| Abstract |
Eastern Africa geothermal fields are associated with the recent to actual volcanism in the Rift region. As a result, the geometry of the geothermal source is complicated. Water and vapor flow in fractures within the lava flows often interlaced with hydrothermal and/or sediment deposits. How geophysical imaging techniques could provide accurate models of the plumbing system and the nature of fluids circulating in it is a central question in geothermal exploration. Electromagnetic techniques and in particular Magnetotelluric (MT) is one of the geophysical technique used in this domain. The method provides information about the electrical conductivity down to several kilometers in depth. Electromagnetic techniques complete seismic techniques for oil, gas, mining, geothermal or hydro-geological exploration. These complementary techniques are particularly useful in areas with complex structures such as geothermal regions. Furthermore, although seismic methods provide the best possible reservoir descriptions, they remain poorly informative on fluids properties, MT methods provide reservoirs electrical resistivity information, thus enabling to better describe and understand fluids in the pore space.. New tree-dimensional imaging (or inversion) tools are now available to improve the accuracy of the conductivity models retrieved from the MT data. For 3-D inversion, it is best to acquire data over a fine 2-D surface grid with a regular spacing. It is in general difficult to acquire MT data on a such a grid over a volcanic geothermal field. In this study, we present results of three-dimensional (3-D) MT inversion for various types of MT sites distribution, including 2-D profiles which show that accurate local 3-D structures can be recovered. We test the approach with synthetic examples and demonstrate the feasibility with a series of 3-D MT studies carried out in Eastern Africa. |