Sound propagation through a fluctuating stratified ocean: Theory and observation

Abstract

We have derived expressions for the mean-square phase and intensity fluctuations and their spectra for cw sound propagating through a channeled fluctuating ocean. The ’’supereikonal’’ approximation reduces to the geometric optics (eikonal) limit for short acoustic wavelengths: λ≪2πL2H/R and λ≪L2V/(R tan2ϑ), where LH and LV are horizontal and vertical correlation lengths of the fluctuations, R is range, and tanϑ is the ray slope, replacing the traditional (and much more severe) Fresnel condition λ≪2πL2/R for a homogeneous isotropic ocean. The results can be expressed in closed form for an exponentially stratified ocean model and associated ’’canonical sound channel,’’ with superimposed fluctuations from an internal wave model spectrum based on oceanographic observations. The parameters are the stratification scale B, the inertial and buoyancy frequencies ωin and n (z), the scale j* of internal wave mode numbers, and the internal wave energy per unit area. The results are in reasonable agreement with numerical experiments based on the parabolic wave equation. For the ’’singlepath’’ 4-kHz transmission over Cobb Seamount the observed and computed rms fluctuations in phase are 1.6 and 2.5 cycles, respectively; in intensity these are 5.5 and 2.2 dB, respectively, with anomalous intensities measured at high frequencies (’’sporadic’’ multipathing?). For the multipath 406-Hz MIMI transmission, we obtain 4×10−3 and 5×10−3 sec−1, respectively, for the experimentally determined and the computed rms phase rates. Subject Classification: [43]30.20, [43]30.40; [43]20.15.