Preface The expedition ARK XIX/3 with the German icebreaking RV ...

Hydrosweep DS2 is installed onboard RV Polarstern. It operates at a frequency of
15.5kHz. In addition to depth measurements, echo amplitudes are recorded by the
system. The amplitudes can be converted into multibeam sidescan and angular
backscatter data. The main application of sidescan is to detect small scale features which
cannot clearly be recognised in the bathymetry (e.g. shallow channels or iceberg plough
marks). In contrast, angular backscatter shows the same resolution as the depth
measurements but supplies additional information about physical properties of the
seafloor (surface- and volume roughness) (Fig. A4.2.2-1).
Fig. A4.2-1: Angular backscatter curve for a) rough and b) smooth seafloor.
Therefore, the backscatter strength and its dependence on the incidence angle of the
acoustic pulse onto the seafloor is considered. Using a terrain model and taking into
account the effect of the water column as well as internal signal processing of the
multibeam, the recorded amplitude values are transformed into angular backscatter
strength. Maps showing the spatial extension of the backscatter data are used as a basis
to segment the seafloor into regions of different backscatter characteristics. The
segments are based on different echo levels and changing slope of the angular
backscatter fall (dB per degree incidence angle). The combination of these maps with
surface samples helps to determine different sediment properties in the segments and the
spatial validity of surface samples can be extended from the sampling spot to the
segmented areas. Thereafter, the sediment data can serve as input for seafloor
classification.
The aim of this study is to investigate the link between seafloor properties and angular
backscatter in order to use Hydrosweep bathymetric measurements for remote sensing of
the seafloor. Sparsely distributed seafloor samples and related seafloor properties will be
utilised to enhance interpretation of the entire area where backscatter data is available.
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