Determination of warm, sensitive permafrost areas in near‐vertical rockwalls and evaluation of distributed models by electrical resistivity tomography

Abstract

Alpine rockwalls with warm permafrost (near 0°C) are the most active rockfall detachment zones in the Mont Blanc massif (MBM, French Alps) with more than 380 recent events. Near-vertical rockwall permafrost is spatially controlled by variations in rock fractures, snow cover, and microtopography. A reliable method to validate the distribution of permafrost in critical and unstable areas does not yet exist. We present seven electrical resistivity tomography (ERT) surveys measured on five near-vertical rockwalls in the MBM from 2012 and 2013 that have been calibrated with measurements on a granite sample in the laboratory. ERT shows consistent measurements of remaining sensitive permafrost relating to inferred temperatures from 0 to À1.5°C. ERT results demonstrate evidence of topographic controls on permafrost distribution and resistivity gradients that appear to reflect crest width. ERT results are compared to two permafrost index maps that use topoclimatic factors and combine effects of thin snow and fractures, where index model spatial resolution is crucial for the validation with ERT. In cryospheric environments, index maps seem to overestimate permafrost conditions in glacial environments. As a consequence, the sensitive areas of permafrost may slightly deviate from the results from distributed models that are only constrained by topoclimatic factors and interpreted with consideration of local fracture and snow conditions. This study demonstrates (i) that the sensitive and hazardous areas of permafrost in near-vertical rock faces can be assessed and monitored by the means of temperature-calibrated ERT and (ii) that ERT can be used for distributed model validation.

Sidewall erosion: Insights from in situ‐produced 10Be concentrations measured on supraglacial clasts (Mont Blanc massif, France)

“…Böhlert et al (2008) and Gallach et al (2018) suggest that the morphology of present-day rockwalls mainly results from a juxtaposition of rockfall scars of variable ages. (Magnin et al, 2015), and rockfall events are mainly triggered by permafrost warming (Gruber and Haeberli, 2007;Deline et al, 2012). Further, 10 Be ages of between less than 0.5 and 60 ka have been measured on the rockwalls (Böhlert et al, 2008;Gallach et al, 2018).…”

Modelling rock wall permafrost degradation in the Mont Blanc massif from the LIA to the end of the 21st century

“…It recorded the rock temperature at depths of 3, 10, 30 and 55 cm until January 2013. The 2012 mean annual rock surface temperature (MARST) at a depth of 3 cm was −1.4 • C. In 2012 and 2013, electrical resistivity tomography (ERT) soundings were conducted along the NW face of the GM and four other sites of the massif (Magnin et al, 2015d). The potential of ERT for qualitative evaluation of 2-D permafrost models has been demonstrated by , as it covers a much wider and deeper rock wall portion than borehole.…”

“…The potential of ERT for qualitative evaluation of 2-D permafrost models has been demonstrated by , as it covers a much wider and deeper rock wall portion than borehole. This makes ERT a potentially better approach with which to evaluate distributed models of rock wall permafrost because it has the capacity to represent the spatial variability of rock wall permafrost (Magnin et al, 2015d). Conversely, direct temperature measurements allow for quantitative evaluation, but have the disadvantage of being only representative for the measurement point.…”

Vertical electrical sounding of soils and permafrost of marine terraces of Gronfjord (Svalbard archipelago)

“…The difference is caused by different amount of gravitational type of water in the topsoil. It has been previously shown that electrical resistivity is decreasing with higher amount of gravitational type of water in soil pores (Samouëlian et al 2005, Pozdnyakov 2008, Magnin et al 2015. The depths of the active layer-permafrost boundary lies on the depth 80-90 cm and are shown in Fig.…”