Abstract
Backscatter mosaics based on a multi-frequency multibeam echosounder survey in the continental shelf setting of the North Sea were compared. The uncalibrated backscatter data were recorded with frequencies of 200, 400 and 600 kHz. The results showed that the seafloor appears mostly featureless in acoustic backscatter mosaics derived from 600 kHz data. The same area surveyed with 200 kHz reveals numerous backscatter anomalies with diameters of 10–70 m deviating between −2 dB and +4 dB from the background sediment. Backscatter anomalies were further subdivided based on their frequency-specific texture and were attributed to bioturbation within the sediment and the presence of polychaetes on the seafloor. While low frequencies show the highest overall contrast between different seafloor types, a consideration of all frequencies permits an improved interpretation of subtle seafloor features.
Highlights
A reliable, repeatable and objective classification of seabeds, comprising both geological and biological habitats, continues to be an important issue for marine spatial planning and management as well as for research
angular response curve (ARC) are less sensitive to small-scale variations in seafloor composition and a strong synergy with backscatter mosaics corrected for the angular dependence exists, albeit this is rarely utilized [7]
The decrease in backscatter intensities is poorly observed in 400 kHz data, and disappears for the 600 kHz mosaic, causing a bluish north-western rim adjacent to sorted bedforms in the multi-frequency mosaic (Figure 2A)
Summary
A reliable, repeatable and objective classification of seabeds, comprising both geological and biological habitats, continues to be an important issue for marine spatial planning and management as well as for research. Acoustic remote sensing by side scan sonar and multibeam echosounder obtains information on seafloor habitats based on measuring the intensity of acoustic signals backscattered from the seafloor [1,2]. The bulk backscattering level measured by the sonar comprises specular reflection, seafloor scatter and volume scatter and depends on the incidence angle and frequency of the acoustic wave. The angular dependence of backscatter levels has been used to characterize different seabeds [6,7]. ARCs are less sensitive to small-scale variations in seafloor composition and a strong synergy with backscatter mosaics corrected for the angular dependence exists, albeit this is rarely utilized [7]
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