3D Magnetotelluric Surveys in Hachijo and Minami-Izu Geothermal Areas, Japan

Abstract

We conducted three-dimensional (3D) magnetotelluric (MT) survey in two geothermal areas in central Japan in 2010 - 2012. The first area is the Hachijo geothermal field, located on a small remote island (Hachijo-jima Island), about three hundreds kilometers south of Tokyo City. A 3.3 MWe geothermal power plant has been in operation since 1999. The second one is the Minami-Izu geothermal area located at a southern part of Izu Peninsula, some 100 km southwest of Tokyo City. A goal of the research project is to understand the relation between the deep high-temperature geothermal reservoir and shallow low-temperature hot spring reservoir by an integrated geoscientific studies, including geophysics and reservoir simulation. The purpose of the MT survey was to provide a detailed electrical resistivity image around the geothermal area including surrounding hot spring sites to the base reservoir model for a geothermal monitoring study. We have applied 3D inversion to the MT data in the two areas. In the Hachijo geothermal field, the 3D resistivity model successfully correlates with the geothermal reservoir, showing low-resistivity clay cap layer and higher-resistivity reservoir core. On the other hand, we could not identify the existence of high-temperature geothermal reservoir in the Minami-Izu area from the MT model.