Time warping and acoustic characterization of the seafloor in horizontally inhomogeneous ocean

Abstract

Time-warping transform is increasingly employed in shallow water acoustics to separate the field due to a compact, broadband sound source or the two-point cross-correlation function of diffuse noise into their normal mode components, and to measure mode travel times as a function of frequency. The time-warping transform was developed for range-independent waveguides, while physical parameters of the ocean are never quite constant in the horizontal plane. Bathymetry variations are typically responsible for the bulk of the waveguide’s range dependence as well as horizontal refraction of sound in the coastal ocean. Simple, exactly solvable models of shallow-water waveguides are used in this paper to illustrate the effects that the range dependence and horizontal refraction have on the performance of the warping transform and on 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. It is found that moderate bottom slopes can lead to large errors in retrieved geoacoustic parameters and cause positive bias in bottom sound speed estimates if horizontal refraction is ignored. [Work supported, in part, by NSF and BSF.]