# Airborne Electromagnetics to Improve Landslide Knowledge in Tropical Volcanic Environments
## 4 Results
### Slope Instability Analyses
#### 4.2.1 Identification of the Best Geotechnical Parameters  
The sensitivity analysis along the cross-sections (Figures 8a and 9b) must reduce the range of values of geotechnical parameters and define the consistency as the best fitting intrinsic for the involved materials. A set of 50 model iterations for each cross-section were carried out (i) with independent variation of each parameter within the range of values and (ii) in dry and fully saturated conditions to obtain the best range of combinations. Tables 5 and 6 give the selected formation and the retained boundaries to perform calculations. For the two sites, cohesion appears to be the most influential parameter in the safety factor modeling followed by the internal friction angle, while the weight bulk density appears to be the least influential, which is often noticed in this type of analysis. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Homogeneous Area**} & \multirow{2}{*}{**Formation**} & \multirow{2}{*}{**Thickness (m)**} & \multicolumn{2}{c}{**\(\boldsymbol{\gamma\) (kN.m\({}^{-3}\))**}} & \multicolumn{2}{c}{**c (KPa)**} & \multicolumn{2}{c}{\(\boldsymbol{\varphi\) (\({}^{\ast}\))}} \\ \cline{3-10}  & & & **IV** & **SV** & **IV** & **SV** & **IV** & **SV** \\ \hline \multirow{3}{*}{S1} & R3 & \multirow{3}{*}{\(>\)20} & 10–30 & 15–20 & 8–16 & _10–15_ & 5–40 & _10–30_ \\  & (andesite) & & infinite & 26–30 & 30 & 40 & 40 & 40 & 40 \\ \hline \multirow{3}{*}{S2} & R2 & 15–20 & 05–29 & _10–15_ & 5–20 & _15–20_ & 1–35 & _10–18_ \\  & Substratum & & infinite & 29–30 & 29 & 31–35 & 35 & 31–35 & 35 \\ \hline \multirow{3}{*}{S3 and S4} & C1 & 1–3 & 12–17 & _12–17_ & 5–15 & _5–10_ & 10–22 & _10–20_ \\  & R1 & 3–10 m & 10–18 & _12–18_ & 1–20 & _10–20_ & 16–25 & _17–22_ \\  & C2 & 5–12 m & 10–18 & _12–18_ & 5–25 & _10–20_ & 18–25 & _18–25_ \\  & R1B & 3–10 m & 10–18 & _12–18_ & 5–25 & _10–20_ & 16–25 & _17–22_ \\  & Substratum & & infinite & 25–30/ & 29 & 50–66/ & 66 & 30–38/ \\  & (basalt) & & 29–30 & 29 & 31–35 & 35 & 31–35 & 38 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Geotechnical values selected after sensitivity analysis for the Morne-Figue area. IV = initial values; SV = selected values; \(\gamma\) = bulk unit weight; c = cohesion; \(\varphi\) = angle of friction; italic values are introduced as triangular probability distributions in ALICE\({}^{\circledR}\). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Formation**} & \multirow{2}{*}{**Thickness (m)**} & \multicolumn{2}{c}{\(\boldsymbol{\gamma\) (kN.m\({}^{-3}\))}} & \multicolumn{2}{c}{**c (KPa)**} & \multicolumn{2}{c}{\(\boldsymbol{\varphi\) (\({}^{\ast}\))}} \\ \cline{3-10}  & & **IV** & **SV** & **IV** & **SV** & **IV** & **SV** \\ \hline R1 & \textgreater{}20 & 25–30 & 30 & 40–50 & 40 & 30–50 & 40 \\ R2 & 2–20 & 10–17 & _11–16_ & 10–37 & 12–35 & 5–30 & _5–25_ \\ R3 & 20–30 & 7–17 & _8–16_ & 6–50 & _6–40_ & 8–35 & _10–30_ \\ C2 & 25–30 & 25–35 & 28 & 35–40 & 37 & 28–47 & 40 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Geotechnical values selected after sensitivity analysis for the La Médaille landslide. IV = initial values; SV = selected values; \(\gamma\) = bulk unit weight; c = cohesion; \(\varphi\) = angle of friction; italic values are introduced as triangular probability distributions in ALICE\({}^{\circledR}\). Therefore, for the La Medaille landslide, the results for rotational failure with a depth of 20 m show that the cohesion must not be below 12 KPa and 6 KPa for layers R2 and R3, respectively, especially if the angle of friction is low for each layer. If the value is below one of these values, the computed FoS remains under 1, corresponding to a recurrent instability that does not correspond to reality. For the Morne-Figue area, there are two cases: shallow and moderately deep-seated landslides. For shallow landslides occurring in the R1 materials, if the angle of friction is higher than 15\({}^{\circ}\), then the slope remains stable. For cohesion, when it is greater than 10 KPa, the slope remains stable.