Magnetotelluric and transient electromegnetic methods in geothermal exploration, with an example from Tendaho geothermal field, Ethiopia

Abstract

Electromagnetic methods (EM) are frequently used in the exploration of geothermal resources for determining the spatial distribution of electrical resistivity (conductivity). Magnetotellurics (MT) and time domain electromagnetic (TEM) methods are especially used for geothermal exploration when using EM methods. Geothermal resources are ideal targets for EM methods since they produce strong variations in underground electrical resistivity (conductivity). Electrical resistivity is directly related to parameters that characterize geothermal systems. In this report, the application of TEM and MT methods and interpretation of data from the Tendaho geothermal field in Ethiopia are discussed. TEM data were used to correct for static shift in MT data from the same site. MT data were analyzed and modelled using 1D Occam inversion of the determinant of the impedance tensor. A 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 with less permeable Afar stratoid series basalts. An observed low-resistivity zone bounded between high-resistivity zones 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. Therefore, it is recommended that additional TEM (with large loop area) and systematically gridded long period MT surveys should be carried out to delineate the size and boundary of the geothermal resource in the Tendaho geothermal field.