Geothermal mapping using shallow hole temperature measurement: A study of magadi prospect, Kenya

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1 Int. J. Int. J. Cur. Tr. Res (2014) 3 (2):1-7 ISSN: Geothermal mapping using shallow hole temperature measurement: A study of magadi prospect, Kenya Richard Gershom Atuya 1*, John Gitonga Githiri 2 and Robert. Kinyua 3 Institute of Energy and Environmental Technology 1,3, Jomo Kenyatta University of Agriculture and Technology, P.O Box Nairobi, Kenya Physics Department 2, Jomo Kenyatta University of Agriculture and Technology, P.O Box , Nairobi, Kenya Received: 5 January 2014/ Accepted: 22 January 2014/ Published online 28 February2014. INJCTR 2014 ABSTRACT In recent years, demand for clean energy has increased prompting the exploitation of renewable energies. These renewable energies, especially geothermal energy, are expensive and would require much exploration before exploitation. Kenya is endowed with geothermal energy and 14 prospects spanning from Barrier in the northern Rift valley to Magadi in the southern rift valley have been singled out as viable for commercial exploitation. In the Magadi prospect, magnetic, gravity and seismic studies have been carried out to determine the hotspot for the geothermal potential. All these studies singled out the Northern part of Magadi as the heat source of geothermal energy in the Magadi area. Heat flux measurements were conducted in the northern part of Lake Magadi and a contour map generated using Golden Software Surfer 8. This contour map was overlaid on to the magnetic and seismic map done in the same region. The heat flow map seems to agree with the seismic and the magnetic maps. From the heat flux contour map generated, it can be concluded that shallow temperature gradient holes are reliable in mapping geothermal prospects and that the heat source for the geothermal energy is likely to be on the north-eastern side of the study area. Key words: Magadi, geothermal mapping, heat flux. Introduction The East African Rift together with the Ethiopian rift forms the larger East African Rift System (EARS). This is a region where tectonic plate activity resulted into faulting and subsequent rifting and formation of a graben. The rift formation process also resulted in previously deep seated hot rocks being brought closer to the surface and thus becoming a heat source while the faults serve as channels of leakage for heated subsurface fluids and form a geothermal system. Geothermal systems are associated with several heat loss features such as hot springs, geysers, mud pools, fumaroles, steaming grounds, hot grounds and altered grounds. These features continuously transfer mass and natural heat from the geothermal system to the atmosphere. Localized thermal gradients can therefore be used to map out geothermal active grounds. Surface manifestations may not reflect potential geothermal conditions at depth in many situations (Mcnitt et al., 1989). This is particularly true when an overlying nongeothermal groundwater horizon masks the geothermal system. For this reason, it is not always possible to use surface heat discharge as a first approximation to the size of the system and its capacity to produce usable energy (Gupta an d Roy, 2007). Corresponding author* E.mail:atuyagershom@gmail.com

2 2 Richard et al. Int. J. Cur. Tr. Res (2014) 3 (2):1-7 Certain hydrological conditions also considerably alter the possible surficial manifestation of geothermal reservoirs. When the top of a geothermal system comes in contact with a deep underground body of water, where the water is stagnant or slowly moving, and the heat from the geothermal system is adequate to cause the groundwater to boil, patches of gently steaming ground would appear. These patches would give no indication of the high pressures that may exist in the geothermal systems. Similarly, when the top of a geothermal system is intersected by a large cold water aquifer, the heat will be swept down gradient (Mcnitt et al., 1989). Therefore, either no evidence of the geothermal system will appear on the surface, or large-volume warm springs may appear at large distances from the centre of the source of heat (Gupta and Roy, 2007). Thermal exploration techniques are extremely useful in assessing the size and potential of a geothermal system. Near-surface temperature gradient and heat flow measurements are routinely made in any geothermal exploration program and are often used as primary criteria for selection of drilling sites (Gupta and Roy, 2007). Heat flow studies are critical to understanding many basic geological and geophysical phenomena. The large scale processes that shape the earth leave thermal signatures that may prove crucial to deeper understanding of processes like plate tectonics and crustal magmatism. Heat flow also provides information on the maturation of hydrocarbons, groundwater flow, and displacement rates on faults (Henrikson and Chapman, 2002). Material and Methods Study Area Magadi, area is located at the Kenya Rift Valley southwest of Nairobi city, in southern Kenya. Magadi is northeast of Lake Natron in Tanzania, bounded by latitude 1 o 53` and longitude 36 o 18` (Figure 1). Magadi soda ash processing plant is located a few meters from the shores of Lake Magadi where trona is mined. Smith and Mosley, (1993) defined the geology of Lake Magadi as being made up of mostly Achaean to early Palaeozoic crystalline basement rocks and rifted related volcanic and sediments. The Magadi area was classified into three formations by Baker, (1963) namely Precambrian metamorphic rocks, Plio- Pleistocene volcanics, the Holocene to Recent Lake and fluvial sediments. The basement rocks consist mainly of regular banded schist, gneisses and muscovite-rich quartzite. The oldest rocks in the area are the quartzite, gneisses and schist of the basement formation which is of Archaean age. In the southern and northern ends of the Lake Magadi area, there is a deposition of irregular interbeddedchert rocks which consists of silicified bedded clays on top of Alkali trachytes (Atmaoni and Hollnack, 2003,;Sequar, 2009). Fig. 1 Location of Magadi Temperature measurements at 1 m depth are inexpensive and quick, and can be used to detect anomalously hot areas.the results obtained from the shallow temperature surveys could guide whether undertaking intermediate-depth, thermal gradient surveys are warranted. The distance between boreholes used for heat flow investigations depends upon the size of the subsurface heat source. Magmatic intrusions, which are usually the sources for economic geothermal fields, cause geothermal disturbances of at least 1 2 km lateral extent. Surface exploration including magnetic, gravity and seismic have been conducted in Magadi prospect singling out the northern part of Magadi as the heat source of geothermal energy in the prospect. It is expected that shallow hole heat flow measurements can help in ascertaining the exact place of the heat source hence reducing chances of drilling wells that are not productive.

Int. J. Cur. Tr. Res (2014) 3 (2): 1-7 Richard et al. 3 Fig.

5 Heat flow contour map overlaid on the seismic map The broader Magadi area is largely

The trachyte lava overlies Pliocene olivine basalts and nephelinites, which, in turn rest

These faults, especially the north-south trending fault scarps, control the occurrence of

3 Int. J. Cur. Tr. Res (2014) 3 (2): 1-7 Richard et al. 3 Fig. 2 Flow of heat from T2 to T1 Fig. 5 Heat flow contour map overlaid on the seismic map The broader Magadi area is largely covered by Holocene sediments that overlie extensive Pleistocene trachyte lavas. The trachyte lava overlies Pliocene olivine basalts and nephelinites, which, in turn rest on the Archean basement. A dense network of grid faults affects the area. These faults, especially the north-south trending fault scarps, control the occurrence of geothermal manifestations (Riaroh and Okoth, 1994). Fig. 3 The contour map for temperature gradient Fig. 6 Heat flow contour map overlaid on the magnetic contour map Fig. 4: A contour map for the heat flux of Magadi

4 4 Richard et al. Int. J. Cur. Tr. Res (2014) 3 (2):1-7 The lake is recharged by saline hot springs (between 26 o C and 86 o C) along the lake margins; most of them lie along the north-western and southern shorelines of the lake (Komolafe et al., 2012). Heat flux calculation Estimation of energy loss was done by obtaining temperature gradient from shallow holes drilled manually using 1 inch diameter by 1.5 m long metal rod and an auger drill. The spacing was 1km between stations. A hammer was used to hit the metal rod to attain the desired depth of the hole. Temperatures were taken at 20cm, 50cm and 70cm depth using the digital laser thermometer. Most of the holes made attained a depth of 50cm. Locations of these holes were read from a portable hand held Global Positioning System (GPS) and recorded in a table. The station coordinates, were mapped out using Quantum Geographical Information System (Q-GIS). Heat flows from a hot object (one at higher temperature) to a colder one (lower temperature) according to Fourier s Law of Conduction =.. (1) Where = h, k is thermal conductivity, A is area, temperature gradient. = = (2) Where q is heat flux, flow of heat per unit time and unit area, and k is thermal conductivity. Materials with k large conduct well - low k materials insulate The thermal conductivity varies with different types of rocks ( Table 1). It is lower in sedimentary rocks (sandstone, shale) than in crystalline rocks (granite, peridotite). This explains the existence of geothermal systems in the deep sedimentary basins (non - convective geothermal systems). The sediments act like insulating blankets and the heat from the interior of the earth is accumulated at the basement below the sediments (Hersir and Bjornsson, 1991). = (3) (Figure 2) = (4) The negative sign indicates that heat flows from the hotter region to the cold region. = (5) Where: T 2 is temperature at depth T 1 is temperature at the surface T is change in temperature L is depth of hole For porous, water-saturated rocks there exists an approximate relationship between in situ ( ), intrinsic or bulk ( ) and water ( ) thermal conductivity and porosity ø (Hersir and Bjornsson, 1991)..( ) ( ) It follows that for low porosity, ø 10% - 20%, the water content only plays a minor role in situ thermal conductivity (Hersir and Bjornsson, 1991). If time variations in T cannot be ignored, equation 6 is not valid and has to be replaced by the diffusion equation for heat production. The temperature distribution in the earth can be described by the heat conduction equation (no convection): Where: + = (7), A The radiogenic heat production in the crust [W/m 3 ] Density [kg/m 3 ] C Heat capacity [J/kg o C] - Thermal diffusivity [m 2 /s] For a one-dimensional stationary state problem ( = 0 ) with constant thermal conductivity and heat production, the solution of equation 7 becomes: = (8) Where T 0 and Q 0 are the temperature and heat flow, respectively, at the earth s surface (z=0). In case of no crustal heat production (A=0) we have: T = 0 and = +., (9) Where T 0 is the annual mean temperature and g the temperature gradient. The temperature gradient was calculated by dividing the change in temperature by depth of hole drilled.

5 Int. J. Cur. Tr. Res (2014) 3 (2): 1-7 Richard et al. 5 A temperature gradient contour map was generated using surfer software.the heat conduction equation (equation (1)) was used to compute the heat flow. The heat flux data was contoured using Golden Software Surfer 8 and the anomalous regions noted. The contoured map was compared and overlaid to Magnetic data generated by (Githiriet al., 2011). The heat flux map was also overlaid to seismic mapped generated by (Ibs -Von Sehtet al., 2001). (Ibs -Von Sehtet al., 2001) noted that maximum focal depths in the northern part were generally shallower than on the southern part. The main reason of selecting this particular study area for the heat flow analysis is the indications strongly suggested by previous research that the heat source of geothermal is in the northern part of little Magadi. Earthquake swarms have commonly been found to be associated with volcanic regions, and their occurrence has been related to the movement of magma (Ibs-Von Sehtet al., 2001). A seismotectonic and crustal structure study by (Ibs- Von Sehtet al., 2001) revealed an earthquake cluster north of Lake Magadi, beside little Magadi; more than 75 per cent of the observed events were on the north of Lake Magadi. Results Temperature readings from shallow holes were recorded in a field work book. Thermal contours were used to demarcate the temperature range areas. Due to the rocky nature of the Magadi prospect, the thermal holes couldn t go as deep as 1 meter using either the auger drill or the metal rod. The temperature gradient was calculated at a depth of 0.5 meters.the thermal conductivity of the rock was taken to be 2W/m o C, and heat flux contour map generated (Figure 4). This contour map was overlaid to the seismic map and clearly showed that the heat source of the geothermal energy is to the north-east of the prospect where there are more epicentres.the heat flux map also correlated with the magnetic contour maps generated by (Githiriet al., 2011). The heat flux map when overlaid on the anomalous areas (coordinates; , ; , ; , ), fitted well with two ( , ; , ) of the anomalous regions also appearing in the heat flux contour map.this is an indication that, the source of the heat can be originating from this side of the lake. A closer look at the heat flow contour map reveals that the heat is concentrated on the northern part of the lake also. Table - 1 Thermal conductivity of various rocks at room temperature(hersir and Bjornsson, 1991) Thermal Rock type conductivity (W/m o C) Dolomite salt 5.0 Peridotite/ pyroxenite Granite Limestone Gabbro/basalt Sandstone Volcanic tuffs(depending on porosity) Shales (depending on water content) Deep-sea sediments (depending on water content) Water 0.6 Discussion The temperature from this study (28 o C 47 o C) is low compared to temperatures measured in other geothermal prospects such as Paka (26.6 o C 93 o C) and Korosi-Chepchuk (27 o C 97 o C) on the northern side of Lake Magadi (Mwawasi, 2011). The temperature gradient on average was between ( o C/m 20 o C/m) which is below that of Paka at: 17 o C/m 23 o C/m (Mwawasi, 2011). Low geothermal gradients may indicate the effects of ground water circulation. Circulating ground water acts as a heat transfer system and thus increases the effective heat transfer which in turn decreases the geothermal gradient. The phenomenon is more pronounced in both zones of higher thermal conductivity and zones of greater fracturing, since the transfer of heat is more efficient. (Reiter, 2007). This is also a clear indication that the heat flow result from Magadi area is slightly lower than that of Paka and Korosi-Chepchuk. Despite the temperature gradient being low as compared to other areas, the temperature gradient contour map (Figure 3) indicates areas of anomalies (the northeastern side of the study area) with high temperature areas being indicated by fading red. This is also replicate in the heat flow map. Ibs-Von Sehtet al.,(2001) concluded that; the epicenter distribution of the cluster events clearly follows the strike direction of the grid faults. The focal depths determined are

6 6 Richard et al. Int. J. Cur. Tr. Res (2014) 3 (2):1-7 mainly shallow; therefore it is most likely that the observed swarm activity is an expression of present day seismic activity in the grid fault system. He continues to say, the swarm activity may be connected to the grid fault system. Frequency-magnitue relations of both the background and the swarm activity results in b-values of approximately 0.75, thus indicating a strong crust and a tectonic origin for the earthquakes. The overlaid map (Figure 5) clearly indicates that the heat anomaly is on the Northeastern part of the study area which correlates with the result gotten by (Ibs-Von Sehtet al., 2001). Magnetic profiles: AA BB and FF, generated by (Githiriet al., 2012) cut across the study area ( Figure 6). The curie point depth determined by (Githiriet al., 2012) along profiles AA, BB and FF were 7.5 km, 7.04 km and 6.08 km, he therefore concluded that; the curie point depths were considered shallow with the shallowest depths on profile AA and FF. This is in line with the heat flow contour map generated. (Githiriet al., 2011) concluded that; The profile anomaly AA in particular indicates sedimentary depth of about 11 km and the reduction to the pole cross-section is characteristic of a thick dyke. The dyke may be a possible heat source causing a thermal anomaly in the area surrounding Lake Magadi and such sedimentary basins may have been filled with sediments to the present time. Such a dyke is suspected to originate from a magma chamber conducting heat to the underground water. A model, whereby the faults in the region provide escape of water as hot springs, is proposed. The overlaid map (Figure-6) correlates with the magnetic map generated by (Githiriet al., 2011). With profiles AA and FF cutting across the temperature anomaly. Conclusion From the heat flow contour map generated, it can be concluded that there are strong indications that the heat source for the geothermal energy is likely to be on the northeastern side of the study area. This means that shallow temperature gradient holes are reliable in mapping geothermal prospect. It is highly recommended that: deep wells be drilled on the prospect considering, the equipments used in this particular research could not drill deep wells. This will ensure climatic changes do not affect the results. Acknowledgement We acknowledge the TATA Chemicals Magadi Company management for giving us the required data and assistance we needed when collecting this information. Also we cannot forget the company s staff for their assistance. We also acknowledge the support given by the Ministry of Energy staff particularly the Chief Geologist Mr. John Omenge and Mr. Barrack Ouma for allowing me to use their equipment during my research and guiding me to understand the heat flow analysis. References Atmaoui N. and D. Hollnack (2003).Neotectonics and extension direction of the Southern Kenya Rift, Lake Magadi area: Tectonophy Baker B.H (1963). Geology of the area south of Magadi. Report Geological survey of Kenya 61. The Government printer, Nairobi. Githiri J.G., J.P. Patel, J.O. Barongo and P.K. Karanja, (2011). Application of Euler Deconvolution Technique in Determining Depths to Magnetic Structures in Magadi Area, Southern Kenya Rift. JAGST, 13(1). Githiri J.G., J.P. Patel, J.O. Barongo and P.K. Karanja, (2012). Spectral analysis of ground magnetic data in Magadi area, southern Kenya rift. Tanz. J. Sci. 38 (1). Gupta, H., and S. Roy, (2007) Geothermal Energy: An alternative resources for the 21 st century. Elsevier, the Netherlands. Henrikson A. and D. S. Chapman (2002). Terrestrial Heat Flow in Utah, University of Utah, Department of Geology and Geophysics, Salt Lake City, Utah March. Hersir G. P. and A. Bjornsson, (1991). Geophysical Exploration for Geothermal resources, principles and application, UNU Geothemal Training Programme, Reykjavik, Iceland. Report 15. Ibs-Von Seht M., S. Blumenstein, R. Wagner, D. Hollnack and J. Wohlenberg, (2001). Seismicity, Seismotectonics and Crustal structure of the southern Kenya Rift-new data from Lake Magadi area. Geophys. J. Int. 146: Komolafe A. A., Z. N.Kuria, T.Woldai, M. Noomen, and A. Y. B. Anifowose (2012). Integrated Remote sensing and Geophysical Investigations of the Geodynamic Activities at Lake Magadi, Southern Kenyan Rift. Hindawi Publishing Coporation, Intern. J.Geophy. McNitt J. R., C. W. Klein, and J. B. Koenig (1989). Probable Subsurface Temperature at lake Magadi, Kenya, As Indicated by Hot Springs Geochemistry, and Potential for development of Geothermal Electric power. Geothermal Ex inc., Richmond, California, USA. November Mwawasi H. M. (2011). Geothermal Mapping Using Shallow Holes Temperature Measurements: A Case

7 Int. J. Cur. Tr. Res (2014) 3 (2): 1-7 Richard et al. 7 Study of Korosi, Chepchuk and Paka. In: Proceedings, Kenya Geothermal Conference (2011). Kenyatta International Conference Center, Nairobi, November 21-22, 2011 Riaroh D. and W. Okoth (1994). The geothermal fields of the Kenya Rift. Tectonophy. 236: Reiter, M. A. (2007). Terrestrial heat flow and thermal conductivity in southwestern Virginia. A thesis submitted to the graduate faculty of the Virginia polytechnic institute. Sequar G.W. (2009) Neotectonics of the East African rift system : new interpretations from conjunctive analysis of field and remotely sensed datasets in the lake Magadi area, Kenya [Msc thesis]: Enschede, ITC Smith M., and P. Mosley (1993). Crustal Heterogeneity and Basement Influence on the Development of the Kenya Rift, East Africa: Tectonics. 12.

SPECTRAL ANALYSIS OF GROUND MAGNETIC DATA IN MAGADI AREA, SOUTHERN KENYA RIFT

SPECTRAL ANALYSIS OF GROUND MAGNETIC DATA IN MAGADI AREA, SOUTHERN KENYA RIFT 1 JG Githiri, 2 JP Patel, 3 JO Barongo and 4 PK Karanja 1 Jomo-Kenyatta University of Agriculture, Science and Technology,