[S-VC29] Hydrothermal systems of volcanoes

[SVC29-09] Analysis and interpretation of DAS-seismic results obtained at the Ohnuma geothermal field in the Hachimantai volcanic region, in Japan

*Junzo Kasahara^1,2, Yoko Hasada^1,3, Kuzume Haruyasu¹ (1.ENAA, 2.Shizuoka University, 3.Daiwa Exploration and Consulting Co. Ltd.)

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

We report seismic data analysis and preliminary interpretation for the distributed acoustic sensor (DAS)-seismic survey obtained at Ohunuma geothermal field in 2020. 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, we carried out a seismic survey as a part of this NEDO project using the DAS-seismic method using an optical fiber system. We deployed the optical fiber in the O-13R geothermal borehole at the Ohnuma power plant owned by Mitsubishi Materials Corporation. The Ohnuma geothermal filed is in Hachimantai volcanic region in the north-eastern part of Japan. We used a middle size active source at nine locations for the survey. In Ohnuma geothermal borehole, the optical fiber extended to 2,037 m-long along the borehole casing (vertical depth = c.a. 1,971 m). The DTS (distributed temperature sensor) temperature in the borehole obtained by the same optical fiber was 235 degree C at around 1,100m-depth. To enhance the signal to noise ratio (S/N) we applied the stacking of the DAS records by 480 times after the removal of outliners and source signature compensation. We separated the DAS records to down-going phases (direct arrivals) and up-going phases (reflected arrivals). We migrated the upgoing arrivals and made a composite migrated image of nine source locations in 3-D space (Fig. 1). We recognized several distinct reflection arrivals. The reflections occur at around 1.1 km, 1.5 km, 2 km and 2.8-3.0 km-depths. The amplitudes of reflected PP arrivals from the 2.8-3.0 km-depth is half of amplitude of direct P arrivals at C8 source location (~700 m distance in the south of O-13R wellhead). The depth of 1.1 km corresponds to the lithological boundary of the dacite welded tuff and the altered andesite, and the depth of 1.5 km does to another lithological boundary of the altered andesite and the silicious shale. The depth of the geothermal production of Ohnuma geothermal power plan is between approximately 1,200 m and 1,700 m depths. The migrated images have similarities to this production zone. We correlated fracture zones obtained by the drilling and logging data of geothermal boreholes (unpublished data of Mitsubishi Materials Co.) and the DAS seismic data, and some of reflections occur around the fracture zones. Because all geothermal boreholes in the Ohnuma geothermal power plant is down to 2,100 m, the distinct reflections from 2.8-3.0 km depth is not resolved by the lithological data of boreholes. One of interpretations for the reflections from the 2.8-3.0 km-depth is presence of hot fluids in fractures of granitic intrusive rock like Sumikawa geothermal field.
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.We also express our great thanks to Mr. Iwasaki and his colleagues of Mitsubishi Materials Techno Corporation for their aggressive discussions.