Conference Agenda

Mountain permafrost is currently undergoing major changes showing clearly detectable ground temperature increase and ground ice content loss. On regional scales, it becomes standard to quantify ground ice loss through electrical, seismic and electromagnetic techniques. Recently developed joint inversion approaches combining different tomographic techniques allow for ground ice loss quantification over time (Steiner et al. 2021, Morard et al. 2024).

The recently funded TREAT project addresses current research questions regarding the future evolution of mountain permafrost such as the existence of tipping points causing irreversible permafrost degradation, the influence of anomalously hot periods (Hauck & Hilbich 2024), the resilience of coarse-blocky substrates to warming events and whether thawing permafrost slopes become wetter or drier in future. Consequently, we further develop geophysical joint inversion techniques and numerically couple a thermo-hydraulic permafrost model to geophysical monitoring data from several observatories in the Alps (Maierhofer et al. 2024). First results of the project and potential paths for future research will be presented.

Hauck C & Hilbich C (2024). Preconditioning of mountain permafrost towards degradation detected by electrical resistivity. Environ.Res.Lett. 19 064010; DOI10.1088/1748-9326/ad3c55

Maierhofer T, Flores Orozco A, Roser N, Limbrock JK, Hilbich C, Moser C, Kemna A, Drigo E, Morra di Cella U & Hauck C (2024). Spectral induced polarization imaging to monitor seasonal and annual dynamics of frozen ground at a mountain permafrost site in the Italian Alps, The Cryosphere, 18, 3383–3414, https://doi.org/10.5194/tc-18-3383-2024

Morard S, Hilbich C, Mollaret C, Pellet C & Hauck C (2024). 20-year permafrost evolution documented through petrophysical joint inversion, thermal and soil moisture data. Environ.Res.Lett., 19; 074074, DOI10.1088/1748-9326/ad5571.

Steiner M, Wagner FM, Maierhofer T, Schöner W & Flores Orozco A (2021). Improved estimation of ice and water contents in Alpine permafrost through constrained petrophysical joint inversion: The Hoher Sonnblick case study. Geophysics, 86(5), WB61-WB75.

Joints and fractures, where large volumes of ground ice can accumulate, play a key role in the thermal regime of bedrock permafrost as they represent a preferential flow path for water and advective heat exchange between the atmosphere and the subsurface. Yet, it is challenging to include them in thermal modelling because their exact location and geometry are commonly unknown. In the Cime Bianche (Aosta valley, Italy) permafrost monitoring area (highly weathered bedrock plateau) at 3100 m a.s.l. borehole temperature measurements and geophysical imaging have shown a high spatial variability in ground temperature and ice content, which suggests the presence of fractured areas affecting the ground thermal regime. We conducted saltwater tracer tests (30-100 l water per injection) at eight different locations at the Cime Bianche site coupled with time-lapse 3D electrical resistivity tomography (ERT) to track the flow of the subsurface amendment and localize flow paths associated to fractured areas. The ERT results show a large difference in flow characteristics between the injection points, even for points in close vicinity (few meters). In five of eight injection points the water infiltrated slowly into the subsurface (over 2-3 hours) resulting in slight conductivity changes over time in the 3D ERT images only close to the injection point. In the other three positions the injected water disappeared after a few seconds resulting in larger conductivity changes, not only directly below the injection point but also to a depth of >10 m, suggesting the presence of fractures. These results demonstrate that our methodology is able to reveal fractures and other preferential flow paths. To further improve the resolution of the resolved fractures more adequate inversion schemes and constraints from complementary data are required.

The causes behind alpine permafrost thawing are multifactorial, with most abrupt active layer thickening observed after hot summers. As intensity of heatwaves increases, it becomes crucial to quantify their impact to identify a potential tipping point beyond which permafrost may not recover. Although existing climatological studies provide several tools for analysing heatwaves (Perkins & Alexander, 2013), these have not been widely applied to Alpine settings. Here we employ the Heat Wave Magnitude Index daily (HWMId) metric (Russo et al., 2015) for temperature analysis from PERMOS and MeteoSwiss stations near prominent Swiss permafrost monitoring sites. Historical and reconstructed data determine site-specific temperature thresholds, as systematic heatwave definitions do not apply uniformly. Beyond observations in borehole temperatures, using the petrophysical joint inversion (Wagner et al. 2019), the HWMId can be compared to time series of ground ice content time series obtained from geophysical data inversions (seismic refraction and electrical resistivity tomographies). Case studies at Schilthorn and Stockhorn highlight the role of the sediment cover and morphology of the site. Analysis of multidecadal temperature and geophysics data shows accordance between decreasing ground resistivity and increasing heatwave frequency and intensity. Further analysis would benefit from integrating metrics like freezing/thawing degree days and considering local geomorphology. Expanding the study to additional PERMOS sites could provide insights into permafrost resilience across landforms.

References:

Perkins, & Alexander (2013): On the Measurement of Heat Waves. J.Climate, 26, 4500–4517, doi :10.1175/JCLI-D-12-00383.1

Russo, et al. (2015): Top ten European heatwaves since 1950 and their occurrence in the coming decades. Environmental Research Letters, 10, 124003, doi:10.1088/1748-9326/10/12/124003

Wagner, et al. (2019): Quantitative imaging of water, ice and air in permafrost systems through petrophysical joint inversion of seismic refraction and electrical resistivity data. Geophysical Journal International, 219 (3), 1866-1875, doi:10.1093/gji/ggz402

In permafrost research, geoelectrical surveys are increasingly used to detect the presence and extent of permafrost and to characterise its stratigraphy and material composition. When repeated, the resulting temporal changes in electrical resistivity can be related to changes in ground temperature and ice content, and therefore also to ground ice loss over time. However, for financial and logistical reasons, only a few continuous electrical resistivity tomography (ERT) monitoring installations on permafrost exist worldwide. An alternative approach are manual but regularly repeated ERT measurements, such as - besides other examples - in the context of the Swiss Permafrost Monitoring Network (PERMOS, 2024). In contrast, there exist many permafrost profiles (estimated to be over 500 in Switzerland) where single ERT measurements have been performed in the past.

In this contribution, we analyse both spatial and temporal variations of the Swiss datasets, which are integrated in the International Database of Geoelectrical Surveys on Permafrost (IDGSP), led by the International Permafrost Association (IPA) Action Group of the same name. Since the launch of the IPA Action Group in 2021, a database has been designed and set up, numerous metadata and data have been collected and homogenised, and public access via a searchable web map is now available (https://resibase.unifr.ch).

The overall goal of the project (supported by the Swiss Geophysical Commission) is to establish a complete database of electrical measurements on permafrost in Switzerland, including ideally all historical measurements. The historical data are re-processed with newly developed filtering and inversion routines and made available to the public to facilitate the repetition of measurements in the context of climate warming induced permafrost degradation, geotechnical studies of permafrost stability, hydrological studies in the context of natural hazards and water availability from thawing permafrost environments, and to serve as a baseline dataset for permafrost distribution and modelling.

This study aims to investigate the connection between different physical parameters of the ground in permafrost environments, namely electrical resistivity, temperature and water content. The area of interest is located on James Ross Island in the north-eastern part of the Antarctic Peninsula. It is underlain by continuous permafrost and experiences semi-arid polar continental climate with a mean annual air temperature around -7 °C. An automated electrical resistivity tomography (A-ERT) setup using a 4POINTLIGHT_10W (Lippmann) device was installed in the vicinity of the Czech Antarctic station Johann Gregor Mendel in late February 2023. The setup measures ground electrical resistivity daily across a 23-metre transect using 47 electrodes with 0.5 m spacing. The transect consists of two distinct lithologies – a Holocene marine terrace and finer-grained Cretaceous sediments. The lower boundary of the A-ERT investigation depth lies approximately 4.5 meters below ground surface in the middle of the profile. Several temperature (sensors placed at 5, 10, 20, 30, 50, 75, 100, 150 and 200 cm) and soil moisture (sensors at 5, 35, 55 and 75 cm depth) profiles are installed at different points along the transect. With data spanning now approximately one year, we demonstrate how different lithologies affect ground thermal and moisture regime and subsequently its resistivity to electrical current flow. Typically, resistivity values are much higher in the winter (ca. 1-2 kΩm) and decrease abruptly as the ground thaws (ca. 10-100 Ωm). Ground temperature and soil moisture both exert direct control over the resistivity changes. During the thawing phase, the progression of the thaw front is clearly visible from the resistivity data. So far, A-ERT has proven a rugged and reliable way to monitor the state of permafrost in the harsh conditions of Antarctica over larger profile distances, providing data beyond the capabilities of the traditional borehole measurement approach.

Pingos are among the most prominent landforms in Arctic permafrost regions. They exist in a variety of shapes and sizes and are classified as either of hydraulic (open-system in terms of groundwater inflow) or hydrostatic (closed-system) origin. Along with similar-looking features such as palsas, lithalsas or frost blisters, they are commonly referred to as permafrost mounds. In northwestern Canada, apart from numerous hydrostatic pingos in the Inuvik-Tuktoyaktuk coastland area, hydraulic pingos are mainly described for the interior region of Yukon. This study investigates the internal structure of different mounds containing permafrost in order to assess their origin and to discuss their relation to the hydrological pattern and the glacial history of the Ogilvie Mountains. Therefore, Electrical Resistivity Tomography measurements were conducted at mounds of different sizes distributed in two river valleys of the Tombstone Territorial Park, central Yukon. Thousands of data points allow the generation of high-resolution quasi-3D models that can provide multidimensional insights about frozen and unfrozen zones. The results show that not just the outward appearance but also geophysical subsurface properties differ remarkably. In the tallest mound of >20 m height even within the frozen part resistivities vary by up to an order of magnitude reflecting different ground ice characteristics. Here, the central core shows values that are typical for massive ice. Just on the opposite side of the valley, in another mound and the adjacent mountain slope, likely unfrozen low-resistivity zones were found. Those taliks can serve as intra-permafrost water pathways that enable the pingo formation on the valley floor. In this region, periglacial climate conditions, mountainous terrain and permanently changing river channels leading to the formation and drying of oxbow lakes, create environments that might have been suitable for both hydraulic and hydrostatic pingo formation.