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A lithological and morphological map created from multibeam backscatter data in challenging circumstances: the Lower Sea Scheldt estuary

M. Mathys, M. Sas, F. Roose

The Lower Sea Scheldt is regularly dredged to maintain the access to the harbour of Antwerp. The Maritime Access Division has the permission to relocate the dredged material within the Scheldt itself on the condition that a monitoring program is performed in order to detect any negative effects on the environment. The creation of a lithological and morphological map of the entire Lower Sea Scheldt is part of an extensive integrated monitoring program.

In the past, the lithological map was created based on the backscatter signal from singlebeam profiles. This technique is fast, but has the disadvantage that areas in between the profiles had to be interpolated. Morphological maps were created based on side-scan sonar data. The disadvantage of this technique is that certain reflections due to sedimentological characteristics of the river bottom could not be distinguished from reflections due to the morphological character of the area. The presented study investigated the feasibility and applied the most appropriate method to create a lithological and morphological map from full coverage multibeam backscatter data which are anyhow available from the regular depth soundings in the Lower Sea Scheldt.

The Lower Sea Scheldt estuary, however, confronts us with challenging circumstances which are not present when mapping offshore areas with multibeam: during a tidal cycle, sediment concentration, salinity and water depth changes significantly. Moreover, due to the morphological character of the Lower Sea Scheldt two different ships had to be used to cover both the deep channels and shallow tidal flats with multibeam. This study describes the sensitivity of the multibeam recording to the extreme environmental circumstances and the consequences of the use of two survey ships on the processing procedure.

The study also presents our experiences with the creation of the lithological and morphological maps itself,  e.g. supervised versus unsupervised classification and gives some recommendations on sampling strategies for ground-truthing of the maps.