Travel time stability in weakly range-dependent sound channels

Multimegameter-range acoustic data obtained by bottom-mounted receivers show significant acoustic energy penetrating several hundred meters into geometric shadow zones below cusps (caustics) of timefronts computed using climatological databases [B. D. Dushaw et al., IEEE J. Ocean. Eng. 24, 202-214 (1999)]. This penetration is much larger than predicted by diffraction theory. Because these receivers are horizontal arrays, they do not provide information on the vertical structure of the shadow-zone arrivals. Acoustic data from two vertical line array receivers deployed in close proximity in the North Pacific Ocean, together virtually spanning the water column, show the vertical structure of the shadow-zone arrivals for transmissions from broadband 250-Hz sources moored at the sound-channel axis (750 m) and slightly above the surface conjugate depth (3000 m) at ranges of 500 and 1000 km. Comparisons to parabolic equation simulations for sound-speed fields that do not include significant internal-wave variability show that early branches of the measured timefronts consistently penetrate as much as 500-800 m deeper into the water column than predicted. Subsequent parabolic equation simulations incorporating sound-speed fluctuations consistent with the Garrett-Munk internal-wave spectrum at full strength accurately predict the observed energy level to within 3-4-dB rms over the depth range of the shadow-zone arrivals.

Section: Axial Shadow-zone Arrivalssupporting

confidence: 84%

The vertical structure of shadow-zone arrivals at long range in the ocean

Uffelen

Worcester

Dzieciuch

et al. 2009

“…4(b) and 4(c) for not very small action I. Figure 4(d) presents the connection between the action variable I and the ray grazing angle at the sound channel axis v. Since functions xðIÞ and x 0 ðIÞ to a significant extent determine the influence of the background (unperturbed) sound speed profile on the dynamics of both perturbed and unperturbed ray paths, 24 we can assume that statistics of rays with grazing angles v >5 may be approximately described using the Wiener process approximation. This means that chaotic rays with starting actions close to I 0 are described using the above relations with B ¼ B(I 0 ).…”

Section: B Wiener Process Approximationmentioning

confidence: 97%

Ray-based description of shadow zone arrivals

Virovlyansky

Kazarova

Lyubavin

2011

Field experiments and numerical simulation show that due to scattering from internal-wave-induced sound speed perturbations, the sound energy at megameter ranges penetrates well below the unperturbed timefront, i.e., into the geometric shadow. Shadow zone arrivals form continuations of cusps of the timefront. In the present paper, this effect is analyzed using a stochastic ray theory derived for statistical description of chaotic rays. Probability density functions for parameters of perturbed rays, including those penetrating into the shadow zone, are evaluated analytically. This made it possible to derive analytical estimates for a vertical extent of shadow zone arrivals and for a coarse-grained distribution of sound energy in the shadow zone. It is shown that the lengths of cusp extensions into the shadow zone grow with range r as r(1/2). A known estimate for the spread of timefront segments in the presence of internal waves is applied for obtaining a criterion of nonoverlapping of the cusp continuations. These results are derived for steep rays whose grazing angles at the sound channel axis exceed 5°.

“…Many observable wave field features are controlled by ␣, both in range-independent and range-dependent environments. [5][6][7] It follows from Eqs. ͑9͒ and ͑10͒ that 7…”

Section: ͑7͒mentioning

confidence: 97%

On the width of a ray

Rypina

Brown

2007

Self Cite

Consistent with earlier work by Kravtsov and Orlov, a simple general expression for the width of a Fresnel zone deltar(F) in a smooth inhomogeneous environment is derived; this is the diffractive contribution to the width of a ray. In a stratified environment at long range, the general Fresnel zone width expression is shown to reduce approximately to one that is proportional to [equation in text] where alpha is the ray stability parameter, sigma is the acoustic frequency, r is the range from the source to the field point of interest, and R is the source to receiver range. In a stratified environment on which a weak small-scale perturbation is superimposed, deterministic rays in the background environment that connect fixed end points break up into bundles of micromultipaths at moderate to long range and a second, scattering-induced, contribution deltar(s) to the width of a ray must be considered. It is shown that deltar(s) is proportional to /alpha/r(R-r) and argued that in a micromultipathing environment the total effective width of a background ray is deltar(tot)= [equation in text] . Theoretical predictions are shown to agree well with travel-time sensitivity kernel calculations.