Horizontal structure of acoustic intensity fluctuations in the ocean.

In April 1989 an acoustic experiment was carried out over the abyssal plain south of Madeira in which transmissions were made, for about an hour, at 482, 680, and 740 Hz from a ship steaming at 5 Kn to a receiving array towed by another ship 65 km away traveling on a parallel course at the same speed. The signals arrived by two paths, an upper path trapped in the surface duct and a lower path via the main sound channel. This paper describes the experiment and analyses of the intensity fluctuations in the signal received by the lower path. This allows the authors to investigate the horizontal structure of acoustic intensity fluctuations in the ocean when these are due principally to internal waves. This aspect of such acoustic intensity fluctuations has received little attention until now. The experimental results are compared with theoretical predictions based on the parabolic moment equations for propagation and scattering in randomly irregular media, and on the standard Garrett-Munk model for ocean internal waves. The experimental results and theoretical predictions agree quite well but the comparison also raises some new questions, in particular, about the correlation of intensity fluctuations as the acoustic transmission frequency is varied.

Optical properties of a planar turbulent jet.

A planar heated air jet was constructed. Its flow properties were characterized and shown to be both reproducible and in good agreement with the results of turbulence theory. The optical properties of the jet were studied with the help of a 632.8-nm He-Ne laser beam. The random phase modulations imposed on the wave front of the beam traversing the jet were measured by interferometric means, and their spectra and variance were determined. The one-dimensional phase fluctuation spectrum obeyed a -8/3 power law as predicted by theory, whereas the phase variance (?(2)) depended on the jet temperature and was studied for values to as high as 0.4 (rad)(2)).

Accurate numerical solutions for the frequency cross correlation of intensity fluctuations in a random medium.

The fourth-moment equation for the cross-frequency correlation of intensity fluctuations of a plane wave propagating in a random medium is solved numerically by using an adaptive grid method. A wide range of frequency ratios r and scattering strength parameters Gamma are considered, and approximate laws for the height and position of the peak of the covariance of intensities are obtained as functions of r and Gamma. Difficulties arising in the analytical solution of the fourth-moment equation are discussed.