Characterizing rockwall permafrost dynamics at Aiguille Du Midi (French Alps) through electrical resistivity tomography monitoring

Climate change significantly impacts high-mountains worldwide, accelerating the degradation of the cryosphere. Over the last decade, numerous rockfall events involving permafrost-affected rockwalls have been recorded, especially in the European Alps. The frequency of these events is expected to increase over time due to the degradation of mountain permafrost. This study investigates permafrost dynamics at the Aiguille du Midi (3840 m a.s.l.) in the French Alps using Electrical Resistivity Tomography (ERT) monitoring over four years. A total of three profiles each 155 m in length, were deployed downwards from the summit in three directions: north-west, south and east. A system for permanent monitoring and remote data acquisition was implemented. A time-lapse inversion technique was employed for data interpretation. Laboratory measurements of electrical resistivity were conducted on granite samples in both unfrozen and frozen conditions to evaluate the temperature-dependency of resistivity. Furthermore, temperature monitoring in three boreholes provides localized information about permafrost dynamic across the site. Our ERT results demonstrate that the temperature-dependence of resistivity in field conditions is less pronounced than in controlled laboratory settings, influenced by the complexity of the site (3D effect, human-made infrastructure, rock heterogeneity (at different scales from fractures to pores) and variable ice content. In field, the freezing temperature fluctuated between -0.5 °C and -2.5 °C. Importantly, we observed that the active layer's thickness varied significantly from one face to another, with implications for the thermal regime and potential geohazards in this mountainous environment. These results are correlated with thermal information measured in boreholes. Notably, our assessments of the hydrogeological system revealed instances of water flux, although the exact pathways of infiltration and drainage remain ambiguous. This research highlights the efficacy of ERT as a low-cost, non-invasive tool for monitoring permafrost dynamics in alpine environments and highlights the need for further methodological refinement to enhance data reliability. These findings contribute to understanding potential geohazards associated with permafrost degradation and emphasize the importance of continuous monitoring in the context of ongoing climate change.