Abstract
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.
Highlights
The North Sea is one of the busiest seas of the world [1], with heavy shipping traffic [2] from major international ports such as Rotterdam, Antwerp, Hamburg, and Bremen
The clearest of these is at ±35° in the 450-kHz data (Figure 5d). This artifact is removed for the remainder of the figures by using the mean of the ARCs that were least affected by the sand ripples, interpolating the area of the dip with a line, and adding the difference between this line and the mean ARC to all the
We found that small-scale repeating seafloor structures have a profound impact on ARCs
Summary
The North Sea is one of the busiest seas of the world [1], with heavy shipping traffic [2] from major international ports such as Rotterdam, Antwerp, Hamburg, and Bremen. Geosciences 2020, 10, 495 point, it connects to the English Channel at the Dover Strait, which is the busiest shipping lane in the world [3]. In addition to the shipping activity on the North Sea, it is one of the most bottom-trawled continental shelves in the world [4]. To maintain a “Good Environmental Status” amidst multiple pressures, the European Marine Strategy Framework Directive (MSFD) requires European states to monitor their marine waters [6]. The use of acoustic methods is an important component of marine monitoring efforts [7]
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