1D joint inversion of TEM and MT data : Suswa geothermal field, Rift Valley, Kenya

Abstract

Suswa geothermal prospect is the southernmost trachyte shield volcanic complex at the base of the Kenyan Rift floor. It is located about 15 and 30 km to the south of Mt. Longonot and Olkaria Domes, respectively, and is characterized by 2 nested calderas. The last volcanic activity in the area occurred less than 400 years ago in the inner caldera. Geothermal surface indicators are in the form of fumaroles, alterations, young lavas and geothermal grass. Unique to Mt. Suswa among the volcanic complexes in the rift axis is its phonolitic composition in addition to trachyte and pyroclastic formations. The main structures in the area trend N-S, ENEWSW and along the caldera ring structures. The main recharge is from the rift flanks to the east and west of the prospect area. The result from resistivity surveys using 1D joint inversion of MT and TEM data indicates the presence of a deep conductor at 4000 m b.s.l. in the Kisharo area, a dominant resistive structure at sea level (0 m) and a conductive layer between 1000 m a.s.l. and 1500 m a.s.l. The deep conductor below the caldera could be the heat source of the prospect. A profile (profile 2) across the southern slopes of Mt. Suswa reveals a deep lying conductor in pockets between 7500 and 3000 m b.s.l., while above 0 m, near parallel conductive layers are prominent. In the Kisharo area at the floor of the outer caldera, with intense fumarolic activities, the resistivity is high near the surface, probably due to dry steam associated with a deep groundwater table.