Resistivity Structure and Hydrothermal System Around the Waita Volcano in Central Kyushu Japan

Abstract

Tensor controlled-source audio-frequency magnetotelluric (CSAMT) and self-potential (SP) surveys were carried out on and around the Waita volcano in central Kyushu, southwest Japan. This work is to figure out resistivity structures and hydrothermal system developed in the study area. Two-dimensional (2-D) forward modeling of both data was conducted separately using the finite element method. Final Tensor CSAMT resistivity models show a remarkable shallow low resistive zone, as low as 1-3 ohm-m, beneath Takenoyu-Hagenoyu geothermal field located in the western foot of the Waita volcano. A cap layer of 20-100 ohm-m underlies this zone. The SP profile calculated for the final model shows an excellent match with the observed one. The resultant 2-D model suggests the extent of fluid flow system around the Waita volcano including surface manifestation area on the foot of the volcano. This work concludes that there is an evidence of subsurface horizontal fluid flows starting from the southeastern part of the Waita volcano to its western foot in Takenoyu geothermal field. Moreover, it can also be concluded that a combination of magnetotelluric and self-potential survey is essential tool for geothermal exploration and reservoir monitoring.