## 3 Landslide Monitoring
[PERSON] et al., 2016 [78] installed a permanent 4D-ERT system at the Super-Sauze clayey landslide (France). They performed a statistical analysis to investigate possible relationships between the rainfall characteristics, the soil hydrological observations and the soil electrical resistivity response (Figure 4). The results highlighted the influence of the temperature in ERT measurements due to the heat exchange between the groundwater, the vadose zone water and the rainwater that hides the variations of resistivity due to variations in the soil water content. Another quite similar experiment was carried out in a landslide area in southwestern China, to map the pathways of rainwater infiltration in the slipping zones ([PERSON] et al., 2016) [79]. [PERSON] et al., 2017 [80] applying electrical resistivity tomography (daily acquisition), rainfall records (since 2006) and boreholes monitoring groundwater level (since 2009), carried out a 9.5-year multiparametric survey on the \"Vence\" (France) landslide. The aim of the survey was to understand fluid circulations within the unstable slope. ERT data were analyzed by using an automated clustering analysis enabled to isolate the ERT section into three groups of apparent resistivity values (Figure 5). The comparison between the  
Figure 3: (**A**) Laval landslide (ORE Draix, South French Alps) with location of the ERT line and of the equipment used for the rain experiment. (**B**) Cross section of the landslide. (**C**) Resistivity changes during the rain experiment (after [73]). variations of these cluster behaviors in time with the variations of groundwater levels on site, allowed the authors to locate distinct hydrogeological units. The authors showed that the qualitative information on the variability of the slope hydrogeological behavior could help improve the conversion of resistivity data into hydrologic quantities. [PERSON] et al., 2017 [81] performed a spray irrigation experiment in a selected Alpine zone interested by shallow landslides. A sequence of 2D ERT images were obtained during the irrigation period. Furthermore, they installed a grid of time-domain reflectometry sensors at different depths for measuring water content, and used a linear diffusion model for simulating the time-dependent variations of the pore pressure. The results allowed them to detect the negative resistivity changes associated with the vertical progressing water. Unfortunately, the measurements and the model were not able to investigate the small-scale variations in pore pressure that may facilitate the triggering of shallow landslides. [PERSON] et al., 2019 [82] carried out a laboratory-scale experiment to simulate the rainfall triggering of shallow landslides. They used the TL-ERT and time-domain reflectometry  
Figure 4: The map above (**a**) shows the location of the investigations carried out on the Super-Sauze landslide (Southeast France). The slope is characterized by non-fissured unit (**b**) and areas affected by a high fissure density (**c,d**). The dashed blue line (**e**) indicates the position of the GEOMON 4D system. The images (**f,g**) illustrate the soil cooling effect due to the rainwater. The percentage change in soil electrical resistivity after a small rainfall event could be due to the difference between rain and initial soil temperature (after [78]). Figure 5: The map at the top reports the location of the survey carried out on the Vence landslide (south-eastern France), a simplified geological cross section (**a**) obtained along the sliding direction and the layout of the field survey carried out in the area. (**b**) Legend. The images below show (**c**) the variations of the daily median values of the three clusters of apparent resistivity measurements identified for the slope in the period 2006-2016, and (**d**) the accumulated rainfall index and groundwater level obtained by direct measurements in C6 and C20 for the same period (after [80]). methods to monitor water infiltration and to evaluate the volumetric water content.