Reinterpretation of Resistivity Data Over the Olkaria Geothermal Prospect (Gregory Rift, Kenya)

Abstract

The apparent resistivity pattern of the Olkaria Field is not as well defined as that of other hot water systems standing in volcanic rocks. Attempts to delineate the lateral extent of the reservoir by resistivity measurements (using arrays with a nominal depth penetration of 750 to produced the confusing picture that several smaller areas with apparent resistivities of occur over an area of about these structures were interpreted as "upf low centres" with the present bore field lying over one of these centres. Until recently A recent re-interpretation of the resistivity data has produced a significantly different resistivity model. Analysis of older Schlumberger sounding an unbroken layer of lake sediments (5 - underlies most of the prospect and separat a non-coherent sequence of surface volcanics (50 - from older volcanics forming the upper reservoir. A t greater depth the whole Olkaria Field is underlain by a coherent low structure. The new model shows t h at the Olkaria system covers a much greater area than previously thought. The Olkaria Field is an example of a hot water system in volcanic rocks which does not exhibit very low resistivities usually found elsewhere over systems in a similarhydrological- geological setting.