An electrical resistivity study of the area between Mt. Suswa and The Olkaria Geothermal field, Kenya

Abstract

The D. C. electrical resistivity study of Suswa-Olkaria region was carried out from July 1987 to November, 1988. The main objective was to evaluate the sub-surface geoelectric structure with a view to determining layers that might be associated with geothermal fluid migrations. The study also aimed at delineating the southern extent of the Olkaria geothermal field and of the structural discontinuities. The study was carried out using the Symmetrical Schlumberger array with a maximumcurrent electrode spacing of 8000m. The resistivity data analysis was carried out by curve matching and fitting, and 1-D iterative computer modelling. Geoelectrical structural models on line profiles have identified horst-graben structures bound by normal to oblique N-S and oblique arcuate E-W trending discontinuities. Some of these discontinuities coincide with fault zones which are permeable zones of complex fluid migration. Geophysical and Geological data synthesi~ has identified four geoelectrical units with considerable variation in thickness. These are the overburden, a resistive cap rock, a conductive unit (3-15 Om) and an electrical basement (50-2000m) . The resistivity of the conductive unit in the southern part of Olkaria is similar to that of Olkaria West. The conductive unit correlates with tertiary pyroclastics rintercalated with tuffs rhyolite and trachytes while the electrical "basement" correlates with Miocene Volcanics. From this study, it is noted that the graben structures with a sub-surface conductive unit which is covered with a resistive "cap" rock offer good prospects for further geothermal exploration. The conductive unit has been attributed to a permeable layer that is part of a convective hydrothermal cell. The conductive unit continues southwards through the present area of study.