Index of content:
Volume 103, Issue 4, April 1998
- UNDERWATER SOUND 
103(1998); http://dx.doi.org/10.1121/1.421332View Description Hide Description
In studies of low-frequency reverberation within the marine environment, a central concern is the relationship between reverberation events and morphological features of the seafloor. A time-domain migration algorithm for the reverberation intensity field is developed that produces scattering coefficient maps coregistered with a bathymetrydatabase. The algorithm is tailored to broadband transient sources with good range resolution, and was developed to analyze an extensive set of reverberation records from a 200–255 Hz source collected on the flanks of the Mid-Atlantic ridge. The precise, sample-by-sample, tracking of wavefronts across elements of the bathymetrydatabase that forms the foundation of the algorithms implementation results in reverberation maps that show a clear and detailed correlation between scattering and morphology with narrow scarp slopes consistently highlighted. Environmentally induced asymmetries in transmission loss and incidence angle are exploited to break the inherent left–right ambiguity of the receiver array. Iterative migration, assuming a dominant dependence of backscatter on grazing angle, produces images, even from individual records, that show good ambiguity resolution. Results from multiple records corroborate the effectiveness of the ambiguity resolution and demonstrate the stability of the scattering coefficient estimates and the acoustic system.
Fast and explicit Wentzel–Kramers–Brillouin mode sum for the bottom-interacting field, including leaky modes103(1998); http://dx.doi.org/10.1121/1.421333View Description Hide Description
A fast and explicit method is presented for calculating the bottom-interacting field by means of a mode sum, using the Wentzel–Kramers–Brillouin approximation for the eigenfunctions and the Tindle–Weston formula for eigenvalue spacing. Both trapped and leaky modes are automatically included, requiring no numerical inversions for either eigenfunctions or eigenvalues. Examples are provided which demonstrate the speed and accuracy of the approach by comparison with reference solutions. The importance of leaky modes, especially in range-dependent environments with variable water depth, is demonstrated. A simple criterion for neglecting leaky modes is developed.
103(1998); http://dx.doi.org/10.1121/1.421334View Description Hide Description
A unified framework is presented for examining the performance of linear mode filtering algorithms. Two common mode filters, samples of the mode shapes and the pseudo-inverse of the mode shapes, are presented in this framework as a tradeoff between sensitivity to other modes and sensitivity to white noise. The maximum a posteriori mode filter is presented as an alternative which gracefully transitions between these extremes, and attains the minimum mean squared error when the modes to be estimated are well modeled as samples of a Gaussian random process. Numerical simulations in both shallow and deep water environments confirm the analytically derived properties of these mode filters.
103(1998); http://dx.doi.org/10.1121/1.421374View Description Hide Description
Data on bubble entrainment and comminution are gathered in three experiments, involving the breakup of a disk of air trapped between two plates, and bubble cloud generation under a waterfall, and a plunging jet. In the second two cases, an automated acoustic system for characterizing the entrainment is employed. The data sets are compared with an existing theory for bubble fragmentation, in which a key parameter is the number of spatial dimensions associated with the insertion of randomly positioned planes which are used to divide up the bubble. While an appropriate best-fit theoretical curve can be obtained for the bubble population histograms generated by air disk comminution, waterfalls and plunging jets produce multimodal distributions which the theory cannot model. The differing roles of shape oscillations and surface waves in bubble fragmentation, and the issues involved with incorporating these into the model, are examined.
Enhancement of the total acoustic field due to the coupling effects from a rough sea surface and a bubble layer103(1998); http://dx.doi.org/10.1121/1.421335View Description Hide Description
At wind speeds higher than a few meters per second, when breaking waves are present, the sea surface roughness is accompanied by an assemblage of microbubblesforming different structures or clouds, varying in range and in time. For simplicity, in terms of an average bubble population, the time and space varying assemblage of microbubbles is often assumed to be uniform and referred to as the subsurface bubble layer. Of fundamental importance is the role that this subsurface bubble layer may play in connection with scattering from a rough air/sea interface. The purpose of this paper is to determine the effect that the rough surface in the presence of this subsurface bubble layer has on the total field at low frequencies. Numerical calculations indicate that at low frequencies the bubble layer shifts the incident angle on the surface to steeper angles. In addition it was found that the enhancement of the total field is a consequence of scattering at the rough surface in the presence of the upper refracting bubble layer. An enhancement of approximately 40 dB with respect to the bubble-free medium was obtained very near the surface for a frequency of 400 Hz, with a nominal grazing angle of 20 deg, and a void fraction at the surface of The enhancement decreases to about 10–15 dB at 10 m below the surface, but is still significant at depth exceeding the bubbly region. For a void fraction at the surface of the enhancement is approximately 5 dB.