Rockwall permafrost dynamics evidenced by repeated and Automated Electrical Resistivity Tomography at Aiguille du Midi (3842 m a.s.l., French Alps)

Abstract

Permafrost degradation significantly affects the stability of rockwalls in high altitude regions. Monitoring rockwall permafrost is essential for assessing potential geohazards. While borehole temperature measurements are the most direct permafrost monitoring approach, they lack sufficient spatial representation in such highly heterogeneous ground conditions. Conversely, geoelectrical measurements can provide more comprehensive insights into these complex patterns and dynamics. This study investigates the permafrost dynamics and intends to detect potential hydrogeological processes at the Aiguille du Midi (3842 m a.s.l. (meter above sea level), French Alps) using repeated and Automated-Electrical Resistivity Tomography (A-ERT) approaches, covering a period of 3.5 years (June 2020–December 2023). A total of three geoelectrical profiles have been installed on three faces of the Aiguille du Midi (N–W, S and E). An automated acquisition system for permanent resistivity monitoring and remote data acquisition is implemented. A time-lapse inversion technique is employed to get the temporal and spatial variations of electrical resistivity at seasonal and interannual time scales. The data revealed significant variations in active layer thickness across rock faces, along with a slight decrease in electrical resistivity at depth, indicating permafrost warming over time. However, they did not provide clear evidence of water pressurization in rock fractures. Using a petrophysical model, calibrated with laboratory measurements of the temperature dependence of electrical resistivity of granite sample, we estimated the temperature within the frozen zone from the resistivity measurements, under favorable conditions at surface in summer and autumn. Validation against direct temperature measurements in a 10 m depth borehole along the NW profile indicates a mean absolute error less than 1 °C within the frozen zone. This research underscores the efficacy of ERT as a promising, non-invasive tool for quantitative monitoring of permafrost dynamics in Alpine environments. It also reveals challenges associated with conducting A-ERT in high mountain rockwalls where the contact resistance is very high (∼500kΩ) and sometimes intermittent due to factors such as thunder strikes and rockfalls.