# Airborne Electromagnetics to Improve Landslide Knowledge in Tropical Volcanic Environments
## 4 Results
### Slope Instability Analyses
#### 4.2.1 Identification of the Best Geotechnical Parameters
On the other hand, when cohesion is close to 5 KPa, the slope switches from stable to unstable according to the saturation scenario. For moderately deep-seated landslides, the situation is more complex, regardless of the conditions, and the slope is close to instability even with high cohesion values and/or high angle of friction values. When the GWL is low and the saturation conditions are null, then the FoS is high, but not higher than 1.5. Therefore, when the angle of friction is less than 10\({}^{\circ}\), the stability remains low under any condition. If the angle of friction is higher than 20\({}^{\circ}\), the slope is stable. For cohesion under 5 KPa, the slope is recurrently unstable (FoS < 1) regardless of the angle of friction. If the angle is greater than 10\({}^{\circ}\), then according to the angle of friction, the stability increases. Tables 5 and 6 show the initial and retained values to be integrated in the spatial modeling for the two case studies. Values in italics are used in ALICE(r) simulations. Values in italics and bold are used with a probability distribution necessary to compute failure probabilities with ALICE(r). The probability distributions taken into account are triangular (Figure 10). This shape of the probability distribution is classically selected for ALICE(r), as explained in [69; 70], and offers the best compromise for testing new hypotheses of slope destabilizations. depth of 3 m and two for deep rotational landslides with a depth of 10 m). The goal was to define the best cell size given the best results, and it was decided to perform the computations taking into account the failure conditions observed in the field (i.e., with a high GWL corresponding to the saturation of the materials, GWL = 1). The comparison and validation of the results is carried out by two statistical tests and expert verification. The tests are classical: (i) the relative error analysis (performed with the observed failure identified) and (ii) the analysis of the area under an ROC curve (ROC-AUC). For the La Medaille site, failures correspond to the main scarps of the landslide (i.e., the scarp between compartments A and C and the scarps identified downstream). For the Morne-Figure, failures correspond to scarps of phenomena. Figures 11 and 12 depict the results for computations carried out with the two cell sizes for each site. For the La Medaille test site, the different scarps are well identified by the models with high probability values of failure in the different scarps. The relative errors are very low, with 0.25 and 0.21 for the 10 m cell size and 5 m cell size, respectively. The two computed ROC-AUCs show that the different models have a high degree of fit with values of 0.89 and 0.91 for a 10 m cell size and a 5 m cell size, respectively. For the Morne-Figure site, for shallow landslides, the different models computed with cell sizes of 10 m and 5 m have a low degree of fit. Indeed, the relative error and ROC-AUC have values of 0.89 and 0.95 and 0.59 and 0.58 for 10 m and 5 m cell sizes, respectively. These results indicate a low degree of model fit and low model representativeness for translational shallow landslides. In contrast, for moderately deep rotational landslides, the results are very good, with low relative errors and high ROC-AUCs of 0.13 and 0.15 and 0.89 and 0.87 for a 10 m or 5 m cell size, respectively. The Morne-Figure failure area is well represented with high probability values from 0.1 and 1, indicating, according to the equations, high failure probabilities when the materials are saturated. Hillshade maps were produced with the Helimap DTM (2013). For both cases, the results few differ between computations with a cell size of 5 m or 10 m, either statistically or visually on the computed maps. Thus, for the two sites, a cell size of 10 m appears to be the best compromise for the next phase. For the Morne-Figure site,  
Figure 11: Comparison of results with cell sizes of 5 m and 10 m for the La Medaille landslide.