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Abstract:
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The acoustic-internal wave interaction in an acoustic waveguide
is investigated using wave techniques. Refractive index fluctuations
due to the vertical displacements of the internal waves create an
inhomogeneous waveguide. The analysis uses weak scattering theory based
upon the Rytov perturbation technique. It is found that the internal
wave field acts as a diffraction grating in permtting only certain
scattered acoustic waves to propagate through the waveguide. Since
the internal waves are continuously distributed in wavenumer space,
the acoustic fluctuations become a statistical average with a bias
toward particular spatial internal wavelengths.
The multimode nature of acoustic propagation precludes the linear
relationship of internal wave statistics to acoustic amplitude and
phase fluctuations. Assuming statistical independence between amplitude
and phase fluctuations within a mode and between different modes,
it is shown that the total phase-rate fluctuation is a weighted sum
of the phase-rate fluctuations in the individual modes.
Using a statistical internal wave model [C. Garrett and W. Munk,
Geophys. Fluid Dynam., 2, 225-264 (1972)] predictions of acoustic
fluctuations are made. Over much of the internal wave frequency band
the slope of the phase rate frequency spectrum is between -0.5 and
-1.0. The depth dependence for the mean-square phase rate fluctuation
has been found. Largest fluctuations occur for shallow and deep receivers.
The predicted fluctuations compare favorably with experimental
data. |