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Mathematical scattering models are derived and compared with data from zooplankton from several gross anatomical groups-fluidlike, elastic shelled, and gas bearing. The models are based upon the acoustically inferred boundary conditions determined from laboratory backscattering data presented in part I of this series ͓Stanton et al., J. Acoust. Soc. Am. 103, 225-235 ͑1998͔͒. The models use a combination of ray theory, modal-series solution, and distorted wave Born approximation ͑DWBA͒. The formulations, which are inherently approximate, are designed to include only the dominant scattering mechanisms as determined from the experiments. The models for the fluidlike animals ͑euphausiids in this case͒ ranged from the simplest case involving two rays, which could qualitatively describe the structure of target strength versus frequency for single pings, to the most complex case involving a rough inhomogeneous asymmetrically tapered bent cylinder using the DWBA-based formulation which could predict echo levels over all angles of incidence ͑including the difficult region of end-on incidence͒. The model for the elastic shelled body ͑gastropods in this case͒ involved development of an analytical model which takes into account irregularities and discontinuities of the shell. The model for gas-bearing animals ͑siphonophores͒ is a hybrid model which is composed of the summation of the exact solution to the gas sphere and the approximate DWBA-based formulation for arbitrarily shaped fluidlike bodies. There is also a simplified ray-based model for the siphonophore. The models are applied to data involving single pings, ping-to-ping variability, and echoes averaged over many pings. (k i •r tan ) where r tan is the tangent to the body axis at point r pos ͑␤ tilt ϭ0 corresponds to broadside incidence to the disk axis at the arbitrary point on the body axis͒. ␤ tilt is not to be confused with the orientation angle, , of the body, although the two are the same when the body axis is straight. ␤ L imaginary part of L ; attenuation coefficient of Lamb wave on elastic shelled sphere

High-frequency acoustic scattering techniques have been used to investigate dominant scatterers in mixed zooplankton populations. Volume backscattering was measured in the Gulf of Maine at 43, 120, 200, and 420 kHz. Zooplankton composition and size were determined using net and video sampling techniques, and water properties were determined using conductivity, temperature, and depth sensors. Dominant scatterers have been identified using recently developed scattering models for zooplankton and microstructure. Microstructure generally did not contribute to the scattering. At certain locations, gas-bearing zooplankton, that account for a small fraction of the total abundance and biomass, dominated the scattering at all frequencies. At these locations, acoustically inferred size agreed well with size determined from the net samples. Significant differences between the acoustic, net, and video estimates of abundance for these zooplankton are most likely due to limitations of the net and video techniques. No other type of biological scatterer ever dominated the scattering at all frequencies. Copepods, fluid-like zooplankton that account for most of the abundance and biomass, dominated at select locations only at the highest frequencies. At these locations, acoustically inferred abundance agreed well with net and video estimates. A general approach for the difficult problem of interpreting high-frequency acoustic scattering in mixed zooplankton populations is described.

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.

What do shipwrecks, schools of fish, singing whales, and foaming seas have in common? They can all be detected and classified with acoustical methods and they are all described under one cover in this book. The seafloor, objects lying on or buried in the seafloor, suspended sediment, marine organisms of many kinds, eddies and turbulence, bubbles, temperature, salinity, and the sea surface all affect the manner in which sound propagates through the ocean. The purpose of this book is to describe methods that exploit the various effects so that sound can be used as a tool to infer important properties of the corresponding objects or processes in the ocean.Why use sound? Light and other forms of electromagnetic radiation do not travel far in the ocean, giving the ocean a dark and mysterious appearance. As a result, we know, in many respects, more about the surface of the moon than of the interior of the ocean on our very own planet! Sound can travel very large distances in the ocean, especially at lower frequencies.Because of this ability, sound has been widely used as a means to probe the ocean's interior. For example, a patent on use of underwater sound was applied for shortly after the steamship Titanic sank due to its collision with an iceberg in 1912. The patent was for using sound for ''detecting the presence of large objects underwater.'' Since then, the applications using sound as a tool to study the ocean grew in number and diversity. Along with the various applications are a multitude of challenges. Both the applications and their corresponding challenges are addressed formally in this book, with many examples given.Fundamentals of Acoustical Oceanography is written in a style appropriate for a broad audience at many levels. Much of the text is written in a simple tutorial manner so that both nonspecialists and people who are just entering the field can understand it. Furthermore, there is enough detail and references made to the literature so that the specialist can also make use of the material. The book spans areas of marine geology, marine biology, physical oceanography, and marine engineering and would be useful in applications involving ecology, commerce, and the military.This book follows Acoustical Oceanography, published in 1977 by the same authors but with author order reversed. The first book was widely used and cited. It had eleven printings, was translated into Russian, and was cited routinely by scientists in varied disciplines and in many different journals. Since 1977, there have been significant advances in the area of acoustical oceanography. This new book incorporates many of the advances along with a new format. The first work reserved advanced topics for appendices at the end of the book while the present book integrates advanced material along with the rest of the text, but denotes the material as optional to read.Acoustical oceanography, as defined by the authors early in the book, involves the so-called ''inverse problem.'' That is, given solutions to acoustic propagation, inverse me...