Integrated geophysical surveys to characterize Tendaho geothermal field in northeastern Ethiopia

Abstract

Tendaho geothermal field is a high temperature geothermal resource in Afar regional state North Eastern Ethiopia. Gravity, magnetic, transient electromagnetic and magnetotelluric survey were carried out. The objective of the geophysical survey was to delineate the boundaries of the geothermal reservoir and geological structures that control flow of geothermal fluids. The complete Bouguer anomaly map has reveled two distinct broad high (in NW sector) and low (in SE sector) Bouguer anomalies. These contrasting Bouguer anomalies may indicate the presence of ENE-WSW trending regional crustal discontinuity (fault). The Tendaho graben is mapped as a NW-SE trending prominent magnetic low feature and corresponding high residual gravity anomaly. The geothermal manifestations at Tendaho geothermal field lie on NW-SE trending fault system on magnetic and gravity surveys. TEM data from the same site was used to correct for static shift in MT data. MT data were analysed and modelled using 1D Occam inversion of the determinant of the impedance tensor. In the MT cross sections, the low resistivity at shallow depth is interpreted as a sedimentary formation, lateral flow of geothermal fluids or a fracture zone. The high resistivity below the low resistivity can be associated to less permeable Afar stratoid series basalts. A low-resistivity zone bounded between high resistivity is interpreted as a fracture zone in the Afar stratoid basalts which may give rise to higher permeability and higher temperature and may indicate upflow of geothermal fluid. The fracture zones inferred from MT correlate with NW-SE trending structures from gravity and magnetic surveys and the surface geothermal manifestations in the area. The fracture zones are not well resolved because of large MT station spacing..