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To explore unknown geothermal reservoirs, we carried out a geothermal seismic study using a distributed acoustic sensor (DAS) at the Ohnuma geothermal power plant ownd by Mitubishi Material Co. in September 2020. The Ohnuma geothermal power station, the third commercial geothermal power plant in Japan, was completed in 1974. We installed an optical fibre system for the DAS measurements, on the site. We deployed the optical fibre system down to a depth of 1973 m in the O- 13R borehole. To enhance the S/N, we stacked the DAS data for 480 times and correlated it with the source signature. By stacking for a long duration, we obtained excellent DAS records down to the bottom of the boreholes. Using 2D migration of observed and synthetic DAS seismic records, we recognised intense seismic reflections from 2.8–3.0 km depth, suggesting the possibility of geothermal reservoirs. The velocity decrease in this zone could be more than 1 km/s, possibly implying that the fracture zone is filled with fluid. The two field studies in the Medipolis and Ohnuma geothermal fields in Japan showed that the DAS-seismic method in the borehole can efficiently image seismically reflective zones, and the findings suggest high possibility of geothermal reservoirs.