Monitoring changes in unfrozen water content with electrical resistivity surveys in cold continuous permafrost

Abstract

For permafrost, information on unfrozen water content is critical for thermal modelling, understanding permafrost evolution and for predicting thaw susceptibility for infrastructure. We utilize electrical resistivity surveys for the detection and monitoring of unfrozen water content in cold continuous permafrost. We develop a data processing scheme based on a common subset of reciprocal measurements to filter data from irregular acquisitions in variable environmental conditions to a standard noise level, such that models recovered from disparate data sets can be compared quantitatively. Using temperature-corrected post-inversion model differencing, changes in recovered electrical resistivity models are used to calculate the relative change in unfrozen water content over common regions of high model resolution. The technique is applied to electrical resistivity data collected seasonally using a permanent electrode installation in cold continuous permafrost alongside airport infrastructure in the Canadian Arctic. The number of healthy data decreases over the experiment duration, with a correlation between data quality and ground surface temperature. Using the resistivity models, relative changes in unfrozen water content are estimated that are consistent with temperature records. Results are compared to an empirical temperature-dependent water content model obtained via calibrated thermal modelling at the study site. When compared to the empirical model, the changes in resistivity overpredict the changes in unfrozen water content due to temperature-induced phase changes alone after one year of acquisition. Prevalent changes in unfrozen water content occur in a zone below the active layer at temperatures of approximately 0 to -4.5 °C, and may be indicative of permafrost evolution involving ice formation or water movement over many months in moderately-saline permafrost beneath airport infrastructure.