## 4 Discussion  
This review helps to highlight the strengths and weaknesses of the TL-ERT method in the study of landslide areas, by considering both the technological aspects of instrumentation and data acquisition, and the phase of data analysis and processing. As shown by the papers analyzed in the previous section, the main and robust contributions of the TL-ERT method in the study of landslides concern the monitoring of rainwater infiltration in the shallow subsurface, and the estimation of the spatial and temporal changes in soil moisture and water content in the top soils and shallow layers of the unstable slope, respectively. In many cases, authors tried to define significant correlations between rainfall, soil hydrological characteristics, and resistivity values [72; 78; 80]. The quantitative analysis of such correlations has always represented a challenge for the scientific community. Very often, the TL-ERT measurements have been integrated and compared with meteorological and hydrological data and, sometimes, with TDR measurements or other geophysical data [81; 82]. The comparison and integration of data coming from different techniques allow the overcoming of some specific limitations of each single method. The results of the integration highlight the capability of TL-ERT to give quantitative information about the water content and to map a preferential pathway for water infiltration. This capability makes the TL-ERT method useful in the future for better evaluating the water saturation effects in the estimation of the rainfall thresholds in landslide early-warning systems. The analyzed papers also confirmed that it is not possible to generalize the results obtained for every geological and geomorphological context. In order to make the landslide monitoring effective by applying the TL-ERT method, an appropriate and correct geological model must be adopted. As reported by other authors, the petrophysical models used to transform resistivity values in hydrological parameters are site-specific [81; 85], and cannot be used to define resistivity thresholds applicable for the activation of landslides in any geological environment. The main evolution of the petrophysical relationships reported in the literature has regarded the change of the number of phases to be considered in the model [86]. Indeed, unlike what is claimed by the petrophysics, the rock formations are unconsolidated, and not composed of a continuous solid phase, but of different phases. So, starting from the one phase (pore water) model proposed by Archie's Law [87], to interpret the electrical properties acquired in different sedimentary contexts, multi-phase models have been considered [88]. Despite the attention paid by the scientific community, the numerous developed petrophysical relationships are still difficult to apply in the study of landslide areas. Often, they require calibration with other parameters (i.e., soil temperature, porosity, water content, etc.) that are not always available or measurable, requiring laboratory analysis for their definition. For this reason, authors working in landslide areas often prefer to consider the raw data and carry out a statistical analysis of them. All the discussed applications were carried out almost exclusively in rural areas, i.e., in areas where the logistical conditions for data acquisition were favorable, and in which the quality of the data was generally high due to the absence of environmental noise sources. Working in rural areas offers some advantages, such as easier positioning of the instrumentation and planning of continuous acquisition over time (the electrodes can be embedded in the ground for long periods without creating problems for the normal activities of a community). The absence of buried objects and underground utilities, that could affect the flow of current in the subsoil, and the good electrode/terrain coupling mean that the acquired data are not very noisy, and do not require high filtering actions.