Seafloor classification using hydroacoustic methods in the shallow coastal North Sea


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Finn.Mielck [ at ] awi.de

Abstract

Seabed environments reveal important insights into marine ecosystems and hydrodynamics. However, the knowledge regarding their distribution in the North Sea is still fragmentary since data interpretation and mapping is very time consuming. Hydroacoustic devices are able to provide rapid and reliable information on the acoustic characteristics of the seafloor that are instrumental for seafloor discrimination. In this study we compare data of two different hydroacoustic devices: Sidescan sonar data provide acoustic images of the seafloor including bedforms and grain-size distribution. The acoustic ground discrimination system RoxAnn (200 kHz) was used to measure backscatter intensities which indicate roughness (first echo return) and hardness (second echo return) parameters of the seafloor. The data were acquired in an exclusively sandy and relatively shallow investigation area in the Sylt-Rømø Basin (SE North Sea) which is typical for the European Wadden Sea. For ground-truthing surface-sediment samples were taken. The results reveal surficial sediments ranging from fine to coarse sand. Finer material is rather restricted to the shallow areas while coarser sediments characterize the deeper tidal channels. The determined roughness and hardness parameters also strongly increase within these inlets following the change in grain size. The sidescan sonar imagery shows flow-transverse subaqueous dunes of different sizes (0.7 to 35 m from crest to crest). Both, ebb- and flood-dominated structures occur. Altogether six physically different environmental units were defined using grain-size data of the grab samples as well as bedform and backscatter data provided by the sidescan sonar. Not all of these units can be discriminated by the RoxAnn. In particular those showing similar grain sizes or superimposed bedforms yield no clear signature in the hardness vs. roughness scatter plots. However, there are some areas characterized by fine sand that reveal completely different roughness and hardness properties than other places with granumlometrically identical fine sand. This is caused by varying amounts of shell fragments on the seafloor which do not become visible in the sidescan records. Both hydroacoustic methods are suitable to identify seafloor properties even in difficult shallow and sandy environments but a combination of the methods yields more information at higher precision especially about shell content and grain-size distribution. Both hydroacoustic methods in combination with occasional grab samples and/or underwater video provide a sound data base for large-scale mapping activities as demanded by the EU government.



Item Type
Conference (Poster)
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Peer revision
Not peer-reviewed
Publication Status
Published
Event Details
AGU Fall Meeting, 03 Dec 2012 - 07 Dec 2012.
Eprint ID
31994
Cite as
Mielck, F. and Hass, H. C. (2012): Seafloor classification using hydroacoustic methods in the shallow coastal North Sea , AGU Fall Meeting, 3 December 2012 - 7 December 2012 .


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