Non-intrusive monitoring using seismic tomography at the Mont Terri rock laboratory

In the framework of a nuclear waste disposal program, we have conducted seismic experiments at the Mont Terri underground rock laboratory in Switzerland. The objective was to explore the possibilities and limitations of seismic tomography for remotely monitoring the changing properties of material filling a small (1 m diameter) tunnel embedded in a highly anisotropic clay formation. Crosshole traveltime tomography allowed the gross properties of the host rock to be determined, but it failed to identify the microtunnel. By comparison, recordings with vertical-component geophones attached to the inside of the microtunnel walls revealed significant traveltime and waveform variations for different fill materials (air and sand) and experimental conditions (dry versus saturated). Data from these geophones were not only diagnostic of the fill, but also of the excavation damage zone (EDZ) surrounding the microtunnel.