Testing 3D electromagnetic data and techniques as a tool for high-resolution bathymetry in tropical coral reef environment.

In tropical coral reef environments, surveying techniques that provide high-resolution data on the seafloor morphology are traditionally associated with high logistical costs (USV, LIDAR or satellite bathymetry) and with high diculties in obtaining shallow bathymetry where submerged hazards are present. Acoustic methods in these environments (using either singlebeam or multibeam echosounders systems) require a big number of routes to reach a high-resolution bathymetry and in addition, submerged obstacles limit their operability. Several studies on land environment have conrmed the accuracy of Structure from Motion (SfM) techniques for high-resolution 3-dimensional (3D) topographic reconstruction and analysis, and in some cases found SfM to be equivalent to LIDAR techniques. In the last 5 years this methodology started to be applied in ultra-shallow marine environment, using snorkeling transect with underwater cameras on coral reefs, where ship-based survey are unfeasible. The small spatial extent of the resulting 3D models, in this particular underwater context, is the main limit of this technique. Our methodology aim to increase the coverage of SfM techniques in ultra-shallow water environment coupling the snorkelling video transect with a commercial Unmanned Aerial Vehicle (UAV) survey in order to obtain: (1) a complete sub-metrical resolution DEM (Digital Elevation Model), and (2) provide unique opportunities to better quantify topography, rugosity and other structural characteristics of this peculiar marine environment. This study utilized SfM 3D reconstruction software tools to create DEM of a coral reef of a small Maldivian island (Magoodhoo Island, Faafu Atoll) and used a proper GIS-based tool to extract geomorphometric parameters. The use of geomorphometric analysis on the resulting DEMs allowed to quantitatively asses the structural complexity of the surveyed reef, that can be integrated with other physiological and ecological parameters to increase our knowledge in understanding the geomorphic process and the evolution of this vulnerable carbonate depositional system.