Modal Theory of Transverse Acoustic Coherence in Shallow Oceans
Formal rept. Nov 2008-May 2012
NAVAL RESEARCH LAB WASHINGTON DC ACOUSTICS DIV
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The coherence of a shallow-water acoustic field transverse to the propagation path is represented using adiabatic normal modes, and path-integral theory is applied to fluctuations of mode amplitudes to obtain the coherence function. The problem is formulated in the horizontal plane where ray paths are nearly straight having no interaction with lateral boundaries, and requires the spectrum of environmental fluctuations that are provided by a shallow-water Garrett-Munk internal-wave model. The coherence function of the acoustic field is shown to depend on phase-structure functions of the normal modes. Environmental fluctuations with wavevectors transverse to the acoustic path were found to govern the structure functions. The impact of internal waves on coherence is proportional to the internal-wave energy, and is strongest for the lowest internal-wave modes in conjunction with those acoustic modes having maxima in the vicinity of high buoyancy. The transverse phase-structure function is found to behave like a sinc function, which provides the transition between limiting solutions for small and large hydrophone separations. Results show the coherence predicted by the theory decays too slowly with hydrophone separation to describe the observed ocean.
- Physical and Dynamic Oceanography