Temperature-calibrated imaging of seasonal changes in permafrost rock walls by quantitative electrical resistivity tomography (Zugspitze, German/Austrian Alps)

Changes of rock and ice temperature inside permafrost rock walls crucially affect their stability. Permafrost rocks at the Zugspitze were involved in a 0.3–0.4 km3 rockfall at 3.7 ka B.P. whose deposits are now inhabited by several thousands of people. A 107 year climate record at the summit (2962 m asl) shows a sharp temperature increase in 1991–2007. This article applies electrical resistivity tomography (ERT) to gain insight into spatial thaw and refreezing behavior of permafrost rocks and presents the first approach to calibrating ERT with frozen rock temperature. High‐resolution ERT was conducted in the north face adjacent to the Zugspitze rockfall scarp in February and monthly from May to October 2007. A smoothness‐constrained inversion is employed with an incorporated data error model, calibrated on the basis of normal reciprocal measurement discrepancy. Laboratory analysis of Zugspitze limestone indicates a bilinear temperature‐resistivity relationship divided by a 0.5 ± 0.1°C and 30 ± 3 kΩm equilibrium freezing point and a twentyfold increase of the frozen temperature‐resistivity gradient (19.3 ± 2.1 kΩm/°C). Temperature dominates resistivity changes in rock below −0.5°C, while in this case geological parameters are less important. ERT shows recession and readvance of frozen conditions in rock correspondingly to temperature data. Maximum resistivity changes in depths up to 27 m coincide with maximum measured water flow in fractures in May. Here we show that laboratory‐calibrated ERT does not only identify frozen and unfrozen rock but provides quantitative information on frozen rock temperature relevant for stability considerations.