| Title | Joint Inversion of TEM and MT Data from Paka Geothermal Prospect in Kenya |
|---|---|
| Authors | Muturia Lichoro, Charles; Wamalwa, Antony M. |
| Year | 2011 |
| Conference | Geothermal Resources Council Transactions |
| Keywords | Resistivity; alteration; static shift; joint inversion |
| Abstract | Joint one-dimensional (1-D) inversion of magnetotelluric (MT) and central loop transient electromagnetic (TEM) data was done by fitting both data sets 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 (> 200 ?m) to about 400 metres below the surface, an intermediate low resistivity zone (~10 ?m) to depths of about 1 kilometre and a deeper high resistivity region (> 50 ?m), up to 3-4 kilometres depth. Below the high resistivity zone a relatively deeper low resistivity zone at depth is evident possibly indicating a high temperature which is a likely heat source for this field. Results from this survey will be used to predict the correlation between resistivity structure and hydrothermal alteration and also to infer temperatures to be expected once drilling is carried out. In a high temperature geothermal system low resistivity is dominated by conductive minerals in the smectite-zeolite zone at temperatures of 100-200 oC. In the temperature range 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. |