The influence of wind-induced bubbles on echo integration surveys

Abstract

An investigation of attenuation of acoustic energy caused by gas bubbles in the surface layers has been carried out. This was done primarily to study the effect on echo integration of fish abundance when using hull-mounted transducers. Two different approaches have been used. The first examines the variation of the echo intensity from an acoustically stable bottom layer and the second measures the total volume reverberation as a function of depths. The bubble density, size distribution, and the attenuation caused by the bubbles is estimated from the measurements done under different weather conditions. The results show that the acoustic attenuation caused by wind-induced gas bubbles in the surface layers appear at a lower wind force and at a greater magnitude than earlier reported and expected. The attenuation is found to increase rapidly with increasing frequency. The results are also used to find the minimum towing depths of a transducer as a function of the wind speed necessary in order to keep the attenuation due to the bubbles below a given number.