Normal-Mode Sound Propagation in Shallow Water

An extension of the normal mode dispersion equation is developed that applies to a variable velocity water layer lying over an absorbing bottom. The effect of bottom absorption on the shape of group-velocity-vs-frequency curves (mode curves) is examined for a special case, showing that there will be no frequency of minimum group velocity (Airy frequency) if the bottom absorption is sufficiently large. A high-frequency cutoff of the mode curves is predicted as the normal consequence of a velocity structure in the water layer. Calculated mode curves are shown to be in good agreement with experimental curves obtained in a shallow-water area of the Bering Sea.