In search of geothermal energy power potential in Tanzania: the role of geophysics

Abstract

A feasibility study was done to evaluate the Southwestern Tanzania geothermal prospects for their power potential. Vertical Electrical Sounding (VES), a geophysical survey method was deployed in this study. VES was done to study the vertical variations of electrical resistivity in the geothermal prospects and to locate the extent of the reservoir/ fracture zones or the outflow regime of the thermal waters. The result showed that at Na John, River-muddy, Bwana Hutu, Mampulo, Mwakalinga and Kasumulo springs (VES 1, 2, 3, 4, 5 and 6 respectively), the apparent resistivities decreased with depth as one moves from the first top layer to the second. It was noted that in VES 3, 4 and 6, the second layers have low resistivity ranging from 3.3-5.0 Ωm. While in VES 5 the apparent resistivity decreased with depth in all three layers encountered. The layer thickness ranged from 0.9-1.2m in the top layers and from 0.9-33.0m in the second layers. VES 3, 4, 5, and 6 proved to be of hot thermal ground. The geothermal manifestation (hot springs) tended to be associated with regions of low resistivity. The Mampulo, Mwakalinga and Kasumulo springs are found to be located at the foot of concealed thermal outflows, which are supposed to be emerging from beneath the Rungwe volcanics. At the Songwe prospect, contrary to the expected huge hydrogeothermal reservoir, which could account for the reported and observed huge deposit of travertine (>150 mill. m 3 ), only a low resistivity zone of maximum depth of less then 10 m was encountered. This can by no means account for the huge travertine deposit and the still on going deposition of calcium carbonate at a rate of 5 g/s. This suggests that the reservoir is further upstream.