1D inversion of magnetotelluric data from Ashute, Butajira geothermal prospect, SE-Ethiopia and its geothermal implications

Abstract

Geophysical surveying is an important part in exploration of geothermal energy. A magnetotelluric (MT) survey was conducted in the Ashute, Butajira geothermal prospect in SE-Ethiopia, where 28 soundings were carried out in 2017. In this project, the 28 MT soundings have been processed and 1D inverted. The results are presented here as resistivity cross-sections and depth-slices, and compared to previous geological findings. The Ashute area is characterized by a conductive shallow layer with resistivity values less than 10 Ωm and an average thickness of about 1,600 m. It reaches down to a depth of 200 m a.sl. The subsurface is composed of volcanic sediments including lithified ash, fine- to coarse-grained sandstone, well laminated conglomerate and debris or lahar. The clasts mostly consist of pyroclastic material derived from a silicic centre from the south. They can be correlated with sediments at shallow depth and the low-resistivity layer. Below the low-resistivity layer there is high resistivity with resistivity values greater than 70 Ωm. This layer is probably related to the basement, fractured basalt and scoria.

The high-resistivity layer below the conductive zone is believed to reflect major fault systems, the tectonic trend of the Debre zait Selti rift graben and the Main Ethiopian Rift (MER). Geothermal fluid at depth emerges to shallow level most likely along a NE-SW discontinuity and NW-SE structures. It flows laterally and enriches the shallow permeable formation, and appears as thermal manifestations in the field. Strike analyses for different frequencies (depth levels) indicate a general NE-SW geo-electrical strike direction for shallow depths which coincides with the major trend of the western rift axis. At greater depths the geo-electrical structure changes. In further studies of the area, it is recommended to add MT and co-located TEM soundings towards the north and west of the Ashute field to define the resistivity structure better, as well as performing TEM soundings at the same location as the already existing MT sites to correct for the static shift of the MT data. Finally, detailed geological mapping of the area is proposed, in particular tectonic mapping, and gravity surveying to delineate geological structures.