Volume 126, Issue 1, July 2009
Index of content:
- UNDERWATER SOUND 
126(2009); http://dx.doi.org/10.1121/1.3143143View Description Hide Description
Green’s function approximation via oceannoise cross-correlation, referred to here as ocean acoustic interferometry, has been demonstrated experimentally for passive noise sources. Active sources offer the advantages of higher frequencies, controllability, and continuous monitoring. Experimental ocean acoustic interferometry is described here for two active source configurations: a source lowered vertically and one towed horizontally. Results are compared and contrasted with cross-correlations of passive noise. The results, in particular, differences between the empirical Green’s function estimates and simulated Green’s functions, are explained with reference to theory and simulations. Approximation of direct paths is shown to be consistently good for each source configuration. Secondary (surface reflection) paths are shown to be more accurate for hydrophones with a greater horizontal separation.
126(2009); http://dx.doi.org/10.1121/1.3147489View Description Hide Description
This paper considers Bayesian inversion of seabed reflection-coefficient data for multi-layer geoacoustic models at several sites, with the goal of studying lateral variability of the seabed. Rigorous uncertainty estimation is carried out to resolve lateral variability of the sediments from inherent inversion uncertainties. The uncertaintyanalysis includes Bayesianmodel selection, comprehensive quantification of data error statistics, and a Markov-chain Monte Carlo approach to transforming data uncertainties to modeluncertainties.Model selection is addressed using the Bayesianinformation criterion to ensure parsimony of the parametrizations. Data error statistics are quantified by estimating full covariance matrices from data residuals, with posterior statistical validation. A Metropolis–Hastings sampling algorithm is used to compute posterior probability densities. Four experiment sites are considered along a track located on the Malta Plateau, Mediterranean Sea, and the inversion results are compared to cores taken at each site. Differences between profile marginal-probability distributions at adjacent sites are quantified using the Bhattacharyya coefficient. Differences that exceed the estimated geoacoustic uncertainties are interpreted as spatial variability of the seabed. The results are compared to an interpretation of geologic features evident in a chirp sub-bottom-profiler section.
Temporal and spatial coherence of sound at 250 Hz and 1659 km in the Pacific Ocean: Demonstrating internal waves and deterministic effects explain observations126(2009); http://dx.doi.org/10.1121/1.3133243View Description Hide Description
The hypothesis tested is that internal gravity waves explain temporal and spatial coherences of sound at 1659 km in the Pacific Ocean for a signal at 250 Hz and a pulse resolution of 0.02 s. From data collected with a towed array, the measured probability that coherence time is 1.8 min or longer is 0.8. Using a parabolic approximation for the acoustic wave equation with sound speeds fluctuating from internal waves, a Monte-Carlo model yields coherence time of 1.8 min or more with probability of 0.9. For spatialcoherence, two subsections of the array are compared that are separated by 142 and 370 m in directions perpendicular and parallel to the geodesic, respectively. Measured coherence is 0.54. This is statistically consistent with the modeled 95% confidence interval of [0.52, 0.76]. The difference of 370 m parallel to the section causes spatialcoherence to degrade deterministically by a larger amount than the effect of internal waves acting on the 142 m separation perpendicular to the section. The models are run without any tuning with data.
126(2009); http://dx.doi.org/10.1121/1.3139901View Description Hide Description
Ray- and mode-based theoretical predictions of the spreads of directionally narrow beams are presented and compared to parabolic-equation-based simulations in deep-ocean environments. Both the spatial and temporal spreads of beams are considered. The environments considered consist of a range-independent deep-ocean background sound channel on which a highly structured sound-speed perturbation, associated with either internal waves or homogeneous isotropic single-scale turbulence, is superimposed. The simulation results are shown to be in good agreement with simple theoretical expressions which predict that beam spreading, in both the unperturbed and perturbed environments, is largely controlled by a property of the background sound channel—the ray-based stability parameter or the asymptotically equivalent mode-based waveguide invariant . These results are consistent with earlier results showing that wavefield structure and stability are largely controlled by (or ).