摘要:

The acoustic attenuation coefficient in shallow coastal waters is of interest to designers and operators of devices such as naval mine-hunting sonars, sidescan surveying sonars, and acoustic Doppler current profilers, typically employing frequencies ranging from tens of kHz to several hundred kHz, possibly up to 1 MHz. At these frequencies attenuation due to suspended particulate matter which characterizes turbid coastal waters is an important contribution to the total attenuation coefficient. In this paper the effect of temperature, pressure, and salinity on the total attenuation coefficient for seawater containing a suspension of solid particles is investigated by employing suitable expressions for the sound speed, density, and viscosity of seawater as functions of temperature, salinity, and pressure. Results presented demonstrate that while there is little variation in the attenuation with pressure in up to a few hundred meters of water, there is significant variation with temperature and salinity over the ranges found globally in the sea.

ISSN:0001-4966    

DOI:10.1121/1.421088

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A 3-D high resolution crosswell acoustic tomography system was designed and tested in a shallow water environment. This system makes use of a damped least-squared inversion technique and is used to construct compressional wave velocity images from measured travel time data. Use of singular value decomposition allowed checks of the linear inversion confirming the validity of the modeled environment. It is shown that when the ratio of width to depth of the cross section is about unity or smaller and the domain has not been overparametrized the inverted image model has sufficiently good figures of merit in resolution and variance to visualize small variations in the magnitude compressional velocity field.

ISSN:0001-4966    

DOI:10.1121/1.421089

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The acoustic scattering properties of live individual zooplankton from several gross anatomical groups have been investigated. The groups involve (1) euphausiids (Meganyctiphanes norvegica) whose bodies behave acoustically as a fluid material, (2) gastropods (Limacina retroversa) whose bodies include a hard elastic shell, and (3) siphonophores (Agalma okeniorelegansandNanomia cara) whose bodies contain a gas inclusion (pneumatophore). The animals were collected from ocean waters off New England (Slope Water, Georges Bank, and the Gulf of Maine). The scattering properties were measured over parts or all of the frequency range 50 kHz to 1 MHz in a laboratory-style pulse-echo setup in a large tank at sea using live fresh specimens. Individual echoes as well as averages and ping-to-ping fluctuations of repeated echoes were studied. The material type of each group is shown to strongly affect both the overall echo level and pattern of the target strength versus frequency plots. In this first article of a two-part series, the dominant scattering mechanisms of the three animal types are determined principally by examining the structure of both the frequency spectra of individual broadband echoes and the compressed pulse (time series) output. Other information is also used involving the effect on overall levels due to (1) animal orientation and (2) tissue in animals having a gas inclusion (siphonophores). The results of this first paper show that (1) the euphausiids behave as weakly scattering fluid bodies and there are major contributions from at least two parts of the body to the echo (the number of contributions depends upon angle of orientation and shape), (2) the gastropods produce echoes from the front interface and possibly from a slow-traveling circumferential (Lamb) wave, and (3) the gas inclusion of the siphonophore dominates the echoes, but the tissue plays a role in the scattering and is especially important when analyzing echoes from individual animals on a ping-by-ping basis. The results of this paper serve as the basis for the development of acoustic scattering models in the companion paper [Stanton et al., J. Acoust. Soc. Am.103, 236–253 (1998)].

ISSN:0001-4966    

DOI:10.1121/1.421469

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

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 (1998)]. The models use a combination of ray theory, modal-series solution, and distorted wave Born approximation (DWBA). 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 case) 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 incidence). The model for the elastic shelled body (gastropods in this case) involved development of an analytical model which takes into account irregularities and discontinuities of the shell. The model for gas-bearing animals (siphonophores) 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. There is reasonable qualitative agreement between the predictions and single ping data, and reasonable quantitative agreement between the predictions and variability and averages of echo data.

ISSN:0001-4966    

DOI:10.1121/1.421110

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A modeling study was conducted to determine the conditions under which fluidlike zooplankton of the same volume but different shapes (spherical/cylindrical) have similar or dramatically different scattering properties. Models of sound scattering by weakly scattering spheres and cylinders of finite length used in this analysis were either taken from other papers or derived and herein adapted for direct comparison over a range of conditions. The models were examined in the very low- (ka≪1,kL≪1), moderately low- (ka≪1,kL≳1), and high-frequency regions (ka≫1,kL≫1), wherekis the acoustic wave number,ais the radius (spherical or cylindrical) of the body, andLis the length of the cylinders (for an elongated body withL/a=10,“moderately low” corresponds to the range0.1≲ka≲0.5). Straight and bent cylinder models were evaluated for broadside incidence, end-on incidence, and averages over various distributions of angle of orientation. The results show that for very low frequencies and for certain distributions of orientation angles at high frequencies, the averaged scattering by cylinders will be similar, if not identical, to the scattering by spheres of the same volume. Other orientation distributions of the cylinders at high frequencies produce markedly different results. Furthermore, over a wide range of orientation distributions the scattering by spheres is dramatically different from that of the cylinders in the moderately low-frequency region and in the Rayleigh/geometric transition region: (1) the Rayleigh to geometric scattering turning point occurs at different points for the two cases when the bodies are constrained to have the same volume and (2) the functional dependence of the scattering levels upon the volume of the bodies in the moderately low-frequency region is quite often different between the spheres and cylinders because of the fact that the scattering by the cylinders is still directional in this region. The study demonstrates that there are indeed conditions under which different shaped zooplankton of the same volume will yield similar (ensemble average) scattering levels, but generally the shape and orientation distribution of the elongated bodies must be taken into account for accurate predictions.

ISSN:0001-4966    

DOI:10.1121/1.421135

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The monopole response of a gas-filled, spherical cavity in a sediment is investigated. The sediment is either a fluid, elastic solid, or saturated poroelastic medium. The present method entails the scattering of an incident displacement field that preferentially excites the monopole resonance of the cavity. The main result demonstrates that a gas-filled, spherical cavity in a saturated poroelastic medium can exhibit two distinct monopole resonances. These resonances arise from the two distinct longitudinal modes of propagation in saturated poroelastic medium as described by Biot’s theory.

ISSN:0001-4966    

DOI:10.1121/1.421111

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A general analytical expression for thetime-dependentmean-square incoherent field scatteredfromorthrough(penetrating) a 2-D fluid–fluid rough interface for a narrow-band incident plane-wave source is derived and expressed in terms of the second moment of the rough interfaceT-matrix. This analytical expression is independent of the scattering solution technique, and for distances greater than only a few wavelengths from the interface, is equivalently expressed in terms of the bistatic scattering cross section per unit area per unit solid angle (differential cross section) of the rough interface. Using this rigorously derived result, the scattered field for a narrow-band point source is heuristically derived. This derivation leads to the usual sonar equation in the limit as the narrow-band signal approaches the cw (continuous wave) case. First-order perturbation calculations for the case of a baseband Gaussian shaped source pulse illustrate narrow-band pulse dispersion effects of the incoherent field for forward scattering into a lossy sediment. For the case of incidence below the critical grazing angle, first-order perturbation computations also show that the incoherent field scattered through a rough interface can be much greater than the zeroth-order field (coherent) transmitted below the corresponding flat-surface depending on loss and receiver depth. These computations for the first-order mean square incoherent field penetrating the rough interface are compared to the results for the flat-surface case, for both plane-wave and point sources.

ISSN:0001-4966    

DOI:10.1121/1.421090

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

An unconsolidated, saturated marine sediment consists of a more or less loose assemblage of mineral grains in contact, with seawater in the interstices. It is postulated that the two-phase medium possesses no skeletal frame, implying that the elastic rigidity modulus of the material is zero. A theory of wave propagation in such a sediment is developed, in which the medium is treated as a fluid that supports a specific form of intergranular dissipation. Two important equations emerge from the analysis, one for compressional wave propagation and the second describing transverse disturbances. For the type of dissipation considered, which exhibits hysteresis or memory, the shear equation admits a wavelike solution, and is thus a genuine wave equation, even though the sediment shows no elastic rigidity. In effect, the medium possesses a “dissipative” rigidity, which is capable of supporting shear. This behavior is distinct from that of a viscous fluid, for which the shear equation is diffusionlike in character, giving rise to critically damped disturbances rather than propagating waves. The new theory predicts an attenuation coefficient for both compressional and shear waves that scales with the first power of frequency, in accord with published data. The wave theory is combined with a model of the mechanical properties of marine sediments to yield expressions relating the compressional and shear wave speeds to the grain size, the porosity, and the density of the medium. These expressions show compelling agreement with a number of measurements from the literature, representing a variety of sediment types ranging from clay to coarse sand.

ISSN:0001-4966    

DOI:10.1121/1.421091

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A recent article [A. R. Kolaini and L. A. Crum, J. Acoust. Soc. Am.96, 1755–1765 (1994)] reported the measurements of the ambient sound generated by laboratory breaking waves over the range 100–20 000 Hz in fresh water. Those observations from both spilling- and plunging-type breakers have been repeated in the same manner and wavemaker/anechoic tank with water that had 25‰ salt in its content. The observations in salt water, just like those in fresh water, reveal that the sources of sound in laboratory spilling breakers are due primarily to single bubble oscillations that can have frequencies lower than a few hundred Hertz. In the case of weak spilling breakers, the sound spectra level in fresh water was due primarily to single bubble oscillation, while the same breakers in salt water have introduced smaller size bubbles with large density. The relatively high-density populated bubble cloud generated by weak breakers shows the evidence of the onset of collective oscillation that was absent for the same breakers in the fresh water. In the case of moderate spilling and plunging breakers, it appears that both individual bubbles and bubble clouds can contribute to the acoustic emissions in fresh and salt water. The average sound spectra reveal that the peak frequencies of the spectra shift from a few kHz (weak, spilling breaker) to few hundred Hz (plunging breaker), and the high-frequency portions have slopes approximately 5–6 dB/oct, which are the slopes observed from the noise spectra of the ocean. Besides the high bubble density and smaller bubble sizes in salt water, all breakers experienced a significant increase in sound-pressure level in all observed frequency range. The ionic structure of the medium alters the sound radiation from bubbles. In this paper some of the observed acoustic signatures from breaking waves are discussed and a plausible explanation of how salt can effect the sound radiation from bubbles is given.

ISSN:0001-4966    

DOI:10.1121/1.421115

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A procedure for estimating acoustic wave velocity and attenuation in ocean sediment using a minimum amount of geological and geotechnical data is demonstrated. First, the Biot–Stoll theory is presented. Next, various asymptotic formulae for the attenuation coefficient are derived for high, low, and intermediate frequencies. These expressions clearly isolate the effects of intergranular Coulomb friction and fluid viscous dissipation on the attenuation of shear and compressional waves. Under the constraint of a minimum amount of geological and geotechnical information, a sequence of empirical equations is compiled to convert basic data, such as blow count number from a Standard Penetration Test or shipboard density, into sediment geoacoustic properties. As a demonstration, two well-known field cases, the Atlantic Generating Station (AGS) site and the Atlantic Margin Coring (AMCOR 6010) site, are examined. By incorporating the uncertainty involved in the data collection, the estimated geoacoustical parameters are provided with a standard deviation.

ISSN:0001-4966    

DOI:10.1121/1.421136

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP