# Airborne Electromagnetics to Improve Landslide Knowledge in Tropical Volcanic Environments
## 4 Results
### Identification of Involved Materials and Geological Models
#### 4.1.3 Geotechnical Models  
From the interpreted resistivity models, different interfaces were derived for each site. Figures 8 and 9 illustrate the different geological models for the two sites. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Name** & **Resistivity (\(\Omega\).m)** & **Thickness (m)** & **Description** \\ \hline C1 & \(<\)5 & 2–4 & Weathered clayey and backfill materials \\ \hline C2 & 1–5 & 5–10 & Highly weathered basalt lavas with preferential water circulation \\ \hline R1 & 5–8 & 3–10 & Basalt lavas that are more or less weathered \\ \hline R1B & 5–8 & 3–10 & Basalt lavas that are more or less weathered that are replaced laterally and at depth C2 \\ \hline R2 & 8–11 & 15–20 & Hyaloclastite formations \\ \hline R3 & \(>\)20 & \(>\)20 & Andesite formations that are more or less weathered \\ \hline \end{tabular}
\end{table} 
Table 4: Characteristics of the different formations observed in the la Morne-Figue area. Figure 8: Geological model for the La Médaille landslide. (**a**) A 2D cross-section representing the different materials selected for physical-based modeling; (**b**) interfaces introduced for ALICE® (hillshade map is produced with Helimap DTM, 2013). For the La Medaille landslide, four layers were defined (Figure 8a): (i) a first layer corresponding to dacitic screes (R2) upstream of the landslide on steep slopes and on compartment A of the landslide. The thickness varies from a few decimeters to approximately 5 m upstream of compartment A. On compartment A of the landslide, the thickness varies from 5 m to 20 m. (ii) A second layer corresponds to andesite screes (R3) with a thickness between 40 m under compartment A and a few meters downstream of compartment C. Therefore, R2 rests on R3 along compartment A and forms compartment C alone. (iii) A third layer corresponds to dacite (R1) located under R2 up to the concave slope failure corresponding to the upstream limit of the landslide. It is limited downstream by the north-south fault. Its thickness varies from 20 to 90 m. (iv) A fourth layer corresponds to andesite (C2) forming the bedrock. The fault observed in the resistivity profile (Figure 7a) and documented in the geological map [26; 27] is not integrated in the geotechnical model. Figure 8a shows the failure hypothesis described in [54] and questioned in [57]. The new information on formations and their thicknesses introduced in the geotechnical model and numerical simulations under ALICE(r) should help to better understand how destabilization occurs. For the Morne-Figue landslide, the area was divided into five homogeneous areas, defined by the main formations identified previously, the structure and the geomorphology (Figure 9a). Thus, S1 corresponds to the hyalocalstite formation (R2) resting on andesite formations (C1); this area corresponds to the main relief. S2 is composed of hyalocalstite (R2) marking steeper slopes. S2 is separated from S3 by a fault identified in the resistivity model and identified in [26; 27]. S3 marks the beginning of the topographic depression of the site. It is characterized by a succession of conductive and more resistive layers (C1, R1, C2, and R1B), including the C2 layer over almost its entire surface. S4, in the southern part  
Figure 9: Geological model of the Morne-Figue area. (**a**) Location of homogeneous areas identified by resistivity data analysis, field observations and boreholes; (**b**) 2D cross-section representing the different materials selected for physical-based modeling; (**c**) interfaces introduced for ALICE® (hillshade map is produced with Litto3D DTM, IGN, 2010). of the site, differs from S3 by an absence of layer C2 on the entire downstream and eastern part. Therefore, downstream of S4, only C1 and R1 are identified. Area S4 is probably limited to the north by an assumed fault separating it from S3. Finally, S5 represents the downstream of the site with gentle slopes covered by alluvial deposits. Faults in this area are not integrated into the geotechnical model.