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

Abstract

Multibeam sonars are now used in increasingly shallower waters, bringing very high resolution imagery and bathymetry from often very varied seabeds. These developments have been associated with technological challenges, such as the adaptation of systems designs for deeper waters to more restricted ranges, often with multiple reflections. They have also brought a step change in acoustic mapping, with at least an order of magnitude in resolutions now attainable, coupled with a higher susceptibility to small-scale variations. This paper will investigate how these changes affect seafloor classification, focusing on the role of acoustic textures of shallow habitats. Maps of marine habitats have several purposes, from ecological (ecosystem health monitoring, marine-protected areas) to socio-economic (resource accessibility and sustainability, changes brought by pollution or offshore activities). Classifications must therefore successfully address the relevant types of information. This will be presented using a high-resolution multibeam dataset acquired on the eastern coast of the island of Malta in May 2012, in water depths of 1.5-400 m. The 70-100 kHz Kongsberg EM710 multibeam echosounder was deployed over diverse terrains including horst and graben alternations, seagrass cover in some areas, gravel and different sediment types. After full processing, the dataset provides multibeam backscatter at 1-m resolution and bathymetry at 2-m resolution, supplemented with photographic ground truth and samples at appropriate locations. The results of acoustic texture analyses will be presented 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)