Acoustic Emission Monitoring of Thermal Cycling in Salt at the Waste Isolation Pilot Plant

A series of field-based multi-physics observations were conducted as part of the Brine Availability Test in Salt (BATS) underground at the Waste Isolation Pilot Plant (WIPP), near Carlsbad, NM. Observations were made of brine production, temperature, electrical resistivity, acoustic emissions (AE), and gas/liquid tracer migration through salt around twin heated/unheated borehole arrays. Here, we present AE results from thermal cycling experiments performed May – July 2021, during which the heater was pulsed several times followed by two longer heated periods. Sixteen AE sensors were installed in three parallel observational boreholes surrounding the central heated borehole. Observed AE demonstrate that heating and cooling cycles significantly impact salt behavior. AE rates increased during heating and increased further upon cooling. Average energy and frequency bandwidth also increased during heating and cooling of salt. Similar behavior was observed for each heating/cooling cycle, suggesting that temperature-enhanced salt healing may have eliminated any threshold loading associated with the onset of AE (i.e., the Kaiser effect). Locations of AE events remained centralized around the borehole. These results show that AE provide valuable information about the thermal behavior of salt, particularly that cooling of salt results in the highest AE behavior.