Subsurface structures and characterization of the silali geothermal system, kenya rift.

Abstract

Electrical resistivity methods are widely applied in geothermal exploration and are the cheapest means of acquiring subsurface data. Further detailed surveys like exploratory drilling which is cost intensive in an area under study is always based on accurate surface exploration results. Several exploration methods are applicable in geophysical prospecting of geothermal resources; however, this study focuses mainly on application of electromagnetic methods namely Transient Electromagnetic (TEM) and Magnetotelluric (MT). During the detailed exploration surveys of Silali geothermal prospect situated in the floor of the Northern Kenya rift, MT and TEM methods were applied. The survey was to map out the subsurface resistivity, which is then interpreted so as to provide information such as fluid filled fractures, the reservoir and the heat source. Joint one dimensional (1-D) inversions of MT and TEM data were done so as to correct for static shift in MT soundings. Results of the joint 1-D inversion of MT and TEM data revealed four main resistivity zones; A shallow high resistivity zone (> 100 Ωm) to about 300 m below the surface which is as a result of unaltered rocks, 2) An intermediate low resistivity zone (10 Ωm) to depths of about 1 km which is as a result of low temperature hydrothermal alteration minerals, 3) A deeper high resistivity (> 50 Ωm), up to 3-4 km depth indicating high temperature minerals occurring at depth. The shallow boundary between an upper, low resistive layer and the underlying intermediate resistivity zone at depths of 800-3500 m appears to mark the change in clay mineralogy from low grade alteration mineralogy represented by smectite to high grade alteration mineralogy represented by chlorite, epidote and actinolite, which is interpreted as the geothermal reservoir zone of this study area, and 4) a deeper low resistivity region, at a depth of about 5000 m below sea level is inferred to be magmatic material or intrusion i.e., the heat source of this study area. The resultant 2-D resistivity models showed significant variations in the resistivity distribution both vertically and horizontally on all profiles indicating major structural controls of the geothermal system. This study concludes that a geothermal resource exists in Silali caldera and the nearly vertical conductors in the resistive zones are fluid filled fracture systems/faults and are best targets for exploratory drilling. It is, therefore, recommended that deep exploration wells be drilled within the caldera floor and outside the caldera to the east to further confirm the nature and potential of the resource at Silali.