We carried out a geophysical study using a distributed acoustic sensor (DAS) and distributed temperature sensor (DTS) at the Ohnuma geothermal power plant owned by Mitsubishi Material Co. in September 2020. We lowered an optical fiber cable down to the depth of 1,973 m in the O-13R borehole. To enhance the S/N, we stacked the DAS data 480 times and correlated them with the vibrator source signature. By stacking for a long duration, we obtained excellent DAS records down to the bottom of the boreholes. Using 3D migration of observed and synthetic DAS seismic records, we recognized intense seismic reflections for a depth range from 2.8 to 3.0 km, suggesting the possibility of geothermal reservoir existence. The density decrease in this zone could be 25%, 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 boreholes could efficiently image seismically reflective zones. Our findings suggest a high possibility of geothermal reservoir existence.
