First-arrival traveltime tomography for monitoring the excavation damaged zone in the Horonobe Underground Research Laboratory

Subsurface excavation results in the formation of a zone called excavation damaged zone (EDZ) around the tunnel wall. An EDZ is a major concern in the field of high-level radioactive waste disposal because it may act as a flow path after the closure of a repository. In this study, first-arrival traveltime tomography was repeatedly conducted on the EDZ at a depth of 350 ​m in the Horonobe Underground Research Laboratory. However, the acquired data was highly affected by the support structure on the drift wall. For proper visualization of the EDZ, information about the structure was incorporated into the inversion by modifying the model constraint. The synthetic study showed that the approach reproduced the EDZ in the model without the artifacts. The method was applied to field data, and the EDZ around the drift was detected. The inversion was extended to a time-lapse inversion to trace the changes in P-wave velocity in the EDZ. The synthetic study demonstrated that temporal changes in the P-wave velocity distribution could be detected. Data obtained from 12 surveys under open-drift conditions were analyzed by time-lapse inversion. The results indicated that the EDZ did not undergo sealing or evolution at the site for approximately seven years.