Analysis of the 20-year long permafrost evolution at the long-term monitoring site Stockhorn, Swiss Alps, by applying a petrophysical joint inversion and a thermal model (Cryogrid3).

The Stockhorn plateau, an east-west oriented crest located at an elevation of around 3’410 m a.s.l. in the Swiss Alps, is a measurement site belonging to the Swiss Permafrost Monitoring Network (PERMOS). In this study we present a combined analysis of thermal and geophysical data by applying the so-called petrophysical joint inversion (PJI) scheme (Wagner et al., 2019). By using the PJI approach with different petrophysical relationships (Archie’s law and Resistivity Geometric Mean model) (see Mollaret et al., 2020), we attempt to quantify the ice and water content changes in the subsurface over the past 20 years and analyse their spatial heterogeneity. The results will be validated with the borehole data.

Many different data sets are available for the Stockhorn plateau and they give evidence of permafrost degradation in the past 20 years. Two boreholes were drilled in 2000 and provide temperature measurements to a depth of 17 m and 100 m, respectively. From 2002 to 2020, the active layer depth has increased by 2 m for the northern borehole and by 3.3 m for the southern borehole. A weather station provides measurements since 2002 (PERMOS, 2021). The meteorological data show an increasing air temperature trend from 2003 to 2018 (Hoelzle et al., 2020). Since 2005, annual geoelectrical surveys (ERT) have been performed with collocated seismic surveys (RST) in almost every year. The geophysical data from 2007 to 2021 show a decreasing trend for specific electrical resistivities and P-wave velocities, but a detailed interpretation of the geophysical data is however not straightforward because of heterogeneous lithology as well as the small-scale topography effects causing a complex thermal regime.

The north-south geophysical profile is hereby situated at the boundary between two different rock formations. This is visible through the occurrence of a conductive anomaly observed in the geoelectrical surveys between the two boreholes. In addition, the plateau is covered by different materials such as fine debris, blocky and fine-grained materials, and bedrock, which implies different porosity values along the geophysical profiles in the subsurface. Due to large spatial heterogeneities in the observed temperature and geophysical data, the impact of permafrost degradation on the ground properties such as water and ice content is unclear. In contrast to the formerly used four-phase model (4PM, Hauck et al., 2011), where ERT and RST inversions are computed individually and a porosity distribution had to be prescribed, the PJI scheme has the advantage of obtaining physically consistent results of water and ice content distributions in the ground by inverting the ERT and RST results simultaneously (Wagner et al., 2019). In addition to the validation of the PJI results with the borehole data, it could be possible to validate the results with the thermal model simulations using Cryogrid3 (Westermann et al., 2016).