Exploration of the structure beneath fumaroles in Kuju Iwouyama, Southwest Japan

Abstract

We carried out electrical resistivity, 30cm depth temperature and gamma-ray spectrum exploration at a fumarole zone in Kuju-Iwouyama, the central Kyushu island, southwest Japan, to clarify a subsurface structure beneath high temperature fumaroles. A survey line, 200m long, was set across fumaroles having a maximum temperature of approximately 260°C. Apparent resistivity of both 'Wenner' and 'Eltran' type four electrodes arrays were measured every 4m along the line. 30cm depth temperature and gamma-ray spectrum of ⁴⁰K, ²¹⁴Bi and ²⁰⁸Tl were measured at 8m intervals. High underground temperature zones (>90°C) were observed at the fumarole zone and up-flowing zone of thermal water. Relatively low gamma-ray intensities and high Bi/K, Bi/Tl values were recognized around the fumarole zone. A steam reservoir beneath the fumarole zone was detected as a high resistivity zone (>1000 ohm, m) which was 60m wide and 30m deep. We have discussed the physical properties of the steam zone in the volcano.