| Abstract |
In the Okuaizu geothermal field, Fukushima, Japan, JOGMEC has conducted a water recharge test since 2015 to stabilize steam production. As part of the test, microseismic monitoring was performed to observe subsurface fluid behavior. In this study, we applied seismic wave travel time tomography and attenuation tomography using microseismic data from two periods: before continuous water recharge operations, and during the recharge test. Both double-difference tomography (Zhang and Thuber, 2003) and double-difference seismic attenuation tomography (Guo and Thurber, 2021) were employed. The results revealed increased seismic velocity and decreased attenuation near the recharge well, likely indicating the saturation of rock pores with recharged water. Conversely, other regions farther from the well showed decreased seismic velocity, which can be attributed to the formation of new microcracks and increased pore pressure. Reduced attenuation in these areas likely reflects fluid saturation in the rock pores. This study demonstrates the potential to estimate geothermal reservoir behavior in response to water recharge by analyzing variations in seismic velocity and attenuation structures. Furthermore, more precise insights into reservoir behavior, such as fluid flow paths and pore water phase, can be gained by incorporating data on microseismic distribution, Vp/Vs ratios, and other relevant parameters. |