To delineate detailed geothermal reservoir structures, we have driven a R&D project for geothermal exploration since 2013. Gravity and electromagnetic surveys are commonly used for geothermal exploration in Japan. However, the resolution of these surveys is much broader than the size of faults or fracture zones we see from well data. Thus, there is difficulty to locate the reservoirs by these surveys alone. To resolve this problem, we focused attention on the seismic method which tends to have higher spatial resolution. As the first step of this project, we reprocessed a few legacy records with present data processing techniques and confirmed improvement in its data quality. Then, we acquired a 3D seismic test data in Yamagawa geothermal field and analyzed it with a variety of methods including 3D visualization of discontinuous structures indicating fractures and integrated analyses with well and other geophysical data. Finally, we conducted the second verification seismic survey in Onikobe geothermal field in 2017 to confirm its effectiveness even in mountainous areas where most geothermal resources are stored. In the mountainous areas, the survey design is crucial because limited number of survey lines are available. We visited and investigated the survey area several times before the survey to find available survey lines as much as possible. Four types of sources and two types of recorders were used according to surface conditions in the data acquisition. These efforts allowed us to obtain 3D seismic volume in deep part which included potential target zones in this field. Analyzed results showed consistency with existing data and added new insights in the reservoir structures.

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