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 the permafrost thermal state is a key task, but it is problematic where temperatures are close to 0 °C. The energy exchange is indeed often 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 of 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 data sets from six sites, where resistivities have been measured on a regular basis for up to twenty 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 data set provides the most valuable insights at the melting point. A prominent permafrost degradation trend is evident for the longest time series (19 years), but also detectable for 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.

A Comparison of Frequency Domain Electro-Magnetometry, Electrical Resistivity Tomography and Borehole Temperatures to Assess the Presence of Ice in a Rock Glacier

“…The ERT data clearly shows the presence of a high resistivity value layer (>15 kΩ m) between 4 and 16 m depth. This layer most likely corresponds to a frozen zone, with values similar to those collected at other Swiss permafrost sites (Vonder Mühll 1996;Mollaret et al, 2019). Unpublished ERT data obtained further East in 1990 at the same site and at borehole B1 (VAW 1991) show similar values.…”

“…Important changes in the ice to water ratio within the permafrost body, which mainly occur close to 0°C, are not discernible from ground temperature data alone. This makes the monitoring of ice-rich permafrost during phase change particularly challenging (Kaab et al, 2007;Jones et al, 2018;Mollaret et al, 2019).…”

“…To extend the area of measurement beyond the pinpoint of a borehole, geophysical measurements such as electrical resistivity tomography (ERT) and refraction seismics are often used (Hilbich et al, 2009;Hauck et al, 2011). The electrical resistivity of the ground can in fact provide indirect information on the active layer thickness (ALT) and particularly, on changes in the ice to water ratio (Mollaret et al, 2019). This is a highly important complement to borehole measurements because changes in ground ice and unfrozen water content cannot be detected using temperature data, due to the zero-curtain effect resulting from latent heat exchange during phase change.…”

“…This is a highly important complement to borehole measurements because changes in ground ice and unfrozen water content cannot be detected using temperature data, due to the zero-curtain effect resulting from latent heat exchange during phase change. ERT is an established method that covers long lines or grids and is operationally applied in mountain permafrost monitoring programs (Mollaret et al, 2019;PERMOS 2007;PERMOS 2019). It is however a laborious method with heavy equipment, requiring the use of multiple steel electrodes with good contact to the substrate to allow the efficient transfer of an electrical current.…”

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, ).…”