Processing and joint 1D inversion of MT and TEM data from Alallobeda geothermal field in Tendaho, NE-Ethiopia

Abstract

Geophysical methods play a big role in geothermal resource exploration, for instance in revealing the electrical conductivity of the subsurface. Magnetotelluric (MT) and time domain electromagnetic resistivity methods are currently the most widely used resistivity methods. Resistivity is an important parameter because it is directly related to the physical properties of the reservoir. Tendaho is one of the high-temperature geothermal fields in the Main Ethiopian Rift, NE-Ethiopia, and a promising area for geothermal development with respect to both its size and location. Alallobeda is one of the three prospects found in Tendaho geothermal field. Understanding the resistivity structure of the Alallobeda prospect could give a good understanding of the subsurface and enhance the possibility of developing the geothermal resource.

A total of 132 MT and Transient Electromagnetic (TEM) sounding pairs were acquired in 2014 and 2015 by the Geological Survey of Ethiopia (GSE) and the Italian consulting company Electro Consult (ELC) financed by the Icelandic International Development Agency (ICEIDA) and the Nordic Development Fund (NDF). Technical supervision for review and monitoring was done by Iceland GeoSurvey (ÍSOR). In this report, 54MT soundings were processed and almost the same number of TEM soundings. The soundings were1D inverted. TEM was used for static shift correction of the MT data from the same site. Some of the MT stations were inverted using MT data only based on a static shift correction given by the Italian consultant. The results are presented in the form of 1D models for individual stations, iso-resistivity maps and cross-sections. Three main resistivity layers were observed. The first is a thin layer of very low resistivity (< 10 Ωm) at shallow depth down to about 300 m b.s.l. which is correlated to sedimentary formation or a smectite- zeolite zone. Below the low resistivity a high-resistivity core appears with values between 100 and 1000 Ωm down to a depth of 1000 to 5000 m b.s.l. which can be correlated to the less permeable Afar Stratoid basalt series and high-alteration minerals like chlorite and epidote. Beneath the high-resistivity layer down to a depth of 5000 m b.s.l. a deep conductor is revealed that could be associated with the heat source.