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Acoustic shadow-zone arrivals at long range in the North Pacific Ocean


Deep acoustic shadow-zone arrivals were first observed on horizontal, bottom-mounted receiving arrays in the North Pacific Ocean in the late 1990s. These receptions revealed significant acoustic energy penetrating an estimated 500- 1000 m into geometric shadow zones below cusps (caustics) of predicted timefronts, much more than predicted by diffraction theory. Two vertical line array receivers deployed in close proximity in the North Pacific as part of the SPICEX experiment, together virtually spanning the water column, show the vertical structure of the shadow- zone arrivals for transmissions from broadband 250-Hz acoustic 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. Chapter II compares a daily average of acoustic timefronts with parabolic equation simulations based on a sound-speed profile measured nearly concurrently with the acoustic data acquisition. Simulations incorporating a range-independent sound-speed profile confirmed the presence of deep shadow-zone arrivals. Receptions from off-axis sources also revealed acoustic energy scattering back up toward the axis at the end of the timefront, referred to as axial shadow-zone arrivals. Simulations incorporating sound-speed fluctuations consistent with the Garrett-Munk internal- wave energy spectrum at full strength accurately predict the vertical extent of and energy contained in both axial and deep shadow-zone arrivals. Chapter III extends the analysis to include acoustic receptions from June to November 2004. Incoherent monthly averages of acoustic timefronts indicate that lower cusps associated with acoustic rays with shallow upper turning points (UTPs), where sound-speed structure is most variable and seasonally dependent, deepen from June to October as the summer thermocline develops. Surface-reflected rays, or those with near-surface UTPs, exhibit less scattering due to internal waves than in later months, when the UTP deepens. Data collected in November exhibits dramatically more vertical extension than previous months. The depth to which the timefronts extend as the seasons change is a complex combination of deterministic changes in the depths of the lower cusps as the range-average profiles evolve and of the amount of scattering, which depends on the depths of the UTPs and the mean vertical gradients at those depths

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