Volume 103, Issue 2, February 1998
 UNDERWATER SOUND [30]


Highfrequency backscattering enhancements by thick finite cylindrical shells in water at oblique incidence: Experiments, interpretation, and calculations
View Description Hide DescriptionImpulse response backscatteringmeasurements are presented and interpreted for the scattering of obliquely incident plane waves by airfilled finite cylindrical shells immersed in water. The measurements were carried out to determine the conditions for significant enhancements of the backscattering by thick shells at large tilt angles. The shells investigated are made of stainless steel and are slender and have thickness to radius ratios of 7.6% and 16.3%. A broadband PVDF (polyvinylidene fluoride) sheet source is used to obtain the backscattering spectral magnitude as a function of the tilt angle (measured from broadside incidence) of the cylinder. Results are plotted as a function of frequency and angle. These plots reveal large backscattering enhancements associated with elastic excitations at high tilt angles, which extend to endon incidence in the coincidence frequency region. Similar features are present in approximate calculations for finite cylindrical shells based on full elasticity theory and the Kirchhoff diffraction integral. One feature is identified as resulting from the axial (meridional ray) propagation of the supersonic leaky Lamb wave. A simple approximation is used to describe circumferential coupling loci in frequencyangle space for several surface waves. The resulting loci are used to identify enhancements due to the helical propagation of the subsonic Lamb wave.

Double wave of Stoneley type on the interface of a stratified fluid layer and an elastic solid halfspace
View Description Hide DescriptionSound propagation from a point timeharmonic source in a stratified water layer lying over an elastic solid halfspace is investigated. It is assumed that the sound speed in the layer is less than the shear speed in the solid bottom, and that it increases with the depth. Numerical examples are given which show that the dependence of the wave field on the range between the source and the receiver can sharply change the character under rather small variations of the frequency. Namely, for some particular frequencies, the sound amplitude shows a periodical dependence on the range, while for other frequencies there is no periodicity. A theoretical explanation of this phenomenon is given in a mathematical development using the normal modes theory and highfrequency asymptotic approximations. The dispersion phase curves are found to have “quasiintersections,” i.e., small domains where two adjacent curves almost intersect. The corresponding frequencies are called the “specific” frequencies. For any nonspecific frequency, there is one interface Stoneley mode, whilst for each specific frequency there are two modes of the Stoneley type with close phase velocities. The periodicity of the field is a result of interference in the two Stoneley modes.

Estimating the compressional and shear wave speeds of a shallow water seabed from the vertical coherence of ambient noise in the water column
View Description Hide DescriptionDue to the multiple bottom reflections encountered in shallow water environments, the spatialstructure of the ambient noise field depends strongly on the geoacoustic properties of the seabed, which are invariant over time scales associated with most measurements. The vertical directionality and coherence are relatively stable features of the noise that are determined primarily by the seabed, rather than temporal variations in the surface source distribution. In this paper, estimates of the compressional and shear wave speeds are determined from ambient noise measurements over shear supporting seabeds. Using a model of windgenerated noise over an elastic seabed, it is shown that the noise is sensitive to the compressional and shear wave speeds in the upper few meters of seabed. An inversion procedure is developed based on a matched field of the complex, broadband coherence from a single hydrophone pair. Using ambient noise data from two shear supporting sites, compressional and shear wave estimates are obtained that are in good agreement with independent surveys. For one site where the bedrock is exposed, a halfspace model of the seabed yields reasonable estimates of the seabed parameters. For the other site, the presence of a thin sedimentary layer results in a low estimate from the halfspace model. However, when the layer is included in the model, the estimates of the underlying bedrock are in good agreement with a seismic survey.

Mode functions for the wideangle approximation to the parabolic equation
View Description Hide DescriptionThe parabolic approximation to the wave equation is examined within the context of normal mode theory. In a layered waveguide, the horizontal propagation constants and modal amplitudes of the field Ψ satisfying the standard parabolic equation (SPE) approximation can be mapped exactly into the amplitudes and wave numbers of the normal modes for the field satisfying the Helmholtz wave equation. However, this is not the case for certain other parabolic approximations, such as the wideangle parabolic equation (WAPE) approximation. Approximate mode functions for the WAPE approximation are developed. These mode functions are then used to decompose rangeindependent soundpressure fields computed using the WAPE approximation. The resulting modal coefficients and eigenfunctions obtained using the WAPE mode functions are compared with those obtained using standard normal mode theory.

Lowfrequency acoustic scattering by gasfilled prolate spheroids in liquids. II. Comparison with the exact solution
View Description Hide DescriptionThis sequential paper describes a comparison between the simple closed solution proposed in a previous paper [, J. Acoust. Soc. Am. 101, 1945–1952 (1997)] with the exact solution for scattering of acoustic waves by gasfilled prolate spheroids at low frequencies. The analysis and comparison show that the lowfrequency acoustic scattering by prolate spheroid gas bubbles has resonance features, and can be rather accurately described by the proposed simple formula. It is shown that the results from the simple analytic solution agree remarkably well with those obtained by the exact solution.

A unified approach to volume and roughness scattering
View Description Hide DescriptionA unified approach is proposed to study scattering from a fluid medium with irregularities of different types: volume inhomogeneities (spatial fluctuations of the compressibility and density) and roughness of the interfaces. The approach considers the roughness as a volume perturbation of a specific kind near flat (unperturbed) interfaces. It permits a description of the scattering problem on the basis of a unique integral equation with a kernel including both types of irregularities. In the case of small perturbations, the firstorder solution of this equation is used to obtain the scattering amplitude and scattering cross section for a stratified randomly inhomogeneous fluid medium with an arbitrary number of rough interfaces. Expressions for the roughness and volume scattering cross section are obtained involving crosscorrelation matrixes between the roughness of different interfaces and between the volume fluctuations of different parameters. Also, an example is considered where roughness–volume correlations are important. The results are generalized to the case when the fluid is placed over an arbitrary basement characterized by its own reflection coefficient and scattering amplitude.
