Marc Roche, Matthias Baeye, Jeroen De Bisschop, Koen Degrendele, Lies De Mol, Sonia Papili, Olga Lopera and Vera Van Lancker (2015)
Backscatter stability and influence of water column conditions: estimation by multibeam echosounder and repeated oceanographic measurements, Belgian part of the North Sea
In: Seabed and Sediment Acoustics: Measurements and Modelling, ed. by Blondel et al., vol. 37(1), pp. 74-84, University of Bath, Bath, Institute of Acoustics. Proceedings of the Institute of Acoustics.
The European Union Marine Strategy Framework Directive requires Member States to
monitor and assess the health status of their marine habitats. Belgium put forward that the
spatial extent and distribution of the major habitats (sandy mud to mud, muddy sands to
sands and coarse-grained sediments), as well as that of gravel beds may only fluctuate
within a margin that is limited to the accuracy of the current sediment distribution maps.
Due to their ability to provide simultaneous bathymetry and backscatter strength (BS),
reflecting seabed nature, multibeam echosounder systems (MBES) provide a time- and costefficient
solution to anticipate on this legal obligation. However, as there is no formal quality
level scale for the BS and consequently no level of reliability of the final decibel values,
evaluating the BS quantitative capabilities to monitor habitat changes remains challenging.
In a monitoring context, it is absolutely necessary to investigate to what extent the mean
MBES-measured BS variation -from one cruise to another- actually represents a significant
change in seabed properties and not only a change in the conditions of the water column
(e.g., variation in near-bed suspensions, biological effects, increased occurrence of micro
bubbles in the sea surface due to wind) at the time of the measurement. In order to evaluate
the relative accuracy and the repeatability of MBES BS and to examine the external sources
of variance in acoustic signatures, repeated measurements combining MBES with an
Acoustic Doppler current Profiler (ADP) have been performed during tidal cycles on
reference areas in the Belgian part of the North Sea. Currents and BS in the water column
and vertical profiling of oceanographic parameters have been investigated allowing studying
the influence of water column conditions on the BS. Measurements were conducted using
two MBES systems to evaluate repeatability between echosounders. Ultimate goal is to arrive
at seabed mapping and analyses protocols to obtain harmonized data products from a
collaborative seabed mapping community.
Open Access, International Redaction Board, Abstract of an Oral Presentation or a Poster
habitat
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