Long-term analysis of ERT monitoring data measured on a critical slope above a high-speed railway gallery

A customized electrical resistivity tomography (ERT) monitoring system was installed on a critical slope above a high-speed railway gallery in March 2022 to monitor the stability condition of the slope. We discuss the results of long-term analysis of the data measured by the system from 24 March 2022 until 21 September 2023. Initial data quality checks were performed to control the quality of the data measured during day and night. We then discuss the optimization of the time-lapse inversion algorithm to generate artifact-free ERT images that can reflect the real distribution of the water content. Using meteorological data, the site-specific model for seasonal variations of the soil temperature at different depths was calibrated and the corresponding resistivity corrections were applied to suppress the effects induced by temperature variations that showed to be important only down to 6 m. Five piezometers were installed along the ERT profile to monitor the water table fluctuations and to correlate the inverted resistivities with piezometric measurements. The complex geology of the site and the complex response of the piezometers to rainfalls suggested focusing the analysis on two target zones identified on ERT images rather than analyzing the average behavior of the resistivity versus the average behavior of the piezometers. A scatterplot for one of these zones, apparently the most sensitive zone to rainfalls and water saturation, was obtained from long-term analysis of the ERT data integrated with the piezometric levels and we could establish a preliminary threshold for addressing excessive water content in this area.