Laboratory and field validated temperature-resistivity relations in bedrock permafrost

Electrical resistivity tomography (ERT) has become a well-established geophysical method for monitoring the thermal state of permafrost sites. However, quantitative interpretation of ERT data requires corresponding temperature information, either from direct borehole temperature measurements or from laboratory-based calibrations. Borehole measurements are costly to implement and remain scarce in alpine environments. Temperature-resistivity relations derived from laboratory experiments are generally site-specific, restricted to individual lithologies, and only rarely validated against field observations.

Here, we present temperature-resistivity relations derived from laboratory experiments on 12 low-porosity rock samples representing different sedimentary, metamorphic, and igneous lithologies. The samples were collected from permafrost-affected summit areas of Zugspitze (DE/AT), Großglockner (AT), Kitzsteinhorn (AT), Gemsstock (CH), Steintälli (CH), Gámanjunni-3 (NOR), Nordnes (NOR), and the Mannen plateau (NOR). The temperature-resistivity pathways are analysed with respect to porosity and mineral composition for unfrozen, frozen, and supercooled conditions. Particular emphasis is placed on the temperature range between -5 and +5 °C, where relevant mechanical changes occur, but also the major electrical transition due to the increasing partial freezing of pore water content.

The transferability of laboratory results to field observations is evaluated using a year-round automated ERT monitoring dataset from the Kitzsteinhorn (3.029 m a.s.l.), complemented by deep borehole temperature measurements along the profile. Deviations between field resistivity values and laboratory values can be explained by temporal and spatial effects. In the field, other than in the lab, seasonal pressurised water flow occurs in fractures, evidenced by piezometric measurements reaching peak values of 1.2 bar, and rock heterogeneities lead to enhanced drying and freezing of disintegrated rock blocks.

We anticipate that our provided temperature-resistivity pathways for different lithologies under unfrozen, frozen, and supercooled conditions will improve quantitative interpretation of ERT monitoring data and the assessment of permafrost warming and associated rock slope instabilities.