Monitoring rock freezing and thawing by novel geoelectrical and acoustic techniques

Automated monitoring of freeze-thaw cycles and fracture propagation in mountain rockwalls is 23 needed to provide early warning about rockfall hazards. Conventional geoelectrical methods 24 such as electrical resistivity tomography (ERT) are limited by large and variable ohmic contact 25 resistances, requiring galvanic coupling with metal electrodes inserted into holes drilled into 26 rock, and which can be loosened by rock weathering. We report a novel experimental 27 methodology that combined capacitive resistivity imaging (CRI), ERT and microseismic event 28 recording to monitor freeze-thaw of six blocks of hard and soft limestones under conditions 29 simulating an active layer above permafrost and seasonally frozen rock in a non-permafrost 30 environment. Our results demonstrate that the CRI method is highly sensitive to freeze-thaw 31 processes; it yields property information equivalent to that obtained with conventional ERT and 32 offers a viable route for non-galvanic long-term geoelectrical monitoring, extending the benefits 33 of the methodology to soft/hard rock environments. Contact impedances achieved with CRI are 34 less affected by seasonal temperature changes, the aggregate state of the pore water (liquid or 35 frozen), and the presence of low-porosity rock with high matrix resistivities than those achieved 36 with ERT. Microseismic monitoring has the advantage over acoustic emissions that events were 37 recorded in relevant field distances of meters to decameters from cracking events. For the first 38 time we recorded about 1000 microcracking events and clustered them in four groups according 39 to frequency and waveform. Compared to previous studies, mainly on ice-cracking in glaciers, 40 the groups are attributed to single- or multiple-stage cracking events such as crack coalescence.