Recent time-reversal experiments with high-frequency transmissions (3.5 kHz) show that stable focusing is severely limited by the time-dependent ocean environments. The vertical focal structure displays dynamic variations associated with focal splitting and remerging resulting in large changes in focal intensity. Numerical simulations verify that the intensity variation is linked to the focal shift induced by phase changes in acoustic waves resulting from sound speed fluctuations due to internal waves. A relationship between focal range shift, frequency shift, or channel depth changes is illustrated using waveguide-invariant theory. Based on the analysis of experimental data and numerical simulations, methods for robust time-reversal focusing are developed to extend the period of stable focusing.

Robust time reversal focusing in the ocean

Report on Echo Trace Classification

Propagation in water with uniform sound velocity but variable-depth lossy bottom

Sound propagation is considered in range-independent environments and environments consisting of a range-independent background on which a weak range-dependent perturbation is superimposed. Recent work on propagation of both types of environment, involving both ray- and mode-based wavefield descriptions, have focused on the importance of $\alpha $, a ray-based ``stability parameter,'' and $\beta ,$ a mode-based ``waveguide invariant.'' It is shown that, when $\beta $ is evaluated using asymptotic mode theory, $% \beta =\alpha .$ Using both ray and mode concepts, known results relating to the manner by which $\alpha $ (or $\beta $) controls both the unperturbed wavefield structure and the stability of the perturbed wavefield are briefly reviewed.

Rays, modes, wavefield structure and wavefield stability

Sound propagation is considered in range-independent environments and environments consisting of a range-independent background on which a weak range-dependent perturbation is superimposed. Recent work on propagation in both types of environments, involving both ray- and mode-based wavefield descriptions, have focused on the importance of α, a ray-based “stability parameter,” and β, a mode-based “waveguide invariant.” It is shown that, when β is evaluated using asymptotic mode theory, β=α. Using both ray and mode concepts, known results relating to the manner by which α (or β) controls both the unperturbed wavefield structure and the stability of the perturbed wavefield are briefly reviewed.

D.E. Weston

Papers

Interference of wide-band sound in shallow water