# Airborne Electromagnetics to Improve Landslide Knowledge in Tropical Volcanic Environments
## 3 Materials and Methods
### Landslide Modeling
#### 3.3.2 Landslide Modeling Protocol  
Landslide modeling, split into three steps, consists of defining the best set of representative parameters for each site.  
1. The first step focuses on the geotechnical characteristics acquired during the drilling campaigns at each site. The goal is to reduce the broad spectrum of geotechnical values because their range is quite large. Indeed, retaining geotechnical values giving recurrent instability under any saturation conditions would not be representative of real conditions, with landslides being episodically unstable. Therefore, a sensitivity study is undertaken by several iterations computing the FoS in dry and fully saturated conditions. Computations are carried out in 2D along a representative cross-section of the study site. Each geotechnical characteristic (cohesion, angle of friction, and bulk weight density) is modified iteratively by increasing its value and keeping constant values for the other two. If the results show recurrent instability under any condition (dry/saturated), the value is excluded. If the results show a recurring stability of the sites under any condition (dry/saturated), then this value is also excluded. Thus, by this sensitivity analysis, the values (cohesion, angle of friction, bulk unit weight) producing a factor of safety lower than one at a high GWL are preserved, and those producing a factor of safety greater than one for a null GWL are also preserved.
2. The second step consists of defining the optimum cell size for ALICE(r) to obtain reliable results. Indeed, for the spatialized model, in raster format, the cell size can generate errors that propagate in the results [74; 75]. Thus, for the two sites, simulations are carried out with cell sizes of 10 m and 5 m to observe if discrepancies can be noted between the simulations. It is not necessary to decrease the resolution because with a lower cell size, the generated profiles are too small, and it is impossible to correctly compute failures shorter than 10 m ([62; 69]). Conversely, performing calculations with a larger cell size results in a loss of spatial precision, sometimes generating areas with a low or high failure probability that is not characteristic of the field reality [62]. Computations are performed with the geological models developed with the new data from the AEM data and interpretation. For this step, the best geotechnical values defined previously are used. The GWL introduced in ALICE(r) is constant and corresponds to full saturation conditions.
3. The third step allows checking the influence of the GWL on the destabilization of the ground. This step takes into account the GWL recorded for the two sites from 1967 to 1968 and from 1987 to 1988 for the La Medaille landslide and Morne-Figue area, respectively. Therefore, following the different records, the maximum GWLis established with a maximum fixed at \(-0.5\) m from the topography surface for the La Medaille landslide, \(-0.5\) m from the topography surface for the moderately deep rotational landslide and a GWL at the level of topography for the translational shallow landslide for the Morne-Figure area. The different saturation ratios increased iteratively from dry conditions (GWL = 0) to full saturation conditions measured for each site (GWL = 1).  
For each step, the failure geometries were fixed following the field observations (Table 2). The validation of simulations is performed by comparison with (i) an expert map derived from field observations and DTM derivatives for the La Medaille landslide [62,76] and (ii) the landslide inventory performed in 2016 for the Morne-Figure area [62]. Different classical statistical tests are computed (relative error and ROC-AUC; [69, 70, 71]) to validate the results. These tests were completed by expert (qualitative) verification.