Near surface geophysics

[NS-07] Evaluation of time lapse behavior of excavation damaged zone by first arrival tomography in Horonobe Underground Research Laboratory

*Yusuke Ozaki¹, Nobukatsu Miyara¹ (1. Japan Atomic Energy Agency (Japan))

The excavation of drift changes in the physical properties around the drift wall through the generation of fractures. The domain enriched by the fractures is called the excavation damaged zone (EDZ) and the enhanced properties concerning mass transportation, such as hydraulic conductivity and diffusive coefficient, is considered as a risk to decline in the isolation feature of repository of the high level radioactive waste. To mitigate the risk, various methods to evaluate the range of EDZ have been developed so far.
In Horonobe Underground research laboratory, we repeatedly performed the travel time tomography to investigate the depth and physical properties of EDZ generated in soft sedimentary rock over time. For the analysis of the acquired data of travel time tomography, we performed numerical simulation and estimated the conceivable change in the arrival time. The results of numerical simulation indicate that the shotcrete on the drift wall has the great impact on the travel time because of its high stiffness whereas the effect of shotcrete fixing the ray-path might be the advantage for the monitoring purpose. The analysis of the data acquired at 350 m suggest that the significant change in EDZ that surpasses the observation limit was not recognized.