Time-Lapse Measurement of P-Wave Velocities Under the Freeze-Thaw Condition in Various Rocks
Authors: Cai Guanglei , Yang Hui , Zhao Qi 
Published: September 2023

ABSTRACT
Global warming issues have led to pronounced cryospheric changes in mountainous areas and cold regions. Freeze-thaw weathering can alter the properties of rock masses and permafrost soils, drastically changing local environmental and geological conditions. So far, although many in-situ and laboratory tests have investigated the variation of mechanical and hydraulic properties of rock under changing temperatures, the effect of temperature variation on the rock seismic properties (e.g., elastic wave velocity, wave amplitude, wave frequency, and wave attenuation) is still insufficiently studied. In this study, we developed a laboratory setup for time-lapse measurement of ultrasonic wave propagation across rocks under freeze-thaw conditions. The P-wave velocities under changing temperature from −20 °C to 20 °C were continuously recorded for four types of rocks (i.e., granite, marble, sandstone, and gabbro) under both dry and saturated conditions. The results demonstrate that the P-wave velocity of all saturated rocks drops significantly when the temperature is above 0 °C. Particularly, saturated sandstone was observed to have the greatest drop in wave amplitude, probably owing to the relatively high water content due to its high porosity. Also, we observe a hysteresis of wave velocity related to the rocks’ temperature gradient during the freeze-thaw cycles. These findings provide new insights into rock dynamic responses related to temperature changes in the cryosphere and cold regions.

INTRODUCTION
Global warming has led to extensive and enduring climate anomalies worldwide, including extremely high air temperatures in summer, fierce seasonal floods, devastating rainfalls, and so forth (Goswami, 2009; Masson-Delmotte, 2022; Rovinaru et al., 2022). The temperature rise also causes the melt of the polar glaciers, permafrost, and mountainous ice in cold regions, negatively influencing the slope stability and infrastructure performance for a long term and resulting in rock-mud flow events and uneven settlement of ground surface (Krautblatter et al., 2013; Mamot et al., 2021; Pläsken et al., 2020).
Since most polar grounds and alpine mountain areas are underlain by permafrost, which is thermally defined as the lithospheric material (except glaciers) that remains below or equal to 0 °C for at least two consecutive years (Tarnocai, 1989), they are considered particularly sensitive to climate change (Nogués-Bravo et al., 2007; Schiavon & Zecchin, 2007). The degrading permafrost due to increasing temperature can trigger the freeze-thaw weathering effects and therefore affect the thermal, hydraulic, and mechanical properties of the rock masses, soil bodies, and their interfaces resulting in a drastic change in local environmental and geological conditions (Gruber & Haeberli, 2007; Krautblatter et al., 2012). The changes in the cryosphere will consequently increase the occurrence and magnitude of alpine rock hazards such as rock slope landslide, abrupt rockfall activities, and rock-based infrastructure damage resulting from slow and large rock deformation (Haeberli et al., 2017; Stoffel & Huggel, 2012).