We carried out a third geothermal seismic study using distributed acoustic sensing (DAS) at the Ohnuma geothermal power plant owned by Mitsubishi Materials Corporation in September 2020. The Ohnuma geothermal power station, the third commercial geothermal power plant in Japan, was completed in 1974; its installed capacity is 9.5 MW. We installed an optical fiber system for the distributed temperature sensor (DTS) and DAS measurements and 26 surface seismometers on the surface of the site. We deployed the optical fiber system down to a depth of 1,973 m in the O-13R borehole. The temperature was measured as 240 °C at around 1,130 m depth using the DTS mode. We operated an IVI EnviroVibe seismic source at nine locations. We repeated the frequency sweep of 10–75 Hz 480 times a day for nine days. To enhance the signal to noise ratio (S/N), we stacked the DAS data and correlated the seismic records with the source signature. By stacking for a long duration, we obtained excellent DAS records down to the bottom of the boreholes. Using the migration of observed and synthetic DAS seismic records, we recognized intense seismic reflections from 2.8–3.0 km depth, suggesting the possibility of geothermal reservoirs. The velocity decrease in this zone might be more than 1 km/s, probably suggesting the presence of a fracture zone filled with fluid. Through two field studies, conducted in the Medipolis and Ohnuma geothermal fields in Japan, the DAS seismic method in the borehole could efficiently image seismically reflective zones that suggest a high possibility of geothermal reservoirs.

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