# Airborne Electromagnetics to Improve Landslide Knowledge in Tropical Volcanic Environments
## 5 Discussion
For example, for the La Medaille landslide, if the results seem conclusive, there are still uncertainties, such as the role of the probable aquifer of the C2 formations and the probable upwelling of water along the discontinuities in C2. The measured levels during the monitoring of the landslide activity show that the water supply to the landslide from the source is not sufficient to activate it. Careful consideration of the AEM results shows low resistivity bodies (C2) with values of approximately 5 \(\Omega\).m under each unstable site for both sites. This low resistivity may come from (i) highly mineralized water; (ii) preferential flow in more or less weathered and fractured materials; and (iii) very weathered materials with likely a high proportion of smectite [30]. Taking into account the recent works in Martinique [12; 30], it is possible to retain hypotheses (i) and (ii) with deep water upwelling in fractured materials feeding shallow water tables in landslide-prone materials, as illustrated in Figure 16b. However, to obtain such a resistivity value, this water must be highly mineralized, and currently, it is difficult to know the mineralization processes [30]. Therefore, this hypothesis needs to be confirmed by deep drilling and less punctual monitoring than those previously undertaken at these sites. Third, the new geological interpretation and physical failure modeling allowed refinement of the landslide susceptibility maps, especially for Morne-Figure. Figure 17 shows the areas identified by the expert approach in 2004 at the 1:20,000 scale of work for all landslide types and the results of numerical failure simulations for an extreme GWL for  
Figure 16: La Médaille conceptualization. (**a**) Before this study; (**b**) after this study (* data not used in the computations of failure probabilities with ALICE®). only moderately deep rotational landslides. The obtained maps are different, particularly (i) the area between the two landslides at Morne-Figue and the 1977 landslide and (ii) the area along the Gue stream or (iii) the areas around the Mont Morne-Figue carved in the andesite formations. One important point to highlight is that Figure 17a integrates all landslide types, and Figure 17b shows only one type of event. At this stage, if the numerical failure modeling resulting from the geotechnical model can help experts create some failure scenarios and better identify some potentially unstable sectors, it is necessary to further model shallow landslides, especially the triggering factors and the material thickness involved. There is a certain amount of uncertainty in the results of the shallow failure probabilities during the calibration phase. Additional investigations must be envisaged, probably based on the work of [15]. The new results of simulated failure probability maps allow better definition of the boundaries of high-hazard areas and improvement of the future final landslide hazard map. This will be achieved during a project to revise the landslide hazard regulatory maps between 2021 and 2022. The goal is not to reject one approach and its results compared to another. Indeed, they are complementary; the expert approach is based on the subjectivity of the expert who, through his or her experience, can intrinsically bring elements not taken into account in the modeling [19; 24]. In addition, the geotechnical model may contain some uncertainties (i.e., interpretation, oversimplification of the GWL, and material not considered) and generate errors in the stability computations under ALICE(r). Therefore, this approach based on interdisciplinarity is in accordance with the landslide hazard mapping strategy proposed in [24] for France with local revisions based on numerical spatialization tools able to provide new information on slope stability quickly for decision support. Therefore, this study complements the study conducted in 2017 on a nearby site for shallow landslides. Figure 17: Landslide hazard maps for the Morne-Figue area.