[S-SS03] New trends in data acquisition, analysis and interpretation of seismicity

[SSS03-04] Fracture mapping using microseismic data analysis at Okuaizu Geothermal Field

*Dian Darisma^1,2、Yusuke Mukuhira³、Naoki Aoyogi⁴、Kyosuke Okamoto⁴、Takuya Ishibashi⁴、Hiroshi Asanuma⁴、Takatoshi Ito³ (1.Graduate School of Environmental Studies, Tohoku University, Sendai, Miyagi, 980-8572, Japan、2.Department of Geophysical Engineering, Universitas Syiah Kuala, Banda Aceh, Aceh, 23111, Indonesia、3.Institute of Fluid Science, Tohoku University, Sendai, Miyagi, 980-8577, Japan、4.AIST-FREA, Koriyama, Fukushima, 963-0298, Japan)

キーワード：microseismicity, hypocenter location, focal mechanism, fracture, fracture network model

In a geothermal field, microseismicity is used to monitor subsurface conditions, especially in a geothermal reservoir. Hypocenter locations could provide us with information about existing fractures as well as permeable zones. The fracture’s location information is then extracted to estimate fracture network models, which benefit reservoir modeling. This study used microseismic data to build the fracture network model for Okuaizu Geothermal Field in Japan. The microseismic monitoring has been operated using nine seismic stations, consisting of five surface stations and four borehole stations. However, the previous study focused on the triggering process during injection for three months of data, and the fracture system of this field has not been well understood. This study analyzed microseismic data during the reinjection process from 2019 to 2021. By manually picking P- and S- arrival times and using a homogeneous seismic velocity model, 10,432 events were located, with most occurring around the bottom of the injection well. Using the Growclust algorithm, 4,235 precise locations were relocated, revealing clusters in the northeast and southern parts of the injection well that some of the clusters correlate with local faults in the NNW-SSE direction. We limited the relocation results using cross correlation above 0.6 and filtered using a frequency of 5-20 Hz. Additionally, this study found that the microseismicity distribution shifted over time. The 2020 microseismic distribution saw less activity in the southern area than the 2019 and 2021 distributions which have almost the same trend. In the next step, we estimate fracture orientation by focal mechanism analysis using P-wave first motion polarity. To improve the results of the focal mechanism, we used S/P amplitude ratio as a constraint.