摘要:

The response of the sediments of Eckernfoerde Bay, Germany to acoustic remote sensing previously has been attributed qualitatively to gas features which were postulated to existinsituwithin the sediment. The existence of features as small as 0.5‐mm equivalent spherical radius has now been confirmed by x‐ray computed tomography of cores taken and scanned atinsitupressures. The interaction of an acoustic pulse from the acoustic sediment classification system (ASCS) with this type of gassy sediment was modeled and the results are presented here. The bubble scattering response model includes both the effects of nonspherical bubbles and of sediment resistance to shear deformation (as expressed by the dynamic shear modulus). Model predictions made using the observed gas bubble distribution agree with normal incidence ASCS data, exhibiting extended returns (i.e., greater than the source pulse length) from the seafloor bubble layers as well as high attenuation within the bubble layers. Spectrograms of ASCS and modeled return pulses were also similar. These results suggest that the acoustic return characteristics of the near surface sediments of Eckernfoerde Bay are dominated by scattering from a distribution of gas bubbles with depth.

ISSN:0001-4966    

DOI:10.1121/1.414500

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The Kaneohe acoustic source transmitted 133‐Hz, 60‐ms resolution signals over 3709 km from Oahu at 183‐m depth to a Naval receiver at 1433‐m depth near northern California. Ray theory successfully models the acoustic multipaths whose travel times are unambiguously tracked between 1983–89, despite the fact that the sound bounces one or more times from the Oahu slope before becoming trapped in the sound channel. The eigenrays are inclined at about 15° at the axis of the sound channel. The upper turning depths of the eigenrays are insensitive to realistic perturbations along the section. This supports the finding that the changes in delay of ∼±0.2 s between 1983–89 are due to temperature and not due to changes in the multipaths. Compared with transmission through a smoothed representation of the ocean’s acoustic waveguide, the mesoscale and submesoscale features vertically scatter axially trapped energy about 200 and 800 m, respectively. The submesoscale structure may be associated with internal waves. Scattering from a combination of the meso and submesoscale is not a perturbation since scattering redistributes sound that would be otherwise axially trapped over 1 km; the scale of the acoustic waveguide.

ISSN:0001-4966    

DOI:10.1121/1.414501

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

In a realistic range‐dependent deep ocean environment that gives rise to ray chaos, a complete set of eigenrays at long ranges is computed using a numerical technique based on the shooting method. The numerical results support and illustrate the known connection between ray chaos and eigenrays, namely, that the number of eigenrays grows exponentially, on average, with increasing range. It is further found that groups of chaotic eigenrays tend to form ‘‘clusters’’ having stable envelopes.

ISSN:0001-4966    

DOI:10.1121/1.414502

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

A new low‐frequency scattering model for small to moderately sized fish schools has been developed. The model, which uses a mathematical formalism based upon the harmonic solution of sets of coupled differential equations, allows a verified swimbladder scattering ‘‘kernel’’ for the individual fish to be incorporated. It includes all orders of multiple scattering interactions between fish, and calculates the aggregate scattering field by coherent summation. Application to simulated ensembles of closely spaced fish indicates significant deviations from incoherent scattering returns. Peak target strength reductions, and shifts in the resonance frequency, appear due to multiple scattering. The target strength also varies strongly with frequency as a result of interference effects. When applied to widely dispersed ensembles, the model reproduces the results of incoherent scattering. For larger ensembles, at greater depth, the model predicts sharply reduced target strength values around the main resonance. The ensemble effects of a school on the scattering of any single individual show more fluctuations as the school size increases. By reducing the viscous damping in the scattering kernel, the model can also describe scattering from small bubble clouds. The model has been applied successfully to fit experimental broadband low‐frequency scattering data from schooling fish.

ISSN:0001-4966    

DOI:10.1121/1.414503

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

This paper describes a model of the noise field generated by waves breaking on the ocean surface and the angle–time–frequency resolution of that noise field. The model takes into account the discrete space‐time structure of the noise sources and thus differs from models based on the continuous source sheet assumption. This is done using a stochastic model to specify the locations, occurrence times, and radiated noise waveforms for the individual breaking waves and then computing the noise field as the superposition of the noise contributions from those breaking waves. The angle–time–frequency resolution of the noise field is obtained by applying a time–frequency resolution algorithm to the output of a beamformer as a function of the steering angle. To describe the properties of the 3‐D noise resolution, we assume a vertical array in a shallow‐water acoustic environment and use recently obtained experimental estimates of the acoustic coverage and the source level and waveform‐duration probability densities to determine the breaking wave source model parameters. The examples presented indicate that the breaking wave noise is largely confined to two angular sectors, the high‐angle sector looking toward the ocean surface and the sector containing the discrete mode propagation energy. The high‐angle noise exhibits a large temporal variability, characterized by a small number of large broadband bursts, and is only weakly dependent on the acoustic environment. On the other hand, the discrete mode noise shows less temporal variability, characterized by a larger number of smaller broadband bursts, and is strongly dependent on the acoustic environment. Finally, the angle–frequency structure of the low‐level noise in the downward‐looking direction represents an example of imaging the bottom with surface‐generated ambient noise and suggests that ambient noise measurements might be used to estimate sediment thickness.

ISSN:0001-4966    

DOI:10.1121/1.414504

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

In this paper the Helmholtz–Kirchhoff approximation for backscattering strength and a Bayesian model of the uncertainty related to acoustic backscatter measurements are integrated into a maximumaposterioriprocessing scheme for the estimation of seafloor roughness parameters. Two processors are developed based on different levels of uncertainty regarding the angles of incidence for the received acoustic signals and the system calibration. Simulations indicate that the new maximumaposterioriprocessors are superior to a maximum likelihood estimation scheme that operates under the assumption that the angles of incidence for the backscattered signals are fixed and treats the calibration factor in a deterministic fashion. Specifically, the new processors produce roughness parameter estimates which are very close to the true values of the parameters, which are known in simulations, whereas the fixed angle processors are shown to result in a substantial bias in the estimation procedure.

ISSN:0001-4966    

DOI:10.1121/1.414505

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The acoustic lance is an instrument developed to obtaininsitucompressional wave velocity and attenuation (Q−1) profiles for a sedimentary layer of several meters thickness at the sediment–seawater interface. The self‐contained instrument consists of ten independent recording channels with a linear array of receivers embedded in the seafloor below a broadband acoustic source. It providesinsiturecording of full waveforms to determine interval velocity and attenuation. The system can be attached to a gravity corer or to a specially designed probe. A comprehensive experiment was carried out in Mid‐Atlantic Ridge sediment ponds where the lance madeinsitumeasurements, and core samples were recovered. Core data agree well withinsitudata in one location, but disagree in other locations. Lance data indicate that the sediment ponds have similarinsituvelocity distributions, with an acoustic channel much thinner than that predicted by earlier investigators.

ISSN:0001-4966    

DOI:10.1121/1.414506

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The probing of a 1‐D (depth‐) inhomogeneous fluid half‐space is carried out from a limited knowledge of plane‐wave reflection coefficients with respect to frequency at two precritical incidences. Born‐type inversion algorithms are investigated. First, index profiles as a function of a density‐dependent pseudodepth are retrieved at two incidences; then density and speed‐of‐sound profiles versus depth are extracted. ABorn‐iterativescheme builds up the solution as the limit of a sequence of solutions of approximate, constrained linear problems. ABorn‐extendedscheme directly yields a linearized solution. Two variants of the Born‐iterative scheme are developed, the first one when data are known in both amplitude and phase, the second one with amplitude‐only data. Possible—but not limiting—applications are in underwater acoustics, once the planar reflection coefficient of a seabed has been found, e.g., from the depth‐dependent Green’s function in the spectral domain. Numerical simulations illustrate the schemes. Good index profiles are obtained with reasonable behavior versus noise or other causes of limitations. As is expected, separate probing of speed and density profiles remains difficult. The computationally costly exact scheme is not better than the cheaper approximate one when good data are available, but handles more complicated situations.

ISSN:0001-4966    

DOI:10.1121/1.414507

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The ability to measure climatic changes in ocean temperature is fundamentally limited by the presence of mesoscale variability. Because ocean acoustic propagation can be used to measure the range‐averaged temperature profile, long‐range acoustic transmissions have been proposed as a means of filtering out mesoscale variability in order to measure a global warming related trend in mean temperature. In this paper, the Cramer–Rao lower bound (CRLB) on the estimation of the mean depth‐dependent temperature profile given a single acoustic transmission is evaluated to provide an indication of the highest accuracy which could be achieved by this experiment. The CRLB derived here applies to broadband vertical arrays of arbitrary length and thus extends previous work. Evaluation of the bound is performed using models of sound‐speed variability derived from real Pacific environmental data. Results indicate that the performance of an acoustic thermometry system is limited by mesoscale variability above a threshold value of observation–time–signal‐to‐noise ratio product and is acoustic noise limited below this threshold. Further, comparisons of the CRLB above this threshold suggest that for a 5000‐km source–receiver separation, ATOC accuracy may vary from between 0.01 and 0.1 °C depending on the shape and uncertainty of the change in mean temperature profile.

ISSN:0001-4966    

DOI:10.1121/1.414536

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Two scatterers at similar range give an echo which may appear to be due to a single scatterer. Methods for determining target strength that depend on resolving single scatterers may fail in this instance. Statistics associated with the described special case of coincidence are derived and illustrated by theoretical computation for the SIMRAD EK500 echo sounder system with the ES38B split‐beam transducer resonant at 38 kHz. Connections to angle measurement in radar and swath bathymetry and to bottom‐scattering‐strength measurement are noted.

ISSN:0001-4966    

DOI:10.1121/1.414537

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP