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Sediment vs topographic micro-roughness: anthropogenic effects on seabed acoustic classification

Ruggero M. Capperucci and Alexander Bartholomä

The ever greater anthropogenic pressure on near-shore coastal ecosystems is leading the increased demand for reliable high-resolution mapping tools. Among the different marine domains, coastal areas are the most accessible ones (for both exploration and exploitation) and the most difficult to be described in details, due to the complexity of natural and human processes in action.

Multiple human activities hang over the Jade system (Wadden Sea, Germany). Fishery and mussel farming are present. A new container terminal (Jade-Weser Port) is under construction since 2008, thus an intense land-reclamation has been coupled with dredging and dumping operations.

A comprehensive research for describing and monitoring habitat dynamics and changes in the Jade area is currently ongoing. Within this frame, a 0.7 km2 area in the Jade Channel has been chosen as case study for assessing the reliability of acoustic ground discrimination systems in such a highly impacted environment.

Multiple acoustic devices have been deployed (multibeam, side-scan sonar and single beam) and sediment samples have been collected in 6 stations (4 replications each). QTC Impact™ and QTC Swathview™ software have been used for processing acoustic data and classify the seabed. Sedimentological results have been analysed using classification and ordination techniques (PAST software). All the data have been analysed using a GIS system.

Results of grain size analysis shows that there are significant differences in sediment composition between replications. No clear relationship occurs between replication closeness and statistical similarity.

Single-beam classification reveals a highly heterogeneous seabed texture with no clear dominant pattern, thus being likely controlled by the distinctive patchiness in sediment distribution. On the contrary, side-scan sonar and multibeam classifications are more dependent on seabed topography (e.g. dredging marks, bedforms).

In conclusion, samples can only partially ground-truth acoustic classifications, which seem to be strongly affected by the topographic micro-roughness rather than by sediments.

Figure 1. The study area (location map top-left corner). A. Shaded-relief map from multibeam data, overlaid by interpreted features (red: bedforms; blue: suction dredging marks; green: back-hoe dredging marks). B: single beam classification (4 classes) and sediment pie-charts (1: gravel; 2: gravelly shells; 3:sand; 4: sandy shells; 5: mud).