## 1 Introduction  
Electrical resistivity tomography (ERT) is a robust and well-consolidated method for near-surface geophysics, with a wide spectrum of applications in the geological, engineering and environmental sciences. This geoelectrical method provides 2D and 3D resistivity images of the subsurface; the basic principle is Ohm's law and the experimental data are the voltage signals produced by the direct electrical currents (f = 0 Hz) injected into the subsoil and filtered by the artificial noise. Technological advances (i.e., multi-channel arrays, innovative sensors) and novel tomographic algorithms for data inversion have rapidly transformed ERT in one of the most popular geophysical methods. Furthermore, the extreme variability of the resistivity parameter (different order of magnitude, from 1-10 Ohm-m of clayey materials to \(10^{5}\) Ohm-m of granite rocks) makes it possible to use the ERT method in many geological environments [1; 2; 3; 4]. The ERT method can also be applied to study the time-dependent changes of the resistivity pattern of the subsoil: we used the analysis of 2D and 3D static resistivity images of the subsurface to the process the time-series of resistivity images (time-lapse approach or 4D tomography). The first laboratory and experimental works on time-lapse electrical resistivity tomography (TL-ERT or 4D-ERT) method date back to the period 1995-2005, when they were mainly devoted to monitor the injection of salt water in soil and aquifers [5; 6; 7; 8]. The results of these controlled experiments stimulated the study of the complex relationship between resistivity values and hydraulic parameters. Since these pioneering activities and seminal papers, the attention of the scientific community to the TL-ERT method has increased considerably, with exponential behavior in the yearly number of articles published in top-leading journals. In the last 20 years, more than 200 papers have been published on ISI journals (Figure 1). The TL-ERT method found an impressive number of applications crossing different disciplines, from hydrogeology [9; 10; 11; 12] to agriculture [13; 14; 15; 16], from engineering geology [17; 18; 19; 20] to geohazards [21; 22; 23; 24], from CO2 storage [25] to the study of the effects of the climate changes on soil and near subsurface [26; 27; 28; 29]. Here, we focus our attention on the application of the TL-ERT method for landslide monitoring. In the last years, climate change and the occurrence of frequent extreme meteorological events have contributed to the triggering of landslide phenomena, with considerable effects on the economic and social life of the affected areas [30; 31; 32]. Indeed, the number of cases in which these phenomena involved strategic infrastructures (e.g., bridges, railways, highways) as well as civil housing and commercial activities [33; 34; 35], sometime causing fatalities [36; 37], has considerably grown. Taking this into account, the need to develop early warning systems allowing officials to monitor the variation of specific subsoil parameters responsible for triggering landslides has become more pressing [38; 39; 40]. Alongside the more classic monitoring systems, based on the use of purely geotechnical sensors, such as inclinometers, piezometers, etc., innovative systems based on indirect measures such as geophysical ones [41] have begun to be experimented with. Geophysical methods allow the measure of physical parameters, whose variation often depends on the variation of parameters directly related to the triggering of landslides (for example the water content, the porosity of the rock, etc.). The ERT method can be considered one of the most widely applied geophysical methods in the study of landslides. Many examples of 2D and 3D ERT applications (Figure 2) for the geometric characterization of the landslide body, the identification of the sliding surface, and the localization of areas with higher water content, are reported in the literature [42; 43; 44].