Abstract
The increased use of backscatter measurements in time series for environmental monitoring necessitates the comparability of individual results. With the current lack of pre-calibrated multibeam echosounder systems for absolute backscatter measurement, a pragmatic solution is the use of natural reference areas for ensuring regular assessment of the backscatter measurement repeatability. This method mainly relies on the assumption of a sufficiently stable reference area regarding its backscatter signature. The aptitude of a natural area to provide a stable and uniform backscatter response must be carefully considered and demonstrated by a sufficiently long time-series of measurements. Furthermore, this approach requires a strict control of the acquisition and processing parameters. If all these conditions are met, stability check and relative calibration of a system are possible by comparison with the averaged backscatter values for the area. Based on a common multibeam echosounder and sampling campaign completed by available bathymetric and backscatter time series, the suitability as a backscatter reference area of three different candidates was evaluated. Two among them, Carré Renard and Kwinte, prove to be excellent choices, while the third one, Western Solent, lacks sufficient data over time, but remains a valuable candidate. The case studies and the available backscatter data on these areas prove the applicability of this method. The expansion of the number of commonly used reference areas and the growth of the number of multibeam echosounder controlled thereon could greatly contribute to the further development of quantitative applications based on multibeam echosounder backscatter measurements.
Highlights
In Northwestern Europe, coastal marine areas have been under increasing pressure from industrial activities for more than three decades
If repeated backscatter measurements undertaken using a single multibeam echosounder (MBES) are incorporated in a seafloor-monitoring program, the resulting backscatter data time-series are intrinsically relative: the backscatter data from a single MBES must be comparable between successive measurements
The benefits of using backscatter natural reference areas are plenty. They provide a simple and pragmatic solution to ensure the repeatability of the MBES measurements
Summary
In Northwestern Europe, coastal marine areas have been under increasing pressure from industrial activities for more than three decades. If repeated backscatter measurements undertaken using a single MBES are incorporated in a seafloor-monitoring program, the resulting backscatter data time-series are intrinsically relative: the backscatter data from a single MBES must be comparable between successive measurements In this case, an absolute reference of the measured backscatter level is not mandatory, but a regular assessment of the backscatter measurement repeatability is required to detect possible drifts of the measuring system sensitivity. Changes of the sedimentary interface at a spatial scale smaller than the MBES elementary measurement resolution (approximately and conveniently given by the extent of the beam footprint on the seafloor) may happen, induced by e.g., changes of benthic fauna and related bioturbation, benthic flora, oscillations of sand ripples Such small variations potentially cause fluctuations of the average backscatter (small causes can here have large effects) without being detectable in the bathymetric data; this illustrates the limitation of a stability assessment based only on the sounder-measured morphology. Regular in situ controls of the seabed interface including sediment sampling and video imaging, in addition to acoustic data, are highly recommended to confirm seabed stability
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