Understanding the effects of permafrost degradation through a multiphysics approach

Abstract

Permafrost is a multiphase porous media that can host matter in all three states (solid, liquid, and gas). The equilibrium between the states of matter within the pore space is largely driven by salinity, pressure, and temperature. The complex interactions between the different thermodynamic processes can lead to a complex pore system that is altered at each subsequent thaw and freeze cycle. The dynamic changes imposed on this porous media alter the mechanical and electrical properties of the samples. These changes can thus be quantified and monitored using ultrasonic and electrical resistivity measurements. The experimental results presented in this work document the impacts of a thawing event on unconsolidated quartz sand samples that are partially saturated with a brine solution. The electrical resistivity and ultrasonic data are acquired simultaneously throughout the experiment, and the spatiotemporal changes within the solid matrix are captured by time-lapse X-ray computed tomography (CT). A total of 39 different samples are investigated. The two independent variables chosen for this study are the grain size and the salinity of the brines. The results indicate a clear transition in electrical and elastic properties as the material in the pore space transitions between two different states. Further results indicate that these transitions are the result of the alteration of the pore network itself. In addition, the study of P-wave velocity, ice fraction, and X-ray CT of two different types of ice that coexist within the pore network is documented. Given the distinct impact of two different types of ice on this cryogenic porous media, it is imperative to thoroughly comprehend the existence of different ice types before undertaking the electroelastic investigation of permafrost.