# Airborne Electromagnetics to Improve Landslide Knowledge in Tropical Volcanic Environments
## 3 Materials and Methods
### AEM Data  
From 29 January to 16 March 2013, SkyTEM ApS conducted a survey over Martinique Island. This survey, supervised by the BRGM (French Geological Survey) and totaling 4233 line-kilometers, was flown along the N-S direction with a 400-m spacing; locally, this spacing was refined to 200 m over areas of interest for the hydrogeology or risk assessment [52]. Along each line, EM measurements were spaced approximately 30 m apart, with an average ground clearance of approximately 64 m due to the sharp topography of the island. Figure 4 shows the location of the flight lines flowing over the two studied sites.  
SkyTEM is an AEM system developed by the Hydro Geophysics Group of Aarhus (Denmark) for hydroenvironmental investigations [53]. This system is composed of (1) a transmitter coil exciting the subsurface, (2) a receiver coil to measure the ground response, (3) a generator as a power source, and (4) several navigation instruments, such as GPS, tiltmeters, and laser altimeters, to locate the loop in space. The SkyTEM system operates in dual transmitter mode. The low moment, with a magnetic moment of approximately 2826 Am\({}^{2}\) and time gates from 11 \(\upmu\)s to 1 ms for the present survey, provides early time data for shallow imaging, and the high moment, with a magnetic moment reaching 144,440 Am\({}^{2}\) and time gates from 109 \(\upmu\)s to 8.9 ms, allows measuring later time data for deeper imaging. Locally, the depth of investigation of the method depends on the emitted magnetic moment, the bandwidth used, the subsurface electrical conductivity, and the signal/noise ratio.  
The AEM method allows imaging of the conductivity contrasts of the subsurface. To obtain usable measurements, several processes are applied to remove couplings with man-made installations and ambient noise from the signal. The processing scheme used is described in [39,40] and is based on singular value decomposition [39,40]. Data are then inverted using the spatially constrained inversion algorithm (SCI) [44]: (i) each usable AEM measurement is translated into a 1D (EM sounding) model divided into n layers, each defined by its thickness and resistivity, and the resistivity variations are displayed according to depth; during the inversion, constraints are applied vertically and spatially between nearby soundings (independently of flight lines). (ii) The ground clearance of the AEM system is also inverted, and the depth of investigation (DOI) is assessed as a final step in the inversion [66]. The results were obtained by running a smooth inversion for 25 layers from 0 m to 300 m deep. Each layer has a fixed and equal thickness. Only the resistivity values vary within a layer [39]. This approach is effective for imaging complex geological structures with the lowest dependency on the starting model, but it only displays a smoothed view of the subsurface. At this step, each flight line can be displayed as a resistivity profile composed of all the associated EM soundings. More 2D information can be obtained by interpolating, on raster grids, the resistivity of layers falling into a depth or elevation range. This interpolation is generally repeated to obtain slices over the entire range of investigations. Slices can then be merged to obtain a 3D resistivity model [45], (i) drawing profiles in any direction for confronting geological data (maps, boreholes, and field observations) and (ii) deriving interfaces for each imaged horizon.