In the second edition of Fundamentals of Ocean Acoustics, Brekhovskikh and Lysanov introduced the concept of a waveguide invariant to a larger audience. They showed how plots of acoustic intensity, mapped in range and frequency, often exhibit striations, contours of constant intensity. They defined a parameter "beta" as a simple function of range, frequency and the slope of the striations. For certain special cases, beta is an invariant quantity. While beta is no longer literally an invariant for more general shallow water waveguides, the concept remains useful. In the present work, the waveguide invariant is formulated as a distribution. The effect of shallow water internal waves on this waveguide distribution is quantified.
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