Obtaining the ocean index of refraction spectrum from the acoustic amplitude fluctuations

Abstract

Ewart and Reynolds [Wave Propagation in Random Media (Scintillation), edited by Tatarski, Ishimaru, and Zavorotny (SPIE, Bellingham, WA, 1993), pp. 100–123] provide a convincing comparison between the ocean spectral model determined from a stochastic inverse using the acoustic phase spectrum (measured in the Mid-ocean Acoustic Transmission Experiment) and the same model derived from oceanographic data. The success of using the acoustic phase for such inverses is based on the high accuracy of Rytov theory to predict the phase spectrum. Accurate measurement of the acoustic phases (travel times) requires fixed sources and receivers, an experimental complication. If it were possible to use Rytov theory for the amplitude fluctuations, the experiment complexity is much reduced and simpler moorings could be used. In this talk results are presented from a study of the viability of this approach using numerical simulations of propagation through internal waves to a vertical array of receivers. With the only requirement being applicability of the Rytov approximation, two-dimensional (depth/time) correlations of the oceanographic model are obtained by stochastic inversion and compared to the input internal wave model. The importance of this result to augmenting oceanographic measurements, and a discussion of the future possibilities, are presented. [Work supported by ONR.]