Resistivity Structure of the North West Olkaria Field Using ID Inversion of Magnetotelluric Data

The Northwest Olkaria prospect is part of the Greater Olkaria Geothermal Area located within the Kenyan rift in Nakuru County. It is bounded by the longitudes 36 0 15 E and 36 0 12 E with an estimated area of 30 Km2. The area has been the least studied sector of the Greater Olkaria Geothermal Area due to its remote location and complex terrain. The study used the Magnetotelluric method to determine the electrical resistivity distribution of the subsurface and locate structural features controlling the geothermal distribution in the prospect area. A total of 52 Magnetotelluric soundings were considered in this research project and 47 corresponding central loop Transient Electromagnetic soundings. To allow the static shift correction in the 1D inversion, the Magnetotelluric data were jointly inverted with Transient Electromagnetic data. The results and interpretations of the Joint 1D inversion of Magnetotelluric and central loop Transient Electromagnetic data of Olkaria Northwest prospect were presented inform of resistivity Iso-maps and cross sections. These inversions were achieved by fitting both data kinds using the same 1D resistivity model. The result of the interpretation shows three main resistivity structures below the prospect area: (1) A shallow superficial lying thin high resistivity layer (greater than 30m) thought to be due to unaltered rock formations and the thick pyroclastic cover is observed across the prospect followed by low resistivity thick dome shaped conductive layer (less than 6m) that extends to about 200 m. a. s. l in the North West field and thins towards the Central field becoming almost uniform in the South East and the rest of the field. This layer is presumed to be dominated by low temperature alteration minerals such as smectite and zeolite that defines the clay cap. (2) A high resistivity layer (resistive core) underlain by a deep lying conductor ( (greater than15-50 m) which is thought to be associated with the high temperature secondary minerals such as epidote, chlorite and biotite that are present in the reservoir. (3) A low conductivity layer that extends to about 6, 000 m. b. s. l. which is associated with the heat source. Among the many recommendations highlighted, the study revealed that the main reservoir to the western side appears to be deeper than the known situation in the eastern field, at great depths, a prominent low resistive cap is observed and it is probably reflecting the presence of heat source of the geothermal system underneath the Northwest Olkaria prospect.