The effect of internal waves on long range acoustic propagation in the ocean

The acoustic-internal wave interaction in an acoustic waveguideis investigated using wave techniques. Refractive index fluctuationsdue to the vertical displacements of the internal waves create an inhomogeneous waveguide. The analysis uses weak scattering theory basedupon the Rytov perturbation technique. It is found that the internalwave field acts as a diffraction grating in permtting only certainscattered acoustic waves to propagate through the waveguide. Sincethe internal waves are continuously distributed in wavenumer space,the acoustic fluctuations become a statistical average with a biastoward particular spatial internal wavelengths.The multimode nature of acoustic propagation precludes the linearrelationship of internal wave statistics to acoustic amplitude andphase fluctuations. Assuming statistical independence between amplitudeand phase fluctuations within a mode and between different modes,it is shown that the total phase-rate fluctuation is a weighted sumof 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 acousticfluctuations are made. Over much of the internal wave frequency bandthe slope of the phase rate frequency spectrum is between -0.5 and-1.0. The depth dependence for the mean-square phase rate fluctuationhas been found. Largest fluctuations occur for shallow and deep receivers. The predicted fluctuations compare favorably with experimentaldata.

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May, 1976