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.