Measuring Centimeter-Scale Sand Ripples Using Multibeam Echosounder Backscatter Data from the Brown Bank Area of the Dutch Continental Shelf
Leo Koop,
Karin J. van der Reijden,
Sebastiaan Mestdagh,
Tom Ysebaert,
Laura L. Govers,
Han Olff,
Peter M. J. Herman,
Mirjam Snellen,
Dick G. Simons
Affiliations
Leo Koop
Acoustics Group, Faculty of Aerospace Engineering, Delft University of Technology, 2629 HS Delft, The Netherlands
Karin J. van der Reijden
Conservation Ecology Group, Groningen Institute for Evolutionary Life Sciences, University of Groningen, P.O. Box 11103, 9700 CC Groningen, The Netherlands
Sebastiaan Mestdagh
Department of Estuarine and Delta Systems, NIOZ Royal Netherlands Institute for Sea Research and Utrecht University, P.O. Box 140, 4400 AC Yerseke, The Netherlands
Tom Ysebaert
Department of Estuarine and Delta Systems, NIOZ Royal Netherlands Institute for Sea Research and Utrecht University, P.O. Box 140, 4400 AC Yerseke, The Netherlands
Laura L. Govers
Conservation Ecology Group, Groningen Institute for Evolutionary Life Sciences, University of Groningen, P.O. Box 11103, 9700 CC Groningen, The Netherlands
Han Olff
Conservation Ecology Group, Groningen Institute for Evolutionary Life Sciences, University of Groningen, P.O. Box 11103, 9700 CC Groningen, The Netherlands
Peter M. J. Herman
Department of Marine and Coastal Systems, Deltares, P.O. Box 177, 2600 MH Delft, The Netherlands
Mirjam Snellen
Acoustics Group, Faculty of Aerospace Engineering, Delft University of Technology, 2629 HS Delft, The Netherlands
Dick G. Simons
Acoustics Group, Faculty of Aerospace Engineering, Delft University of Technology, 2629 HS Delft, The Netherlands
Backscatter data from multibeam echosounders are commonly used to classify seafloor sediment composition. Previously, it was found that the survey azimuth affects backscatter when small organized seafloor structures, such as sand ripples, are present. These sand ripples are too small to be detected in the multibeam bathymetry. Here, we show that such azimuth effects are time dependent and are useful to examine the orientation of sand ripples in relation to the flow direction of the tide. To this end, multibeam echosounder data at four different frequencies were gathered from the area of the Brown Bank in the North Sea. The acoustic results were compared to video and tide-flow data for validation. The sand ripples affected the backscatter at all frequencies, but for the lowest frequencies the effect was spread over more beam angles. Using the acoustic data made it possible to deduce the orientations of the sand ripples over areas of multiple square kilometers. We found that the top centimeter(s) of the seafloor undergoes a complete transformation every six hours, as the orientation of the sand ripples changes with the changing tide. Our methodology allows for morphology change detection at larger scales and higher resolutions than previously achieved.
