Magnetotelluric and transient electromagnetic methods in geothermal prospecting, with examples from Menengai, Kenya

Abstract

Electromagnetic (EM) methods are used to determine variations in electrical conductivity of the sub-surface, both laterally and with depth. Included in these techniques are natural-field methods (magnetotelluric and audiomagnetotellurics) and controlled-source induction methods in both the frequency and time domain. EM methods are more sensitive to conductive (low-resistivity) structures compared to direct current (DC) techniques. This advantage, together with the ability of the magnetotelluric technique to probe several tens of kilometres into the sub-surface, has made these methods very useful in developing a resistivity model for Olkaria geothermal field and also in defining possible geothermal regimes in Olkaria Domes, Longonot and Menengai geothermal prospects that are currently targeted for development. The Menengai prospect is used as an example in this report. The Menengai geothermal system as defined by resistivity structures seems to exceed 150 km2 at 1300 m a.s.l. A low-resistivity cap underlain by a high-resistivity core delineates the geothermal system, showing boundaries in all directions except northwest. MT resistivity cross-sections show low resistivity at deeper levels that could be related to heat sources. To define the field better more TEM and MT soundings need to be done west and north of the prospect.