Understanding and Utilizing Waveguide Invariant Range-frequency Striations in Ocean Acoustic Waveguides
MASSACHUSETTS INST OF TECH CAMBRIDGE
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The range-frequency waveguide invariant describes striations that often appear in plots of acoustic intensity versus range and frequency. Signal processing techniques based on the waveguide invariant are able to exploit the effects of the ocean acoustic waveguide without requiring detailed knowledge of the sound speed profile or of the seafloor. This thesis develops a framework for understanding and calculating the waveguide invariant, and uses that framework to create signal processing techniques for the waveguide invariant which may be applicable to passive and active sonar, time-reversal mirrors, and array processing. A method for passively estimating the range from an acoustic source to a receiver is developed, and tested on experimental data. Heuristics are developed to estimate the minimum source bandwidth and minimum horizontal aperture required for range estimation. A semi-analytic formula for the waveguide invariant is derived using WKB approximation along with a normal mode description of the acoustic field in a range-independent waveguide. This formula illustrates how the SSP and the seafloor properties affect the value of the waveguide invariant. Array processing techniques designed specifically for the purpose of observing range-frequency striations are developed and demonstrated. Finally, a relationship between the waveguide invariant and wavenumber integration is derived.