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.

Estimation of sound speed and temperature fields from tomographic data requires knowledge of the accuracy of the forward model. For this reason, time domain results using three models derived from the acoustic wave equation are compared here. The models chosen for the analysis are normal modes (NM) and the parabolic equation (PE) and ray approximations. (The NM and PE time series are obtained via Fourier synthesis). The analysis is done using a depth-dependent sound-speed profile where only refracted and surface-reflected/refracted energy is allowed to propagate. Because the NM representation is theoretically exact in this environment, it is used as the comparative base. Travel-time sequences generated using the PE and ray approximations are compared to those generated using NM. For the angular aperature analyzed ( --F_ 9 to --F_ 16.5 deg), phase errors in the PE models investigated limit reliable predictions of travel time to distances of about 1000 km. However, the ray approximation is found to be adequate for estimating arrival times at ranges out to 4000 km for frequencies of 100 Hz and above.

The Heard Island transmissions were received 9140 km away at Ascension Island by an irregular array of bottom-mounted hydrophones. The single-hydrophone signal-to-noise ratio sometimes exceeded 30 dB in a 1-Hz band, confirming the detectability of 57-Hz underwater sound at global distances. The arrival-time pattern consists of a single broad pulse about 10 s long, whose fine structure decorrelates in about 12 min, in sharp contrast with the stable, discrete sequences observed over shorter, midlatitude paths. The amplitude fluctuations of both the fine arrival structure and the unmodulated receptions are uncorrelated between hydrophones as little as 3.4 km apart. Phase varies less than one cycle during a 1-h transmission after correcting for source motion, and the rms phase difference between hydrophones is about 3 rad averaged over the array. Phasor diagrams suggest that the effects of both source motion and ocean dynamics vary over the array. The probability density functions of the real and imaginary parts of a downshifted cw transmission are nearly Gaussian.

No abstract