Geophysical investigation of permafrost conditions in a thermokarst-prone area in Fairbanks, Alaska

Abstract

Permafrost degradation is a significant environmental concern for cold regions, posing risks to communities and infrastructure. The warming of near-surface permafrost leads to diverse topographic variations in the Arctic and sub-Arctic communities. The varying rates of thawing permafrost, influenced by ground ice content, give rise to geologic hazards like thermokarst, which causes ground subsidence. This gradual or sudden subsidence endangers existing infrastructure and economic activities in cold regions. A subsurface 2D Electrical Resistivity Tomography survey was conducted in the West Ridge area of the University of Alaska Fairbanks upper campus to advance our understanding of frozen ground conditions. The objective was to monitor the ground thermal regime and permafrost conditions over a year and characterize permafrost conditions at a thermokarst-prone site. Various data filtering and processing techniques were employed for analysis, including the Depth of Investigation index, optimal smoothing parameters, and the utilization of two array types. Additionally, borehole data and the Very Low-Frequency Electromagnetic method were integrated to provide further insights into subsurface features and resistivity contrasts. This approach aimed to mitigate potential misinterpretation or overinterpretation of inversion results. By employing the 2D Electrical Resistivity Tomography method, we gained valuable information on the spatial variability of transient processes, such as the movement of freezing and thawing fronts. Analysis of the time-lapse data revealed a pronounced increase in resistivity within the active layer during the winter, followed by a decrease during summer. Resistivity profiles across the site prone to thermokarst depression exhibited distinct variations in permafrost conditions, with both low and high resistive anomalies observed along the transects. These anomalies, representing taliks and ice wedges, were characterized by resistivity values below 50 Ωm and above 700 Ωm, respectively. The Very Low-Frequency Electromagnetic method results demonstrated similar resistivity trends, although the anomaly patterns differed from those observed in the ERT data. The outcomes of this study contribute to an improved understanding of permafrost, which is vital for engineering applications and infrastructure stability.