Long-standing problems with acoustical terminology in fisheries applications such as echo-integration indicate the need for a more consistent approach. Based where possible on existing terms, a scheme of explicitly named quantities is proposed, backed by clearly stated definitions and preferred symbols. The emphasis is on scattering phenomena because the terminology in this area presents the main source of difficulty. Starting with the scattering equations for a small target, the volume, area, and line coefficients relevant to multiple, distributed targets are defined, leading to practical formulas for the important application of remote biomass estimation from echo-integration. The aim is to incorporate, as far as possible, common practice in fisheries-acoustics terminology and related fields. The developed scheme has been commended by the ICES Fisheries Acoustics Science and Technology Working Group as a constructive approach to better communication standards in fisheries-acoustics publications.

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A consistent approach to definitions and symbols in fisheries acoustics

Geostrophic contour-following bottom currents involved in the deep thermohaline circulation of the world ocean appear to be the principal agents which control the shape of the continental rise and other sediment bodies.

https://science.sciencemag.org/content/sci/152/3721/502.full.pdf

Shaping of the continental rise by deep geostrophic contour currents.

Over the past two decades the application of acoustics to the measurement of small-scale sediment processes has been gaining increasing acceptance within the sedimentological community. This has arisen because acoustics has the potential to measure non-intrusively, with high temporal and spatial resolution, profiles of suspended sediment size and concentration, profiles of flow, and the bedform morphology. In the present article we review the capability of acoustics to deliver on its potentiality to make a valuable and unique contribution to the measurement of small-scale sediment processes. The article introduces the reasons for using acoustics, the physics underlying the approach, a series of examples illustrating collected data, a discussion on some of the difficulties encountered when applying acoustics and finally a look to the future and possible new developments.

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A review of acoustic measurement of small-scale sediment processes

http://misclab.umeoce.maine.edu/documents/stramskietal2004.pdf

The role of seawater constituents in light backscattering in the ocean

This paper is based on material presented at the start of a Health Protection Agency meeting on ultrasound and infrasound. In answering the question ‘what is ultrasound?’, it shows that the simple description of a wave which transports mechanical energy through the local vibration of particles at frequencies of 20 kHz or more, with no net transport of the particles themselves, can in every respect be misleading or even incorrect. To explain the complexities responsible for this, the description of ultrasound is first built up from the fundamental properties of these local particle vibrations. This progresses through an exposition of the characteristics of linear waves, in order to explain the propensity for, and properties of, the nonlinear propagation which occurs in many practical ultrasonic fields. Given the Health Protection environment which framed the original presentation, explanation and examples are given of how these complexities affect issues of practical importance. These issues include the measurement and description of fields and exposures, and the ability of ultrasound to affect tissue (through microstreaming, streaming, cavitation, heating, etc.). It is noted that there are two very distinct regimes, in terms of wave characteristics and potential for bioeffect. The first concerns the use of ultrasound in liquids/solids, for measurement or material processing. For biomedical applications (where these two processes are termed diagnosis and therapy, respectively), the issue of hazard has been studied in depth, although this has not been done to such a degree for industrial uses of ultrasound in liquids/solids (sonar, non-destructive testing, ultrasonic processing etc.). However, in the second regime, that of the use of ultrasound in air, although the waves in question tend to be of much lower intensities than those used in liquids/solids, there is a greater mismatch between the extent to which hazard has been studied, and the growth in commercial applications for airborne ultrasound.

What is ultrasound

Fundamentals of Acoustical Oceanography an important reference for specialists in acoustics, oceanography, marine biology, and related fields. This book also encourages a new generation of scientists, engineers, and entrepreneurs to apply the modern methods of acoustical physics to probe the unknown sea. The book is an authoritative, modern text with examples and exercises. It contains techniques to solve the direct problems, solutions of inverse problems, and an extensive bibliography from the earliest use of sound in the sea to present references. The book provides background to measure ocean parameters and processes, find life and objects in the sea, communicate underwater, and survey the boundaries of the sea. Fundamentals of Acoustical Oceanography explains principles of underwater sound propagation, and describes how both actively probing sonars and passively listening hydrophones can reveal what the eye cannot see over vast ranges of the turbid ocean. This book demonstrates how to use acoustical remote sensing, variations in sound transmission, in situ acoustical measurements, and computer and laboratory models to identify the physical and biological parameters and processes in the sea.

Fundamentals of Acoustical Oceanography

Acoustic fish models should represent the fish body form. The Atlantic cod were used to model the acoustic scattering function of teleost fish. The model provides a basis for choices of sonar carrier frequencies. Anesthetized live Atlantic cod ranging from 156 to 380 mm (SL) were ‘‘soft’’ x‐rayed to image inflated swimbladders and skeletal elements. Maximum body heights and widths were 0.18 and 0.13 of fish lengths. Lengths and diameters of swimbladder were approximately 0.25 and 0.05 of the fish lengths. A series of short‐length fluid‐filled cylinders were used to represent body flesh. For carrier frequencies above the breathing mode resonance, swimbladders were modeled as a series of short gas‐filled volume elements of cylinders. A Kirchhoff‐ray approximation was used to compute the high‐frequency acoustic scattering. A low mode solution for a gas‐filled cylinder was used to compute the low‐frequency ‘‘breathing mode resonance.’’ All contributions were added coherently. The scattering lengths L, or target strength=20 log‖L/L0‖ (where L0 is reference length) were sensitive to fish orientation relative to the sonar beam. Theoretical target strengths were compared to the 38‐kHz cod data. Agreement was good.

Acoustic models of fish: The Atlantic cod (Gadus morhua)

By heuristically extending the previously developed ray solution [Stanton et al. J. Acoust. Soc. Am. 94, 3454–3462 (1993)] to predict the scattering by cylinders over all angles of incidence, approximate expressions are derived which describe the echo energy due to sound scattered by finite cylinders averaged over orientation and length. Both straight and bent finite length cylinders of high aspect ratio are considered over the full range of frequencies (Rayleigh through geometric scattering). The results show that for a sufficiently broad range of orientation, the average echo is largely independent of the degree of bend—that is, the results are essentially the same for both the straight and bent cylinders of various radii of curvature (provided the bend is not too great). Also, in the limit of high frequency (i.e., the acoustic wavelength is much smaller than the cross‐sectional radius of the object), the averages are independent of frequency. The resultant formulas derived herein are useful in describi...

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Average echoes from randomly oriented random‐length finite cylinders: Zooplankton models

Acoustic technology is an accepted and important component of aquatic research and resource management. Despite the widespread use of echosounders, few guidelines aid in the choice of appropriate sonar system parameters for acoustic surveys. Choice of acoustic carrier frequency is analogous to the choice of spotlight colour used to illuminate a painting. Thr ee primary biological factors influence the scattering of sound by aquatic organisms: swimbladder presence, organism length, and organism behaviour. We illustrate the influence of these factors on the amplitude of backscattered echoes using a Kirchhoff-ray mode scattering model to quantify fish and zooplankton backscatter as a function of carrier frequency, fish length, and swimbladder aspect. Model results illustrate that echo amplitudes from aquatic organisms are largely dependent on the presence or absence of a swimbladder. Target strengths generally increase with increasing carrier frequency and organism length. Swimbladder angle relative to the incident sound wave affects scattering amplitudes at all frequencies. Measurements of backscatter from swimbladdered fish are relatively robust when the ratio of fish length to acoustic frequency wavelength ranges between 2 and 10. As fish length to frequency wavelength ratios increase, echo amplitudes become more dependent on aspect and peak when the swimbladder is perpendicular to the acoustic wavefront.

/pdf/sonar-systems-and-aquatic-organisms-matching-equipment-and-59ea92z4uf.pdf

Sonar systems and aquatic organisms: matching equipment and model parameters

Data indicate that certain important types of marine organisms behave acoustically like weakly scattering fluid bodies (i.e., their material properties appear fluidlike and similar to those of the surrounding fluid medium). Use of this boundary condition, along with certain assumptions, allows reduction of what is a very complex scattering problem to a relatively simple, approximate ray‐based solution. Because of the diversity of this problem, the formulation is presented in two articles: this first one in which the basic physics of the scattering process is described where the incident sound wave is nearly normally incident upon a single target (i.e., the region in which the scattering amplitude is typically at or near a maximum value for the individual) and the second one [Stanton et al., J. Acoust. Soc. Am. 94, 3463–3472 (1993)] where the formulation is heuristically extended to all angles of incidence and then statistically averaged over a range of angles and target sizes to produce a collective echo involving an aggregation of randomly oriented different sized scatterers. In this article, a simple ray model is employed in the deformed cylinder formulation [Stanton, J. Acoust. Soc. Am. 86, 691–705 (1989)] to describe the scattering by finite length deformed fluid bodies in the general shape of elongated organisms. The work involves single realizations of the length and angle of orientation. Straight and bent finite cylinders and prolate spheroids are treated in separate examples. There is reasonable qualitative comparison between the structure of the data collected by Chu et al. [ICES J. Mar. Sci. 49, 97–106 (1992)] involving two decapod shrimp and this single‐target normal‐incidence theory. This analysis forms the basis for successful comparison (presented in the companion article) between the extended formulation that is averaged over an ensemble of realizations of length and angle of orientation and scattering data involving aggregations of up to 100’s of animals.

/pdf/ray-representation-of-sound-scattering-by-weakly-scattering-2ks8yq4599.pdf

Clarence S. Clay

Papers

Fundamentals of Acoustical Oceanography

Acoustic models of fish: The Atlantic cod (Gadus morhua)

Average echoes from randomly oriented random‐length finite cylinders: Zooplankton models

Sonar systems and aquatic organisms: matching equipment and model parameters

Ray representation of sound scattering by weakly scattering deformed fluid cylinders: Simple physics and application to zooplankton