Volume 109, Issue 1, January 2001
Index of content:
- UNDERWATER SOUND 
109(2001); http://dx.doi.org/10.1121/1.1329622View Description Hide Description
Signals received by low-frequency multibeam echosounders are strongly affected by sound penetration inside the upper sediment layers and by backscattering from buried layers down to depths of a few meters; this may lead to serious ambiguities and misinterpretations of experimental data. These phenomena are modeled here using a concept of equivalent input backscattering strength (EIBS), based on a combination of classical models of local backscattering strength and propagation inside fluid layered media. The local backscattering strength at a buried interface is expressed first to account for the impedance adaptation due to the overlying layers, for the angular refraction effects due to the velocity profile, and for the layered structure of the underlying medium. It is then transferred to the upper water–sediment interface, accounting for propagation inside the layered stack; the transfer coefficient is obtained from the classical theory of plane wave propagation in layered media. The volume backscatteringeffects are processed in the same way and account for the finite thickness of the layers. The various contributions are finally summed to give the backscattering strength, at the upper interface, that features the various effects of propagation and attenuation inside the layered structure.
Bubble clouds and their transport within the surf zone as measured with a distributed array of upward-looking sonars109(2001); http://dx.doi.org/10.1121/1.1331108View Description Hide Description
A collaborative, multi-institute experiment called the Scripps Pier Experiment was conducted in the vicinity of the Scripps pier in La Jolla, California, in March 1997 to study the fate of bubbles in the surf zone and the effects of these bubbles on acoustic propagation. This paper discusses data gathered by the Applied Physics Laboratory, University of Washington, using a set of four upward-looking sonars (frequency 240 kHz), which simultaneously measured vertical profiles of acoustic volume scattering from bubbles at four locations. The transport of bubbles via rip currents emerged as an important, though episodic and localized, feature of the acoustic environment in the surf zone. Images of volumetric backscattering strength vs time and depth reveal the episodic events (of increased scattering level) lasting between 5 and 10 min caused by the passage of bubble clouds over the sonar. Time lags for the onset of increased scattering at the four locations are consistent with a seaward velocity of the bubble clouds of order 10 cm/s, and the length scales of these bubble clouds in the seaward direction are inferred to be in the range 50–100 m. The influence of the incoming surface wave field is also discussed.
109(2001); http://dx.doi.org/10.1121/1.1329624View Description Hide Description
High-frequency underwater acoustic transmissions across the Strait of Gibraltar were used to examine acoustic scattering caused by the unique internal wave field in the Strait. Internal solitary waves of 100 m in amplitude propagate along the interface between an upper layer of Atlantic water and a lower layer of Mediterranean water. The interface is also strongly modulated by internal tides of comparable amplitude. As internal solitary waves cross the acoustic path, they cause sharp soundspeed gradients which intermittently refract acoustic rays away from normal sound channels. Internal tides vertically shift soundspeed profiles for additional travel time variability. Although the acoustic scattering is quite complicated, it is also surprisingly robust, making it a good candidate for modeling. Key features of the acoustic arrival pattern can be accounted for in some detail by a model description of the complex hydraulics in the Strait.
A least squares method of estimating length to target strength relationships from in situ target strength distributions and length frequencies109(2001); http://dx.doi.org/10.1121/1.1329621View Description Hide Description
A least squares method is presented for estimating length to target strength relationships for a target species and associated species using a series of target strength distributions and associated trawl catches. A by-product of the estimation procedure is an objective determination of the correspondence between modal lengths in the trawl catches and the modal lengths in the associated target strength distributions. The method is illustrated by applying it to a data set collected to determine the length to target strength relationship for hoki (Macruronus novaezelandiae).
A design study of an acoustic system suitable for differentiating between orange roughy and other New Zealand deep-water species109(2001); http://dx.doi.org/10.1121/1.1332381View Description Hide Description
Using the simple slab-cylinder acoustic model for fish, developed by Clay and Horne [J. Acoust. Soc. Am. 96, 1661–1668 (1994)], the target strengths of three of New Zealand’s deep-water fish species, namely orange roughy, black oreos, and smooth oreos, have been derived. The target strengths derived for the model fish have been found to be in reasonable agreement with currently accepted target strength values. These three model fish were used in a study to test the species discrimination of a chirp sonar system. Chirps of center frequencies 40, 80, and 160 kHz and bandwidth of 10, 20, and 40 kHz have been used to acoustically illuminate the three fish species listed above and the matched, filtered responses to the chirps determined. The effect of the response of transducer or system bandwidth has also been investigated. It has been found that the bandwidth of the chirp is much more important for resolving detail in a fish target than the chirp center frequency. A bandwidth of at least 20 kHz, and preferably 40 kHz, produces matched filtered responses for black and smooth oreos and orange roughy which are quite clearly species specific. Results suggest that with orange roughy it may be possible to size and even sex the targets acoustically.