A New Scheme for Acoustical Tomography of the Ocean

Award #: N00014-95-F-0046 http://www.etl.noaa.gov LONG-TERM GOAL The long-term purpose is to develop a new scheme of the acoustical tomography of the ocean of mesoto global scales which is based on measurements of horizontal-refraction angle (HRA) related to different acoustic modes rather than travel time along different rays. OBJECTIVES To develop robust inversion scheme for retrieving 3-D ocean inner structure based on measurements of HRA. In spite of its small value HRA angle can be easily measured with the help of pair of moderesolving line vertical arrays situated about 10 km apart (ocean interferometer). As a first approximation adiabaticity of mode propagation should be assumed. Then the scheme should be generalized to the case of non-adiabatic propagation with mode interaction taken into account in a "N 2-D" approximation, and appropriate computer simulations should be performed. Scattering of acoustic signals from internal waves should be also considered, and its effect on the accuracy of sound speed field retrieval should be estimated. APPROACH A low frequency tonal sound source (F = 30-100 Hz) is assumed to be towed by a vessel around the area of interest with typical horizontal scale of the order of 1000 km. The transmitted signal is received by acoustic interferometers located inside or outside the area. Thus, the area is exposed from different directions, and HRA is known as a function of source position. Those data are then used for tomography inversion. In the general case, acoustic mode interaction due to water mass inhomogeneity should be taken into account. This is accomplished with the help of iterations. In the first approximation mode interaction is neglected, and HRA are inverted into sound speed profiles assuming adiabatic propagation. The inversion proceeds in two stages: 1) 2-D tomography which retrieves propagation constants of different modes at the nodes of horizontal rectangular grid covering the area. 2) 1-D tomography which retrieves sound speed profile (in terms of expansion with respect to a set of empirical orthogonal functions) at each node of horizontal grid based on already determined values of propagation constants. Then the contribution to horizontal refraction due to mode interaction are calculated with respect to retrieved inhomogeneous medium using propagation code which takes into account mode interactions in

Coupled mode transport theory for sound transmission through an ocean with random sound speed perturbations: Coherence in deep water environments

The article of record as published may be found at http://dx.doi.org/10.1121/1.4818779Second moments of mode amplitudes at fixed frequency as a function of separations in mode number, time, and horizontal distance are investigated using mode-based transport equations and Monte Carlo simulation. These second moments are used to study full-field acoustic coherence, including depth separations. Calculations for low-order modes between 50 and 250 Hz are presented using a deep-water Philippine Sea environment. Comparisons between Monte Carlo simulations and transport theory for time and depth coherence at frequencies of 75 and 250 Hz and for ranges up to 500 km show good agreement. The theory is used to examine the accuracy of the adiabatic and quadratic lag approximations, and the range and frequency scaling of coherence. It is found that while temporal coherence has a dominant adiabatic component, horizontal and vertical coherence have more equal contributions from coupling and adiabatic effects. In addition, the quadratic lag approximation is shown to be most accurate at higher frequencies and longer ranges. Last the range and frequency scalings are found to be sensitive to the functional form of the exponential decay of coherence with lag, but temporal and horizontal coherence show scalings that fall quite close to the well-known inverse frequency and inverse square root range laws.Office of Naval Research Ocean Acoustics Program Code (322)Office of Naval Research Ocean Acoustics Program Code (322