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Mulwa JK. Integrated geophysical study of Lake Bogoria basin, Kenya: Implications for geothermal energy prospecting. Mulwa JK, of Prof. Justus Barongo(University of Nairobi DG), of Prof. Jayanti Patel(University of Nairobi DP), Prof. Derek Fairhead(Leeds University and MD GETECH), of and Prof. Greg Houseman(Leeds University IGT), Dr Nicholas Mariita(Kenya Electricity Generating Company OGP), eds. Nairobi/Leeds: University of Nairobi/Leeds University; 2011. Abstract

The Lake Bogoria basin, herein referred to as ‘the study area’, is located in the greater Baringo-Bogoria basin (BBB), about 250 km from the city of Nairobi on the floor of Kenya Rift Valley (KRV). It is bound by latitudes 0o 00’ and 0o 30’N and longitudes 35o45’E and 36o15’E within the rift graben. The study area is characterised by geothermal surface manifestations which include hot springs, spouting geysers, fumaroles/steam jets and mud pools. The area is overlain by Miocene lavas mainly basalts and phonolites, and Pliocene to Recent sediments and pyroclastics such as tuffs, tuffaceous sediments, superficial deposits, volcanic soils, alluvium and lacustrine silts. The terrain is characterized by extensive faulting which forms numerous N-S ridges and fault scarps.

Gravity and magnetotelluric (MT) surveys were undertaken in the area in order to determine the heat source and evaluate the geothermal resource potential of the basin for generation of geothermal power. The gravity data used was from the University of Texas at El Paso and Leicester University gravity data bases. New gravity measurements’ comprising 260 data points was undertaken for the purpose of this study. In addition, magnetotelluric data comprising about fourty sites was also acquired in the study area.

Gravity survey results indicate Bouguer anomaly having an amplitude of ~40 mGals aligned in a north-south direction and this has been interpreted to be due to a series of dyke injections and hence the heat source in the basin. The dyke injections occur at depths of 3-6 km on average, but at 1 km depth at the shallowest. The gravity models show a north-south gradual variation in thickness of the uppermost low density layer comprising rift-fill volcanics from 1-4 km on average. The variation in thickness of this layer from south-north suggests that these volcanic deposits are as a result of volcanic eruption(s) outside Lake Bogoria basin such as Menengai to the south.

The MT survey results show three distinct relatively thick layers in the basin. The first of these layers, which is overlain by high resistivity (50-1000 m) thin (100-500 m) layers, is ~3 km thick and has resistivities ranging between 4-30 -m. This layer is interpreted as the geothermal reservoir and the low resistivities are due to a combination of circulating hot mineralized geothermal fluids, hydrothermal alteration and saline sediments. The second layer is ~10 km thick and resistivity values range between 85-2500 -m and is interpreted to be a fractured and hydrothermally altered basement metamorphic rock. The relatively high degree of fracturing has considerably enhanced circulation of water which gets heated by the underlying dyke injections and thus inducing convective heat transport to the geothermal reservoir. The substratum is characterized by resistivities ranging between 0.5-47 -m and is interpreted as hot dyke injections at depths of about 6-12 km, which are the heat sources for the geothermal system.

Consequently, a heat source and a geothermal reservoir exist in Lake Bogoria basin. The heat source(s) is/are due to cooling dyke injections occurring at depths of 3-6 km on average, but 1 km depth at the shallowest near Arus where steam jets and fumaroles occur. The magnetotelluric method, however, favours depths of 6-12 km for the heat source and this may be attributed to lack of significant resistivity contrast between the dyke injections and the basement rocks where the former have intruded the latter rocks.
More gravity data is warranted so as to precisely define the geometry and areal extent of the heat source in Lake Bogoria basin. However, based on the results of this study, it is recommended that:- 1) exploratory drilling be undertaken in the area near Arus steam jets, 2) even though the study area is not prone to any pre-historic eruptions, microgravity and seismic monitoring be undertaken so as to help in tracking possible magma migration and variations in magma input. Such data could, in turn, play an important role in predicting future eruptive events in Lake Bogoria basin.

Mulwa J, Barongo J, Fairhead D, Mariita N, Patel J. "Integrated Geophysical Study of Lake Bogoria Basin, Kenya: Implications for Geothermal Energy Prospecting.". In: Proceedings: World Geothermal Congress 2010. Bali, Indonesia: World Geothermal Congress; 2010. Abstract

The Lake Bogoria basin, here in referred to as the study area, is located in the greater Baringo-Bogoria basin (BBB), about 100 km to the north of Menengai geothermal prospect on the floor of Kenya Rift Valley (KRV). It is bound by latitudes 0o 00’ and 0o 30’N and longitudes 35o45’E and 36o15’E within the rift graben. The study area is characterized by geothermal surface manifestations which include hot springs, spouting geysers, fumaroles/steam jets and mud pools. The area is overlain by Miocene lavas lavas, mainly basalts and phonolites, and Pliocene to recent sediments and pyroclastics such as tuffs, tuffaceous sediments, superficial deposits, volcanic soils, alluvium and lacustrine silts. The terrain is characterized by extensive faulting forming numerous N-S ridges and fault scarps.
Gravity and magnetotelluric (MT) surveys were undertaken in the area to determine the heat source, characterize the geothermal reservoir, and evaluate the geothermal resource potential of the basin.
Gravity survey results indicate Bouguer anomaly having an amplitude of ~40 mGals aligned in a north-South direction and interpreted to be due to a series of dyke injections and hence the heat source in the basin. The interpretation of Bouguer anomaly has been constrained by using previous seismic results. The MT survey results show three distinct layers in the basin. The first layer, overlain by high resistivity thin layers, is ~3 km thick and has resistivities ranging between 4-30 -m. This layer is interpreted to be due to a combination of saline sediments and circulation of high temperature geothermal fluids. The second layer is ~10 km thick and resistivity values range between 85-2500 -m. This layer is interpreted to be fractured basement metamorphic rocks hosting a steam reservoir where circulating fluids are heated by underlying dyke injections. The substratum is characterized by resistivities ranging between 0.5-47 -m and is interpreted as hot dyke injections which are the heat sources for this geothermal prospect. The magnetotelluric results in this study are consistent with results of previous microseismic study in Lake Bogoria basin by Young et al. (1991).
On the basis of gravity and MT results, the heat source in Lake Bogoria basin is due to cooling dyke injections occurring at depths of ~6 – 12 km in the subsurface. Gravity method however favours depths of ~3 – 6 km. The geothermal reservoir is probably two-phase and is attributed to condensation of high temperature steam from the underlying fractured basement metamorphic rocks.

Mwega BW, Mati B, Mulwa JK, Kituu GM. "Identification of groundwater potential zones using remote sensing and GIS in Lake Chala watershed, Kenya.". In: Mechanical Engineering Annual Conference on sustainable research and innovation. Jomo Kenyatta University of Agriculture and Technology, Thika, Kenya; 2013. Abstract

Groundwater is a natural resource of the earth that sustains and supports domestic, agricultural and industrial activities. It is distributed fairly and evenly throughout the world and over half of the world’s population depends on groundwater for drinking water supplies. Its usage is increasing due to rapid population growth, high rate of urbanization, industrial growth and agricultural utilizations. This has resulted to rapid depletion of groundwater which leads to water stress and degradation of these resources. The situation is further worsened by inadequate information on groundwater resource which has been and is still a big obstacle to the proper management of these resources. Remote sensing and GIS techniques have emerged as very effective and reliable tools in the assessment, monitoring and conservation of groundwater resources. This paper has made an attempt to identify and delineate groundwater potential zones in Lake Chala Basin in Kenya using Remote sensing and GIS. In the process of groundwater delineation in the area, different thematic maps on lithology, land use/land cover, drainage density, slope and rainfall were prepared, assigned with different weighting values as per their importance on groundwater occurrence and overlaid using spatial analyst tool in ArcGis 10 to generate groundwater potential map. The generated groundwater potential zone map was classified into four groundwater potential zones namely, very high, high, moderate and low. The study revealed that the area has very high groundwater potential. The generated groundwater potential map will be used for further groundwater exploration, proper planning, sustainable utilization and management of groundwater resources in the Lake Chala Watershed.

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