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

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).