INVESTIGATIONS INTO THE PERMEABILITY AND TECTONIC LINEAMENTS OF HOMA HILLS GEOTHERMAL PROSPECT, USING GROUND MAGNETIC METHOD

Transcription

1 INVESTIGATIONS INTO THE PERMEABILITY AND TECTONIC LINEAMENTS OF HOMA HILLS GEOTHERMAL PROSPECT, USING GROUND MAGNETIC METHOD ADERO BERNARD OTIENO I56/11475/2008 A thesis submitted in partial fulfilment of the requirements for the award of the degree of Master of Science in the School of Pure and Applied Sciences of Kenyatta University September, 2012

2 ii DECLARATION This thesis is my original work and has not been presented for the award of a degree or any other award in any University Adero Bernard Otieno Signature Date Department of Physics Kenyatta University P.O BOX NAIROBI- KENYA This thesis has been submitted with our approval as University Supervisors Dr. Willis J. Ambusso Signature Date Department of Physics Kenyatta University P.O BOX NAIROBI - KENYA Dr. Githiri J. Gitonga Signature Date Department of Physics Jomo Kenyatta University of Agriculture and Technology P.O BOX NAIROBI - KENYA

3 iii DEDICATION This thesis is dedicated to my son Nevine.

4 iv ACKNOWLEDGEMENTS Firstly, I am indeed grateful to the Almighty God for the sustenance, strength and the successful completion of my Msc program. My profound gratitude goes to my research supervisors, Dr. W.J. Ambusso and Dr. J.G. Githiri for their technical guidance, constructive comments, critical reading and directions. I want to thank the chairman of the department of physics, Kenyatta University, Dr. Njoroge and the entire physics department lecturers for their efforts in ensuring a successful running of the Msc program. I cannot but appreciate my Principal at Olembo Secondary School, Mr. J.K. Otiende and his entire teaching staff for their understanding and cooperation during this Msc program. I must thank my boyhood friend Mr. J. Oriema and his family for their hospitality and kindness in accommodating me during numerous short impromptu trips I made to Nairobi. God bless you abundantly. Undoubtedly, my unreserved gratitude goes to my dear wife Mrs. Evelyne Otieno and my son Nevine for their patience and unbroken link during my Msc program. I am highly indebted to my parents Mr. and Mrs. Adero for having given me the education foundation and the desire to aim for the best in life. Many thanks to Pastor Kennedy Angayah and members of Chrisco Church Katito for their spiritual and moral supports during my Msc study.

5 v To my fellow researcher, Mr. Odek Antony, I can t but appreciate the understanding and co-operation we displayed during field work amid scorching sun and sometimes empty bellies. To all my colleagues in Geophysics class, 2009, I say thank you.

6 vi TABLE OF CONTENTS Table of Contents DECLARATION... ii DEDICATION... iii ACKNOWLEDGEMENTS... iv TABLE OF CONTENTS... vi LIST OF TABLES... ix LIST OF FIGURES... x LIST OF ABBREVIATIONS... xii ABSTRACT... xiii CHAPTER ONE INTRODUCTION Background to the study Study area location Geology and tectonic settings of the Nyanzian Rift Geology of Homa Hills Structure Statement of research problem Objectives of the research project Main objective Specific objectives Rationale of the study CHAPTER TWO LITERATURE REVIEW Role of faults and fractures on crustal fluids Geothermal system: Process and model Magnetic method in geothermal fields Magnetic surveys in Kenya (Olkaria) Previous exploratory works in Homa Hills CHAPTER THREE THEORETICAL BACKGROUND Introduction... 19

7 vii 3.2 Theory of the Earth s magnetic field Elements of the Earth s magnetic field The geomagnetic field Rock magnetism Choice of the exploration method CHAPTER FOUR MATERIALS AND METHODS Ground magnetic survey Introduction Field instruments Global positioning system Proton-precession magnetometer Field measurements Magnetic data processing Introduction Diurnal variations corrections Removal of geomagnetic field Data enhancement techniques Introduction Vertical derivatives Euler deconvolution Residual anomaly processing (trend analysis) CHAPTER FIVE INTERPRETATION OF MAGNETIC DATA Introduction Qualitative interpretations Interpretation of Homa Hills TMI map Interpretation of residual magnetic map Interpretation of vertical derivative map D Interpretation of the magnetic data along the profiles Quantitative interpretations Parasnis method of direct interpretation Parameter determination Forward modelling Assumptions and guidelines to interpretations Models interpretations Discussion... 68

8 viii CHAPTER SIX CONCLUSIONS AND RECOMMENDATIONS Introduction Conclusions Limitations Recommendations REFERENCES APPENDIX I RAW TOTAL FIELD MAGNETIC DATA APPENDIX II OBSERVED MAGNETIC VALUES AFTER CORRECTING FOR DIURNAL APPENDIX III BASE STATION DIURNAL CURVES APPENDIX IV RESIDUAL MAGNETIC VALUES FOR SELECTED PROFILES... 89

9 ix LIST OF TABLES Table 3.1: Basic patterns of alignment of atomic magnetic moments 27 Table 4.1: I.G.R.F components for Homa Hills area using model Table 5.1: The determined parameters using Parasnis direct interpretation method 60 Table 5.2: Modelled parameters of the causative bodies 65

10 x LIST OF FIGURES Figure 1.1: Homa Hills geothermal prospect 3 Figure 1.2: Location of Homa Hills and other geothermal prospects 4 Figure 1.3: Geological map of Homa Hills geothermal prospect 7 Figure 2.1: Schematic representation of geothermal system 13 Figure 2.2: Geothermal model of Homa Hills prospect 18 Figure 3.1: Geomagnetic elements 20 Figure 3.2: The Earth s dipole field 22 Figure 3.3: Vector diagrams representing the Earth s magnetic field 23 Figure 3.4: Magnetic pole of strength +M at a distance r from observation point 25 Figure 4.1: Schematic diagram of Proton- Precession Magnetometer 31 Figure 4.2: Magnetic field stations in the study area 33 Figure 4.3: Diurnal curve at base station on 17/04/ Figure 4.4: Total Magnetic Intensity map for Homa Hills 37 Figure 4.5: Residual magnetic map of Homa Hills 39 Figure 4.6: Vertical derivative map of Homa Hills 41 Figure 4.7: Flow chart describing Euler convolution analysis 43 Figure 4.8a: Residual magnetic anomalies on profile AA 45 Figure 4.8b: Residual magnetic anomaly on profile BB 46 Figure 4.8c: Residual magnetic anomaly on profile CC 46 Figure 4.8d: Residual magnetic anomaly on profile DD 47 Figure 4.8e: Residual magnetic anomaly on profile EE 47 Figure 5.1: 2D Euler solutions along profile AA 52

11 xi Figure 5.2: 2D Euler solutions along profile BB 53 Figure 5.3: 2D Euler solutions along profile CC 54 Figure 5.4: 2D Euler solutions along profile DD 54 Figure 5.5: 2D Euler solutions along profile EE 55 Figure 5.6: Anomaly due to a thin, sheet-like body 56 Figure 5.7: General magnetic profile across a thin sheet 59 Figure 5.8: Forward model along profile AA 61 Figure 5.9: Forward model along profile BB 62 Figure 5.10: Forward model along profile CC 63 Figure 5.11: Forward model along profile DD 63 Figure 5.12: Forward model along profile EE 64

12 xii LIST OF ABBREVIATIONS BS AC DC GPS IGRF PPM TMI UTM 2D RTP ABS-MAG RAW-MAG OBS-MAG REG-MAG RES-MAG ST MT TEM Base Station Alternating Current Direct Current Global Positioning System International Geomagnetic Reference Frame Proton-Precession Magnetometer Total Magnetic Intensity Universal Transverse Mercator Two-Dimension Reduced to the Pole Absolute magnetic field Raw magnetic field Observed magnetic field Regional magnetic field Residual magnetic field Station Magnetotelluric Transient Electromagnetics

13 xiii ABSTRACT In this study, the tectonic lineaments and permeability around Homa Hills geothermal prospect, Nyanzian rift were investigated in order to explore its geothermal potential using ground magnetic method. A proton-precession magnetometer model G-856 of accuracy of ±0.1nT was used to take total magnetic field intensity of the earth at every station. A total of 86 magnetic stations were established over an area of about 76km 2. The magnetic data at each station was corrected for both diurnal variations and geomagnetic corrections. The geomagnetic field was calculated using the mathematical model of earth s magnetic field called International Geomagnetic Reference Field (I.G.R.F) model using Potent software at each station and subsequently subtracted. This model was calculated based on average geomagnetic field of 33420nT, inclination of o and declination of 0.9 o. Total magnetic Intensity (TMI) map and residual magnetic map of Homa Hills were then plotted using Surfer 8.0 software. Qualitative interpretations of the TMI map, residual magnetic map and vertical derivative map revealed fracture/fault lineaments on the survey area trending NW-SE, NNW-SSE and N-S represented by distinct broad negative magnetic anomalies suggesting demagnetisation due to fluid-rock interactions, thus, showing these regions to be relatively permeable. There exist relatively quiet magnetic anomalies on the north eastern and eastern parts of the survey area suggesting absence of faults/fracture, thus impermeable. Modelling of selected profiles revealed presence of intrusive bodies on the southern, central and northern regions with the subsurface structures being as shallow as 60m and as deep as 511m. These bodies display magnetic susceptibilities as high as SI units and as low as SI units, suggesting them to be carbonatite sills, dykes and plugs of different kinds based on geologic units of the area. 2D Euler solutions revealed subsurface faulting activities up to a depth of about 250m and the presence of fluid-filled zones within the survey area which are marked by the absence of magnetic sources. The zones trend in the NW-SE, NNW- SSE and N-S within the northern, central and southern parts of Homa Hills. This method confirms that faults/fractures-like structures trending NW-SE, NNW-SSE and N-S on the northern and southern parts of the area serve as fluid conduits which support the upward flow of the geothermal fluid and that the heat sources are shallow intrusive bodies such as dykes, plugs and sills taping from deeper magmatic bodies and that these intrusive bodies form along fracture zones.

14 1 CHAPTER ONE INTRODUCTION 1.1 Background to the study Almost all geophysical methods can be used to investigate geothermal systems and the results of two or more methods can be used to give more accurate information on the nature of the system. The success of each method depends on the geological and hydrological system (Palmasson, 1975). The permeability of a geothermal reservoir is an important constraint for any conceptual model of a geothermal prospect and for resource assessment. The permeability of a geothermal system in terms of the nature of its tectonic lineaments can be assessed in moderately steep terrain from geophysical surveys, in particular using ground magnetic method. This is because significant contrast in physical parameters such as magnetisation contrast exists between reservoir rocks and the surrounding country rocks as a result of fluid/rock interactions (Hochstein and Soengkono, 1996). The uprising of magma to the surface during the rifting process often result in different geodynamic activities such as the surface expressions of tectonic lineaments and manifestations of geothermal resources. These lineaments such as faults and fractures play important roles in the study of the evolution and dynamism of the rift zones. Investigations of tectonic lineaments and permeability of a geothermal system are crucial to the understanding of the geothermal activities and processes associated with these active regions. According to Blewitt et al. (2002), continuous accumulation of tectonic strain helps to maintain faults and fractures as conduits for fluid flow thereby sustaining

15 2 geothermal systems. The transportation of hydrothermal fluids in form of hot springs in these active regions is largely dependent on the permeability and existence of lineaments (Babiker and Gudmundsson, 2004). This study did review application of ground magnetic method for investigations of permeability and tectonic lineaments of Homa Hills geothermal prospect. Hydrothermal fluids are described as convecting water in the crust of the earth due to density difference, which in a bounded space, transfers heat through a pore space from a heat source to the free space (Mary and Mario, 2004). These fluids, which are mostly generated from the deep geothermal source, are sometime manifested in the surface in the form of hot springs, geysers and fumaroles through different conduits like faults and fractures in the subsurface. Such faults and fractures which are connected to one another to form series of networks, serve as conduits through which fluids are transported to the surface. Critically stressed fractures and faults have been discovered to play important role in most geothermal fields (Blewitt et al., 2002; Noorollahi et al., 2007). This environment is generally associated with various tectonic/faulting occurrences due to magma intrusion and volcanism. Homa Hills geothermal environment has a clear surface manifestation of hot springs, altered ground and geothermal plant. The potential of its geothermal resources anchors however, on the subsurface network of fluid conduits, connectivity and the thermal intrusive structures. This study was therefore undertaken to investigate the geometry of the subsurface faults and fractures including the heat sources in order to know the distribution and the flow path of hydrothermal fluids; this would help to understand its potential.

16 3 1.2 Study area location The project was undertaken at the Homa Hills and the surrounding area. The prospect is located about 50 km North of Kendu Bay in the Nyanza Rift and is bounded by the area marked in the map below. The resource area lies within geographical location (Easting m to m and Northing of m to m), which is approximately 155 km 2 (Lagat, 2010). Figure 1.1: Homa Hills geothermal prospect by Lagat (2010)

17 4 1.3 Geology and tectonic settings of the Nyanzian Rift The Kenya Rift is one of the most spectacular topographic expressions of active continental extension with a length of about 500 km, a width of km and an average cross-sectional relieve of more than 1000 m (Figure 1.2). Figure 1.2: Location of Homa Hills and other geothermal prospects by Simiyu and Keller (2000)

18 5 The Kenya Rift consists of the Turkana and Kenya Rift zones (Bosworth et al. 1992) and broadly follows the boundary between the Proterozoic Mozambique Belt and the Archean Tanzania Craton (Smith and Mosley, 1993). The northern, central, and southern segment of the Kenya Rift have half-graben geometries due to earlier faulting and pronounced subsidence along the western boundary faults (Chapman et al., 1978). The N80 E trending Nyanza half-graben branches with the main Kenyan rift at Menengai where there is a triple junction and disappears westward under Lake Victoria. The rift floor is generally at 1100 m, the immediate shoulders being at 1600 m. The main arm of the East African Rift valley geothermal prospect have been explored and the potential documented, however the Nyanzian rift (Figure 1.2) has not been fully explored and its geothermal potential is not well known, hence the need to carry out surface geothemal exploration survey in the area. 1.4 Geology of Homa Hills Homa Mountain, the site of an active volcano in Tertiary and Pleistocene times, dominates the country west of Kendu and occupies most of the Homa peninsula, which protrudes into the Winam Gulf and forms the east flank of Homa Bay. The Homa Mountain is a cone sheet complex comprising a number of carbonatite cone sheets of large and small scales. Most of carbonatite-alkaline rocks, except those composing the carbonatite ijolite complex in the south-eastern part of this area, are distributed in an oval area approximately 6km long in the NE-SW direction and 5km wide. The main carbonatite cone sheet of Homa Mountain, the largest of all, is located slightly to the southwest of the center of the oval area and composes the major structural element of the

19 6 cone sheet complex. A series of intrusive activities of these cone sheets have resulted in domal uplifting of the Nyanzian Metavolcanics to an elevation 500m above the surrounding ground. The main cone sheet of the Homa Mountain, where its structures are well exhibited, is encircled by cliffs steeply standing out above the surrounding ground. These circular cliffs correspond to the contact between the carbonatite and the Nyanzian metavolcanics. The inside of the cone sheet, approx. 2.5km across in diameter (Saggerson, 1952), has a concentric structure in plan and it is well observed in the field that the carbonatite sheets dip 40 o -60 o towards the center of the cone. Modes of occurrences of various facies of the carbonatite suggest that the present level of erosion stays still in a relatively upper part of the carbonatite complex. The carbonatites adjacent to the ijolites in the Ndiru Hills and a group of carbonatites dykes in the south-eastern part of this area are presumed to be of relatively deeper facies judging from distribution of sovite. Figure 1.3 shows the geologic map of Homa Hills geothermal prospect with hot springs on the northern and southern parts of the area. The mountain has a well defined radial drainage, the stream flowing into the Winam Gulf and Homa bay. Kent (1944) mentions the fact that Homa is situated on the Line of the Kendu Fault. The Samanga fault passes to the south east of the mountain where it has disturbed the phonolite lavas at Homa Bay the north westerly throw of the fault must be small as no prominent features are apparent. The location of the mountain is therefore a weakened zone near the intersection of these two sets of fractures. The Nyanzian rift is closely associated with the formation of the Homa Mountain and the Crater Lake Simbi.

20 Figure 1.3: Geological map of Homa Hills geothermal prospect by Saggerson (1952) 7

21 8 1.5 Structure There are two major Tertiary faults, the Kanyamwa fault and the Mfangano fault. The Kanyamwa fault is apparently a normal fault down-throwing to the north-west. The line of the fault bears no relation to the strike of the underlying schists, being north-east to south-west. The estimated throw in the centre section, opposite Kanyamwa and Kiambo fault is over 1,500ft. The escarpment of the Kanyamwa fault is eroded, and the state of dissection corresponds with the old marginal fault-line escarpments of the East African Gregory Rift Valley. It is a fault-line scarp, the actual line being obscured by the Lambwe alluvium in the southwest of the study area. The probable age of the faulting is suggested as late Pliocene or early Pleistocene by Shackleton (1951). The Mfangano fault has the same trend as Lambwe fault but throws down to the south. In spite of the sheer nature of the scarp, it is considered to be a fault-line scarp and that itself lies well into the lake. It is likely that the faulting of the Kavirondo and Lambwe rift valleys may also have a close connection with volcanism, for the greatest displacements in these two rifts are seen, first in the Kiambo-Kanyamwa and Mfangano escarpments exactly opposite the Kaksingiri volcano, and secondly in the sector east of Kisumu containing the vast Tinderet nephelinite volcano. It is clear from the field evidence that the history of this Tertiary rift valley commenced with the emplacement of a chain of diatremes along a linear zone prior to the Lambwe faulting. Down-warping or faulting may have taken place previously and no doubt some surface volcanics were produced, but these have been completely obliterated. It is the cause of the development of such linear zones in the continental shield, characterized by recurrent episodes of diatreme activity, surface volcanic activity and down-warping or faulting that still presents a problem.

22 9 Minor fractures running north-westwards can be detected from the orientation of aligned phonolitic nephelinite plugs running N-S, NNE-SSW, NNW-SSE and NW-SE obliquely joining the SW-NE Kanyamwa fault-line. They are possibly a posthumous reflection of older lines of weakness in the Precambrian rocks, or may be due to complex fracturing near the junction of the Kavirondo and Lambwe troughs. 1.6 Statement of research problem Homa Hills prospect is an undeveloped geothermal field located in western Kenya region near the shore of lake Victoria with surface thermal manifestations characterized by discharges of hot water having temperatures of up to 80 o C, an indication of a geothermal reserve. However, little research has been undertaken to study the main flow channels of the geothermal water in the area and thermal structures acting as heat sources. Due to ever dropping depth of water during drought in the dams that produce hydroelectric power, which subsequently lead to power rationing and high cost of electricity to the consumers, it is important to explore alternative sources of energy like geothermal which is the most affordable and environmentally benign energy source to complement or better substitute the existing sources of energy. It is also of great importance to utilize the geothermal resource in the area for direct applications such as in farming, development of spa pool and heating among others. This study was therefore necessary to be undertaken in this area in order to strengthen the global resolve of accelerating the development of geothermal energy for a better world, thus, steering Kenya towards achieving Vision 2030.

23 Objectives of the research project Main objective The main objective of this study was to assess the permeability of Homa Hills geothermal prospect by mapping subsurface structural features based on the magnetic properties of the subsurface rocks Specific objectives The specific objectives of this study were: (i) (ii) (iii) To conduct ground magnetic intensity measurements of Homa Hills. To carry out reduction on the magnetic intensity data. To generate 2D profiles across anomalies, carry out depth estimates and magnetic modeling to determine the depth of intrusive structures. (iv) To interpret the models in order to delineate the permeability and tectonic lineaments of the Homa Hills geothermal system.

24 Rationale of the study Insufficient energy still clogs regional economic wheels including Kenya, yet, we are sitting on immense geothermal reserves. Consequently, energy is the bedrock on which industrial thrive is anchored and as the world looks to building low carbon economies by replacing heavy polluting fossils, there is need to making geothermal Kenya s base load energy, thus, the reason why this study was carried out in Homa Hills prospect. The information on the permeability and tectonic lineaments of a geothermal system is an important constraint for resource assessment since it determines the possibility of harnessing geothermal energy to help reduce the cost of energy consumption and provide alternative source of energy as well as for direct utilization of the geothermal resource.

25 12 CHAPTER TWO LITERATURE REVIEW 2.1 Role of faults and fractures on crustal fluids The roles of faults and fractures on crustal fluids have been of major interest in earth sciences, including geology, seismology, hydrogeology and geothermal exploration (Gudmundsson et al., 2001). The static and dynamic effects of different stress on rock often produce change in rock mass such as fractures, faults and in general permeability which in turn control the flow of fluids in the earth crust. According to Lerner and Cengage (2003), fractures and faults are planes of tensile or shear failure at microscopic to regional scales in brittle rocks. These faults and fractures are developed mostly in competent rocks within the earth crust. In case of fractures, they are usually developed when the stress applied exceeds the elastic limit of the rock (Lerner and Cengage, 2003). These two deformations are of great importance in crustal fluid distributions and control. The movement of subsurface fluids (in this case, hydrothermal) to the surface from the reservoir rock depends on the pressure, temperature and most importantly the presence of active faults and fractures in the subsurface which are extended to the surface. 2.2 Geothermal system: Process and model Literally, geothermal energy can be described as the natural heat stored within the earth s crust which could be exploited for use. Geothermal resources according to Mary and Mario (2004) are generally associated with tectonically active region which are generated as a result of temperature differences between the different parts of the athenosphere (below the lithosphere) where convective movements are formed. According to Kent

26 13 (1944) Homa Hills is located in tectonically active region along Kendu fault. This slow convective movement is said to be maintained by the radioactive elements and heat core of the earth. The less dense deep hotter rocks tend to rise with the movement towards the surface while the colder but heavier rocks close to the surface tend to sink, re-heat and rise again. Figure 2.1: Schematic representation of geothermal system by Mary and Mario (2004) Generally, hydrothermal system is made up of the heat source, the reservoir, the recharge area and the connecting paths such as faults and fractures through which fluids percolates to the reservoir (the host rock) and in most cases are escaped to the surface as fumaroles, geysers and hot springs. The heat source is generally magmatic intrusion that has reached shallow depths (5-10km), (Mary and Mario, 2004). The reservoir rocks are permeable rocks through which fluids circulates and extracts heat from the heat source. This is often overlain by impermeable rocks and also connected through medium such as faults and

27 14 fractures to a surficial recharge area from which meteoric water replaces or partly replace the fluids which escape from the reservoirs through springs or by drilling. Large amounts of chemicals are carried out along due to alteration of reservoir rocks caused by fluid/rock interactions as the geothermal water percolates through the host rock; this dissolution of rock minerals mostly contributes to the demagnetization of such rocks and the salinity of most hot springs. Geothermal activities are associated with most of the Kenyan Rift Valley and to a smaller extent Nyanzian Rift. In Kenya, geothermal manifestations at different locations have been identified (Simiyu and Keller, 2000) (Figure 1.2), the most active and currently producing is the Olkaria geothermal field, the southern part of the Kenya rift. However, exploratory wells are currently being dug at Menengai Geothermal field and steam has been realized. 2.3 Magnetic method in geothermal fields Ground magnetic surveys are used in geothermal exploration to map subsurface structural features and as an aid to geological mapping where outcrops are scarce. The purpose of magnetic surveys is to detect rocks or minerals possessing contrasting magnetic properties, which reveal themselves by causing disturbances or anomalies in the intensity of the earth s magnetic field. According to Palmasson (1975) there is a good correlation found in some high temperature geothermal areas between altered ground and the reduced intensity of magnetisation caused by the alteration of magnetic minerals. In the low temperature geothermal fields of Iceland, ground magnetics are extensively used for

28 15 tracing hidden dykes and faults that often control the flow of geothermal water to the surface (Bjornsson and Hersir, 1981). For detection of shallow hydrothermal demagnetisation patterns ground magnetic data are usually very useful. This is because magnetic signal associated with such rather thin near surface anomalous bodies remain strong with lower height (Soengkono and Hochstein, 1995). Near surface alteration zones can often be detected by normal magnetic surveys, particularly when the gradient of magnetic field components is measured during the survey. The phenomenon that volcanic rocks at the top of some geothermal systems have been partially demagnetised is a recent discovery. However, studies carried out in Te Kopia geothermal field, central north island, and New Zealand show that demagnetisation of shallow rocks is common (Hochstein and Soengkono, 1996). At present, it is believed that the phenomenon is caused mainly by the same process which causes demagnetisation of high standing surface extrusions, namely demagnetisation induced by acid steam condensates (Perez-Ramos, 1993). This study attempted to employ ground magnetic method to assess whether some basement structures could be located, which could be interpreted as heat sources and conduits for geothermal water to the surface of Homa Hills geothermal area. High temperature, water dominated geothermal systems hosted by volcanic rocks according to Hochstein and Soengkono (1997) are often associated with distinctive magnetic anomalies. These anomalies represent the demagnetisation of reservoir rocks by the actions of thermal fluids altering ferromagnetic minerals (such as magnetite) into non-

29 16 magnetic minerals (such as pyrite). Such a phenomenon is common in the Taupo Volcanic Zone, an area of Quaternary volcanism in New Zealand, where broad negative residual anomalies occur over more than ten geothermal systems (Hochstein and Soengkono, 1997), this has also been observed elsewhere. For instance, in the USA, a ground magnetic survey over the Coso volcanic field in California recorded low anomalies associated with the geothermal prospect according to studies carried out by Roquemore (1984). Hence, according to Soengkono (2001) studies of magnetic anomalies are often useful for investigating geothermal systems hosted by volcanic rocks. Since different types of volcanic rocks often have significant magnetization contrasts due to differences of magnetic properties, magnetic anomalies could thus be used to assess the permeability and tectonic lineaments of geothermal reservoir in Homa Hills region. 2.4 Magnetic surveys in Kenya (Olkaria) Exploration of the Olkaria geothermal field located in the rift valley, Kenya, began in 1960 s and has led to the commissioning of a 45 MW power station and plans for further power station development of up to 240MW. Currently, the Olkaria Geothermal Power Stations have a combined generation capacity of 115MW. Aeromagnetic and ground magnetic surveys have been used to investigate the structure of a number of geothermal fields worldwide. The rationale behind these surveys is the expectation that hydrothermal alteration at upper levels of the geothermal system will be manifested as demagnetised zones. However, this interpretation is complicated by the common occurrence of the acidic volcanics, with reduced magnetization, which can produce a similar magnetic signature (Soengkono, 1985). An aeromagnetic survey of the Olkaria geothermal field

30 17 was carried out by Geosurvey for the Kenya Power Company in the year E-W traverse lines, at a 1Km spacing were flown at a height of 300m above the surface, with N-S tie lines every 5Km. According to Geosurvey report (1987) modeling of the aeromagnetic data from Olkaria geothermal field has indicated the presence of a large body of varying degrees of reduced magnetisation, trending EN-WSW across the rift valley. The subdued variation in the magnetic anomaly pattern over this body indicates a fairly uniform rock magnetisation, which could be caused by general magnetisation from widespread hydrothermal alteration. Ground magnetic survey was used in this study to map subsurface structural features and determine the permeability of Homa Hills geothermal area. 2.5 Previous exploratory works in Homa Hills A series of geological mapping at a scale of 1:250,000 of the area were done by Saggerson (1952) and McCall (1958). A further detailed data have as well been published as a result of the Le Bas s research work (1977). A preliminary investigation was carried out by a Finnish Team to evaluate phosphate and niobate resources for possible exploitation. The team investigated in detail a small carbornatite body at the Ndiru Hill at the sourthern foot-hill of the Homa Mountain. Japan International Cooperation Agency (JICA) and Metal Mining Agency of Japan (1988) further did investigations on the Rare Earth Minerals and other Minerals of Economic values in the area extending to Ndiru Hill. Until recently (2010), no previous work for geothermal investigations had been done. Earlier works have all noted the presence of hot Springs at Abundu and Kakdhimu in the North and South of Homa Hills respectively, giving indications of geothermal

31 18 resources underneath. Previous surface exploration for geothermal resources was carried out in the area by Geothermal Development Company (2010) using MT/TEM geophysical resistivity methods as part of multidisciplinary approach that included geological and geochemical studies of the area. In their geophysical report prepared by Lagat (2010), it was deduced that orientation of the anomalies found agreed with the major geological feature namely Homa hills as mapped on surface. It was also deduced that the heat source is associated with shallow intrusive along fractures. Thus, a viable geothermal resource exists, which can only be confirmed by drilling. A feasible geothermal model of Homa Hills is shown in Figure 2.2. Figure 2.2: The geothermal model of Homa Hills prospect after Lagat (2010) This study therefore, investigated the permeability and tectonic lineaments around Homa Hills using ground magnetic method in order to further deduce the potentiality of geothermal resource for economic exploitation.

32 19 CHAPTER THREE THEORETICAL BACKGROUND 3.1 Introduction The purpose of magnetic surveying is to identify and describe regions of the Earth s crust that have unusual (anomalous) magnetisations. In the realm of applied geophysics the anomalous magnetizations might be associated with local mineralization that is potentially of commercial interest, or they could be due to subsurface structures that have a bearing on the location of geothermal heat source or even oil deposits. The magnetic method involves the measurement of the Earth s magnetic field at predetermined points, correcting the measurements for known changes and comparing the resultant value of the field with expected value at each measurement station. Magnetism, like gravity, is a potential field. Anomalies in the earth s magnetic field are caused by induced or remanent magnetism. Induced magnetic anomalies are the result of secondary magnetisation induced in a ferrous body by the Earth s magnetic field. The shape, dimensions, and amplitude of an induced magnetic anomaly is a function of orientation, geometry, size, depth, and magnetic susceptibility of the body as well as the intensity and inclination of the Earth s magnetic field in the survey area. In a geothermal environment, existence of faults and fractures in the geologic units creates magnetic variations and can cause anomaly in magnetic measurement. This is because most magnetic rocks must have been altered and converted from magnetite to pyrite which in turn results in lower magnetic anomaly than the unaltered zones. In general the presence of fluid within the faults and fractures would reduce or have no magnetic response.

33 Theory of the Earth s magnetic field Elements of the Earth s magnetic field The geomagnetic field, like any magnetic field is a vector field. At any point on the Earth s surface it is represented by a vector pointing in the direction of force on a positive pole, and having a length proportional to the strength of the field at that point. Its components are called magnetic elements. Among the magnetic elements, the direction of the field is the element least sensitive to changes in the dimensions and the magnetic properties of the subsurface body. The various magnetic elements are B Z, B H, B T, D and I which describe the Earth s magnetic field. These elements are represented in the parallelepiped in Figure 3.1. The angle between the magnetic and the geographic meridians is the magnetic declination D while that between the total geomagnetic field vector and the horizontal plane is the magnetic inclination I. These geomagnetic elements vary all over the Earth s surface. The line where inclination I is zero is the magnetic equator and points where the inclination is +90 and -90 are the North and South magnetic poles respectively. F = Total magnetic intensity vector Z = Vertical component of geomagnetic field H =Horizontal component of geomagnetic field D = Angle of declination I = Angle of inclination Y = Geographic East X = Geographic North Figure 3.1: Geomagnetic elements

34 21 The total field vector B has a vertical component Z and a horizontal component H in the direction of the magnetic north. The vertical component Z is positive north of the magnetic equator and negative south of it (Parasnis, 1986). The dip of B is the inclination I of the field. B varies in strength from about 25000nT in equatorial regions to about 70000nT at the magnetic poles The geomagnetic field This is the magnetic field of the earth, which can be measured at any part on the earth s surface. The magnetic field on the earth at a given place and time may be considered to consist of three parts. These are the main field which is slowly changing, a diurnal part that changes with time which is approximately repeated in daily cycles and the anomaly part caused by inhomogeneities of the earth s crust. The main field is the undisturbed component of the earth s field which to the first approximation can be mathematically represented as a dipole field. The best fitting dipole has its axis inclined at to the earth s rotation axis, and its centre is displaced about 400km away from the geometric centre of the earth towards the south-western pacific (Petrova, 1980), the displacement reflecting the symmetry of the magnetic field on the earth s surface as illustrated in Figure 3.2. The dipole moment is approximated as 7.94x10 22 Am 2 (Petrova, 1980). There are areas over the earth s surface where the actual field deviates from the dipolar one. Three of the areas are located in the northern hemisphere and three others in the southern hemisphere. These areas are about the same size as the continents and have been called continental or world anomalies. The

35 22 geomagnetic field undergoes slow changes in intensity and direction with periods from 20 up to 8,000 years called the secular variation. There also exists a westward drift of the magnetic field, to the first approximation contours of the continental anomalies and the phase of the secular variation are drifting westward at a rate of 0.2/yr (Petrova, 1980). N Geographic North Pole Geomagnetic North Pole Geomagnetic Equator Geographical Equator 11.5 o Geomagnetic South Pole S Geographic South Pole Figure 3.2: The earth s dipole field (after Militzer and Weber, 1984) The origin of the main field and its secular variation is commonly believed to be the liquid outer core, which cools at the outside as a result of which the material becomes denser and sinks towards the inside of the outer core and new warm liquid matter rises to the outside, thus, convection currents are generated by liquid metallic matter which move through a weak cosmic magnetic field which subsequently generates induction currents (Nettleton, 1976). It is this induction current that generate the earth s magnetic field

36 23 (Telford et al., 1976). By slow convective movements, electric currents are produced in the core; these maintain the magnetic field, as in a self-exciting dynamo. Diurnal variations are small but more rigid oscillations in the earth s field with a periodicity of about a day and amplitude averaging 25nT (Dobrin, 1988). The first variations of magnetic field that takes place within the course of the day are connected with phenomena occurring on the sun. These variations are influenced by conditions in the atmosphere. The highly ionised layer of upper atmosphere above 80Km altitude which in turn is affected by the solar emissions. Normally, steady ring currents are present in the ionosphere. In addition the outer layers of the sun corona erupt occasionally emitting corpuscular rays consisting of protons and electrons. When the corpuscles impinge upon the ionosphere, the ring currents are greatly disturbed and this affects the magnetic field of the earth (Fukushima and Kaminde, 1973). I H magnetic north H+ H I magnetic north H H Z+ Z B+ B α Z B (a) (b) (c) Figure 3.3: a, b, and c vector diagram representing the Earth s magnetic field (Keary and Brooks, 1984). The magnetic anomaly consists of that part of the magnetic field which is caused by irregularities in the distribution of magnetised material in the outer crust of the earth. The

37 24 magnetised rock produces a magnetic field around itself. If the rock is close enough to the earth s surface, its magnetic field will combine with the earth s field. The field from the rock constitutes the anomalous field and because fields are vectors, the combined field may be greater or smaller than the geomagnetic field acting alone. If the field from the magnetised body lies more or less in the same direction as the earth s magnetic field at the site, the two fields will reinforce each other, and the total field will be greater than the earth s field alone and the resulting anomaly is a positive anomaly. If the two fields are opposite in direction, they will cancel each other and the total field will be smaller than the earth s field alone, the resulting anomaly being negative. A magnetic anomaly is detected when the measured magnetic field at the earth s surface differs from the undisturbed geomagnetic field. This implies presence of a magnetised material below the subsurface. All magnetic anomalies caused by rocks are superimposed in the main field of the earth. The description of the magnetic anomaly below was extracted from Keary and Brooks (1984). A magnetic anomaly is now superimposed on the earth s field causing a change B in the total field vector B. Let the anomaly produce a vertical component Z and a horizontal component H at an angle to H as shown in Figure 3.3 (b). Only that part of H in the direction of H, namely H', will contribute to the anomaly. H' H cos 3.1 Also, ( B Z B) ( H H ') ( Z ) 3.2 Expansion of the above equation ignoring the negligible terms in 2 yields, B Z( Z / B) H'( H / B) 3.3

38 25 Substituting the above equation with angular descriptions of geomagnetic element ratios yields, B Z sin I H cos 3.4 The above approach can be used in calculating the magnetic anomaly caused by a small magnetic pole of strength m, defined as the effect of this pole on a unit positive pole at the observation point. The pole is situated at depth z, a horizontal distance x and radial distance r from the observation point and is the angle between a line joining the pole to the observation point to the horizontal as illustrated in Figure 3.4. The force of repulsion Br on the unit positive pole in the direction r is given by, F 2 o m1m 2 / 4 Rr 3.5 Where m1 and m2are magnetic poles of strengths m1 and m 2 separated by a distance r and and are constants corresponding to the magnetic permeability of vacuum and o R relative permeability of medium separating the poles. The S.I unit for is Hm -1 and is dimensionless. Z Br x o R θ H B Depth z r +M Figure 3.4: Magnetic pole of strength +M at a distance r from observation point (Keary and Brooks, 1984).

39 26 Substituting R = 1 since relative permeability for air is close to unity then, 2 Br = Cm / r 3.6 Where C = / 4. Assuming the profile lies in the direction of magnetic north so that o the horizontal component of the anomaly lies in this direction, then in their relevant directions are given by, B and Z resolved 2 3 H Cmcos / r Cmx / r Z Cmsin / r Cmz / r B Cmz sin I / r Cmxcos I cos / r 3.9 Where α is zero if profile is in the direction of the magnetic north. If the converse is the case, α would represent the angle between the magnetic north and the profile direction Rock magnetism All rocks become magnetised because they contain magnetic minerals. Such minerals are magnetite, hematite, pyrrhotite, ilmenite, maghematite and leucoxenes but magnetite is far the most common of these minerals. Therefore for most practical purposes, rocks are said to be magnetic if they contain magnetite and their magnetic properties depend on the amount of magnetite disseminated among the non-magnetic minerals making up the principal material of the rock. Magnetite is a representative of the cubic minerals with spontaneous magnetisations comparable to the familiar ferromagnetic metals (Fe, Co, Ni). Hematite is representative of the more weakly magnetic, uniaxial minerals, in which the oppositely magnetised sub-lattices of interacting Fe 3+ ions are equally balanced, that is, anti-ferromagnetic but centred at a small angle to give slight spontaneous magnetisation perpendicular to the ion moments. The magnetism of a rock may either be

40 27 induced by the earth s field or remanent which may have occurred during cooling or deposition in the rock s history. The ionic moments of the magnetic domains in different type of materials determine their net spontaneous magnetisation (Stacy, 1977). This is displayed in Table 3.1. Table 3.1: Basic patterns of alignment of atomic magnetic moments by mutual interaction (source: Stacy, 1977) TYPE EXAMPLE IONIC MOMENTS NET SPONTANEOUS MAGNETISATION Ferromagnetic Fe,Co,Ni Antiferromagnetic NiO,MnO Zero Ferrimagnetic Canted antiferromagnetic Magnetite Hematite Induced magnetisation refers to the action of the field on the material where the ambient field of the earth is enhanced and the material itself acts as a magnet. This magnetisation is directly proportional to the intensity of the ambient field. I i kf 3.10 Where k is the volume magnetic susceptibility, F the ambient field intensity and I i is the induced magnetisation per unit volume. The magnetic susceptibility of a rock containing magnetite is simply related to the amount of magnetite it contains. The remanent magnetisation I r is a permanent magnetisation often predominant in many igneous rocks. This magnetisation depends upon thermal, mechanical and magnetic history of the material and is independent of the field in which it is measured. The remanent magnetisation of a rock may not be in the same direction as the present earth s field for the field is known to have changed its orientation in geologic time.

41 Choice of the exploration method Magnetic method is utilised in this study because it is fast, cover a large area within a short period of time, its ability to delineate the geological structure and basement relief. Magnetic surveying is ideal for both reconnaissance and focused surveys. It is expedient and cost effective, covers more ground in less time, and requires a minimum of field support. The portability of the instruments makes magnetic surveying well suited to sites with topographic variations. Estimate of the cost of a ground magnetic survey is derived from the parameters designed for the survey.

42 29 CHAPTER FOUR MATERIALS AND METHODS 4.1 Ground magnetic survey Introduction Ground magnetic measurements were made around Homa Hills geothermal field in Nyanzian rift in the month of April, The survey covered an area of approximately 76km 2, and consisted of 16 profiles spaced 500m apart and measurements were made at 500m intervals along each line except where there was inaccessibility due to extrusion of geological features or where there was several surface thermal manifestations of geothermal resource station spacing were made closer. Each profile had a total length of 2000m with a bearing normal to the regional structure. In the area of study there are several Hills and the terrain is rugged, because of this motor bicycle was used in this field study for movement from one magnetic intensity station to the other. Since there is little basement exposure near the springs, the purpose of the magnetic survey was to investigate whether some basement structures could be located, which could be interpreted as conduits for geothermal water to the surface. Such structures are commonly found to be dykes, faults and fractures. Understanding the relations between dykes, faults, and fractures and the flow of geothermal fluids is key to successful exploration for new geothermal resources at Homa Hills.

43 Field instruments Global positioning system The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S Department of Defence. GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user s exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user s position and display it on the unit s electronic map. A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user s 3D position (latitude, longitude and altitude). Once the user s position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more. Today s GPS receivers are extremely accurate, thanks to their parallel multi-channel design. Garmin s 12 parallel channel receivers are quick to lock onto satellites when first turned on and they maintain strong locks, even in dense foliage or urban settings with tall buildings. Garmin GPS receivers are accurate to within 15 meters on average.

44 Proton-precession magnetometer For land-based magnetic surveys, the most commonly used magnetometer is the proton precession magnetometer (PPM). The proton precession magnetometer only measures the total amplitude (size) of the earth s magnetic field. These types of measurements are usually referred to as total field measurements. Figure 4.1 shows a schematic of the proton precession magnetometer. Figure 4.1: Schematic diagram of Proton Precession Magnetometer The sensor component of the proton precession magnetometer is a cylindrical container filled with a liquid rich in hydrogen atoms surrounded by a coil. Commonly used liquids include water, kerosene, and alcohol. The sensor is connected by a cable to a small unit in which is housed a power supply, an electronic switch, an amplifier, and a frequency counter. When the switch is closed, a DC current delivered by a battery is directed