MAGNETOTELLURIC ANALYSIS OF THE OLKARIA GEOTHERMAL SYSTEM, KENYA

Olkaria volcano is the biggest producing geothermal field in Africa with current installed capacity of 687.5 MWe. It is a young Quaternary volcano located on the central region of the Kenya portion of the East African rift system. The unclear outline of its caldera makes it a case study to try understand the volcanic evolution history such as magma chambers morphology and associated geothermal systems. Surface structural features such as arcuate series of domes and major faults intersecting them is assumed to be the rim of a collapsed caldera. Recent volcanic activity, less than 200 years ago indicate a young active system and a major resource for power generation. The total potential energy capacity for the field is estimated to be six times present power generation hence additional continuous geo-scientific studies to understand the reservoir system is vital in expanding production. We aim to use the results from the analysis of magnetotelluric (MT) data and transient electromagnetic data to image the subsurface structure of the caldera. Analysis of 186 MT data indicates a predominant 3D dimensionality of long period data and the regional electrical strike analysis indicates a regional NE orientation. From our results it is evident that shallow magmatic intrusions power the reservoir systems located between 1 to 3.5 km deep as depicted by high electrical resistivity values determined by 2D inverse models. Low electrical resistivity values at approximately 0.7 km is interpreted as the cap over the geothermal system and its thickness ranges at different locations. Magma intrusions, laccoliths alignment at depth seems to have a surface imprint of a stress field with minor fault mostly NE orientation.