[S-EM12] Electric, magnetic and electromagnetic survey technologies and scientific achievements

[SEM12-P13] High-density AMT survey in Unzen geothermal area, Japan

*Nagi Yamashita¹, Tada-nori Goto¹, Keiichi Ishizu¹, Kodo Umakoshi², Hiroshi Sasaki³ (1.University of Hyogo, 2.Nagasaki University, 3.Unzen City)

Keywords:magnetotellurics, geothermal area, Unzen

In geothermal areas, hydrothermal fluids are known to upwell along fractures (active faults), and geothermal reservoirs exist beneath geothermal alteration layers. The Unzen Onsen (geothermal area with hot springs) is a volcanic hot spring area located in the middle of the Unzen Volcano. In Unzen Onsen, geothermal alteration layers expose on the surface at several places (called "Jigoku"), where fumaroles exist. However, the distribution of geothermal alteration layers and the relationship between fractures and hydrothermal circulation in the shallow subsurface of the Unzen area are not clarified. Audio-frequency magnetotelluric (AMT) survey is often used to estimate resistivity structure down to several kilometers below the surface in geothermal areas. Previous studies have revealed the distribution of geothermal alteration layers and geothermal reservoirs from the resistivity structure. In this study, we carried out high-density AMT explorations at 83 sites with a site separation of about 50 to 150 meters in Unzen hot spring area, then estimated the detailed subsurface three-dimensional (3D) resistivity structure shallower than 1km depth. We present the distribution of conductive zones below the surface, which reach to the surface near hot springs area and fumaroles area. It is concluded that high-density AMT surveys can reveal shallow subsurface geothermal systems.