# Airborne Electromagnetics to Improve Landslide Knowledge in Tropical Volcanic Environments
## 4 Results
### Identification of Involved Materials and Geological Models
#### 4.1.2 Morne-Figue  
Figure 6c,d show two resistivity grids at two different depths, between 2 and 4 m and between 19 and 24 m. The resistivity range is globally low (\(<\)50 \(\Omega\).m). At the surface, the highest resistivity corresponds to andesites and volcanic-sedimentary formations. Weather basalts have a lower resistivity (\(<\) 8 \(\Omega\).m). Landslides occur in the majority of these materials described in [62; 63]. In between, it is possible to define the hyaloclastites. Resistivity profiles are displayed in Figure 7b and Figure S2. Nearby boreholes are also shown. Six horizons are delineated: a conductive layer (C1; \(<\)5 \(\Omega\).m) corresponding to a very weathered clayey layer and backfill materials that is more or less saturated following the groundwater level, and a more resistive layer (R1; from 5 \(\Omega\).m to 8 \(\Omega\).m) corresponding to less weathered clay materials. Locally, this layer disappears in lieu of a more conductive layer C2 (from 2 \(\Omega\) m to 5 \(\Omega\) m).  
According to borehole data (Figure 7b), this layer (C2) would correspond to highly weathered lavas, where water was observed [63]. C2 disappears progressively downstream of the slope and is less weathered at depth (R4) in favor of a few resistive layers (R1B and R1). Under R1, C1 and R1B, R2 shows higher resistivity values (between 8 \(\Omega\).m and 11 \(\Omega\).m). This layer, located 25 m below the topographic surface, would correspond to hyaloclastites. However, it remains difficult to know the exact nature of this layer due to the lack of deep boreholes. Finally, a more resistive layer (R3), with values higher than 20 \(\Omega\).m, corresponds to andesite formations overlying the hyaloclastite formations in some places. It should be noted that two sectors, where resistivity values drop sharply, probably correspond to faults delimiting the topographic depression at the center of the study site. Table 4 gives the different characteristics of each formation.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Name**} & **Resistivity** & \multirow{2}{*}{**Thickness (m)**} & \multirow{2}{*}{**Description**} \\  & **(\(\Omega\).m)** & & & \\ \hline R1 & \(>\)50 & \(>\)20 & Very deep layer with high resistivity values corresponding to dacite \\ \hline C1 & 15 & 1–3 & Surficial layers corresponding to recent weathered materials rich in clay \\ \hline R2 & 50–55 & 4–20 & Layer with high resistivity values corresponding to dacitic screes and debris implemented by an ancient debris avalanche \\ \hline R3 & 15–30 & 20–30 & Layer under R2 with lower resistivity values corresponding to mixed materials (breccias or fossilized screes) \\ \hline C2 & \(<\)15 & 25–30 & Layer with low resistivity values and a thickness of approximately 30 m corresponding to weathered andesite at the bottom, which was probably caused by hydrothermal water \\ \hline R4 & 20–25 & not identified & Formation with high resistivity values and likely corresponding to the bedrock (andesite) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Characteristics of the different formations observed in the La Médaille landslide.