Resolution capacity of geophysical monitoring regarding permafrost degradation induced by hydrological processes

Abstract

Abstract. Geophysical methods are often used to characterize and monitor the subsurface composition of permafrost. The resolution capacity of standard methods, i.e. electrical resistivity tomography and refraction seismic tomography, depends not only on static parameters such as measurement geometry, but also on the temporal variability in the contrast of the geophysical target variables (electrical resistivity and P-wave velocity). Our study analyses the resolution capacity of electrical resistivity tomography and refraction seismic tomography for typical processes in the context of permafrost degradation using synthetic and field data sets of mountain permafrost terrain. In addition, we tested the resolution capacity of a petrophysically based quantitative combination of both methods, the so-called 4-phase model, and through this analysed the expected changes in water and ice content upon permafrost thaw. The results from the synthetic data experiments suggest a higher sensitivity regarding an increase in water content compared to a decrease in ice content. A potentially larger uncertainty originates from the individual geophysical methods than from the combined evaluation with the 4-phase model. In the latter, a loss of ground ice can be detected quite reliably, whereas artefacts occur in the case of increased horizontal or vertical water flow. Analysis of field data from a well-investigated rock glacier in the Swiss Alps successfully visualized the seasonal ice loss in summer and the complex spatially variable ice, water and air content changes in an interannual comparison.

Petrophysical Joint Inversion Applied to Alpine Permafrost Field Sites to Image Subsurface Ice, Water, Air, and Rock Contents

“…Of course, the PJI resolution capacity is directly driven by the resolutions of the input geophysical data. According to Mewes et al (2017), a coarse sensor spacing (up to 8 m in our cases) may also lead to poor spatial resolution of small-scale anomalies, which may be improved by smaller electrode and geophone spacings. However, for all presented profiles, the sensor spacing was not chosen to focus on the thaw layer.…”

“…• A time-lapse joint inversion constraining the rock content being constant with time could be a significant improvement of the method for the sites having both resistivity and P-wave velocity monitoring data available (e.g., SCH or LAH, cf. Hilbich, 2010;Hauck et al, 2017;Mewes et al, 2017). The assumption of a constant rock content with time (in case of absence of excess ice, i.e., not applicable for e.g., rock glaciers) would reduce the number of model parameters and thereby decrease the uncertainties.…”

“…New approaches include the possible estimation of ground ice variation through subsidence measurements using reflected GNSS signals (Liu and Larson, 2018) or through spaceborne gravity measurements (e.g., Vey et al, 2012). Since information from one method is often ambiguous, many cryogeophysical studies use a combination of different geophysical methods (e.g., Hausmann et al, 2007;Kneisel et al, 2008;Doetsch et al, 2012;Buchli et al, 2013;Parsekian et al, 2015;Briggs et al, 2016;Merz et al, 2016;Pellet et al, 2016;Emmert and Kneisel, 2017;Léger et al, 2017;Mewes et al, 2017).…”

“…The so-called four-phase model (4PM) developed by Hauck et al (2011) estimates ice, water and air contents from simple petrophysical relations involving electrical resistivities and seismic velocities. The 4PM was applied successfully in several recent permafrost studies (Pellet et al, 2016;Hauck et al, 2017;Mewes et al, 2017). However, the approach requires to prescribe the porosity and is based on individual seismic and electric inversions with the danger of introducing physical inconsistencies in the final results.…”

Low‐Frequency Induced Polarization of Porous Media Undergoing Freezing: Preliminary Observations and Modeling

“…These methods provide nonintrusive tomographic techniques that can be used to image temperature changes (Hermans et al, ; Revil, Ghorbani, et al, ) and therefore complement in situ temperature point or line measurements using temperature probes such as thermocouples or optical fiber. Geophysical methods have also been used to monitor the evolution of rock glaciers and permafrost (e.g., Hauck et al, , ; Hilbich et al, ; Kellerer‐Pirklbauer & Kaufmann, ; Mewes et al, ; Mollaret et al, ; Springman et al, ).…”