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Oceanic Engineering, IEEE Journal of

Issue 4 • Date Oct 1990

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Displaying Results 1 - 15 of 15
  • An electromagnetic current meter-based system for application in unsteady flows

    Page(s): 373 - 379
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    A flexible configuration for an array of sensors has been developed for measurements in energetic regions of the ocean where the flow has high spatial and temporal variability. Central to the system is a two-axis electromagnetic current meter that measures the flow through a ducted volume containing a uniform magnetic field and nonprotruding electrodes flush with the duct surfaces. This geometry minimizes the electrode boundary-layer effect on the output, thus improving calibration and reducing fouling and damage potential. An inexpensive amplifier designed for low signal-to-noise ratios is employed to produce very low zero drift during operation View full abstract»

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  • Vertical directionality of ambient noise at 32°N as a function of longitude and wind speed

    Page(s): 335 - 339
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    Measurements of the ambient-noise field between 50 and 300 Hz with vertical arrays at 32°N (124°W, 136°W, and 150°W) have been made. Substantial differences in the vertical distribution of noise have been measured (especially at the higher frequencies), which can be interpreted in the context of attenuation by seawater sound absorption of coastal shipping noise. Under low wind-speed conditions, the vertical distribution of ambient noise is clearly concentrated within approximately ±15° of the horizontal. High wind speed has the effect of filling in the higher vertical angles, while leaving the level within the low-angular region unchanged View full abstract»

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  • Inverted echo sounder measurement of dynamic height through an ENSO cycle in the central equatorial Pacific

    Page(s): 380 - 383
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    A four-year record from an inverted echo sounder deployed near Palmyra Island at 6°N in the central Pacific Ocean is compared with a simultaneous record of subsurface pressure from this island lagoon. A factor m, converting round-trip acoustic travel time to surface dynamic height relative to a deep pressure level, was estimated from the ratio of the spectra of the two records in the energetic synoptic oscillation band. Year-to-year variation in m was not statistically significant. For the overall record, m was found to be -70±8 dynamic m/s, where the error bounds represent a 90% confidence interval. This is consistent with first-baroclinic-mode excitation View full abstract»

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  • Experimental investigation of the wave-interaction mechanism for ambient noise

    Page(s): 282 - 285
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    An experimental investigation of the wave-interaction mechanism for wind-generated ambient noise is presented. Predictions based on dispersion theory for frequencies above 10 Hz fall below experimental data and do not account for wind-speed dependence. The discrepancy may rest either in the linear wave-model or the mechanism. Laboratory measurements of wind waves show low spatial coherence and no evidence of dispersion at high frequencies, suggesting that the surface dynamics are more like convective turbulence than linear waves. While spectral estimates based on this model do not yet account for field data, the effects of nonlinear phenomena noted in the experiments remain to be examined View full abstract»

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  • Ambient noise data logger buoy

    Page(s): 286 - 291
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    A buoy for measuring wind speed and the ambient noise sound pressure level from 10 to 1500 Hz with 1-Hz resolution is described. The measurement buoy was deployed in a remote fjord in southeastern Alaska from October to December, 1989. The results from the data collected show that, for a wind speed of 5 kn, the measured ambient noise level at 900 Hz lies well below the Knudsen curve for open-ocean, wind-generated noise. As the wind speed increases from 5 to 10 kn, the measured ambient noise level approaches the Knudsen curve, increasing at 4 dB/kn compared to 1 dB/kn for the Knudsen curve. Above 10 kn, the measured ambient noise level matches the Knudsen curve View full abstract»

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  • Acoustically relevant bubble assemblages and their dependence on meteorological parameters

    Page(s): 340 - 349
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    A detailed physical model of the life history of a typical bubble plume, from its formation by a breaking wave to its dissipation into the background bubble population, is given, and the relationship between the early, acoustically relevant stages in bubble-plume development and the associated, remotely detectable whitecap is described. The manner in which the fraction of the sea surface covered by stage A spilling crests and by stage B mature whitecaps depends upon wind speed and upon wind stress or friction velocity is investigated. Formal expressions are given whereby near-surface bubble concentrations can be estimated from observations of fractional whitecap coverage or from measurements of the 10-m elevation wind speed View full abstract»

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  • Depth dependence of noise resulting from ship traffic and wind

    Page(s): 292 - 298
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    Under conditions of distantly generated noise, the noise level is found to decrease with depth in the mid-northeastern Pacific. These data show a decrease in noise level greater than 25 dB between critical depth and the ocean bottom. A result of this decrease is that locally wind-generated noise can be detected on near-bottom receivers for wind speeds less than 10 kn. It is shown that the noise level generated form local sources such as wind and nearby shipping is almost independent of receiver depth. The differences in spectra shape between the distant shipping noise and wind-generated noise and the low noise levels detected near the ocean bottom allow the measurement in the frequency band at 200-500 Hz of local wind noise level for wind speeds less than 10 kn View full abstract»

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  • Deep-ocean vertical noise directionality

    Page(s): 324 - 334
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    The structure of beam noise measured at the output of a vertical array in a range dependent ocean basin was investigated using the modified wide-angle parabolic equation (PE). Noise sources were distributed throughout the basin, and the field due to each noise source at an array located in the midbasin was calculated. The response of the array to the superposition of the noise sources was found by beamforming. An efficient and direct approach that superimposes the noise sources on the PE field as the field is marched toward the array was developed. Downslope calculations of the midbasin vertical directionality were made between 50 and 400 Hz with this technique. Use of a geoacoustic model shows that the bottom behaves as a low-pass filter View full abstract»

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  • Low-frequency acoustic wave-scattering phenomena under ice cover

    Page(s): 361 - 372
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    Studies on low-frequency acoustic wave-scattering phenomena due to under-ice roughness made by utilizing a rough, thin-ice plate model are presented. The model naturally divides the reflected field solution into specular and off-specular components. The model for specular components can give an excellent propagation loss prediction if the combined effects of under-ice roughness scattering, ice absorptions, and ice thickness are taken into account. The model for scattered or off-specular components is evaluated for a point source and point receiver geometry to study various spreading phenomena View full abstract»

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  • Matched field processing of deep-water ambient noise

    Page(s): 316 - 323
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    Matched-field ambiguity surfaces produced by deep-water ambient-noise data are discussed and quantified in terms of power levels and correlation values. Two processors were implemented (Bartlett and minimum variance) using data at 35 and 95 Hz with similar but distinct vertical angular distributions. In general, the ambiguity surfaces have both diffuse and discrete components. The diffuse distribution extends across the sound channel with correlation values increasing with distance. The discrete sources have higher correlation values and are distributed in a convergence-zone sidelobe structure. Local wind conditions appear to affect the received power but not the correlation values or the processor output power View full abstract»

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  • Characteristics of SeaMARC II phase data

    Page(s): 350 - 360
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    The dispersion of SeaMARC II phase-difference samples is discussed. They appear to be a function of signal direction, range, noise level, and backscatter strength of the bottom. Field data from a lava flow area and from sedimented areas at different depths are compared. The temporal distribution of the phase-difference samples was skewed and asymmetrical about the model. The angular distribution was symmetrical about the mode, with some phase wrap-around. The field data show the presence of a complicated noise interference field. The amount of phase-difference dispersion was larger than that calculated by using a simple Gaussian isotropic noise model, possibly suggesting an additional phase-dispersion process caused by bottom roughness. The method used to produce bathymetry data from the phase-difference samples was evaluated in light of the phase-difference sample distribution View full abstract»

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  • Underwater noise emissions from bubble clouds

    Page(s): 275 - 281
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    By means of an effective equation model for the propagation of pressure waves in a bubbly liquid, the normal modes of oscillation of regions of bubbly liquid in an otherwise pure liquid are calculated for some simple geometries. It is shown that the frequencies of oscillation of such bubble clouds can be much lower than those of the constituent bubbles in isolation and fall well within the range where substantial wind-dependent noise is observed in the ocean. A comparison with some experimental data strongly supports the theoretical results View full abstract»

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  • Low-frequency ambient-noise measurements in the South Fiji basin

    Page(s): 311 - 315
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    The effect of wind speed on ambient noise has been measured in an experiment carried out in the South Fiji basin. The noise data in the band 15-250 Hz are well correlated with the variations in the local wind speed. The relationship between noise level N and wind speed ν is expressed by N=B+20n log ν. The constants B and n have been estimated by fitting the data using this model. The analysis indicates that there are two types of behavior: for ν>15 kn, a value of n=1.5 is obtained for the entire band, whereas for ν<15 kn, there is no correlation with wind speed observed in the data. The results suggest that there is a delay of 40-120 min for the effect of wind on the hydrophone noise level View full abstract»

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  • Measuring the vertical directional spectra caused by sea surface sound

    Page(s): 299 - 310
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    The results of the initial deployment of a wideband (40-4000 Hz) vertical acoustic antenna system in the Tongue of the Ocean, the Bahamas are discussed. The instrumentation system consisted of seven octavely nested, four-wavelength linear apertures covering the above frequency range, and a VCR-based subsurface recording system that carried out a programmed sequence of recordings. The objective of the measurement system is the development of a database of directional spectra obtained in an acoustically isolated area where the ambient is principally controlled by the local (observable) sea-surface conditions. The purpose of the database is to identify a model of the space-time statistics of the acoustic ambient caused by sea-surface sound View full abstract»

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  • Bubbles as sources of ambient noise

    Page(s): 268 - 274
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    It has been shown that the main mechanism which produces the Knudsen region of the ambient noise spectrum is the free oscillations of bubbles. Some experimental results which seem to confirm these facts and to refute various alternative theories involving spray impacts and turbulent forcing of bubble oscillations are described. The results show that the mechanism which excites the bubbles is their formation at the surface; once a bubble has been formed and has radiated the excess energy resulting from its formation, it is more or less silent. It is possible for extremely violent conditions to re-excite bubbles by breaking them into smaller fractions, but it is not clear how important this process would be in the ocean. How the entrainment process imparts energy to the bubble is discussed View full abstract»

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Aims & Scope

The IEEE Journal of Oceanic Engineering (ISSN 0364-9059) is published quarterly by the IEEE Oceanic Engineering Society (IEEE OES). The scope of the Journal is the field of interest of the IEEE OES, which encompasses all aspects of science, engineering, and technology that address research, development, and operations pertaining to all bodies of water. This includes the creation of new capabilities and technologies from concept design through prototypes, testing, and operational systems to sense, explore, understand, develop, use, and responsibly manage natural resources.

Full Aims & Scope

Meet Our Editors

Editor-in-Chief
N. Ross Chapman
School of Earth & Ocean Sciences
University of Victoria
3800 Finnerty Road
Victoria, BC V8P 5C2 Canada
chapman@uvic.ca