# Airborne Electromagnetics to Improve Landslide Knowledge in Tropical Volcanic Environments
## 3 Materials and Methods
### Landslide Modeling  
#### 3.3.1 ALICE Presentation  
Landside modeling is performed with ALICE(r) (Assessment of Landslide Induced by Climatic Events) developed by the French Geological Survey (BRGM, [69]). This tool supports landslide susceptibility mapping for areas ranging from local sites ( catchments) to large areas (several municipalities; [15; 69; 70; 71]). Developed in a GIS environment (MAPINFO(r)), it is a SPBM described in [69] and summarized below (Figure 5).  
The geometry of the studied area is introduced in raster format with (i) the topography and (ii) the geometry of different material layers. Geomechanical characteristics, cohesion (c), friction angle (\(\varphi\)) and specific bulk unit weight (\(\gamma\)), are given for each lithology and surficial formation. These parameters can be implemented by a constant value or by probabilistic distributions to take into account environmental variability and uncertainties [69; 70]. The tool supports different landslide geometries and failures (i.e., rotational, translational, and complex with different lengths and depths). Two triggering factors can be used: (i) the groundwater level (GWL) or (ii) seismic acceleration. The GWL represents the saturation ratio (m = h/z), where h is the height of the water table and z is the depth of the material(s) taken into account. The GWL can be implemented empirically in one or several formations by increasing the saturation level from 0 (dry conditions) to 1 (saturated conditions) or with the help of a hydrogeological model taking into account the effective rainfalls. Seismic acceleration is represented by the peak ground acceleration (PGA).  
The slope stability computation is based on a limit equilibrium method (LEM) and slice theory described in [72] and in Figure S1. The iteration process is based on the concept of reducing the number of iterations about the interslice function and therefore the computation time [73]. The hypothetical failure surface is divided into \(n\) vertical slices,  
Figure 5: ALICE® concept (adapted from [69]).  
and each slice \(I\) is subject to the normal shear interslice forces and to the shear resistance (Figure S3), where:  
\[R_{i}=[w_{i}cos\alpha_{i}-u_{i}b_{i}\;\sec\alpha_{i}]tan\varphi_{i}^{\prime}+c_{ i}^{\prime}b_{i}\sec\alpha_{i}\]  
and the moving forces:  
\[T_{i}=w_{i^{\text{in}}\alpha_{i}}\]  
where \(W_{i}\): weight; \(\alpha_{i}\): base inclination; \(u_{i}\): average water pressure; \(b_{i}\): width of the slice; \(\varphi_{i}\): effective friction angle; \(c^{\prime}_{i}\): cohesion along the base; and \(R_{i}\): sum of the shear resistances, except the normal shear interslice forces. Ti is the component tending to cause instability. The potential failure is expressed by the factor of safety (FoS). If the FoS is below 1, the slope (i.e., the computation cell) is considered instable. The slope stability assessment is performed on regularly spaced 2D profiles automatically produced over the whole area and based on maximum gradient lines from the DTM. Two types of computation are possible, (i) the computation of the FoS or (ii) the computation of the probability, to obtain an FoS below 1 for each cell. In the second case, the tool performs a random selection of each geotechnical value following probabilistic distributions and Monte Carlo simulations [69; 70; 71].