MAGNETOTELLURIC ANALYSIS OF THE OLKARIA AND EBURRU GEOTHERMAL FIELDS -KENYAN RIFT, EAST AFRICAN RIFT SYSTEM

The Olkaria and Eburru geothermal fields are within a chain of volcanoes occurring on the floor of Kenyan rift segment of the eastern branch of the East African rift system. These Quaternary-aged volcanoes are characterized by an active eruption history of commenditic rhyolites for Olkaria field and pantellelites for Eburru field. The Olkaria geothermal field has been generating power since 1981, while the Eburru geothermal field has been operational since 2012. Their current capacity 583 Mwe and 2.5 Mwe respectively. To increase capacity for more steam production from the fields, exploration has been recently undertaken to establish new drilling sites. In aid in determining the drilling sites, new broadband magnetotelluric (MT) data has been acquired, and to date there are more than 100 soundings to image the subsurface resistivity structure under both volcanoes using 2D and 3D inversion techniques. The inversion models will be investigating the depth and morphology of magma intrusions powering the geothermal systems, regions with hydrothermally altered volcanic rocks, and the location of cap rocks overlying the geothermal system. Our goa in conjunction with gravity modeling is to understand of the plumbing system so as to optimize production by siting additional wells as well as guiding the management of the reservoirs over time. The MT data were on average over an 18 hour period and were processed into impedance tensors using robust Fourier transform methods. The electrical strike direction and dimensionality of individual soundings were estimated in order to determine the electrical structure of the volcanic regions and to rotate the individual stations for optimization 2D inversions. The known geothermal fields at both volcanoes are highly fractured and the results from the resistivity will give insight to fluid flow including the recharge and outflow zones for the which play a vital role in the life of a geothermal reservoir. These volcanoes are in close proximity, hence we look also to compare their resistivity signature to infer on the present and past thermal regime.