Measuring Active Layer Thickness and Permafrost Shear-Wave Velocity as a Potential Proxy for Thaw Stability Using Surface-Wave Methods

Abstract Two of the most important design considerations of civil infrastructure in cold regions are active layer thickness and permafrost thaw stability. This study examines whether noninvasive surface-wave methods, specifically the multichannel analysis of surface waves, can effectively measure active layer thickness and permafrost shear-wave velocity ( V s ) as a potential proxy for its thaw stability. To address these questions, time-lapse surface-wave measurements were performed at six sites along the Steese Highway between Fairbanks and Circle, Alaska. The first set was performed in August 2023, around maximum thaw, and the second in January 2024, when the active layer was frozen. Data acquisition was designed to address the limitations of previous studies and to enable recommendations for performing surface-wave methods in cold regions. Key recommendations include collecting surface-wave measurements using multiple wavefield polarities, performing preliminary data processing in the field, acquiring data with different receiver spacings, and using a physics-informed mode assignment procedure. As invasive geotechnical measurements could not be made, the surface-wave results are validated by comparisons with local geology, typical V s values, and time-lapse V s measurements made at the same location. When following the aforementioned recommendations, surface-wave methods have been shown to be effective at characterizing active layer thickness and the V s of frozen ground. Furthermore, while based on a limited number of sites, frozen ground with V s values much less than pure ice (around 400 m/s) is shown to correlate to geologies with thaw stability (i.e., coarse-grained soils), whereas frozen ground with much higher values of V s (above 800 m/s) is shown to correlate to geologies with thaw instability (i.e., fine-grained soils).