Frozen ground dynamics resolved by multi-year and year-round electrical resistivity monitoring at three alpine sites in the Swiss Alps

摘要

Frozen ground characteristics resolved by annual, seasonal/monthly and daily electrical resistivity monitoring are presented based on case studies from three alpine sites in the Swiss Alps with dif- ferent surface conditions and subsurface process dynamics. Data acquisition is achieved by different set-ups ranging from low-cost to automated and more expensive monitoring strategies. To ensure the reproducibility of measurement results a robust set-up is required within the rough surface conditions of alpine environments, and this constitutes a fundamental precondition for time-lapse measurements. The selected different monitoring approaches allow for a detection and analysis of processes related to frozen ground dynamics on varying timescales. The interpretation of the geophysical data is improved by temperature measurements from various data loggers and borehole data. All three approaches allowed detection of the interface between frozen and unfrozen ground. The variation of the frequency of measurements enabled exploration of the specific permafrost-related problems. At one site, the multi-annual resistivity distribution at the end of the thawing period revealed fairly stable permafrost conditions, while at the second site, year-round measurements showed extremely divergent evolution of resistivity values in the subsurface throughout the measurement period, which could be ascribed to different site-specific environmental parameters. Using measurements with daily resolution at the third field site, the rapid decrease in subsurface resistivity values due to the infiltration of meltwater in spring could be documented. The presented results show that the different monitoring set-ups have their justification and are able to monitor timedependent subsurface dynamics within the scale of their temporal resolution. The operation of an automated monitoring system allows for very efficient observation especially of short-time processes within the active layer and the frozen ground below, the major advantage in comparison to non-automated monitoring approaches. However, the system is cost-intensive, requires an extensive infrastructure, and is more prone to environmental forces. For monitoring the inter-annual and long-term permafrost evolution, application of a fixed monitoring set-up that is accessible throughout the year and measured manually has proven to be a robust and cost-efficient alternative. Focusing on the long-term permafrost evolution, set-ups using fixed electrodes and measurements conducted as needed with a brought-along cable is a legitimate approach. Hence, for studies in alpine permafrost environments, choice of the monitoring set-up remains a question of the scientific problem, infrastructure facilities, and cost-efficiency.