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May 1990

Volume 87, Issue S1, pp. S1-S164

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back to top Session X. Acoustical Oceanography III, Underwater Acoustics IV, and Animal Bioacoustics I: Water Mass Boundaries I
Invited Papers
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Anatomies of oceanic fronts (A)

Christopher N. K. Mooers

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S58-S58 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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Oceanic fronts have attributes in common with atmospheric fronts: for example, strong horizontal gradients in temperature and/or salinity and strong horizontal velocity shears and convergences. Their forcing mechanisms include tidally induced mixing, the advection of plumes, atmospherically driven upwelling, and deformation fields in oceanic flows. Some fronts are confined to the surface or bottom turbulent boundary layers; others exist as internal features of the ocean. Some are thousands of kilometers long, extend to great depth, and are semipermanent; others occur over short horizontal and vertical distances for brief durations. Due to strong convergences often found at fronts, strong vertical velocities can be induced. Also, internal gravity waves are often intense near fronts, and the consequent shear instabilities induce turbulent mixing and fine structure. Thus fronts can be major foci for vertical transfers, including between the surface mixed layer and the ocean interior, especially in situations where a surface front is an outcrop of an oceanic pycnocline. Due to their convergences, fronts are biologically significant: They concentrate nutrients and phytoplankton, and, consequently, zooplankton, fish, marine mammals, marine birds, and probably “marine snow.” Hence, oceanic fronts can be expected to scatter, as well as refract, acoustic energy.
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Acoustical mapping of biomass in frontal oceanic systems (A)

Ole A. Mathisen

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S58-S58 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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Primary and secondary production in the oceans and hence abundance of macrofauna is limited by light or more often by nutrients. Any oceanic system that injects new or regenerated nutrients into a body of water creates a center of high biological productivity. While classical oceanographers have identified such production areas and depth zones by measurements of physical parameters and net sampling, the fishermen have done it empirically with sonar and other acoustic devices available to them after the end of World War II. The fisheries biologists started extensively to employ acoustical techniques in their abundance estimations, first with analog and later with numerous digital integrators and echo counters using dual or split beam transducers. This development is illustrated with examples from the Peruvian and Northwest Africa upwelling systems, krill abundance at the ice edge, and mapping of superswarms in Antarctica, and salmon and herring populations in Alaska. A next generation of acoustical systems will be used to map the patchiness of herbivores and carnivores foraging on primary and secondary producers. Once patches have been identified, it is possible to measure the physical characteristics of the water masses with patches and hence study the microstructure of front and boundary layers.
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Lagrangian studies of ocean currents using SOFAR (A)

T. Rossby

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S58-S59 (1990); (2 pages)

Online Publication Date: 13 Aug 2005

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Acoustically tracked neutrally buoyant floats are being used to study two extremes of ocean motion: the very energetic flows of the Gulf Stream off the east coast of the U.S. and Canada, and the very weak flows of the deep South Pacific. In the first case, where the particle velocities are much greater than the phase velocity of the meandering current, there is a strong tendency for the floats to remain in the stream. This has allowed the study of the advective properties of the current and the pathways of fluid exchange to both sides. In the deep ocean, where the particle velocities are much smaller than the phase velocity of the planetary wave field, the floats tell us about the time scales of deep ocean motions, and how particles move and disperse with time. Linear FM at 260 Hz (1.5‐Hz bandwidth) and a few watts power (with moored sound sources built by Webb Res. Corp.) give a useful (off‐axis, shallow) range of 1000 km in the Gulf Stream and somewhat more (off‐axis, deep) in the deep South Pacific. A new high‐powered resonant pipe projector is expected to give ocean basin scale tracking capabilities.
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Measurements of zooplankton biomass with acoustic Doppler current profilers (A)

Charles N. Flagg

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S59-S59 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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Acoustic Doppler current profilers, ADCPs, arrived on the oceanographic scene a decade ago as a development of ship‐mounted Doppler speed logs to measure vertical profiles of horizontal water velocities. Backscattered acoustic intensity is one of the parameters that can be recorded and early anecdotal observations indicated that intensity variations had distinctly biological characteristics such as diel zooplankton migrations. These early observations led to an ongoing study of the conditions under which ADCPs might be able to make quantitative estimates of zooplankton biomass. The attraction of ADCPs, and the motivation for the study, is not so much the ADCPs' ability to discern details about zooplankton populations but rather their ease of use and general availability. The results of the study to date, with operating frequencies of 150 and 300 kHz, have shown that ADCPs produce biomass estimates to within ± 15% throughout their acoustic range when the individual transducers have been calibrated and proper care is exercised in processing the results. ADCPs provide the ability to conduct nonintrusive large‐scale surveys, in either time or space, of zooplankton biomass providing input, for example, to ecological models and as guides to more conventional sampling schemes.
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Investigations into biological/water mass interactions in the Alaskan Beaufort Sea using hydroacoustic techniques (A)

Richard E. Thorne and Gary E. Johnson

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S59-S59 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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Hydroacoustic techniques were used to study the biological activity associated with water mass boundaries. Both fish and zooplankton were found to be most abundant along the boundary between a cold, high‐salinity water mass and a warmer, low‐salinity water mass in the nearshore region. Biological abundance was found to be greatest where the boundary between the water masses intersected the bottom.
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An acoustical technique to detect and possibly classify fish in confined reverberant environments (A)

P. H. Patrick, J. J. Kowalewski, A. E. Christie, and B. L. Jennette

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S59-S59 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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Investigations were conducted in the laboratory on the potential of using acoustical resonance as a technique to detect and possibly classify fish in shallow, fast flow environments. Present high‐frequency acoustical approaches are limited in these situations because of reverberation. An initial experiment with bubble packing material indicated an insertion loss of approximately 26 dB. Further experiments involving live walleye and rainbow trout showed similar results. For rainbow trout, there was a direct relationship between an increase in insertion loss with a corresponding increase in fish density. The estimated resonant frequency for each density of fish was approximately 412 Hz. Insertion loss ranged from approximately 7 dB for 3 fish to approximately 32 dB for 65 fish. These preliminary results were encouraging and indicated the potential of using this technique in highly reverberant environments. Research activities involving high‐frequency acoustics will also be discussed. [Work supported by Ontario Hydro.]
Contributed Paper
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Covariance analysis of bioacoustic volume reverberation (A)

C. S. Clay

J. Acoust. Soc. Am. Volume 87, Issue S1, pp. S60-S60 (1990); (1 page)

Online Publication Date: 13 Aug 2005

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Covariance analysis of the echoes or volume reverberation from macro zooplankton and small nekton is practical with the use of digitally recorded sonar data. Numerical simulations of echoes from an ensemble of randomly located identical scatterers have the same covariance function as an individual. Fourier transformations of the covariances give the (power) spectrum of the scattering function that is within the envelope of the (power) spectrum of the incident ping. Part of the motivation for this research came from new signal‐processing capabilities of multichannel digital sonar, and part of the motivation for considering time‐domain echo processing came from papers that displayed the time‐domain signals scattered by simple objects and revisions to Acoustical Oceanography. [This research was supported in part by NSF (OCE‐8817171) and ONR (N00014‐89‐J‐1514).]
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