGE OF BASIC AND APPLIED SCIENCES 

 

CHARACTERIZATION    OF    FAULT    SYSTEMS    USING    

GEOPHYSICS    IN    THE SOUTHWESTERN PARTS OF THE 

AKUAPEM – TOGO RANGE, SOUTHEAST GHANA 

 

 

 

BY 

 

KOFI DUKU 

10279112 

 

 

 

 

 

THIS THESIS IS SUBMITTED TO UNIVERSITY OF GHANA, 

LEGON, IN PARTIAL FULFILMENT OF THE REQUIREMENT 

FOR THE AWARD OF MPHIL GEOLOGY DEGREE 

 

 

JUNE, 2015

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DECLARATION 

This thesis is the result of research work undertaken by Duku Kofi in the Department of 

Earth Science, University of Ghana, under the supervision of Dr. Thomas Armah and Dr. 

Paulina E. Amponsah in partial fulfillment for the award of a Master of Philosophy of 

Science in Geology.   

 

 

 

 

........................................     Date:……………… 

DUKU KOFI 

STUDENT   

 

 

 

…………………………………….    Date:……………… 

DR. THOMAS ARMAH  

SUPERVISOR 

 

 

.......................................     Date:………………  

DR. PAULINA E. AMPONSAH 

SUPERVISOR 

 

 

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ABSTRACT 

The Akwapim Fault zone is one of the two major fault zones in the southeastern part of 

Ghana. Its junction with the Coastal Boundary Fault happens to be epicenter for most of 

the earthquakes that occur in Ghana. The earliest work done in this region was by the Gold 

Coast Geological Survey on a regional scale in mapping the area to determine the 

geological structures. With most of geological deformational features being inferred, it is 

prudent to carry out more research to identify more features to substantiate or disprove the 

inferences that have been made through different modes of investigation. In this study, 

regional resolution magnetic data, time and frequency domain electromagnetic data and 

gravity data are some geophysical data that are used to help define the structural setting of 

the Akwapim fault zone. Ground geophysical survey in addition to geological field 

mapping is carried out in the area to augment the information given by the aero geophysical 

data.  The electrical resistivity methods comprising the Square array, the Schlumberger 

vertical electrical sounding and the Wenner azimuthal sounding were the geophysical 

survey methods used. The data obtained from the surveys were processed and interpreted. 

The data from the geological field mapping is plotted and analyzed to identify the attitudes 

of the geological structures associated with deformational terrain. From the aero 

geophysical data processed, grids are made to distinguish features relating to the 

deformation occurring within the study area and also identify a relation between the results 

of the surveys carried out. The occurrence of relatively high amounts of uranium from the 

radiometric concentration grid produced indicates the occurrence of tectonism within the 

area. The various results obtained from the ground geophysical survey were compared to 

see which array or combination of arrays best describes the subsurface. Plots from the 

various geo-electric arrays produced results to indicate a consistent layer of very low 

apparent resistivity for all the survey points. The Square array and the azimuthal array 

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were used to identify fractures and their properties while the Square array and the 

Schlumberger array were used to locate the bedrock at the various sample point. The 

square array had a higher penetration effect than the schlumberger array. Data from the 

azimuthal array was used to plot possible fault lines on a geological map of the area 

indicating that the faulting is not limited to just one geological domain. The square array 

tends to require less space than the two other arrays used and as such can be singularly 

used to do both depth soundings and azimuthal surveys to locate fractures at depth. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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DEDICATION 

To God, for giving me Rose Ackah and Dr. Moses H. Duku 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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ACKNOWLEDGEMENTS 

My unreserved gratitude goes to my mum for all the encouragement and advice in starting 

this academic pursuit. My appreciation also goes to Dr. Thomas Armah and Dr. Paulina 

Amponsah for the guidance, help and encouragement throughout the period of writing this 

thesis.  

I would want to appreciate Prof. Prosper M. Nude for the initial advice on the topic of the 

thesis. I express profound gratitude to Mr. Sampson Renner, Mr. Patrick Banahene and 

Mr. Emmanuel Haruna for their technical inputs and support in the writing of this thesis. 

Also to be acknowledged are Mr. Obeng, Mr. Prosper Apedo, Mr. Moses Mensah, Mr. 

Opuni Antwi-Bosiako, Mr. Ben Aidoo, Mr. Sampson  Ebiasah, Team Chez, Team Tapes 

and Hammers and Team Four-Geos. 

My indebtedness goes to the Geological Survey Department of Ghana for assisting me 

with secondary  

Aerogeophysics data and to the University of Ghana for allowing me the opportunity to 

undertake a master’s degree program 

 

 

 

  

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TABLE OF CONTENTS 

Content           Page 
DECLARATION .................................................................................................................................. i 

ABSTRACT ........................................................................................................................................ ii 

DEDICATION ................................................................................................................................... iv 

ACKNOWLEDGEMENTS ................................................................................................................... v 

TABLE OF CONTENTS ...................................................................................................................... vi 

LIST OF TABLES ............................................................................................................................... ix 

LIST OF FIGURES .............................................................................................................................. x 

LIST OF APPENDIXES ...................................................................................................................... xii 

CHAPTER ONE.................................................................................................................................. 1 

1.1 Introduction .......................................................................................................................... 1 

1.2 Objective ............................................................................................................................... 2 

1.3 Study Area ............................................................................................................................. 2 

1.3.1 Location and size ............................................................................................................ 2 

1.3.2 Accessibility .................................................................................................................... 3 

1.3.3 Physiography .................................................................................................................. 3 

1.4 Geological Setting ................................................................................................................. 5 

CHAPTER TWO ............................................................................................................................... 11 

LITERATURE REVIEW ..................................................................................................................... 11 

2.1 Geology ............................................................................................................................... 11 

2.1.1 The Pan-African cross section ...................................................................................... 12 

2.1.2 The Western Boundary Fault ....................................................................................... 16 

2.1.3 Faults Parallel or Subparallel To the Coast ................................................................... 17 

2.1.4 Transverse Faults.......................................................................................................... 19 

2.1.5 Fault Patterns in the Akwapim Range and Related Earthquakes ................................. 20 

2.2 Geological Structures .......................................................................................................... 22 

2.2.1 Joints ............................................................................................................................ 22 

2.2.2 Folds ............................................................................................................................. 24 

2.3 Geophysical Investigation Methods .................................................................................... 24 

CHAPTER THREE ............................................................................................................................ 30 

METHOLOGY ................................................................................................................................. 30 

3.1 Introduction ........................................................................................................................ 30 

3.2 Desk Study ........................................................................................................................... 30 

3.3 Aero – Geophysical Data: Acquisition and Processing ........................................................ 30 

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3.3.1 Magnetic Data .............................................................................................................. 31 

3.3.2 Radiometric Data ......................................................................................................... 37 

3.4 Geological Structural Mapping ............................................................................................ 38 

3.5 Ground Geophysical Surveys ............................................................................................... 38 

3.5.1 Geophysical Resistivity Methods ................................................................................. 38 

3.5.2 Square Array ................................................................................................................. 42 

3.5.3 Schlumberger Array ..................................................................................................... 45 

3.5.4 Wenner Azimuthal Array .............................................................................................. 46 

3.5.5 Azimuthal Apparent Resistivity Measurements ........................................................... 47 

CHAPTER 4 ..................................................................................................................................... 48 

RESULTS AND DISCUSSION ............................................................................................................ 48 

4.1 Aeromagnetic Data ............................................................................................................. 48 

4.1.1 Microlevelling ............................................................................................................... 48 

4.1.2 Butterworth Filter ........................................................................................................ 50 

4.1.3 First Vertical Derivative filter ....................................................................................... 51 

4.1.4 Analytical Signal ........................................................................................................... 52 

4.1.5 Directional Cosine Filter ............................................................................................... 54 

4.1.6 Downward Continuation Filter ..................................................................................... 55 

4.1.6 Upward Continuation Filter.......................................................................................... 56 

4.1.7 Inferred Fault/ Fracture Lines ...................................................................................... 57 

4.2 Radiometric Data .......................................................................................................... 59 

4.3 LITHOLOGICAL DISTRIBUTION ............................................................................................. 60 

4.3.1 Foliation............................................................................................................................ 61 

4.3.2 Joints ............................................................................................................................ 64 

4.4 GROUND GEOPHYSICAL SURVEY ......................................................................................... 65 

4.4.1 Square Array Method ................................................................................................... 66 

4.4.2 Schlumberger Sounding ............................................................................................... 71 

4.4.3 Azimuthal Sounding ..................................................................................................... 90 

4.5 Comparism of the Geophysical Electrical Resistivity Methods Used ................................ 100 

CHAPTER FIVE .............................................................................................................................. 102 

CONCLUSION AND RECOMENDATIONS ...................................................................................... 102 

5.1 Conclusion ......................................................................................................................... 102 

5.2 Recommendation .............................................................................................................. 103 

REFERENCES ................................................................................................................................ 105 

APPENDIX 1A ............................................................................................................................... 110 

APPENDIX 2A ............................................................................................................................... 113 

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APPENDIX 2B ............................................................................................................................... 117 

APPENDIX 2C ............................................................................................................................... 126 

 

 

 

 

 

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LIST OF TABLES 

Table 1: A table showing the parameters used for the butterworth filtering process. ...... 33 

Table 2: Apparent resistivity measurements at point KP3 ............................................... 67 

Table 3: Apparent resistivity measurements taken at point KP5 ...................................... 68 

Table 4: Apparent resistivity measurements taken at point KP6 ...................................... 69 

Table 5: Apparent resistivity measurements from point KP7 .......................................... 69 

Table 6: Apparent resistivity measurements at point KP8 ............................................... 70 

Table 7: Apparent resistivity values taken at point KP11 ................................................ 71 

Table 8: Table showing the survey parameters for the GTK aero geophysical survey .. 111 

Table 9: Table showing the projection parameters in the aero geophysical data 

processing ....................................................................................................................... 112 

Table 10: Table showing the local datum shift parameters ............................................ 112 

Table 11: Apparent resistivity measurements taken at point KP1 .................................. 113 

Table 12 : Apparent resistivity measurements taken at point KP2 ................................. 113 

Table 13: Apparent resistivity measurements taken at point KP4 .................................. 114 

Table 14: Apparent resistivity measurements taken at point KP9 .................................. 114 

Table 15: Apparent resistivity measurements taken at point KP10 ................................ 114 

Table 16: Apparent resistivity measurements taken at point KP11 ................................ 115 

Table 17: Apparent resistivity measurements taken at point KP12 ................................ 115 

Table 18: Apparent resistivity measurements taken at point KP13 ................................ 115 

Table 19: Apparent resistivity measurements taken at point KP14 ................................ 116 

Table 20: Apparent resistivity measurements taken at point KP15 ................................ 116 

  

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LIST OF FIGURES 

Figure 1: Geological Map of the Study Area ...................................................................... 3 

Figure 2: The Romanche transform margin ...................................................................... 13 

Figure 3: Schematic cross section of the Pan-African Dahomeyide orogeny and the 

eastern margin of the WAC in southeastern Ghana. ......................................................... 13 

Figure 4: Stereographic polar diagram of joints in Togo Series ....................................... 23 

Figure 5: (a) Theoretical azimuthal apparent-resistivity ellipse plotted as a polar diagram. 

(b) After removal of a datum value the ellipse takes on a characteristic double-lobed 

form. .................................................................................................................................. 28 

Figure 6: Map of the study area showing the survey points ............................................. 41 

Figure 7: Electrode positions for Square Array measurements ........................................ 42 

Figure 8: Electrode positions for the first and second orientation of the Square array .... 42 

Figure 9: Symmetrical Expansion of the square about the center point. .......................... 44 

Figure 10: A schematic representation of the Schlumberger array Configuration ........... 45 

Figure 11: A map grid of the microlevelled total magnetic intensity (TMI) data of the 

study area. ......................................................................................................................... 49 

Figure 12: Butterworth filter grid of the total magnetic intensity (TMI) data of the study 

area. ................................................................................................................................... 50 

Figure 13: First Vertical Derivative grid of the total magnetic intensity (TMI) data of the 

study area. ......................................................................................................................... 52 

Figure 14: Analytical signal grid of the total magnetic intensity (TMI) data of the study 

area. ................................................................................................................................... 53 

Figure 15: Directional cosine filter grid of the total magnetic intensity (TMI) data of the 

study area. ......................................................................................................................... 54 

Figure 16: Downward Continuation grid of the total magnetic intensity (TMI) data of the 

study area. ......................................................................................................................... 55 

Figure 17: Upward Continuation grid of the total magnetic intensity (TMI) data of the 

study area. ......................................................................................................................... 56 

Figure 18: A grid of the aeromagnetic data of the area showing traces of possible fault/ 

fracture lines in the area .................................................................................................... 58 

Figure 19: Uranium concentration map of the study area. ............................................... 59 

Figure 20: An outcrop of the quartzites along the beach from Langma to Nyanyano 

showing multiple joints..................................................................................................... 61 

Figure 21: A stereographic plot to pole of the dips and dip direction of the foliation ..... 62 

Figure 22: A stereographic plot of the dips and dip direction of the foliation in the 

quartzites. .......................................................................................................................... 62 

Figure 23: An equal area plot of the dips and dip directions of the foliations in the 

quartzites. .......................................................................................................................... 63 

Figure 24: A contour plot of the dips and dip directions of the foliation of the quartzites.

 .......................................................................................................................................... 63 

Figure 25: A rose plot of the dips and dip directions of the joints of the quartzites......... 64 

Figure 26: A contour plot of the dips and dip directions of the joints of the quartzites ... 65 

Figure 27: VES curve of schlumberger sounding data from Survey Point KP3 .............. 73 

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Figure 28: VES curve of Schlumberger data taken at point KP3a, 10 meters north of 

point KP3 .......................................................................................................................... 73 

Figure 29: VES curve of Schlumberger data taken  at point KP3b, 10 meters south of 

survey point KP3 .............................................................................................................. 74 

Figure 30: Pseudo-section of points KP3a, KP3 and KP3b .............................................. 74 

Figure 31: VES curve of the Schlumberger sounding data taken from survey point KP5

 .......................................................................................................................................... 76 

Figure 32: VES curve of Schlumberger data taken at point KP5a, 10 meters north of 

survey point KP5 .............................................................................................................. 76 

Figure 33: VES curve of Schlumberger data taken at point KP5b, 10 meters south of 

survey point KP5 .............................................................................................................. 77 

Figure 34: Pseudo section of points KP5a, KP5 and KP5b .............................................. 77 

Figure 35: VES curve of the Schlumberger sounding data taken from survey point KP6

 .......................................................................................................................................... 78 

Figure 36: VES curve of Schlumberger data taken at point KP6b, 10 meters north of 

survey point KP6 .............................................................................................................. 79 

Figure 37: VES curve of Schlumberger data taken at point KP6b, 10 meters south of 

survey point KP6 .............................................................................................................. 79 

Figure 38: Pseudo section of points KP6a, KP6 and KP6b .............................................. 79 

Figure 39: VES curve of the Schlumberger sounding data taken from survey point KP7

 .......................................................................................................................................... 81 

Figure 40: VES curve of Schlumberger data taken at point KP7a, 10 meters north of 

survey point KP7 .............................................................................................................. 82 

Figure 41: VES curve of Schlumberger data taken at point KP7b, 10 meters south of 

survey point KP7 .............................................................................................................. 82 

Figure 42: Pseudo section of points KP7a, KP7 and KP7b .............................................. 82 

Figure 43: VES curve of the Schlumberger sounding data taken from survey point KP8

 .......................................................................................................................................... 84 

Figure 44: VES curve of Schlumberger data taken at point KP8a, 10 meters north of 

survey point KP8 .............................................................................................................. 84 

Figure 45: VES curve of Schlumberger data taken at point KP8b, 10 meters south of 

survey point KP8 .............................................................................................................. 85 

Figure 46: Pseudo section of points KP8a, KP8 and KP8b .............................................. 85 

Figure 47: VES curve of the Schlumberger sounding data taken from survey point KP11

 .......................................................................................................................................... 86 

Figure 48: VES curve of Schlumberger data taken at point KP11a, 10 meters north of 

survey point KP11 ............................................................................................................ 86 

Figure 49: VES curve of Schlumberger data taken at point KP11b, 10 meters south of 

survey point KP11 ............................................................................................................ 87 

Figure 50: Pseudo section of points KP11a, KP11 and KP11b ........................................ 87 

Figure 51: Proposed Geological map of the study area showing the various resistivity 

curve types recorded at the various survey points ............................................................ 89 

Figure 52: Plots of Wenner Azimuthal resistivity sounding at survey point KP3 ............ 90 

Figure 53: Plots of Wenner Azimuthal resistivity sounding at survey point KP5 ............ 92 

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Figure 54: Plots of Wenner Azimuthal resistivity sounding at survey point KP6 ............ 94 

Figure 55: Plots of Wenner Azimuthal resistivity sounding at survey point KP7 ............ 95 

Figure 56: Plots of Wenner Azimuthal resistivity sounding at survey point KP8 ............ 96 

Figure 57: Plots of Wenner Azimuthal resistivity sounding at survey point KP11 .......... 97 

Figure 58: A proposed geological map of the area showing the inferred fracture lines 

deduced from the azimuthal geophysical survey .............................................................. 98 

Figure 59: VES curves and pseudo-section for KP1a, KP1 and KP1b .......................... 117 

Figure 60: VES curves and pseudo-section for KP2, KP2a and KP2b .......................... 118 

Figure 61: VES curves and pseudo-section for point KP4, KP4a and KP4b ................. 119 

Figure 62: VES curves and pseudo-section for the point KP9, KP9a and KP9b ........... 120 

Figure 63: VES curves and pseudo-section for schlumberger sounding at KP10, KP10a 

and KP10b ...................................................................................................................... 121 

Figure 64: VES plots and pseudo-section of schlumberger sounding at KP12, KP12a and 

KP12b ............................................................................................................................. 122 

Figure 65: VES plots and pseudo-section of schlumberger sounding at KP13, KP13a and 

KP13b ............................................................................................................................. 123 

Figure 66: VES plots and pseudo-section of schlumberger sounding at KP14, KP14a and 

KP14b ............................................................................................................................. 124 

Figure 67: VES plots and pseudo-section of schlumberger sounding at KP15, KP15a and 

KP15b ............................................................................................................................. 125 

Figure 68: Wenner Azimuthal array plots for KP1 ........................................................ 126 

Figure 69: Wenner Azimuthal array plots for KP2 ........................................................ 126 

Figure 70: Wenner Azimuthal array plots for KP4 ........................................................ 127 

Figure 71: Wenner Azimuthal array plots for KP9 ........................................................ 128 

Figure 72: Wenner Azimuthal array plots for KP10 ...................................................... 128 

Figure 73: Wenner Azimuthal array plots for KP12 ...................................................... 129 

Figure 74: Wenner Azimuthal array plots for KP13 ...................................................... 129 

Figure 75: Wenner Azimuthal array plots for KP14 ...................................................... 130 

Figure 76: Wenner Azimuthal array plots for KP15 ...................................................... 130 

 

 

LIST OF APPENDIXES 

Appendix 1:  Details on the Aeromagnetic data. ……..…………………………….. 110 

Appendix 2a: Apparent resistivity measurements for the Square array. ..…….......... 113 

Appendix 2b: VES curves for the Schlumberger sounding. ….………………….…. 117    

Appendix 2c: Polar plots for the Azimuthal array measurements. …………………. 126

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CHAPTER ONE 

1.1 Introduction 

The Akwapim Fault is one of the two major fault zones in the southeastern part of Ghana. 

Its junction with the Coastal Boundary Fault happens to be epicenter for most of the 

earthquakes that occur in Ghana (Bacon and Quaah, 1981)  

The earliest work done in this region was by the Gold Coast Geological Survey on a 

regional scale in mapping the area to determine the geological structures in the area 

(Amponsah et al., 2002). They recognized various structures that gave the indication of a 

fault occurring in this region. 

Until recently, geophysical methods have not been used in the study area due to the fact 

that most of the geophysical investigations have been targeted at mineral exploration in 

other areas. Geological field mapping as well as geochemical methods have been carried 

out to identify the occurrence certain feature associated with deformation. With no exact 

knowledge of the subsurface, inferences made from these previous study have been 

accepted as the best description of the subsurface character in the study area.  

Regional resolution magnetic data, time and frequency domain electromagnetic data and 

gravity data are some geophysical data that can be used to help define the structural setting 

of the Akwapim fault zone. Further analysis can be done to further define and delineate 

the structures associated with the main fault as well as the minor faults. This can be done 

using softwares such as ArcGIS, MapInfo and Oasis Montaj. Regional field data of the 

area with emphasis on the geological structures will be a boost in the development of a 

good interpretation of the nature of the Fault zone. 

 

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1.2 Objective 

The main objectives of the study are to: 

 identify and characterise the Western Boundary Fault and its associated minor 

faults in the Akwapim Fault zone in Southeast Ghana; 

 determine the influence of the Akwapim Fault zone on the seismicity in the study 

area and 

 determine which geophysical method best characterizes the fault structures in the 

subsurface, data from which will be used to support the geological data already 

known. 

 

1.3 Study Area 

1.3.1 Location and size 

The Akwapim-Togo Range is a narrow belt of ridges and hills in Ghana. It extends in a 

southwest-northeast line for about 320 km from the mouth of the Densu River (near Accra) 

on the Atlantic coast to the boundary with Togo. Averaging 460 m in height, the hills 

continue eastward to the Niger River as the Togo Mountains in Togo and as the Atakora 

Mountains in Benin, and they contain isolated peaks near the Togo border. The study area 

(figures 1 and 6) falls within the Ga South Municipal area and the Awutu Senya East 

District. 

 

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1.3.2 Accessibility 

The study area is mainly accessible by a first class road (Accra to Winneba road), second 

class roads (Kasoa main town to Nyanyano) as well as feeder roads (Weija Toll Booth 

Junction to Tuba, Kokrobite and Langma and surrounding towns).  

 

Figure 1: Geological Map of the Study Area (After Geological Survey Department Of Ghana, 2010)  

 

1.3.3 Physiography 

Three major rivers that drain the area are Densu, Nsaki and Onyasana rivers. The largest 

of the three, the Densu drains down from the Eastern Region through the western part of 

the Municipality to Weija where it enters the sea. The land area consists of gentle slopes 

interspersed with plains in most parts and generally undulating at less than 76 m above sea 

level. The slopes are mostly formed over the clay soils of the Dahomeyan gneiss with 

alluvial areas surrounding the coastal lagoons generally flat. 

The Akwapim range and the Weija hills rise steeply above the western edge. The crest of 

the Akwapim range lies generally at 300 m elevation southwards. This line of hills 

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continues through to the Weija hills with the highest point reaching 192 m near Weija. 

There are three major rivers namely; the Densu, Nsaki and Ponpon River that drain the 

district. The largest of the three is the Densu which drains down from the Eastern Region 

through the western portions of the district to Weija where it enters the sea. It is the main 

source of water supply to over half the entire population of the Accra Metropolis. Other 

water bodies mostly tributaries of the Densu are the Adaiso, Doblo, Ntafafa and the 

Ponpon River. 

Most parts of the study area are well drained although there are vast lowland areas present. 

There are two major hills with the south-western portion of the Akwapim-Togo range also 

present in the area. The south-western part of the Weija Lake occurs in the northeastern 

part of the area and in addition to that the Weija estuary is also present at the south-western 

part of the area. 

The study area lies wholly in the coastal savannah agro-ecological zone. The rainfall 

pattern is bi-modal with an annual mean varying between 790 mm on the coast to about 

1270 mm in the extreme north. The annual average temperature ranges between 25.1°C in 

August and 28.4°C in February and March. Humidity is generally high during the year. 

Average humidity figures are about 94% and 69% and 15:00 hours respectively (Ministry 

of Local Government and Rural Development, 2006). The Municipal lies wholly in the 

coastal savanna agro-ecological zone. The land area is underlain by shallow rocky soils 

and are extensively developed on the steep slopes of the Akwapim range and the Weija 

hills as well as the basic gneiss inselbergs. On the Akwapim range the soils are mainly 

pale and sandy with brushy quartzite occurring to the surface in most places. The abundant 

sandstone and limestone present in the area are good source of material for the construction 

industry (Muff and Efa, 2006). 

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They are typically loamy in texture near the surface becoming more clay below. The red 

soils are porous and well drained and support road development and also provide ample 

moisture storage at depth for deep-rooting plants. Nutrients supplies are concentrated in 

the humus top-soil. 

1.4 Geological Setting 

About two thirds of the land surface of Ghana is covered by the Birimian rocks of the 

Paleoproterozoic age (Ahmed et al., 1977). The Birimian rocks form the eastern end of the 

West African Craton which has remained stable since 1.7Ga. The Birimian rocks are made 

up of the Volcanic Belts and the Sedimentary Basins.   The rocks of the Dahomeyide and 

the Voltaian lie in the eastern portions of Ghana. The Dahomeyide is made up of the 

Dahomeyan, Togo and Buem structural units (Ahmed et el, 1977, Muff and Efa, 2006). 

The study area falls in the area of the Togo rock units. 

The Togo rock formation is generally made up of quartzites, quartz-schists, sericite schists 

and phillites (Junner and Service, 1936). The sedimentary rocks of the Togo form a 

prominent northeast-southwest trending mountain range known as the Akwapim-Togo 

range. The Togo structural unit is characterized by structures such as foliation, folds, faults 

and joints caused by the Pan African Orogeny (Ahmed et al., 1977). The Pan African 

Orogeny resulted in the intense folding and faulting of the supracrustal sedimentary rocks 

in the Togo. The mountain range is bounded to the east by the Dahomeyan rocks and to 

the west by the Cape Coast granitoid complex, the Voltaian and the Buem Structural Units. 

Two major thrust faults bound the Akwapim-Togo range. The fault on the east is at the 

contact with the Dahomeyan while the other fault also known as the Western Boundary 

fault is at the contact with the Cape Coast granitoid complex, the Voltaian and the Buem 

(Ahmed et al., 1977).  

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The Togo Structural unit is of the late Proterozoic age. It consists of siliciclastic sediments 

that have been subjected to a low- to intermediate grade type of metamorphism. The Togo 

Structural unit forms the greatest part of the north-north-east trending Akwapim Range 

and the Weija Mountains (Ahmed et al., 1977). In the work done by Muff and Efa (2006) 

the Togo Structural has been subdivided into three main units which reflect their 

lithological properties. These units are: 

a) Phyllite/Phyllonite Unit 

b) Quartz-Schist Unit  

c) Quartzite Unit 

(a) Phyllite/Phyllonite Unit 

This unit consists mainly of phyllite and phyllonite, which are often talcy. Phyllonites 

make up a large part of this unit. Phyllonites are phyllitic rocks of mylonitic origin and 

they belong to the group of rocks called fault rocks. The phyllonites show thin, stretched 

out lenses of grey or white quartz in a very fine grained foliated matrix. Sometimes thin, 

planar layers exhibit clear signs of mylonisation (Ahmed et al., 1977). The phyllonites 

often possess a cherty aspect and sometimes they have a soapy feeling to the touch. The 

terms cherty phyllite, quartz-phyllite or finely-laminated quartzite describe their texture 

and composition best.   

Phyllonites underlie large parts of the eastern part of the Akwapim Range and they form 

the low-lying country rock in the Achimota area. Phyllonite has been found in many 

excavation pits along the Togo Structural unit/Dahomeyan System contact to the northeast 

of the Ghana Atomic Energy Commission offices and fragments of phyllonites are often 

found in the top of shallow elevations where red soil formed over mica schists of the 

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Dahomeyan System. The fragments are relics of phyllonite layers which now have been 

eroded.   

Mapping done by Efa and Muff (2006) in the coastal area showed that phyllonites are 

exposed southwest of Gomoa Fetteh, where they are associated with augen schists of the 

Dahomeyan Structural unit at a thrust fault. Phyllonite also occurs in a track leading to the 

top of a hill immediately west of Gomoa Fetteh, and it outcrops over several tens of meters 

in a ditch in the dirt road leading from Gomoa Fetteh to Petuduase and Senya Beraku 

(Ahmed et al., 1977). 

The occurrence of phyllonite and fragments of phyllonites in soil over Dahomeyan rocks 

indicates that much of the Dahomeyan terrane was formerly overlain by the Togo 

Structural. The phyllonites are interpreted as products of the thrust of the Togo Structural 

unit over the Dahomeyan Formation along a thrust fault. The rocks belonging to the Togo 

Structural which are exposed in the area to the east of the Akwapim Range are interpreted 

as infolded or infaultet klippen of the Togo Series (Kesse, 1985). 

 

(b) Quartz-Schist Unit  

This unit encompasses mainly sericitic quartz-schist and chlorite-schist and contains layers 

of quartzite, phyllite or chlorite schist. The sequence is well bedded and strongly jointed. 

The thickness of argillitic layers is several millimeters to some centimeters, the arenaceous 

layers are some centimeters to a few meters thick. Schistosity is well developed and 

parallel to bedding (Ahmed et al., 1977). Quartz-schist may also have been formed by 

mylonisation. This type of quartz-schist possesses a strong slip cleavage and it consists of 

planar recrystallized quartzite layers of 0.5 to 2 centimeters thickness which alter with 1 

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to 3 millimeters thick layers of phyllosilicates which are often green or grey. The planar 

surfaces are sometimes thrown into small crinkles, called crenulation folds. The 

wavelengths and amplitudes of the crinkles are measured in millimeters (Effah and Muff, 

2006). 

 

(c) Quartzite Unit 

This unit comprises mainly quartzite and the amount of quartz-schist and phyllite is very 

variable. In some places, the quartzite possesses aspects of vaguely banded chert. Quartzite 

may occur as massive layers of up to 2 m width, or it may be laminated and with increasing 

lamination it may grade into phyllonite (Muff and Efa, 2006). 

Three types of quartzite are recognized. These are:  

a) Metaquartzite 

b) Micaceous quartzite 

c) Cherty quartzite 

Metaquartzite: It is very hard, exhibits splintery fracturing and is medium- to thick-

bedded with interlayers of phyllite. Fresh surfaces have glassy luster and are grey or dark 

grey. Rounded or stretched out quartz grains are visible.  

Micaceous quartzite: It is very-thin to thin-bedded and contains very thin layers of 

phyllite between quartzitic layers. In some places it is cherty. When fresh it is grey. 

Weathered surfaces are often white.   

Cherty quartzite: It is grey to white, sometimes with faint dark bands. It forms 

lenses which are 1 to 2 m thick and 10 to 20 m long which are parallel to the bedding. The 

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cherty quartzite is interpreted as flinty crush rock, which is a fault rock often associated 

with dynamic metamorphism.   

The Togo Structural unit has been intensely tectonically stressed and strongly brecciated 

(Ahmed et al., 1977). The rocks which make up the Akwapim Range represent the 

lowermost portion of a once much more thick sequence, of which the top part has been 

eroded away.   The strata generally dip towards the east-south-east and the intercalations 

of erosion resistant quartzites and quartz-schists with soft phyllites cause an asymmetric 

topography of steep cliff-slopes facing the west-northwest, and gentle dip-slopes facing 

the east-southeast.  

Because the bottom of the scree has not been exposed by creeks, the eastern contact 

between Togo Series and the Dahomeyan System has to be inferred from exposures some 

hundred meters apart.  Fractured rocks of the Quartzite and Quartz-Schist Unit are very 

permeable for groundwater. The Phyllite/Phyllonite Unit is very prone to erosion when 

slopes are graded or cleared at construction sites.   

The only location, where Togo Structural unit overlies the granitoids is at the northwest 

side of the mountainous block south of the Weija Lake. The “V-type” pattern of the 

granitoid/Togo Structural unit contact in valley incisions indicates a gentle southeast dip 

of the contact at the regional scale (Muff and Efa, 2006). The contact itself is exposed in 

the ditch south of the major road which leads to Winneba and beyond. The Togo Structural 

unit consists of contorted quartz and chlorite schists, and the underlying granitic rocks are 

strongly sheared. Although the contact at this specific location dips approximately 10 

degrees to the northeast, a generally low southeast dip is inferred from the outcrop pattern.  

Kesse (1985), indicate that the contact between Togo Structural unit and Dahomeyan 

Structural unit can best be studied at low tide along the coast between Gomoa Fetteh and 

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Nyanyano. Augen schist, sericitic quartz-schists, chloritic and amphibolitic rocks 

belonging to the Dahomeyan Structural units are tightly folded and overlain by fine-

laminated, light-colored cherty phyllonites. The amplitudes and wavelengths of the folds 

measure several decimeters, and they are overturned so that their axial planes dip shallowly 

to the east-south-east. The small folds are interpreted as drag folds which indicate that the 

overlying strata have been pushed towards the west-north-west direction. At some 

locations along the beach from Kokrobite to Nyannyano, garnet porphyroblasts are found 

in the Augen gneisses as well as in sericitic quartz-schist and phyllonites of the Togo 

Structural unit (Muff and Efa, 2006).    

The economic importance of the Togo structural unit is not just limited to the mining of 

the laminated sericite-quartz-schists in many places to be used as decorative floor and wall 

tiles and paving but also brecciated quartzites are important as crushed aggregate material 

for road and housing foundations. 

 

 

 

 

 

 

 

 

 

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CHAPTER TWO 

LITERATURE REVIEW 

2.1 Geology 

Birimian rocks of the Paleoproterozoic age cover about two-thirds of the land surface of 

Ghana. The Birimian rocks form the eastern end of the West African Craton which has 

remained stable since 1.7 Ga (Ahmed et al., 1977). The Birimian rocks are made up of the 

Volcanic Belts and the Sedimentary Basins.   The rocks of the Dahomeyide and the 

Voltaian lie in the eastern portions of Ghana. The Dahomeyide is made up of the 

Dahomeyan, Togo and Buem structural units. The study area falls in the area of the Togo 

rock units. The Dahomeyide resulted from the Pan African Event. The term Pan-African 

is used to describe tectonic, magmatic, and metamorphic activity of Neoproterozoic to 

earliest Palaeozoic age, especially for crust that was once Part of Gondwana. Because of 

its tremendous geographical and temporal extent, the Pan-African cannot be a single 

orogeny but must be a protracted orogenic cycle reflecting the opening and closing of large 

oceanic realms as well as accretion and collision of buoyant crustal blocks (Attoh and 

Ekwueme, 1997). Pan-African events culminated in the formation of the Late 

Neoproterozoic supercontinent Gondwana. 

The Romanche transform margin, offshore Ghana (as shown in Figure 3) is an 

oblique transect of the boundary between the West African Craton (WAC), underlain by 

Paleoproterozoic (Birimian) rocks, and the Pan-African orogenic belt (Attoh and 

Ekwueme, 1997). Along this transect the principal tectonic elements of the onshore 

geology include prominent structures such as:  

(i) the Pan-African front (PF), representing the western limit of deformation 

in the external zone, and  

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(ii) the Pan-African suture (PS) represented by a ductile shear zone at the base 

of high-pressure (HP) mafic granulites which mark the eastern edge of 

WAC (Attoh et al., 1997).  

The Romanche Fracture Zone (RFZ) intersects the coastline east of the suture zone and 

projects into the Pan-African dextral shear zone (TSS). To the west of the Pan-African 

front, the WAC is underlain by Birimian granitoids and greenstone belts. 

  

2.1.1 The Pan-African cross section 

A schematic geological cross-section of the southern segment of the Pan-African 

Dahomeyide Orogeny (Figures 2 and 3) allows comparison of structures in the seismic 

sections with those inferred from onshore geology (Attoh et al., 1997). It shows the 

boundary between the WAC, to the west, and the Dahomeyide orogeny east of the Pan-

African front (PF). The Pan-African external zone comprises of west-verging Atacora 

nappes composed of quartzites and quartz schists locally imbricated with low-strain 

mylonites derived from granitoids of the deformed margin of the 2.1 Ga West African 

Craton. To the east of the Pan-African suture zone (PS) are the Neoproterozoic rocks that 

comprise the accreted (Attoh et al, 1997). 

 

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Figure 2: The Romanche transform margin, offshore Ghana showing the major tectonic elements of 
onshore geology and offshore seismic lines referred to in this study: PF = Pan-African Front, PS = Pan-
African Suture zone (suture zone gneiss shown in striped pattern) 

 

 

Figure 3: Schematic cross section of the Pan-African Dahomeyide orogeny and the eastern margin of the 
WAC in southeastern Ghana. Symbols and abbreviations as in Fig. 3 after Attoh et al, 2005 

 

 

 

 

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Although Ghana is remote from the major earthquake zones, it is moderately active 

seismically, with a history of earthquakes damaging the capital, Accra. Seismic recording 

done during 1977 to 1980 has been used to develop a crustal velocity model and locate 

epicenters. Work done by Bacon and Quaah (1981) indicate that most of the earthquakes 

occur in an area to the west of Accra around the junction of two major fault systems, the 

east-west trending Coastal Boundary fault and the NNE trending Akwapim fault zone. 

Fault plane solutions suggest a mixture of normal faulting on the Akwapim fault zone (in 

agreement with geological evidence) and strike-slip faulting, in response to a regional ESE 

tensional stress perhaps caused by strike-slip movement along the Romanche fracture 

zone. The Akwapim Fault is one of the two major fault zones in the southeastern part of 

Ghana. Its junction with the Coastal Boundary Fault happens to be epicenter for most of 

the earthquakes that occur in Ghana.  

The Akwapim Fault Zone (AFZ) is made up of a northeast–southwest trending system of 

faults outlined by the Western Boundary Fault (WBF) on the western side and the Eastern 

Boundary Fault (EBF) on the eastern side as described by Ahmed et al., (1977). Both faults 

are overthrusts of Neo- Proterozoic age. Edwards et al., (1997) suggest that the fault 

contact of the West African Craton with the Pan-African Province can be extrapolated 

beneath the continental shelf although the bathymetric maps do not show a continuation 

of the Akwapim Range out into the shelf area. Recent large-scale mapping exercises in the 

southern part of the Akwapim fault zone (Muff and Efa, 2006) shows that at a later stage, 

the Akwapim Togo Belt has been subjected to a block-tectonic style of deformation and 

that many normal faults of local extension have developed in recent times. 

Coastal Boundary Fault (CBF) Seismic studies established the occurrence of sedimentary 

basins and crustal blocks separated by normal faults in the Ghana shelf area (Blundell and 

Banson, 1975). They named the most prominent normal fault in the shelf area coastal 

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boundary fault. It strikes northeast at a distance of 3–5 km from the coast and downthrows 

the block south of it for several 1000 m. Sedimentation contemporaneous with and 

subsequent to the down-throw leveled the horizontal displacement. The coastal boundary 

fault is the northern boundary of a basin filled with sediments of Upper Jurassic to recent 

age. The fault was probably active throughout the entire time of deposition. West of Accra 

it bends to strike E-W and intersects the AFZ. Blundell (1976) suggest that the coastal 

boundary fault forms the northern margin of the Keta Basin. 

The basin geometry and sediment fill is dominated by the NE–SW striking Fenyi-Yakoe 

and Adina Faults (Akpati, 1978). Cyclic deposition commenced in the Cretaceous and 

continued until Holocene times, indicating that the Fenyi-Yakoe and Adina Faults may 

still be tectonically active (Akpati, 1978). Akpati (1978) interprets the Fenyi-Yakoe Fault 

as the extension of the Romanche Fracture Zone, while Blundell (1976) suggests that the 

Fenyi-Yakoe Fault is the eastern extension of the coastal boundary fault. The Romanche 

Fracture Zone (RFZ) represents an offshore fault system related to the opening of the 

Atlantic Ocean. The fracture zone is 6–11 km wide and runs approximately parallel to the 

coast. It represents an inactive transform fault of the Mid-Atlantic Ridge which separates 

continental from oceanic crust (Deltail et al., 1974; Mascle and Sibuet, 1974; Edwards et 

al., 1997; Attoh and Brown, 2008). Transform motion at the Romanche Fracture Zone 

(RFZ) off- shore Ghana ended after West Africa had migrated along and beyond the 

spreading center lying at the opposite side of the transform fault and the oceanic 

lithosphere south of the Romanche Fracture Zone. The continental lithosphere to the north 

of it assumed the same sense of movement. Necessarily, the forces related to the Atlantic 

spreading are ruled out as a source for the present day seismic activities. The eastward 

projection of the Romanche Fracture Zone has been speculated in many studies without 

arriving at a consensus (Akpati, 1978; Blundell and Banson, 1975; Bacon and Quaah, 

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1981; Attoh et al., 1997, 2005). This dividedness on the eastward continuation of the 

Romanche Fracture Zone was mainly caused by the fact that previous investigators treated 

the entire transform domain as a generally straight single segment. Recent interpretation 

of multichannel seismic reflections (Antobreh et al., 2009) shows that the Romanche 

Fracture Zone is composed of several segments which developed under distinct conditions. 

Antobreh et al. (2009) provide evidence that the Romanche Fracture Zone does not make 

landfall in south-east Ghana as previously thought and they are of the opinion that the AFZ 

are splays of the Romanche Fracture Zone instead of continental extensions. 

 

2.1.2 The Western Boundary Fault 

This separates unmetamorphosed siliciclastic rocks of the Voltaian Structural unit from 

metamorphosed sedimentary rocks of the Togo Structural unit in the northern part of the 

study area. The fault itself cannot be observed but its existence is inferred on the grounds 

that unmetamorphosed rocks are juxtaposed to dynamically metamorphosed rocks. 

Deformation in the rocks adjoining the Western Boundary Fault is present as tilting of 

strata and brittle brecciation. The fault appears to be restricted to a relatively narrow zone 

and the straight fault line favors a steep to mediocre dip of the fault. The location of 

unmetamorphosed sedimentary rocks next to metamorphic rocks suggests that the eastern 

bloc of the fault moved upwards (Muff and Efa, 2006). Also, the general regime of a 

compressive tectonic style calls for a reverse or thrust fault. Extensive mylonisation in a 

wide zone, comparable to that observed in the Eastern Boundary Fault, has not been 

observed and is suggestive of an entirely different faulting style.   

To the north of the Weija Lake, the western scarp of the Akwapim Range is considered an 

erosion scarp. The occurrence of a steeply dipping dip-fault located at the foot of the scarp 

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is ruled out, as the cliff slope represents only the present stage of a receding scarp caused 

by ongoing lateral erosion of the Densu River drainage system. Field evidence at the foot 

of the western scarp delivers no evidence of major tectonic processes, such as intense 

deformation or brecciation. The outcrop pattern follows the rule of V’s and indicates a 

planar contact dipping shallowly to the southeast (Muff and Efa, 2006).    

At the western side of the mountain block between the Weija Lake and Kokrobite, no rocks 

belonging to the Voltaian System have been found to fringe the Togo Series. As Voltaian 

rocks are found again further to the southwest along the coast, they have possibly been 

eroded. To the south of the Weija Lake, the continuation of the Western Boundary Fault 

which north of the lake separates Togo and Voltaian rocks, possibly runs some distance 

away from the scarp in the granitoid rocks, possibly in the topographic depression linking 

Galilea (immediately south of Weija Lake), Gidan-Tuba and Nyanyano, from where it 

extends into the ocean. In the accompanying map, it is shown as an inferred fault.   

Here it appears appropriate to look at the nomenclature which in the past has been used 

when describing faults in the Akwapim Range. The term “Akwapim Fault Zone” has been 

applied in the past by various authors with varying meanings. Some refer to it as a 

compound name for all faults occurring in the Akwapim Range, for others it is a synonym 

for either the Eastern Boundary Fault, the Western Boundary Fault, or a thrust fault which 

has not been properly described and defined.  

 

2.1.3 Faults Parallel or Subparallel To the Coast     

They are tectonically important, because some of them cause major displacements in such 

a way, that the south block was displaced downward. This can be seen in the Kokrobite 

and Langma area, where shattered quartzites and quartzschists of the Togo Series came to 

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lie adjacently to granitoids along the coast. The downthrow is at least 100 m, because close 

to the coast deeply weathered granitoid rocks are exposed on top of a 100 m high Langma 

Hill, while along the beach, and a few meters above sea level, rocks of the Togo Series are 

exposed.    

In the littoral zone in front of the Kako Lagoon near Gomoa Fetteh a fault is inferred, 

because sandstones of the Voltaian System form two large rocks which are exposed during 

low tide (Muff and Efa, 2006).   

The faults described above are parallel to a major off-shore fault which has been termed 

Great Boundary Fault, and later Coastal Boundary Fault:   

Seismic studies (Blundell & Banson, 1975; Blundell, 1976) established the presence of the 

Coastal Boundary Fault. It runs approximately parallel to the west-north-west extending 

coast at a distance of 3 to 5 km. South of Accra it curves to an east-west direction and 

intersects with the coastal area near the village of Nyanyano. The Coastal Boundary Fault 

separates a sedimentary basin filled with Jurassic and younger sediments on the seaward 

side from Devonian to Carboniferous and older rocks on the continental side Subsidence 

along this fault continues to the present time. Blundell & Banson (1975) state that the 

downthrow of the southern side of the Coastal Boundary Fault since Jurassic times is some 

several thousand meters.   

Edwards et al. (1997) suggest that the contact of the West African Craton with the Pan-

African Belt can be extrapolated beneath the continental shelf. The bathymetric maps do 

not show a continuation of the Akwapim Range out into the shelf area and it is likely that 

the mountain range has been downthrown along the coast and is being buried by recent 

sediments.   

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2.1.4 Transverse Faults   

Efa and Muff (2006) acknowledged the presence of transverse faults in the area with an 

orientation that may vary from east-west to north-northwesterly directions. High-angle 

normal and reverse faults were observed in the areas of Kokrobite, Gomoa Fetteh, 

Nyanyano, and Weija, south of the Accra - Winneba road.    

In the Akwapim Range, their occurrence has been inferred from topographical features or 

erosion patterns. When erosional valleys are developed on both slope sides of a strike ridge 

in such a way that the valleys cut across the ridge, the occurrence of a fault is inferred 

because such a feature develops preferably when a weakened rock zone transects the strike 

ridge giving way to the formation of erosional valleys on both sides of the ridge. This 

topographic feature has been used to map several short inferred faults which cross the 

ridges of the range (Ahmed et al, 1997).   

Some major roads follow valleys formed by transform faults. The road leading from Accra 

to Ablekuma, the railway and main road to Amasaman, and the road from Kwabenya to 

Mayera are such examples. Also, the Winneba road seems to follow a transverse fault on 

the stretch that crosses the ridge of the range near the Weija Lake (Muff and Efa, 2006). 

Transverse faults result in block tectonics in the Togo Series and the individual blocks 

appear to have been lightly tilted. This is indicated by slightly varying attitudes of the 

contacts, either observed or trigonometrically calculated, between the Togo Series and the 

granitoid basement rocks.  The transverse faults are considered to be tear-faults related to 

the formation of the longitudinal faults. They are probably due to inhomogeneous simple 

shear and formed to accommodate differential movement of tectonic blocks which formed 

by the breaking of the brittle metasedimentary and sedimentary rocks.   

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Work done by Efa and Muff (2006) indicate the presence of a notable fault which is 

transverse to the Akwapim Fault Zone and is known as the Weija Fault. It strikes west-

north-west and cuts across the Akwapim Range at the Weija Dam were it marks a deep 

gorge in strongly indurated quartz-schists. It is buried by recent sands and continental 

deposits and appears to connect to faults which have been mapped in the Accraian Series. 

Ahmed et al., (1977) quote two reasons for inferring this fault, although no direct 

observation exists.  

These two reasons are: 

 (1) the Densu River turns sharply to the east and cuts a deep gorge into the highly 

indurated Togo rocks to cross the mountain range, and  

(2) Seismic activity along this inferred fault is conspicuous.     

 

2.1.5 Fault Patterns in the Akwapim Range and Related Earthquakes   

Southern Ghana is an earthquake-prone area and for this reason it appears appropriate to 

elaborate on the tectonic structure in the area and the possible causes of the earthquakes 

(Blundell, 1976). Amponsah (2002) points out that seismic activity in southern Ghana is 

concentrated in the junction area of the Akwapim Fault and Coastal Boundary Fault.     

Active tectonic movement at the foot of the Akwapim Range was observed during the 

earthquake in 1939 when a nearly 10 km long fissure opened immediately west of the hills 

south of the Weija Lake. The geodynamic forces which strain this fault zone are speculated 

about, but consensus converges on the idea that the forces are related to the Romanche 

Fracture Zone (Muff and Efa, 2006).   

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The Coastal Boundary Fault, which runs approximately parallel to the coast at a distance 

of 3 km and south of Accra turns towards west-northeast to continue inshore at the village 

of Nyanyano, has been shown to be active by Blundell and Banson (1975) and Blundell 

(1976). They suggest a connection between the Coastal Boundary Fault and the Romanche 

Fracture Zone.   

The Romanche Fracture Zone and its continuation, the Côte d’Ivoire – Ghana Transform 

Margin, have often been cited as sources of earthquakes in southern Ghana. Deltail et al. 

(1974); Mascle and Sibuet, (1974); Edwards et al. (1997) observed that the fracture zone 

is at a distance of approximately 65 km from the coast of Accra. They state that it is 6 to 

11 km wide and runs approximately parallel to the coast. From their study, they indicate 

that the Romanche Fracture Zone represents a transform fault of the Mid-Atlantic Ridge 

which separates continental from oceanic crust. The development of this fault started with 

active transform motion when Africa and South America separated. After the spreading 

center had migrated along and beyond the fault, movement along this fault ceased. The 

forces related to the Atlantic spreading which formed the Romanche Fracture Zone, are 

also ruled out as a source for the present day seismic activities (Muff and Efa, 2006).   

Pickett and Allerton (1998) note another tectonic episode at the Côte d’Ivoire – Ghana 

Transform Margin during the Eocene. Following the Eocene, the transform margin 

subsided to its present depth. Pickett and Allerton (1998) relate the subsidence with a 

change from uncoupled to coupled linkage at the continental-oceanic contact.   

Geological investigations in the coastal Keta Basin, some 80 km to the east of the 

investigation area, reveal a sedimentary succession from Devonian to Pliocene Holocene, 

interrupted by several unconformities (Akpati, 1978). Akpati (1978) relates the basin 

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development and sedimentary accumulation with the development of a graben related to 

the Romanche Fracture Zone.   

The foregoing discussion on the geo-dynamic situation in the region shows that seismic 

activities are concentrated along the Coastal Boundary Fault and the Pan African structures 

of the Akwapim Fault Zone. The forces responsible for seismic activity along the Coastal 

Boundary Fault could possibly be related to isostatic equilibration along the Côte d’Ivoire 

– Ghana Transform Margin, while no answer is available as to which forces cause the 

present day seismic activities related to the Pan-African structural elements. 

 

2.2 Geological Structures  

2.2.1 Joints  

Joints are the commonest kind of fractures in rocks and they are of utmost importance in 

promoting erosion and thus shaping the topography. The surface configuration of 

mountain slopes and summits, drainage patterns, elbow turns in rivers and coastal cliffs 

are strongly governed by rock fractures (Muff and Efa, 2006).   

The formation of joints is the result of stress on a rock mass and for this reason, joints 

show typical patterns, that is, they occur in distinctive sets.    

The density or intensity of joints, expressed in number of joints per meter normal to the 

joint surface, depends mainly on the rock type. Brittle rocks like quartzite, quartzitic 

sandstone and granite, show a higher joint density than softer rocks which have been 

exposed to the same stress. Strength and permeability of a rock mass depends strongly on 

the presence and nature of fractures. When loads are normal to a joint set, deformation 

increases in proportion to joint density (Muff and Efa, 2006).   

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Quartzites and quartz-schists of the Togo Structural unit, quartz sandstones of the Voltaian 

Structural unit, and granites display most joints in the study area. The joints are open and 

have not been cemented by mobilized secondary minerals (Muff and Efa, 2006)     

The joints observed in quarries or excavations in the study area often occur as sets of one 

or more directions. They occur as straight, continuous, parallel joints. Non-systematic 

fractures that form irregular patterns occur in addition to the systematic fractures present.   

Muff and Efa, (2006) show that joint orientations are widely scattered and very variable 

(Figure 4). A stereographic plot of 73 joints evenly distributed in the Togo Series in the 

area of investigation reveals that several clusters occur. Joints are generally steeply 

dipping, their strike directions vary widely. One set possesses strike directions ranging 

from east to north, and another direction strikes to the north-northwest. Near-horizontal 

sheet joints are rare. 

  

Figure 4: Stereographic polar diagram of joints in Togo Series, plotted on equal area net, lower 

Hemisphere.  Contour intervals: 2, 4, 8 %. Max.: 13.7 % (after Efa and Muff, 2006). 

 

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The wide spread of values is may be due to the fact that the plotted joints were sampled in 

a large area. The clear distinction of definite joint sets gets lost because of tilting or 

translation of tectonic blocks and possible rotation of certain areas due to later tectonic 

processes.    

 

2.2.2 Folds  

Muff and Efa (2006) state that folds do not play an important part as a structural element 

in the tectonic development of the brittle rock units. They appear to be related to local 

tectonic processes and faulting. Folds in the Togo Structural unit are exposed in several 

places. They are open or closed folds, they often show axial plane cleavage, and their 

amplitude is often between one to a few meters. Monoclines related to faults also occur. 

Phyllites and quartzschists often show small crenulation folds.   

In the Dahomeyan Structural unit, recumbent drag folds at the tectonic contact between 

Dahomeyan schists and overlying phyllonites of the Togo Series are exposed along the 

beach southwest of Gomoa Fetteh. They indicate shearing along a thrust fault (Ahmed, 

1977 and Muff and Efa, 2006). 

 

2.3 Geophysical Investigation Methods 

The advancement of science has resulted in the use of faster and more efficient methods 

in solving problems. A limitation in geological field mapping is now solved by the use of 

geophysical investigation methods. These include: seismic, electromagnetic, magnetic and 

electrical resistivity methods. Electrical resistivity methods tend to be cheaper and easier 

to access hence its popularity.  

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Electrical resistivity methods is made up of a number of arrays which are mainly 

differentiated by the position of the electrodes. Some of the popular electrical resistivity 

arrays include the schlumberger, wenner, dipole-dipole, pole-dipole and the gradient array. 

Optimized forms of some of the popular arrays are the square array (from the wenner), the 

azimuthal resistivity array (made up of the wenner azimuthal, the equatorial dipole array 

and the arrow array) (Schmutz et al., 2006). 

Habberjam, (1979) proposed nonconventional arrays such as the square array which could 

also be used for azimuthal resistivity measurements. The square array and its modified 

version (Habberjam, 1979) have also proven to be very effective in the investigation of 

ground inhomogeneity and anisotropy, (Habberjam, 1967, 1975). The use of square arrays 

also provides a fourth parameter, the ‘‘anisotropic non-compatibility ratio’’ (Habberjam, 

1979), which is an indication of whether the subsurface is anisotropic or if fracturing or 

other in-homogeneities prevail. The Square array method was originally developed as an 

alternative to Wenner or Schlumberger arrays for work on dipping subsurface, bedding or 

foliation planes. It is used to detect fractures within the earth. The square array resistivity 

sounding method is more sensitive to a given rock anisotropy than the more usually used 

Schlumberger and Wenner arrays. An advantage of the square array over the other arrays 

is that it requires just about 65 percent less land area than the equivalent survey using 

Schlumberger or Wenner array.  

For field techniques, three conventional methods are used for resistivity analysis of the 

subsurface. They are the vertical electric sounding (VES), the constant separation method 

(CST) and the combined procedures which utilize characteristics of both VES and CST 

(Cardimona, 2002).  The vertical electric sounding (VES) employs collinear arrays that 

are configured to produce a 1-D vertical apparent resistivity versus depth model at the 

points of investigation. The induced current passes through progressively deeper layers at 

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greater electrode spacing and as such a series of potential differences are acquired at 

successively greater electrode spacing while maintaining a fixed central reference point. 

For the VES, Schlumberger array or Wenner array can be used. The measured potential 

differences are directly proportional to the changes with depth (Cardimona, 2002). The 

Square-array method (Habberjam and Watkins, 1967) was used to determine the presence 

and azimuth of subsurface fracture zones in the volcanic tuff.  

Over the years azimuthal apparent resistivity has been advanced as a methodology for 

measuring fracture orientation of sub-vertical fracture sets. Fractures usually occur in 

preferred orientation that induces anisotropic physical properties on the rock mass. If the 

measurements are repeated over a period of time, any variations indicate an alteration of 

the physical properties of the rock mass, one of which would be changes in dilatancy 

within the fracture network.  

The theoretical development of the response of a homogeneous, but anisotropic, rock mass 

to a collinear apparent-resistivity measurement has been covered by Taylor and Fleming 

(1988). Lane et al., (1995) extended the analysis to a non-linear square array. The apparent 

resistivity for any one electrode spacing, obtained by expanding the electrode array along 

each azimuth, are plotted against azimuth in a polar diagram (Figure 5). If this is circular 

then either there are no measurable fracture sets or the volume of rock investigated was 

insufficient (because the electrode-array spacing was too small) for the rock to behave 

anisotropically. If a distinct ellipse results then the major axis of the ellipse is coincident 

with the strike of the fractures. This is true, regardless of whether the fracture-fill is more, 

or less, resistive than the host rock (Nunn, et al., 1983), because the resistivity along strike 

is the arithmetic mean and is always higher than the resistivity across strike, which is the 

harmonic mean. Due to the paradox of anisotropy (Keller and Frischknecht 1966,), the 

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measured apparent resistivity normal to the fractures is equal to the true resistivity parallel 

to the fractures. 

The coefficient of anisotropy l is defined by Habberjam and Watkins, (1967) as 

      𝜆 =  √
𝜌𝑦

𝜌𝑥
 =  

𝜌𝑥𝑎𝑝𝑝

𝜌𝑦𝑎𝑝𝑝

   (1) 

Where 

ρy is the true resistivity normal to the fractures, 

ρx is the true resistivity parallel to the fractures, 

ρxapp is the apparent resistivity parallel to the fractures and 

ρyapp is the apparent resistivity normal to the fractures. 

The measured apparent resistivity values for the various points are then subjected to a 

filtering technique to highlight the isotropic and anisotropic characters of the subsurface. 

Lilian and Niels (1994) indicate that the filtering is done by subtracting the function:

 
𝜌𝑎𝑚𝑖𝑛−1

10(𝜌𝑎𝑚𝑎𝑥− 𝜌𝑎𝑚𝑖𝑛)
               (2) 

The results are plotted against angular axes to generate a resultant anisotropic Figure. 

Using radar charts, the data is plotted from minimum to maximum values on the outer 

radial axis. If a multiple-peaked pattern is observed then the azimuth of the peaks indicates 

the strike of more than one fracture set. Since the degree of anisotropy is usually small, a 

datum value is commonly removed from the data before plotting in order to emphasize the 

anisotropy.  

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Figure 5: (a) Theoretical azimuthal apparent-resistivity ellipse plotted as a polar diagram. (b) After removal 

of a datum value the ellipse takes on a characteristic double-lobed form. (After Busby, 2000) 

 

From the polar plots, circular plots indicate no measurable fracture sets or that the volume 

of the rock investigated were insufficient because the structure and material sampled were, 

to the limit of the measurement, isotropic. Elliptical plots show a defined principal fracture 

set. Habberjam and Watkins (1967) write that for collinear arrays, the major axis of the 

ellipse coincides with the strike of the fractures whereas for non-collinear arrays such as 

square array, the minor axis of the ellipse is parallel to the strike fracture.  

Two situations that need to be distinguished are firstly, the apparent resistivity varying 

with electrode array orientation but independent of coordinates (Bolshakov et al, 1997) 

and secondly apparent resistivity dependent on both coordinates and orientation of 

electrodes. As a result of the paradox of anisotropy (Keller and Frischknecht, 1996), the 

measured apparent resistivity of normal to the fractures is equal to the true resistivity 

parallel to the fracture.        A 

fractured layer is electrically anisotropic thus resistivities in any of the three orthogonal 

directions can be different from each other (Iddirisu and Armah, 1997). The variation in 

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the horizontal permeability within the subsurface generate electrical anisotropy 

characteristics in surface electrical resistivity. Taylor and Flemming (1988) and Lillian 

and Niels (1994) show that, for a given set of vertical fractures, the potential distribution 

in the horizontal plane is given by the expression by Taylor and Flemming, (1988) as:  

𝑉 =  
𝐼𝜌𝑚

2𝜋𝑟√(𝑐𝑜𝑠2𝜗+ 𝜆2𝑠𝑖𝑛2𝜗 
        (3) 

Where V is the potential at a point defined by M = (x, y, 𝜗) with polar coordinates (r, 𝜗, 

0),  

ρm is the mean bulk resistivity, 

I is the magnitude of the input current and  

λ is the coefficient of anisotropy which is greater than unity (Keller and Frischknecht, 

1966). 

For situations where anisotropy is due to the fracturing, the fracture orientation is directly 

observed from the data. Single lobbed ellipses indicate only one primary fracture set that 

is parallel to the minor axis of the fitted ellipses (Lane, 1995). Double lobbed ellipses show 

two fracture set orientations based on visual inspection of the polar plots. 

The minimum axes which indicate the direction of high resistivity in the anisotropic 

medium within the subsurface rocks also represents the direction of subsurface conductive 

features having a high probability of very good porosity and permeability (Iddirisu and 

Armah, 1997). Keller and Frischknecht (1996) tag this occurrence as the paradox of 

anisotropy. 

 

 

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CHAPTER THREE 

METHOLOGY 

3.1 Introduction 

The project was carried out in three stages, namely desk study, field and laboratory work 

and data analysis. The methods employed in each stage are described out in details below. 

 

3.2 Desk Study 

The project consisted of a preliminary desktop study and processing of the regional 

geophysical data to help target areas for subsequent field work. Good regional resolution 

magnetic and radiometric data was assembled and assessed to constrain the structures for 

the field work.  

 

3.3 Aero – Geophysical Data: Acquisition and Processing 

Over the years, the methods of geophysical survey have improved with time. The advent 

of airborne geophysical survey has been a major step in the improvement of geophysical 

surveys as a whole. Presently the use of airborne geophysics helps in the survey and 

mapping of areas that may be inaccessible by ground or having unfavorable conditions for 

ground geophysics.  

Airborne geophysical survey was carried out in the southern parts of Ghana within 

November 1999 and 2000 by High Sense Geophysics Incorporated of Canada. The survey 

comprised magnetic, radiometric and VLF surveys of four selected areas. 

 

 

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3.3.1 Magnetic Data 

The following steps were carried out prior to levelling:  

1. Base-station correction to account for diurnal variations in the geomagnetic 

field.  

2.  Lag correction to correct for positioning error between the GPS and mag-

sensor.  

3. Heading correction to correct for errors caused by the aircraft’s position 

relative to the geomagnetic field.  

4. IGRF correction. Subtraction of the IGRF-field in the area. 

Levelling was made in two steps:  

1. Standard tie line leveling.  

2. “Micro- Levelling” as described by Geosoft (Geosoft levelling system, Tutorial 

and user guide, Chapter 6, p. 53-54). 

The micro- levelling was made according to the Geosoft procedure, except that a limit was 

set to the correction allowed. This was made to minimize the effect on anomalies of 

geological origin. Typically a maximum correction of +/- 3 nT was accepted. 

 

3.3.1.1 Processing and Integration 

The initially processed airborne geophysical data was further processed to suit the 

objectives of the study. Under this, various principles were applied to the data in order to 

produce the expected graphical map representation. The various principles used in the 

processing of the data are discussed as follows: 

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3.3.1.2 Microlevelling 

Working with complex datasets such as airborne magnetic or radiometric data requires one 

key processing objective that is to eliminate levelling problems which were not removed 

during the regular processing (such as lag correction, tie line levelling and base level 

correction (Reeves, 2005). In the study, although the airborne magnetic data set had 

already been microlevelled, the microlevelling was carried out on the data to cross check 

for any errors in the first microlevelling process. This was to increase the confidence level 

of the data by minimizing all levelling errors as much as possible. This was done using 

Geosoft’s Oasis Montaj software. 

 

3.3.1.4 Butterworth Filter 

The Butterworth filter is good for applying straight forward high-pass and low-pass filters 

to data due to the fact that the degree of filter roll-off can be controlled leaving the central 

wave number fixed. It is the simplified alternative of the Cosine filter (COSN). For the 

butterworth filter, the mathematical principle used is defined by Reeves, (2005) as: 

 L(k) = 
1

[1+(
𝑘

𝑘𝑜
)

𝑛
]
       (4) 

Where: ko is the central wave number of the filter 

n is the degree of the Butterworth filter function (By default = 8) 

0/1 is used to flag the pass required. It specifies if a residual (0) high pass or a regional             

(1) low pass is required. By default, a regional filter is applied  

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The total magnetic intensity map was produced by gridding the magnetic anomaly channel 

of the magnetic geodatabase using the minimum curvature gridding method at a chosen 

grid cell size of 100 m. The grid cell size is normally set to be ½ to ¼ the nominal line 

spacing; in this case, ¼ of the sample interval (400 m) was used (Reeves, 2005). A 

minimum curvature surface is the smoothest possible surface that will fit the given data 

values. This process serves as the basic processing method that can be performed on a 

magnetic dataset (Reeves, 2005).  However, the disadvantage of applying this method in 

gridding the data is that the minimum curvature does not entirely account for or remove 

noise. As a result, a smoothening technique (butterworth filter) had to be adopted to correct 

for these error using the parameters as shown in table below: 

 

Table 1: A table showing the parameters used for the butterworth filtering process. 

Butterworth Filter 

Cut Off Wavelength 400 m 

Filter Order 8 

High / low Pass Low  

 

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3.3.1.5 First Vertical Derivative 

The vertical derivative is commonly applied to total magnetic field data to enhance the 

shallowest geologic sources and suppress deeper sources in the data. It computes the 

vertical rate of change in the magnetic field and suppresses the long wavelengths. It is 

defined by multiplying the amplitude spectra of a field by a factor of a form  

1

𝑛
{(𝑢2 +  𝑣2)

1

2}𝑛       (5) 

where n is the order of differentiation of the vertical derivative, μ is the rate of change of 

the magnetic field in the “x” plane and ν is the rate of change of the magnetic field in the 

“y” plane with the “x” plane and the “y” plane perpendicular to each other (Reeves, 2005).  

The order of differentiation defines the high wavenumber components to enhance; 

increasing the order will enhance higher wavenumber components of the spectrum. The 

first vertical derivative is theoretically equivalent to the measurement of the magnetic field 

at two points perpendicularly above each order; subtracting the data and dividing the 

outcome by the vertical spatial separation of the measurement points. (Milligan and Gunn, 

1997). 

 

3.3.1.6 International Geomagnetic Reference Field (IGRF) 

The magnitude of the magnetic field F generally varies between 20,000 and 70,000 nT on 

the Earth’s surface (Telford et al., 1990; Reeves, 2005). It does also have local variations 

of several hundred nT (sometimes, but less often, several thousand nT) imposed on it by 

the effects of the magnetization of the crustal geology (Telford et al., 1990). Anomalies 

produced usually indicate a magnitude about two orders smaller than the original total field 

value.  

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The IGRF provides the means of subtracting on a rational basis the expected variation in 

the main field to leave anomalies that may be compared from one survey to another, even 

when surveys are conducted several decades apart and when, as a consequence, the main 

field may have been subject to considerable secular variation (Telford et al., 1990). Its 

removal thus involves subtracting of close to 99% of the measured value and will require 

a precise definition if an accuracy of the residual is to be retained.  The IGRF channel 

routine in Oasis Montaj was applied to calculate the inclination and declination of the study 

area related to the period with which the data was acquired. The IGRF model year which 

is calculated every 5 years, was set to the most recent year  

2005. After these parameters were used the calculated declination (D) and inclination (I) 

values were obtained to be –4.5° and -15.5° respectively.  

 

3.3.1.7 Analytic Signal 

The analytic signal is another quantitative method developed for interpretation of potential 

field data and combines the horizontal and vertical derivatives of the anomaly (Bilim and  

Ates, 2003). The analytical signal can be useful for locating the edges of remanently 

magnetized bodies and for centering anomalies over their causative bodies in areas of low 

magnetic latitudes (MacLeod et al., 1993). It is defined as the square root of the sum of 

squares of the derivatives in the x, y, and z directions.  

(𝐴|𝑥, 𝑦|) =  ({(
𝜕𝑚2

𝜕𝑥
)

2

+  (
𝜕𝑚2

𝜕𝑦
)

2

+ (
𝜕𝑚2

𝜕𝑧
)2})

1
2⁄      (6) 

 (A|x, y|) is the amplitude of the analytical signal at a given point (x,y) and m is the 

observed magnetic field at (x, y) while z is the depth of that point.  

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The analytical signal is not a measurable quantity and is not dependent on the direction of 

magnetization and the direction of the earth’s magnetic field. All bodies with the same 

geometry have the same analytical signal (Milligan and Gunn, 1997) only differing in the 

contrast of their susceptibilities.  

The distortion caused by a body’s magnetization remains in the analytical signal but 

produces a better result in terms of amplitudinal signatures if the analytical signal is 

applied to a reduced to pole anomaly (Bilim and Ates, 2003). The amplitude of the 

analytical signal of the total magnetic field produces a maxima over magnetic bodies 

regardless of the direction of magnetization (MacLeod et al., 1993). This is due to the fact 

that the strength of the magnetization of rocks is inversely related to the amplitude of their 

analytical signals 

  

3.3.1.8 Upward and Downward Continuation 

The upward continuation filtering technique is considered a “clean filter” because it 

produces almost no side effects that may require the application of other filters or processes 

to correct and is thus frequently used to minimize or clean shallow source and noise effects 

in grids.  

Unlike many other filtering processes, upward continued data may be interpreted 

numerically and with modelling programs. Because it has a smooth shape and does not 

alter the energy spectrum below the start of roll off, it is usually used for simple low-pass 

operations.  

Gunn (1997) described the filter as having a frequency response of  

𝑒−ℎ (𝑢2 + 𝑣2)1/
2         (7) 

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37 
 

where u and v are the changes in magnetic field in the “x’ and “y” planes respectively and 

“h”, the elevation. This implies that upward continuation smoothens out high frequency 

anomalies relative to lower frequency ones and can thus be used for suppressing the effect 

of shallow anomalies when emphasis is to be placed on deeper anomalies (Gunn, 1997). 

The downward continuation process is similar but enhances shallower features which are 

of interest. 

 

3.3.2 Radiometric Data 

The occurrence of naturally occurring gamma radiation serves as a tool which can be used 

in the study of faults and fractures within the earth (Nsiah-Akoto et al, 2013). It is believed 

that these radioactive gases emanate from within the earth and get to the surface through 

gaps and crevices mostly generated by the break in continuity of the underlying rocks 

(Nsiah-Akoto et al, 2013). This serves as a good tool to track shear zones, faulted zones 

and other unconformities that result in the break in the continuity of the underlying rocks. 

There are more than fifty naturally occurring existing radioactive isotopes though only a 

few such as Uranium (238U), Thorium (232Th) and Potassium (40K) emit significant 

amounts of gamma radiation. The abundance of these gases is dependent on the geology, 

soils, regolith and surface cover. The purpose of radiometrics is to determine either the 

relative or absolute values of U, Th, or K in surface rocks and soils (Graham, 2013). 

Regionally, gamma ray spectrometry can be used to delineate geologic provinces, fold 

belts, sedimentary basins and tectonic and structural detail of shear zones (Graham, 2013).  

The information contained in the aero geophysical data includes the coordinate system, 

and the mode of measurements of the radioelements.    

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The radioelements (uranium, thorium and potassium were gridded using Geosoft’s 

minimum curvature gridding technique to produce separate maps that showed individual 

radioactive signatures in the study area. The grids produced were then correlated with the 

geological units, patterns and trends from which analysis were made. 

 

3.4 Geological Structural Mapping 

Geological structural mapping was carried out at areas with rock exposures within the 

study area. The rocks in the study are predominantly made up of the rocks of the Togo 

structural unit bounded to the west by the cape coast granitoids and to the east by the 

Dahomeyan structural unit. 

The structural features that were targeted are joints, foliation, folds and faults. With most 

parts of the area falling within the lowland areas, the number of exposures seen are few. 

Also, the high rate of urban settlement development within the area in addition to other 

anthropogenic activities have left the area with few outcrop exposures although they are 

representative of the area. The prominent geological structures present are the foliation 

and joints. The structural measurements were done in concordance with the right-hand-

rule using a Konustar compass. Repetitive measurements were taken at each point to 

ensure a high level of accuracy.  

 

3.5 Ground Geophysical Surveys 

3.5.1 Geophysical Resistivity Methods 

The study involved the use of various electrical resistivity techniques to acquire data. 

These methods included the Square Array Method, the Schlumberger Sounding method 

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39 
 

and the Azimuthal Sounding method. These resistivity investigation methods were 

selected because of their effectiveness in fracture and fault zones mapping. The points for 

the geophysical survey were selected based on the maps generated from the aeromagnetic 

and radiometric data. The points were selected to fall within the fault and fracture zone 

and are representative of the whole study area.  

 Resistivity is a measure of a materials resistance to the flow of electrical current. 

Field measurement result in apparent resistivity (ρa) values that are generally related to the 

rock type and water content, which are often related to grain-size distribution as finer 

grained materials usually have a lower water content than coarse grained materials and 

consolidated rock (Cardimona, 2002) The values are also affected by the location and 

spacing of the electrode arrays. The larger the spacing of the electrode array, the larger the 

sample volume covered and as such the deeper depth reached. The variation of electrical 

properties of material with direction (anisotropy) also affects the resistivity measured 

based on the direction of measurement (Habberjam, 1975). Horizontally stratified rocks 

have their plane of anisotropy generally parallel to the surface unlike dipping layered rock 

units which have their plane of anisotropy being dependent on the orientation. The plane 

of anisotropy of fractured rock units such as those in the study area follows the same 

principle as that of dipping layered rock units. The instrument used is the Scintrex 

Automated Resistivity Imaging System (SARIS). This was used together with electrical 

cables, alligator clips, banana plugs and electrodes. The electrode positioning was varied 

based on the geophysical electrical resistivity array being carried out. For every sample 

point (as shown in Figure 6), the Square array, Schlumberger soundings and Wenner 

Azimuthal Soundings were carried out.  

 

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40 
 

For this survey, electrical resistivity methods were chosen because: 

i. They are relatively rapid to carry out and are flexible. The field time 

increases with the intended depth of reach. 

ii. Minimal expenses are made in this kind of survey other than on personnel. 

iii. The equipment used are relatively light and portable. 

iv. There is a relatively better response to different material properties than 

other geophysical survey methods. 

v. Qualitative interpretation of the data is straightforward. 

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41 
 

 

Figure 6: Map of the study area showing the survey points 

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42 
 

3.5.2 Square Array 

For this study, the interpreted results of the Square Array method is supported by 

Azimuthal Wenner Array Sounding data and Schlumberger Sounding data. 

 

Figure 7: Electrode positions for Square Array measurements 

 

Figure 8: Electrode positions for the first and second orientation of the Square array 

Current electrode configuration for Crossed Square Array 

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43 
 

 Alpha  1 – 4 = ρa1 

 Alpha’ 5 – 8 = ρa2 

 Beta  1 – 2 = ρa3 

 Beta’ 5 – 6 = pa4 

 Gamma 1 – 3 

 Gamma’ 5 – 7 

Using the Square-array method, the azimuth of the existing structures is generally 

indicated by a decrease in resistivity along a particular azimuth relative to the other 

azimuths.  

The direct- current survey using the Square-array method is conducted in a similar manner 

as the traditional collinear arrays. In this study, the positioning is similar to the traditional 

Wenner method. The position of the measurement is assigned to the center point of the 

square (figure 8). The array size, defined by the symbol “A”, is the length of the side of 

the square. Expansions of the square are made symmetrically about the center point in 

increments of A (2)1/2 (Habberjam and Watkins, 1967), so that the soundings can be 

interpreted as a function of depth. The electrode positions are connected to the Resistivity 

meter such that electrodes A and B are the transmitter electrodes or the current electrodes 

while the electrodes M and N are the receiver electrodes or the potential electrodes. The 

depth of investigation is generally considered to be approximately equal to the side of the 

square but varies with resistivity (Habberjam, 1975). 

For each square, two perpendicular measurements (alpha, α and beta, β) were taken in 

addition to one d