Acoustic textures and multibeam mapping of shallow marine habitats - Examples from eastern Malta

Habitat mapping is expanding into deeper waters, for which existing tools like sidescan sonars and Multibeam Echo-Sounders (MBES) are already tried and tested, to increasingly shallower waters. The adaptation of systems designed for deeper waters to more restricted ranges, often with multiple reflections, and more varied seabeds, often with overlaying vegetation or habitats, has been associated with significant technological challenges. The resulting step change in acoustic mapping has brought much higher spatial resolutions, at least an order of magnitude better, but also a higher variability to small-scale variations and survey settings. Maps of marine habitats have several purposes, from ecological (ecosystem health monitoring, marine-protected areas) to socioeconomic (resource accessibility and sustainability, changes brought by land-based processes, pollution or offshore activities). Classifications must therefore successfully address the relevant types of information. A large part of these classification activities relies on similar techniques, looking at variations in backscatter (in particular textures) and/or bathymetry to identify seabed and habitat types1,2,3,4,5. Previous studies have shown that interpretation could be affected by large-scale slopes not accounted for during processing6 or by changes in survey speed/sonar depth7. These changes may affect subsequent classifications and interpretations, and the present study aims at presenting the results of acoustic texture analyses in the context of data acquisition choices (e.g. pulse lengths, survey speeds), terrain morphology (role of slopes and large-scale types) and multi-scale terrain variability (bathymetry and backscatter), using a typical MBES map of a complex, shallow-water area offshore Eastern Malta