Monitoring Changes in Bentonite at the Grimsel Test Site Using Crosshole Seismic Tomography

We have been exploring the feasibility of monitoring the swelling of bentonite using non-intrusive crosshole seismic tomography. Our study has included numerical simulations and extensive field experiments at the Grimsel Test Site in the central Swiss Alps. Numerical simulations have demonstrated that changes in bentonite's elastic properties resulting from variations in water-saturation conditions should yield measurable effects in crosshole seismic data. Based on the simulation results, a relatively thin block of encased bentonite at the end of a tunnel was surrounded by six inclined boreholes. Seismic energy sequentially generated in three of the holes was detected by hydrophones in the other holes. The recorded seismic wavefields, characterized by high signal-to-ambient-noise ratios and coherent frequencies of 1-4kHz over propagation distances exceeding 20m, have provided sufficient information for meaningful high resolution tomographic inversions. Varying the water-saturation conditions resulted in marked changes in the transmitted and reflected wavefields that were readily observable in the raw data. Source and receiver coupling effects also varied significantly during the course of our experiments, and unusual phases due to wave propagation in the source and receiver cables and to borehole-related phenomena were prominent features of the seismic sections.