Investigation of the Ayrobera geothermal field using 3D magnetotelluric data inversion, Afar depression, NE Ethiopia

摘要

The Afar Depression, a region of active extensional tectonics and volcanism in the northeastern section of the Ethiopian Rift, hosts a high-enthalpy geothermal area, the Ayrobera geothermal field. High temperatures recorded from wells drilled in this area suggested the existence of this geothermal system. In this study, magnetotelluric data acquired from 199 stations in Ayrobera at different times since 2007 were inverted to produce a new resistivity distribution model to image geothermal structures. Our new 3-D magnetotelluric model revealed three main resistivity features: a near-surface conductive first layer (C1) interpreted as clay-rich sediment at shallow depths down to 600m and basaltic rocks (600-1200 meters) corresponding to the smectite-zeolite zone, followed by a high resistivity region (R1 and R2) below the C1 conductive layer, associated with hightemperature alteration minerals such as chlorite and epidote. The chlorite and epidote were detected below 800 meters in the deep well TD-3, drilled in the southern section of the Ayrobera geothermal field. Fumaroles with high temperatures recorded on the surface near the NW-SE faults (F1 and F2) and hot ground anomalies from shallow ground temperature measurements above the resistive zones (R1 and R2) indicate an upflow region underneath the Ayrobera field. This also implies the possibility of two reservoirs within the resistive zones (R1 and R2) inside the fractured Afar Stratoid Series. At depth, the resistivity model reveals another conductive region (C2) below 8 km, interpreted as a partial melt (heat source) based on previous studies including seismic and geochemical analysis.