On the echo interference in sound backscattering by densely aggregated targets

It is important to understand the volume backscattering by dense aggregations of a variety of scattering objects such as bubbles or biological targets. This paper addresses the interference of the echoes from randomly distributed targets. The main motivation of the paper is to understand the conditions under which the echo interference may affect the accuracy of the abundance and/or the biomass estimation in fisheries and zooplankton acoustics significantly. Our approach consists of two parts. The first includes an analytical approach, which describes explicitly the dependence of the echo interference on the pulse shape of the transmitted signals. Because of the limitations of the analytical approach, numerical computations based on Monte Carlo simulations of acoustic backscattering by three-dimensional target distribution were performed as a second approach. The impacts of the echo interference were studied numerically over a wide range of frequencies, for different pulse shapes and directivity patterns of the acoustic systems, and for various spatial distributions of the targets (abundance), as well as the corresponding target strengths. Using analytical and numerical approaches it was demonstrated that for targets that are uniformly distributed in space, the influence of echo interference on the observed volume-backscattering strength is strongly controlled by three main parameters. These are the number of targets in the sampling volume, the product of sound frequency and pulse duration, and the degree of tapering of the applied pulses. A numerical examples of the abundance estimation of marine organisms are presented.