Volume 115, Issue 3, March 2004
Index of content:
- UNDERWATER SOUND 
115(2004); http://dx.doi.org/10.1121/1.1648321View Description Hide Description
This paper presents an original way to perform wave number inversion from simulated data obtained in a noisy shallow-water environment. In the studied configuration an acoustic source is horizontally towed with respect to a vertical hydrophone array. The inversion is achieved from the combination of three ingredients. First, a modified version of the Prony algorithm is presented and numerical comparison is made to another high-resolution wave number inversion algorithm based on the matrix-pencil technique. Second, knowing that these high-resolution algorithms are classically sensitive to noise, the use of a holographic array processing enables improvement of the signal-to-noise ratio before the inversion is performed. Last, particular care is taken in the representations of the solutions in the wave number space to improve resolution without suffering from aliasing. The dependence of this wave number inversion algorithm on the relevant parameters of the problem is discussed.
115(2004); http://dx.doi.org/10.1121/1.1648320View Description Hide Description
Travel time stability is investigated in environments consisting of a range-independent background sound-speed profile on which a highly structured range-dependent perturbation is superimposed. The stability of both unconstrained and constrained (eigenray) travel times is considered. Both general theoretical arguments and analytical estimates of time spreads suggest that travel time stability is largely controlled by a property ω′ of the background sound-speed profile. Here, is the range of a ray double loop and I is the ray action variable. Numerical results for both volume scattering by internal waves in deep-ocean environments and rough surfacescattering in upward-refracting environments are shown to confirm the expectation that travel time stability is largely controlled by ω′.
Geoacoustic inversion in range-dependent ocean environments using a plane wave reflection coefficient approach115(2004); http://dx.doi.org/10.1121/1.1646405View Description Hide Description
A new, efficient, versatile ray-based model is presented that performs geoacoustic inversions in range-dependent oceanwaveguides faster than alternative forward models for which the computation time becomes extremely long, especially for broadband inversions. The water propagation is approximately separated from the seabed interaction using predetermined bathymetry and a possibly range-dependent water sound speed profile. The geometrical optics approximation is used to calculate eigenrays between sources and receivers, including bottom reflecting paths. Modeled broadband pressure fields are obtained by computing the plane wavereflection coefficient at specific angles and frequencies and by then linking this result with the bottom reflected eigenrays. Each perturbation of the seabed requires a recalculation of the plane wavereflection coefficient, but not a recalculation of the eigenrays, resulting in a highly efficient method. Range-independent problems are treated as a limiting case of the approach. The method is first described and then demonstrated with a few simple range-independent theoretical models. The versatility of addressing range-dependence in the bottom seabed is demonstrated with a simulated data set. Finally, the new model is applied to inversion from a measured data set, taken with impulsive sources, for both range-independent and range-dependent continental shelf environments.
Criteria for discretization of seafloor bathymetry when using a stairstep approximation: Application to computation of T-phase seismograms115(2004); http://dx.doi.org/10.1121/1.1643361View Description Hide Description
Acoustic solutions for numerical models in which an overly coarse discretization of a stairstep boundary is employed to simulate smoothly varying bathymetry are degraded in a way that simulates scattering. Geometrical optics approximations are used to derive discretization criteria for simulating a smoothly sloping interface for the case of a source embedded in either an acoustic or an elastic seafloor, and applied to modeling T-phases. A finite difference time-domain modeling approach is used to synthesize T-phases for both smoothly sloping and rough seafloor boundaries. It is shown that scattering at a rough seafloor boundary yields ocean-borne acoustic phases with velocities near those of observed T-phase, while smooth seafloor models yield T-phases with slower horizontal velocities. The long duration of the computed T-phases for both the rough acoustic and elasticmodels is consistent with energy being scattered into the sound channel both as it transits the ocean/crust boundary, as well as at several subsequent seafloor reflections. However, comparison between the elastic and acoustic modelingsolutions indicates that the T-phase wavetrain duration decreases with decreasing impedence contrast between the ocean and seafloor.