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 geoscientific study of the Alalobeda geothermal field, was funded by the Icelandic International Development Agency (ICEIDA) and the Nordic Development Fund (NDF), and 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 NNE-trending MER normal faults. These fracture zones are characterised by a shallow conductive anomaly and a more resistive deeper anomaly. The shallow conductor is interpreted as a low-temperature clay cap, resulting from the fluid upflow coming from a deeper high-temperature resistive horizon. 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