A phase-compensated distorted wave Born approximation representation of the bistatic scattering by weakly scattering objects : application to zooplankton

Author Posting. © Acoustical Society of America, 1999. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 106 (1999): 1732-1743, doi:10.1121/1.428036.The distorted wave Born approximation (DWBA) method has been successfully used in modeling the acoustic backscattering by weakly scattering zooplankton [Stanton et al., J. Acoust. Soc. Am. 94, 3463–3472 (1993), Wiebe et al., IEEE J. Ocean. Eng. 22(3), 445–464 (1997)]. However, the previously developed DWBA model ignores the imaginary part of the scattering amplitude and thus results in a zero-extinction cross section. As a consequence, the model fails to predict the scattering-induced attenuation which could be important under certain circumstances. In this paper, a phase-compensated DWBA-based approximation is presented. The improved method allows us to compute not only the scattering strength but also the acoustic attenuation. The new scattering model is validated by comparing with the existing exact solution for certain representative finite objects. The results from this study can be applied to bioacoustic applications where the attenuation due to scattering and/or multiple scattering by zooplankton is relevant, and where this information might be used to infer the acoustic properties of live animals.This work was partially supported by the National Science Foundation under Grant No. OCE-9730680

Application of pulse compression techniques to broadband acoustic scattering by live individual zooplankton

Author Posting. © Acoustical Society of America, 1998. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 104 (1998): 39-55, doi:10.1121/1.424056.Distinct frequency dependencies of the acoustic backscattering by zooplankton of different anatomical groups have been observed in our previous studies [Chu et al., ICES J. Mar. Sci. 49, 97–106 (1992); Stanton et al., ICES J. Mar. Sci. 51, 505–512 (1994)]. Based mainly on the spectral information, scattering models have been proposed to describe the backscattering mechanisms of different zooplankton groups [Stanton et al., J. Acoust. Soc. Am. 103, 236–253 (1998b)]. In this paper, an in-depth study of pulse compression (PC) techniques is presented to characterize the temporal, spectral, and statistical signatures of the acoustic backscattering by zooplankton of different gross anatomical classes. Data collected from various sources are analyzed and the results are consistent with our acoustic models. From compressed pulse (CP) outputs for all three different zooplankton groups, two major arrivals from different parts of the animal body can be identified: a primary and a secondary arrival. (1) Shrimplike animals (Euphausiids and decapod shrimp; near broadside incidence only): the primary one is from the front interface (interface closest to the transducer) of the animal and the secondary arrival is from the back interface; (2) gas-bearing animals (Siphonophores): the primary arrival is from the gas inclusion and the secondary arrival is from the body tissue ("local acoustic center of mass"); and (3) elastic shelled animals (Gastropods): the primary one is from the front interface and the secondary arrival corresponds to the subsonic Lamb wave that circumnavigates the surface of the shell. Statistical analysis of these arrivals is used to successfully infer the size of the individual animals. In conjunction with different aspects of PC techniques explored in this paper, a concept of partial wave target strength (PWTS) is introduced to describe scattering by the different CP highlights. Furthermore, temporal gating of the CP output allows rejection of unwanted signals, improves the output signal-to-noise ratio (SNR) of the spectra of selected partial waves of interest, and provides a better understanding of the scattering mechanism of the animals. In addition, it is found that the averaged PWTS can be used to obtain a more quantitative scattering characterization for certain animals such as siphonophores.This work was supported by the National Science Foundation under Grant No. OCE-9201264 and the U.S. Office of Naval Research under Grant Nos. N00014-89-J-1729, N00014-94-1-0452, and N00014-95-1-0287

Sound scattering by rough elongated elastic objects. II: Fluctuations of scattered field

Author Posting. © Acoustical Society of America, 1992. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 92 (1992): 1665-1678, doi:10.1121/1.403906.Sonar echoes from unresolved features of rough objects tend to interfere with each other. Because of these interferences, properties of the echoes, such as its envelope level, will vary from realization to realization of stochastically rough objects. In this article, the nature of the fluctuations of the backscattered echo envelope of rough solid elastic elongated objects is investigated. A general formulation is initially presented after which specific formulas are derived and numerically evaluated for straight finite-length cylinders. The study uses both the approximate modal-series- and Sommerfeld–Watson-transformation-based deformed cylinder solutions presented in the first part of this series [T. K. Stanton, J. Acoust. Soc. Am. 92, XXX (1992)]. The fluctuations of the backscattered echo envelope are related to the Rice probability density function (PDF) and shown to depend upon δ/a and [script L]/L in the Rayleigh scattering region (ka≪1) and kδ and [script L]/L in the geometric region (ka≫1), where δ is the rms roughness, a is the radius of the cylinder, [script L] is the correlation length of the roughness, L is the length of the cylinder, and k is the acoustic wave number in the surrounding fluid. There are similarities shown between these fluctuations in the geometric region and those from rough planar interfaces. In addition, analytical expressions and numerical examples show that the fluctuation or ``incoherent'' component of the scattered field is random only in amplitude—its phase approaches a constant value, in phase with the mean scattered field, which needed to be taken into account in the formulation. Finally, applications of the theory developed in this article to backscatter data involving live marine shrimp-like organisms are discussed.This work was supported by the U.S. Office of Naval Research Grant Nos. N00014-89-J-1729 and N00014-90-J-1804

Comment on ``Multiple scattering in a reflecting cavity: Application to fish counting in a tank [J. Acoust. Soc. Am. {\bf 109}, 2587-2597 (2001)]"

This paper presents a comment on the recent work on fish counting in a tank (J. Acoust. Soc. Am. 109, 2587-2597 (2001)). It is pointed out that there are ambiguities with the counting method.Comment: 2 page

A preliminary study of shallow-water sonar issues : signal motion loss and reverberation noise

This preliminary investigation addresses key program elements for sonar sensing in a shallow-water environment to establish bounds on possible solutions and to reduce program uncertainty. The modeling and experimental program focuses on two issues - the potential degradation of sonar data due to signal masking by shallow-water reverberation and signal loss caused by extreme platform motions. The research program combines theoretical analysis, experimental validation in a shallow-water environment, and development of a computer model to explore parametric sensitivity. Results from an initial dock-side test show good agreement with the theoretical predictions. From the shallow-water experiments and acoustic modeling we conclude that: (1) Signal motion loss can influence the reverberation level significantly but is not the dominant factor in target detection for sonars in the frequency range of interest (>200 kHz); a high-quality (velocity-aided) inertial navigation and attitude system will be sufficient to correct for geometric distortions caused by platform motion. (2) Although surface reverberation and multipath noise can be a factor, partcularly in shadow-mode imaging, reverberation levels are rapidly attenuated at the frequencies of interest and beam patterns can be manipulated to reject most interferences; echo-mode imaging is still dominated by the contrast between target strength and bottom reverberation.Funding was provided by the Mitre Corporation and the Office of Naval Research under Contract N00014-92-C-602

Differences between sound scattering by weakly scattering spheres and finite-length cylinders with applications to sound scattering by zooplankton

Author Posting. © Acoustical Society of America, 1998. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 103 (1998): 254-264, doi:10.1121/1.421135.A modeling study was conducted to determine the conditions under which fluidlike zooplankton of the same volume but different shapes (spherical/cylindrical) have similar or dramatically different scattering properties. Models of sound scattering by weakly scattering spheres and cylinders of finite length used in this analysis were either taken from other papers or derived and herein adapted for direct comparison over a range of conditions. The models were examined in the very low- (ka ≪ 1, kL ≪ 1), moderately low- (ka ≪ 1, kL ≳ 1), and high-frequency regions (ka ≫ 1, kL ≫ 1), where k is the acoustic wave number, a is the radius (spherical or cylindrical) of the body, and L is the length of the cylinders (for an elongated body with L/a = 10, "moderately low" corresponds to the range 0.1 ≲ ka ≲ 0.5). Straight and bent cylinder models were evaluated for broadside incidence, end-on incidence, and averages over various distributions of angle of orientation. The results show that for very low frequencies and for certain distributions of orientation angles at high frequencies, the averaged scattering by cylinders will be similar, if not identical, to the scattering by spheres of the same volume. Other orientation distributions of the cylinders at high frequencies produce markedly different results. Furthermore, over a wide range of orientation distributions the scattering by spheres is dramatically different from that of the cylinders in the moderately low-frequency region and in the Rayleigh/geometric transition region: (1) the Rayleigh to geometric scattering turning point occurs at different points for the two cases when the bodies are constrained to have the same volume and (2) the functional dependence of the scattering levels upon the volume of the bodies in the moderately low-frequency region is quite often different between the spheres and cylinders because of the fact that the scattering by the cylinders is still directional in this region. The study demonstrates that there are indeed conditions under which different shaped zooplankton of the same volume will yield similar (ensemble average) scattering levels, but generally the shape and orientation distribution of the elongated bodies must be taken into account for accurate predictions.This work was supported by the U.S. Office of Naval Research Grant No. N00014-95-1-0287 and the National Science Foundation Grant No. OCE-9201264

On the echo interference in sound backscattering by densely aggregated targets

Author Posting. © The Authors, 2004. This is the author's version of the work. It is posted here by permission of Elsevier B. V. for personal use, not for redistribution. The definitive version was published in ICES Journal of Marine Science 62 (2005): 771-778, doi:10.1016/j.icesjms.2004.12.021.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.The work was sponsored by the Institute of Oceanology, Polish Academy of Sciences (sponsor programme 2.7), and by the USA's National Science Foundation (Grant # 010562900)

Non-Rayleigh acoustic scattering characteristics of individual fish and zooplankton

Author Posting. © IEEE, 2004. This article is posted here by permission of IEEE for personal use, not for redistribution. The definitive version was published in IEEE Journal of Oceanic Engineering 29 (2004): 260-268, doi:10.1109/JOE.2004.828208.It has long been known that the statistical properties of acoustic echoes from individual fish can have non-Rayleigh characteristics. The statistical properties of echoes from zooplankton are generally less understood. In this study, echoes from individual fish and zooplankton from a series of laboratory measurements from the past decade are investigated. In the experiments, acoustic echoes from various individual organisms were measured over a wide range of frequencies and orientations, typically in 1/spl deg/-3/spl deg/ increments. In the analysis in this paper, the echoes from most of those measurements are grouped according to ranges of orientation, which correspond to typical orientation distributions of these organisms in the natural ocean environment. This grouping provides a distribution of echo values for each range of orientation. This approach, in essence, emulates a field experiment whereby distributions of echoes would be recorded for different distributions of orientations of the organisms. For both the fish and zooplankton data, there are conditions under which the echoes are strongly non-Rayleigh distributed. In some cases, the distribution is quantitatively connected to the physics of the scattering process while, in other cases, the connection is described qualitatively. Exploitation of the animal-specific statistics for classification purposes is suggested

Estimation of biological parameters of marine organisms using linear and nonlinear acoustic scattering model-based inversion methods

Author Posting. © Acoustical Society of America, 2016. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 139 (2016): 2885, doi:10.1121/1.4948759.The linear inversion commonly used in fisheries and zooplankton acoustics assumes a constant inversion kernel and ignores the uncertainties associated with the shape and behavior of the scattering targets, as well as other relevant animal parameters. Here, errors of the linear inversion due to uncertainty associated with the inversion kernel are quantified. A scattering model-based nonlinear inversion method is presented that takes into account the nonlinearity of the inverse problem and is able to estimate simultaneously animal abundance and the parameters associated with the scattering model inherent to the kernel. It uses sophisticated scattering models to estimate first, the abundance, and second, the relevant shape and behavioral parameters of the target organisms. Numerical simulations demonstrate that the abundance, size, and behavior (tilt angle) parameters of marine animals (fish or zooplankton) can be accurately inferred from the inversion by using multi-frequency acoustic data. The influence of the singularity and uncertainty in the inversion kernel on the inversion results can be mitigated by examining the singular values for linear inverse problems and employing a non-linear inversion involving a scattering model-based kernel.This work was supported by the National Science Foundation under Grant No. OCE-0928801 and the NOAA National Marine Fisheries Service, Northwest Fisheries Science Center. G.L.L. was partially supported by NOAA Cooperative Agreement Nos. NA09OAR4320129 and NA14OAR4320158 through the NOAA Fisheries Quantitative Ecology and Socioeconomics Training (QUEST) program

Calibration of broadband active acoustic systems using a single standard spherical target

Author Posting. © Acoustical Society of America, 2008. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 124 (2008): 128-136, doi:10.1121/1.2917387.When calibrating a broadband active acoustic system with a single standard target such as a sphere, the inherent resonances associated with the scattering by the sphere pose a significant challenge. In this paper, a method is developed which completely eliminates the source of resonances through isolating and exploiting the echo from the front interface of a sphere. This echo is relatively insensitive to frequency over a wide range of frequencies, lacking resonances, and is relatively insensitive to small changes in material properties and, in the case of spherical shells, shell thickness. The research builds upon the concept of using this echo for calibration in the work of Dragonette et al. [J. Acoust. Soc. Am. 69, 1186–1189 (1981)]. This current work generalizes that of Dragonette by (1) incorporating a pulse compression technique to significantly improve the ability to resolve the echo, and (2) rigorously accounting for the scattering physics of the echo so that the technique is applicable over a wide range of frequencies and material properties of the sphere. The utility of the new approach is illustrated through application to data collected at sea with an air-filled aluminum spherical shell and long broadband chirp signals (30–105 kHz).This work was supported by the U.S. Office of Naval Research Grant Nos. N00014-04-1-0475 and N00014- 04-1-0440 and the J. Seward Johnson Chair at WHOI