Hydrogeology Inferred from the Self-Potential Distribution, Kirishima Geothermal Field, Japan

Abstract

Self potential (SP) surveys were carried out in the Kirishima geothermal field, southern Kyushu, Japan. SP anomalies of positive polarity (about 100 mV in amplitude and 3 km in spatial extent) were found to overlie fumarole areas. On the basis of electro kinetic mechanisms for source free fluid flow, the depth and the strength of a current source are estimated for the anomaly overlying the Ginyu fumarole area. Positive charge carried by the hot water up flow from depth is probably accumulated around a zone where separation between the vapor and the liquid phases occur. We infer subsurface hydrology from SP data: evaluating parameters such as the hydraulic radius and the permeability of the formation where the fluid is up flowing. In addition to SP generated by thermally driven flows, negative and positive anomalies, probably generated by nonthermal groundwater flows, were observed., We present a possible electrokinetic mechanism of producing these anomalies.