[H-CG20] 原子力と地球惑星科学

[HCG20-P04] 広域地下水理モデリングのためのプローブ・パーミアメーター測定とボーリング検層データの統合解析

*久保 大樹¹、Scibek Jacek、小池 克明¹ (1.京都大学大学院工学研究科)

キーワード：透水性、花崗岩、地下水流動、地層処分

To evaluate the safety of geological disposal of radioactive waste, it is important to understand the regional hydraulic structure in underground. The permeability of crystalline rock is primarily influenced by the distribution of connected and permeable parts of facture network that is related to tectonic structure. There are also highly porous and permeable domains of granite, from past hot hydrothermal activity, where the hydraulic property is not dominated by brittle fractures. Furthermore, when the stability on the very long-time scale is considered, the properties of the porosity and permeability of the "unfractured" rock matrix and the altered part also contribute to hydraulic properties. In this study, we applied a borehole core measurement technique utilizing variable non-destructive measurement devices, such as a probe permeability meter (permeameter) and 3D scanner. The combined analysis of the permeameter and 3D scanner is well-suited for identifying changes in permeability within rock matrices and small alteration zones, which are difficult to extract in hydraulic tests using a borehole. We selected Mizunami area in Gifu Prefecture as the case study site. Many geological investigations were carried out in this site by JAEA to understand the deep underground properties. Kubo et al. (2013, 2019) were proposed the method for 3D fracture modeling (GEOFRAC) and hydraulic parameters estimation. In this presentation, additional measurement data and statistical analysis results are added to them, and a new composite geological model which integrates them is proposed. We conducted measurements on over 100 boring core samples. From the measurement results, the spatial variation of hydraulic conductivity obtained in hydraulic tests was improved to a more detailed one. In addition, detailed observations using a microscope revealed the characteristics of rock properties and their distribution, which have a significant effect on permeability besides fractures. Permeameter measurements unveiled that a hydrothermally-leached granite region with numerous macroscopic connected voids of Episyenite composition, had high porosity and permeability. Other deep altered zones also exist along unpapped fault zones in Toki granite, from "regional" (DH) drill hole core samples that we studied. It is thought that such areas can form a different flow pattern than groundwater flow dominated by a fracture system. Groundwater flow simulation was performed using a hydrogeological model constructed by integrating observed data with the GEOFRAC model. The results explained on the flow system around the major faults in the study area.

Acknowledgment:
This work was supported by the Ministry of Economy, Trade and Industry (METI) of Japan.

Kubo, T., Koike, K., Liu, C., Kurihara, A., Matsuoka, T., 2013. 3D hydraulic conductivity modeling of fractured granitic body using geostatistical techniques and its application to regional groundwater flow analysis. Journal of Geography. 122, 139-158. https://doi.org/10.5026/jgeography.2012ap02 (Japanese with English abstract)
Kubo, T., Matsuda, N., Kashiwaya, K., Koike, K., Ishibashi, M., Tsuruta, T., Matsuoka, T., Sasao, E., Lanyon, G.W., 2019. Characterizing the permeability of drillhole core samples of Toki granite, Central Japan to identify factors influencing rock-matrix permeability. Eng. Geol. 259, 105163. https://doi.org/10.1016/J.ENGGEO.2019.105163