[S-VC29] Hydrothermal systems of volcanoes

[SVC29-P07] DAS-Seismic and temperature data acquisition using DAS and DTS measurements at the Ohnuma, geothermal field, Tohoku, Japan

*Junzo Kasahara^1,2, Yoko Hasada^1,3, Kuzume Haruyasu¹, Hitoshi Mikada⁴, Yoshihiro Fujise⁵ (1.ENAA, 2.Shizuoka University, 3.Daiwa Exploration and Consulting Co. Ltd., 4.Graduate School of Kyoto University, 5.WELMA)

Keywords:Geothermal exploration, supercritical water, DAS, seismic reflection, migration imaging, geothermal reservoir

The New Energy and Industrial Technology Development Organization (NEDO) in Japan has promoted the use of supercritical water for geothermal power generation under a project called “Research and Development of Supercritical Geothermal Resources. In September, 2020, we carried out a seismic survey as a part of this NEDO project using distributed acoustic sensor (DAS) method by an optical fiber system. We deployed an optical fiber system in the O-13R borehole at the Ohnuma geothermal power plant owned by Mitsubishi Materials Corporation. The Ohnuma geothermal power station, the third commercial geothermal power plant in Japan, was completed in 1974; its installed capacity is 9.5 MW. The optical fiber reached to the depth of 1,973 m. The temperature was measured as 235 °C at around 1,130 m depth using the DTS mode. We operated a medium size 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 in or above basement rock and/or igneous intrusions (Fig. 1).

Acknowledgements

This study was supported by the New Energy and Industrial Technology Development Organization (NEDO). We greatly appreciate Mitsubishi Materials Corporation for their kind permission for fieldwork in their power plant. Mr. Kimura provided us with Schlumberger hDVS measurements. YK Engineering Co. and Fujitsu Research Institute Co. provided DAS data. WELMA Co. provided us with a borehole fiber-optic system and measured the temperatures in the borehole. The staff of the Kawasaki Geological Engineering Co. and Daiwa Exploration and Consulting Co. assisted our surface seismological measurements during the field study. The IVI EnviroVibe operation was performed by JGI Inc.

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