Geophysical Investigations of the Gergedi Thermal Springs, North West of Wonji, Main Ethiopian Rift

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Addis Ababa Universty


2D electrical resistivity imaging and total field magnetic surveys were carried out at Gergedi Thermal springs, located northwest of Wonji, in the Main Ethiopian Rift, with the aim of characterizing the thermal setting of the area. The area is geologically characterized by alluvial and lacostrine deposits, basaltic lava, ignimbrites and rhyolites and hydrogeologically all potential aquifers of low moderate and high permeability groups are present in the area. The prominent structural feature in the Rift, the SW to NE trending structures of the Wonji Fault Belt System, crosses over the study area. Three traverse lines of imaging data, encompassing the active thermal springs, and about 500 magnetic data, covering the area with random distribution, were collected. From the analysis of the electrical survey data it is observed that the area is covered by low resistivity response region at shallow depths which is believed to result from saline moisturized soil subsurface horizon. Relatively medium and high resistivity responses subjected to weathered basalt, rhyolites and ignimbrites are also mapped. The electrical survey and the magnetic profiles have also indicated the presence of structures in most part oriented following the main structural units of the Wonji Fault System. The magnetic data have been qualitatively interpreted to show the presence of structures, which could act as paths for heat and fluids for the manifested springs, and to characterize the degree of thermal alteration of the area. Results from these investigations suggest that the Gergedi thermal springs area are controlled by fault systems oriented parallel and sub parallel to the main tectonic lines of the Rift and potential sites of shallow thermal water resource, other than the current active thermal springs, are also detected that can be used for further exploitation of the resource.