Geophysical survey of a high-temperature field, Olkaria

Abstract

Olkaria Geothermal field is a high temperature geothermal resource in the Kenyan Rift Valley which has been used for electricity generation since 1981. Geophysical exploration for the resource during the early stages of development included dipole, Schlumberger, electromagnetic, head on, gravity, seismic and magnetics and various levels of success were achieved. It was noted that whereas resistivity was the most important in identifying the reservoirs, depth of penetration was low for dipole and Schlumberger while interpretation of head-on data was ambiguous.Moderm geophysical methods such as Magnetotelluric (MT) and Transient Electromagnetic (TEM) were used and great success has been achieved.

This paper presents the results of Olkaria Geothermal field geophysical data analysis and interpretation. MT and TEM data is continually collected in the area for the improvement of the geophysical model and also determine the extent of the resource. From the results of the joint inversion of MT and TEM data of the area, a fairly good correlation with the available geological information is noted and the following resistivity structure can be deduced as seen on cross-sections and isoresistivity maps. Generally, the area is characterised by a thin shallow layer of high resistivity on the surface especially on higher grounds. This is interpreted as being caused by unaltered rock formations on the surface possibly due to the thick pyroclastic cover from the adjacent Longonot volcano. Underlying this layer is a low resistivity (<15Ωm) layer that extend to approximately 1000 m a.s.l. This layer is presumed to be dominated by low temperature alteration minerals such as smectite and zeolite and defines the clay cap. A deep high resistivity (resistivity core) layer with values greater than 100Ω m is observed underlying the clay cap. This is a zone where high temperature hydrothermal alteration minerals such as epidote, chlorite and actinolite are observed and is interpreted as the reservoir zone. The existence of a high resistivity core indicates reservoir temperatures exceeding 250°C, which has been confirmed by the drilled wells and this zone is probably dominated by pore fluid conduction. Further information has been achieved by combining MT, seismics and gravity in regard to the heat sources.