Joint 1-D Inversion of TEM and MT Data from Olkaria Domes Geothermal Area, Kenya

Abstract

Joint one-dimensional (1-D) inversion of magnetotelluric (MT) and central loop transient electromagnetic (TEM) data was done by fitting both data kinds using the same 1-D resistivity model. It is well known that in the presence of small-scale surface or near-surface resistivity inhomogeneities the magnetotelluric (MT) apparent resistivity can be shifted by a multiplicative factor which is independent of frequency. In this regard TEM has been used to correct for the static shift factor to restore the MT curve where it should have been without the effect of shift for this project. The 1-D joint inversion results reveals three main resistivity zones, a shallow high resistivity zone (over 100 m) to about 300 metres below the surface, an intermediate low resistivity zone (10 m) to depths of about 1 kilometre and a deeper high resistivity (over 50 m), up to 3-4 kilometres depth. Below the high resistivity zone a relatively low resistivity zone at depth is evident possibly indicating a high temperature which is a likely source of crustal fluids for this field. A good correlation is found between the resistivity structure, hydrothermal alteration and reservoir temperatures. The low resistivity is dominated by conductive minerals in the smectite-zeolite zone at temperatures of 100-200°C. In the temperature range of 200-240 °C zeolites disappear and smectite is gradually replaced by resistive chlorite. At temperatures exceeding 250 °C chlorite and epidote are the dominant minerals and the resistivity is probably dominated by the pore fluid conduction in the high-resistivity core provided that hydrothermal alteration is in equilibrium with the present temperature of the reservoir.