The MATE acoustic frequency cross correlations of intensity

In the Mid‐Ocean Acoustic Transmission Experiment (MATE), measurements were made of the fluctuating phase and amplitude of signals propagated 18.1 km at four frequencies (2,4,8, and 12.5 kHz) over a wholly refracted Fermat path. Extensive oceanographic instrumentation of that transmission path permits evaluation of the space–time correlations of the index of refraction. The scattering regimes span the important region from well before the medium focus to beyond it, but do not extend to the very far field. In the past two years, solutions for the propagation of the fourth moment of the complex amplitude of a wave propagating in a random field have become available; these solutions are applicable at all ranges. Ewart and Reynolds discuss MATE and compare the acoustic results with theory for the intensity autospectra [J. Acoust. Soc. Am. 75, 785–802 (1984)]. The theoretical solutions have recently been extended to include acoustic frequency dependence. In this paper predictions from the new theory, based on space–time medium correlations where both internal waves and low wavenumber finestructure have been considered, are compared with the measured acoustic results from MATE. These comparisons include the cross correlations and cross spectra of intensity as a function of acoustic frequency separation. The agreement between theory and experiment is shown to be somewhat poorer for the cross spectra than for the autospectra. It is postulated that the lack of agreement arises either from the sensitivity of the cross spectral predictions to the precise form of the medium correlation function or from the fact that the first order correction to the evaluation of the fundamental solution (which has not been evaluated for the two frequency case) is more significant for the cross spectra of intensity than for the autospectra.