New full-wave approximation for ocean acoustic travel time predictions

Abstract

A new full-wave parabolic approximation is introduced that is valid for a wide range of grazing angles. By Fourier synthesis it yields travel times of ocean acoustic multipaths that are insensitive to a reference speed of sound. After depths and sound speeds are transformed to new coordinates, the highly efficient ‘‘split-step Fourier’’ algorithm is used to solve the new approximate wave equation for forward propagation. Accuracy of the new approximation has been tested by comparison to a broadband normal mode model in a range-independent environment. At 1000 km range and with a pulse of resolution 20 ms at center frequency 75 Hz, computed travel times of 24 multipaths agreed with maximum difference 3.4 ms, mean difference 0.9 ms, and rms difference 1.5 ms. This approximation may prove to be an efficient method for accurate travel time predictions of multipaths over a wide range of acoustic frequencies and for basin scale distances.

Regions that influence acoustic propagation in the sea at moderate frequencies, and the consequent departures from the ray-acoustic description

“…Results based on the analytical solution are compared with those derived from the sound-speed insensitive parabolic approximation. 9 If these results are similar, then results previously given for this approximation are probably accurate. 8 James Bowlin pioneered the use of the Huygens-Fresnel principle ͑pp.…”

“…The sound-speed insensitive parabolic approximation 9 yields accurate travel times, is efficient due to its split-step algorithm, and obeys reciprocity. It is important that reciprocity is obeyed because a proof 1 for reciprocity is provided by the integral theorem of Helmholtz and Kirchhoff.…”

“…We compare the region of influence at 100 Hz using the exact solution of the wave equation ͑normal modes͒ with the solution using the sound-speed insensitive parabolic approximation 9 ͓Figs. 8͑a͒ and 8͑b͔͒.…”

Acoustic Tomography with Navy Sonars

“…1 and Spiesberger et al, 2002). The predictions for coherence time were based on the time evolution of a linear internal wave field whose spectrum is given by Garrett and Munk (1972), a realistic bottom, our best guess of the geoacoustic properties of the sub-bottom, a range-dependent background of sound speed derived from Levitus' data base (1982), and the sound speed insensitive parabolic approximation (Tappert et al 1995). Neither the one-half or normal energy level of the Garrett-Munk spectrum fits all the data.…”

“…Traditional means to process the signals will be done including beamforming, coherent averaging (when dealing with periodic signals), correcting for Doppler shifts (when dealing with mobile sonars), and matched filtering (when a replica with the emitted waveform is available). The data will be interpreted using rays and the sound speed insensitive parabolic approximation (Tappert et al 1995) in collaboration with Frederick Tappert and Andrew Jacobson. The acoustic models will be used in conjunction with oceanographic models that contain the best available digital data sets for bathymetry, sound speed fields that vary with range and depth, and internal waves.…”