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Reducing Sound Impact From Multibeam Bathymetry

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Beyer, A. , Niederjasper, F. , Graffe, D. and Schenke, H. W. (2006): Reducing Sound Impact From Multibeam Bathymetry , AGU Joint Assembly, Baltimore, Maryland, USAMay. .
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Abstract:

Multibeam bathymetry systems use the travel time of acoustic pulses as observable to determine the depth of the seafloor. In order to provide appropriate echo quality necessary for depth determination and seafloor shape recording, a high acoustic source level is needed. The main factors affecting the sound level of the emitted sonar pulse during the travel through the water column are geometrical spreading, absorption and the backscattering strength of the seabed. The reception threshold of the acoustic receiver determines the sound level needed to detect the acoustic pulse. Common deep sea multibeam systems use the determined depth as an indication to control the source level necessary for the successive ping.Lowering the transmission level of hydroacoustic sources is a general demand to reduce the impact of sonar signals on the marine life. In order to protect marine mammals we studied the capability of RV Polarstern's Hydrosweep multibeam system. However, reducing the source level finally limits the ability of accurate depth determination. The recent upgrade of Hydrosweep provides the possibility to adjust the source level based on the echo quality. Automatic and manual controls can be used to reduce the source level while achieving a given limit of data quality for selected beam angles. We present first results of this feasibility study focusing on the applicability of different methods used for reducing the acoustic source level. The influence of the source level reduction on data quality and accuracy is evaluated on the basis of terrain models derived from a test survey in a special calibration area in the Bay of Biscay.

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