Application of time-warping transform for separation of acoustic normal modes in horizontally inhomogeneous waveguides

Time-warping transform is often used in underwater acoustics to separate normal mode components of a broadband signal recorded by a single hydrophone. An important application of time warping is retrieval of modal dispersion curves for subsequent inversion of the measured mode dispersion for unknown geoacoustic parameters. The time-warping transform was developed for range-independent shallow-water waveguides. Physical parameters of the ocean are never quite constant in the horizontal plane, with bathymetry variations being typically responsible for the bulk of the waveguide's range dependence as well as horizontal refraction of sound in the coastal ocean. We use simple, exactly solvable models of shallow-water waveguides to illustrate the effects that the range dependence and horizontal refraction have on the performance of the warping transform and the inferred geoacoustic parameters. Horizontal refraction due to generic bathymetric variations is addressed in the adiabatic approximation using perturbation techniques. Theoretical predictions are verified using numerical simulations.