Tokyo University of Marine science and Technology, Minato, Tokyo 108-8477, Japan
An error is introduced by the influence of sea surface and bottom reverberations in quantifying surface fish using fisheries scanning sonar. Minimization of the error is the purpose of this study. The sonar equation is derived to measure the surface backscattering strengths (SS) of the sea surface and bottom reverberations. The characteristics of the equivalent beam angle in the equation is investigated, after clarifying the beam directivity relative to the sea surface and bottom. This equation was applied to an experimental quantitative sonar system, and the SS of the sea surface and bottom reverberations were observed in Tateyama Bay, Japan. The signal to reverberation ratio (SRR) for the average volume scattering measurement of fish is investigated using the characteristics of SS. Combining the SRR and the signal to noise ratio (SNR), an appropriate tilt angle and measurement range are introduced. Then the influence of sea surface and bottom reverberations can be minimized in the abundance measurement by the echo integration method using a sonar.