(HYDRO INTERNATIONAL: 28/02/2014) It is widely recognised that air bubbles may have a negative effect on echosounding systems. Klaus von Bröckel from GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany, has written an article about dealing with air bubbles from sweep-down and cavitation processes and outlining experiences with the last three large research vessels (RVs Polarstern, Meteor and Maria S. Merian). The practical experience resulted in the development of an optimised new hull form for the replacement of RV Sonne. The new ship is now under construction at a German shipyard and will be delivered in autumn 2014.
The basis of all echo sounding systems is the directed transmittance of sound into the water at different frequencies and the recording of its reflection through suitable receivers. The reflection (echo) occurs at boundary layers e.g. within the water column, at sediment surfaces or fishes. Even boundary layers within the sediment can be recorded in this way. Naturally, any air or other gas bubble within the water possesses a rather sharp sound-reflecting boundary layer too. So, as long as there is only water through which the sound is travelling, everything is fine and records are worth reading. But as soon as air or any other gas (e.g. methane) in the form of small and large bubbles (size range might be between less than 1mm to more than several centimetres) gets in the way of the sound, it is stopped and/or redirected. The result is only poor echosounding records, or none at all. Hereinafter, all ‘gas bubbles’ are referred to as ‘air bubbles’ for easier reading.
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