# Airborne Electromagnetics to Improve Landslide Knowledge in Tropical Volcanic Environments
## 2 Martinique and Study Sites
### Study Sites: Presentation and Previous Works
#### 2.2.1 La Medaille Landslide  
The La Medaille landslide is located in Morne Balthazar to the north of the Fort-de-France Municipality (Figures 2a and 3a). With a volume of approximately 260,000 m\({}^{3}\) and an area of approximately 8 ha (Table 1), the landslide occurred in 1916 [54; 55], removing the old village of La Medaille and causing five casualties. The landslide is bounded to the north and south by two water courses marking two parallel faults and by a cliff of approximately 110 m representing a normal fault in the west. It is probably located on a paleolandslide (rock avalanche) represented by dacitic deposits in the small-scale lithological map [26; 27]. Since 1916, several peaks of activity have been recorded (i.e., 1958, 1966, and 1993; [54; 55; 56; 57; 58; 59; 60; 61; 62]), each time with the rehabilitation or the relocation of road RN3 due to its crossing the moving mass. It is a slow-moving landslide but is likely to experience accelerations due to GWL variations. More recently, a debris slide in weathered andesitic formations occurred in the upper part of the cliff overhanging the landslide (Figure 2b,c). Since the 1960s, several investigations (i.e., field surveys, geotechnical studies, and monitoring; Table 1) were engaged to improve the knowledge of the landslide [54; 55; 56; 57; 58; 59; 60; 61; 62; 63], especially for the upper part (compartment A). Indeed, the topography of the lower part (compartment C) of the landslide is chaotic and complex, and the very dense tropical vegetation prevents the deployment of different investigation devices. The different field surveys allowed delineation of four main compartments functioning more or less independently. The lower compartment (C) seems to have an influence on the upper compartment (A), while the compartments on the edges are considered independent of the main landslide stricto sensu [58]. All drilling campaigns (five campaigns between 1967 and 1996) were located in compartment A of the landslide, which presents a high blockage potential (Figure 2). Thus, this compartment benefited from 14 boreholes reaching a depth of approximately 20 m. These campaigns located the faults bordering the landslide (Figure 2d). Figure 3: Morne-Figure area; (**a**) picture of the limit north of the Morne-Figure landslide (the bumps on the road are produced by the shear cracks of the landslide); (**b**) main scarp of the Morne-Figure landslide in 1989; (**c**) panorama of the Morne-Figure area; (**d**) main morphological landslide features (the map was produced with an orthophoto from IGN, 2015, and hillshade map from Litto3D DTM, IGN, 2010). The first campaign in 1967 revealed a more or less clayy upper layer, up to 3 m thick, followed by approximately 19 m of dacitic screes upstream and 3 m of dacitic screes downstream of compartment A (Figure S1). Under this layer, a thin clay layer of a thickness of 0.5 to 0.9 m was observed. Finally, the lower layer was interpreted as andesite bedrock that was more or less fractured [54; 55]. [PERSON] et al. [57] questioned this statement following new observations based on new boreholes. Indeed, the last layer would be a mixture of weathered andesites and breccias and could be part of the landslide, which would then be composed of two superimposed bodies. This information corroborates the conceptual scheme of landslide development established in [56; 57; 63], which mentioned the implementation of a rock avalanche in andesites. This layer would have been weathered and would then have been covered by a thick layer of dacite debris (breccias), which would have also been weathered. The different boreholes defined a groundwater level between 1967 and 1968 in the upper part of the landslide with a maximum piezometric level of approximately 10 m under the topography. The fault system and a nearby spring probably feed this water level [57]. Finally, a series of rainfall and piezometric records attempted to prove the relationship between precipitation, groundwater level (GWL) and landslide activity.