Influence of target composition on the relationship between echo energy and target quantity

Abstract

The knowledge of the relationship between echo energy and target quantity is essential for fish stock assessment by energy investigation of echoes. The linearity principle of fisheries acoustics states that a linear relationship exists between the mean echo energy scattered by randomly distributed targets and the quantity of those targets. The validity of the linearity principle depends not only on the number density of targets under investigation, but also on their physical structure. This is evidenced by a study of the echo energy scattered by a random distribution of underwater spherical targets. Numerical results show that under the same conditions (measuring system, signal frequency, target size, etc.), the linearity principle may hold for spheres made of one material (a fluid, for example), but fail for spheres made of other materials (soft, rigid, and steel spheres in our case). In cases where the target number density is relatively high, the normalized total scattering cross section of targets may be an important parameter in determining whether the linearity principle holds. The interference of direct echoes, the shadowing effect, and the second-order scattering are taken into account in this work.