Surface Exploration of a Viable Geothermal Resource in Mbeya Area, SW Tanzania. Part III: Geophysics

Abstract

Based on the geochemical results presented by Kraml et al. (this volume) the proposed high-temperature reservoir was traced by resistivity methods. To cover an exploration depth down to approximately 10 km a combination of the transient electromagnetic (TEM) and magnetotelluric (MT) methods have been applied.
2-dimensional resistivity models have been calculated and used for horizontal resistivity maps at several depth levels. The resulting resistivity structure may be interpreted as a high-temperature geothermal reservoir:
• high resistivity at shallow depth down to approximately 500 m,
• very low resistivity between 500 and 1000 m depth interpreted as clay cap fingering out laterally, and
• localized slightly increasing resistivities interpreted as advancement into the hottest part of the reservoir at about 2200 m depth below the surface.
At greater depth around 4000 m below surface the major NW-SE rift trend within the high-resistive Precambrian basement can be distinguished by its resistivity structure.
The interpretation of an intense alteration in the subsurface only to the W of Ngozi volcano deduced from MT and TEM measurements is supported by a prominent magnetic low with coinciding lateral extent, indicating demagnetization by alteration of the originally magnetic volcanic rocks forming the clay cap of the geothermal reservoir.
Deep exploration wells (about 2200 m) could be located according to the resistivity model derived. However, additional MT measurements are recommended to define the southward extension of the geothermal upflow zone more precisely. Additionally, deep temperature gradient wells reaching the top of the clay cap in about 500 m depth would further constrain the presented model and reduce resource related risk for the drilling of deep exploration wells.