Seismic Study at the Mori Geothermal Power Plant, Japan Using DAS and DTS in a Geothermal Well

Abstract

As the third field study using an optical fiber seismic sensor array in the geothermal well, we conducted geophysical exploration at the Mori geothermal field in Hokkaido, Japan. We used an optical fiber system in the geothermal well to the depth of 2,100 m located in the Nigorikawa caldera basin. Active seismic sources were deployed in the caldera basin surrounding the F-01 well, whose temperature profile showed a maximum temperature of 206 °C at the bottom of the well. The DAS seismic records with 30-second sweeps between 10 and 75 Hz 500 times were stacked. The DAS records indicate numerous seismic reflected phases below 1,000 m depth, the first arrival discontinuity at 1,060 m, and the P first arrival amplitude attenuation below 1,060 m depth. We conducted the Vp travel-time tomography along the east-west and north-south lines using the P fist arrivals of DAS and surface seismometers data. The observed travel time discontinuity was partly explained by the model of a rectangular shape zone of 50 m thick and 250 m long filled with water at the 1,060 m depth. The layer at 1,060 m could be caused by the vertical fracture zones filled with the injected wastewater and pre-existing water-filled fractures before injection. We also made the migration image along the east-west line using DAS reflection arrivals. The seismic reflection zones at a depth between 2,000 and 3,000 m imply a fracture system located in the production zone of the power plant. The current results can well explain the existing geothermal data owed by the Mori Geothermal Power Plant.