Effects of ship motion and survey conditions on volume back-scattering strength in acoustic survey

The influence of the ship motion and survey conditions, such as ship speed, wind direction, and wind speed, on volume back-scattering strength (SV value) was investigated by measuring echoes from an acoustically stable sea-bed. Using hull-mounted transducers, the examination was carried out by varying the speed of the vessel and the course of the vessel relative to the wind direction. The multiple regression analysis was made, with SV values as a dependent variable and the motion of the vessel and conditions of survey as independent variables. The variation of SV values had a close relation to the speed of the vessel and the course of the vessel relative to the wind, which had considerable effect on the motion of the vessel. These factors affected SV values complexly. It was concluded that the motion of the vessel induced an acoustic attenuation due to the bubbles caused by roll and pitch motions and to the beam swing caused by roll motion. The multiple regression equations obtained will be also useful as a correction of SV value in echo surveys for estimating the abundance of fish stocks.