Interpretation of MT and SP Survey Results at Minami-Izu Geothermal Field, Japan

Abstract

The Minami-Izu geothermal field, Shizuoka, Japan has several hot spring wells which depths are around 150 m. Aiming at finding an up-flow zone of the hot fluid from a depth to the shallower aquifer, Self-Potential (SP) survey was conducted. The result shows a dipolar SP anomaly across a nearly vertical N-S fault. A simplified subsurface structure model, which indicates cause of streaming potential, shows that a dipolar SP anomaly across a fault may be due to a combination of lateral flow in shallow aquifer across the fault and an up-flow from a depth along the fault. Based on the horizontal distance between the dipolar SP peaks, the depth of the lateral flow is calculated to be 100 to 150 m with a simple analytical model. An MT survey result in this area shows low resistivity zone around the fault and shallow aquifer zone in the east of the fault. The possible interpretation of MT and SP survey results in terms of thermal fluid flow of this area is as follows; 1) hot fluid from a depth rises along the fault, 2) hot fluid change its flow direction toward east at a depth of 100-150 m. This study concludes that a simple analytical SP model may give us information on flow pattern and flowing depth.