Acoustic propagation through an internal wave field in a shallow water waveguide

Abstract

This paper addresses the problem of predicting and interpreting acoustic wave field properties in a stochastic ocean waveguide, for which the sound-speed variability within the water column is treated explicitly as a random process. It is assumed that the sound-speed distribution is composed of three components: a deterministic, time-independent profile and two stochastic components induced by internal wave activity. One random contribution represents a spatially diffuse Garrett–Munk field whose spectrum is constrained by the shallow water waveguide, while the second corresponds to spatially localized soliton packets. A high-angle elastic parabolic equation method is applied to compute single frequency realizations of the pressure field using this three-component representation of the sound-speed distribution. Ensemble-averaged transmission loss and scintillation index measures for the full pressure field and its modal components are estimated for different source depths and for both flat and sloping bottoms. Probability distributions of the mode amplitudes for different ranges are also presented. These statistical measures are incorporated into the analysis of range-dependent mode coupling between the internal wave and acoustic fields, and evidence is presented which supports a recent prediction that the scintillation index grows exponentially with range due to the competition between mode coupling and mode stripping found in shallow water waveguides. Full-field estimates of the scintillation index are also presented for a shallow water region on the continental slope off the New Jersey coast.

Analysis of internal wave induced mode coupling effects on the 1995 SWARM experiment acoustic transmissions

“…For small perturbations in temperature, the associated sound-speed fluctuations can be approximated by Creamer [18] considers from 10 -6 to 10 -s to be typical values, and Tielbuerger et al [9] report a value of 5 x 10-' for their model.…”

“…This paper generated interest in the subject, and further strides have been made in the several investigations that have followed [1,2,7,8,9]. …”

Statistics of normal mode amplitudes in an ocean with random sound-speed perturbations: Cross-mode coherence and mean intensity

“…An acoustic observable that has received much attention since the seminal paper by Dozier and Tappert ͑1978a, 1978b͒ is the mode energy or ͉͗a n ͉ 2 ͘ ͑Creamer, 1996; Colosi and Flatte, 1996;Tielburger et al, 1997;Wage et al, 2005͒. For the case of weak attenuation a state of equipartitioning of modal energy is understood to be a modal manifestation of full saturation ͑Flatté et al, 1979͒. However, theoretical work to date has not been able to address the influences of the cross-mode coherences on the modal energy evolution.…”

“…Examples in deep-water problems are the depth broadening of the acoustic finale ͑Worcester et Colosi et al, 1994;Colosi and Flatté, 1996;Worcester et al, 1999͒ and the so-called deep shadow zone arrivals ͑Dushaw et al, 1999;Flatté and Colosi, 2008;Van Uffelen et al, 2009͒. For shallow-water problems, on the other hand, the acoustic arrivals are seen to have significant time spreading ͑Tielburger et al ., 1997;Fredricks et al, 2005͒ which could be due to both random linear internal waves and nonlinear internal solitary waves. Lacking, however, has been a theoretical understanding of the dominant acoustic scattering physics leading to the mean redistribution of the acoustical energy.…”

Workshop on Imaging of Complex Media with Acoustic and Elastic Waves

“…For more complete accounts of internal wave dynamics and statistics, see [40] and [41]. For treatments of the problem of acoustic scattering by internal waves, see [42,43,44].…”