GEOTHERMAL PROSPECTING OF OLKARIA DOME AREAS IN , COUNTY USING GRAVITY METHOD

JOSEPH AYIETA WAREGA I56/CE/24502/2012

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD OF THE DEGREE OF MASTER OF SCIENCE (PHYSICS) IN THE SCHOOL OF PURE AND APPLIED SCIENCES OF KENYATTA UNIVERSITY

NOVEMBER 2019 ii

DECLARATION

This thesis is my work and has not been presented for award of a degree at any other University,or for any other award.

Signature……………………………………Date:……………………………………

Joseph Ayieta Warega I56/CE/24502/2012

SUPERVISORS

This thesis has been submitted with our approval as University supervisors:

Signature…………………………………Date:………………………………………

Dr. Willis. J. Ambusso Department of Physics, Kenyatta University

Signature:………………………………….Date………………………………….. Dr. Gitonga .J. Githiri Department of Physics, Jomo Kenyatta University of Agriculture and Technology

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DEDICATION

This thesis is dedicated to my family, my cousins: Mr. Odhiambo Jayoro and Mr.

John Oringo, whose exemplary support sustained me during the entire period of research and the period of schooling.

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ACKNOWLEDGEMENTS

I wish to thank most sincerely all those who have contributed in any way the completion of this study. Firstly, my most heartfelt gratitude goes to Dr. Githiri John whose most lively lectures in geophysics triggered off in me the interest to pursue applied geophysics in gravity. As my lecturer and supervisor he gave me both invaluable guidance and assistance at every stage as the project work developed. He was a source of constant encouragement and I benefited much from his wealthy experience in geophysical prospecting techniques. Secondly, I wish to thank my main supervisor Dr. Willis Ambusso for his invaluable assistance he offered in this research. He guided me throughout my work more sincerely during development of my proposal.

Thirdly, I wish to thank Odhiambo Jayoro and John Oringo for their financial support and encouragements during my high school and University education, God bless you abundantly. My sincere thanks go to my wife, Jedida Ayieta and the entire family for their continual moral and spiritual support throughout the course of this research work amid social-economic challenges. Not forgetting Charles Otieno of Kengen Naivasha,

Kapis Otieno and Thomas Obunga, my former principal for their tremedius contributions and encouragements. On conclusion, I wish to extend my gratitude to

God for sustaining my life and the lives of my supervisors, lecturers from physics department and any other person who contributed indirectly to the completion of this work. God has remained faithful to all of us. May glory be unto Him forever.

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TABLE OF CONTENTS

DECLARATION ...... ii

DEDICATION ...... iii

ACKNOWLEDGEMENT ...... iv

TABLE OF CONTENTS ...... v

LIST OF TABLES ...... viii

LIST FIGURES ...... ix

LIST OF PLATES ...... xi

ABBREVIATION AND ACRONYMS ...... xii

ABSTRACT ...... xiii

CHAPTER ONE: INTRODUCTION ...... 1

1.1 General background ...... 1

1.2 Statement of Research Problem ...... 4

1.3 Objectives ...... 4

1.3.1 Main objectives ...... 4

1.3.2 Specific objectives ...... 5

1.4 Rational of the study ...... 5

CHAPTER TWO: LITERATURE REVIEW ...... 6

2.0 Geology ...... 6

2.1 Regional geology ...... 6

2.2 Geology of the Greater Olkaria volcanic complex ...... 7

2.3 Subsurface geology of greater Olkaria ...... 7

2.4 Greater Olkaria volcanic Structure complex ...... 10

2.5 Geophysical survey methods ...... 11

2.5.1 Seismic surveying ...... 12

2.5.1.1 Seismic monitoring ...... 12

2.5.2 Magnetics ...... 14

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2.5.3 Resistivity method ...... 15

2.5.3.1 Transient Electromagnetic Method (TEM) ...... 16

2.5.3.2 Theory of Transient Electromagnetic Technique ...... 17

2.5.3.3 Magnetotellurics (MT) method ...... 18

2.5.4 Gravity Method ...... 19

CHAPTER THREE: MATERIALS AND METHODS ...... 20

3.1 Auto grav C.G 5 Gravimeter ...... 20

3.1.2 Data acquisition ...... 21

3.1.2.1 Gravity data correction ...... 22

3.1.2.2 Instrumental drift ...... 22

3.1.2.3 Correction for Bouguer ...... 22

3.1.2.4 Correction for latitudes effect ...... 23

3.1.2.5 Free air correction ...... 23

3.1.2.6 Terrain correction...... 24

CHAPTER FOUR: RESULTS AND DISCUSSION ...... 25

4.1 Processing Data ...... 25

4.1.1 The map for the Bouguer anomaly ...... 25

4.1.2 Interpretation of the observed anomalies ...... 27

4.1.2.1 Introduction ...... 27

4.1.2.2 Qualitative interpretation of gravity anomaly ...... 28

4.1.2.3 Selection of profiles ...... 29

4.1.2.4 Residual anomalies isolation and description of the profiles...... 30

4.1.3 Quantitative interpretation of the gravity anomaly ...... 35

4.1.3.1 Introduction ...... 35

4.1.3.2 Direct interpretation ...... 35

4.1.3.2.1 Limiting depth ...... 35

4.1.3.2.2 Half-width Ratio method X1/2 ...... 35

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4.1.3.2.3 Amplitude gradient ratio approach ...... 37

4.1.4 Indirect methods...... 40

4.1.4.1 Euler Deconvolution: Principles ...... 40

4.1.4.2 Euler deconvolution along selected profiles ...... 41

4.1.4.3 Forward Modeling: Principles ...... 43

4.1.4.3.1 Forward modelling for the profiles ...... 43

4.2 Discussion ...... 48

CHAPTER FIVE: CONCLUSION AND RECOMMENDATION ...... 50

5.1 Conclusion ...... 50

5.2 Recommendation ...... 51

REFERENCES ...... 52

APPENDICES ...... 55

Appendix I: Gravity data reductions ...... 55

Appendix II: Residual Gravity for Profiles ...... 61

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LIST OF TABLES

Table 3.1: Repeatability determination ...... 21

Table 4.0: Shows a sample of the data processed used to draw contour map ...... 25

Table 4.1: Shows a sample of residual data for profile LL’ ...... 30

Table 4.2: Shows a sample of residue data for profile QQ’ ...... 32

Table 4.3: Shows a sample of data for Profile SS’...... 33

Table 4.4: Shows a sample of data for Profile TT’ ...... 34

Table 4.5: Empirical method result for the obtained depth ...... 40

Table 4.6: Results from Euler Deconvolution ...... 42

Table 4.7: Modelling Results ...... 47

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LIST OF FIGURES

Figure 1.1: Olkaria Geothermal Field location ...... 3

Figure 2.1: Map showing geology of the study area ...... 9

Figure 2.2: Structural map of Olkaria, faults in black colour were mapped in the past

while faults in blue were mapped on June 2014 ...... 10

Figure 2.3: Seismic distribution across Olkaria ...... 13

Figure 3.1: Bouguer correction ...... 23

Figure 3.2: Free air correction ...... 24

Figure 4.1: Complete Bouguer contour map of Olkaria Domes ...... 26

Figure 4.2: Three dimentional map of complete Bouguer anomaly ...... 27

Figure 4.3(a): Complete Bouguer anomaly profile ...... 31

Figure 4.3(b): Residual gravity anomaly profile LL’ ...... 31

Figure 4.4(a): Complete Bouguer anomaly profile QQ’...... 32

Figure 4.4 (b): Residual gravity anomaly profile QQ’ ...... 32

Figure 4.5(a): Complete Bouguer anomaly profile SS’ ...... 33

Figure 4.5(b): Residual gravity anomaly profile SS’ ...... 33

Figure 4.6(a): Complete Bouguer anomaly map profile TT’...... 34

Figure 4.6(b): Residual gravity anomaly for profile TT’ ...... 34

Figure 4.7(a): Half-width method along profile SS’ ...... 36

Figure 4.7(b): Half-width method along profile TT’ ...... 37

Figure 4.7(c): Gradient amplitude ratio method along profile SS’ ...... 38

Figure 4.7(d): Gradient- amplitude ratio method along profile TT’ ...... 39

Figure 4.8: Euler Deconvolution solutions for profile SS’ ...... 41

Figure 4.9: Euler Deconvolution solutions along profile TT’ ...... 42

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Figure 4.10: Observed, Computed anomaly and forward model of 2-D body for

profile SS’ ...... 44

Figure 4.11: Observed, calculated anomaly and forward model of 2-D body for

profile TT’ ...... 45

Figure 4.12: Calculated, Observed anomaly and forward model of 2-D body for

profile LL’ ...... 46

Figure 4.13: Computed, Observed anomaly and model of 2-D body for profile QQ’ 47

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LIST OF PLATES

Plate 1: Layout of topography resistivity survey…………………………………….16

Plate 2: A photograph showing Auto Grav CG 5……………………………………20

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ABBREVIATION AND ACRONYMS

BC - Bouguer Correction

CBA - Complete Bouguer Anomaly

ENE - East North East

GPS - Global Position System

HZ - Hertz

IGF - International Gravity Formula

KPL - Kenya Power and Lighting

MT - Magnetotelluric

MW - Mega Watts

NE - North East

NNE - North North East

NW - North West

SBA - Simple Bouguer Anomaly

SSW - South South West

TC - Terrain Correction

TEM - Trascient Elecromagnetic

UNDP - United Nations Development Programme

WSW - West South West

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ABSTRACT

The survey used Autograv.C.G 5 type Gravimeter over short wavelength by marking stations 310m intervals. The data was processed to remove all other effects independent of the subsurface changes in density. The complete bouguer anomaly was computed and Surfer 11 software has been used to draw contour anomaly map of the study area. Quantitative analysis of the contuor map indicates regions of gravity highs which were analysed as bodies of high density within the earth’s crust. Four profiles were drawn. The gravity anomaly was interpreted by inspection of profiles and separating the residual anomaly from the regional gravity field. 2D Euler deconvolution was done on the data profiles, indicated subsurface bodies and faults at depth between 10m and 50m. A 2D gravity model along the four profiles were generated by the computer application based on algorithm in the Grav. 2dc.The obtained results revealed presence of dense body intrusions with the contrasting density ranging from 0.22g/cm3 to 0.50g/cm3. These bodies were interpreted as intrusive dykes that have higher density than surrounding rocks and probably are conduits of heat from the geothermal reservoir imaged at bottom depth of between 500m – 1000m below the surface. Advance methods of gravity data analysis such as Tensor Euler deconvolution is recommended to be carried out in Olkaria Domes to verify the results since this technique hoonours responses from many dimensions and deconvolution without gridding. Collection of more gravity data over steep and wild animal habitat areas is also required for deeper probing on longer profiles.

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CHAPTER ONE

INTRODUCTION

1.1 General background

The future of geothermal energy in Kenya is clearly a subject of great interest and importance to the people and economy of the country. This is because geothermal energy is one of the stable sources of energy with lower cost of production as compared to hydroelectric energy. From what has been learned from past experiences there is little doubt the large geothermal fields of Rift valley of Kenya offer the most energy proposition of electricity for future development if properly tapped (Bhogal, and Skinner,

1971). The megawatts of energy so far reached in those fields cannot be considered sufficient to have exhausted the geothermal energy so more exploration needs to be carried out.

The term Rift Valley geothermal field is used for the extensive region under volcanic activities in southern, central and northern parts of Rift Valley. In Kenya many geothermal exploration sites have been established and are situated in rift system of

Kenya.

Geothermal exploration began in 1950’s in Kenya with major investigations in the area between and Olkaria upper rift ( Keary and Brooks, 1991). Wells were drilled at depth which met high temperatures. The exploration then gathered speed with support from United Nations Development Programme (UNDP) that made many more geophysical probes undertaken and more wells drilled (Marrita, 1995).

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The geophysical studies done in the area included various resistivity techniques

(Schlumberger, eletromagmatic, dipole, headon), seismics and magnetics. These activities contributed to the building and setting up of first geothermal power plant of 45

MW capacity between 1981 and 1985 (Marrita, 1995). Technological advancement made it possible for the application of modern geophysical method such as magnetotellurics

(MT) and transient electromagnetics (TEM) suitable for deep and shallow conductors to be correctly mapped and so additional geothermal models evolved (Meju, 1996). More research saw the evolution of the second, 70MW geothermal power plant North-East region. The 12 MW Olkaria III geothermal power plant in the Olkaria west region has since been built with another 2.0 MW at the nearby Oserian farm.

The Olkaria West, central and East geothermal prospect areas have been explored and their potential established. However, Olkaria dome area has not been surveyed using gravity technique to attest to other geophysical methods applied in order to locate a geothermal reservoir. The gravity survey was then carried out and dykes which are associated with heat sources have been identified. These dykes act as conduit for the heat from a geothermal reservoir located at bottom depth of 1000m below the surface. The location of Olkaria geothermal field is shown in Figure 1.1.

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Figure 1.1: Olkaria Geothermal Field location modified from Simiyu, 1997

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1.2 Statement of Research Problem

Geothermal energy in Kenya has become one of the most reliable sources of energy besides solar energy and hydroelectric power energy in Kenya. However solar being energy derived from the sun, may not supply enough energy for use in heavy commercial industries. The solar panel and solar energy storage gadget have also limited period of lifespan. Hydroelectric energy production which depends on water levels of rivers and fuels has not been able to produce enough power apart from being unfriendly to the environment. Therefore geothermal energy remains a better option for both industrial and domestic use if properly tapped and required megawatts is produced to meet the Kenyan population electricity consumption demand. In order to achieve this, more exploration work has to be done for more geothermal reservoirs to be identified and hence more wells to be drilled.

Geophysical techniques such as seismic, magnetic and resistivity have been employed in the study area to give details of geological boundaries of faults, fluid flow and

Earthquake monitoring. However, little work has been done by gravity survey. Gravity study as a technique of geothermal prospecting was used to outline the expected source of heat and other crustal states (i.e faults) beneath the surface in Olkaria dome areas.

1.3 Objectives

1.3.1 General objective

The main general objective was to conduct a geophysical exploration of Olkaria dome geothermal field using gravity method.

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1.3.2 Specific objectives

i. To carry out gravity data reductions.

ii. To draw contour anomaly map.

iii. To interpret the anomalies from the contour map

iv. To select gravity data profiles.

v. To determine location of geothermal of heat source reservoir by developing

gravity models.

1.4 Rational of the study

The main challenge facing geothermal firms is how to cheaply and effectively predict the most suitable sites for drill wells so as to offer productive geothermal fluid reservoirs deep below the surface.

Gravity method depends on the principle that different rocks locally affect the earth’s gravitational fields and produce anomalies which are either negative or positive and is important in detecting the heat source and fault systems in the subsurface. It has been discovered that heat sources below the surface leads to increase in density and is mapped by the gravity method as gravity highs. Unlike the main Olkaria geothermal prospects which have been explored and its potential documented, Olkaria dome areas have not been explored for geothermal potentiality using gravity.

Geophysical studies and detailed mapping by other techniques on the area have revealed zones of lava flows, fractures and presence of hot springs which are evidence of presence of geothermal heat reservoir, hence the reason to conduct a reconnaissance gravity study in order to determine the location for fluid reservoir.

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CHAPTER TWO

LITERATURE REVIEW

2 Geology

2.1 Regional geology

Development of rift system of African East regions was structurally restrained which saw faults penetrating the collisional contact zone Proterozoic orogenic and Tanzanian

Archean craton belts (Mosley and Smith, 1993).The rifting process within Kenya began in the late Oligocene (30 Ma) in the region to day dubbed the Turkana rift (Mohr and

Kampunzu, 1991). Volcanism related to splitting began at time of Miocene. The uplifting of domes of about 400m, out broke phonolites (Baker and Wohlenberg, 1971). The volume of eruptive rocks related to rifting approximated to be greater than 220,000 km3

(Williams, 1972; Baker, 1987). The subsequent faulting of the Miocene volcanics gave birth to large trachytic ignimbrites eruption in the central area forming the Mau tuffs.

Another faulting chapter tailed eruption of ignimbrites that formed what is known today as the graben structure. During the development of the graben, fissure eruptions of trachytes, trachyandesites, basalts and basaltic happened. The developing graben was filled by the plateau rocks which were faulted in block to form normal faults of high angle in rift floor (Hay, D.E., and Wendlandt, R.F., 1995). The cracks acted as conduits for the volcanic activity of mafic to felsic formation. The most powerful volcanic activity happened at the centre of the rift where volcanic series is suggested to be of the sequence

5km in thickness. The approximated thickness is from seismic data (Henry et al., 1990,

Simiyu., 1997) correlation of stratigraphy (e.g. Baker et al, 1972), and discerned materials from drilled wells in geothermal Olkaria fields.

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2.2 Geology of the Greater Olkaria volcanic complex

The larger volcanic field of Olkaria is portrayed by several centres of volcanicity with existence of comendite on the surface. Other volcanic centres include Suswa caldera to the south, Longonot to the southeast and the Eburru volcanic complex to the north of

Olkaria (Figure 1.1). Volcanic complex of Olkaria does not have distinct caldera correlation whereas the other volcanoes are related to caldera of varying sizes. The occurrence of a ring of volcanic domes in the south, southwest and east has been used to invoke the occurrence of a buried caldera (Naylor, 1972, Virkir, 1980, Clarke, 1990,

Mungania, 1992). Studies on Seismic wave decay in Olkaria region reveal also an anomaly in the region corresponding with the suggested caldera.

Magmatic activity of Olkaria started at the time of the late Pleistocene and continued to recent as shown by Oloolbutot comendite, which is dated at 180±50 years B.P using carbon 14 (Clarke, 1990).

2.3 Subsurface geology of greater Olkaria

The litho-stratigraphic data collected from geothermal wells and regional geology revealed that the Olkaria geothermal area is categorized into six major groups; namely

Upper Olkaria volcanics, Pre-Mau volcanics, Olkaria basalt, Proterozoic ‘‘basement’’ formation and plateau trachytes (Omenda, 2000). The layouts are shortly discussed below. The ‘‘basement’’ rock in the region is of the Proterozoic amphibolite grade gneisses and schists and related marble and quartzites of the Mozambiquan group

(Shackleton, 1986). The far flanks of the rift are the rocks outcrops, particularly, towards

Magadi area in the south. Rock composition of gneisses and schists are largely found in the southern section of the Kenya rift. Gravity, seismic and geological correlation show

8 that the ‘‘basement’’ is 6 km deep in the rift (Simiyu et al., 1995 and Keller, 1997).

Gravity together with seismic studies reveal occurrence of dense magmatic intrusion in the metamorphic ‘‘basement’’ rocks (Wohlenberg and Baker, 1971).

Pre-Mau formation is evident on the rift cliffs in the southern regions of Kenya rift as rocks outcrop. The rocks are basalts, ignimbrites and trachytes composition of undetermined thickness as shown in figure 2.1. The rocks are covered by the Mau tuffs.

Mau tuff rocks are absent but are present in the west because of an angle dipping fault eastwards passing into Olkaria Hill (Omenda 1994, 1998). These rocks differ in texture from consolidated to ignimbritic and the main reservoir of geothermal energy bearing rocks in the Olkaria west area have been noticed from the geothermal boreholes drill fragments in the area. Trachytes are main formation of Pleistocene age and occur between 1000m to more than 2600m in depth but minor basalts, tuffs, and rhyolites are also present (Ogoso-Odongo, 1986; Omenda, 1994, 1998). The Olkaria Hill region to the east where a graben occurred before they erupted is characterized by occurrence of

Plateau trachytes and are the geothermal bearing rocks in the Olkaria east. (Omenda,

1994, 1998). These are the geothermal bearing rocks in the Olkaria east.

The formation in upper Olkaria is composed of lavas from comendite, ashes from

Longonot and Suswa volcano and trachytes occur about 500m deep from the surface

(Thompson and Dodson, 1963). This formation is dominated by comendite rocks of which the youngest of the lavas is the Ololbutot comendite dated 180±50 years (Clarke,

1990). The vents for these young lavas and pyroclastics were structurally controlled with many centres occurring along N-S faults and a ring structure (Figure 2.2).

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Figure 2.1: Map showing geology of the study area modified from Omenda, 1998

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2.4 Greater Olkaria volcanic Structure complex

The volcanic structure of greater Olkaria complex include; ring structure, ENE-WSW, N-

S, NNE-SSW, WNW-ESE, NW-SE trending faults and Ol’Njorowa gorge (Simiyu,

1997). Prominent faults are present in the west, northeast and east of Olkaria areas but are

rare in the Olkaria Domes area, possibly because of the thick pyroclastics cover. The

WNW-ESE and NW-SE faults are thought to develop during the rifting and so are the

oldest. The Gorge Farm fault is the most visible of these faults which bounds the

geothermal areas in the north-eastern part and extends to the Olkaria Dome areas. The N-

S and the NNE-SSW faults are most recent structures (Figure 2.2).

(M)

NORTHINGS

EASTINGS (M)

Figure 2.2: Structural map of Olkaria, faults in black colour were mapped in the past while faults in blue were mapped on June 2014

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2.5 Geophysical survey methods

Geophysical techniques can be categorized into two main categories namely; those that require earth’s natural field and the ones that use input energy generated artificially underground. Natural field techniques are magnetics, Magnetolluric (TM) gravity. Man- made techniques entail generating electro-magnetic, seismic or electrical energy whose signature underground give details on the geological boundaries at depth. There are numerous geophysical methods for geothermal energy exploration together with micro- monitoring reservoirs under exploration (Ndombi, 1981; Marrita, 1995). The application of these methods depends on the type of physical property. At Olkaria, Magnetotelluric

(MT), electrical resistivity, magnetics, Transient Electromagnetic (TEM), seismic and gravity have been employed. Normally these techniques are applied in fusion.

For instance, gravity is carried out together with magnetic. At the time of explanation, results from one survey may often be compared with results from a second survey technique in order to remove ambiguity that may arise. Although several geophysical techniques that are used involve complicated computation in data processing and analysis, much information is evolved from a simple assessment of the survey data.

In addition to using the above methods to do a geothermal survey, they are also used in micro-monitoring of geothermal reservoir. For instance, in the recent past Olkaria has witnessed problem to maintain its intended output of 45 MW. Therefore under such conditions it is important to monitor which part of the field is being affected most by fluid withdrawal so as to recommend re-injection strategies (Ambusso Karingithi 1993).

Long term changes, which take over months and years, are monitored by using micro- gravity and micro-seismicity methods.

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2.5.1 Seismic surveying

In seismic method of survey, seismic waves are sent into the earth and travel times of the waves that return to the surface after reflection or refraction at the geologic boundaries are measured. Then these travel times are converted into depth values and the subsurface interfaces distribution of geologic interest is imaged. Active seismicity is employed to image faults for possible targets for drilling at Olkaria (Hamilton, 1973).

2.5.1.1 Seismic monitoring

From 1996, a continuous micro-monitoring network has been run in Olkaria to provide information on the distribution of earthquake and wave characteristics over the geothermal field (Marrita et al., 1996; Simiyu, 1999). Also seismic stations have been started near geothermal sites of Suswa, Longonot and Olkaria Domes. The main purpose is to analyse wave parameters to determine location of earthquake so as to correlate the sites to the existence of structures that permit exit of fluid from reservoir in particular patterns (Figure 2.3). Then, these seismic characteristics are associated directly to physical parameters such as temperature as well as pressure (Hamilton.,R.M Smith B.F., and Knopp F., 1973). This method has been used in interference tests for feeder zone location in wells, discharging wells and barriers that control fluid flow (Mariita, Otieno and Shako, 1996).

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Figure 2.3: Seismic distribution across Olkaria

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2.5.2 Magnetics

The main purpose of using this method is to analyse underground geology based on the magnetic field of the earth anomalies that result from the characteristics of the underlying magnetic rocks. The type of rock and the environment in which it is affect its magnetic susceptibility. Faults, dykes and lava flows are some of the common causes of magnetic anomalies. For a geothermal field, the susceptibility decreases because of high temperatures.

Magnetic technique works best in combination with other methods such as gravity method. The magnetic effects caused by terrain, volcanism, hidden lavas with reversely magnetised rocks or strong magnetization can adversely affect interpretation of anomalies due aeromagnetic survey over a geothermal area. On the other hand in the world, there are instances (e.g., Iceland) where a one-off negative magnetic anomalies over geothermal fields have been caused by hydrothermal demagnetisation. Both aeromagnetic and ground methods have been applied to examine the occurrence of a geothermal resource in blend with blend over Olkaria area (Bhogal and Skinner, 1991). On the aero- magnetc maps numerous anomalies can vividly be associated with surface signatures from volcanism due to cones, domes, craters, plugs or lavas. Many centres of volcanic origin lie in regions with magnetic highs (positive) or an overlapping magnetic low

(negative) exists from these maps; but this is generally weak or zero.

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2.5.3 Resistivity method

The resistivity data interpretation delineates variation in resistivity with depth. The earth is assumed to have electric homogeneity and its resistivity is only depth variant and compares with thermal and hydrogeological structures which are correlated with reservoirs for geothermal. Following numerous soundings a two dimensional model is built using computer inversion programs. A resistivity layout is shown in plate 2.

There is thermal alteration of the geothermal area, according to the resistivity survey of the area. High temperatures increase minerals alteration in the subsurface rocks hence decreasing their resistivity. Direct current resistivity methods have been employed in

Olkaria area for follow up mapping, faults location for targets drilling to image the borders of geothermal reservoirs. Magnetotelluric (MT) and Transient Electromagnetic

(TEM) sounding techniques have been favoured in the recent past. In the Transient

Electromagnetic method a man-made electromagnetic field is sent in the ground and secondary fields are recorded at the surface (Dimitrios, 1989).

The products from the soundings are usually allotted by apparent resistivity as time dependent. This is then put in computer inversion program. The TEM soundings do not penetrate to very great depths as it is being restricted by the range of frequencies that can be generated and detected. Time for the signal received to be traced before it is decayed by noise and geology of the area also plays a role. For the Olkaria situation it is discovered that the maximum depth is about 500m to 1 km. The interpretation of resistivity data from the Olkaria geothermal field indicates that the low resistivity (less than 20 Ωm) anomalies of about 1km define the geothermal resource boundaries and are controlled by linear structures in the the NW-SE and NE-SW directions.

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Plate 1: Layout of topography resistivity survey

2.5.3.1 Transient Electromagnetic Method (TEM)

TEM method of geophysical exploration is used to determine the resistivity of the subsurface structures. Transient Electromagnetic soundings also help in the static shift correction of the Magnetolluric data (Onacha, 1993).

This method has proven to be superior to the convectional DC methods for the following reasons:

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 The TEM transmitter is inductively coupled to the earth so no current is injected into

the ground, this is important in the areas where the earth’s surface is highly resistive.

 The monitored signal in TEM is a decaying magnetic field rather than electric field;

hence it is useful in geothermal areas which are associated with low resistivity.

2.5.3.2 Theory of Transient Electromagnetic Technique

The theory of the TEM Central loop is based on Maxwell’s equations for the electromagnetic field caused by a magnetic dipole at the surface in the case of horizontally layered earth. Current is transmitted through the loop or dipole and when it is turned off, a current is induced into the ground by the decaying magnetic field which in turn induces a secondary magnetic field. The decay rate of the secondary magnetic field is monitored through magnetic field sensors. The frequency done is for 16Hz twice then

4Hz once. The rate of decay of the magnetic field is a function of resistivity structure and may be used to evaluate the subsurface resistivity structures. The depth of penetration of a signal depends on how long the induction in the receiver coil (loop) may be traced, the effective areas of the receiver and transmitter loops and the resistivity structure. The upper limit of the depth of penetration is controlled by the earliest sample time of the measuring instrument while the lower limit is time the signal decays into noise determinant. The induced current distribution after current turn off may be viewed as diffuse current ring which propagates downwards and outwards from the source loop.

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2.5.3.3 Magnetotellurics (MT) method

The Magnetotelluric resistivity is the latest method ever been used for geothermal energy exploration in Kenya. The technique employs natural current field which is induced in the earth variations of the earth’s magnetic field. Both the magnetic and electric fields are measured. The method does provide more information on subsurface structure, as its depth of penetration is much larger than Transient Electromagnetics.

The depth of penetration depends on frequency and the resistivity of the substrate.

Consequently, the apparent resistivity varies with frequency and depth seeping is increasing as frequency decreases. The apparent resistivity calculation for several decreasing frequencies gives information of resistivity at progressively increasing depths.

Although the Magnetelluric technique has the ability to probe a number of tens of kilometres, the data may be distorted by distortions manifesting as frequency- independent static shifts of the apparent resistivity curves due to the presence of small- size surficial heterogeneities, as can be expected in the weathered and volcanic-covered basement terrain of Olkaria. The Transient Electromagnetic method gives a logical shallow-depth (<1km) compliment to Magnetolluric and also works correction for

Magnetolluric static shifts (Meju, 1996). The TEM-MT approach has therefore been embraced as the method with optimum potential in Olkaria. Initial analysis of

Magnetolluric data from the Olkaria area postulates the occurrence of enhanced conductivities beneath Olkaria-Domes however, no quantitative data modeling has been done and the actual physical parameters of the proposed zone of significantly enhanced conductivities are not yet known.

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2.5.4 Gravity Method

Gravity survey method is based on Newton‟s Law of Gravitation, which states that the force of attraction F between two masses m1 and m2, whose dimensions are small with respect to the distance r between them, is given by,

…………………………….…………….2.0 where G is the gravitational constant(6.67x10-11m3kg-1s-2). For a small mass m on the surface of a spherical, non-rotating, homogeneous Earth of mass M and radius R, the gravitational attraction it experiences is given by

………………………………………2.1

The term is known as gravitational acceleration (gravity). On such an

Earth, gravity would be constant (Keary and Brooks, 1991). However, the Earths oblate ellipsoidal shape, rotation, irregular surface relief and internal mass distribution cause gravity to vary over its surface. The gravitational potential (V) due to the field is given as

…………………………………………………2.2

The acceleration of any point mass towards the centre of the earth is the derivative of the potential V. The acceleration is as if the whole mass of the earth were concentrated into a point mass at the centre (Telford and Geldart, 1976).

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CHAPTER THREE

MATERIALS AND METHODS

3.1 Auto grav C.G 5 Gravimeter

The CG5 Auto Grav is an advanced digital instrument used for measuring gravity developed after an extensive use of Lacoste Romberge model. The instrument has a resolution of 1 microGal with a deviation that is less than 5 microGals. The sensor of the instrument depends on elastic system of fused quartz. The spring and relatively small electrostatic restoring force are used to balance the gravitational force on the proof-mass.

The strength and elastic properties that form part of fused quartz, together with the limit stops the proof-mass without clamping (Plate2).

Plate 2: A photograph showing Auto Grav CG 5

The CG 5 sensor is sealed in a temperature established double stage vacuum chamber to shield it from the changes of ambient temperature. The temperature sensor signal in close contact with the elastic system was employed to do a software correction for small residual changes in temperature and atmospheric pressure.

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3.1.2 Data acquisition

Before delving into field measurements it was necessary to establish whether the gravimeter was providing valid readings. This was done by taking several readings in a succession in one location and the repeatability noted as shown in Table 3.1. The successive readings were found to be less than 0.005 mGals.

Table 3.1: Repeatability determination Trials Gravimeter reading(mGal) 1 1496.063 2 1496.066 3 1496.062 4 1496.064 5 1496.068 6 1496.065 7 1496.067 8 1496.062 9 1496.060

The gravity data was collected from about 100 gravity stations established over 10km2 target areas in Olkaria Dome areas with 310m spacing. The stations were profiled at

310m interval in grid system whose positions were measured using a Global Position

System (GPS) while a Gravimeter was used to conduct gravity measurements. In order to observe and monitor drifting of the instrument as well as to determine the complete gravity value in every observation point, base station readings were recorded beginning and after the survey at station interval of every three stations. The measured drifts for the instrument were then extrapolated linearly from the reading differences made at the base station. CG 5 Autograv has SCTUTIL data transfer software which was used to perform data transfer to a personal computer each day of operation.

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3.1.2.1 Gravity data correction

Since readings in the gravity survey basically reflects the gravity field from all the earth’s masses and the effect of rotation of the earth, numerous corrections were done on the field gravity readings. For good interpretation of gravity data, all known effects of gravity not identified with the changes due subsurface density were isolated. Each reading had to be corrected for the terrain, elevation, bouguer and latitude and to carry out the corrections, a reading for gravity is first taken on a flat ground surface (geoid or datum).

The corrections are put on to cater for the differences from the datum.

3.1.2.2 Instrumental drift

Due to sensitivity of the gravimeters, correction for instrumental drift was carried out by having a base station periodically visited in a day.

In Olkaria all mapped areas have been looped to one station point whose gravitational acceleration value is known to be 975953.804 mGal. This value was added to all gravity readings for all the stations in order to get gravity observed values.

3.1.2.3 Correction for Bouguer

This is applied to account for rock slab attraction present between the datum (geoid) and the observation point which is detected by the instrument. It isolates the influence by estimating the rock below the observation point as horizontal slab with thickness same as the height of the observation point above the datum. A factor B.C = 0. 4191 ph g.u. was subtracted from the measured gravity. Where ρ. (2.67g/cm3), crustal density average and h – is the height above datum.

23

h station

GEOID

Figure 3.1: Bouguer correction

∆ɡ = 3.0

∆ɡ = ( 3.1

Where ρ=2.67g/cm3 and ∆ɡ is acceleration due to gravity.

3.1.2.4 Correction for latitudes effect

The earth’s non spherical shape and because the angular velocity of a point on the earth’s surface decreases from maximum at the equator to zero at the poles causes variation of gravity with latitude. Latitude correction is given by the international gravity formula

2 2 (IGF) (Clairaut’s formula). The value (gØ = 9.78049 (1 +0.0052884sin Φ-0.0000059sin

2Φ) m/s2 where Φ in radians was subtracted from the measured value to remove latitude effect.

3.1.2.5 Free air correction

The pull of gravity decreases with increasing distance from centre of the earth.

Correction to account for this is called free air correction.

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The obtained value was added to gravity observed (gobs) to correct it to what would have been measured at the ‘geoid’ (mean sea level) as shown in figure 3.2.

Station

gØ

Geoid

Figure 3.2: Free air correction

.2

3.1.2.6 Terrain correction

Correction for terrain accounts for variations in the observed gravitational acceleration caused by topographical variations near each observation point. Terrain correction is positive regardless of whether the local topography consists of a mountain or a valley.

Grav CG 5 terrain correction software was enabled and the effect of terrain on each point was automatically corrected.

Since these corrections are assumed to have accurately accounted for the variations in gravitational acceleration, any remaining variations in the gravitational acceleration related to the terrain corrected bouguer anomaly can be assumed to be caused by geologic structure. The complete bouguer anomaly is in form of

Bouguer anomaly = gravity observed – latitude correction +free air correction– bouguer correction + terrain correction.

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CHAPTER FOUR

RESULTS AND DISCUSSION

4.1 Processing Data

4.1.1 The map for the Bouguer anomaly

After gravity values for complete bouguer anomaly were acquired as shown in table 4.0,

a contour map was drawn as shown in figure 4.1 using sufer11software. Surfer is

contouring and 2D-3D mapping program that runs under Microsoft windows. It quickly

and easily converts gravity data into outstanding contour surface, wireframe vector,

image and shaded relief. The map was drawn at a contour interval of 2 miligal, with

Easting and Northing being metres. The hachured contours represented gravity highs.

Density inhomogeneity is distinct in the manner the contour lines are distributed (Figure

4.1)

Table 4: Shows a sample of the data processed used to draw contour map

ST EAST NORTH Height GRAV LATgø GRAV obs FAC BC SBA TC CBA

AN1 202953 9899899 2051.83 1482.18 978050.282 977435.99 633.203687 229.602 -210.69501 52.509013 -158.19

U.G 203137 9899862 2052.60 1481.69 978050.282 977435.49 626.880782 227.31 -215.22020 54.699205 -160.52

AN2 203234 9899480 2051.86 1482.54 978050.282 977436.35 624.042896 226.281 -216.17206 55.267061 -160.91

AN3 203338 9899862 2031 1482.30 978050.282 977436.10 628.196344 227.787 -213.77167 50.989672 -162.78

AN4 203523 9898893 2022.17 1485.84 978050.282 977439.64 644.134299 233.566 -200.06988 36.331881 -163.74

AN5 203526 9898562 2035.63 1483.25 978050.282 977437.06 646.019228 234.249 -201.45544 38.226443 -163.23

AN6 203859 9898449 2087.28 1470.86 978050.282 977424.66 640.049361 232.085 -217.65861 56.800610 -160.86

AN7 204133 9898557 2093.39 1469.23 978050.282 977423.04 628.325339 227.833 -226.75445 63.696451 -163.06

AN8 204459 9898738 2074.04 1473.47 978050.282 977427.27 640.177739 232.131 -214.96578 52.268783 -162.70

AN9 204520 9899252 2036.05 1482.42 978050.282 977436.22 638.97173 231.694 -206.78248 44.805487 -161.98

AN11 204410 9898893 2074.46 1471.67 978050.282 977425.48 639.726874 231.968 -217.04516 54.572162 -162.47

AN12 204450 9898554 2070.55 1472.96 978050.282 977426.77 637.189565 231.048 -217.37343 52.253432 -165.12

AN13 204442 9898238 2073.00 1473.02 978050.282 977426.83 639.448826 231.867 -215.87438 50.793389 -165.08

AN14 204635 9898000 2064.78 1474.90 978050.282 977428.70 629.544926 228.276 -220.31009 54.536091 -165.77

AN15 204604 9898739 2072.10 1472.50 978050.282 977426.30 630.508066 228.625 -222.09819 56.350190 -165.75

AN16 205166 9898755 2040.00 1480.79 978050.282 977434.59 620.972944 225.167 -219.88783 54.879836 -165.01

DG25 205083 9898954 2043.12 1479.63 978050.282 977433.44 617.34998 223.854 -223.35109 58.547097 -164.80

AN17 204877 9899428 2012.23 1485.70 978050.282 977439.51 617.443794 223.888 -217.21930 52.588300 -164.63

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(METRES)

NORTHINGS

Figure 4.1: Complete Bouguer contour map of Olkaria Domes

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Figure 4.2: Three dimentional map of complete Bouguer anomaly

4.1.2 Interpretation of the observed anomalies

4.1.2.1 Introduction

After corrections for all the geological effects, that include correction for latitude,

Bouguer slab, instrumental drift and terrain, Bouguer anomaly should therefore have details regarding the underlying density alone.

The Bouguer anomaly map figure 4.1 displays un evenly spaced contour signatures due to in homogeneity in density distribution in the subsurface rock materials and contains

28 information of the subsurface causative body density. Analysing the data involved establishing anomalies from Bouguer contour map which was analysed on basis of the knowledge that, low values anomaly reveal low density material below the measured points and high anomaly values show high density material below the measured point

(Qualitative analysis). Anomaly profiles were selected figure 4.1. Through calculating anomaly fields and correlating with measured values (Quantitative analysis), a body of suitable form was obtained. This further aided density, depth determination besides extent of the causative body.

4.1.2.2 Qualitative interpretation of gravity anomaly

This entails careful dissecting Bouguer anomaly map for the regions with anomalies. It was done to approximate the causative body’s extent and identify the long and short regional trend wavelength for every profile which was isolation for more analysis.

Bouguer map of the area of study figure 4.1 reveals contours that are ascending to the centre (gravity high), giving Bouguer positive anomaly which is interpreted as caused by a body with higher density (density contrast) than the surrounding rocks. It envelops a region of about 7 km2 which is under Olkaria Domes.

The gravity highs contain maximum peak of -163mgal at co-ordinates (202000,

9900000) and (204000, 9898500) in the South and peak minimum of about -170mgal to the East at co-ordinates (205000, 9901500) within Olkaria Dome area. These could be dense materials of magmatic intrusion. This is encircled by gravity lows of a bout -

176mgal.

29

Along wavelength regional trend extending in the East–West sense is mapped in the contours, with overlapping short wavelength ones elongated in the NW-SE orientation.

High density causative body is imaged compared to the underlying rocks in the neighbourhood resulting from gravity field perturbation for instance shallowly situated density structures correspond to the short wavelength anomaly. For deeply seated density bodies could emanate from long wavelength regional effects. However, the study is concerned with gravity anomalies with short wavelengths for shallow exploration. It is important therefore to isolate the regional trends from residual ones. That could make it impossible to analyse residuals with short wavelength hence to accentuate the anomaly more vividly (Griffin, 1949). This is carefully performed not to remove valuable data in the process.

Gravity high in the NW-SE orientation is a fault indicator formation along the profile

SS’. This possibly is a geothermal reservoir present in the area. Another fault trending

W-E direction emanates from the NW-SE fault. A gravity low intrusion is also imaged at the grid co-ordinates centred (202000, 9901500) and (206500, 9900000) which could be as a result of sedimentation.

4.1.2.3 Selection of profiles

Profiles LL’, QQ’, SS’ and TT’ are selected from areas mostly covered by the gravity data points figure 4.1. Profile LL’ is oriented towards SWNE direction cutting through high gravity anomalies. It is centred co-ordinates (203500, 9900000). QQ’ profile is traversing NWSSE orientation and cuts the positive gravity anomaly centred grid coordinates (203000, 9899000). Profile TT’ is oriented towards NS direction centred at co-ordinates (2035000, 9899000). It targets the NW-SE fault of high gravity area

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possibly the geothermal reservoir along the Easting 203500. Profile SS’ is NWSE

inclined and passes through regions centred at (203500, 9902000) of varied gravity lows

and highs.

4.1.2.4 Residual anomalies isolation and description of the profiles

Profiles LL’, TT’, QQ’, and SS’ reveals details on the regional long wavelength and the

residual short wavelength anomalies. The regional trend was removed by estimating it

with a straight line for better anomaly analysis. The obtained values were removed

absolute Bouguer anomaly in order to separate the residual for good anomaly analysis.

Profiles SS’ and TT’ relatively gives sharp gravity curves which are located in areas

with gravity anomaly highs. Figures 4.5(a) and 4.6(a) account for absolute Bouguer

anomalies before isolating the regional while figures 4.5(b) and 4.6(b) display curves of

residual anomaly. Besides high gravity anomalies, there are profiles LL’ and QQ’ that

show both low and gravity highs. Figures 4.3 (a) and 4.4(a) both display absolute

Bouguer anomalies and figures 4.3(b) and 4.4(b) display LL’ and QQ’ residuals

respectively.

Table 4.1: Shows a sample of residual data for profile LL’

Distance(m) Residual(mgl) Distance( Distance(m) Residual m) Residual(mgl) 0 0.296393 602.2798 1.087572 1204.56 5.106464 26.18608 0.308478 628.4658 1.197644 1230.746 4.216531 52.37215 0.321763 654.6519 1.323333 1256.932 3.564436 78.55823 0.332949 680.838 1.457123 1283.118 2.690902 104.7443 0.34264 707.0241 1.609833 1309.304 1.831515 130.9304 0.356739 733.2101 1.766178 1335.49 1.130323 157.1165 0.36609 759.3962 1.943639 1361.676 0.543013 183.3025 0.37873 785.5823 2.141384 1387.862 0.142277

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PROFILE LL’

Regional Profile LL' -158 0 1 2 3 4 5 6 -160

-162 REGIONAL OBSERVED -164

-166

-168 mgl -170

CBA( -172

-174 DISTANCE(KM) Figure 4.3(a): Complete Bouguer anomaly profile Profile LL' Residual

8

)

6 (mgl

4 RESIDUAL

2

0

Residual gravity Residualgravity 0 1000 2000 3000 4000 5000 6000

-2

-4 DISTANCE (METRES) Figure 4.3(b): Residual gravity anomaly profile LL’

The residual anomaly along the first section of this profile has mapped a high sharp

peaked causative body of high density with a width of about 1500m.Subsequently, low

dense materials stretching to cover the other section as shown by negative values have

been revealed.

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PROFILE QQ’

Table 4.2: Shows a sample of residue data for profile QQ’

Distance(m) Residual Distance(m) Residual (mgl) Distance(m) Residual (mgl) (mgl) 0 -0.18909 491.9148 1.842378 983.8295 6.672096 16.39716 -0.16126 508.3119 2.074891 1000.227 6.727458 32.79432 -0.14611 524.7091 2.30639 1016.624 6.788717 49.19148 -0.16317 541.1062 2.496087 1033.021 6.770243 65.58863 -0.18003 557.5034 2.679994 1049.418 6.680203 81.98579 -0.1982 573.9005 2.865165 1065.815 6.564453 98.38295 -0.21767 590.2977 3.055793 1082.212 6.422993 114.7801 -0.23853 606.6949 3.249215 1098.61 6.249189 131.1773 -0.25863 623.092 3.393833 1115.007 6.001537 147.5744 -0.28982 639.4892 3.538895 1131.404 5.705724 163.9716 -0.3238 655.8863 3.695525 1147.801 5.385956 180.3687 -0.35881 672.2835 3.870959 1164.198 5.042234 196.7659 -0.39248 688.6807 4.055215 1180.595 4.674557

Profile QQ' Regional -160 REGIONAL

0 0.5 1 1.5 2 2.5 3 3.5 -162 OBSERVED

-164 (mgl) -166

CBA -168 -170 DISTANCE (KM)

Figure 4.4(a): Complete Bouguer anomaly profile QQ’

) 10 Profile QQ' Residual

(mgl RESIDUAL

5

0 (mgl) 0 500 1000 1500 2000 2500 3000 3500

-5

Residual gravity Residualgravity gravity gravity

DISTANCE (M)

Figure 4.4 (b) Residual gravity anomaly profile QQ’ Residual

A long this profile, big dense causative bodies of varied positive gravity values have been

imaged covering a stretch of about 3500m.

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PROFILE SS’ Table 4.3: Shows a sample of data for Profile SS’ Distance(m) Residual Distance(m) Residual Distance(m) Residual (mgl) (mgl) (mgl) 0 -0.26824 694.2995 0.450618 1388.599 -2.47021 27.77198 -0.17555 722.0714 0.294582 1416.371 -2.58785 55.54396 -0.08147 749.8434 0.090397 1444.143 -2.6954 83.31594 0.01377 777.6154 -0.11831 1471.915 -2.79904 111.0879 0.109877 805.3874 -0.34157 1499.687 -2.89191 138.8599 0.206978 833.1594 -0.56761 1527.459 -2.98167 166.6319 0.304168 860.9313 -0.78592 1555.231 -3.05928 194.4039 0.401733 888.7033 -1.0054 1583.003 -3.13407 222.1758 0.498195 916.4753 -1.12846 1610.775 -3.19442

Profile SS' Regional -162 0 1 2 3 4 5 6 -164 REGIONAL -166 OBSERVED

-168

-170

(mgl) -172

CBA -174

DISTANCE (KM)

Figure 4.5(a): Complete Bouguer anomaly profile SS’

) Pofile SS' Residual 2 RESIDUAL 0 0 1000 2000 3000 4000 5000 6000

-2 gravity (mgl gravity -4 -6 -8

Residual -10 DISTANCE (METRES) Figure 4.5(b): Residual gravity anomaly profile SS’

The residual gravity anomaly has revealed a positive gravity along small section of profile SS’ which is interpreted as dense matter intrusive of width about 1000m at co-ordinate (201500, 9901900) of anomaly map. The rest of the section of the profile mirror anomaly of gravity low.

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PROFILE TT’ Table 4.4: Shows a sample of data for Profile TT’

Distance(m) Residual Distance(m) Residual (mgl) Distance(m) Residual (mgl) (mgl) 0 0.42101 485.3537 9.501865 970.7075 8.153641 19.41415 0.656139 504.7679 9.512017 990.1216 8.144555 38.8283 0.891176 524.182 9.48988 1009.536 8.13721 58.24245 1.126122 543.5962 9.467853 1028.95 8.172558 77.6566 1.443383 563.0103 9.445936 1048.364 8.20793 97.07075 1.773749 582.4245 9.404457 1067.778 8.243326 116.4849 2.103994 601.8386 9.358819 1087.192 8.310601 135.899 2.479644 621.2528 9.313248 1106.607 8.386687 155.3132 2.884666 640.6669 9.258999 1126.021 8.462822 174.7273 3.289566 660.0811 9.198146 1145.435 8.553039 194.1415 3.711666 679.4952 9.137332 1164.849 8.655795 213.5556 4.165547 698.9094 9.074605 1184.263 8.758622 232.9698 4.61934 718.3235 9.007615 1203.677 8.8624

Profile TT' Regional -162

-164 0 0.5 1 1.5 2 2.5RESIDUAL3 3.5 4 4.5

-166 OBSERVED -168 -170

CBA(mgl) -172 -174

DISTANCE (KM) Figure 4.6(a): Complete Bouguer anomaly map profile TT’

Profile TT' Residual

12 10 RESIDUAL 8 6 4 2 0 0 500 1000 1500 2000 2500 3000 3500 4000 4500 Residual gravity (mgl) Residualgravity -2

DISTANCE (METRES) Figure 4.6(b): Residual gravity anomaly for profile TT’

The anomaly along this profile reveals a dense causative body between co-ordinates

(203500, 9899500) and (203500, 9898750) of the anomaly map with a sudden change in

residual gradient at the end. This could map presence of a fault.

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4.1.3 Quantitative interpretation of the gravity anomaly

4.1.3.1 Introduction

After regional trend with long wavelength has been removed the residual gravity anomaly along the profiles remains. The obtained data is subjected to both direct and indirect analysis procedures. This aimed at establishing origins of the unusual residuals that might cause shallow density bodies below the surface.

4.1.3.2 Direct interpretation

Direct interpretation gives information on the anomalous body directly from the gravity anomalies, which is largely devoid of the actual shape of the body. Usually, direct interpretation is used as top depth determinant of an anomaly. Both limiting depth and half-width methods have been used in the study.

4.1.3.2.1 Limiting depth

This is topmost depth at which a causative structure would rest but still produces an observable anomaly. Decaying gravity anomalies is inversely proportional to the distance from their source such that deeply seated structures are caused by anomalies of lower displacement and larger breadth than those produced by shallow causative bodies. The wave length –amplitude to compute maximum depth is quantified to the top depth where the anomaly would be lying.

4.1.3.2.2 Half-width Ratio method X1/2

This refers to length measured from anomaly’s vertical height to where the anomaly’s maximum value has reduced to half. Half-width ratio for 2-D is as follows

Z < X1/2 ; Z body’s limiting depth……………………

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The approach was applied on profiles SS’ and TT’ as follows;

PROFILE SS’

X 1/2 RESIDUAL GRAVITY (mgal)

DISTANCE (METRES)

Figure 4.7(a): Half-width method along profile SS’

Maximum height distance=425m, distance from maximum height to half its maximum value=150m

Difference (425m-150m) is Z< 275M Shows that the causative body is at a depth less than 275m.

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PROFILE TT’

RESIDUAL GRAVITY (mgal) X 1/2

DISTANCE (METRES)

Figure 4.7(b): Half-width method along profile TT’

Z< 250 M. reveals that the anomaly is imaged at a depth less than 250m.

4.1.3.2.3 Amplitude gradient ratio approach

It involves calculation of amplitude upper limit (Amax) of anomaly and upper limit

| horizontal gradient (A max)

For a 2-D body.

Z< 0.65 Amax

1 A MAX

| Where Amax = maximum amplitude and A max = maximum slope

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PROFILE SS’

PROFILE SS1

1

A Max

A Max RESIDUAL GRAVITY( Mgal) GRAVITY( RESIDUAL

DISTANCE (METRES)

Figure 4.7(c): Gradient amplitude ratio method along profile SS’

Gradient = 0.00375

Z < 0.65( )

Z < 260 m

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PROFILE TT’

mgal)

(

1 A max

Amax RESIDUALGRAVITY

DISTANCE (METRES)

Figure 4.7(d): Gradient- amplitude ratio method along profile TT’

Gradient= =0.02

Z <0.65( )

Z< 325m

This method has approximated the causative body along the profile to be at a depth less than 325m.

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Table 4.5 indicates the results of the empirical methods of depth determination

Table 4.5: Empirical method result for the obtained depth Profile Depth (m)

Half-width Gradient-amplitude ratio

SS1 275 260

TTI 250 325

The empirical method result has revealed that the causative anomaly along profiles SS1 and TT1 is at estimated depth less than 300m below the surface.

4.1.4 Indirect methods

4.1.4.1 Euler Deconvolution: Principles

The technique uses potential field derivatives to map subsurface depth of a magnetic or gravity source (Hsu, 2002). Mushayandebbvu et al (2001) described three dimensional space Euler’s deconvolution equation as

………………………….4.1

Vertical gravity anomaly segment TZ of a structure is uniform gravity field.

The xo, yo, zo are the source unknown co-ordinates to be calculated while X,Y,Z are the

1 real position of the detected point of gravity as well as gradients. TZX , TZY and TZZ values are recorded as gravity gradients in the direction of x, y, z. BZ and N are regional value and structural index of gravity respectively being approximated.

Equation 5 is modified further for a 2-D body as,

------4.2

41

Euler deconvolution technique has been done on 2-D profile data because the 2-D

estimations results in outcomes with major scatter compared with the grid based

approach (Sideris and Zhang, 1996).

4.1.4.2 Euler deconvolution along selected profiles

This method helps automatic estimation of location of a causative body and its depth

within the earth’s subsurface. Therefore, the boundary of the said resource and its depth

from the surface was located by Euler deconvolution. Trends and depths description are

the most important products of Euler deconvolution. A structural index of 1.0 was used

in this study to delineate cracks and intruding dykes in the subsurface to which heat

sources are related. The structural index becomes an exponential integer in a power law

giving off field strength drop against source distance.

PROFILE SS’

Euler’s solution for profile SS’ captured a shallow depth structure of between 10m and

750m (Figure 4.8). The top depth of the intrusion, possibly lie at 10m and 750m as its

base depth. The profile density body is 500m wide (Figure 4.8) imaged at co-ordinate

(201500, 9901900) in the anomaly map as shown in figure 4.1.

Residual(mgl) Gravity

Distance (km)

Depth in (km) Depth Figure 4.8: Euler Deconvolution solutions for profile SS’

42

PROFILE TT’

The depth estimation technique reveals solutions clustering as from shallow depth of

20m to deep depth of about 1000m (Figure 4.9) which are at co-ordinates (203500,

9899500) and (203500, 9898750) in the anomaly map. The slight gap imaged is a

characteristic of formation of a fault. It has also shown few ignorable spurious solutions

near the surface. Therefore, the intrusive crest is possibly lying at 20m in depth while

foot depth is 1000m. The profile density structure is over 2000m broad (Figure 4.9).

Residua Gravity (mgl) Gravity Residua

Distance (km)

Depth (km) Depth Figure 4.9: Euler Deconvolution solutions along profile TT’

PROFILES LL’ AND QQ’ Euler Deconvolution solutions for profiles LL’ and QQ’ give no solution for structural

index of 1.0 which was used to delineate fractures and intruding dykes associated with

heat sources in the subsurface.

Table 4.6: Results from Euler Deconvolution

Profiles Depth (m) Width (m)

Top (m) Bottom (m) SS’1 10 800 500 SS’2 40 500 800 TT’1 20 1000 1500 TT’2 25 950 2000

TT’3 50 1250 1750

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4.1.4.3 Forward Modeling: Principles

Forward modelling involves determination of the causative body parameters such as its geometry, density contrast and the depth of burial. In the study, two dimensional forward modeling techniques was applied to generate the source body parameters using Grav.2dc software (Gordon Cooper). Gravitational force of attraction at each observation point due polygonal shaped models was calculated by the software, where each model was assigned a specific density value. A two dimensional arbitrary model in this method is assumed to have an infinite strike length. A body is estimated by use of polygons whose gravity effects are added up in an algorithm in the Grav.2dc software. Iteratively adjustments on initial begin model was done until an acceptable fit was obtained for the models. The initial body start parameters for modeling process were derived from Euler deconvolution, spectral analysis and previous geophysical study of the area which depth and shape of causative body inferred from the structural index 1.0 used.

4.1.4.3.1 Forward modelling for the profiles

PROFILE SS’

Profile SS’ is to model high gravity anomaly centered at co-ordinate (2035000,

9902000).

Forward modelling result reveals dyke structures lying at top depth of 33m, density contrast 0.22487g/cm3 and 801m wide for body 2 which is at co-ordinate (201500,

9901900) of the anomaly map, maximum top depth as 40m, contrasting density

0.245250g/cm3 and a width of 501m for body 3.

The first and the forth bodies with density contrast -0.228374g/cm3,width 1184m and density contrast -0.0051541g/cm3, width of 1854m respectively are imaged at the surface.

44

Figure 4.10 shows a fit between the calculated gravity anomaly curve and the observed

anomaly curve and the observed anomaly for profile SS’.

RESIDUAL GRAVITY (mgal) GRAVITY RESIDUAL Distance (km)

m)

k DEPTH ( DEPTH

Body 2 Body 4 Body 1 Body 3 Figure 4.10: Observed, Computed anomaly and forward model of 2-D body for profile SS’

PROFILE TT’

Profile TT’ models centered at grid co-ordinates (2035000, 9899000), as shown in

Figure 4.11 reveals three subsurface intruding bodies. Body number one at co-ordinate

(203500, 9899500) has density contrast 0.2415g/cm3, width of 1526m and mapped at

40m deep.

The second body has a contrasting density of 0.22239g/cm3, width 1829m and imaged at

25m in depth while the third body has density contrast of 0.1488g/cm3, width 540m and

mapped at maximum depth to the top as 20m at co-ordinate (203500, 9898750) in the

anomaly map as shown in figure 4.1.

45

Figure 4.11 Calculated anomaly curve and the observed anomaly for profile TT’.

Distance (km)

Residual Gravity (mgl) Gravity Residual

Depth (km) Depth

Body 1 Body 2 Body 3 Figure 4.11: Observed, calculated anomaly and forward model of 2-D body for profile TT’

PROFILE LL’

Models on LL’ profile centered at grid co-ordinates (2035000, 9900000) as shown in

Figure 4.12 maps one subsurface intruding body of density contrast 0.252g/cm3, breadth

of 2697m and imaged at 40m below the surface. Bodies structures of densities -

0.2335g/m3, width 1182m and - 0.03179g/m3, and width of 2480m are imaged at the

surface presumed to be composed of sediments also revealed.

46

DISTANCE (KM)

RESIDUAL GRAVITY (mgal) GRAVITY RESIDUAL

DEPTH (km) DEPTH

Body 2 Body 1 Body 3 Figure 4.12: Calculated, Observed anomaly and forward model of 2-D body for profile LL’

PROFILE QQ’

Profile QQ’ models centered at grid co-ordinates (203000, 9899000) are shown in

Figure 4.13 have two dense subsurface bodies intrusive revealed. The first dense body

has density contrast of 0.478545g/cm3 width of 1026m being imaged 44m deep. The

second dense body has a density contrasting of 0.325694g/cm3 width 1473m and mapped

41m in depth. The calculated anomaly curve and the observed fitting for QQ’ profile is

shown in figure 4.13.

At the surface are two bodies of density contrast of -0.842062g/cm3, width 1221m and -

1.171403g/cm3, and width of 39m respectively which are presumed to be composed of

materials of low density like sedimentary rocks.

47

) OBSERVED

(mgal CALCULATED

Distance (km)

ResidualGravity

) Depth (km Depth

Body 2 Body 1 Body 3 Body 4

Figure 4.13: Computed, Observed anomaly and model of 2-D body for profile QQ’

Table 4.7: Modelling Results Profile Body Depth (m) Breadth (m) Contrasting density Top (g/cm3) SS’ Body 1 0.0000 1184 -0.228374 Body 2 33 801 0.22487 Body 3 40 501 0.245250 Body 4 0.0000 1854 -0.051541 TT’ Body 1 40 526 0.2415 Body 2 25 1829 0.22239 Body 3 20 540 0.1488 LL’ Body 1 0.0000 1182 -0.23352 Body 2 40 2697 0.2524 LL’ Cont. Body 3 0.0000 2480 -0.001589 QQ’ Body 1 44 1026 0.478545 Body 2 0.0000 1221 -0.842062 Body 3 0.0000 993 -1.171403 Body 4 41 1473 0.325694

48

4.2 Discussion

Profile SS’ is inclined towards NWSE on the upper part of the study area and traverses the high and low areas centred at (203500, 9902000) as shown in Figure 4.1. The direct analysis that is curve shapes dependant and the limiting depth approach, approximates the SS’ profile body to be less than 267.5m in depth (Table 4.2). This is in line with the top depth of the modelled structures which is at about 40m. The outcome of the forward modelling reveals dyke like structures at the maximum depth to the top as 33m, density

2.89g/cm3 and a width of 801m for body 2, maximum depth to the top as 40m, density

2.92g/cm3 and a width of 501m for body 3. The structures are lying slightly vertical and spread to 801m in breadth. This as was revealed earlier by Euler’s results that estimated depth of 40m below the surface and a width of 800m (Figure 4.8).The first and the forth bodies with density 2.34g/cm3, width 1184m and density 2.66g/cm3, width of 1854m respectively are imaged at the surface (Figure 4.10). These positive gravity anomalies could be hot intrusives of density high at 800m in depth emanating from the mantle under the volcanic complexes located at co-ordinate (201500, 9901900) of the anomaly map which agree well with the geology of the area and quantitative analysis results, hence they could be the heat sources at the basin.

The TT’ profile is inclined towards NS direction on the central part of the study area and centred at grid co-ordinates (2035000, 9899000) as shown in Figure 4.1. It targets the

NW-SE fault of high gravity area possibly the geothermal reservoir along the Easting

203500. It reveals a subsurface relatively dense body whose direct analysis, has average crest depth of 287.5m.

Model on profile TT’ as shown in figure 4.11 reveal three subsurface intruding bodies.

Body one has density 2.92g/cm3 extensive width of 526m and imaged at a shallow depth

49 of 40m. Body two has a density 2.89g/cm3 width 1829m and imaged at a shallow depth of 25m while the third body has density 2.82g/cm3, width 540m and imaged at maximum depth to the top as 20m. These were presumed to be hot dense bodies composing phonolitic trachytes imaged under magma at bottom depth of 1000m which are attesting to a heat source at the basin. The trachytes occurring in the east of Olkaria

Hill are the host for the geothermal reservoir (Omenda, 1994, 1998) could also be the heat bearing rocks in the dome areas under the survey. The formation of these rocks has pyroclastic minor and basalt flows. Thickness of the formation changes from 100m -

500m and is regarded as the rock cap for geothermal system of Olkaria (Haukwa, 1984).

Profile LL’ is inclined SWNE towards positive anomaly, centered at co-ordinates

(203500, 9900000), shown in Figure 4.1. Models on LL’ as shown in Figure 4.12 mirrors one subsurface intrusive body of density 2.92g/cm3, extensive width of 2697m and imaged at a depth of 40m. Body structures of densities 2.44g/m3, width 1826m and

2.63g/m3, width of 2480m are imaged at the surface, presumed to be composed of sediments also revealed.

Profile QQ’ is drawn in the NWSSE direction and traverses the positive gravity anomaly centred at grid coordinates (203000, 9899000) as in Figure 4.1.Profile QQ’ models shown in figure 4.13, displays three dense intrusive structures below the surface. First dense structure bears 3.049g/cm3 density, width of 1026m, detected at 44m deep. The second dense body bears 3.096g/cm3, width 1473m and mirrored at 41m deep.

At the surface are two bodies of density 1.649g/cm3, width 1221m and 1.498g/cm3, and width of 993m respectively which are presumed to be composed of materials of low density like sedimentary rocks.

50

CHAPTER FIVE

CONCLUSIONS AND RECOMMENDATIONS

5.1 Conclusions

The map of anomaly shows large area with anomalies encircling most of the area, while three dimensional map gives a view of 3-D gravitational field changes in the study area.

Olkaria Dome prospect area possess high geothermal gradient, which is evident by presence of hot springs and steaming, has a potential heat source covering approximately

10km2. It is characterized by major faults, veins, sills, fractures, craters dykes, gorge and inferred ring structures.

The possible source of heat imaged at co-ordinate (201500, 9901900) and (203500,

9899500) in the area has shallow sharp dykes manifestation, modelled at 10m, 25m, 30m, and 50 m deep as in Figure 4.10 and 4.11 according to Euler deconvolution results and

11m, 20m, 25m, 33m and 40 m according to results obtained by Forward modelling shown in Figure 4.12 through 4.13. These depth results are attest to what empirical methods have postulated and agree with the geology of the area. The shallow imaged fault by Euler deconvolution Figure 4.9 directs the flow of water from the rift scarps to the hot masses underground. This possibly led to the trapping of a heat source in the area as evidenced by the hot springs. The possible uncaptured deep heat source was because of length of profiles constrain occasioned by steepness and thick vegetation cover. This limited taking more readings from some sections of the area. The low lying dykes causing heating effect on the geothermal fluid in Olkaria Dome were well captured. Due to radioactivity within the crust and great pressure the over lying rocks exert, these dykes gradually cool down. Gravity method was able to locate these body dikes as positive gravity anomalies within Earth’s subsurface.

51

It is postulated that intrusives, in the form of dykes could be the conduit of heat from large magma bodies at few kilometres from the surface. Therefore this study was able to detect gravity highs that show evidence of a buried dense body compared to the surrounding rocks along profiles SS’ and TT’. The buried dense bodies were interpreted as intruding dyke injections within the subsurface which could be heat sources.

5.2 Recommendations

This gravity study was conducted to gather information on the possible existence of geothermal heat sources in Olkaria Dome area. It has provided information which will be used as a start point for future detailed geophysical study. This study recommends the application of other geophysical techniques like seismic, resistivity, magneto telluric

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

Advance techniques of gravity data analysis such as Tensor Euler deconvolution

approach to be carried out in Olkaria Domes to verify the results. Usage of calculated

gravity gradients in convectional Euler is overcome by Tensor Euler deconvolution that

deploys the measured ones. This honours responses from many dimensions and the, the

deconvolution can be executed minus gridding. Over the steep and wild animal habitat

areas, collection of more gravity data is required for deeper probing on longer profiles.

52

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Baker B.H., Mohr, P.A., and Williams, L.A.J., 1972: Geology of the Eastern Rift System of Africa. Geological Society of America, Special Paper 136, 1-67.

Baker, B.H., 1972: The structural pattern of the Afro-Arabian rift system in relation to plate tectonics. Philosophical Transactions, Royal Society of London, Series A267, 383-391.

Baker, B.H., Williams, L.A.J., 1971: Sequence and geochronology of the Kenya rift volcanics. Tectonophysics, 11, 191-215.

Bhogal P. and Skinner M., 1971: Magnetic surveys and interpretation in the Olkaria Geothermal Area. Report prepared for the Kenya Power Company Ltd, 15pp.

Clarke, M.C.G., 1990: Geological, volcanological and hydrogeological controls of the occurrence of geothermal activity in the area surrounding , Kenya. Ministry of Energy, Kenya, report.

Dimitrios G. 1989: Magnetotelluric studies in geothermal areas in Greece and Kenya. Ph.D. Thesis, University of Edinburgh.

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Hamilton, R.M., 1973: Earthquakes in Geothermal Areas near Lakes Naivasha and Hannington (Bogoria), Kenya. Unpublished report to the UNDP/EAP&L.

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Henry, W.J., Mechie, J., Maguire, P.K.H., Khan, M.A., Prodehl, C., Keller, G.R., and Patel, J., 1990: A seismic investigation of the Kenya rift valley. Geophys. J. Int., 100, 107-130.

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Mariita, N.O., Otieno, C.O. and Shako, J.W., 1996: Micro-seismic monitoring at the Olkaria Geothermal field, Kenya. KenGen Internal Report. 15 p.

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Omenda, P.A., 2000: Anatectic origin for Comendite in Olkaria geothermal field, Kenya Rift; Geochemical evidence for syenitic protholith. African Journal of Science and Technology. Science and Engineering series, 1, 39-47.

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Simiyu, S.M., Mboya, T.K., Oduong, E.O, and Vyele, H.I. 1997: Seismic Monitoring of the Olkaria Geothermal Area, Kenya. KPC internal report.

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55

APPENDICES

Appendix I: Gravity data reductions

ST EASTINGS NORTHINGS Height GRAV RD LAT(°) LAT(Rn) SIN^2(2ø) LATgø GRAV obs FAC BC SBA TC CBA

AN1 202953 9899899 2051.83 1482.18 0.90466 0.01578 0.0009960 978050.282 977435.99 633.203687 229.602 -210.695013 52.509013 -158.19

U.G 203137 9899862 2052.60 1481.69 0.90466 0.01578 0.0009960 978050.282 977435.49 626.880782 227.31 -215.220205 54.699205 -160.52

AN2 203234 9899480 2051.86 1482.54 0.90466 0.01578 0.0009960 978050.282 977436.35 624.042896 226.281 -216.172061 55.267061 -160.91

AN3 203338 9899862 2031 1482.30 0.90466 0.01578 0.0009960 978050.282 977436.10 628.196344 227.787 -213.771672 50.989672 -162.78

AN4 203523 9898893 2022.17 1485.84 0.90466 0.01578 0.0009960 978050.282 977439.64 644.134299 233.566 -200.069888 36.331881 -163.74

AN5 203526 9898562 2035.63 1483.25 0.90466 0.01578 0.0009960 978050.282 977437.06 646.019228 234.249 -201.455443 38.226443 -163.23

AN6 203859 9898449 2087.28 1470.86 0.90466 0.01578 0.0009960 978050.282 977424.66 640.049361 232.085 -217.658610 56.800610 -160.86

AN7 204133 9898557 2093.39 1469.23 0.90466 0.01578 0.0009960 978050.282 977423.04 628.325339 227.833 -226.754451 63.696451 -163.06

AN8 204459 9898738 2074.04 1473.47 0.90466 0.01578 0.0009960 978050.282 977427.27 640.177739 232.131 -214.965783 52.268783 -162.70

AN9 204520 9899252 2036.05 1482.42 0.90466 0.01578 0.0009960 978050.282 977436.22 638.97173 231.694 -206.782487 44.805487 -161.98

AN11 204410 9898893 2074.46 1471.67 0.90466 0.01578 0.0009960 978050.282 977425.48 639.726874 231.968 -217.045162 54.572162 -162.47

AN12 204450 9898554 2070.55 1472.96 0.90466 0.01578 0.0009960 978050.282 977426.77 637.189565 231.048 -217.373432 52.253432 -165.12

AN13 204442 9898238 2073.00 1473.02 0.90466 0.01578 0.0009960 978050.282 977426.83 639.448826 231.867 -215.874389 50.793389 -165.08

AN14 204635 9898000 2064.78 1474.90 0.90466 0.01578 0.0009960 978050.282 977428.70 629.544926 228.276 -220.310091 54.536091 -165.77

AN15 204604 9898739 2072.10 1472.50 0.90466 0.01578 0.0009960 978050.282 977426.30 630.508066 228.625 -222.098190 56.350190 -165.75

AN16 205166 9898755 2040.00 1480.79 0.90466 0.01578 0.0009960 978050.282 977434.59 620.972944 225.167 -219.887836 54.879836 -165.01

DG25 205083 9898954 2043.12 1479.63 0.90466 0.01578 0.0009960 978050.282 977433.44 617.34998 223.854 -223.351097 58.547097 -164.80

AN17 204877 9899428 2012.23 1485.70 0.90466 0.01578 0.0009960 978050.282 977439.51 617.443794 223.888 -217.219300 52.588300 -164.63

AN18 205031 9899802 2000.49 1486.57 0.90466 0.01578 0.0009960 978050.282 977440.37 620.360064 224.945 -214.496482 46.445452 -168.05 AN19 204869 9900622 2000.79 1485.85 0.90466 0.01578 0.0009960 978050.282 977439.66 619.244784 224.541 -215.921357 46.583357 -169.34

AN20 204638 9900943 2010.24 1483.49 0.90466 0.01578 0.0009960 978050.282 977437.29 605.29236 219.482 -227.178571 57.863571 -169.32

56

Appendix I Cont:

AN21 203885 9901142 2006.63 1484.56 0.90466 0.01578 0.0009960 978050.282 977438.36 602.381954 218.426 -227.967652 59.611652 -168.36

DG59 203325 9901638 1961.41 1494.06 0.90466 0.01578 0.0009960 978050.282 977447.86 633.521854 229.718 -198.618215 31.443215 -167.17

AN22 202949 9901572 1951.98 1494.58 0.90466 0.01578 0.0009960 978050.282 977448.38 623.038403 225.916 -204.781321 34.624321 -170.16

DG27 202958 9899762 2052.89 1479.81 0.90466 0.01578 0.0009960 978050.282 977433.61 622.057673 225.561 -220.174434 58.234434 -161.94

AN31 203305 9898930 2018.92 1484.12 0.90466 0.01578 0.0009960 978050.282 977437.92 613.016927 222.283 -221.623966 56.551966 -165.07

DG41 203090 9898689 2015.74 1484.85 0.90466 0.01578 0.0009960 978050.282 977438.65 612.442005 222.074 -221.260419 56.934419 -164.33

AN32 202769 9898388 1986.45 1492.40 0.90466 0.01578 0.0009960 978050.282 977446.21 613.94427 222.619 -212.748882 47.852882 -164.90

AN33 202801 9898180 1984.58 1492.69 0.90466 0.01578 0.0009960 978050.282 977446.50 613.090682 222.309 -213.005955 47.284955 -165.72

AN34 202545 9898661 1989.45 1492.64 0.90466 0.01578 0.0009960 978050.282 977446.45 622.536311 225.734 -207.034352 42.711352 -164.32

AN35 202353 9899000 1986.68 1492.73 0.90466 0.01578 0.0009960 978050.282 977446.54 619.032467 224.464 -209.178687 44.109687 -165.07

AN36 204538 9899468 2017.29 1482.31 0.90466 0.01578 0.0009960 978050.282 977436.11 613.808486 222.57 -222.930430 55.694330 -167.24

DG38 204470 9899573 2005.94 1484.48 0.90466 0.01578 0.0009960 978050.282 977438.28 612.946258 222.257 -221.311010 53.300010 -168.01

DG29 204277 9899836 1989.01 1487.29 0.90466 0.01578 0.0009960 978050.282 977441.09 619.875562 224.77 -214.084302 45.817302 -168.27

AN37 204344 9900104 1986.22 1483.37 0.90466 0.01578 0.0009960 978050.282 977437.17 615.376483 223.138 -220.869996 46.314996 -174.55

AN38 205022 9900602 2008.67 1481.90 0.90466 0.01578 0.0009960 978050.282 977435.71 623.72257 226.165 -217.017236 45.500236 -171.52

AN39 204666 9900450 1994.09 1485.01 0.90466 0.01578 0.0009960 978050.282 977438.81 616.37789 223.501 -218.592703 46.941703 -171.65

DGC1 204058 9900441 2021.14 1478.24 0.90466 0.01578 0.0009960 978050.282 977432.04 615.677676 223.247 -225.813016 56.323016 -169.49

AN40 204293 9900708 1997.34 1484.08 0.90466 0.01578 0.0009960 978050.282 977437.88 616.347338 223.49 -219.545176 49.719176 -169.83

AN41 204204 9900560 1995.07 1484.44 0.90466 0.01578 0.0009960 978050.282 977438.24 614.516414 222.826 -220.348199 48.643199 -171.71

AN42 204465 9900687 1997.24 1483.67 0.90466 0.01578 0.0009960 978050.282 977437.47 616.305986 223.475 -219.977534 49.862534 -170.12

AN43 204483 9900439 1991.30 1485.16 0.90466 0.01578 0.0009960 978050.282 977438.96 622.384789 225.679 -214.615932 42.520932 -172.09

DG04 204742 9901263 1997.10 1483.35 0.90466 0.01578 0.0009960 978050.282 977437.15 625.719212 226.889 -214.298585 42.498585 -171.80

AN44 204998 9901073 2016.80 1478.54 0.90466 0.01578 0.0009960 978050.282 977432.34 604.880688 219.332 -232.393970 61.337970 -171.06

AN45 205147 9901046 2027.61 1476.61 0.90466 0.01578 0.0009960 978050.282 977430.41 604.213186 219.09 -234.749432 65.863434 -168.89

AN46 203231 9901384 1960.08 1492.05 0.90466 0.01578 0.0009960 978050.282 977445.85 600.328529 217.682 -221.784496 51.133496 -170.65

57

Appendix I Cont:

AN47 203196 9901111 1957.92 1492.68 0.90466 0.01578 0.0009960 978050.282 977446.49 609.863035 221.139 -215.072243 43.487243 -171.59

AN48 203290 9901807 1945.33 1494.96 0.90466 0.01578 0.0009960 978050.282 977448.76 594.30404 215.497 -222.713479 50.923479 -171.79

AN49 203615 9901620 1976.23 1488.34 0.90466 0.01578 0.0009960 978050.282 977442.14 587.903059 213.176 -233.415437 63.538437 -169.88

AN50 202471 9901097 1925.81 1499.04 0.90466 0.01578 0.0009960 978050.282 977452.84 579.536604 210.142 -228.049178 57.977178 -170.07

AN51 202078 9901182 1905.07 1504.42 0.90466 0.01578 0.0009960 978050.282 977458.23 578.78146 209.869 -223.142504 55.113504 -168.03

AN52 201969 9901503 1877.95 1510.43 0.90466 0.01578 0.0009960 978050.282 977464.23 578.672524 209.829 -217.206939 47.510939 -169.70

AN53 201849 9901969 1875.51 1510.37 0.90466 0.01578 0.0009960 978050.282 977464.17 583.99896 211.761 -213.874892 47.200892 -166.67

AN54 201945 9902248 1875.15 1509.80 0.90466 0.01578 0.0009960 978050.282 977463.61 583.240113 211.485 -214.920578 49.362578 -165.56

AN55 200825 9900620 1892.41 1508.83 0.90466 0.01578 0.0009960 978050.282 977462.63 580.735207 210.577 -217.495194 50.191194 -167.30

AN56 200996 9900799 1889.96 1509.12 0.90466 0.01578 0.0009960 978050.282 977462.92 581.257049 210.766 -216.867574 50.071574 -166.80

AN57 201361 9901152 1881.84 1509.93 0.90466 0.01578 0.0009960 978050.282 977463.73 581.751735 210.946 -215.746264 47.655264 -168.09

AN58 202053 9902646 1883.53 1507.01 0.90466 0.01578 0.0009960 978050.282 977460.81 583.755303 211.672 -217.387198 51.697198 -165.69

AN59 202332 9902884 1885.13 1507.59 0.90466 0.01578 0.0009960 978050.282 977461.39 584.990801 212.12 -216.017697 49.396697 -166.62

AN60 203083 9903202 1891.62 1505.98 0.90466 0.01578 0.0009960 978050.282 977459.79 589.394214 213.717 -214.818981 44.880981 -169.94

AN61 203627 9903308 1895.63 1505.39 0.90466 0.01578 0.0009960 978050.282 977459.19 591.189032 214.368 -214.266972 43.980972 -170.29

AN62 203381 9903042 1909.90 1502.19 0.90466 0.01578 0.0009960 978050.282 977455.99 590.777977 214.219 -217.731977 47.655977 -170.08

AN63 203009 9902794 1915.71 1500.55 0.90466 0.01578 0.0009960 978050.282 977454.36 588.142533 213.263 -221.044797 50.954797 -170.09

AN64 202714 9902422 1914.38 1500.66 0.90466 0.01578 0.0009960 978050.282 977454.47 586.322101 212.603 -222.095132 50.559132 -171.54

AN65 202421 9902167 1905.84 1502.55 0.90466 0.01578 0.0009960 978050.282 977456.36 594.194179 215.457 -215.189504 43.489504 -171.70

AN66 202248 9901902 1899.94 1503.86 0.90466 0.01578 0.0009960 978050.282 977457.66 620.080472 224.844 -197.384693 26.082693 -171.30

AN68 202849 9902119 1925.45 1498.65 0.90466 0.01578 0.0009960 978050.282 977452.45 615.533869 223.195 -205.493678 33.832678 -171.66

AN69 202579 9900017 2009.33 1484.08 0.90466 0.01578 0.0009960 978050.282 977437.89 608.331145 220.584 -224.648663 60.874663 -163.77

AN70 202465 9900158 1994.60 1485.82 0.90466 0.01578 0.0009960 978050.282 977439.62 599.335763 217.322 -228.647281 63.397228 -165.25

AN67 202315 9900157 1971.26 1491.80 0.90466 0.01578 0.0009960 978050.282 977445.60 609.909016 221.156 -215.929935 50.580935 -165.35

AN71 202416 9900393 1942.11 1495.88 0.90466 0.01578 0.0009960 978050.282 977449.69 607.567977 220.307 -213.336103 43.514103 -169.82

58

Appendix I Cont:

DG07 203048 9900663 1976.37 1487.76 0.90466 0.01578 0.0009960 978050.282 977441.56 601.560461 218.128 -225.290268 56.659268 -168.63

AN72 202900 9900615 1968.79 1489.72 0.90466 0.01578 0.0009960 978050.282 977443.52 600.996031 217.924 -223.687033 54.826033 -168.86

AN73 202661 9900639 1949.32 1483.53 0.90466 0.01578 0.0009960 978050.282 977437.33 590.644353 214.17 -236.478148 57.406148 -179.07

AN74 202411 9900615 1947.49 1493.62 0.90466 0.01578 0.0009960 978050.282 977447.43 582.362763 211.167 -231.661797 62.937797 -168.72

AN75 202152 9900768 1913.95 1501.80 0.90466 0.01578 0.0009960 978050.282 977455.61 601.777098 218.207 -211.107184 41.446184 -169.66

AN76 202162 9901040 1887.11 1507.96 0.90466 0.01578 0.0009960 978050.282 977461.76 602.011942 218.292 -204.801496 34.151496 -170.65

AN77 202772 9900826 1950.02 1492.22 0.90466 0.01578 0.0009960 978050.282 977446.02 599.049382 217.218 -222.429818 49.443818 -172.99

DG18 202550 9900773 1950.78 1492.28 0.90466 0.01578 0.0009960 978050.282 977446.08 627.980324 227.708 -203.931362 33.047362 -170.88

AN78 202489 9900641 1941.18 1494.98 0.90466 0.01578 0.0009960 978050.282 977448.78 638.86588 231.656 -194.289956 25.501956 -168.79

AN79 205191 9901162 2034.93 1472.38 0.90466 0.01578 0.0009960 978050.282 977426.19 648.141779 235.019 -210.972538 40.143538 -170.83

AN80 205384 9901266 2070.21 1464.39 0.90466 0.01578 0.0009960 978050.282 977418.20 665.85511 241.442 -207.672137 38.675137 -169.00

AN81 205588 9901393 2100.27 1457.45 0.90466 0.01578 0.0009960 978050.282 977411.25 673.21306 244.11 -209.925212 39.493212 -170.43

AN82 205805 9901281 2157.66 1444.82 0.90466 0.01578 0.0009960 978050.282 977398.63 681.215675 247.012 -217.450380 49.000380 -168.45

AN83 206046 9901162 2181.51 1440.39 0.90466 0.01578 0.0009960 978050.282 977394.20 660.756113 239.593 -234.924216 66.875216 -168.05

AN84 206553 9900994 2207.44 1435.99 0.90466 0.01578 0.0009960 978050.282 977389.80 660.344132 239.444 -239.583811 68.441811 -171.14

AN85 206684 9900241 2141.14 1448.12 0.90466 0.01578 0.0009960 978050.282 977401.92 649.458575 235.496 -234.398218 58.418218 -175.98

AN86 206310 9900227 2139.81 1449.22 0.90466 0.01578 0.0009960 978050.282 977403.02 638.940253 231.683 -240.001551 67.094551 -172.91

AN87 205955 9900246 2104.53 1457.76 0.90466 0.01578 0.0009960 978050.282 977411.56 628.030934 227.727 -238.414103 65.706103 -172.71

AN88 205729 9900024 2070.45 1465.30 0.90466 0.01578 0.0009960 978050.282 977419.11 666.139948 241.545 -206.582583 34.990583 -171.59

DG57 205465 9899973 2035.10 1473.34 0.90466 0.01578 0.0009960 978050.282 977427.15 652.451995 236.582 -207.267225 34.555225 -172.71

JMC1 206113 9898702 2158.59 1445.31 0.90466 0.01578 0.0009960 978050.282 977399.11 634.13782 229.941 -246.973601 79.461601 -167.51

AN89 206198 9898921 2114.23 1456.79 0.90466 0.01578 0.0009960 978050.282 977410.59 646.65155 234.479 -227.519404 56.603404 -170.92

AN90 205609 9898795 2054.89 1471.22 0.90466 0.01578 0.0009960 978050.282 977425.03 623.828728 226.203 -227.629571 57.317571 -170.31

DG44 204174 9898794 2095.44 1460.82 0.90466 0.01578 0.0009960 978050.282 977414.62 611.83468 221.854 -245.681525 79.500525 -166.18

AN91 203672 9900378 2021.48 1477.85 0.90466 0.01578 0.0009960 978050.282 977431.65 612.501874 222.096 -228.227259 61.446259 -166.78

59

Appendix I Cont:

AN92 203146 9900547 1982.61 1483.36 0.90466 0.01578 0.0009960 978050.282 977437.16 619.659542 224.691 -218.153992 46.877992 -171.28

AN93 203255 9900467 1984.78 1484.61 0.90466 0.01578 0.0009960 978050.282 977438.41 629.498019 228.259 -210.632990 40.006990 -170.63

AN94 203242 9900280 2007.97 1480.64 0.90466 0.01578 0.0009960 978050.282 977434.45 628.427485 227.871 -215.277343 45.743343 -169.53

AN95 203170 9900190 2039.85 1474.11 0.90466 0.01578 0.0009960 978050.282 977427.92 633.299353 229.637 -218.703036 52.070036 -166.63

AN96 203219 9899983 2036.38 1476.54 0.90466 0.01578 0.0009960 978050.282 977430.34 637.297575 231.087 -213.732587 45.321587 -168.41

AN97 203117 9899930 2052.17 1473.37 0.90466 0.01578 0.0009960 978050.282 977427.17 644.361738 233.648 -212.397920 44.442920 -167.95

AN98 203140 9899219 2065.13 1469.94 0.90466 0.01578 0.0009960 978050.282 977423.75 634.767364 230.169 -221.938332 52.089332 -169.85

AN99 203032 9899398 2088.02 1464.56 0.90466 0.01578 0.0009960 978050.282 977418.36 635.418818 230.406 -226.908098 60.393098 -166.51

AN100 203137 9899532 2056.93 1472.36 0.90466 0.01578 0.0009960 978050.282 977426.16 637.891939 231.302 -217.532742 47.999742 -169.53

AN101 202974 9899541 2059.04 1472.18 0.90466 0.01578 0.0009960 978050.282 977425.99 633.252446 229.62 -220.661934 51.735934 -168.93

AN102 202844 9899350 2067.05 1469.70 0.90466 0.01578 0.0009960 978050.282 977423.51 589.099193 213.61 -251.285026 83.676026 -167.61

AN103 202709 9899195 2052.02 1472.87 0.90466 0.01578 0.0009960 978050.282 977426.68 625.103863 226.665 -225.167805 57.124805 -168.04

DG11 202162 9901258 1908.94 1476.53 0.90466 0.01578 0.0009960 978050.282 977430.33 620.450484 224.978 -224.477849 29.066849 -195.41

AN104 203192 9898293 2025.61 1476.04 0.90466 0.01578 0.0009960 978050.282 977429.84 621.853688 225.487 -224.076453 51.709453 -172.37

AN105 203148 9898087 2010.53 1480.16 0.90466 0.01578 0.0009960 978050.282 977433.97 624.451791 226.429 -218.293433 46.178433 -172.12

AN106 203055 9898231 2015.08 1478.69 0.90466 0.01578 0.0009960 978050.282 977432.49 643.992035 233.514 -207.314567 35.156567 -172.16

AN107 203227 9898373 2023.50 1476.40 0.90466 0.01578 0.0009960 978050.282 977430.20 634.816431 230.187 -215.452057 42.330057 -173.12

DG47 203913 9898295 2086.82 1468.24 0.90466 0.01578 0.0009960 978050.282 977422.04 606.830114 220.039 -241.448414 77.185414 -164.26

AN108 203769 9898148 2057.09 1468.24 0.90466 0.01578 0.0009960 978050.282 977422.04 606.283275 219.841 -241.796967 68.200967 -173.60

AN109 201874 9900181 1966.40 1488.27 0.90466 0.01578 0.0009960 978050.282 977442.07 602.12921 218.335 -224.413749 56.970749 -167.44

AN110 201929 9900332 1964.63 1487.94 0.90466 0.01578 0.0009960 978050.282 977441.74 603.57531 218.859 -223.823012 55.677012 -168.15

AN111 202014 9900589 1951.16 1490.47 0.90466 0.01578 0.0009960 978050.282 977444.27 605.648485 219.611 -219.974579 50.690579 -169.28

AN112 202050 9900390 1955.85 1489.61 0.90466 0.01578 0.0009960 978050.282 977443.41 613.348981 222.403 -215.926317 45.611317 -170.31

AN113 202037 9900217 1962.57 1488.77 0.90466 0.01578 0.0009960 978050.282 977442.57 643.561538 233.358 -197.508964 27.507964 -170.00

AN114 202181 9900122 1987.52 1483.75 0.90466 0.01578 0.0009960 978050.282 977437.55 644.797481 233.806 -201.737179 32.866179 -168.87

60

Appendix I Cont:

AN115 204542 9898839 2085.42 1459.59 0.90466 0.01578 0.0009960 978050.282 977413.40 634.921972 230.225 -232.190785 59.956785 -172.23

AN116 204577 9898885 2089.43 1458.54 0.90466 0.01578 0.0009960 978050.282 977412.35 640.734762 232.333 -229.535739 57.895739 -171.64

AN117 204579 9899095 2057.43 1466.88 0.90466 0.01578 0.0009960 978050.282 977420.68 584.41125 211.91 -257.100100 85.236100 -171.86

AN118 204178 9898936 2076.26 1462.06 0.90466 0.01578 0.0009960 978050.282 977415.86 585.531777 212.316 -261.204882 91.685882 -169.52

AN119 201654 9900337 1893.75 1502.88 0.90466 0.01578 0.0009960 978050.282 977456.68 585.920304 212.457 -220.136236 47.486236 -172.65

AN120 201709 9900382 1897.38 1501.36 0.90466 0.01578 0.0009960 978050.282 977455.16 583.747451 211.669 -223.044203 49.772203 -173.27

AN121 201809 9900563 1898.64 1500.63 0.90466 0.01578 0.0009960 978050.282 977454.43 583.734799 211.665 -223.778267 49.554267 -174.22

AN122 201780 9900788 1891.60 1501.85 0.90466 0.01578 0.0009960 978050.282 977455.66 583.7477 211.669 -222.547594 48.702594 -173.84

AN123 201856 9900976 1891.56 1501.06 0.90466 0.01578 0.0009960 978050.282 977454.86 583.7354 211.664 -223.347894 47.803894 -175.54

61

Appendix II: Residual Gravity for Profiles

PROFILE LL’

Distance(m) Residual(mgl) Distance( Distance(m) Residual m) Residual(mgl) 0 0.296393 602.2798 1.087572 1204.56 5.106464 26.18608 0.308478 628.4658 1.197644 1230.746 4.216531 52.37215 0.321763 654.6519 1.323333 1256.932 3.564436 78.55823 0.332949 680.838 1.457123 1283.118 2.690902 104.7443 0.34264 707.0241 1.609833 1309.304 1.831515 130.9304 0.356739 733.2101 1.766178 1335.49 1.130323 157.1165 0.36609 759.3962 1.943639 1361.676 0.543013 183.3025 0.37873 785.5823 2.141384 1387.862 0.142277 209.4886 0.38908 811.7684 2.358113 1414.048 0.056214 235.6747 0.40002 837.9545 2.608143 1440.234 0.233994 261.8608 0.419095 864.1405 2.873396 1466.42 0.423063 288.0468 0.434896 890.3266 3.175813 1492.606 0.551433 314.2329 0.457311 916.5127 3.506112 1518.792 0.602118 340.419 0.478964 942.6988 3.87809 1544.979 0.715758 366.6051 0.505478 968.8848 4.305432 1571.165 0.767588 392.7911 0.543799 995.0709 4.741149 1597.351 0.797437 418.9772 0.582606 1021.257 5.211598 1623.537 0.803013 445.1633 0.63182 1047.443 5.637653 1649.723 0.784141 471.3494 0.682917 1073.629 6.015018 1675.909 0.795216 497.5355 0.743614 1099.815 6.376028 1702.095 0.788237 523.7215 0.818533 1126.001 6.442512 1728.281 0.789468 549.9076 0.897923 1152.187 6.265703 1754.467 0.785595

62

APPENDIX II cont:

PROFILE QQ’

Distance(m) Residual Distance(m) Residual (mgl) Distance(m) Residual (mgl) (mgl)

0 -0.18909 491.9148 1.842378 983.8295 6.672096 16.39716 -0.16126 508.3119 2.074891 1000.227 6.727458 32.79432 -0.14611 524.7091 2.30639 1016.624 6.788717 49.19148 -0.16317 541.1062 2.496087 1033.021 6.770243 65.58863 -0.18003 557.5034 2.679994 1049.418 6.680203 81.98579 -0.1982 573.9005 2.865165 1065.815 6.564453 98.38295 -0.21767 590.2977 3.055793 1082.212 6.422993 114.7801 -0.23853 606.6949 3.249215 1098.61 6.249189 131.1773 -0.25863 623.092 3.393833 1115.007 6.001537 147.5744 -0.28982 639.4892 3.538895 1131.404 5.705724 163.9716 -0.3238 655.8863 3.695525 1147.801 5.385956 180.3687 -0.35881 672.2835 3.870959 1164.198 5.042234 196.7659 -0.39248 688.6807 4.055215 1180.595 4.674557 213.1631 -0.3829 705.0778 4.20836 1196.993 4.268243 229.5602 -0.38891 721.475 4.35945 1213.39 3.84177 245.9574 -0.41008 737.8721 4.519084 1229.787 3.396781 262.3545 -0.4305 754.2693 4.693004 1246.184 2.933275 278.7517 -0.44923 770.6664 4.879677 1262.581 2.451253 295.1489 -0.36475 787.0636 5.031847 1278.978 1.988142 311.546 -0.2759 803.4608 5.178245 1295.376 1.547463 327.9432 -0.20334 819.8579 5.333922 1311.773 1.105851 344.3403 -0.10834 836.2551 5.503015 1328.17 0.663307 360.7375 0.009094 852.6522 5.693459 1344.567 0.219831 377.1346 0.210491 869.0494 5.842216 1360.964 -0.10368

63

APPENDIX II cont:

PROFILE TT’

Distance(m) Residual Distance(m) Residual (mgl) Distance(m) Residual (mgl) (mgl) 0 0.42101 485.3537 9.501865 970.7075 8.153641 19.41415 0.656139 504.7679 9.512017 990.1216 8.144555 38.8283 0.891176 524.182 9.48988 1009.536 8.13721 58.24245 1.126122 543.5962 9.467853 1028.95 8.172558 77.6566 1.443383 563.0103 9.445936 1048.364 8.20793 97.07075 1.773749 582.4245 9.404457 1067.778 8.243326 116.4849 2.103994 601.8386 9.358819 1087.192 8.310601 135.899 2.479644 621.2528 9.313248 1106.607 8.386687 155.3132 2.884666 640.6669 9.258999 1126.021 8.462822 174.7273 3.289566 660.0811 9.198146 1145.435 8.553039 194.1415 3.711666 679.4952 9.137332 1164.849 8.655795 213.5556 4.165547 698.9094 9.074605 1184.263 8.758622 232.9698 4.61934 718.3235 9.007615 1203.677 8.8624 252.3839 5.075401 737.7377 8.940635 1223.091 8.968575 271.7981 5.553308 757.1518 8.872253 1242.506 9.074828 291.2122 6.031169 776.566 8.781182 1261.92 9.180703 310.6264 6.508985 795.9801 8.690127 1281.334 9.26394 330.0405 6.9874 815.3943 8.599089 1300.748 9.347237 349.4547 7.465901 834.8084 8.517877 1320.162 9.430594 368.8688 7.944369 854.2226 8.439065 1339.576 9.481918 388.283 8.382051 873.6367 8.360251 1358.99 9.523406 407.6971 8.791036 893.0509 8.297836 1378.405 9.56492 427.1113 9.199924 912.465 8.248901 1397.819 9.583313 446.5254 9.476813 931.8792 8.199961 1417.233 9.579474 465.9396 9.489218 951.2933 8.16273 1436.647 9.575636

64

APPENDIX II cont:

PROFILE SS’

Distance(m) Residual Distance(m) Residual Distance(m) Residual (mgl) (mgl) (mgl)

0 -0.26824 694.2995 0.450618 1388.599 -2.47021 27.77198 -0.17555 722.0714 0.294582 1416.371 -2.58785 55.54396 -0.08147 749.8434 0.090397 1444.143 -2.6954 83.31594 0.01377 777.6154 -0.11831 1471.915 -2.79904 111.0879 0.109877 805.3874 -0.34157 1499.687 -2.89191 138.8599 0.206978 833.1594 -0.56761 1527.459 -2.98167 166.6319 0.304168 860.9313 -0.78592 1555.231 -3.05928 194.4039 0.401733 888.7033 -1.0054 1583.003 -3.13407 222.1758 0.498195 916.4753 -1.12846 1610.775 -3.19442 249.9478 0.593471 944.2473 -1.21515 1638.547 -3.25299 277.7198 0.685764 972.0193 -1.22244 1666.319 -3.29372 305.4918 0.772667 999.7912 -1.19066 1694.091 -3.33383 333.2637 0.850458 1027.563 -1.17043 1721.863 -3.35262 361.0357 0.916135 1055.335 -1.15952 1749.635 -3.37128 388.8077 0.973403 1083.107 -1.18107 1777.407 -3.36936