THREE DIMENSIONAL GEOPHYSICAL MODELLING OF THE ALALOBEDA GEOTHERMAL FIELD
The Alalobeda geothermal system is located in the Afar region (Northern Ethiopia) along the western margin of the Tendaho Graben. A multidisciplinary study of the Alalobeda geothermal field, funded by the Icelandic International Development Agency (ICEIDA) and the Nordic Development Fund (NDF) was carried out by the Geothermal Division of the Geological Survey of Ethiopia under the supervision of ELC Electroconsult (Milan, Italy) In this paper, we show the results of the integrated geophysical survey, including 123 magnetotelluric and transient electromagnetic soundings and 300 gravity records. The high density station coverage over a regular grid allowed the successful application of 3D MT and gravity inversion, producing detailed and realistic models of the resistivity and density distributions. The final 3D gravity model images of the bedrock surface hindered by the sediments reveal first order normal faults controlling the graben main structures. Beneath the graben shoulder, a positive density contrast +300 kg m 3 was modelled at depths of 1 2 km and interpreted as a propylitic alteration of the bedrock surface. The 3D resistivity model revealed three NNE trending fracture zones beneath the shoulder of the Tendaho Graben, where topography and structural observations indicate the presence of NNEtrending MER normal faults. These fracture zones are characterised by a shallow conductive anomaly (interpreted as the "low temperature" clay cap) and a more resistive deeper anomaly (interpreted as the "high temperature" alteration) The fracture zones do not prosecute towards the plain and appear to be limited by the NW trending main normal faults of the Tendaho Graben