| Title | Multidimensional Inversion of Electromagnetic Data from Alalobeda, Tendaho Geothermal Field in NE-Ethiopia and Its Geothermal Significance |
|---|---|
| Authors | Getenesh Hailegiorgis ABEBE, Gylfi Páll HERSIR, ÃsdÃs BENEDIKTSDÓTTIR, Halldór GEIRSSON |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | Tendaho, Alalobeda, geothermal, resistivity structure, joint inversion, 3D inversion, joint interpretation |
| Abstract | Tendaho is one of the high-temperature geothermal fields within the Main Ethiopian Rift (MER) in the Afar depression, NE-Ethiopia. It is located in one of the most important geothermal zone of the world. Due to its location and size, Tendaho is a promising area for geothermal development. Alalobeda is one of the three geothermal prospects in Tendaho, located on the southwest shoulder of Tendaho Graben. Detailed surface exploration studies were carried out jointly by Geological Survey of Ethiopia (GSE) and ELC-Electroconsult in Italy financed by the Icelandic International Development Agency (ICEIDA) and the Nordic Development Fund (NDF). With financial support by ICEIDA/NDF in Alalobeda prospect recently including; gravity, micro-seismics and combined use of MT and TEM soundings. The main objective of the survey was to come up with a detailed resistivity model and image the deep resistivity structure, detect and characterize the possible geothermal reservoir of the Alalobeda geothermal prospect and propose drilling sites. The static shift correction of the MT data was made by jointly inverting the MT and TEM data from the same site. Joint 1D inversion of 108 MT/TEM sounding pairs and a 3D inversion of the off-diagonal impedance tensor of 107 MT soundings have been done. The results of joint 1D inversion of MT/TEM and 3D inversion of the resistivity distribution from Alalobeda prospect gave comparable results at shallow depths. However, at deeper level 3D inversion reveals much more consistent details confirming that the resistivity structure in the area is highly three dimensional. From both inversion approaches three main resistivity structures were observed. The first one is a thin layer of very low resistivity ( less than 10 Ωm) at shallow depth down to 300 m b.s.l, which is correlated with sedimentary formation or smectite zone. The second layer is a resistive core between the depths of 1000 m to 4000 m b.s.l, which correlates with less resistive Afar Stratoid Series or the chloride-epidote zone. Beneath this resistive core a deep conductor is imaged that could be associated with the heat source. From the 1D and 3D approaches lithological contacts and lineaments were identified. Sharp resistivity contacts or fault lines with an orientation of NE-SW transverse faults and NW-SE fault were observed. These identified faults and lineaments are in good agreement with gravimetric and micro-seismic results. |