# Airborne Electromagnetics to Improve Landslide Knowledge in Tropical Volcanic Environments
## 5 Discussion  
AEM data have already been used to study large landslides in temperate alpine environments or in Japan [38; 48; 49; 50; 51]. Few studies have been carried out on multiple types of landslides simultaneously in complex tropical environments with superimposed lavas that are more or less weathered at depth. Despite this lithological and structural complexity, through this study, AEM data appear to be of primary interest to assess these phenomena generating damages and losses in the West Indies [1]. In the following paragraphs, the AEM method, the role of water in slope failures and the differences with regulatory landslide hazard maps are discussed. Thus, first, the AEM data allowed a better definition of the internal structure and the materials prone to landslides for both the La Medaille and Morne-Figue sites. For the La Medaille site, some hypotheses [57; 58; 59; 60] that were difficult to validate due to the lack of deep boreholes were confirmed, such as the existence of two superimposed main bodies (one associated with a very low resistivity \(<\)5 \(\Omega\).m) composing the landslide body. The extracted information also allows building a specific geotechnical model integrated into ALICE(r). Figure 16 gives an overview of the conceptualization of the La Medaille landslide gathering ancient information with the information derived from the AEM data. Without this latter, the geological model should be different (Figure 16), with downstream and upstream compartments (A and C) defined in [54; 55] and located on one type of thinner material. Using this model (Figure 16a), the results of the numerical failure simulations would probably show that only the upper part of the landslide is likely to be unstable. Conversely, taking into account the interpretations derived from AEM data, it is possible to show that the shear surface identified in [54; 55] and questioned in [56; 57] turns out to be true concerning failure probability computations. For the Morne-Figue site, the data from the AEM allowed us to clarify the geological structure of the site: (i) the hyaloclastite footprint and (ii) the different successive low and high resistive horizons in the basalts with likely different weathered levels. The derived geological model was able to reproduce the unstable behavior of highly saturated materials, especially for moderately deep rotational slides. The results obtained for shallow landslides are less conclusive. However, a previous study conducted in [15] showed that AEM data and modeling under ALICE(r) could give satisfactory results for shallow landslides. Here, this is not the case, and we decided to stop the modeling of shallow phenomena because of the mediocrity of the results. To improve the results for this type of landslide, two points should be emphasized: (i) the shallow landslide is located on a preferential location of water circulation mentioned in [77; 78]. The use of a static GWL in ALICE(r) did not allow faithful reproduction of the real influence of water at this location. (ii) The landslide was triggered in complex materials mixing road embarkments and likely weathered lavas. This type of material was not investigated during the geotechnical laboratory tests, and at this location, the simplification of the model meant that these specific materials were not included in the study. Second, beyond these considerations, the results of physical numerical modeling show that water (i.e., introduced by the GWL) remains the main factor of slope destabilization in this environment. For both sites, the GWL has to be high to obtain high failure probabilities. This statement confirms the field observations and the punctual measurements and monitoring during the 1960s and 1980s for the La Medaille landslide and Morne-Figue area, respectively. However, the question of the origin of this water remains.