i

GEOPHYSICAL INVESTIGATION OF GEOTHERMAL POTENTIAL OF THE GILGIL AREA, COUNTY, USING GRAVITY METHOD

ERICK RAYORA NYAKUNDI [B.Ed. (Sc.)] I56/CE/24486/2012

A thesis submitted in partial fulfilment of the requirements for the award of the degree of Master of Science in the School of Pure and Applied Sciences of Kenyatta University

MAY, 2017 ii

DECLARATION

This thesis is my original work and has not been presented for the award of a degree or any

other award in any other university

Erick Rayora Nyakundi Signature Date

Department of Physics

Kenyatta University ……………………. …………………

I confirm that the work reported in this thesis was carried out by the candidate under my

supervision

Dr. Willis Ambusso Signature Date

Department of Physics ………………….. ………………………… Kenyatta University

Signature Date Dr. John Githiri

Department of Physics Jomo Kenyatta University of Agriculture and Technology ………………….. ………………………… iii

DEDICATION

This thesis is a special dedication to my beautiful daughters; June and Emma, lovely wife;

Judy and my dear parents; Hellen and Vincent.

iv

ACKNOWLEDGEMENTS

First, I thank the Almighty God for taking care of me throughout this Master of Science

programme. He protected me, gave me good health and strength which ensured

accomplishment of this work.

My special thanks goes to my research supervisors, Dr. Willis Ambusso and Dr. John

Githiri for their technical guidance, positive criticism, critical reading, suggestions and

encouragement during this research work. I want also to thank the entire physics

department lecturers for their support in ensuring a successful completion of this Master of

Science program.

I wish to thank the Department of Geology, Ministry of Energy Kenya for allowing me to

use the ministry’s CG-5 gravimeter during data collection. I also thank the entire team

especially Chief Geophysicist Mr. Barrack Ouma who accompanied me to the field for

technical assistance on the operation of the CG-5 gravimeter.

My special gratitude goes to my lovely better half Judy for her tolerance and motivation

throughout this programme. I appreciate my beautiful daughters Emma and June for giving

me humble time to complete this work. Further appreciation is to my dear mum Hellen and

dear dad Vincent for their moral and financial support during this programme.

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TABLE OF CONTENTS

Contents DECLARATION ...... ii DEDICATION ...... iii ACKNOWLEDGEMENTS ...... iv TABLE OF CONTENTS ...... v LIST OF FIGURES ...... viii ABBREVIATIONS, ACRONYMS AND SYMBOLS ...... x ABSTRACT ...... xi CHAPTER ONE ...... 1 INTRODUCTION ...... 1 1.1 Background to the study ...... 1 1.2 Regional geological setting ...... 3 1.3 Statement of research problem ...... 6 1.4 Objectives ...... 6 1.4.1 General objective ...... 6 1.4.2 Specific objectives ...... 7 1.5 Rationale of the study ...... 7 CHAPTER TWO ...... 9 LITERATURE REVIEW ...... 9 2.1 Gravity method ...... 9 2.2 Geothermal exploration ...... 10 CHAPTER THREE ...... 14 MATERIALS AND METHODS ...... 14 3.1 Introduction ...... 14 3.2 Theory of the Earth’s gravitational field...... 14 3.3 Geopotential ...... 16 3.4 Gravity measurement ...... 16 3.5 Data collection ...... 17 3.6 Gravity instrumentation ...... 18 3.6.1 CG-5 Autograv gravimeter ...... 18 3.6.2 Euler deconvolution ...... 20 3.6.3 Forward modeling ...... 21 vi

CHAPTER FOUR ...... 23 DATA AND PROCESSING PROCEDURES ...... 23 4.1 Introduction ...... 23 4.1.1 Drift correction ...... 23 4.1.2 Free air correction (FAC) ...... 24 4.1.3 Bouguer correction (BC) ...... 25 4.1.4 Terrain correction (TC) ...... 26 4.2 Regional density ...... 27 4.3 The Bouguer anomaly map ...... 27 4.4 Selection of profiles...... 33 4.4.1 Cross section PP’ ...... 35 4.4.2 Cross section QQ’ ...... 36 4.4.3 Cross section RR’ ...... 38 4.4.4 Cross section SS’ ...... 39 4.4.5 Cross section TT’ ...... 41 CHAPTER FIVE ...... 43 RESULTS PRESENTATION AND ANALYSIS ...... 43 5.1 Introduction ...... 43 5.2 Quantitative interpretation ...... 44 5.2.1 Euler solutions along profile PP’ ...... 44 5.2.2 Model fit along profile PP’ ...... 45 5.2.3 Euler solutions obtained along profile QQ’ ...... 46 5.2.4 Model fit along profile QQ’ ...... 47 5.2.5 Euler solutions obtained along profile RR’ ...... 48 5.2.6 Model fit along profile RR’ ...... 49 5.2.7 Euler solutions obtained along profile SS’ ...... 50 5.2.8 Model fit along profile SS’ ...... 51 5.2.9 Euler solutions obtained along profile TT’ ...... 52 5.2.10 Model fit along profile TT’ ...... 53 5.3 Discussion ...... 55 CHAPTER SIX ...... 57 CONCLUSIONS AND RECOMMENDATIONS ...... 57 6.1 Conclusions ...... 57 vii

6.2 Recommendations ...... 58 REFERENCES ...... 60 APPENDICES ...... 62 APPENDIX I: DATA COLLECTED AND COMPLETE BOUGUER ANOMALY ...... 62 APPENDIX II: PROFILES DATA ...... 72 (a) DATA FOR PROFILE PP’ ...... 72 (b) DATA FOR PROFILE QQ’ ...... 73 (c) DATA FOR PROFILE RR’ ...... 74 (d) DATA FOR PROFILE SS’ ...... 75 (e) DATA FOR PROFILE TT’ ...... 77 APPENDIX III: SOME DRIFT CURVES ...... 80

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LIST OF FIGURES

Figure 1.1: Map displaying the geology of the study area...... 5

Figure 3.1: Body of mass m on the Earth’s surface...... 15

Figure 3.2: Measurement point distribution for the gravity survey in Gilgil area...... 17

Figure 3.3: The CG-5 Autograv gravimeter……………...... 19

Figure 4.1: Drift curve for data collected on 24/04/2015………...... 24

Figure 4.2: Contour map for the Gilgil area …………………………………….….……..28

Figure 4.3: An image of a contour map of Gilgil area from the South.....…...... 29

Figure 4.4: An image of a contour map of Gilgil area from the West …...... 30

Figure 4.5: An image of a contour map of Gilgil area from the North…...... 31

Figure 4.6: An image of a contour map of Gilgil area from the East………...... 31

Figure 4.7: 2-D Relief map of Gilgil area…...... 32

Figure 4.8: 3-D relief map of Gilgil area……………………………………………….....33

Figure 4.9: Contour map profiles. …...... 34

Figure 4.10: Observed and regional anomalies along PP’…………...... 35

Figure 4.11: Residual anomaly along PP’……………………...... 36

Figure 4.12: Observed and regional anomalies along QQ...... 37

Figure 4.13: Residual anomaly along QQ’………………...... 37

Figure 4.14: Observed and regional anomalies along RR’……………………..…………38

Figure 4.15: Residual anomaly along RR’………………………...... 39

Figure 4.16: Observed and regional anomalies along SS’……………………..……….....40

Figure 4.17: Residual anomaly along SS’. ………………...... 40

Figure 4.18: Observed and regional anomalies along TT’…………………...... 41 ix

Figure 4.19: Residual anomaly along TT’………………...……………………….……...42

Figure 5.1: Euler solutions obtained along profile PP’……………….…..……………….45

Figure 5.2: Model fit on residual bouguer anomaly PP’…………...………...……………46

Figure 5.3: Euler solutions obtained along profile QQ’……………..……………………47

Figure 5.4: Model fit on residual bouguer anomaly QQ’…………...……………...……...48

Figure 5.5: Euler solutions obtained along profile RR’………………...…………………49

Figure 5.6: Model fit on residual bouguer anomaly RR’………………..………………...50

Figure 5.7: Euler solutions obtained along profile SS’……………………………………51

Figure 5.8: Model fit on residual bouguer anomaly SS’………………..…………………52

Figure 5.9: Euler solutions obtained along profile TT’………………..………………….53

Figure 5.10: Model fit on residual bouguer anomaly TT’………………………...………54

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ABBREVIATIONS, ACRONYMS AND SYMBOLS a acceleration

B Base station

BC Bouguer Correction

CBA Complete Bouguer Anomaly

F Force

FAC Free Air Correction

G Universal gravitational constant

GPS Global Positioning System

gz Earth’s gravitational field strength

m Mass of the body

Me Mass of the Earth

MT Magnetotellurics

Re Radius of the Earth

S Station SBA Simple Bouguer Anomaly

TC Terrain Correction

TEM Transient Electromagnetic

흆 Density contrast

흆풂 Average density

흆풃 Density of an intruding body

xi

ABSTRACT

In this study, gravity survey method was used to investigate the geothermal potential field in Gilgil area, , Kenya. No previous geophysical study has been done in Gilgil area. Gilgil area is volcanic and gravity survey in volcanic regions has shown that it gives useful information about the density changes within the Earth’s subsurface associated with the heat source. Gravity highs are related to centres of volcanism, faults and geothermal activity. The discovery of geothermal reservoir in Gilgil area will provide an alternative source of clean energy in Kenya, increase the amount of electricity supplied to the national grid thus lowering the cost of electricity and ensuring it is enough to meet the market demand of electricity in Kenya. The specific objectives of this study were to conduct ground gravity measurements of Gilgil area, determine gravity anomalies and figure out the size, depth and form of the underground body that produces the gravity anomaly. The ground based CG-5 Autograv gravimeter was used to accurately measure gravity at each field station. A total of 147 gravity stations were established over an area of about 68 km2 and gravity corrections done. The complete bouguer anomaly was computed and a contour map for the study area plotted using surfer 8.0 software. Qualitative interpretation of the map shows gravity highs in the study area which were interpreted as dense intruding bodies within the subsurface. Five profiles along the gravity highs were drawn and positioned in the southwest-northeast, northwest-southeast and almost north-south directions for analysis. The regional trend of the profiles was subtracted from the observed data yielding the residual anomaly. 2D Euler deconvolution was done on the profile data and revealed subsurface faults and bodies at a depth range of 790 m – 4331 m. It was discovered that the deep faults transport thermal fluids from deep parts of the Earth to the subsurface. The shallow faults in the Earth’s subsurface direct the flow of thermal fluids on the upper part of the basement. The top faults direct the flow of water from rift scarps to the hot masses underground. This faults were concluded to be responsible for underground movement of thermal fluids. Forward modelling of selected profiles using Grav 2DC software revealed presence of dense intrusive bodies on the northern and southern parts of the study area with the density contrast range of 0.25 − 0.28. These bodies were interpreted as intrusive dykes that have higher density than surrounding rocks. Thus the high heat flow observed in the area as evidenced by hot springs could be due to these intruding bodies within the rift floor faults. This intrusive in form of dykes was concluded to be tapping heat from large magma bodies at few kilometers from the surface. Such intrusive dykes may be geothermal heat sources.

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CHAPTER ONE

INTRODUCTION

1.1 Background to the study

Gilgil is located between and Nakuru in Nakuru County, Kenya. It lies 121km north of . Gilgil area is in the Kenyan rift where a number of geothermal fields lie.

Preliminary surface investigations have been carried out in Suswa, Longonot, ,

Eburru, , Bogoria, Baringo, , Silali and Emurangogolak geothermal fields

(Clarke, 1990). Drilling has been done in Eburru and Olkaria. The present power station is in Olkaria. Thus this study was carried out to establish the potential of Gilgil area as a geothermal reservoir.

Geothermal reservoir can be studied by most geophysical techniques such as gravity, magnetic and seismic. The outcome from these techniques can be used to provide a precise data on the nature of the reservoir. The geology of the area plays a key role to any geophysical method employed. Geology information can suggest specific areas where detailed study should be carried out. Geothermal occurs along major fracture lines because these faults direct the flow of underground fluids within the Earth’s subsurface. When these fluids come into contact with a hot magma material from the mantle, they generate a heat source. Also geothermal may occur in quiet volcanic craters. This craters implies that volcanic activity took place and pushed the hot magma materials from the mantle of the

Earth to the upper crust of the Earth. By using geophysical technique, these features can be outlined and located for exploration (Telford et al., 1990).

Gravity surveying has been done to gain information on geothermal potential areas. Gravity technique in geophysical exploration deals with measurements of changes in the Earth’s 2 gravitational field strength (Sharma, 2002). Gravity measurements and observations are done on the earth’s surface. The gravimeter is an instrument used to measure changes in the Earth’s gravitational field on the Earth’s surface and records its values in milligals. It helps to find bodies within the subsurface of the Earth which have greater or lesser density than the surrounding host rocks. Gravity can also constrain data during interpretation of other geophysical techniques such as seismic and magnetic.

Gravitational field is natural on the Earth’s surface similar to magnetic and radioactivity.

It is a natural field technique that uses gravitational field of the earth. There is no energy required to be put into the subsurface to gain information (Dickerson, 2004). It reveals change in these natural gravitational field that is attributed to economic feature of concern within the subsurface. This feature portrays a subsurface area of anomalous mass and causes localized change in gravity referred to as gravity anomaly.

A major advantage of the gravity method over other geophysical methods in underground work is that any anomaly can be attributed only to an increase or decrease in density. Also gravitational field strength does not change significantly with time like magnetic field strength (Kearey et al., 2002). Variations in the distribution of densities of rocks within the subsurface causes changes in the gravitational field strength. This changes in gravitational field strength can reveal either a gravity high or a gravity low. Gravity high shows an underlying causative body within the subsurface of greater density than the surrounding host rock. Gravity low shows an underlying causative body within the subsurface of less density than the surrounding host rock. Heat sources are associated with bodies of high density hence gravity highs. Therefore, gravity is a natural function of mass or density.

Change in mass or density results in gravity variations within the Earth’s subsurface. 3

No geophysical study has been done in Gilgil area hence gravity method provided quick information which may be used for future detailed study. Gravity method is less expensive unlike other geophysical methods thus was embraced for initial reconnaissance. Gravity survey in volcanic regions has essentially shown that gravity technique gives useful information about the density changes within the Earth’s subsurface associated with the heat source. Gravity highs are related to centers of volcanism, faults and geothermal activity. Gravity has been used in many studies. It can be used to study the whole Earth or a localized area of the upper surface for construction reasons (Telford et al., 1990). Gravity analysis can show the arrangement of rocks and their depth that shade light on the area’s subsurface geology. Gravity surveying gives a quick and cheap information on subsurface geology unlike drilling boreholes. A large ground area can be covered within a short time.

Therefore, gravity method is better than other geophysical techniques because data can be obtained within a short time, it does not harm the ecosystem, the instrument used during data collection can be easily operated and carried during the study and does not require the ground to be energized to collect data. This reveals that gravity technique can be easily used to discover and depict resources within the subsurface of the Earth of economic attention (Lowrie, 1997).

1.2 Regional geological setting

The geology of Gilgil area is the outcome of volcanic and tectonic activities of the rift valley. The volcanic activity of the rift preceded and accompanied the rift tectonic activities. Gilgil area is dominated by quaternary volcanic ash and diatomaceous silts in the plain areas and some volcanic tuff, lava flow and diatomite deposits in the higher escarpments (Mccall, 1967). Alkaline volcanism composed of pumiceous pyroclastics, 4 ashes, trachytes, ignimbrites, phonolites and phonolitic trachytes, tuffs, agglomerates and acid lava dominates Gilgil area. Also volcanic soil and diatomite deposits dominate the area with trona impregnated silts bordering (Mccall, 1967). The area is also characterized by repeated volcanicity followed by movement. The eruptives in each episode start with basalt (Thompson and Dodson, 1963). The southern part of Gilgil is within the Olkaria volcanic complex. Craters, fumaroles, hot springs and steam vents are found in several places within the Olkaria and Eburru area. The earlier tectonic geology is reflected in the step-faults of Satima and Kinangop generating Kinangop plateau. Grid faulting generated Gilgil plateau while the is as a result of fault flexures.

The major fault escarpments influence topography of the rift floor that influence the drainage flow pattern (Onywere et al., 2012). Figure 1.1 describes the geology of this study area and shows the distribution of various geological features in the study area. 5

Figure 1.1: Map displaying the geology of the study area. (Ministry of Mining, Kenya)

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1.3 Statement of research problem

Gilgil area lies in the Kenyan rift which has a greater potential of geothermal power. It is

42 km north of Olkaria which is Kenya’s main geothermal reservoir. Gilgil area is characterized by faults, volcanic craters and hot springs which indicate the presence of heat reservoir underground. No geophysical study has been done in Gilgil area hence gravity method provided a quick information which may be used for future detailed study. Gravity method is cheap unlike other geophysical methods thus was embraced for initial reconnaissance. Gravity survey in volcanic regions has essentially shown that gravity technique gives useful information about the density changes within the Earth’s subsurface associated with the heat source. Gravity highs are related to centres of volcanism, faults and geothermal activity. Geophysical investigation of geothermal potential field was carried out in this prospect area to identify specific points where detailed research work should be done. The discovery of a geothermal reservoir in the area will open up the place and raise the living standards of the residents. It will attract investments and create employment opportunities to the residents. It will also provide an alternative source of energy in Kenya, increase the amount of electricity supplied to the national grid thus lowering the cost of electricity and ensuring it is enough to meet the market demand of electricity in Kenya.

1.4 Objectives

1.4.1 General objective

The general objective of this research was to map the geothermal potential field in Gilgil area, Nakuru County, Kenya using gravity. 7

1.4.2 Specific objectives

The specific objectives of the study were: i) To conduct ground gravity measurement of Gilgil area. ii) To determine gravity anomalies that would show the presence of geothermal potential field. iii) To determine the size, depth and form of the underground body that produces the

gravity anomaly.

1.5 Rationale of the study

The knowledge concerning the subsurface geology is crucial as it relates changes in gravity with the distribution of masses within the Earth’s subsurface (Sherriff, 1994). Gravity method outlines underground resources within the subsurface of the Earth. Gravity method does not require any energy to be put into the ground in order to acquire information. It uses the variations in natural gravitational field that arises due to underground resources.

Gravity analysis can show the arrangement of rocks and their depth that shade light about the area’s subsurface geology. Gravity surveying gives a quick and cheap information on subsurface geology unlike drilling boreholes. A large ground area can be covered within a short time. Therefore, gravity method is better than other geophysical techniques because data can be obtained within a short time, it does not harm the ecosystem, the instrument used during data collection can be easily operated and carried during the study and does not require the ground to be energized to collect data. This reveals that gravity technique can be easily used to discover and depict geothermal resource within the subsurface of the

Earth which is of economic attention. Discovery of a geothermal resource will create employment opportunity for the locals, provide alternative source of energy for the country 8 and encourage the use of clean energy which does not pollute the environment. It will also increase the amount of electricity supplied to the national grid thus lowering the cost of electricity. This reduces the cost of production thus lowering the cost of living.

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CHAPTER TWO

LITERATURE REVIEW

2.1 Gravity method

In geothermal studies, gravity method plays a crucial role in understanding the mass distributions within the Earth’s subsurface. If there were no mass distributions within the

Earth’s subsurface, the gravity anomaly would be zero. Gravity anomaly is obtained after gravity reductions have been applied to the observed data. Gravity anomaly can either be positive or negative depending on the causative body. Positive gravity anomaly which is a gravity high implies an underlying dense body within the subsurface of the Earth. Negative anomaly which is a gravity low implies a less dense body within the Earth’s subsurface compared to the host materials. Geothermal reservoir occurs in gravity highs as materials coming from the mantle of the Earth have higher density than the surrounding materials

(Telford et al., 1990).

In gravity technique, the geology is examined on the foundations of changes in the Earth’s gravitational field emerging from deviations of mass within the underlying rocks. The fundamental concept is the idea of a causative body, which is a rock of unusual density from the host masses. This causative body portrays a subsurface region of abnormal density and results in change in the Earth’s gravitational field called gravity anomaly (Kearey et al., 2002). Gravity can be used to study the whole earth or a localized area of the upper surface for construction reasons. Gravity analysis can show the arrangement of rocks and their depth that shade light on the area’s subsurface geology. Gravity surveying gives a quick and cheap information on subsurface geology unlike drilling boreholes. A large ground area can be covered within a short time. Therefore, gravity method is better than 10 other geophysical techniques because data can be obtained within a short time, it does not harm the ecosystem, the instrument used during data collection can be easily operated and carried during the study and does not require the ground to be energized to collect data

(Dickerson, 2004). This reveals that gravity technique can be easily used to discover and depict resources within the subsurface of the Earth of economic attention.

2.2 Geothermal exploration

Mulwa et al. (2010) carried out a gravity and magnetotelluric (MT) prospects in Lake

Bogoria area to ascertain the heat source, characterize the geothermal reservoir, and evaluate the geothermal resource potential of the basin. Gravity prospect outcome showed bouguer anomaly with an amplitude of approximately 40 mGals aligned in a North-South direction and were interpreted as a result of a series of dyke injections and hence the heat source in the basin. The MT prospect outcome revealed three distinct layers in the basin.

The first layer, overlain by high resistivity slim layers, was about 3 km thick and had resistivities ranging between 4-30 Ωm. This layer was interpreted to be due to a combination of saline sediments and circulation of high temperature geothermal fluids. The second layer was approximately 10 km thick and resistivity values range between 85-

2500 Ωm. This layer was interpreted to be fractured basement metamorphic rocks hosting a steam reservoir where circulating fluids are heated by underlying dyke injections. The substratum is characterized by resistivities ranging between 0.5-47 Ωm and was interpreted as hot dyke injections which were the heat sources for this geothermal prospect. The study concluded that the heat source in basin is due to cooling dyke injections occurring at depths of about 6 – 12 km in the subsurface. Gravity method however favoured 11 depths of about 3 – 6 km. The geothermal reservoir was probably attributed to condensation of high temperature steam from the underlying fractured basement metamorphic rocks.

Yu et al. (2009) carried out a 2D magnetotelluric (MT) and gravity prospects in twelve prospect areas in Hungary to locate potential heat sources for alternative source of electricity. Szentloric prospect region was chosen because geothermal drilling plans were in place. This study of 2D magnetotelluric (MT) and gravity was geared towards identifying exact position of geothermal potential area within Mesozoic geothermal reservoir system for drilling purposes. It was assumed that the north south main faults in the Szentloric prospect area occurred in the deep floor. Dense fractures also occurred in the top floors of the two prospect regions. The heat source of geothermal reservoir was perceived to be due to thermal fluids which were transmitted up along the fault structures from the deep Earth within the subsurface. A collection of heavy build tertiary deposits was found to be positioned above the reservoir. It was discovered that fractured karst limestone and dolomite deeply buried in the Mesozoic system contained the targeted geothermal reservoirs. On the foundations of both constrained inversions of magnetotelluric (MT) and gravity data, it was concluded that the underground layer of water bearing porous stone was characterized by a relatively low apparent resistivity and low density, while the higher porosity and permeability formations are unique for faults and fractured zones.

Santos et al. (2009) investigated gravity survey contribution to geothermal exploration in

El Salvador. The study considered three areas of Berlin, Ahuachapan and San Vincente. In

Berlin geothermal field, a total of 400 measurement points were covered. A density of 12

2.3 푔/푐푚3 was used during data reductions. Residual bouguer anomaly map was drawn and exhibited gravity highs in the study area. It was found that these gravity highs contained the geothermal reservoir. Propilization as a result of mineral deposition during the underground fluid movement and decrease of porosity caused density increase in the study area. In San Vicente geothermal area, over 480 measurement points were covered.

Gravity data was analysed along MT resistivity layer using the wing link software. The graven faults imaged agreed with the gravity anomaly contrast. It was found that the MT resistivity basement associated with fractured andesitic lavas with propylitic alteration agreed to a 2.4 g/cm3 density gravity layer. It was concluded that this layer represents the producer of the high temperature reservoir within the subsurface of the study area. In

Ahuachapan geothermal field, more than 330 gravity stations were covered. Data was collected using CG-3 digital gravimeter and a density of 2.2 푔/푐푚3 was used during data correction. The residual Bouguer map showed that an Ah-core field located on the NW end of a shallow gravity high with an ENE-WSW trend, which agreed to the regional structural trend in the prospect area. This explained the structural set up of the region. The gravity high was interpreted to be due to dense bodies under the geothermal reservoir.

Searle (1970) carried out a gravity survey within the Kenyan Rift valley to the south, covering the Menengai, Longonot and Suswa volcanoes. It was revealed from the survey that the axial high continue but changes in width and amplitude. The anomaly was interpreted to be as a result of dense intruding material from the mantle 20 km wide extending from a depth of 20 km to width 2 to 3 km of the Rift floor, thus representing

‘extreme thinning of the lithospheric plate’. Similar interpretation was presented by Baker and Wohlenberg (1971) for a long, roughly equatorial profile through menengai, but with 13 narrower (10km) intrusion in the crust and a wedge-shaped low-density zone in the upper mantle for the broad negative bouguer in the Rift valley.

Riaroh and Okoth (1994) carried out specific studies of geothermal fields in the Kenya Rift valley. It was found that heat flow in the Kenya Rift valley is high but variable. There is spatial association among high heat flow and quaternary volcanism, faulting and hydrothermal manifestations, strongly suggesting that high heat flow is ultimately magmatic in origin redistributed in the shallow crust by hydrothermal activity.

Therefore, gravity method has been used in many areas around the world for geothermal exploration as pointed out from the literature review. It is used for reconnaissance before detailed geophysical work is done. This is because gravity method is less expensive, easy to conduct due to the nature of its instrumentation and can locate gravity high regions which are associated with geothermal resource. Also gravity method can image underground fractures which control and direct fluid movement within the Earth’s subsurface. Since no geophysical work has been done in Gilgil area, gravity method was employed in this study area to gain information on the possibility of occurrence of geothermal reservoir.

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CHAPTER THREE

MATERIALS AND METHODS

3.1 Introduction

In this chapter, materials and methods used to collect and analyze data are discussed. In gravity technique, the geology is examined on the foundations of changes in the Earth’s gravitational field emerging from deviations of mass within the underlying rocks. The fundamental concept is the idea of a causative body, which is a rock of unusual density from the host masses. This causative body portrays a subsurface region of abnormal density and results in change in the Earth’s gravitational field called gravity anomaly (Kearey et al., 2002).

3.2 Theory of the Earth’s gravitational field.

The rotation of the Earth results in centrifugal acceleration. The centrifugal acceleration vanishes at the poles and becomes maximum at the equator. This centrifugal acceleration causes flattening of the earth at the poles. At each point on the Earth’s surface, the centrifugal and gravitation act together (Telford et al., 1990). The vector of their sum is known as gravitational field. Gravitational field is a vector quantity and acts vertically downwards towards the centre of the Earth.

Newton’s law of universal gravitation and his second law of motion explains the fundamental of gravity technique in geophysical exploration. Newton’s universal law of gravitation states that any two bodies in the universe attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square 15 of the distance between them (Kearey et al., 2002). Consider a body of mass m on the surface of the Earth as shown in figure 3.1;

Figure 3.1: Body of mass m on the Earth’s surface.

Then,

(3.1)

Newton’s second law of motion states that the force acting on a body is equivalent to the product of mass and acceleration of that body as shown in equation (3.2);

F = ma (3.2)

F = mgz (3.3)

Equating equation (3.1) and (3.3), the force is given by equation (3.4);

(3.4)

Where,

16

3.3 Geopotential

The potential V is work done against gravitational force for a unit mass to leave gravitational field of a body. When a body is moved in the equipotential surface, no work is done. Equipotential surface is a surface with constant potential field. Equipotential surfaces surround each other forming a shell. The orthogonal trajectories of the equipotential surfaces are known as field lines. Therefore, gravitational potential V is given by equation 3.5;

(3.5)

The geoid is a highly acknowledged equipotential surface, where in all locations is horizontal, that is perpendicular to the direction of gravitational acceleration. (Telford et al., 1990).

3.4 Gravity measurement

The international system (SI) unit of measuring gravity is metre per second squared (푚⁄푠2).

The average value of gravity at any point on the Earth’s surface is approximately 9.8푚⁄푠2

(Kearey et al., 2012). Gravity variations are measured in millgals (mgal) because changes in gravity resulting from differences in density of masses within the subsurface of the upper crust of the Earth is so small. The relations below show how milligals (mgal) used in this study compare to other units of gravity;

1m/s2 = 100cm/s2

1cm/s2 = 1gal 17

1gal = 1000mgals

1m/s2 = 105mgals

1mgal = 10 g. u

3.5 Data collection

An area of approximately 68푘푚2 was covered during this study. Data was gathered from

147 measurement points as displayed in figure 3.2 using CG-5 gravimeter. The base stations were formed for the purpose of drift corrections. Stations were spaced at 500m apart. At each station the time, northing, easting, altitude and gravity value in milligals was recorded.

Figure 3.2: Measurement points distributions for gravity survey in Gilgil area. 18

3.6 Gravity instrumentation

3.6.1 CG-5 Autograv gravimeter

The CG-5 Autograv gravimeter in figure 3.3 was used for relative measurements on the

Earth’s surface. This gravimeter measured gravity differences between different locations on the surface of the Earth. The CG-5 gravimeter is designed to level itself before the reading is taken. The working principle of a CG-5 gravimeter is founded on a fused quartz elastic system. The minimal electrostatic recovering force and a spring neutralizes the gravitational force on the proof mass. The gravity variations due to underground mass distribution within the Earth’s subsurface changes the location of the mass which is identified by the device’s transducer system. The mass is reset to its original location by an automatic set up which feeds a DC signal to the recovering force. The signal looped back to the control system which is a measure of a gravity variations within the Earth’s subsurface at the gravity station is changed to a digital data. This data is sent to the information section of the machine for further processing, storing and showing on its screen for observation. 19

Figure 3.3: The CG-5 Autograv gravimeter used for field measurement of gravity.

The natural ability and stretchable features of fused quartz with controlled movements near the proof mass allow the device to be used without holding it firmly. The device levels itself automatically. Also the device is cushioned from external disturbances or forces by impact absorber set up which is connected to its body.

The CG5 gravimeter can be used in both detailed geophysical studies to regional reconnaissance studies. This is because the device’s system is made in a way that the signal looped back covers a set of gravity values more than 8000 mGals without setting it back to its initial state. Also the device uses a minimal noise digital pattern with exact analogue to digital changer generating a degree of fineness of 0.001 mGal. 20

The CG5 gravimeter applies automatically terrain correction to the observed gravity data.

Terrain correction is as a result of surface irregularities near each observation point. There could be hills in the vicinity of observation point which exert an upward force on the gravimeter while there could be valleys around the observation point which do not apply a downward force on it. The fundamental concept of this device to apply terrain corrections is founded on a standard hammer chart with four regions near the observation station. The calculation needs the area density and tilt information from identified regions at specific distances from every gravity station.

The CG5 gravimeter has inside tilt devices that correct for errors arising from a fluctuating ground. The error arises because the device cannot balance on the fluctuating ground and it is corrected automatically for each gravity station.

The CG5 gravimeter has a system that enables taking the data from a gravity station without unsettling the meter. Also the CG5 gravimeter has a global positioning system (GPS) that enables the observer to get the latitude, longitude and altitude of the gravity station.

3.6.2 Euler deconvolution

Euler deconvolution is a data improvement method for approximating location and depth to gravity anomaly source. It associates the gravity field and its gradient components to the location of the anomaly source with the intensity of homogeneity expressed as a structural index and it is an appropriate technique for outlining anomalies caused by isolated and multiple sources (El Dawi et al., 2004). With good structural index and proper analysis of gravity field gradients, 2-D Euler deconvolution can determine the depth of a causative body and the nature of faults within the subsurface of the Earth (Williams et al., 2006). In 21 this study, a structural index of 1.0 was used as it best represents sills or dykes and faults, which are associated with geothermal reservoirs. It is based on the Euler equation of homogeneity shown in equation 3.6;

(푋 − 푋0)푇푍푋 + (푌 − 푌0)푇푍푌 + (푍 − 푍0)푇푍푍 = 푛(퐵푍 − 푇푍) (3.6) for the gravity anomaly vertical component Tz of a body having a homogeneous gravity field, (푋0, 푌0, 푍0) are the unknown co‐ordinates of the source body centre to be estimated.

(X, Y, Z) are the known co‐ordinates of the observation points of the gravity and gradients.

The values 푇푍푋, 푇푍푌, 푇푍푍 are the measured gradients along the x‐, y‐ and z‐directions, n is the structural index and 퐵푍 is the regional value of the gravity to be estimated. In 2‐D equation 3.6 reduces to equation 3.7.;

(푋 − 푋0)푇푍푋 + (푍 − 푍0)푇푍푍 = 푛(퐵푍 − 푇푍) (3.7)

Euler deconvolution technique provided automatic approximations of a causative body location and its depth within the Earth’s subsurface. Therefore, Euler deconvolution located the boundary of the said resource and its depth from the surface. The most important outcome of Euler deconvolution is the description of trends and depths (Chenrai et al., 2010).

3.6.3 Forward modeling

This was done using GRAV2DC software in surfer 8 computer programme. Modelling entailed construction of an appropriate model based on geological information of the study area. The cross section data was transferred to GRAV2DC software for Forward modelling.

The parameters determined by Euler deconvolution acted as start-up parameters for the 22 model bodies. The model’s gravity anomaly was computed and compared to the observed anomaly. Features of the model were altered to increase the correspondence of observed anomaly and computed anomaly. The use of this program entails trial and error procedure to get a good fit between computed anomaly and observed anomaly. Therefore, Forward modelling marks the calculation of the gravitational field produced by certain source mass underlying within the Earth’s subsurface (Hirt, 2015). In this Forward modelling, the depth and density contrast of a causative body was determined.

23

CHAPTER FOUR

DATA AND PROCESSING PROCEDURES

4.1 Introduction

After collection of data as described in chapter 3, it was processed for analysis. This chapter

4 explains how data was processed before analysis was done. Changes in the Earth’s gravitational field which do not come from the differences of density in the underlying masses within the subsurface was corrected. This is called gravity reduction. After all the gravity reductions have been done, gravity anomaly is obtained. The gravity anomaly will be zero if there were no mass distribution within the subsurface of the Earth and it will be either positive or negative if there were mass distribution with different densities within the Earth’s subsurface. This gravity anomaly can either be positive or negative depending on the density of the underlying resource. Positive gravity anomaly implies a dense underlying body within the Earth’s surface which is associated with the heat source. The following corrections were done to the observed data for the purpose of determining the gravity anomalies.

4.1.1 Drift correction

The gravimeter reads different values in same station at different times. This is called instrumental drift. This is because of gravitational effect of the sun and the moon, attraction due to tides, expansion, contraction and creeping of the spring. This is corrected by frequent reading of the base station within one hour throughout the day. The gravimeter reading is then plotted against time. The drift graph is taken to be a straight line between consecutive base readings. The gravity value at any time is either added or taken away from the observed value for correction. For example, the graph in figure 4.1 was plotted to correct 24 for instrumental drift of the five stations shown. This was done to the other 147 gravity stations that were surveyed.

Figure 4.1: Drift curve for data collected on 24/04/2015.

4.1.2 Free air correction (FAC)

Free air correction (FAC) accounts for reduction in gravity as the elevation increases.

Gravity reduces with height because the distance from the centre of mass of the Earth increases as height increases. The correction is positive for an observation point above the sea level hence it is added to the observed reading. This study was done above the sea level and therefore free air corrections were positive. This values were added to each of the 147 observed readings. Gravity at sea level is given by equation 4.1;

(4.1) 25

Gravity at a height h is given by equation 4.2;

(4.2)

(4.3)

(4.4)

Therefore;

(4.5)

For example, using equation 4.5 in station 1

FAC = −0.3086 ∗ 1913 = −590.3518 mgals

This was done to the 147 observed points.

4.1.3 Bouguer correction (BC)

Bouguer correction (BC) accounts for gravity outcome due to the presence of masses between observation point and the geoid. The free air correction takes into consideration only the decrease in gravity with height. It does not recognize the gravity effect of the masses present between observation point and geoid. The Bouguer correction (BC) takes into consideration this outcome by taking the rock layer below the observation point to an indefinitely large flat material of height equivalent to the elevation of the observation point above the sea level. In this study area bouguer correction was taken away from observation value as the masses between observation point and geoid increase the gravity value. 26

The Bouguer Correction (BC) is given by equation (4.6);

BC = 2πGρh = 0.4191ρh g. u = 0.04191ρh mgals (4.6)

For example, using equation 4.6 in station 1,

BC = 0.04191 ∗ 2.67 ∗ 1913 = 214.0641 mgals

This was done to all 147 gravity stations covered during the study.

4.1.4 Terrain correction (TC)

Terrain correction (TC) accounts for topographic relief near each observation point. The bouguer correction assumes that the terrain near each observation point is flat. This is not case as the terrain near the observation point would have valleys and hills. The terrain correction must be done to remove this effect near the observation points. The terrain correction is positive because bouguer correction assumed indefinitely large flat material between observation point and geoid. This means if there was a valley round the observation point, then this valley was assumed to contain a material which is not the case.

Therefore, terrain correction should add this effect which was removed by bouguer correction. Also if there was a hill round the observation point, then it was excluded during bouguer correction. This hill applies upward attraction at the observation point resulting in gravity decrease. Therefore, this gravity decrease is restored by adding terrain correction to the observed value. The terrain correction was done automatically by the CG-5 gravimeter to all 147 observed gravity stations. 27

4.2 Regional density

Gravity variations arise from the changes in density of underlying rocks within the subsurface. If density of underlying rocks within the subsurface was constant, then gravity anomalies would not emerge after the gravity corrections. Therefore, understanding the region’s density is important during data processing and interpretation. The average density of rocks in the study area 휌푎was taken as 2.67g/푐푚3 (Mccall, 1967). Density of an intruding body 휌푏 ranges from 2.70 푔/푐푚3 - 3.20푔/푐푚3 (Kearey et al., 2002). Density contrast of the bodies ranges from 0.28 – 0.34 (Githiri et al., 2005). Density contrast is given by equation 4.7;

휌 = 휌푏 - 휌푎 (4.7)

Density contrast range was found to be 0.03 푔/푐푚3 – 0.53 푔/푐푚3 and was employed for modelling. Body of density contrast 0.03 푔/푐푚3 – 0.53 푔/푐푚3 is associated with heat source at its basin because it best forms at plumes and hotspots below the continent. Mainly occurs as an effusive body for example lava flow. Also may occur as intrusive rocks for example dyke (Mccall, 1967).

4.3 The Bouguer anomaly map

The contour map for the study area was constructed as shown in figure 4.2. The data was put in Microsoft Excel. After the corrections were done, the complete bouguer anomaly

(CBA) was determined and then transferred to surfer 8 new worksheet. The reduced data was used to generate a contour map shown in figure 4.2;

28

193000 194000 195000 196000 197000 198000 199000 200000 EASTINGS Figure 4.2: Contour map for the Gilgil area.

The contour map in figure 4.2 has been used for qualitative interpretation of the study area.

Gravity anomalies are obtained after reductions have been done to the observed gravity data. If there were no mass distribution within the Earth, the gravity anomaly would be zero. The contour map in figure 4.2 was generated from processed gravity data. This map shows contour intervals of 1푚푔푎푙 with the highest value at −181푚푔푎푙 and the least value at −211푚푔푎푙. The contour map was used for qualitative analysis of the study area.

Qualitative analysis involves identification of regions with positive gravity anomaly and regions with negative gravity anomaly. Positive gravity anomaly means a gravity high which implies a more dense underlying body than the surrounding host rock within the 29 subsurface while negative gravity anomaly means gravity low which implies a less dense underlying material than the surrounding host rocks within the subsurface.

To the Northeast, the map reveals gravity highs with few gravity lows. To the southeast, the map reveals gravity lows with a few gravity highs. To the northwest, the map reveals gravity lows with a small part of gravity high. An intruding rock has density ranging from

2.70 푔/푐푚3 - 3.20푔/푐푚3 which is a gravity high (Kearey et al., 2002). Geothermal reservoir is associated with a gravity high because materials coming from the Earth’s mantle are of higher density than materials found in the Earth’s crust.

Figures 4.3 - 4.6 displays the contour map (figure 4.2) generated from the collected data in three dimensions as observed from four different points.

Figure 4.3: An image of a contour map of Gilgil area from the south. 30

Figure 4.4: An image of a contour map of Gilgil area from the west.

Figure 4.5: An image of a contour map of Gilgil area from the North. 31

Figure 4.6: An image of a contour map of Gilgil area from the east.

Figures 4.3 -4.6 shows hills and lows from the collected reduced gravity data. The hills represent gravity high regions. This are points with the possibility of occurrence of a geothermal reservoir. 32

193000 194000 195000 196000 197000 198000 199000 200000 EASTINGS Figure 4.7: 2D Relief map of Gilgil area.

Figure 4.7 shows the altitude above the sea level for various points within the study area.

It reveals the highest point at 2040 m above sea level and the lowest point at 1780 m above sea level. 33

Figure 4.8: 3-D relief map of Gilgil area.

Generally, the map in figure 4.8 shows a lowland to the northwest and the altitude increases towards the southeast of Gilgil area. A gravity high occurring in a low altitude implies an underlying dense material within the subsurface. This could be linked to a heat source for detailed study.

4.4 Selection of profiles.

Profiles were chosen based on the information from the bouguer anomaly map in figure

4.2. This map indicated the gravity highs within the study area. Heat sources are found in gravity highs as materials from the mantle have higher density than the surrounding materials within the subsurface. Five Profiles were taken along these gravity highs for the reason of constructing a model to determine the required parameters. The five profiles PP’,

QQ’, RR’, SS’ and TT’ chosen in this study area are as shown in figure 4.9. 34

193000 194000 195000 196000 197000 198000 199000 200000 EASTINGS Figure 4.9: Contour map profiles. + Sign on the map show field stations.

The data was put in Microsoft Excel. After the corrections were done, the complete bouguer anomaly (CBA) was determined and then transferred to surfer 8 new worksheet.

The data in surfer 8 was used to create a ‘bln’ file that was sliced to construct profiles. The profiles enabled the removal of regional trend from observed anomaly generating residual anomaly. The residual anomaly data was loaded in ‘Euler’ surfer 8 software for outlining 35 fractures present in the study area and estimating depths of causative bodies. Then the data was transferred to GRAV2DC software for modelling which determined the depth, size and form of the underlying resource.

4.4.1 Cross section PP’

Figure 4.10: Observed and regional anomalies along PP’.

36

Figure 4.11: Residual anomaly along PP’.

This cross section in profile PP’ shows observed anomaly and regional trend for the area.

It was constructed along two positive gravity anomalies as shown in figure 4.10. Residual anomaly which is due to local masses was calculated by taking away regional anomaly which is due to deep masses in the study area. The residual profile was plotted as shown in figure 4.11. The residual anomaly data was transferred to Euler software for fracture imaging and depth estimation of a causative body. It was then transferred to GRAV2DC software for modelling.

4.4.2 Cross section QQ’

Figure 4.12: Observed and regional anomalies along QQ’. 37

Figure 4.13: Residual anomaly along QQ’.

The cross section in gravity profile QQ’ as shown in figure 4.12, shows observed anomaly and regional trend which cuts across a positive gravity anomaly. Residual anomaly which is due to local masses was calculated by taking away regional anomaly which is due to deep masses in the study area. The residual profile was plotted as shown in figure 4.13.

The residual anomaly data was transferred to Euler software for fracture imaging and depth estimation of a causative body. It was then transferred to GRAV2DC software for modelling. 38

4.4.3 Cross section RR’

Figure 4.14: Observed and regional anomalies along RR’.

Figure 4.15: Residual anomaly along RR’.

The cross section in profile RR’ shown in figure 4.14 was plotted across a gravity high region to the southeast direction of the study area. It displays observed anomaly and 39 regional anomaly of the area. Residual anomaly which is due to local masses within the subsurface was calculated by taking away regional anomaly which is due to deep masses in the study area. The residual profile was plotted as shown in figure 4.15. The residual anomaly data was transferred to Euler software for fracture imaging and depth estimation of a causative body. It was then transferred to GRAV2DC software for modelling.

4.4.4 Cross section SS’

Figure 4.16: Observed and regional anomalies along SS’. 40

Figure 4.17: Residual anomaly along SS’.

The cross section in gravity profile SS’ shown in figure 4.16 was oriented in SW-NE direction for the purpose of constraining the density contrast and depth of a causative body.

The residual anomaly which is a result of local masses was computed by taking away the regional anomaly which is a result of deep masses from the observed anomaly. The residual anomaly for the area was plotted as shown in figure 4.17. The residual anomaly data was transferred to Euler software for fracture imaging and depth estimation of a causative body.

It was then transferred to GRAV2DC software for modelling. 41

4.4.5 Cross section TT’

Figure 4.18: Observed and regional anomalies along TT’.

Figure 4.19: Residual anomaly along TT’. 42

The cross section in gravity profile TT’ shown in figure 4.18 was directed in a north south position for the purpose of constraining the density contrast and depth of a causative body.

The residual anomaly which is a result of local masses was computed by taking away the regional anomaly which is a result of deep masses from the observed anomaly. The residual anomaly for the area was plotted as shown in figure 4.19. The residual anomaly data was transferred to Euler software for fracture imaging and depth estimation of a causative body.

It was then transferred to GRAV2DC software for modelling.

43

CHAPTER FIVE

RESULTS PRESENTATION AND ANALYSIS

5.1 Introduction

In this chapter 5, results obtained from the processed data in chapter 4 are presented and analyzed. Gravity results presentation is based on models arising from gravity anomalies while gravity analysis is based on interpretation of these gravity anomalies. Gravity anomalies are obtained after reductions have been applied to the raw gravity data. If there were no mass distribution within the Earth’s subsurface, the gravity anomaly would be zero. There are two major methods used to interpret gravity anomalies namely qualitative and quantitative interpretations.

Qualitative interpretations outlines areas within the study area with positive gravity anomaly or negative gravity anomaly. Positive gravity anomaly (gravity high) indicates a dense underlying body within the Earth’s subsurface than the surrounding material.

Negative gravity anomaly (gravity low) indicates a less dense underlying body within the

Earth’s subsurface than the surrounding material. Northeast part of this study area shows gravity highs with few gravity lows. To the southeast, the region has gravity lows with a few gravity highs. To the northwest, the area shows gravity lows with a small part of gravity high. An intruding rock has density ranging from 2.70 푔/푐푚3 - 3.20푔/푐푚3 which is a gravity high (Kearey et al., 2002). Geothermal reservoir is associated with a gravity high because materials coming from the Earth’s mantle are of higher density than materials found in the Earth’s crust. Gravity profiles were constructed along these gravity highs for detailed quantitative interpretations. 44

Quantitative interpretations involve obtaining the true shape, size and depth of the causative body within the subsurface that produces the gravity anomaly. Quantitative interpretations proceed in either direct or indirect method. With direct methods, assumptions are made about the supposed body and its depth inverted. In indirect methods, gravity effect of an assumed model is calculated and compared to the observed gravity effects. In this study, quantitative interpretation was done.

5.2 Quantitative interpretation

In this study, quantitative interpretation was done by Euler deconvolution and Forward modelling techniques. This was done using Euler and GRAV2DC software in surfer 8 computer programme. Euler deconvolution entailed imaging of faults, fractures and depth approximation of causative bodies. Modelling entailed construction of an appropriate model based on geological information of the study area. The parameters determined by

Euler deconvolution acted as start-up parameters for the model bodies. The model’s gravity anomaly was computed and compared to the observed anomaly. Features of the model were altered to increase the correspondence of observed anomaly and computed anomaly. In this interpretation, the depth and density contrast of a causative body was determined.

5.2.1 Euler solutions along profile PP’

Euler solutions along profile PP’ as shown in figure 5.1 suggest a causative body which occurs at maximum depth of 2053.74 m. It reveals a fault at 1000 m and 2000 m along the profile. It also shows a causative body at 1000 m and 3000 m along the profile which has a material of higher density than the host rock.

45

Figure 5.1: Euler solutions obtained along profile PP’

5.2.2 Model fit along profile PP’

Profile PP’ is on the northern part of the study area as shown in figure 4.9. It cuts across a gravity high anomaly region trending in a NW – SE direction. Models on profile PP’ as shown in figure 5.2 reveals two subsurface intrusive bodies. The first body has a density of 2.92 푔/푐푚3 and imaged at a depth of 169.48 푚 while the second body has the same density of 2.92 푔/푐푚3 and imaged at a depth of 159.51 푚. This positive gravity anomaly could be a result of hot intrusive bodies of high density from the mantle under the volcanic complexes which are probably feeding the hot spring in the area hence there could be a heat source at the basin. 46

………………………………… Observed anomaly Calculated anomaly

Figure 5.2: Model fit on residual bouguer anomaly PP’

5.2.3 Euler solutions obtained along profile QQ’

Euler solutions along profile QQ’ as shown in figure 5.3 reveals a causative body which occurs at a maximum depth of 792.74 m. It has imaged a body of higher density than the surrounding rock at 1000 m along the profile. At 200 m along the profile, there is a shallow causative body.

47

Figure 5.3: Euler solutions obtained along profile QQ’

5.2.4 Model fit along profile QQ’

Profile QQ’ is on the southern part of the study area as shown in figure 4.9. It cuts across a gravity high anomaly region trending in a NW – SE direction. Models on profile QQ’ as shown in figure 5.4 reveals an intrusive body of density 2.95 푔/푐푚3 and imaged at a depth of 50.33 푚. This was presumed to be due to phonolitic trachytes during the lower

Pleistocene period in the study area. Presence of recent volcanic soil shows there was volcanic activity which deposited high density materials close to the surface. Probably there was a volcanic activity in the area which stopped hence the imaged body could be a cooling dyke injection. 48

Figure 5.4: Model fit on residual bouguer anomaly QQ’

5.2.5 Euler solutions obtained along profile RR’

Euler solutions along profile RR’ shown in figure 5.5 shows a causative body which occurs at a maximum depth of 1194.21 m. The body occurs between 1200 m and 2000 m along the profile and is denser than the surrounding material within the subsurface. It also reveals a fault between 1200 m and 2000 m along the profile.

49

Figure 5.5: Euler solutions obtained along profile RR’

5.2.6 Model fit along profile RR’

Profile RR’ is on the north western part of the study area as shown in figure 4.9. It cuts across a gravity high anomaly region trending in a NW – SE direction. Models on profile

RR’ as shown in figure 5.6 shows an intrusive body of density 2.92 푔/푐푚3 and imaged at a depth of 370.22 푚. This was presumed to be a dense body imaged under a volcano which is probably a hot intruding dyke hence a heat source at the basin. 50

Figure 5.6: Model fit on residual bouguer anomaly RR’

5.2.7 Euler solutions obtained along profile SS’

Euler solutions along profile SS’ shown in figure 5.7 shows an intrusive body which occurs at maximum depth of 2837.15 m. It has also imaged faults at about 2000 m and 4000 m along the profile which is filled by a material of higher density than the surrounding host rock. 51

Figure 5.7: Euler solutions obtained along profile SS’

5.2.8 Model fit along profile SS’

This model shown in figure 5.8 was oriented in a SW-NE direction to constrain the density contrast and depth of profile RR’ and PP’. It produced the same values as obtained in figure

5.2 and figure 5.6. The density contrast of the intruding bodies was found to be 0.25푔/푐푚3.

The depth for body 1 was 365.99푚 and body 2 was 169.48푚. 52

…………………… Observed anomaly Calculated anomaly Figure 5.8: Model fit on residual bouguer anomaly SS’

5.2.9 Euler solutions obtained along profile TT’

Euler solutions along profile TT’ shown in figure 5.9 reveals a causative body which occurs at a maximum depth of 4331.38 m. It has also imaged a fault at about 2000 m to 3000 m along the profile and is denser compared to the surrounding material.

53

Figure 5.9: Euler solutions obtained along profile TT’

5.2.10 Model fit along profile TT’

The model fit shown in figure 5.10 was oriented in a nearly N-S direction to constrain the density contrast and depth of profile PP’ and QQ’. It produced the same results as in figure

5.2 and figure 5.4. The density contrast for body 1 was 0.25 푔/푐푚3 and body 2 was 0.28

푔/푐푚3. The depth was 129.16 푚 for body 1 and 50.61 푚 for body 2. 54

Figure 5.10: Model fit on residual bouguer anomaly TT’

Profile SS’ shown in figure 4.9 was drawn to cut across profile RR’ and profile PP’ for the purpose of constraining the density contrast and depth of imaged body. Also profile TT’ in figure 4.9 was drawn to cut across profile PP’ and QQ’ for the purpose of constraining the density contrast and depth of imaged body. Profile SS’ and profile TT’ gave the same density contrast and depth as profile PP’, profile QQ’ and profile RR’ as shown in figure

5.8 and figure 5.10. The two profiles have imaged faults and massive intrusions which could be heat sources. Due to hydrothermal activity and imaged fractures in the area, probably this hot intrusive bodies for profile PP’ and RR’ are responsible for the hot spring in the area. 55

5.3 Discussion

The Gilgil prospect area is located in the Kenyan rift where a number of geothermal fields lie. Fractures resulting from extensional tectonics of continental rifting provide a good structural set up that allows water from the rift scarps to penetrate deep into the crust, towards the hot magmatic bodies as modelled under the volcanoes and normal faults conducting hot fluids from deep into possible geothermal reservoirs at shallower depth.

The deep water circulation would therefore collect heat from the bodies and discharge it through hot springs along faults and fractures as observed in the study area.

The 2D Euler deconvolution was done on the profile data and revealed subsurface faults and bodies at a depth range of 790 m – 4331 m. It was discovered that the deep faults transport thermal fluids from deep parts of the Earth to the subsurface. The shallow faults in the Earth’s subsurface direct the flow of thermal fluids on the upper part of the basement.

The top faults direct the flow of water from rift scarps to the hot masses underground. This faults were concluded to be responsible for underground movement of thermal fluids.

Forward modelling of selected profiles using Grav 2DC software revealed presence of dense intrusive bodies on the northern and southern parts of the study area with the density contrast range of 0.25 − 0.28. This range was compared with the density contrast of an intruding body that ranges from 0.03 – 0.53 (Kearey et al, 2002) and density contrast of the bodies ranges from 0.28 – 0.34 (Githiri et al., 2005). These bodies were interpreted as intrusive dykes that have higher density than surrounding rocks. Thus the high heat flow observed in the area as evidenced by hot springs could be due to these intruding bodies within the rift floor faults hence heat source at the basin.

56

Geothermal occurs along major fracture lines, inactive volcanic craters and where there are hot springs. Therefore, the modelled bodies across the selected gravity profiles lie relatively at shallower depths as shown from the models in figure 5.1 - 5.10. Thus, the high heat flow observed in the area as evidenced by hot springs could be due to shallow intruding bodies within the rift floor faults. It is postulated that intrusives, in the form of dykes would be tapping heat from large magma bodies at few kilometres from the surface (Mulwa et al.,

2010).

The gravity method become an important choice to many problems that involve subsurface mapping, especially in high terrain environments as can be found in common geothermal fields in Kenya rift valley. This was possible due to a good measurement precision level

(up to 0.01 mGal), faster measurement and fully portable instrument. Gravity surveying identified the intrusive bodies and major faults that are associated with heat sources and permeability structures of geothermal prospects, respectively. The ability to identify changes in gravity as a result of mass distribution within the Earth’s subsurface was provided by precisely measuring the acceleration of gravity in the vertical direction for each observed station. The gravity anomaly was generated after gravity data reduction was done. Interpretation revealed underlying dense bodies within the Earth’s subsurface of the study area. Probably these dense intruding bodies shown in figure 5.1 – 5.10 could be magma materials pushed from the mantle into the Earth’s subsurface. These magma materials could be heat sources hence a geothermal reservoir.

57

CHAPTER SIX

CONCLUSIONS AND RECOMMENDATIONS

6.1 Conclusions

The main objective of this study was to map the geothermal potential field in Gilgil area,

Nakuru county, Kenya using gravity. This study discovered that there are regions of positive gravity anomalies (gravity highs) and regions of negative gravity anomalies

(gravity lows) in Gilgil prospect area. Gravity high regions in volcanic areas are geothermal potential fields.

The specific objectives of this study were to collect gravity data, apply gravity reductions to the observed gravity data and determine the depth, size and form of underground body within the Earth’s subsurface that causes the gravity anomalies. Data was collected from a total of 147 measurement points covering an area of approximately 68 푘푚2. Data reduction was done to determine gravity anomalies. The reduced data was processed by Euler deconvolution and Forward modelling methods to determine the depth, size and form of underground body that causes the gravity anomalies.

The 2D Euler deconvolution revealed subsurface faults and bodies at a depth range of 790 m – 4331 m. These faults are at different depths from the surface. There are those at the deep basement while others at the shallow subsurface. The deep faults transport thermal fluids from deep parts of the Earth to the subsurface. Also the shallow faults in the Earth’s subsurface direct the flow of thermal fluids on the upper part of the basement. The top faults direct the flow of water from the rift scarps to the hot masses underground. These faults converge at gravity high regions hence a geothermal potential field. 58

Models along gravity highs to the north eastern part of the prospect area are hot intrusive bodies of higher density from the mantle under the volcanic complexes which are feeding the hot spring in the area. The gravity high to the south eastern part of the study area was presumed to be due to phonolitic trachytes during the lower Pleistocene period in the study area. Presence of recent volcanic soil shows there was volcanic activity which deposited high density materials close to the surface. The gravity high to the north western part of the prospect area was presumed to be a dense body imaged under a volcano which is probably a hot intruding dyke. Forward modelling along these gravity highs showed bodies of density contrast range of 0.25 − 0.28 at various depths within the Earth’s subsurface. These gravity high anomalies are concluded to be intruding dyke injections hence they could be heat sources.

6.2 Recommendations

This gravity study was done to gather information on the possibility of geothermal occurrence in Gilgil area. No previous geophysical study has been done in this area, hence this study has provided information which will be used as a start point for future detailed geophysical work. Gravity technique is ambiguous and this implies that any anomaly could be a result of many possible sources. It could be due to a geothermal resource or any other underground resource. To reduce this ambiguity during interpretation, this study recommends the application of other geophysical techniques like seismic, magnetotelluric

(MT) and magnetic for outcome comparisons. This ensures confirmation of this gravity results before drilling is done which is expensive.

The geology survey in this area was done more than fifty years ago and probably many activities have taken place which may have altered the geological information. For 59 example, earthquakes may cause more fractures and volcanic activities in the area.

Volcanic activity will deposit new materials and faults may change the movement of underground fluids. Therefore, this study recommends more geological research to be conducted to improve the interpretation of these gravity anomalies in Gilgil area.

60

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Clarke M.G., (1990). Geological, Volcanological and hydrogeological controls on the occurrence of geothermal activity in the area surrounding . Ministry of Energy, Kenya. 7-12.

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62

APPENDICES

APPENDIX I: DATA COLLECTED AND COMPLETE BOUGUER ANOMALY Complete Observed bouguer Gravity anomaly Station Time Eastings Northings Altitude Readings CBA (Hrs) (m) (m) (m) (mGals) (mGals) Base 01 12:32 199500 9945000 1926 774.829 -202.960 -203.195 1 12:54 199000 9945000 1913 777.156 -203.436 2 13:10 198500 9945000 1915 776.525 -204.122 3 13:24 198160 9945000 1914 776.040 Base -202.960 01 13:46 199500 9945000 1926 774.848 -204.076 4 14:07 199500 9945500 1916 775.704 -203.003 5 14:23 199000 9945500 1914 777.175 -202.709 6 14:36 198500 9945500 1914 777.471 Base -202.960 01 14:51 199500 9945000 1926 774.866 Base -203.533 02 17:05 199500 9946000 1925 774.452 -205.317 7 17:21 199000 9946000 1907 776.204 -204.414 8 17:34 198500 9946000 1908 776.907 -206.254 9 17:49 198988 9946500 1904 775.850 Base -203.533 02 18:04 199500 9946000 1925 774.436 Base -203.080 01 09:28 199500 9945000 1926 774.709 -202.749 10 09:59 198500 9944500 1944 771.499 -203.419 11 10:29 198000 9944500 1914 776.730 Base -203.080 01 10:44 199500 9945000 1926 774.709 -202.753 12 11:11 199000 9944500 1922 775.811 63

-203.017 13 11:44 199000 9944000 1932 773.566 -204.128 14 12:12 199500 9944500 1920 774.804 Base -203.080 01 12:23 199500 9945000 1926 774.668 Base -203.838 03 15:36 198500 9943500 1954 768.443 -204.263 15 15:55 199000 9943500 1935 771.761 -205.240 16 16:16 199500 9943500 1930 771.776 -206.028 17 16:43 199500 9944000 1925 771.981 Base -203.838 03 17:01 198500 9943500 1954 768.474 -204.625 18 17:25 198500 9944000 1943 769.849 -205.313 19 17:43 198210 9944000 1946 768.570 -204.859 20 18:06 198500 9943000 1973 763.711 Base -203.838 03 18:19 198500 9943500 1954 768.469 Base -205.393 04 11:35 197500 9945000 1903 776.920 -205.173 21 12:24 197080 9945000 1920 773.754 Base -205.393 04 12:34 197500 9945000 1903 776.869 -203.999 22 12:52 197028 9945500 1896 779.641 -203.925 23 13:05 197500 9945500 1904 778.141 -202.929 24 13:15 197500 9946000 1904 779.138 Base -205.393 04 13:25 197500 9945000 1903 776.872 -203.799 25 13:45 197500 9944510 1917 775.704 -204.290 26 14:05 197500 9944000 1929 772.853 -205.704 27 14:15 197500 9943500 1949 767.499 -204.622 28 14:33 198000 9944000 1921 774.091 64

Base -205.393 04 14:45 197500 9945000 1903 776.859 Base -205.266 05 16:54 198000 9943500 1927 772.326 -204.356 29 17:11 198000 9943000 1935 771.659 -206.233 30 17:26 197500 9943000 1967 763.486 -204.961 31 17:42 198000 9942500 1948 768.492 Base -205.266 05 17:54 198000 9943500 1927 772.317 Base -206.866 06 10:30 198500 9940500 1944 767.382 -204.299 32 10:56 198000 9940500 1972 764.437 -206.042 33 11:23 198000 9940000 1962 764.657 -207.474 34 11:50 198500 9940000 1956 764.402 Base -206.866 06 11:58 198500 9940500 1944 767.369 Base -204.536 07 17:02 198000 9941500 1956 767.352 -205.213 35 17:13 198000 9942000 1951 767.663 -206.166 36 17:36 198517 9942000 1936 769.672 -206.608 37 17:54 198500 9941500 1935 769.435 Base -204.536 07 18:04 198000 9941500 1956 767.382 Base -206.178 08 10:17 198500 9941000 1938 769.250 -205.167 38 10:38 198000 9941000 1971 763.772 -205.052 39 10:47 197500 9941000 1976 762.905 -205.733 40 11:03 197500 9941500 1993 758.881 Base -206.178 08 11:16 198500 9941000 1938 769.256 -206.111 41 11:30 199000 9941500 1965 764.010 -205.698 42 11:51 199000 9942000 1967 764.029 65

-204.109 43 12:06 199000 9942500 1954 768.174 Base -206.178 08 12:20 198500 9941000 1938 769.251 -208.520 44 12:39 198500 9939500 1957 763.156 -207.631 45 12:50 198000 9939500 1968 761.872 -206.600 46 13:02 197500 9940000 1976 761.321 Base -206.178 08 13:15 198500 9941000 1938 769.206 Base -206.178 08 14:01 198500 9941000 1938 769.196 -208.422 47 14:15 199000 9940000 1956 763.409 -208.529 48 14:45 198995 9939500 1953 763.887 49 15:05 199500 9940000 1977 758.658 -209.034

50 15:15 199500 9940500 1976 760.109 -207.778 Base -206.178 08 15:26 198500 9941000 1938 769.181 51 15:44 199000 9941015 1955 765.953 -196.063

52 15:56 199000 9940500 1957 763.607 -208.016 53 16:13 199500 9941000 1983 761.234 -205.277 Base 08 16:31 198500 9941000 1938 769.185 -206.178 Base -208.093 09 09:27 197000 9947500 1852 784.251 -206.569 54 09:44 197000 9948000 1860 784.203 -204.467 55 10:00 196500 9948000 1840 790.240 -203.658 56 10:17 196500 9947500 1831 792.820 -203.243 57 10:35 196500 9947000 1817 795.990 Base -208.093 09 10:55 197000 9947500 1852 784.256 -203.109 58 11:11 197000 9947000 1873 785.103 -203.154 59 11:28 197000 9946500 1882 783.280 66

-203.247 60 11:43 196500 9946500 1837 792.032 Base -208.093 09 11:57 197000 9947500 1852 784.229 -203.768 61 12:14 197500 9947500 1895 780.091 -203.584 62 12:42 197500 9946500 1896 780.071 -203.519 63 12:49 197500 9947000 1896 780.134 Base -208.093 09 12:59 197000 9947500 1852 784.211 Base -203.784 10 13:55 198000 9946500 1927 773.808 -203.717 64 14:08 198006 9946000 1936 772.101 -202.979 65 14:27 197981 9945500 1930 774.017 -203.197 66 14:45 198500 9945500 1915 776.745 Base -203.784 10 14:55 198000 9946500 1927 773.797 Base -203.974 11 15:21 198000 9947500 1913 776.372 -204.224 67 15:37 198000 9948000 1911 776.519 Base -203.974 11 15:50 198000 9947500 1913 776.379 Base -200.466 12 09:51 196000 9946500 1814 799.353 -202.561 68 10:07 196000 9946000 1808 798.445 -203.647 69 10:26 196000 9945500 1816 795.794 Base -200.466 12 10:36 196000 9946500 1814 799.373 Base -206.791 13 11:30 197500 9949500 1894 777.292 -207.865 70 11:53 198000 9949493 1911 772.871 -206.800 71 12:06 198000 9950000 1914 773.344 -206.041 72 12:18 197593 9950056 1906 775.675 Base -206.791 13 12:28 197500 9949500 1894 777.284 67

Base -206.108 14 13:12 198500 9949000 1923 772.271 -205.464 73 13:30 198500 9949500 1926 772.312 -206.167 74 13:44 199000 9949000 1947 767.469 -205.412 75 14:00 198982 9948484 1923 772.934 Base -206.108 14 14:06 198500 9949000 1923 772.234 Base -206.108 14 15:04 198500 9949000 1923 772.231 -205.506 76 15:13 198500 9948500 1914 774.599 -204.934 77 15:24 198000 9948500 1916 774.774 -205.691 78 15:38 198500 9948000 1906 775.979 -206.618 79 15:54 199000 9948000 1911 774.063 Base -206.108 14 16:04 198500 9949000 1923 772.209 -208.905 80 16:21 199000 9949500 1951 763.899 -206.903 81 16:41 198000 9949000 1908 774.354 -205.412 82 16:51 197500 9949000 1890 779.384 Base -206.108 14 17:02 198500 9949000 1923 772.194 Base -207.502 15 17:21 199500 9947500 1931 769.303 -208.742 83 17:36 199500 9948000 1961 762.162 -211.016 84 18:08 200000 9948000 2036 745.135 -208.787 85 18:27 199500 9948500 1977 758.970 Base -204.032 16 10:26 195500 9946000 1795 799.524 -203.735 86 10:46 195000 9946000 1809 797.064 -203.285 87 11:00 195000 9946500 1794 800.463 -203.904 88 11:11 194500 9946500 1795 799.645 68

-202.355 89 11:24 195000 9947000 1792 801.783 Base -204.032 16 11:38 195500 9946000 1795 799.514 -202.553 90 11:58 195500 9945500 1833 793.513 -202.125 91 12:21 195500 9945000 1838 792.950 -204.061 92 12:42 196000 9945000 1831 792.384 Base -204.032 16 13:00 195500 9946000 1795 799.489 -200.369 93 13:14 196000 9947000 1809 800.392 -199.852 94 13:29 196000 9947500 1803 802.083 -201.346 95 13:48 196000 9948000 1815 798.22 Base -204.032 16 14:05 195500 9946000 1795 799.461 Base -195.485 17 13:40 194000 9946500 1805 806.104 -196.122 96 13:54 193500 9946500 1812 804.108 -195.951 97 14:17 193500 9946000 1813 804.115 -195.951 98 14:36 194000 9946000 1814 803.943 Base -195.485 17 14:48 194000 9946500 1805 806.196 Base -195.485 17 15:54 194000 9946500 1805 806.212 -196.344 99 16:09 194000 9947000 1797 806.936 -195.994 100 16:19 194500 9947000 1792 808.276 -195.119 101 16:31 194500 9947500 1790 809.553 -195.631 102 16:45 195000 9947500 1802 806.689 Base -195.485 17 16:57 194000 9946500 1805 806.253 -198.058 103 17:13 193000 9946500 1804 803.872 -197.480 104 17:25 193500 9947000 1795 806.217 69

-196.912 105 17:36 193500 9947500 1787 808.356 -196.425 106 17:49 193569 9948000 1787 808.840 -196.227 107 17:58 194000 9948000 1785 809.429 Base -195.485 17 18:09 194000 9946500 1805 806.234 Base -195.880 18 10:25 195500 9946500 1790 808.660 -195.597 108 10:44 195500 9947000 1785 809.925 -194.766 109 11:08 195452 9947500 1786 810.560 Base -195.880 18 11:19 195500 9946500 1790 808.659 -195.553 110 11:51 194508 9946000 1824 802.309 -194.097 111 12:30 194500 9945500 1844 799.905 Base -195.880 18 12:56 195500 9946500 1790 808.694 Base -195.880 18 13:43 195500 9946500 1790 808.667 -194.631 112 13:58 196500 9946000 1857 796.739 -195.561 113 14:09 196500 9945500 1856 796.009 -196.398 114 14:22 196500 9945000 1842 797.927 -198.344 115 14:36 196500 9944500 1852 794.017 Base -195.880 18 14:47 195500 9946500 1790 808.679 Base -194.947 17 09:07 194000 9946500 1805 806.642 -196.478 116 09:53 193984 9945500 1834 799.376 -196.848 117 10:22 193500 9945500 1814 802.921 Base -194.947 17 10:37 194000 9946500 1805 806.582 -195.474 118 11:26 195000 9945500 1835 800.140 Base -194.947 17 11:51 194000 9946500 1805 806.561 70

Base -190.750 19 11:00 197507 9948000 1896 792.939 -190.891 119 11:21 197500 9948676 1893 793.384 Base -190.751 19 11:54 197507 9948000 1896 792.928 -192.657 120 12:15 198599 9947500 1907 788.842 -191.566 121 12:38 198458 9947000 1916 788.145 Base -190.751 19 12:52 197507 9948000 1896 792.884 -191.657 122 13:07 198000 9946974 1917 787.828 -191.647 123 13:26 198480 9946936 1917 787.814 -191.261 124 13:41 198385 9946490 1922 787.197 Base -190.751 19 13:49 197507 9948000 1896 792.812 Base -185.580 20 10:30 197500 9939500 1974 782.767 -185.087 125 11:06 197000 9939500 1958 786.403 -184.978 126 11:25 197000 9940000 1958 786.509 Base -185.580 20 11:48 197500 9939500 1974 782.758 -188.213 127 12:12 199500 9939500 1988 777.369 -187.762 128 12:26 200000 9939500 1997 776.049 -186.763 129 12:35 200000 9940000 1997 777.048 -186.895 130 12:49 200000 9940500 2007 774.948 Base -185.580 20 13:06 197500 9939500 1974 782.753 Base -184.521 21 15:01 196000 9939500 1937 791.104 -184.610 131 15:25 196500 9939500 1956 787.289 Base -184.521 21 15:41 196000 9939500 1937 791.122 -184.734 132 15:58 195500 9939500 1923 793.671 71

133 16:16 195000 9939500 1929 794.623 -182.610 134 16:35 194500 9939500 1928 795.296 -182.144 Base -184.521 21 17:07 196000 9939500 1937 791.165 Base -180.944 22 13:25 195500 9942500 1883 805.303 135 13:51 195959 9941932 1928 794.046 -183.372 136 14:16 196500 9942000 1955 788.967 -183.162 137 14:36 196500 9941500 1926 795.937 -181.915 Base -180.944 22 14:57 195500 9942500 1883 805.386 138 15:13 195484 9942014 1876 804.064 -183.638 139 15:34 195007 9942492 1872 806.566 -181.916 -185.261 140 16:17 195485 9942998 1909 795.931 141 16:27 195355 9943000 1868 805.542 -183.712 Base -180.944 22 16:35 195500 9942500 1883 805.358 Base -180.996 23 11:35 196000 9943500 1881 805.644 142 11:49 196000 9943000 1846 811.741 -181.792 143 12:09 196500 9943000 1888 800.674 -184.609 Base -180.996 23 12:25 196000 9943500 1881 805.675 -182.701 144 12:47 196000 9944466 1846 810.851 145 13:02 196500 9944000 1875 802.95 -184.894 146 13:20 196000 9944000 1896 799.084 -184.626 Base -180.996 23 13:35 196000 9943500 1881 805.662 147 13:52 196500 9943500 1892 799.514 -184.985 Base -180.996 23 14:01 196000 9943500 1881 805.671

72

APPENDIX II: PROFILES DATA

(a) DATA FOR PROFILE PP’

Distance Anomaly (m) (mgal) 0 0.000449276 15.82274919 0.035479591 121.2029954 0.359358556 156.9795725 0.487991753 283.0550946 1.13702256 298.1363959 1.229508051 439.2932193 2.406928384 444.9071939 2.461389094 580.4500426 4.185298941 606.7592931 4.53746041 721.606866 6.54683338 768.6113924 7.231214986 862.7636893 8.751114812 930.4634916 9.378084629 1003.920513 9.599740634 1092.315591 9.795786885 1145.077336 9.50043166 1254.16769 9.400878688 1286.234159 9.125545521 1416.019789 8.146329158 1427.390983 7.982997116 1568.547806 5.346866881 1577.871889 5.125510697 1709.704629 2.443356714 1739.723988 1.90960832 1850.861453 0.78629767 1901.576087 0.400054984 1992.018276 0.480355009 2063.428186 0.747326453 2133.175099 1.624865961 2225.280286 3.447334863 2274.331923 4.548087793 2387.132385 7.28184156 2415.488746 7.900927106 2548.984484 10.21349601 73

2556.64557 10.29545413 2697.802393 11.20591402 2710.836583 11.24726552 2838.959216 10.78211217 2872.688682 10.52610695 2980.11604 9.247108182 3034.540782 8.410931301 3121.272863 6.920350544 3196.392881 5.398488047 3262.429686 4.06566334 3358.24498 1.828828781 3403.58651 0.938596486 3520.097079 -1.34615119 3544.743333 -1.60440484 3681.949179 -1.83932896 3685.900156 -1.84139297 3827.05698 -1.44917980 3843.801278 -1.38518358 3968.213803 -0.81242094 4005.653377 -0.60145431 4107.481872 0.000422519

(b) DATA FOR PROFILE QQ’

Distance Anomaly (m) (mgal) 0 -2.4E- 08 81.89056166 -0.15260757 134.3132329 -0.25308039 236.9659919 -0.35428624 280.3797499 -0.28562473 392.041422 0.340964957 426.4462668 0.592243664 547.1168522 1.813151249 572.5127838 2.124707768 702.1922824 4.208576668 718.5793007 4.505110978 857.2677126 7.573325852 74

864.6458177 7.715185846 1010.712335 8.93500123 1012.343143 8.917398897 1156.778852 5.207281445 1167.418573 4.999434391 1302.845369 2.358498755 1322.494003 2.106521867 1448.911885 0.624955601 1477.569433 0.523676659 1585.470678 -3.1147E-08

(c) DATA FOR PROFILE RR’

Distance Anomaly (m) (mgal) 0 4.8E- 08 42.37312693 0.106184679 109.3303886 0.297336012 163.4808254 0.46177425 284.5885238 0.868817907 330.1999026 1.021318699 405.6962223 1.275906509 526.8039207 1.668523473 551.0694167 1.740050964 647.9116191 2.014280121 769.0193176 2.284091258 771.9389308 2.289269777 890.127016 2.414710304 992.8084449 2.49754412 1011.234714 2.489473882 1132.342413 2.517682457 1213.677959 2.632941968 1253.450111 2.595098081 1374.55781 2.480628357 1434.547473 2.582034895 1495.665508 2.654258787 1616.773207 3.001409133 1655.416987 3.068038105 1737.880905 3.322277482 75

1858.988604 3.388287964 1876.286501 3.330551238 1980.096302 3.035161325 2097.156015 2.288236305 2101.204 2.260784513 2222.311699 1.273509133 2318.02553 0.433825652 2343.419397 0.234474534 2388.419945 1.07482E- 06

(d) DATA FOR PROFILE SS’

Distance Anomaly (m) (mgal) 0 -3.9E- 08 37.85315097 0.159776796 91.74808062 0.405013862 207.0156639 0.875360819 310.1198186 1.278236053 322.2832471 1.327649959 437.5508303 1.783053787 552.8184136 2.239449063 582.3864861 2.347793173 668.0859968 2.646725657 783.35358 2.974783112 854.6531537 3.105262027 898.6211633 3.202171383 1013.888746 3.266859163 1126.919821 3.342725264 1129.15633 3.343443526 1244.423913 3.504533957 1359.691496 3.839014066 1399.186489 3.972185575 1474.959079 4.13561596 1590.226663 4.328964592 1671.453156 4.429436284 1705.494246 4.439314394 1820.761829 4.342845624 76

1936.029412 3.99227384 1943.719824 3.95329551 2051.296996 3.42394097 2166.564579 2.709867596 2215.986492 2.364244578 2281.832162 1.931223978 2397.099745 1.130713255

2488.253159 0.49385719 2512.367329 0.350356837 2627.634912 -0.12199477 2742.902495 -0.48537962 2760.519827 -0.53407123 2858.170078 -0.74211387 2973.437661 -0.86462197 3032.786494 -0.84007642 3088.705245 -0.78478034 3203.972828 -0.43505844 3305.053162 0.179267232 3319.240411 0.265986239 3434.507994 1.327353668 3549.775578 2.841787826 3577.31983 3.302051398 3665.043161 4.594643677 3780.310744 6.522794382 3849.586497 7.764449137 3895.578327 8.450855185 4010.845911 10.2473028 4121.853165 12.37067946 4126.113494 12.40336675 4241.381077 10.06608036 4356.64866 6.368779897 4394.119832 5.365684693 4471.916244 3.352902454 4587.183827 1.129868552 4666.3865 0.099097999 4702.45141 -0.34090760 4817.718993 -0.92758218 4932.986576 -0.86497699 77

4938.653167 -0.85193218 5048.25416 -0.51669927 5163.521743 -0.10224116 5210.919835 0.040456175 5278.789326 0.260536188 5394.056909 0.448576186 5483.186503 0.432226865 5509.324493 0.417216884 5624.592076 0.165860619 5674.300581 -1.7406E-06

(e) DATA FOR PROFILE TT’

Distance Anomaly (m) (mgal) 0 -4.3E-08 3.12604805 0.005417863 21.04318025 0.037003396 127.9848126 0.216298264 234.9264449 0.281562259 341.8680772 0.199192144 448.8097095 0.144747526 555.7513418 0.206514412 662.6929741 0.537176494 769.6346064 1.981530252 876.5762387 4.194956555 983.517871 6.759303884 1090.459503 9.563820004 1197.401136 12.02202977 1304.342768 12.98959479 1385.173523 13.3124507 1411.2844 13.38755765 1518.226033 13.71337114 1625.167665 14.01650626 1732.109297 14.09527901 1839.050929 13.610771 1945.992562 12.96170654 2052.934194 12.30789789 2159.875826 11.41808065 78

2266.817459 9.789769388 2373.759091 7.648746197 2480.700723 5.586821159 2587.642356 3.860528364 2694.583988 2.739306805 2767.220998 2.602814462 2801.52562 2.531574902 2908.467252 2.809846249 3015.408885 3.25878012 3122.350517 3.712915587 3229.292149 4.005635785 3336.233782 4.07794651 3443.175414 4.04807294

3550.117046 3.999313285 3657.058679 3.986271686 3764.000311 4.049826226 3870.941943 4.1826437 3977.883576 4.342371187 4084.825208 4.477332853 4149.268473 4.506302463 4191.76684 4.52546368 4298.708472 4.454750218 4405.650105 4.304497049 4512.591737 4.144625209 4619.533369 4.016655987 4726.475002 3.91333649 4833.416634 3.809226147 4940.358266 3.722817192 5047.299899 3.666181236 5154.241531 3.619562039 5261.183163 3.555259808 5368.124795 3.472933563 5475.066428 3.394746056 5531.315948 3.365193414 5582.00806 3.339407973 5688.949692 3.318434724 5795.891325 3.335432058 5902.832957 3.383242783 79

6009.774589 3.448009673 6116.716222 3.50727479 6223.657854 3.487245281 6330.599486 3.27843338 6437.541119 3.004341403 6544.482751 2.722756926 6651.424383 2.455087218 6758.366015 2.243667298 6865.307648 2.10879418 6913.363423 2.07368557 6972.24928 2.036123087 7079.190912 2.056672548 7186.132545 2.319053306 7293.074177 3.601238609 7400.015809 5.501313509 7506.957442 7.622242086 7613.899074 9.86540851 7720.840706 10.82982031 7827.782339 8.512740982 7934.723971 5.966820431 8041.665603 3.669455175 8148.607235 1.683347843 8255.548868 0.321027495 8295.410898 0.102429886 8362.4905 -0.258524788 8469.432132 -0.477425414 8576.373765 -0.362872354 8662.754015 -1.6059E-07

80

APPENDIX III: SOME DRIFT CURVES

(a) Drift Corrections for Base 1

(b) Drift Corrections for Base 2

81

(c) Drift Corrections for Base 3

(d) Drift Corrections for Base 5

82

(e) Drift Corrections for Base 6

(f) Drift Corrections for Base 7

83

(g) Drift Corrections for Base 9