Three Dimensional Seismic Explorations in Kijiyama Geothermal Field in Akita, Japan Using the DFOS (Distributed Fiber Optic Sensing) System in Four Geothermal Boreholes in 2022, 2024, and 2025

Abstract

In 2022, 2024, and 2025, we carried out a DFOS seismic studies in the Kijiyam geothermal field in north-eastern Japan. In 2022and 2024, we installed an optical fiber cable to 2,000 m into the NS and EW strike geothermal boreholes. We operated 24vertical seismic sources around the Kijiyama constructing geothermal plant. In 2024, we also used the N-S and E-W horizontal vibrations at two stations. The DTS temperature was 294ºC at 2,000 m depth. Based on the DAS data and synthetic seismic waveforms, we determined 3D Vp depth distribution in this area. The extract Vp profiles along five boreholes are compared to borehole geology and we confirmed the reasonable fitting. Migrated reflection shows intense reflections from 1 km to 4 km depth. We made 3D composite diagram using Vp, seismic reflection images and lost-circulation, flow-in and flow-out locations along the well trajectories. We obtained anisotropy using the horizontal vibrations. The Vp/Vs for N-S direction was approximately 2.0, in contrast to 1.95 for E-W direction. This suggests the fracture diction is E-W direction down to 1,500m depth. Currently six new boreholes are in drilling state using our 3D Vp and reflection image. We are also conducting the new seismic survey in 2025 using two boreholes for two months continuously. One of new drilling met the fracture zone and circulation loss at one of the seismic reflection zones. We will interpret the geothermal structure in the Kijiyama geothermal field using three years datasets of 2022, 2024 and 2025.