Fracture distribution in bedrock has a great influence on the groundwater flow, whose characteristics should be understood for resource exploration and deep geological repository and so on. Previous studies have been attempting to grasp the hydraulic characteristics on a local scale using boreholes and so on. However, it is considered to be difficult to grasp the hydraulic permeability without drillings. In this study, we tried to estimate the permeability of rocks in deep underground from physical property values like resistivity and Young's modulus in order to grasp macroscopic hydraulic characteristics of rock including fracture. The validity of the present estimation method was examined, by comparing this estimated value with the measured value. The resistivity and Young's modulus of exposed granitic rock mass at depth of 300-500m were measured in the underground research gallery at the Mizunami Underground Research Laboratory, Japan Atomic Energy Agency in 2017. We estimated the permeability using a rock physics model. The permeability was also measured in 2015 along the same gallery, therefore compared with the estimated permeability from our geophysical measurements.
Nuclear power plants are used in many countries. But disposal of radioactive waste is an issue. It is said that geological disposal is most practical method to solve the issue. But his method has a leakage risk radioactive waste by groundwater flow. So, it is necessary to grasp the hydraulic characteristics for evaluation of leakage risk of radioactive material by groundwater flow. However, it is considered to be difficult to grasp the hydraulic permeability without drillings. In this study, we tried to estimate the permeability of rocks from measured physical properties in Mizunami Underground Research Laboratory. And we compared measured permeability and estimated one and discussed its relevance.
We measured exposed bedrock walls along the research tunnels (depth 300 m, 500 m) of the Mizunami underground research labolatory of the Japan Atomic Energy Agency, located in Mizunami-city, Gifu Prefecture. In this area, a lot of logging data have been published and the fracture distribution in the gallery was also examined in detail (e.g., Ishibashi et al., 2016). Furthermore, permeability of the rock wall was measured in 2015 (Sato, 2015). In this study, we measured resistivity using a small electric surveying device (4-pole method with electrode interval of about 10 cm) and the Young's modulus using a Schmidt-lock hammer. The measured data are shown Fig.1.