3D magnetotelluric exploration of Sete Cidades Volcano, São Miguel Island, Azores

Abstract

Geothermal energy accounts for 43% of the electricity expenditure of São Miguel Island, Azores Archipelago. All production comes from the Ribeira Grande (RG) high-enthalpy geothermal field. To meet the growing energy demand in the island, it is necessary to extend the exploration efforts to new areas. We evaluated the results of a broadband magnetotelluric reconnaissance survey conducted at Sete Cidades Volcano, placed only 30 km westward of the RG field. The resistivity structure of the Sete Cidades geothermal system was obtained through a simultaneous 3D inversion of the full impedance tensor and tipper. The bathymetry and the topography of the island were treated as fixed features in the model. The geothermal reservoir at Sete Cidades is outlined as a northwest–southeast elongated resistive anomaly, geologically controlled by the Terceira Rift fracture zone. We have also identified high-conductivity zones between 1000 and 4000 m below mean sea level, probably associated with clay cap rocks overlying the geothermal reservoir.