3D gravity inversion constrained by a 3D resistivity model: Implication for the conceptual models of Fiale caldera and Gale-le-Koma geothermal prospects in the Asal Rift (Republic of Djibouti)

Geothermal systems in the context of very active rifting, such as in the Republic of Djibouti, are complex to characterize. Despite the numerous geoscientific studies conducted over decades in recognized prospects such as Fiale Caldera and Gale-le-Koma in the Asal Rift, we do not understand well the mechanism of the geothermal resources, their relationships with the heat source at depth and the recharge of water. In an attempt to progress in this understanding, we present an analysis of 3-D gravity data inversion guided by a new three-dimensional electrical model to obtain constrained density models for each geothermal site. These models along with the resistivity model revealed highly fractured underground structures primarily influenced by two processes: geothermal activity from the rift and structural control from the active rift tectonics. These models are in good agreement with the lithology from boreholes, as well as with geochemical and hydrogeological data. The study presents revised conceptual models for the two geothermal systems. These conceptual models consist of a shallow reservoir and a deep reservoir at each site (Fiale Caldera and Gale-le-Koma geothermal prospects), primarily recharged by seawater from the Bay of Ghoubbet and possibly by regional meteorological groundwater recharges.