Mountain permafrost degradation documented through a network of permanent electrical resistivity tomography sites

Abstract

Abstract. Mountain permafrost is sensitive to climate change and is expected to gradually degrade in response to the ongoing atmospheric warming trend. Long-term monitoring of the permafrost thermal state is a key task, but problematic where temperatures are close to 0 ∘C because the energy exchange is then dominantly related to latent heat effects associated with phase change (ice–water), rather than ground warming or cooling. Consequently, it is difficult to detect significant spatio-temporal variations in ground properties (e.g. ice–water ratio) that occur during the freezing–thawing process with point scale temperature monitoring alone. Hence, electrical methods have become popular in permafrost investigations as the resistivities of ice and water differ by several orders of magnitude, theoretically allowing a clear distinction between frozen and unfrozen ground. In this study we present an assessment of mountain permafrost evolution using long-term electrical resistivity tomography monitoring (ERTM) from a network of permanent sites in the central Alps. The time series consist of more than 1000 datasets from six sites, where resistivities have been measured on a regular basis for up to 20 years. We identify systematic sources of error and apply automatic filtering procedures during data processing. In order to constrain the interpretation of the results, we analyse inversion results and long-term resistivity changes in comparison with existing borehole temperature time series. Our results show that the resistivity dataset provides valuable insights at the melting point, where temperature changes stagnate due to latent heat effects. The longest time series (19 years) demonstrates a prominent permafrost degradation trend, but degradation is also detectable in shorter time series (about a decade) at most sites. In spite of the wide range of morphological, climatological, and geological differences between the sites, the observed inter-annual resistivity changes and long-term tendencies are similar for all sites of the network.

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

“…The input data of the joint inversion problem consist of compressional wave travel times and apparent electrical resistivities. Apparent resistivity data were acquired through a Geotom instrument (Geolog, Germany) or a Syscal resistivity meter (Iris Instruments, France) and were filtered following the procedure described in Mollaret et al (2019). Seismograms were recorded through a Geode system (Geometrics, USA).…”

“…Annual geophysical measurements of the same profile are performed since 2013 (Pogliotti et al, 2015;Pellet et al, 2016;Mollaret et al, 2019). In the tomogram in Figure 2D, the lowest velocities (corresponding to a high air content) occurring in the near-surface are interpreted to be a porous debris layer.…”

“…Geophysical measurements were performed along two orthogonal profiles: the Lapires horizontal profile (LAH, Hilbich, 2010;Mollaret et al, 2019;PERMOS, 2019a) and the Lapires vertical profile (LAV, PERMOS, 2013; Scapozza, 2013). Hilbich (2010) investigated and jointly interpreted seasonal changes in electrical resistivities and seismic travel times at LAH.…”

Ice content and interannual water storage changes of an active rock glacier in the dry Andes of Argentina

“…Iterative tomographic inversions of apparent resistivities measured at different electrode locations and separations along a profile finally yield a two-dimensional model of the specific resistivity of the subsurface. The application of ERT has become a standard technique to image and monitor 220 variations of electrical resistivity in the near subsurface of mountain permafrost due to its sensitivity to the state of water (liquid or solid) in unfrozen and frozen conditions (Hauck, 2013;Hilbich et al, 2008;Mollaret et al, 2019).…”

“…ice-rich permafrost in rock glaciers are less sensitive to climatic warming than permafrost with low ice content (Scherler 55 et al, 2013) due to complex interactions of advective and convective heat fluxes and latent heat effects during freeze and thaw processes (Scherler et al, 2010). However, ice melt is also predicted for rock glaciers (Marmy et al, 2016) and already observed in the Alps (Mollaret et al, 2019).…”