Recently developed broadband acoustic methods were used to study mixed assemblages of fish spanning a wide range of lengths and species. Through a combination of resonance classification and pulse-compression signal processing, which provides for high-range resolution, a modified commercial broadband echosounder was demonstrated to provide quantitative information on the spatial distribution of the individual size classes within an assemblage. In essence, this system spectrally resolves the different size classes of fish that are otherwise not resolved spatially. This method reveals new insights into biological processes, such as predator-prey interactions, that are not obtainable through the use of a conventional narrowband high-frequency echosounder or previous broadband systems. A recent study at sea with this system revealed aggregations containing bladdered fish 15-30 cm in length (Atlantic herring (Clupea harengus) and silver hake (Merluccius bilinearis)) and a variety of species of smaller fish 2-5 cm in length. These observations infer that the smaller 2-5 cm fish can be colocated in the same aggregations as their predator, the larger silver hake, as well as pre-spawning herring. While this technological advancement provides more information, there remain challenges in interpreting the echo spectra in terms of meaningful biological quantities such as size distribution and species composition.Résumé : Des méthodes acoustiques à large bande récemment mises au point nous servent à étudier des peuplements mixtes de poissons couvrant un large éventail de longueurs et d'espèces. Par une combinaison de classification des résonances et de traitement des signaux de compression des impulsions qui fournit un haut degré de résolution, un échosondeur commercial modifié à large bande s'avère capable de fournir des données quantitatives sur la répartition spatiale des classes d'âge individuelles au sein d'un peuplement. Spécifiquement, le système permet une résolution spectrale des différentes classes de taille des poissons qui autrement ne peuvent être distinguées dans l'espace. La méthode ouvre de nouvelles perspectives sur les processus biologiques, tels que les interactions prédateurs-proies, qu'on ne peut obtenir par l'emploi d'un échosondeur ordinaire de haute fréquence à bande étroite, ni par les systèmes à large bande plus anciens. Une étude récente en mer avec ce système montre des rassemblements de poissons à vessie natatoire de longueurs de 15 à 30 cm (harengs atlantiques (Clupea harengus) et merlus argentés (Merluccius bilinearis)), ainsi qu'une variété d'espèces de poissons plus petits de longueurs de 2-5 cm. Ces observations laissent croire que les poissons plus petits de 2-5 cm peuvent être localisés ensemble dans les mêmes rassemblements que leurs prédateurs, les merlus de plus grande taille et les harengs avant la fraie. Bien que cette avancée technologique fournisse plus d'information, il reste des défis pour interpréter les spectres des rétrodiffusions en regard de quantités biologiques sig...

Sea ice is a heterogeneous material whose acoustic properties are functions of time and space. Results of a crosshole tomography experiment conducted in multi-year ice with the objective of determining the spatial structure of the compressional and shear wave speeds from travel time measurements made with high-frequency pulses are presented here. The results of the experiment indicate that the wave speeds can be determined from such a crosshole experiment with good resolution. The compressional and shear wave speed contour maps indicate that the spatial variations of the wave speeds are complex with regions of low speed. Low-speed regions observed are likely caused by high brine volume content. Resolution and variance studies performed on the estimates are also presented. Material properties such as Poisson’s ratio, salinity, and elastic and shear moduli of sea ice are obtained from the estimates of compressional and shear wave speeds. By measuring the amplitude of the transmitted and received signals along specific paths, estimates of the attenuation coefficients at different depth intervals are obtained. Spatial variability observed in the estimates is believed to be due to scattering by inhomogeneities in the material.

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Ultralow frequency (0.02-2 Hz) acoustic ambient noise was monitored from January to April 1991 at six ocean bottom stations off the eastern U.S. coast. The depths of the stations ranged from about 100 m to 2500 m. The measured spectra are in good agreement with predictions made using Cato's theory [J. Acoust. Soc. Am. 89, 1076-1095 (1991)] for noise generation by surface-wave orbital motion after extending the calculations to incorporate horizontally stratified environments. Contributions from both the linear, single-frequency (virtual monopole) and the nonlinear, double-frequency (dipole) mechanisms are clearly recognizable in the data. The predictions make use of directional wave data obtained from surface buoys deployed during the SWADE experiment and an ocean bottom model derived from compressional wave speed data measured during the EDGE deep seismic reflection survey. The results demonstrate conclusively that nonlinear surface-wave interactions are the dominant mechanism for generating deep-ocean ULF noise in the band 0.2-0.7 Hz.

An experiment to study acoustic backscattering from deep-ocean sediments was conducted in July 1993 as part of the Acoustic Reverberation Special Research Program sponsored by the Office of Naval Research. An acoustic source transmitting chirp signals in the frequency range 250–650 Hz and a 24-element vertical receiving array attached to the source were suspended near the seafloor over a sediment pond in the vicinity of the Mid-Atlantic Ridge. The array steered in the normal incidence (endfire) direction is used to determine the sediment structure, while the array steered at oblique angles is used to determine backscattering strength. It is found that the sediment is stratified with gentle horizontal changes, except for two irregular regions, with thicknesses of about 20 m, located approximately 18 and 60 m beneath the seafloor. These inhomogeneous regions are shown to be the dominant influence on the backscattered field, and the backscattering strength of each region as a function of grazing angle is estimated. It is found that the removal of coherent reflections from subbottom stratified layers is essential to the analysis of bottom backscattering phenomena.