[H-CG21] Nuclear Energy and Geoscience

[HCG21-P07] Fracture characteristics of hot spring discharge: an example from the Kamiyu hot spring, Kii peninsula

*Hakoiwa Hioaki¹, Koji Shimada¹, Makoto Kawamura¹, Nariaki Nishiyama¹, Akira Goto² (1.Tono Geoscience Center, Japan Atomic Energy Agency, 2.OYO Corporation)

Keywords:fracture domain, discharge of hot spring, fluid pathway

Understanding and detection of fluid flow pathways in the earth's crust is important for evaluation of rock mass and groundwater managements for geological disposal projects. We consider hot spring outcrops where outflow is still hot from fractures in the bedrock as fractures that act as high-speed flow paths for fluids and investigate the characteristics of fracture distribution. Fracture data were obtained by the scanline method at outcrops along the Kamiyu river of the Kamiyu hot spring. Strike and dip of fracture planes crossing on measuring tape and distance are gained. The strike data is treated as a vector of unit length with the orientation range from the north through the east to the south. For the graphical presentation, the first vector starts at the origin of the cartesian coordinates with the east is x-axis and the north is y-axis. Then, the end of previous vector is the starting point of current vector. Repeated of this procedure leads to a line graph of the sequential strike vector. If the same unit of the x- and the y-axes, the direction of a part of the line graph shows an average direction of the strike of these fractures. Some trends and bend points of them can be depicted in the graph. Each trend corresponds to a fracture domain with fractures with similar strike. The current discharge of the hot spring locates at one of the bending points, i.e., at the fracture domain boundary. Paleo discharge fractures are also located at fracture domain boundaries. Distance and dip information of the fractures does not appear in this line graph. Distance of constant fracture number, inverse of fracture frequency, can also be used to domain boundary detection. The paleo discharge of the hot spring locates at of the sudden change point of the distance of fractures which correspond the change of clustering feature of adjacent domains. A line of intersection of neighbor two fractures can also be treated as a unit vector make us construct another graph of sequential intersection line vector. Above mentioned current and paleo discharge locate at bending points, domain boundaries, of this graph. These simple procedures depict fracture domain boundaries and locations of flow paths in fractured rock mass as an example from the Kamiyu hot spring. An advantage of this method is availabilities without any modification in drilling surveys with borehole televiewer logging which is taken for granted in the forthcoming prospective study.

Acknowledgements:
This study was funded by the Ministry of Economy, Trade and Industry (METI), Japan as part of its R&D supporting program titled “Establishment of Advanced Technology for Evaluating the Long-term Geosphere Stability on Geological Disposal Project of Radioactive Waste (JPJ007597) (Fiscal Year 2022)”.