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A long, vertical line array was deployed off Monterey, California during the Heard Island Feasibility Test to measure the modal content of the received signals. The array contained 32, equally spaced hydrophones spanning from 345 to 1740-m depth. The multichannel data were recorded through a tether to the R/V Point Sur. The measurements had very low signal to noise ratios and indicated the cw transmission losses were approximately 140 dB for a source/receiver range of 17 000 km. Modal content was analyzed using (i) the modal extent versus depth, (ii) frequency-vertical wave-number spectra, (iii) modal beamforming and (iv) least squares fitting. All led to the conclusion that the modal population is surprisingly rich. There was strong evidence of population up to at least mode seven in the data.

Significant three-dimensional (3-D) environmental variability exists in the vicinity of the shelf break along the mid-Atlantic Bight. This study examines the influence of azimuthal coupling due to this variability on acoustic propagation in this region. Numerical studies employing a 3-D ray code, a hybrid ray-mode code, and a 3-D parabolic equation model are used to study the significance of azimuthal coupling on various propagation paths. These paths include up-slope, slant-slope, and cross-slope propagation. The numerical analysis suggests that, for the propagation ranges less than 60 km examined, the influence of azimuthal coupling is negligible compared to the inherent uncertainty in the environment itself.

Experiments from several shallow-water areas are summarized. Coherent sound transmission results, particularly wavenumber spectra, are compared to range-dependent calculations that use oceanographic and geophysical characteristics from measurements and archives as bounded inputs to the propagation codes. In general excellent agreement was obtained between the measured and calculated results for both narrowband and broadband transmissions between 50 Hz and 1 kHz to ranges of 40 km. A relative signal gain (RSG) method for the estimation of horizontal coherence length was applied to measured RSG results and yielded coherence lengths on the order of 30λ at 400 Hz at distances of 40 km. Perturbation theory was applied to the shallow-water waveguide under the condition of adiabatic normal modes and expressions were derived for the phase structure function that was simplified by the use of Gaussian correlation functions. These analytical results, along with estimates of the variances of the environmental variables permitted the estimation of the coherence function and the RSG. The calculated coherence function and RSG were found to be consistent with measured RSG and replica correlation results. The fluctuations in the oceanic water volume were found to be the dominant factor in the loss of coherence.

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