Coda wave interferometry to monitor the evolution of rock mass damage and saturation in Opalinus Clay: a study at the Mont Terri Underground Laboratory

We explore the potential to monitor the evolution and extent of structurally controlled rock mass damage in faulted Opalinus Clay shale at the Mont Terri Underground Rock Laboratory (MT URL) using coda wave interferometry. In a dedicated experiment, a borehole was drilled through the main fault of the MT URL in October 2020. The borehole was continuously ventilated for 21 months and then resaturated from July 2022 onward. We performed repeated time-lapse seismic experiments using 12 acoustic emission sensors and 12 ultrasonic transmitters distributed across four monitoring boreholes. The first-arriving P waves remained largely unchanged, and their velocities provide valuable insights into the initial health state of the Opalinus Clay. However, the later-arriving coda waves show clear changes in seismic velocity and waveform. Using probabilistic inversion, we link these changes to variations in rock saturation and pore pressure, as well as to geometric alterations of the rock mass surrounding the experimental borehole caused by new fracture formation, slip along pre-existing tectonic faults, and borehole overbreaks.