Echo statistics of individual and aggregations of scatterers in the water column of a random, oceanic waveguide

Abstract

The relative contributions of various physical factors to producing non-Rayleigh distributions of echo magnitudes in a waveguide are examined. Factors that are considered include (1) a stochastic, range-dependent sound-speed profile, (2) a directional acoustic source, (3) a variable scattering response, and (4) an extended scattering volume. A two-way parabolic equation model, coupled with a stochastic internal wave model, produces realistic simulations of acoustic propagation through a complex oceanic sound speed field. Simulations are conducted for a single frequency (3 kHz), monostatic sonar with a narrow beam (5 À3 dB beam width). The randomization of the waveguide, range of propagation, directionality of the sonar, and spatial extent of the scatterers each contribute to the degree to which the echo statistics are non-Rayleigh. Of critical importance are the deterministic and stochastic processes that induce multipath and drive the one-way acoustic pressure field to saturation (i.e., complex-Gaussian statistics). In this limit predictable statistics of echo envelopes are obtained at all ranges. A computationally low-budget phasor summation can successfully predict the probability density functions when the beam pattern and number of scatterers ensonified are known quantities. [http://dx

Broadband classification and statistics of echoes from aggregations of fish measured by long-range, mid-frequency sonar

“…While this formulation was developed for direct path scattering by point scatterers, the Chu-Stanton theory has been applied to finite-length extended scatterers at long ranges by accounting for waveguide effects. 16 These effects include the attenuation of high-propagation-angle energy and the squaring of the oneway response due to reciprocity of forward and backpropagated acoustic paths. The term finite-length extended scatterer implies that the scattering volume is large compared with the correlation length of the incident pressure field, yet small enough to subtend an angle that can be approximated as a constant value.…”

“…Specifically, the propagation model used here (described in detail in Ref. 16) incorporates internal-wave (IW) induced perturbations to the mean sound-speed field to capture the natural randomness of the ocean environment. Modeling of internal wave fields 30 is based on the mean observed sound speed profile (Fig.…”

“…Finally, PDFs of echo magnitudes are predicted using a continuous wave (CW) phasor-summation model that accounts for sonar system characteristics, waveguide effects, and the scattering response of resonant fish schools. 16 Probability density functions are calculated using a Monte-Carlo method in which the location of the fish aggregation within the beam, the scattering response, and noise are random variables.…”

“…For example, it is not practical to continuously or uniformly sample a large region with trawls. In addition, the combination of a narrow beam and a refractive waveguide can lead to a highly non-uniform distribution of acoustic energy in the water column 16 potentially causing significant underestimates in abundance calculations. Furthermore, since the depths of the fish are generally not known, it can be difficult to discriminate between seafloor reverberation and scatterers in the water column.…”

“…The echo-envelope distributions are well modeled by a computationally efficient calculation that accounts for waveguide effects and beampatterns of the source and receiver. 16 This article is organized as follows: Section II describes the 2011 Gulf of Maine experiment. Section III describes the acoustic models used to predict frequency-dependent scattering by fish swimbladders, propagation of the long-range sonar signal, and distributions of the magnitudes of echoes from aggregations of fish.…”