摘要:

The characteristics of acoustic signals backscattered from inside-corner and outside-corner oceanic crust are investigated using acoustic reverberation data from the 1993 Acoustic Reverberation Special Research Program (ARSRP) Acoustics Cruise. Specifically, the seafloor dip distribution, as measured from Hydrosweep bathymetry data is compared in areas of each crustal type and a correlation between seafloor dip and seafloor scattering strength is sought. Beamformed and match-filtered acoustic data from the site A (run 1) monostatic, wideband, LFM (linear frequency modulated) experiment are used to find the scattering strength corresponding to specific areas of the seafloor. Scattering strength is determined as the average of intersecting beams from different source locations in order to reduce the left–right ambiguity inherent in beamsteering of a linear array. When overlaid on the bathymetry map, high-scattering strengths are found to correspond to steep flanks of seafloor features and can be used to determine their shape and orientation. Some of these feature shapes are characteristic of specific crustal regions. Cross-plotting scattering strength with true grazing angle or seafloor dip shows a trend increasing at a rate of about 0.1 dB/deg. This trend is not, however, sufficiently constrained to be a useful predictor. It is concluded that the seafloor dip, on the scale of a few hundreds of meters, that can be resolved with Hydrosweep bathymetry data, influences but does not determine, scattering strength. Since, in detail, the observed variations in scattering strength are larger than can be explained by data error or seafloor dip, it is suggested that other characteristics of steeply dipping areas, such as subsurface properties or smaller scale surface features, strongly affect the level of backscattered signals.

ISSN:0001-4966    

DOI:10.1121/1.418002

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The paper describes an ultrasonic backscatter device for measuring vertical profiles of the concentration of suspended sand in water in the presence of air bubbles. The use of six frequencies permits a simultaneous measurement of the concentration of suspended sand, the grain size, and the concentration of air bubbles. For measuring in the near field of a transducer, the sensitivity of an ideal piston-like transducer in the near field is calculated and compared with measurements. A simple, but fast and efficient inversion algorithm to convert the backscattered intensities into sediment concentration and grain size is described. Measurements of known suspensions with concentration and grain size are presented, they exhibit a low error (20% rms) over a grain radius from 40 to 300 μm.In-situmeasurements of the concentration of suspended sediment in the surf zone are compared with the results of two optical backscattering sensors (OBS).

ISSN:0001-4966    

DOI:10.1121/1.418003

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The acoustical characteristics of bubbles injected into a fully developed turbulent flow field are studied. By injecting an “adult” bubble into a flow, generated by a submerged axisymmetric horizontal water jet, the acoustic reexcitation of the bubble with and without breakup may occur in the shear-induced flow region. Bubbles of various sizes were introduced into jets of various speeds by means of interchangeable hypodermic needles. Results of the role of the turbulent flow characteristics in determining the acoustic bubble response are discussed. The characterizations of both the acoustical and the dynamics of the bubbles encountering a turbulent flow field depend upon the estimated integral and microlength scales, the corresponding Reynolds numbers, and the critical Weber numbers for both bubble distortion and breakup. A critical Weber number was estimated to be 0.52 for an “adult” bubble acoustic reexcitation without breakup and 1.10 for bubble breakup. A simple model is given to account for the bubble acoustic radiation. The Rayleigh–Plesset equation was altered by incorporating the turbulent pressure fluctuation of the flow as a driving force of the bubble. The turbulent energy spectrum obtained experimentally was used to predict the monopole acoustic emission by the bubble. Some conclusions are given about the nature of the turbulent flow field and conditions under which the bubble acoustic reexcitation may occur.

ISSN:0001-4966    

DOI:10.1121/1.418015

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

This paper presents a theoretical study of sound wave propagation through media whose properties change along one of the coordinate rectangular axes, and remain constant in the plane orthogonal to this axis. The uppermost sea bottom layer could be modeled as such an inhomogeneous medium where the density, sound velocity, and absorption of acoustical energy varies along one axis. The results were applied to obtain the reflection coefficient of the water-bottom surface considering that there is a transitional inhomogeneous layer with density, velocity, and absorption gradient, and without discontinuity with the bottom. It was concluded that the reflection coefficient of the bottom surface shows a frequency variation, dependent on the physical property gradients in the uppermost sea bottom layer.

ISSN:0001-4966    

DOI:10.1121/1.418005

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

There are two distinct mechanisms associated with compressional wave absorption in lossy media, internal relaxation and internal friction. For the special case of propagation in an homogeneous, unbounded medium, both mechanisms can be modeled by adopting the convention of a complex sound speed and are, in this sense, equivalent. For the more realistic case of propagation in a stratified medium, the convention of complex sound speed does not give a correct description for losses which modify the linearized equation of motion, such as internal friction. In the presence of boundaries, internal friction can be modeled by the introduction of a complex quiescent density in addition to complex sound speed. Propagation models which use complex sound speed only in the presence of boundaries make the tacit assumption that seafloor losses are caused by internal relaxations only. A solution is developed for propagation in a lossy Pekeris channel where absorption in the lower fluid is caused by internal friction. The example that has been considered yields a sound level 3 dB less than the standard description over a 50-km path.

ISSN:0001-4966    

DOI:10.1121/1.417995

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

This paper addresses the problem of matched-field source localization in the presence of uncertainties in the ocean environment. Because signal wavefront mismatch can cause anomalous source location estimates, development of robust localization methods is critically important. In this paper, a robust maximum-likelihood estimator is proposed. It is based on a decomposition of the field into predictable and unpredictable subspaces of the acoustic normal mode representation. The estimator uses the predictable subspace for source localization. Identification of the predictable modes is made according to the second-order joint statistics of the horizontal wave numbers. The performance of the method is evaluated and compared to other matched-field methods using simulations and acoustic array data from the Mediterranean Sea. In the presence of mismatches, the algorithm has superior probability of correct localization than the maximum-likelihood, matched-mode-processing, and Bartlett methods.

ISSN:0001-4966    

DOI:10.1121/1.417996

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The schlieren technique can be used to visualize the two-dimensional ultrasonic standing wave fields associated with circular cylindrical shells under resonance conditions. However, the interpretation of the schlieren image is not simple due to the complex relationship between the acoustic and optical fields. A model for predicting the optical image in an ideal schlieren system is presented and used to investigate the influence of the acoustic pressure and optical spatial filtering on the resultant image, demonstrating the conditions under which the image is a meaningful representation of the acoustic wave field. A low-frequency (>100 kHz), wide-aperture, laboratory schlieren system is used to image the fluid column resonances of a circular cylindrical shell. Experimental results agree well with the predictions, validating the theory. Although the schlieren image is two-dimensional, limiting investigations to targets having translational symmetry, the technique is noninvasive and can potentially provide greater insight into the acoustic resonance behavior of more complex scattering geometries.

ISSN:0001-4966    

DOI:10.1121/1.417976

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

In this work the response of an embedded fiber optic Fabry–Perot ultrasound sensor has been investigated. Calculations were performed that model the fiber as a cylindrical elastic inclusion subjected to obliquely incident harmonic plane waves. The phase shift of the light traveling in the fiber was calculated along the principal strain axes as a function of ultrasonic frequency and incident angle. Experiments were performed to validate the calculations. Induced phase shifts were measured for the cases of normally incident ultrasound impinging upon a fiber sensor submerged in water as well as embedded in epoxy. The case of obliquely incident ultrasound at angles up to 4 deg was also investigated for a fiber embedded in epoxy. The experimental measurements were performed over a bandwidth from 2 to 8 MHz.

ISSN:0001-4966    

DOI:10.1121/1.417997

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Surface roughness changes the acoustic backscatter signal due to the microstructure of a sample. These changes have been previously reported for the backscatter power under the restrictive assumption that the microstructural length scales are much smaller than any other length scale in the problem. In this study this restriction is removed and an approximate analytic series solution is presented that describes the effects of surface roughness on the power and mean square of the acoustic backscatter from samples whose microstructure is characterized by an autocorrelation lengthLm, which may be smaller than, comparable to, or larger than either the wavelength or the autocorrelation lengthLsthat describes the surface roughness. Both focused and unfocused phase-sensitive transducers are considered. A surprising result emerged. For focused probes at normal incidence and for moderate roughness, the backscattered power at the focal depth is almost entirely determined by the frequency and the rms surface height; it is independent ofLm,Ls, and the radius of the transducer.

ISSN:0001-4966    

DOI:10.1121/1.418006

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The effects of surface roughness on the signal-to-noise ratio (SNR) for the phase-sensitive detection of an inclusion in a sample with microstructure is explored in a simple model. It is assumed that a phase-sensitive transducer is used in pulse-echo mode, that the sample is immersed in a water bath, and that the noise is dominated by scattering from the sample’s microstructure. Since there is interest in roughness-inducedchangesin the SNR, calculations are reported of the normalized signal-to-noise ratio (NSNR), i.e., the SNR with the rough surface divided by the SNR with the smooth surface. The dependence of the NSNR on transducer parameters (radius and focal length), on surface parameters (root-mean-square height and autocorrelation length), and on material parameters (scattering strength, average crystalline size) is reported. In this paper, approximate analytic series solutions, previously obtained for the signal and the noise, are combined to estimate the NSNR as a function of the inclusion’s depth beneath the surface. It is found that for unfocused transducers even moderately rough surfaces can substantially degrade the NSNR for the most typical case, a relatively large inclusion in a background of small microcrystallites. The calculations predict two other results that initially surprised us. First, surface roughness is predicted toincreasethe SNR for a relatively small strongly scattering inclusion in a sample with relatively large but weakly scattering microstructure. Second, moderate roughness is predicted to have little effect on the SNR for focused transducers at the focal depth.

ISSN:0001-4966    

DOI:10.1121/1.418007

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP