Integrated geophysical imaging of the Alalobeda geothermal field (Ethiopia)

Abstract

A geothermal exploration project was founded by the Icelandic International Development Agency (ICEIDA) and the Nordic Development Fund (NDF), with the aim of assessing the geothermal resources of Alalobeda (Ethiopia). The Alalobeda geothermal system is located along the western margin of the Tendaho Graben, where the NW tectonic structures of the graben intersect the NNE trending Main Ethiopian Rift (MER) lineaments yielding intense rock fracturation. An integrated geophysical survey was carried out during in 2014-15, over an area of about 140 km2. The geophysical campaign comprised 123 coincident magnetotelluric and time domain soundings and 300 gravity stations. 2-D and 3-D gravimetric models inferred two NW trending major normal faults hidden by the graben sediment infill. Between them, the basaltic bedrock shows a structure made by second order horsts and grabens. The 3-D electrical resistivity model reveals that the basalts cropping out and the sediments covering the bedrock in the graben depression show exceptionally low resistivities (sometimes lower than 1 Ohm m). In this general frame, three NNE trending fracture zones were inferred by their enhanced conductivity in the 3D model. The fracture zones are located in the shoulder sector of the NW trending Tendaho Graben, where the topography and the structural observations indicate the presence of NNE trending MER normal faults. All the three inferred fracture zones are put into evidence by more resistive deeper anomaly, interpreted as high-temperature alteration, and a shallower very conductive layer, interpreted as a low-temperature clay cap. The fracture zones do not continue towards the plain and appear to be limited by the NW Tendaho Graben main normal fault. The resistivity pattern appears compatible with a main fluid path trending NNE along the MER fracture zones while orthogonal flow is likely driven by the intersecting NW Tendaho faults.