Seismic Monitoring of a Simulated Radioactive Waste Repository During Water Saturation

In the framework of a radioactive waste disposal and monitoring program as part of the EC Integrated Project ESDRED, we have conducted for the NDA (UK) in co-operation with Nagra (Switzerland) high resolution (0.2 - 4 kHz frequencies) seismic experiments in an anisotropic clay unit at the Mont Terri underground rock laboratory in Switzerland. The objective was to explore the possibilities and limitations of seismic measurements for remotely monitoring, at distances of tens of metres, the changing properties of material filling a small scaled–down version of a repository (1 m diameter tunnel) embedded in a clay formation. Recordings using vertical-component geophones attached to the inside wall of the microtunnel revealed significant waveform variations for different fill materials (empty and sand-filled) and experimental conditions (dry, wet, and pressurised). Initially, the presence of water weakens the clay, but at a later stage the seismic measurements indicate swelling of the clay material, which likely causes healing in the excavation damage zone. On the basis of our results, it is judged worthwhile to develop wireless seismic sensors that could be employed for non-intrusive monitoring during the water saturation of a repository.