ASSESSMENT OF THE GEOTHERMAL POTENTIAL OF LAKE
BARINGO AREA USING MAGNETIC METHOD
SEUREY PAUL (B.ED SC)
156/CE/25984/2011
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE AWARD OF THE DEGREE OF MASTER OF
SCIENCE IN THE SCHOOL OF PURE AND APPLIED SCIENCES OF
KENYATTA UNIVERSITY
JULY, 2018
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
Signature …………………………………………... Date……………………….. Seurey Paul - I56/CE/25984/2011 Department of Physics
SUPERVISORS We confirm that the work reported in this thesis was carried out by the candidate under our supervision.
Signature …………………………………………... Date……………………….. Dr. Willis J. Ambusso Department of Physics, Kenyatta University
Signature …………………………………………... Date……………………….. Dr. Githiri J. Gitonga Department of Physics, Jomo Kenyatta University of Agriculture and Technology
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DEDICATION
I dedicate this work to my parents Mr and Mrs Philip Kibii Seurey. My lovely wife
Tecla and my children Samantha, Carson and Cedrick
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ACKNOWLEDGEMENTS
I thank the Almighty God for the sufficient grace he gave me all the time throughout my research.
I wish to appreciate my research supervisors, Dr. W.J. Ambusso and Dr. J.G. Githiri for their support and guidance during my research. Their guidance and encouragement gave me the drive to work on this research. My gratitude also goes to National Commission for Science, Technology and Innovation for giving postgraduate research fund.
My gratitude also to my family members for support they gave me during my Msc program.
My appreciation also goes to the family of my nephew Mr. Bethwel Kosgei for warmly welcoming me every time I came to Nairobi in pursuit of my research work.
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TABLE OF CONTENTS
DECLARATION ...... ii
DEDICATION ...... iii
ACKNOWLEDGEMENTS ...... iv
TABLE OF CONTENTS ...... v
LIST OF TABLES ...... ix
LIST OF FIGURES ...... x
LIST OF ABBREVIATIONS ...... xi
ABSTRACT ...... xii
CHAPTER ONE ...... 1
INTRODUCTION ...... 1
1.1 Background to the study ...... 1
1.2 Study area location ...... 3
1.3 Geology of Lake Baringo area ...... 5
1.4 Geological structures ...... 6
1.5 Statement of research problem ...... 7
1.6 Objectives of study ...... 8
1.6.1 General objective ...... 8
1.6.2 Specific objectives ...... 8 vi
1.7 Rational of study ...... 8
LITERATURE REVIEW ...... 9
2.1 Magnetic survey ...... 9
2.3 Magnetic surveys in Kenya ...... 11
2.4 Previous geophysical work in Lake Baringo ...... 13
CHAPTER THREE ...... 15
THEORETICAL BACKGROUND ...... 15
3.1 Introduction ...... 15
3.2 Magnetism of the Earth ...... 17
3.2.1 Nature of the geomagnetic field ...... 17
3.2.2 The magnetic elements ...... 18
MATERIALS AND METHODS ...... 22
4.1 Ground magnetic survey ...... 22
4.1.1 Introduction ...... 22
4.2 Field instruments ...... 22
4.2.1 Global positioning system ...... 22
4.3 Field measurements ...... 24
4.4 Magnetic data processing ...... 26
4.4.1 Introduction ...... 26 vii
4.4.2 Diurnal variations corrections ...... 26
4.4.3 Removal of geomagnetic field ...... 28
4.5 Data enhancement techniques ...... 32
4.5.1 Introduction ...... 32
4.5.2 Reduction to the Pole ...... 32
4.5.3 Boundary analysis by vertical and horizontal gradients ...... 33
4.5.4 Euler deconvolution ...... 33
CHAPTER FIVE ...... 35
INTERPRETATION OF MAGNETIC DATA ...... 35
5.1 Introduction ...... 35
5.2 Profile selection and removal of regional trend ...... 35
5.2.1 Profile selection ...... 35
5.2.2 Removal of regional trend ...... 37
5.3 Qualitative interpretations ...... 42
5.3.1 Interpretation of Lake Baringo area magnetic intensity map ...... 42
5.3.2 2D Interpretation of the Euler solutions along the profiles ...... 42
5.4 Quantitative interpretations ...... 46
5.4.1 Forward modelling ...... 46
5.4.1.1 Models interpretations ...... 51 viii
5.5 Discussion ...... 52
CHAPTER SIX ...... 56
CONCLUSIONS AND RECOMMENDATIONS...... 56
6.1 Introduction ...... 56
6.2 Conclusions ...... 57
6.3 Recommendations ...... 58
REFERENCES ...... 59
APPENDIX I: RAW TOTAL FIELD MAGNETIC DATA ...... 63
APPENDIX II: CORRECTED MAGNETIC FIELD ...... 68
APPENDIX III: DIURNAL VARIATIONS CURVES ...... 73
APPENDIX IV: RESIDUAL MAGNETIC FIELD ALONG THE PROFILES ON THE
CONTUR MAP ...... 78
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LIST OF TABLES
Table 3.1: Basic patterns of alignment of atomic magnetic moments by mutual interaction ...... 19
Table 4.1: I.G.R.F element for Lake Baringo area using model 2000-2015 ...... 26
Table 5.1: Modelled parameters of the causative bodies………………………… 47
x
LIST OF FIGURES
Figure. 1.1 Location of the Lake Baringo Geothermal Prospect in the Kenya Rift .... 4
Figure 3.1: Histogram showing ranges in susceptibility of common rock types...... 15
Figure 3.2: Geomagnetic elements ...... 17
Figure 4.1: Schematic diagram of Flux-gate Magnetometer ...... 22
Figure 4.2: Magnetic field stations in the study area ...... 23
Figure 4.3: Diurnal curve at base station on 26/04/2014 ...... 25
Figure 4.4: Reduced vertical magnetic Intensity contour Map ...... 27
Figure 4.5: 3-D view of the magnetic anomalies in the study area ...... 28
Figure: 5.1. Vertical intensity map with the selected profiles ...... 33
Figure 5.2a: Residual magnetic anomaly on profile AA‟ ...... 35
Figure 5.2b: Residual magnetic anomaly on profile BB‟ ...... 36
Figure 5.2c: Residual magnetic anomaly on profile CC‟ ...... 37
Figure 5.2d: Residual magnetic anomaly on profile DD‟ ...... 38
Figure 5.3: Euler depth solutions along magnetic anomaly profile AA‟ ...... 40
Figure 5.4: Euler depth solutions along magnetic anomaly profile BB‟ ...... 41
Figure 5.5: Euler depth solutions along magnetic anomaly profile CC‟ ...... 41
Figure5.6: Euler depth solutions along magnetic anomaly profile DD‟ ...... 42
Figure 5.7: magnetic model along profile AA‟ ...... 43
Figure 5.8: magnetic model along profile BB‟ ...... 44
Figure 5.9: magnetic model along profile CC‟ ...... 45
Figure 5.10: magnetic model along profile DD‟…..…………………………...... 46 xi
LIST OF ABBREVIATIONS
BS Base Station
GPS Global Positioning System
IGRF International Geomagnetic Reference
FGM Flux gate Magnetometer
MI Magnetic Intensity
2D Two-Dimension
RTP Reduced to the Pole
C.V Corrected value
RAW-MAG Raw magnetic field
VMICM Vertical magnetic intensity contour map
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ABSTRACT
Ground magnetic survey in Lake Baringo area was carried out to examine underlying geology in relation to anomalies in the magnetic field of the earth arising from the magnetic properties of subsurface lithology. This has been done to delineate areas associated with high temperature reservoir in the study area and establish if it relates with the occurrence of geothermal manifestations. The field stations were selected carefully along a profile in the study area and were positioned using Global Positioning System (GPS). The profile and station separation was set at about 800m and 500m respectively. The vertical magnetic field intensities were then measured using Flux Gate magnetometer. Data reduction was done to correct for diurnal and geomagnetic variations. Surfer 8.0 software was then used for gridding and to plot contour map to establish the anomaly signature of the area. Qualitative and quantitative interpretations has been done using the reduced vertical magnetic field intensity. Qualitative interpretation from the magnetic intesity contour map of the area showed that NE region is more dominated by long-wavelength component, that could be caused by deeper sources of magnetic anomalies. The Euler deconvolution method has been effectively used in estimating depth to the top of magnetic bodies. 2D Euler solutions revealed subsurface faulting activities and the presence of fluid-filled zones within the survey area which are marked by the absence of magnetic sources. Quantitative interpretation by forward modeling using Mag2dc software, has been used along selected profiles in the study area. The average modeled depth for the near surface magnetic anomaly sources of the area is 86.57m, while that of the deep seated anomaly sources is 349.25m. The bodies display susceptibility as high as 0.5301 SI units to as low as –0.841 SI units. The models show extensive lava flows. They are interpreted to be basaltic sills and dykes of different types based on geologic unit of the area. They may be possible heat source causing a thermal anomaly in the area west of Lake Baringo and such may have been magmatic intrusive that remained at the subsurface.
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CHAPTER ONE
INTRODUCTION
1.1 Background to the study
Geophysical prospecting for geothermal potential is carried out using different methods. Palmasson, (1975), found out that the effectiveness of any methods depends on the geothermal and hydrological system of the survey area. An important physical parameter to be considered is the magnetic anomaly that originates from the subsurface formation and the variation in size and shape of rocks with magnetic content.
Magnetic prospection is therefore done to determine subsurface formation with regard to the underlying anomalies that is of magnetic contrast to the host rocks.
Generally, the magnetic susceptibility of subsurface formation varies depending on the rock type. Magnetic anomalies present themselves in the form of dykes, faults and lava flows. Subsurface geology with high magnetic susceptibility, leads to greater magnetic field as measured on the ground, whereas in cases of low magnetic susceptibility, weaker magnetic field is registered. Magnetic susceptibility decreases with increase in temperatures of the geothermal systems.
Remanent magnetization leads to anomalies in the earth's magnetic field. Induced magnetic anomalies are the result of secondary magnetization induced in a ferrous body by the earth‟s magnetic field. Magnetic prospection targets contrast in the 2
magnetic field of the earth that are caused by changes in the subsurface geologic structure or by differences in the magnetic properties of near-surface rocks.
Some areas in the Kenya Rift Valley are rich with hydrothermal manifestation and are possible host for geothermal systems. Hydrothermal activity in the Lake Baringo prospect area has several manifestations in form of hot springs that discharge along the shore line, altered grounds and extensive occurrence of fumaroles. The prospect area also has ground-water boreholes that are thermally anomalous. Chepkoiyo borehole was drilled in April 2004 and self-discharged water at 980C. The discharged fluids indicate possible input from a geothermal reservoir.
The detection of the presence of a fault or intrusive body and most importantly their form or depth is important. Also the permeability of a geothermal system in terms of the nature of tectonic lineament can usually be assessed in gentle steep terrain from geophysical surveys. This is because the significant contrast in physical parameters exists between reservoir rocks and the surrounding rock as a result of fluid and rock interaction (Hochstein and Soengkono, 1996). Magnetic data has been useful because the intrusive is more magnetic than the underlying lava flows. The hydro thermal system temperature and oxygen volatility will determine the quantity present loadstone in the area of faults and therefore, their magnetic response. Blewitt et al, ( 2002), found out that continuous accumulation of tectonic strain helps to maintain faults and fractures as conduit for fluid flow, thereby sustaining the geothermal system. The transportation of hydrothermal fluids in form of hot springs in these active regions is largely dependent on permeability and existence of 3
lineaments (Babiker and Gudmundsson, 2004). 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. A geothermal system 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 using ground magnetic method. This would help to assess the potential of geothermal in the study area, which when exploited would provide a sustainable and reliable green energy which is an integral component in social- economic development in Kenya.
1.2 Study area location
The study was undertaken on the western side of Lake Baringo area. The prospect area is located within the eastern floor of the Kenyan rift valley, which is part of the
East African Rift System. It is bounded by latitudes 00 30‟N and 0045‟N and longitudes 35059‟E and 360 10‟E. It is one of the important areas on the Kenyan rift floor that are associated with possible occurrence of geothermal resource. The 4
Tugen hills, an uplift fault block of volcanic and metamorphic rocks lies west of the lake. The Laikipia escarpment lies to the east.
Figure. 1.1: Location of the Lake Baringo Geothermal Prospect in the Kenya
Rift Valley, (Omenda, 2007).
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1.3 Geology of Lake Baringo area
The geology of the area is dominated by intermediate lavas (trachytes and trachy- phonolites) in the west and east sectors of the prospect area and basalts in the north.
The southern sector is, however, dominated by fluvial and alluvial deposits. The
Baringo trachyte was described by Martyn (1969) as the „Lake Baringo Trachyte‟.
The Baringo trachyte outcrops over 12km2 west of KampiyaSamaki and was the major source material for artifacts found in Kapthurin basin (Tallon, 1978). The striking appearance on aerial photographs is due to lineaments which reflect pressure ridges resulting from the congealing of highly viscous lava. The trachyte is fine grained, black, streaked with green and locally speckled, an expression of glomerophyric clusters of dark minerals. Sanidine forms about 60% of the mode: otherwise aegirine-angite, aenigmatite and zeloilites are commonly observed in this section (Dagley et al ,1978).Thin basalt flows occur on the west side of the lake
(Truckle, 1977) forming the greater part of the outcrops which Martyn (1969) reffered to „Basalts of Nginyang type‟. They are well exposed on the lake shores promontories between Kampi ya Samaki and west Bay, and further north at
Logoratibim. The Baringo basalts typically weather black with a rough texture due to vascularity. Columnar or rough blocky jointing is usual. The lake has several small islands, the largest being Olkokwe island which is an extinct volcanic center related to Korosi volcano, north of the lake. The temperature of the discharged fluids indicates possible input from a geothermal reservoir.
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1.4 Geological structures
The dominant geological structure in the prospect area is the young N to NNE trending fault pattern that form a dense fault swarm restricted to the rift axis. Within the prospect, the faults dip west and east for those to the east and west of Lake
Baringo, respectively.
The NW trending faults are more common in the area to the west of Lake Baringo and in a few locations in the east. According to Dunkley et al. ( 1993), the structures are older and coincide with the major NW shear zones, which are considered to reflect reactivation of the Precambrian Mozambiquan belt formation. The important structures are the Ol Arabel and Loboi/Marmanet lineaments. The interaction between these faults and the NNE ones has created an intensely sheared/fractured zone to the west of Lake Baringo within the Kaparaina area.The shearing has resulted in the Baringo Trachytes in the vicinity of Kampi Ya Samaki. The N to NE-
SW trending faults is younger and cut through the NW faults/fractures. The northern extension of the structure has series of weak fumaroles discharging through structural fissures.
The most intense NE-SW faulting episode occurred after the emplacement of
Baringo Trachytes, Loyamarok Trachyphonolites, and Baringo Basalts. The
precambrian metamorphic rocks of the Mozambiquan formation outcrop
along the Elgeyo fault and a small exposure is present along the Saimo Fault,
west of the prospect area. These are major rift forming faults that have large 7
throws. Above the metamorphic lie the Miocene Plateau lavas that include
basalts, trachytes, and phonolites that were erupted on the crest of the
uplift.The that the intrusions under the Kaparaina Domal area and those
underlying other volcanoes are Pleistocene or younger in age.
1.5 Statement of research problem
Due to erratic and unpredictable weather patterns and climate change globally, the main source of power which is dependent on the water levels in the dams has been unreliable. The power supplier has had to resort to power rationing and producing power using other methods that are not environmentally friendly. This is expensive and extra cost of production has always been passed on to the consumers.
Lake Baringo area is an unexploited geothermal resource located in the northern part of the Kenyan Rift Valley. It has several geothermal manifestations in the form of hot spring that discharge along the shore line and also fumaroles. In Kenya, the installed capacity of power generation is way below the ever increasing demand and this has led to procurement of emergency power to meet the shortfall. It is therefore necessary to carry out research in this prospect area to assess its geothermal potential.
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1.6 Objectives of study
1.6.1 General objective
To carry out reconnaissance study for geothermal potential in Lake Baringo area by imaging the subsurface formation using magnetic method.
1.6.2 Specific objectives
(i) To carry out ground magnetic intensity measurements of Lake Baringo area
(ii) To identify anomalous regions, generate 2D profiles across them and carry
out depth estimates of fault structures that act as fluid reservoir and conduits.
(iii) To develop and interpret forward models of magnetic anomalous bodies that
may be geothermal heat sources
1.7 Rational of study
Kenya is endowed with vast geothermal resource potential along the world
Kenya Rift that transects the country from north to south. This notwithstanding,
insufficient energy supply is still a major impediment to economic growth in
Kenya. Since development in the economy has to be anchored on the provision
of reliable, sustainable and environmentally friendly source of energy, there is
need to make a deliberate effort to harness geothermal potential in Lake Baringo.
The knowledge on the permeability of a geothermal reservoir is an important
parameter in determining the viability and possible exploitation of a geothermal
resource. It reduces the overall cost of geophysical prospecting and improves the
overall quality of a site investigation.
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CHAPTER TWO
LITERATURE REVIEW
2.1 Magnetic survey
Magnetic survey is done to locate or detect the presence of subsurface structures or bodies and determine their size, shape, depth, and physical properties. These structures are magmatic intrusives that can act as heat sources in a geothermal system of an area. Magnetic method requires measurements of the magnetic field intensitry at specific points, called magnetic stations, distributed regularly throughout the survey area of interest. Most rocks are not magnetic, however certain types of rocks contain enough minerals to originate significant magnetic anomalies.
Blakely, (1995), observed that data interpretation that reflects difference in local abundance of magnetization is especially useful to locate faults and geologic contacts. Buried metallic objects, submerged marines in oceans, magmatic intrusives like dykes can be located by carrying out a magnetic survey.
In a geothermal system, magnetic survey can be used to locate the possible heat sources and permeability of a geothermal reservoir. The magnetic anomalies in the earth‟s magnetic field are the results of contributions from variations in concentrations of ferromagnetic materials within the locality of the magneto meters‟ sensor.
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2.2 Magnetic survey in geothermal prospection
The magnetic field produced by the anomalous source which may be a magmatic intrusive of interest is measured in magnetic survey, after removing the contribution from other sources not of interest. Rocks of igneous origin are magnetic and affect the geomagnetic magnetic field. This method therefore can be used to map intra- sedimentary magnetic sources like shallow volcanics or intrusives that perturb the normal sedimentary sequence. Most variation in magnetic field measured from at the surface result from topographic or lithological changes associated with igneous intrusives. Magnetic materials become demagnetized when their temperature reaches Curie point temperature and acquire their magnetization when they cool below this temperature. Due to earth‟s internal heat, rock magnetic properties can be measured only at temperature cooler than Curie point, which limits the depth to which a magnetic anomaly source can be mapped. Palmasson, (1975), from his research discovered that there is a marked relationship between altered ground and the reduced intensity of magnetisation of some high temperature geothermal areas.
Bjornsson and Hersir, (1981), in their research findings in the low temperature geothermal fields of Iceland used ground magnetics extensively to trace hidden dykes and faults that often control the flow of geothermal water to the surface. Mary and Mario, (2004) explained that the convective movements within the earths subsurface are formed because of temperature differences between the different parts of the asthenosphere. This gives rise to tectonically active regions that are closely linked to geothermal systems. Because of increase in temperature with depth 11
from the surface of the earth, there is an upwelling of 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.
A geothermal system is made up of heat source, the reservoir, the recharge area and the connecting paths such as faults and fractures through which fluids enter to the reservoir and escape to the surface as hot springs.
Gudmusson et al., (2001), in their study observed that faults and fractures play a critical role in the movement of crustal fluids. Their study has been of great interest in geology, seismology, hydrogeology and geothermal exploration, Lerner and
Cengage, ( 2003), found out that fractures are formed when the elastic limit of the rock is exceeded through application of tensile stress.
Mitchell, (2009) used magnetic method to image the subsurface within the Bushveld complex in Limpopo, South Africa. Using a magnetometer, an area that consisted of
30 lines that were 800 meters long was surveyed in search of changes in magnetic field. A major dyke was imaged, with a NE-SW strike and having magnetic value peaking at 4667 nT. Other three smaller dykes were also imaged, but were also deeper in subsurface.
2.3 Magnetic surveys in Kenya
In Kenya, the magnetic technique, both aeromagnetic and ground magnetic, has been used to locate or detect the presence of subsurface structures or bodies and determine their size, shape, depth, and physical properties (magnetic susceptibility) and fluid content. In 1972, Wohlenberg and Bhatt did some research in Magadi and 12
Hannington to find out whether it was possible to detect, by magnetic method, the tops of intrusive bodies beneath the extensive lava flows that cover most parts of the rift floor. From the findings of Lake Magadi aeromagnetic survey, revealed strong anomalies on the eastern border of the survey area, near Olorgesailie volcano and on the north near Suswa volcano. The intense anomalies correlated with exposure of volcanic rocks that have much stronger magnetic susceptibility than otherlava of the area.
Adero, (2012), used magnetic method to investigate the permeability and tectonic lineaments of Homa Hills geothermal prospect. The study helped to map and delineates lineaments with intrusives. Soengkono, (1985), in his study observed that in magnetic survey, hydrothermal alteration at upper levels of the geothermal system is manifested as demagnetized zones. However, this interpretation is complicated by the common occurrence of acidic volcanic, with reduced magnetization, which can produce a similar magnetic signature. Simiyu (2010), noted that several stakeholders like Kenya Electricity Generating Company limited (KenGen) and Geothermal
Development Company (GDC) have been working in conjuction with the ministry of energy to carry out detailed surface studies of most of the prospects areas in the
Kenya rift.
He further observed that the Kenya government was working on expanding geothermal power and the energy generation to meet the demand as it focuses on vision 2030. 13
2.4 Previous geophysical work in Lake Baringo
Geophysical methods for geothermal prospection have been carried out within and around the prospects area since early 1980s. The techniques applied include seismics, gravity, magnetics and resistivity surveys. Dunkley et al. (1993), observed that a few gravity measurements had been performed in the Baringo area by university research groups. Gravity surveys in the region have confirmed that this method show imaged shallow subsurface variations in density. The density contrasts are related to the magmatic intrusions and structural history of volcanic eruption fields. This explains the association of recent volcanism, intensive faulting and geothermal activity with gravity highs.
Regional gravity profile models along the rift axis using seismic refraction velocity, deep drill-hole data, and petrology data as density constraints (Mariita, 2003) have since been constructed. The models confirm the presence of high density/velocity formations below the volcanic spots. Lagat et al, (2005) carried out gravity survey and found out that gravity highs are evident to the eastern and northern rims of the study area. The gravity high to the west of the lake trends approximately N-S coincident with the western fault zone and passes through the Chepkoiyo bore hole.
The low gravity signals are recorded in regions covered by fluvial sediments, especially the southern part of the lake.
Aeromagnetic survey over the whole rift zone was done at 300m above ground in
1987 for the National Oil Corporation of Kenya. A qualitative examination of this data by Mariita (2003) indicated positive anomalies for the north rift volcanic 14
centres superimposed over a negative regional background. Such anomalies could be due to lava flows, dykes and faults. Mariita and Kilele (1989) demonstrated that a few schlumberger resistivity measurements were carried out north of Baringo in the late 1980s by the Ministry of Energy personnel. This was part of a reconnaissance study for geothermal potential in the region. An analysis of the soundings from these work indicated a discrete anomaly less than 20 Ωm at depths of 1000 m lodged northwest of Lake.
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CHAPTER THREE
THEORETICAL BACKGROUND
3.1 Introduction
Geophysical techniques such as magnetic surveys provide a way of measuring the physical properties of a subsurface formation. These measurements are translated into geologic data such as structure, stratigraphy, depth and position. The practical value in geophysical surveys is in their ability to measure the physical properties of rocks that are related to potential traps in reservoir rocks as well as documenting regional structural trends and overall basin geometry.
The basis of magnetic studies is to examine underlying geology in relation to anomalies in the magnetic field of the earth arising from the magnetic properties of subsurface lithology. Magnetic survey is a branch of geophysics that studies how the properties or effects of magnetic fields change in different places on the earth (Mc
Carthy and Rubdige 2005). Adero, (2012), in his research used magnetic method to measure the Earth‟s magnetic field at predetermined points then correct the measurements for known changes and comparing the resultant value of the field with the expected value at each measurement station. In general, the magnetite content and, hence, the susceptibility of rocks is extremely variable and overlaps between different lithologies.The histogram shown in the figure 3.1 illustrates the susceptibilities of common rock types. It is not usually possible to identify with certainty the causative lithology of any anomaly from magnetic information alone. 16
However, sedimentary rocks are effectively non-magnetic unless they contain a significant amount of magnetite in the heavy mineral fraction. Kearey and Brook,
(1984), observed that where magnetic anomalies are observed over sediment covered areas the anomalies are generally caused by an underlying igneous or metamorphic basement, or by intrusions into the sediments. Common causes of magnetic anomalies include dykes, faulted, folded or truncated sills and lava flows, massive basic intrusions, metamorphic basement rocks and magnetite ore bodies.
Magnetic anomalies range in amplitude from a few tens of nT over deep metamorphic basement to several hundred nT over basic intrusions and may reach an amplitude of several thousand nT over magnetite ores.
Figure 3.1: Ranges in susceptibility of common rock types. (After Dobrin & Savit
1988).
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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.
3.2 Magnetism of the Earth
3.2.1 Nature of the geomagnetic field
In exploration geophysics, the earth‟s geomagnetic field has three parts. They include: (i). The main field, which varies relatively slowly and is of internal origin.
The main field caused by convection currents of conducting material circulating in the liquid outer core. The earth‟s core is assumed to be a mixture of iron and nickel, which are both good electrical conductors. The magnetic source is a self-excited dynamo in which highly conductive fluid moves because of convective mechanical motion by heat generated from radioactivity in the core. According to Nettleton,
(1976), 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. It is this induction current that generate the earth‟s magnetic field (Telford et al., 1976). (ii). Diurnal variations, a small field
(compared to the main field) which varies rather rapidly and originates outside the earth. This part of the field is associated with electric currents in the ionized layers 18
of the upper atmosphere. Time variations of this portion are more rapid than the main field. Dobrin and Savit, (1988), from their research findings observed that diurnal variations are small but more rigid oscillations in the earth‟s field with a periodicity of about a day and amplitude averaging 25nT. 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).
iii). Spatial variations of the main field, which are usually smaller than the main field, are nearly constant in time and space, and are caused by local magnetic anomalies in the near-surface crust of the earth. Local changes in the main field result from variations in the magnetic mineral content of the near surface rocks.
These magnetic anomalies are occasionally large enough, but do not persist over great distances. Thus magnetic maps do not exhibit large-scale regional features.
The source of local magnetic anomalies cannot be very deep because at large depth, temperatures go beyond curie point, and the rocks lose their magnetic properties.
Hence, local anomalies must be associated with features in the upper crust. These are the targets in magnetic prospecting.
3.2.2 The magnetic elements
The earth‟s magnetic field is described by the magnetic elements which include BZ,
BH, BT, D and I. These elements are shown in the figure 3.2.
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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 3.2: Elements of the Earth’s magnetic field (Whitham, 1960)
The angle between the horizontal component of the magnetic field intensity and the geographic north meridians is called the angle of declination, D, while that between the total magnetic field vector and the horizontal component of the magnetic field intensity is the angle of inclination or the dip angle, 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.
The total magnetic field, its horizontal and vertical components are related by equation 3.1.
(3.1)
The angle of inclination can be expressed as
(3.2)
Hence, 20
( ) (3.3)
The field can be described in terms of the vertical component, Z, which is positive downwards and the horizontal component, H, which is always positive. (Parasnis,
1986).
3.2.3 Rocks and Minerals magnetism
Magnetic anomalies are caused by magnetic minerals, like magnetite and pyrrhotite contained in the rocks. The rock formation present in the earths subsurface can be considered on the basis of their behavior when within the vicinity of an external field. When the field of a rock is dominated by atoms whose orbital electrons are oriented to oppose the external field, then it is said to exhibit negative magnetic susceptibility. Such a rock formation is said to be diamagnetic.
Diamagnetism arises if the net magnetic moment of all the atoms is zero when H is zero. This situation is characteristic of atoms with completely filled electron shells.
Graphite, marble, quartz and salt are the examples of diamagnetic formations. A substance with positive magnetic susceptibility is paramagnetic. In this case, its magnetic moment when H is zero is not equal to zero. Iron, cobalt and nickel are examples of paramagnetic substances with strong magnetic interaction that enable the moments to align within fairly large domain. They are therefore said to be ferromagnetic. 21
Ferromagnetism decrease with increase in temperature and disappear at curie point temperature. The domains of some ferromagnetic substances may be subdivided into subdomains that align in opposite direction. In a case their moments nearly cancel and are called antiferromagnetic. Such substances have relatively low magnetic susceptibility. When the magnetic subdomains align in opposite direction, but their net moment is not equal to zero, the substance is called ferrimagnetic. This can arise either because one set of the subdomains has a stronger magnetic alignment than the other or there are more subdomains of one type than the other.
The ionic moments of the magnetic domains in different type of materials determine their net spontaneous magnetization (Stacey, 1977). This is displayed in Table 3.1.
Table 3.1Basic patterns of alignment of atomic magnetic moments by mutual interaction (source: Stacey, 1977)
22
CHAPTER FOUR
MATERIALS AND METHODS
4.1 Ground magnetic survey
4.1.1 Introduction
In ground magnetic survey, the portable magnetometer is used. The magnetic data collection was carried out in Lake Baringo geothermal prospect area between 26th
April 2014 to 3rd May 2014 using a fluxgate magnetometer that measures the vertical component of the field. The area covered was approximately 100 km2. The magnetic data were measured from 105 magnetic stations and 15 profiles. The profile and station separation was set at distances that would allow the mapping of the subsurface formation of magnetic contrast with the host rock. It varied depending on the accessibility. Some areas with geological outcrops where there were several surface thermal manifestations of geothermal resource, station spacing were made closer.
4.2 Field instruments
4.2.1 Global positioning system
The Global Positioning System (GPS) is one of the field equipment that is used to measure the co-ordinates of the base stations in latitude and longitude. It has to 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). The satellite receiver tracks and records every available pass at a given location. 23
4.2.2 Flux gate magnetometer
The flux gate magnetometer consists of two ferromagnetic cores of very high susceptibility aligned in the direction of the earth‟s field. The two high susceptibility strips are wound in opposite directions and are energized by an AC of about 1000
Hz with magnetization of the two coils being in opposite directions. These two coils are enclosed together in another coil carrying direct current. If there is no external field, the distortion of the magnetizing winding of the two coils will be equal and opposite resulting in no output in the pulse transformer. In the presence of an ambient magnetic field, the hysteresis curves for the two strips are displayed in opposite directions. The pulses from the two windings no longer balance each other and there is a net output pulse to the amplifier. This output is directional and changes sign when ambient field changes sign. The net output pulse is proportional to the ambient field intensity at the magnetometer element. This net output activates a balancing circuit, which changes the current in the d.c coil bringing the net field to zero. The change in the dc current is recorded digitally (Nettleton, 1976).
24
Figure 4.1: Schematic diagram of Flux-gate Magnetometer (After Kearey and
Brook, 1984)
4.3 Field measurements
The position of magnetic stations was measured using a hand held Global
Positioning System (GPS) Garmin model. The ground magnetic measurements were taken along fifteen profiles as shown in figure 4.2. Using a Flux-gate magnetometer, magnetic measurements were then taken along the profiles at intervals of approximately 350 m, though it could reduce or increase depending on the topography of the survey area such as large outcrops and thick prosobis juliflora forest cover. The total vertical magnetic field intensities were recorded in the field book for each magnetic station and the corresponding time of measurement and the coordinates throughout the whole survey.
25
82000
80000
78000
76000
74000 HING (m) HING
72000 NORT
70000
68000
66000 166000 168000 170000 172000 174000 EASTING (m)
Figure 4.2: Magnetic field stations in the study area
A total of 105 magnetic intensity stations were measured covering the survey area of approximately 100 km2. The subsequent profiles were identified roughly parallel to 26
the previous one at intervals of approximately 600 m. During the field survey, a few repeat readings were taken to ensure the accuracy of the measurement. The base station was selected and readings taken after about every one to two hours from the base station to correct for diurnal variations. Appendix I represents values of raw magnetic measurements in the survey area.
4.4 Magnetic data processing
4.4.1 Introduction Data correction and processing is a series of steps taken to rid the magnetic data of noise from sources not related to the geology of the earth‟s crust. This is necessary to prepare it for analysis and interpretation. Data reduction process included diurnal variations and the main (geomagnetic) field corrections.
4.4.2 Diurnal variations corrections
Diurnal variations are small but more rapid oscillations in the earth‟s field with a periodicity of about a day. Riddibough, ( 1971), described diurnal variations as variation of earth‟s magnetic field with time, due to the rotation of the earth and with respect to the solar wind, which may last several hours to one day. Lilley, 1992,
Milligan, (1995), observed that the best results are obtained if the base station is close to the survey area, the diurnal variations are small and smooth and electromagnetic induction effects are minimal. Diurnal variations corrections were done to remove contributions due to solar activity. The effects of diurnal variations 27
were removed by taking magnetometer readings at fixed base stations periodically throughout the day ensuring that the first and the last readings were taken from the base station. The base stations within the survey area, away from magnetic noise, were selected for each survey day. After the data collection, a diurnal variation curve of magnetic field versus time of measurement was plotted. Diurnal corrections were effected by subtracting magnetic readings extrapolated from diurnal curve from the field reading measured at the same time of the day. This was repeated for each survey day. Figure 4.3 shows a diurnal curve at base station for the first survey day,
(26/04/2014).
10790) nT
10780
, , (
10770Bz 10760 BZ 10750 Detum
10740 X mag field , field mag
line 10730 10720
10710Vertical 838 1038 1238 1438 1638 1838 Time of day (hrs)
Figure 4.3: Diurnal curve at base station on 26/04/2014
Diurnal variation reduction at station S15 is equivalent to the change in magnetic field represented by letter x (i.e. -36nT) in the diurnal curve in figure 4.3. The observed magnetic value at this station is obtained by removing the effect of diurnal 28
variation from the raw magnetic value measured in the field using the linear relation given as:
Observed value = raw magnetic value – diurnal variation ( 4.1)
Hence the observed magnetic field for station S15 is given by;
Observed magnetic value = 10686 – (-36)
= 10722 nT , where 10686nT is the raw magnetic field as indicated in the Appendix 1.
The above process was repeated for every raw magnetic measurement in every other station for each day. This effectively removes diurnal variation.
The Tables in Appendix II show the observed magnetic field values after correcting for diurnal variations for all the stations and their corresponding base station diurnal curves are shown in Appendix III.
4.4.3 Removal of geomagnetic field
The value of the main field was determined from international geomagnetic reference field (IGRF) charts. These charts are produced from observatories that monitor the earths‟ field throughout the year for many years. They are able to predict the value of the main field at a particular location at a particular time. The
IGRF readings for the stations were subtracted from the observed readings to determine the residual magnetic field due to anomalous contribution from local magnetic sources in the area. The IGRF model is calculated based on the dates, elevation and geographical locations (latitudes and longitudes) of the observed 29
magnetic data with the generated average total vertical magnetic intensity field of
11300 nT, inclination of -19.5670 and declination of 1.1450. Table 4.1 shows values of IGRF elements used in this geomagnetic correction.
Table 4.1: I.G.R.F elements for Lake Baringo area using model 2000-2015
Element Value
Declination 1.1450
Inclination -19.5670
Total vertical Intensity 11300 nT
From the corrected data, a computer Software, Golden Surfer, was then used for gridding and to plot the magnetic intensity contour map, in Figure 4.4, to establish the magnetic signature of the area.
30
MAGNETIC INTENSITY MAP OF LAKE BARINGO AREA
82000
nT 80000 1000 950
78000 900 850 800
76000 750
) 700
650 74000 600 550 500 72000 NORTHING (M NORTHING 450
400
70000 350 300
250 68000 200
66000 166000 168000 170000 172000 174000 EASTING (M)
Figure 4.4: Reduced verticalEASTING magnetic Intensity (m)contour Map of Lake Baringo area
31
Figure 4.5: 3-D view of the magnetic anomalies in the study area 32
4.5 Data enhancement techniques
4.5.1 Introduction
The magnetic data reduction procedure facilitated the drawing of a 2-D contour map. Further, the processed data was transformed through some procedures to assist in its interpretation. The transformations that were carried out on the obtained magnetic data include reduction to pole, boundary analysis by vertical and horizontal gradients and Euler deconvolution.
4.5.2 Reduction to the Pole
This simplifies the interpretation of anomalies by removing the symmetry introduced due to its induction by the inclined main field. The main field is only vertical (and induced anomaly symmetric) at the north and south magnetic poles. In reduction to the pole procedure, the measured total field anomaly is transformed into the vertical component of the field caused by the same source distribution magnetized in the vertical direction. The acquired anomaly is therefore the one that would be measured at the north magnetic pole, where induced magnetization and ambient field both are directed downwards (Blakely, 1995).The Euler software was used in reducing to the pole the magnetic profile data. This was important for outlining magnetic units and positioning magnetic discontinuities, which may correspond to faults. Reduction to the pole is usually unreliable at low magnetic latitudes, where northerly striking magnetic features have little magnetic expression.
Some bodies have no detectable magnetic anomaly at zero inclination (Blakely, 33
1995). The validity of the reduction to the pole is doubtful for inclinations lower than approximately 15º. The average inclination in the survey area was -19.567º and therefore reduction to the pole was considered reliable.
4.5.3 Boundary analysis by vertical and horizontal gradients
This quantifies the spatial rate of change of the magnetic field in vertical or horizontal direction. Derivatives essentially enhance and sharpen high frequency anomalies to low frequencies. This numerical filtering method is effective in enhancing anomaly due to shallow sources; it narrows the width of anomalies and very effective in locating source bodies and defining the boundaries of magnetic bodies.
4.5.4 Euler deconvolution
Euler deconvolution is used to estimate location and depth to magnetic anomaly source. It relates the magnetic field and its gradient components to the location of the anomaly source with the degree of homogeneity expressed as a structural index.
El Dawi et al.,( 2004) observed that Euler deconvolutiona suitable for mapping anomalies caused by isolated and multiple sources. Equation 4.2 represents Euler deconvolution.
( ) ( ) ( ) ( ) (4.2)
34
When the Euler equation is applied on the 2D source and x-coordinate is taken as a measure of the distance along the profile and y-coordinate set to zero along the entire profile, then the Euler Equation 4.2 reduces to:
( ) ( ) ( ) (4.3)
where ( xo, z0 ) is the co-ordinate of a 2D magnetic source whose total field T is detected at (x, z) . El Dawi et al.,( 2004), in their research found out that the total field has a regional value of B , and n is a measure of fall -off rate of the magnetic field. n is directly related to the source slope and is referred to as the structural index and depends on the geometry of the source.
35
CHAPTER FIVE
INTERPRETATION OF MAGNETIC DATA
5.1 Introduction
Potential field data can be interpreted either using qualitative or quantitative technique. Both approaches are used in this study. Qualitative technique makes use of either a map form as in Fig. 4.4 or from selected profiles as in Fig. 5.1. The map gives the general magnetic signature of the study area.
In quantitative interpretation, estimates of inherent parameters of the causative bodies were determined. Due to the ambiguity in the interpretation of potential field data, constrains had to be imposed. This required that the geology of the area to be considered. It allowed interpretation of data as recorded through analysis of magnetic anomaly profiles. mag2DC software was then used to estimate the depth of the body, depth extent, thickness of the body its dip and magnetic susceptibility.
5.2 Profile selection and removal of regional trend
5.2.1 Profile selection
Four profiles AA‟, BB‟, CC‟ and DD‟ were selected over the magnetic anomalies as shown in figure 5.1. Each profile was selected so as to form a dipolar magnetic anomaly by cutting a positive anomaly and an adjacent negative anomaly. A comparison of a topographical map of the area and figure 5.1 showed no marked correlation between the magnetic anomalies and the terrain. Magnetic anomalies in the study area are therefore independent of the terrain. 36
MAGNETIC INTENSITY MAP AND PROFILES
82000 D' nT
80000 1000 D 950 78000 900
)
850 (M) B' m A' 800 ( 76000
750 700
G NORTHING 650 74000 N C C'
I 600 550
H 72000 B 500 T 450 R 400 70000 O 350 300 N A 250 68000 200
66000 166000 168000 170000 172000 174000 EASTINGEASTING (m) (M)
Figure: 5.1. Vertical magnetic intensity contour map with the selected profiles
37
5.2.2 Removal of regional trend
The regional trend is the component of the magnetic anomaly having long wavelength and low spatial frequency. It is caused by deep-seated structural features that distort the effects of shallow structures. This is because the anomaly with short wavelength and high spatial frequency superimpose on the residual anomaly. The residual fields are responsible for the relatively shallow local anomaly of interest in the study which is to be interpreted. The mathematical approach of isolating the residual field is by fitting a linear or a polynomial and then subtracting it from the observed magnetic field interpolated from contours along a profile. The linear equations (5.1) to (5.4) were used to obtain the regional values for the total component of the magnetic intensity where x is the station position, for the profiles
AA‟, BB‟, CC‟ and DD‟ respectively as shown in Fig. 5.1 of VIM map of the study area. y=0.0042x+494 (5.1) y= -0.017x + 629 (5.2) y= 0.0019x + 638 (5.3) y=-0.012x + 436 (5.4)
Residual field = Corrected field data – Regional field (5.5)
The residual values were obtained using equation (5.5) by subtracting the regional values from the corrected field data. Figure 5.2a (ii) to 5.2d (ii) represent the residual field. The tables in Appendix IV show residual magnetic values for the four profiles. 38
PROFILE AA’
)
nT
1000 900 800 700 600
500 Magnetic Anomaly ( Anomaly Magnetic 400 Tot Vert. Mag. 300 Reg. Mag. 200 100 0 0 2000 4000 6000 8000 10000 12000 Distance (m)
(i)
500
)
400 nT
300
200 Res Mag. 100
0 Magnetic Anomaly ( Anomaly Magnetic 0 2000 4000 6000 8000 10000 12000 Distance (m) -100
-200 (ii)
Figure 5.2a (ii): Residual magnetic anomaly on profile AA’
39
PROFILE BB’
(i)
Figure 5.2b (ii): Residual magnetic anomaly on profile BB’ (ii)
40
PROFILE CC’
700
600
500
400
300 Tot. Vert. Mag
(i) Anomaly (nT)Anomaly
Reg. Mag 200
100 Distance Magnetic Magnetic 0 0 2000 4000 6000 8000
50 Distance (m) 0 0 2000 4000 6000 8000 -50 -100
-150 (ii)
Anomaly (nT)Anomaly Res. Mag -200
-250
-300 Magnetic Magnetic -350
Figure 5.2c (ii): Residual magnetic anomaly on profile CC’
41
PROFILE DD’
500 450
400
) 350 300 250 Tot. Vert. Mag. 200 Reg. mag. 150
100 Magnetic Anomaly (nT Anomaly Magnetic 50 0 (i) 0 1000 2000 3000 4000 5000 6000 7000 8000
100
50
0 0 2000 4000 6000 8000
Res. Mag. Anomaly(nT)
-50
Magnetic -100
(ii)
-150
Figure 5.2d (ii): Residual magnetic anomaly on profile DD’
42
5.3 Qualitative interpretations
5.3.1 Interpretation of Lake Baringo area magnetic intensity map
The vertical magnetic intensity contour map (VMICM) in Figure 4.4 was generated using corrected magnetic data. Qualitative interpretation of the vertical magnetic intensity contour map (Fig. 4.4) shows high magnetic signature trending NW and SW. Magnetic field decreases gradually to 250 nT on the NE region. This could be caused by deep seated bodies. The NW and SW parts have relatively high magnetic signature, magnetic field of
700nT and 900nT. Their trend (NW-SW) show significant magnetic relief and reflect the presence of both shallow and deep magnetic sources.
5.3.2 2D Interpretation of the Euler solutions along the profiles
The Euler solutions for magnetic anomaly along profile AA‟ are as shown in
Figure 5.3. From the calculated euler solutions the depth to the subsurface structure are estimated. A structural index of 1.0 was used since it best represents sill edge, dike, or fault. The horizontal and vertical gradients fluctuate at a distance of 1.5 km to 2.0 km, 3.5 km 4.0 km and at 5.5 km. This may represent abrupt lateral change in magnetization over the distance range.
There is relatively low magnetic anomaly signature along the profile AA‟ at 0
-1 km and 4 km -5.5 km. The shoulder of RTP curves within these ranges outlines the edges of possible faults and evidence of the presence of the fluids.
The depth to the magnetic structure is approximately 0.6 km. 43
……………….. RTP
Residual Field
.……………. Vertical gradient
Horizontal gradient
Figure 5.3: Euler depth solutions along magnetic anomaly profile AA’
From magnetic profile BB‟ in Figure 5.4, Euler solution cluster is observed at
1.5 km and at 2 km.There is an abrupt change in both horizontal and vertical gradients at 1.8 and at 4.5 km profile distance. At the same location the shallowest depth of approximately 50 m is attained. From the magnetic profile
CC‟ in Figure 5.5 the Euler solution have the shallowest depth at 250 m and the deepest at 750m. The RTP curve display a magnetic high at 500m and at 3250 m and a magnetic low at 1500 m which represent rocks of high and low magnetic susceptibility respectively relative to the host rocks. Abrupt changes in 44
horizontal and vertical gradients at 400m indicate possible change in magnetization. The Euler magnetic analysis along profile DD‟ in Figure 5.6 reveals a magnetic structure to a maximum depth of approximately 0.5 km. The
RTP curve has a magnetic high at 1.75 km and at 5.75 km which could be indicative of the edges of the causative bodies. The horizontal and vertical gradients fluctuate at a profile distance of 1.25km and 5.5km which are also the points of inflection of RTP. This suggests the top of a magnetic body.
Figure 5.4: Euler depth solutions along magnetic anomaly profile BB’
45
Figure 5.5: Euler depth solutions along magnetic anomaly profile CC’
Figure5.6: Euler depth solutions along magnetic anomaly profile DD’
46
5.4 Quantitative interpretations
5.4.1 Forward modelling
mag2dC computer program was used to draw 2-D models. It calculates the anomalous field caused by a collection of 2–D magmatic intrusive bodies.
Mag2dc for Windows allows the forward modelling and inversion of magnetic data. The bodies are modelled through a trial and error procedure to obtain a good fit between the calculated and observed anomalies. Geophysical constrains like known surface geology are also imposed because of ambiguity in magnetic data. Figures 5.7 to 5.10 show the 2-D modelled bodies of the subsurface geological formation.
47
Calculated magnetic anomaly
………………………. Observed magnetic anomaly
(a)
(c) (b)
Figure 5.7: Magnetic model along profile AA’
48
Calculated magnetic anomaly ……… . … Observed magnetic anomaly
(a)
(b)
Figure 5.8: magnetic model along profile BB’
49
Calculated magnetic anomaly
...... Observed magnetic anomaly (a)
(d) (C ) (e)
(b)
Figure 5.9: magnetic model along profile CC’
50
Calculated magnetic anomaly
………..Observed magnetic anomaly
(a)
(e)
(b) (f) ( C)
(d)
Figure 5.10: magnetic model along profile DD’
51
The parameters of the modeled bodies are shown in Table 5.1.
Table 5.1: Modelled parameters of the causative bodies
Profile Causative Modeled Modelled name bodies depth, a, (m) susceptibility, k, (SI) AA‟ b 170.15 -0.841 c 255.23 -0.578 BB‟ b 244.78 -0.677 CC‟ b 86.57 -0.600 c 182.09 0.353 d 223.88 -0.632 e 164.18 0.3803 DD‟ b 228.09 0.5301 c 237.313 -0.297 d 349.25 -0.095 e 147.07 0.067 f 277.61 -0.257
5.4.1.1 Models interpretations
The profile AA‟ in magnetic intensity map traverses across a region of high and a low magnetic signature region towards SW towards the NE respectively. The modeled bodies along profile AA‟ show two subsurface magmatic intrusive bodies which are basaltic sills and dykes forming along the fault zones. At a distance of 2022m and 6020m along the profile correspond to magnetic highs, and a basaltic dyke and basaltic sill form at depths of 170m and 255m from the surface and their susceptibilities are -0.841 and -0.578 SI respectively.
Profile BB‟ trends NE-SE in the magnetic intensity map. It has a magnetic low at a distance of 1597m along the profile. The models on profile BB‟ suggests a 52
body at a depth of 245m from the surface. This is could to be a magmatic intrusive, and more likely a basalt sill. Its magnetic susceptibility is –o.677 SI.
The magnetic model along profile CC‟ illustrated by fig. 5.9 has two magnetic highs at 250m and 3348m along the profile. At these points modeled bodies are at depths of 87m, 224m and 164m while their magnetic susceptibilities are -
0.600 SI,-0.632 SI and 0.3803 SI respectively. These bodies are suggested to magmatic intrusive, likely to be basaltic dykes. At a distance of 1598m along the profile is a magnetic low.The modeled body at this point is a horizontal magmatic intrusive of depth 182m and magnetic susceptibility of 0.353 SI. It is suggested to be a volcanic sill formed along the fractures zones because of its orientation.
Profile DD‟ is on the northern region of the magnetic intensity map. It trends in the direction NEE-SWW in the study area. On this profile, five bodies of different magnetic susceptibility, depth and geometry are modeled. They are magmatic intrusive arising from extensive lava flow. They are suggested to be dykes.
5.5 Discussion
The rift floor in the Lake Baringo prospect is marked by extensive North-to-
North East trending normal faults, which are a result of frequent tectonic activity dominated by extensional rifting process. This structural set-up results to permeable (fault) zones in the subsurface. The rift graben at the area around 53
Baringo is asymmetric deep graben with the western margin delineated by the
Saimo boundary faults and the eastern flank bound by the Mukutan Platform.
Recharge in this area is possibly by hydraulic gradient created by meteoric water from the scarps, which are at much higher elevation. This may be much enhanced along the NNW trending regional structures cut through the rift (and extend to depth). These structures are represented by the OlArabel and
Marmanent regional lineaments. Dunkley (1993) suggests that Lake Baringo could be recharging the geothermal systems in the area and estimated northward outflow from the lake to be in the order of 108 m3 annually.
It is envisaged that the reservoir is hosted within the Plio-Pleistocene rift floor lavas and their associated pyroclastics. The intense fracturing as a result of extensional stress is expected to provide good permeability at the subsurface.
Shallow lying anomalous regions were delineated by magnetic measurement implying zones of intense hot fluid/rock interactions.
Geothermal reservoir(s) therefore do exist at the subsurface in the Lake Baringo prospect area. The NNE trending fault zone is a possibly a transmitter of geothermal fluids at depth and require direct probe by drilling to establish whether it might be hosting a geothermal reservoir.
Surface geology show extensive thick fluvial and alluvial deposits in the area and covering most of the rift graben. The tuffs and pyroclastics layers exposed around KampiYaSamaki indicative of a widespread cover of the region by material from large eruption center in the vicinity, e.g., Korosi. This could be 54
good cap-rock reinforcement. Older extensive hydrothermal activity west of this area shows that subsurface self-sealing of fracture openings may be possible, especially for the formations around the prospect that have been further buried by alluvial deposits. Occurrence of manifestation preferentially along fault/fracture is indicative of a well-capped system that leakage is only along the fractures.
From the final results generated by forward modeling, the depth extend of the intrusives was determined. The results of the analysis and modeling suggest that magnetic features underlie the study area. The magnetic anomalies along the profiles AA‟, BB‟ and CC‟ correlate well in position with the local resistivity findings and results of previous geophysical measurements. By direct interpretation, these anomalies were found to have nearly the same magnetization intensity and from their dimensions of width, they were suggested to be prismatic intrusives. This is evident as from KRISP 90 seismic refraction investigation that the upper and middle crustal layers from eastern to western rift flank are mostly intruded by dykes and sills beneath the rift consisting of metamorphic basement rocks and an igneous mafic residuum accreted to the base of the crust derived from the upper mantle.
Lake Baringo prospect appears to host a geothermal system on the western side of Lake Baringo. Major structures like faults NE-SW along the rift floor tend to strongly influence the magnetic anomaly distribution in the area. Subsequently, its noted that the discharge of thermal fluids is evident in the western and central 55
parts of the lake. Nonetheless as the lake forms a natural sink for regional drainage, there is a high probability that the discharge features around the lake could be due to the extensive subsurface drainage of thermal fluids with probable sources on the western region.
Modelled bodies across the selected magnetic profiles lie relatively at shallower depths as shown in Table 5.1. Thus, the high heat flow observed in the area as manifested by hot springs could be due to shallow intrusives as shown by the occurrence of dykes that lie in-trend with the rift floor faults. According to
Riaroh and Okoth, 1994, the springs issue from the base of fault scarps bounding the lake. The source of heat conducted to the underground water could be dyke- like intra-crustal structures penetrating from the magma chambers.
56
CHAPTER SIX
CONCLUSIONS AND RECOMMENDATIONS
6.1 Introduction
The measured magnetic field is a composite of anomalies of varying frequencies. The highest frequency events of interest are those created by geological conditions in the shallow subsurface and the lowest frequency events are caused by magnetic property contrasts at or beneath the basement surface. Intermediate frequency events are created within a sedimentary section. There are a number of geological causes for local distortions in the
Earth's magnetic field. Causes especially important to this study are those that put near-surface formations with contrasting magnetic properties in contact with one another. The variations and contrasts in magnetic properties of the shallow formations are brought about singularly or by some combination of faulting, deposition and mineralization associated with structural displacement. Structuring the near-surface formations gives rise to a characteristic high wave-number magnetic expression.
The processing and analysis of the survey area's total spectrum of magnetic responses, it was possible to deduce the picture of subsurface structure.
Magnetic surveys have the capacity to let us view the distribution and trending of shallow structure relative to deep structure for an expanded perception of the structural setting. This study sought to map permeable areas within the study region, locate and identify intrusive structures. 57
6.2 Conclusions
The magnetic prospecting method used in this research work was found to image the subsurface formation of contrasting magnetic field that could be heat sources rooted from the asthenosphere. From the Euler solutions, the subsurface geometry of the western part of Lake Baringo has been delineated. Presence of hydrothermal fluids filled structures are identified. This is confirmed by absence of magnetic sources along the profiles. 2-D magnetic bodies were modeled assuming uniform magnetization higher than the surrounding rock. The confidence level in the resultant model was depended not only on the degree of fit but also geophysical constrains imposed during modeling. The visual inspection and analysis of the magnetic intensity map, discontinuities in Euler solution cluster along the profiles and the models revealed that Lake Baringo prospect area is generally characterized by broad and low magnetic signatures at the western and north western parts.
The modeled depth for the near surface magnetic anomaly sources of the area was found to be 86.57m, while that of the deep seated anomaly sources is 349.25m. The ground magnetic study of this area has helped to delineate lineaments and target zones with magmatic intrusives. The major subsurface structures delineated
(faults/fractures, sills and dykes) will aid the geothermal exploration work in the area. Also, the near linear nature of the anomalies in this prospect area suggests that the rocks may be bounded by faults. The results further support the delineation of faults/fractures and heat sources associated with shallow intrusive along structures.
These subsurface constituents of faults and the impact they have on the geothermal 58
resource are manifest on the surface around the lake. Major structures like faults
NE-SW along the rift floor may be the major reservoir control/host in the area. The heat sources for the geothermal systems in the area are possible shallow intrusive bodies (dykes) associated with faults below Lake Baringo. The subsurface permeability is mainly along fractures and fault systems and are important hydrogeological controls of fluids at depth.
6.3 Recommendations
The magnetic study carried out in Lake Baringo area will act as an important indicator to the potential of geothermal prospecting. It will motivate further research works using other geophysical methods. Indeed Langat et al,
(2005), observed resistivity, magnetics and gravity data may offer guidance to the existence of deep-seated networks of faults that may act as channels for thermal fluids, reservoir zone and possible heat sources. Through the use of multiple techniques, it would be possible to establish the existence and viability of a geothermal resource in the Baringo Prospect area.
Further follow up to calculated heat sources depth should be done using borehole logging to get information on the geothermal potential Lake
Baringo area.
59
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63
APPENDIX I
RAW TOTAL FIELD MAGNETIC DATA
DAY ONE :26/04/2014
ELEVATION EASTING NORTING RAW MAG. TIME STATION 838 BS 1034 167830 73068 10762 845 S11 1016 167830 73323 10683 857 S12 1010 167830 73952 10866 912 S13 1008 167830 74775 10974 924 S14 1016 167830 75274 10688 957 BS 1034 167830 73068 10730 1012 S15 1042 167830 72702 10686 1023 S16 1052 167830 72297 10755 1032 S17 1069 167830 71947 10848 1052 BS 1034 167830 73068 10718 1122 S21 1018 168120 73037 10587 1134 S22 1020 168120 72738 10855 1146 S23 1036 168120 72261 10979 1156 S24 1047 168120 71989 10383 1218 BS 1034 167830 73068 10730 1314 S25 1021 168120 71265 10676 1338 S26 1027 168120 70578 10553 1407 S27 1011 168120 70403 10725 1429 BS 1034 167830 73068 10773 1451 S28 1076 168120 73503 10958 1513 S29 1004 168120 73761 10853 1537 BS 1034 167830 73068 10781 1552 S31 1036 169763 73068 10691 1619 S32 1022 169763 73195 10781 1644 S33 1031 169763 73416 10674 1709 S34 1026 169763 73874 10390 1732 BS 1034 167830 73068 10730
64
DAY TWO: 27/04/2014
TIME STATION ELEVATION EASTING NORTHING RAW MAG. 814 BS 1011 166935 74322 10670 838 S11 1032 166935 74057 10304 911 S12 1065 166935 73721 10820 944 S13 1073 166935 73427 11012 1007 S14 1069 166935 72849 10828 1033 BS 1011 166935 74322 10677 1056 S15 1041 166935 74722 10877 1119 S16 1029 166935 75257 10797 1148 S17 1019 166935 75647 10724 1223 BS 1011 166935 74322 10650 1249 S21 1010 166421 74737 10784 1316 S22 1010 166421 75299 10741 1348 S23 1006 166421 75727 10680 1417 BS 1011 166935 74322 10643 1434 S24 1019 166421 74017 10803 1452 S25 1017 166421 73395 10822 1514 S26 1034 166421 72718 10872 1553 S27 1056 166421 72207 10727 1558 BS 1011 166935 74322 10653 1617 S31 1023 165949 74766 10784 1642 S32 1021 165949 75193 10770 1707 S33 1029 165949 75675 10694 1715 S34 1042 165949 76127 10537 1728 BS 1011 166935 74322 10688
65
DAY THREE: 28/04/2014
TIME STATION ELEVATION(M) EASTING NORTHING RAW MAG. 821 BS 1029 167194 68861 10793 858 S11 1017 167194 68436 10771 923 S12 1026 167194 68045 10743 1008 S13 1041 167194 67569 10802 1044 BS 1029 167194 68861 10766 1121 S14 1043 167194 69181 10633 1149 S15 1032 167194 69479 10836 1210 S16 1041 167194 69742 10817 1237 S17 1014 167194 70059 10784 1306 BS 1029 167194 68861 10801 1343 S21 1033 166434 68484 10823 1416 S22 1029 166434 68012 10757 1452 S23 1049 166434 67564 10736 1526 S24 1038 166434 67254 10706 1603 BS 1029 167194 68861 10741 1648 S25 1031 166434 66786 10804 1712 S26 1023 166434 66392 10749 1736 S27 1018 166434 65909 10794 1804 BS 1029 167194 68861 10758
DAY FOUR : 29/04/2014
TIME STATION ELEVATION EASTING NORTHING RAW MAG. 900 BS 1012 16739 71432 10712 921 S11 1008 16739 70967 10667 948 S12 1025 16739 70696 10652 1006 S13 1030 16739 70329 10601 1045 BS 1012 16739 71432 10703 1113 S14 1018 16739 71837 10846 1130 BS 1012 16739 71432 10752 1254 S15 1019 16739 69374 10763 1342 S16 1024 16739 68976 10795 1412 S17 107 16739 68619 10758 1536 BS 1012 16739 71432 10721 66
DAY FIVE : 30/04/2014
TIME STATION ELEVATION NORTHING RAW MAG. EASTING 918 BS 1006 168334 71480 10682 956 S11 1011 168334 71541 10826 1134 S12 1013 168334 71841 11054 1203 S13 984 168334 72104 10735 1258 BS 1006 168334 71480 10706 1329 S21 1028 169137 71680 10732 1354 S22 1023 169137 72013 10703 1436 S23 1033 169137 72389 10686 1518 BS 1006 168334 71480 10639 1534 S24 1017 169137 71271 10674 1604 S25 1008 169137 70811 10688 1634 BS 1006 168334 71480 10659
DAY SIX : 01/05/2014
TIME STATION ELEVATION EASTING NORTHINGS RAW MAG. 841 BS 1003 168783 77644 10878 932 S11 1014 168783 77924 10763 1006 S12 1009 168783 78205 10781 1029 S13 1007 168783 78491 10634 1118 BS 1003 168783 77644 10836 1144 S21 1021 169881 77538 10804 1231 S22 1032 169881 77967 10734 1307 S23 1026 169881 78301 10752 1341 S24 1017 169881 78579 10638 1426 BS 1003 168783 77644 10804 1533 S25 1016 169881 79068 10738 1617 S26 1004 169881 79603 10736 1654 S27 1009 169881 80129 10803 1748 BS 1003 168783 77644 10868
67
DAY SEVEN :02/05/2014
TIME STATION ELEVATION EASTING NORTHING RAW MAG. 824 BS 1013 167849 77670 10794 941 S11 1026 167849 78191 10802 1018 S12 1009 167849 78673 10709 1054 S13 1036 167849 79160 10839 1130 BS 1013 167849 77670 10771 1224 S14 1017 167849 79573 10924 1256 S15 1011 167849 80065 10647 1343 BS 1013 167849 77670 10833 1415 S21 1033 166841 77844 10862 1442 S22 1016 166841 78356 10948 1518 S23 1008 166841 78838 10775 1621 BS 1014 167849 77670 10803 1659 S24 1O17 166841 79235 10878 1727 S25 131 166841 79728 10637 1809 BS 1026 167849 77670 10786
DAY EIGHT :03/05/2014
TIME STATION ELEVATION EASTING NORTING RAW MAG. 815 BS 1017 171934 80248 10738 839 S11 1023 171934 80765 10846 921 S12 1046 171934 81247 10869 958 S13 1069 171934 81715 10751 1024 BS 1017 171934 80248 10778 1049 S14 1031 171934 82239 10803 1136 S15 1009 171934 82701 10698 1212 BS 1017 171934 80248 10719 1238 S21 1018 172016 81396 10736 1256 S22 1042 172016 81864 10627 1324 S23 1011 172016 82407 10714 1352 BS 1017 171934 80248 10746 1418 S24 1016 172016 82733 10817 1514 S31 1052 174294 81054 10806 1538 S32 1046 174294 81618 10945 1612 BS 1017 171934 80248 10733
68
APPENDIX II
CORRECTED MAGNETIC FIELD
Diurnal corrections on 26/04/2014
RAW MAG. DIURNAL TIME STATION CORRECTION C.V ANOMALY 838 BS 10762 0 10762 530 845 S11 10683 -3 10686 602 857 S12 10866 -9 10875 407 912 S13 10974 -14 10988 283 924 S14 10688 -21 10709 557 957 BS 10730 -32 10762 530 1012 S15 10686 -36 10722 574 1023 S16 10755 -38 10793 508 1032 S17 10848 -40 10888 417 1052 BS 10718 -44 10762 530 1122 S21 10587 -40 10627 665 1134 S22 10855 -39 10894 402 1146 S23 10979 -37 11016 285 1156 S24 10383 -35 10418 885 1218 BS 10730 -33 10763 529 1314 S25 10676 -13 10689 623 1338 S26 10553 -5 10558 762 1407 S27 10725 5 10720 602 1429 BS 10773 11 10762 529 1451 S28 10958 13 10945 341 1513 S29 10853 16 10837 445 1537 BS 10781 19 10762 530 1552 S31 10691 13 10678 610 1619 S32 10781 -1 10782 504 1644 S33 10674 -10 10684 599 1709 S34 10390 -21 10411 867 1732 BS 10730 -32 10762 530
69
Diurnal corrections on 27/04/2014
DIURNAL TIME STATION ELEVATION CORRECTION C.V ANOMALY 814 BS 1011 0 10670 608 838 S11 1032 1 10303 979 911 S12 1065 2 10818 468 944 S13 1073 4 11008 280 1007 S14 1069 6 10822 474 1033 BS 1011 -16 10693 585 1056 S15 1041 2 10875 399 1119 S16 1029 -4 10801 467 1148 S17 1019 -12 10736 527 1223 BS 1011 -20 10670 608 1249 S21 1010 -22 10806 468 1316 S22 1010 -24 10765 502 1348 S23 1006 -26 10706 558 1417 BS 1011 -26 10669 609 1434 S24 1019 -25 10828 455 1452 S25 1017 -24 10846 445 1514 S26 1034 -21 10893 405 1553 S27 1056 -20 10747 556 1558 BS 1011 -18 10671 607 1617 S31 1023 -12 10796 479 1642 S32 1021 -4 10774 496 1707 S33 1029 2 10692 572 1715 S34 1042 3 10534 747 1728 BS 1011 4 10684 594
70
Diurnal corrections on 28/04/2014
DIURNAL TIME STATION RAW MAG. CORRECTION C.V ANOMALY 821 BS 10793 0 10793 548 858 S11 10771 -6 10777 568 923 S12 10743 -12 10755 595 1008 S13 10802 -20 10822 534 1044 BS 10766 -27 10793 548 1121 S14 10633 -18 10651 687 1149 S15 10836 -11 10847 487 1210 S16 10817 -6 10823 508 1237 S17 10784 1 10783 544 1306 BS 10801 8 10793 548 1343 S21 10823 3 10820 527 1416 S22 10757 -14 10771 581 1452 S23 10736 -28 10764 593 1526 S24 10706 -40 10746 615 1603 BS 10741 -52 10793 548 1648 S25 10804 -46 10850 516 1712 S26 10749 -42 10791 580 1736 S27 10794 -39 10833 543 1804 BS 10758 -35 10793 548
71
Diurnal corrections on 29/04/2014
DIURNAL TIME STATION RAW MAG. CORRECTION C.V ANOMALY 900 BS 10712 0 10712 876 921 S11 10667 -2 10669 925 948 S12 10652 -5 10657 939 1006 S13 10601 -6 10607 993 1045 BS 10703 -9 10712 876 1113 S14 10846 21 10825 759 1130 BS 10752 39 10713 875 1254 S15 10763 29 10734 878 1342 S16 10795 25 10770 846 1412 S17 10758 19 10739 882 1536 BS 10721 9 10712 876
Diurnal corrections on 01/05/2014
DIURNAL TIME STATION RAW MAG. CORRECTION C.V ANOMALY 841 BS 10878 0 10878 358 932 S11 10763 -21 10784 449 1006 S12 10781 -36 10817 412 1029 S13 10634 -39 10673 554 1118 BS 10836 -42 10878 358 1144 S21 10804 -22 10826 409 1231 S22 10734 -11 10745 485 1307 S23 10752 -3 10755 471 1341 S24 10638 5 10633 590 1426 BS 10804 16 10788 448 1533 S25 10738 7 10731 486 1617 S26 10736 1 10735 475 1654 S27 10803 -3 10806 399 1748 BS 10868 -5 10873 363
72
Diurnal corrections on02/05/2014
DIURNAL TIME STATION RAW MAG. CORRECTION C.V ANOMALY 824 BS 10794 0 10794 443 941 S11 10802 -9 10811 420 1018 S12 10709 -14 10723 503 1054 S13 10839 -19 10858 361 1130 BS 10771 -23 10794 443 1224 S14 10924 1 10923 292 1256 S15 10647 16 10631 578 1343 BS 10833 39 10794 443 1415 S21 10862 33 10829 407 1442 S22 10948 28 10920 311 1518 S23 10775 21 10754 471 1621 BS 10803 9 10794 443 1659 S24 10878 3 10875 346 1727 S25 10637 -2 10639 576 1809 BS 10786 -8 10794 443
Diurnal corrections on 03/05/2014 DIURNAL TIME STATION RAW MAG. CORRECTION C.V ANOMALY 815 BS 10738 0 10738 462 839 S11 10846 7 10839 354 921 S12 10869 20 10849 339 958 S13 10751 31 10720 461 1024 BS 10778 40 10738 462 1049 S14 10803 27 10776 401 1136 S15 10698 1 10697 473 1212 BS 10719 -19 10738 462 1238 S21 10736 -12 10748 438 1256 S22 10627 -7 10634 547 1324 S23 10714 1 10713 461 1352 BS 10746 8 10738 462 1418 S24 10817 5 10812 358 1514 S31 10806 -1 10807 378 1538 S32 10945 -3 10948 231 1612 BS 10733 -5 10738 462
73
APPENDIX III
DIURNAL VARIATIONS CURVES
Base station diurnal curve on 26/04/2014
10790
10780
10770
10760 BZ 10750 Detum line
10740 mag field ,Bz , , (nT) ,Bz field mag
10730
10720 Vertical 10710 Time of day(hrs) 838 1038 1238 1438 1638 1838
Base station diurnal curve on 27/04/2014 74
10680
10675
10670
10665 BZ 10660 Detum line 10655
10650
10645
Vertical mag field ,Bz , (nT), ,Bzfieldmag Vertical 10640 814 1014 1214 1414 1614 1814Time of day(hrs)
Base station diurnal curve on 28/04/2014
10810
10800
10790
10780 BZ 10770 Detum line 10760
10750
10740 Vertical mag field ,Bz , , (nT) ,Bz field mag Vertical 10730 Time of day(hrs) 814 1314 1814 2314
75
Base station diurnal curve on 29/04/2014
10760
10750
10740
BZ 10730 Detum line 10720
10710
10700 Time of day ( 900 1100 1300 1500 1700
Base station diurnal curve on 30/04/2014
10710
10700
10690
10680
10670 BZ Detum line 10660
10650
10640 Vertical,fieldmag,Bz (nT) 10630 Time of day(hrs 918 1118 1318 1518 1718
76
Base station diurnal curve on 01/05/2014
10910
10900
10890
10880 BZ 10870 Detum line 10860
10850 Vertical mag fied ,Bz , , (nT) ,Bz fied mag Vertical
10840
10830 841 1041 1241 1441 1641 1841
Base station diurnal curve on 02/05/2014
10840
10830
10820
10810 Bz 10800 Detum line 10790
10780
Vertical mag field ,Bz , , (nT) ,Bz field mag Vertical 10770
10760 824 1324 1824 2324
77
Base station diurnal curve on 03/05/2014
10790
10780
10770
10760
10750 Bz Detum line 10740
10730
10720
10710 815 1015 1215 1415 1615
78
APPENDIX IV
RESIDUAL MAGNETIC FIELD ALONG THE PROFILES ON THE
CONTUR MAP
Residual magnetic field along profile AA’ TOTAL REGIONAL MAGNETIC MAGNETIC RESIDUAL MAGNETIC DISTANCE (m) FIELD (nT) FIELD (nT) FIELD (nT) 0 494.060256 494.0603 -4.3969E-05 20.04791396 492.1871375 494.1443478 -1.957210273 136.1050689 483.7649536 494.6308993 -10.86594575 219.0076152 478.8273173 494.9784553 -16.15113801 417.9673164 471.2306505 495.8125629 -24.58191238 463.5894432 469.6067605 496.0038266 -26.39706603 616.9270176 467.930788 496.6466705 -28.71588249 791.0738175 459.5130538 497.3767538 -37.86369998 815.8867189 458.1148724 497.480778 -39.36590559 1014.84642 456.304705 498.3148856 -42.01018065 1118.558192 466.2446077 498.7496811 -32.50507338 1213.806121 482.6063917 499.1489932 -16.54260145 1412.765823 523.4898411 499.9831007 23.50674033 1446.042566 542.6165522 500.1226083 42.49394387 1611.725524 690.9758881 500.8172083 190.1586798 1773.52694 881.6059684 501.4955356 380.1104329 1810.685225 916.8775214 501.6513159 415.2262055 2009.644926 948.5454477 502.4854235 446.0600243 2101.011315 944.90886 502.8684628 442.0403972 2208.604627 921.6475541 503.319531 418.3280231 2407.564329 883.335391 504.1536386 379.1817524 2428.495689 878.3317199 504.2413901 374.0903299 2606.52403 826.119343 504.9877462 321.1315968 2755.980063 770.6711861 505.6143173 265.0568688 2805.483731 751.1779848 505.8218537 245.356131 3004.443432 668.93369 506.6559613 162.2777287 3083.464437 638.264652 506.9872446 131.2774075 3203.403134 599.1528754 507.4900689 91.66280657 3402.362835 518.6524569 508.3241764 10.32828047 3410.948812 520.722397 508.3601718 12.36222515 79
3601.322536 428.2614871 509.158284 -80.8967969 3738.433186 467.7669122 509.7330991 -41.96618688 3800.282237 527.9962728 509.9923916 18.00388119 3999.241938 490.2870583 510.8264991 -20.53944081 4065.91756 477.6875239 511.1060263 -33.41850241 4198.20164 472.5602717 511.6606067 -39.10033497 4393.401935 490.1765179 512.4789536 -22.30243564 4397.161341 490.5480837 512.4947143 -21.94663056 4596.121042 514.5553058 513.3288219 1.226483947 4720.886309 526.7591619 513.8518808 12.90728106 4795.080743 532.183851 514.1629294 18.02092159 4994.040445 543.7317418 514.997037 28.73470476 5048.370683 547.0757284 515.2248081 31.85092032 5193.000146 555.6940449 515.8311446 39.86290036 5375.855057 576.6369502 516.5977353 60.03921492 5391.959847 578.4560437 516.6652521 61.79079161 5590.919548 619.4513952 517.4993597 101.9520355 5703.339432 652.4111801 517.9706626 134.4405176 5789.879249 683.1476372 518.3334673 164.8141699 5988.838951 753.4848366 519.1675748 234.3172618 6030.823806 762.141846 519.3435898 242.7982562 6187.798652 788.1964147 520.0016824 268.1947323 6358.30818 786.6720716 520.7165171 265.9555546 6386.758353 783.4079663 520.83579 262.5721763 6585.718054 762.2977933 521.6698975 240.6278958 6685.792555 752.6547447 522.0894443 230.5653004 6784.677756 740.2467846 522.5040051 217.7427795 6983.637457 718.6784278 523.3381127 195.3403151 7013.276929 715.7641808 523.4623716 192.3018093 7182.597158 697.1325282 524.1722203 172.960308 7340.761303 681.4454565 524.8352988 156.6101577 7381.556859 677.1680288 525.0063278 152.161701 7580.516561 657.7206129 525.8404354 131.8801775 7668.245677 649.7356037 526.2082261 123.5273776 7779.476262 639.3460748 526.674543 112.6715318 7978.435963 622.1103978 527.5086505 94.60174728 7995.730052 620.6991986 527.5811533 93.11804534 8177.395664 605.796677 528.3427581 77.45391888 8323.214426 594.8894485 528.9540806 65.93536798 80
8376.355365 590.9356631 529.1768657 61.75879742 8575.315067 577.4471545 530.0109732 47.4361813 8650.6988 572.9098578 530.3270078 42.58285003 8774.274768 565.5320167 530.8450808 34.68693591 8973.234469 555.409278 531.6791884 23.73008958 8978.183175 555.1966227 531.6999351 23.49668765 9172.19417 546.7700032 532.5132959 14.25670721 9305.667549 542.2033341 533.0728623 9.130471788 9371.153872 539.8755475 533.3474035 6.528143974 9570.113573 534.3427406 534.1815111 0.161229491 9576.603044 534.2090615 534.2087172 0.000344357 Residual magnetic field along profile BB’ TOTAL REGIONAL RESIDUAL DISTANCE MAGNETIC MAGNETIC MAGNETIC (m) FIELD (nT) FIELD (nT) FIELD (nT) 0 629.3681663 629.3682 -3.3701E-05 62.41899307 630.9359974 628.2984633 2.637534074 234.8453066 636.2142897 625.3434211 10.87086852 407.2716201 642.6714783 622.388379 20.28309934 579.6979336 650.773548 619.4333368 31.34021122 752.1242471 661.1591332 616.4782947 44.68083855 924.5505605 674.6256793 613.5232525 61.10242682 950.9749325 677.2694818 613.0703916 64.19909019 1096.976874 691.3420794 610.5682103 80.77386906 1269.403188 712.3996486 607.6131682 104.7864804 1441.829501 738.414919 604.658126 133.756793 1614.255815 769.4052329 601.7030839 167.7021491 1786.682128 804.4185851 598.7480417 205.6705434 1957.345221 840.0990278 595.8232176 244.2758102 1959.108442 840.4727981 595.7929995 244.6797986 2131.534755 871.5692416 592.8379574 278.7312843 2303.961069 892.6250612 589.8829152 302.742146 2476.387382 901.3294616 586.927873 314.4015886 2648.813696 897.9623592 583.9728309 313.9895283 2821.240009 880.4313406 581.0177887 299.4135519 2963.715509 852.3967718 578.5760436 273.8207282 2993.666323 845.9395477 578.0627466 267.8768011 3166.092636 799.8377026 575.1077044 224.7299982 3338.51895 765.5096335 572.1526622 193.3569712 81
3510.945263 763.1488626 569.1976201 193.9512425 3683.371577 800.5215159 566.2425779 234.2789379 3855.79789 859.6204403 563.2875358 296.3329046 3970.085797 885.0896914 561.3288696 323.7608218 4028.224203 896.7248401 560.3324936 336.3923465 4200.650517 902.9992769 557.3774514 345.6218255 4373.07683 896.9202995 554.4224093 342.4978902 4545.503144 884.3645176 551.4673671 332.8971505 4717.929457 865.2775496 548.512325 316.7652247 4890.355771 850.2459516 545.5572828 304.6886688 4976.456085 834.9256584 544.0816956 290.8439628 5062.782084 816.9183265 542.6022406 274.3160858 5235.208398 766.2121768 539.6471985 226.5649784 5407.634711 720.2835593 536.6921563 183.591403 5580.061025 656.8754675 533.7371142 123.1383533 5752.487338 613.2445194 530.782072 82.46244736 5924.913652 582.7812593 527.8270298 54.95422942 5982.826374 574.5705608 526.8345216 47.73603914 6097.339965 555.7721704 524.8719877 30.90018272 6269.766279 528.0027958 521.9169455 6.085850284 6442.192592 495.0165969 518.9619034 -23.94530644 6614.618906 454.2251319 516.0068612 -61.78172929 6787.045219 410.9422564 513.051819 -102.1095626 6959.471533 383.6168035 510.0967769 -126.4799733 6989.196662 388.7843327 509.5873476 -120.8030149 7131.897846 414.8527556 507.1417347 -92.28897912 7304.32416 513.7826584 504.1866925 9.595965809 7476.750473 693.8769612 501.2316504 192.6453108 7649.176787 861.9563808 498.2766082 363.6797726 7821.6031 712.6621944 495.3215661 217.3406283 7994.029414 557.9179335 492.3665239 65.55140963 7995.56695 557.105919 492.3401736 64.76574536 8166.455727 467.551884 489.4114817 -21.85959774 8338.882041 411.7510384 486.4564396 -74.7054012 8511.308354 422.3982758 483.5013974 -61.10312159 8683.734668 445.0026732 480.5463553 -35.54368202 8856.160981 471.2912166 477.5913131 -6.300096475 8892.280593 476.9718611 476.9722952 -0.000434137
82
Residual magnetic field along profile CC’ TOTAL REGIONAL RESIDUAL DISTANCE MAGNETIC MAGNETIC FIELD MAGNETIC (m) FIELD (nT) (nT) FIELD (nT) 0 637.969793 637.9698 -6.97E-06 64.77570207 635.137143 638.0919864 -2.954843457 234.6913742 626.8648239 638.4124983 -11.5476744 286.0072107 624.0548963 638.5092954 -14.45439912 500.6834314 609.8720822 638.9142392 -29.04215695 507.2387194 609.3669576 638.9266044 -29.55964679 728.470228 589.1713477 639.3439134 -50.17256572 766.6754887 585.4254225 639.41598 -53.9905575 949.7017367 567.5455084 639.7612224 -72.21571396 1032.667546 561.9295487 639.9177208 -77.98817206 1170.933245 552.7407117 640.1785314 -87.43781965 1298.659603 548.0421424 640.4194616 -92.37731924 1392.164754 543.4846472 640.5958404 -97.1111932 1564.65166 533.280975 640.9212024 -107.6402274 1613.396263 529.091007 641.0131494 -111.9221424 1830.643718 503.5518055 641.4229432 -137.8711378 1834.627771 502.9112595 641.4304584 -138.5191988 2055.85928 472.7822053 641.8477674 -169.0655621 2096.635775 467.8940764 641.9246841 -174.0306077 2277.090789 453.5591869 642.2650764 -188.7058895 2362.627832 443.8526336 642.4264249 -198.5737913 2498.322297 430.7026494 642.6823853 -211.979736 2628.619889 412.5964056 642.9281657 -230.3317601 2719.553806 402.1736255 643.0996943 -240.9260689 2894.611947 376.0301694 643.4299065 -267.3997372 2940.785315 371.8743521 643.5170033 -271.6426512 3160.604004 334.873407 643.9316473 -309.0582404 3162.016823 334.7893937 643.9343123 -309.1449186 3383.248332 335.7461323 644.3516213 -308.605489 3426.596061 351.1622791 644.4333882 -293.271109 3604.479841 394.4822389 644.7689303 -250.2866914 3692.588118 417.9345127 644.935129 -227.0006163 3825.711349 440.5819252 645.1862393 -204.6043141 3958.580176 458.9089555 645.4368698 -186.5279143 4046.942858 469.9350521 645.6035483 -175.6684962 83
4224.572233 488.5040804 645.9386106 -157.4345302 4268.174367 493.9538156 646.0208573 -152.0670417 4489.405875 519.014343 646.4381663 -127.4238233 4490.56429 519.121284 646.4403514 -127.3190675 4710.637384 543.1214061 646.8554753 -103.7340692 4756.556347 547.8301134 646.9420922 -99.11197889 4931.868892 570.9233116 647.2727843 -76.34947265 5022.548405 582.6752014 647.4438331 -64.7686317 5153.100401 604.9633212 647.6900933 -42.72677206 5288.540462 623.4876045 647.9455739 -24.45796935 5374.33191 637.6808235 648.1074023 -10.42657874 5554.532519 653.2852576 648.4473147 4.83794295 5595.563418 655.9099465 648.5247113 7.385235196 5816.794927 654.6699052 648.9420203 5.727884916 5820.524576 654.5338912 648.9490555 5.584835644 5903.780568 649.1042255 649.1061013 -0.001875772 Residual magnetic field along profile DD’ TOTAL REGIONAL RESIDUAL DISTANCE MAGNETIC MAGNETIC MAGNETIC (m) FIELD (nT) FIELD (nT) FIELD (nT) 0 436.0172635 436.0173 -3.6502E-05 117.7776232 430.7855447 434.6404796 -3.854934858 204.8014892 417.4143146 433.6231706 -16.20885603 305.5087794 402.7395102 432.4459024 -29.7063922 493.2399356 392.2864377 430.2513252 -37.96488741 608.7067245 387.9393589 428.9015184 -40.96215948 680.9710918 388.2818904 428.0567479 -39.77485756 868.7022479 375.1304661 425.8621707 -50.73170459 1012.61196 353.6424299 424.1798662 -70.53743634 1056.433404 347.7286296 423.6675935 -75.9389639 1244.16456 316.9091265 421.4730163 -104.5638898 1416.517195 332.4655226 419.458214 -86.99269136 1431.895717 336.2797531 419.2784391 -82.99868595 1619.626873 392.1726763 417.0838619 -24.91118558 1807.358029 428.5020565 414.8892846 13.61277182 1820.42243 430.1874524 414.7365618 15.45089059 1995.089185 453.1210139 412.6947074 40.42630648 2182.820341 464.6727987 410.5001302 54.17266851 2224.327666 465.6938523 410.0149096 55.67894269 84
2370.551497 468.2891249 408.305553 59.98357192 2558.282654 465.4462171 406.1109758 59.33524134 2628.232901 462.7674255 405.2932574 57.47416807 2746.01381 457.9422683 403.9163986 54.02586973 2933.744966 446.4939319 401.7218213 44.77211053 3032.138136 438.5731638 400.5716052 38.00155864 3121.476122 432.1420764 399.5272441 32.61483222 3309.207278 417.785429 397.3326669 20.45276203 3436.043372 411.426937 395.849953 15.57698403 3496.938435 409.169148 395.1380897 14.03105827 3684.669591 408.8847418 392.9435125 15.94122929 3839.948607 409.8181895 391.1283008 18.68988875 3872.400747 409.8972965 390.7489353 19.14836122 4060.131903 410.4544682 388.5543581 21.90011019 4243.853842 409.2919816 386.4066486 22.88533302 4247.863059 409.2672885 386.3597808 22.90750762 4435.594215 407.0709951 384.1652036 22.90579144 4623.325372 402.6980381 381.9706264 20.72741166 4647.759078 401.8643618 381.6849964 20.1793654 4811.056528 397.0982735 379.7760492 17.3222243 4998.787684 389.0951296 377.581472 11.51365762 5051.664313 386.0168779 376.9633442 9.053533743 5186.51884 380.3560531 375.3868948 4.969158388 5374.249996 370.9358691 373.1923175 -2.256448413 5455.569548 365.8740048 372.241692 -6.367687156 5561.981153 365.2012782 370.9977403 -5.796462091 5749.712309 382.1296871 368.8031631 13.32652395 5859.474783 415.1252468 367.5200398 47.60520704 5937.443465 433.5434803 366.6085859 66.93489443 6125.174621 446.1000047 364.4140087 81.68599605 6263.380019 446.1060679 362.7983876 83.3076803 6312.905777 444.2544945 362.2194315 82.03506306 6500.636933 430.6590314 360.0248542 70.63417713 6667.285254 414.7185093 358.0767354 56.64177395 6688.36809 412.2270865 357.830277 54.39680945 6876.099246 390.6536359 355.6356998 35.01793605 7063.830402 368.7964182 353.4411226 15.35529562 7071.190489 367.9408146 353.3550832 14.58573141 7207.842974 351.7574591 351.7576156 -0.000156541 85