An experimental study into the viability of remote crosshole seismic monitoring of potential nuclear waste repositories has been undertaken. Full-waveform repeatability measurements to assess source, receiver and coupling effects were carried out at the Grimsel hard rock laboratory in Switzerland. Numerical modelling simulations showed that changes in the anomalous feature to be monitored (bentonite plug) resulted in measurable changes in the seismic waveforms. However, the use of waveform inversion to extract changes in medium properties requires that the changes not be overshadowed by recording variations. We found that a sparker source was highly repeatable up to frequencies of several kilohertz for propagation distances out to several tens of meters. In contrast, we observed large variations of the hydrophone coupling to the host rock when the hydrophone streamer was removed and re-inserted into the boreholes. Our investigations have outlined a quantitative methodology to assess the data quality requirements for successful monitoring. We suggest that seismic full-waveform tomography can be used to monitor radioactive waste repositories provided that careful attention is paid to receiver coupling differences.


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