Hydrothermal System Model around the Waita Volcano Based on a Self-potential Survey

Abstract

Self-potential (SP) survey was carried out at 420 measurement points around the Waita volcano-Takenoyu area, Kyushu, Japan. This survey region covers the summit of the Waita volcano and the Takenoyu hot spring area of which elevation differs by 750 m. SP tends to decrease according to the elevation but high negative anomaly up to six-hundred millivolts can be observed at the middle height of the Waita volcano. Negative peak anomaly of an SP profile has been generally interpreted as an indicator of a recharge zone. However, a numerical modeling of the subsurface fluid flow and its corresponding SP shows that a negative peak anomaly at the middle height of a mountain can not be explained by a model with a higher permeability in the middle height nor by a homogeneous permeability model. Further simulation suggests the existence of a higher permeability column underneath the summit of the Waita volcano. The hot water flows up beneath the volcano and flows horizontaly toward the geothermal system which has been developed at the foot of the volcano.