The use of geoelectrical and geo-electromagnetic (e.g., magnetotelluric) methods in geothermal exploration is based on the fact that the resistivity of hydrothermal groundwater in the rocks decreases significantly at high temperatures and that geothermal activity can produce conductive alteration minerals. The resistivity of these rocks observed in geothermal areas is lower than in surrounding rocks, indicating the presence of a considerable resistivity contrast that can be investigated by the magnetotelluric method (Brown, 1994).