Coincident swath acoustic backscatter and bathymetry for the interpretation of shallow-water sediment composition and processes

Two surveys from the west of Ireland demonstrate how the combination of high-resolution, geo-referenced, spatially coincident, swath acoustic bathymetric and backscatter data is effective for understanding underwater geological processes and assisting the design of environmental monitoring programmes. One case study corroborates terrestrial observations of Quaternary glacial cycles around Clew Bay by identifying sea-bed morphology that is consistent with a glacial advance from east to west, followed by deglaciation and a subsequent readvance to the NW during the Last Glacial Maximum. The bathymetric data identify a palaeo-coastline at water depths of c. 30 m below present sea level and the underwater extension of an onshore end-moraine. The backscatter data, corroborated by grab sampling, identify mixed sediment outwash associated with the ice limit, as well as sea-bed depressions caused by subglacial erosion. The other case study in Lough Corrib, an inland lake, identifies areas of mud from backscatter data. Swath bathymetry, sub-bottom profiling and grab samples suggest that the mud accumulates in bedrock depressions and is the preferred location of several elements related to eutrophication. The combination of spatially coincident swath bathymetry and backscatter data is more effective for geological interpretations than backscatter data from conventional side-scan sonar.