Monitoring mountain permafrost evolution using electrical resistivity tomography: A 7‐year study of seasonal, annual, and long‐term variations at Schilthorn, Swiss Alps

Abstract

[1] A combined geophysical and thermal monitoring approach for improved observation of mountain permafrost degradation is presented. Time-lapse inversion of repeated electrical resistivity tomography (ERT) measurements allows both active layer dynamics and interannual permafrost conditions to be delineated. Analysis of a comprehensive ERT monitoring data set from a 7-year study at Schilthorn, Swiss Alps, confirmed the applicability of ERT monitoring to observations of freezing and thawing processes on short-term, seasonal, and long-term scales. Long-term resistivity changes were evaluated on the basis of seasonal resistivity variations showing an annual cycle with high resistivities in frozen and low resistivities in unfrozen state. One important result is the detection of a sustained impact of the extraordinarily hot European summer of 2003 on the permafrost regime, which is more severe than previously assumed from borehole temperatures. Combined analyses of ERT monitoring and borehole temperature data revealed substantial ground ice degradation as a consequence of the 2003 summer, which did not recover in the following years despite suitable subsurface temperature conditions. Resistivity changes that are nonconforming to long-term temperature evolution are attributed to the limited availability of liquid water and/or changes in material characteristics (e.g., pore volume changes as a result of subsidence).Citation: Hilbich, C., C. Hauck, M. Hoelzle, M. Scherler, L. Schudel, I. Völksch, D. Vonder Mühll, and R. Mäusbacher (2008), Monitoring mountain permafrost evolution using electrical resistivity tomography: A 7-year study of seasonal, annual, and long-term variations at Schilthorn, Swiss Alps,

Meltwater infiltration into the frozen active layer at an alpine permafrost site

“…In the observation period from October 2003 to October 2004, the active layer extended to a depth of 7 m to 9 m below the surface. This is much deeper than in preceding years, reflecting the exceptionally warm summer of 2003 in the European Alps (see Schär et al, 2004;Hilbich et al, 2008).…”

“…However, these inaccuracies develop predominantly at greater depths, where the sensitivity of the method is diminished, below the area of largest resistivity increase or decrease (Rings et al, 2008;Marescot et al, 2003;Hilbich et al, 2008).…”

“…Thus, low resistivity readings indicate high water content or high ionic strength, while high readings indicate low water content or low ionic strength. Assuming that pore space distribution and ionic strength of a measured soil profile do not change significantly between two measurements, resistivity differences over time can yield semi-quantitative information about changes in water or ice content (Hilbich et al, 2008). Two-dimensional ERT was used to give a rapid, qualitative overview of the evolution of soil moisture over a relatively large area.…”

“…In 1999, a semi-automatic electrical resistivity array was permanently installed at the site. The array is situated on a small plateau perpendicular to the gradient (see Hilbich et al, 2008, Figure 2) and is used to monitor water and ice content distribution in the uppermost 10 m (Hauck, 2002;Hilbich et al, 2008).…”

“…Therefore, in this study, we used electrical resistivity tomography (ERT) measurements combined with borehole temperature logs as alternative means to assess infiltration (French and Binley, 2004). This method can investigate spatial differences in the evolution of water and ice content over time spans of days to years (Hauck, 2002;Hilbich et al, 2008).…”