An electrical resistivity survey in the area south of Suswa volcano

Abstract

The area south of Suswa volcano is underlain by volcanic and volcano-sedimentary rocks of the Pliocene-Pleistocene-Recent Periods. These rock types are common waterbearing formations, especially the wea~hered, fractured or transition zones along faults and at interfaces of the various volcanic flows. Consequently, any groundwater investigations are usually based on methods aimed at detecting such underground structures. Electrical resistivity studies conducted in the area have indicated low resistivities which appear to be associated with geothermal fluids migrating from the Suswa volcano region. Hot grounds and fumaroles within the perimeter of Suswa caldera indicate that . the geothermal system is likely to be directly beneath Suswa volcano. The resistivities are further rioted to increase southwards. The low resistivities are attributed to high fluid temperatures and/or conductive minerals within the water-bearing units which are either weathered or fractured. The actual temperatures of these reservoirs are between 700 and 1000 C which are considerably below those expected for high temperature geothermal systems. High temperature geothermal areas normally border low temperature geothermal areas and the present area of study borders the more productive high temperature Olkaria geothermal fields. The analysis of resistivity data was first done by partial curve matching prior to the use of linear inverse theory in the determination of the most probable physical parameters of the subsurface layers necessary for two-dimensional interpretation of the underlying geology. From the results, three distinctive layers overlying a resistive half-space were recognized in terms of their resistivities and thicknesses. The main conductive layer, which is one of those three layers" immediately overlies the resistive half-space