Ocean acoustic wave propagation and ray method correspondence: Internal wave fine structure

Abstract

Acoustic wave fields propagating long ranges through the ocean are refracted by the inhomogeneities in the ocean's sound speed profile. Intuitively, for a given acoustic source frequency, the inhomogeneities become ineffective at refracting the field beyond a certain fine scale determined by the acoustic wavelength. On the other hand, ray methods are sensitive to infinitely fine features. Thus, it is possible to complicate arbitrarily the ray dynamics, and yet have the wave field propagate unchanged. This feature raises doubts about the ray/wave correspondence. Given the importance of various analyses relying on ray methods, a proper model should, at a minimum, exclude all of the fine structure that does not significantly alter the propagated wave field when the correspondence to the ray dynamics is integral. We develop a simple, efficient, smoothing technique to be applied to the inhomogeneities-a low pass filtering performed in the spatial domain-and give a characterization of its necessary extent as a function of acoustic source frequency. We indicate how the smoothing improves the ray/wave correspondence, and show that the so-called ''ray chaos'' problem remains above a very low frequency (ϳ15-25 Hz).

Clustering in randomly driven Hamiltonian systems

“…The rays in a coherent cluster do arrive at the fixed range at closed times and move coherently when propagating through the fluctuating ocean different from the clusters to be described in Refs. ͓18,16,17,19͔. It should be emphasized that similar strips have been found in the field experiments ͓8,14,15,21͔.…”

North Pacific Acoustic Laboratory

“…One suggests that chaotic behavior is predicted unrealistically soon because inhomogeneities that do not appreciably alter the propagating wave, i.e., those smaller than the vertical projection of a wavelength, are included in ray models. 16 It is shown that excluding this fine structure by smoothing it, or filtering it out, leads to much longer predictability horizons. Another argues that while chaotic rays are expected to become random, travel times retain some of the same properties as nonchaotic rays.…”

Wavefront intensity statistics for 284-Hz broadband transmissions to 107-km range in the Philippine Sea: Observations and modeling

“…Features in the sound speed field that are much smaller than the acoustic wavelength will have a negligible effect on the propagation of acoustic waves; see e.g., Hegewisch et al (2005). The criterion used to determine convergence was the normalized sum of squared errors (A1)…”