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 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 forming numerous N-S ridges and fault scarps.
Gravity and magnetotelluric (MT) surveys were undertaken in the basin in an attempt 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. Seismic velocities were converted to densities using the expression derived from Gardner et al. (1974). The MT survey results show that the geothermal prospect in Lake Bogoria basin is overlain by high resistivity (50-1000 -m) thin (100-500 m) layer which forms a cap rock for the geothermal reservoir, which is subsequently underlain by three distinct relatively thick layers within the geothermal prospect. The first of these thick 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 circulating hot mineralized geothermal fluids. The underlying 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 rocks. The relatively high degree of fracturing allows deep circulation of water where it gets heated up by the underlying dyke injections, and 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 which are the heat sources for this geothermal prospect.
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 for the heat source. The geothermal reservoir is probably two-phase and the temperature ranges between 150-400oC (Karingithi, 2006). Previous microseismic studies by Young et al. (1991), Tongue (1992) and Tongue et al., (1992, 1994) show that Lake Bogoria basin geothermal prospect is characterised by high frequency of low magnitude (< 3) seismic events which are correlated with surface faulting and multiple episodes of dyke injections. The gravity and MT results in this study are therefore fairly consistent with results from previous microseismic studies undertaken in the basin.