Appraisal of waveform repeatability for crosshole and hole-to-tunnel seismic monitoring of radioactive waste repositories

Abstract

Countries worldwide are seeking solutions for the permanent removal of high-level radioactive waste from the environment. Surrounding the waste with multiple engineered barriers and emplacement in deep geological repositories is widely accepted as a safe means of isolating it from the biosphere for the necessary 10 5 -10 6 years. As a precautionary measure, society demands that repositories be monitored for 100-300 years after they are backfilled and sealed. Effective monitoring that does not compromise the engineered and natural barriers is challenging. To address this issue, we investigate the viability of crosshole and hole-to-tunnel seismic methods for remotely monitoring high level radioactive waste repositories. Measurements are made at two underground rock laboratories in Switzerland, one within granitic rock and one within clay-rich sediments. Numerical simulations demonstrate that temporal changes of the monitored features ͑i.e., bentonite plug, excavation damage zone, sand-filled microtunnel͒ should produce significant changes in the seismic waveforms. Nevertheless, inversion for medium-property changes requires that true seismic waveform changes are not overwhelmed by recording variations. We find that a P-wave sparker source is highly repeatable up to frequencies of 3 -4 kHz for propagation distances out to tens of meters involved in repository-scale monitoring. Hydrophone repeatability is limited by incoherent high frequency noise and variable hydrophone-borehole coupling conditions, but firmly grouted geophones within the tunnels yield consistent recordings. Three kinds of coherent noise contaminate the data: ͑1͒ mechanically induced electrical effects in the hydrophone chains; ͑2͒ high currents in the sparker cable, which cause it to oscillate radially as a line source; and ͑3͒ tube waves. Our investigations outline a quantitative methodology to assess data-quality requirements for successful monitoring. We suggest that full waveform seismic tomography can be used to monitor radioactive waste emplacement tunnels, provided that careful attention is paid to instrument fidelity and noise suppression.

Laboratory measurements of the longitudinal and transverse wave velocities of compacted bentonite as a function of water content, temperature, and confining pressure

“…We measured the wave speeds at an ultrasonic frequency of ~0.1 MHz, whereas in a realistic repository monitoring, seismic frequencies on the order of 1–10 kHz would be employed [ Marelli et al ., ; Manukyan et al ., ]. Due to the velocity dispersion, we expect a quite large mismatch between velocities measured at ultrasonic and seismic frequency.…”

“…Changes in water content, swelling pressure, and temperature may have a profound influence on the elastic properties of the bentonite: its longitudinal and transverse wave velocities ( V P and V s ), density ( ρ ), and attenuation (1/ Q ). If this sensitivity to elastic properties is measurable, seismic imaging will be a useful technique for nonintrusive monitoring of the barrier [e.g., Manukyan et al , ; Marelli et al ., ].…”

Multi-Disciplinary Monitoring Networks for Mesoscale Underground Experiments: Advances in the Bedretto Reservoir Project

“…We did not find a geophone suitable for the BRP objectives (pressure resistance, small diameter, and suitable for cementation), especially because many manufacturers could not provide information on the absolute instrument response summarizing both the sensor and the housing. For the borehole installation, we therefore asked the Institute of Mine Seismology (IMS), Somerset West/South Africa, to build a custom-made geophone using 3 uniaxial GS-100 sensors from Geospace (natural frequency 100 Hz), which were tested in our department [ 38 ], and to combine it with the IMS geophone borehole housing for high pressures (<20 MPa) typically used for the IMS 14 Hz geophone. The GS-100 sensor is spurious free up to 600 Hz.…”

Longitudinal and Shear Wave Velocities Measurement in Compacted Bentonite for Water Content

“…Ultrasound is one of the elastic waves and is a useful technique for the laboratory experiments. Recently, ultrasonic velocity in compacted bentonite has been investigated for laboratory experiments [1], and seismic imaging technique for nonintrusive monitoring of the barrier in repositories [2] has been studied. For these reasons, in this study, the non-destructive ultrasonic measurement technique for monitoring of water saturation level in compacted bentonite was developed in our laboratory.…”