O.C. Rodríguez and S.M. Jesus
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SiPLAB-FCT, Universidade do Algarve
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Ref.: Proc. ECUA 2002, (ISBN 83-907591-8-7), p.515-520, Gdansk (Poland), June 2002.
Abstract : Ocean Acoustic Tomography is an important method developed in the context of Underwater Acoustics, oriented to the monitoring of ocean environments. The method intends to estimate the variations of the physical properties of an ocean waveguide as, for instance, temperature and sound speed, by taking advantage of the sensitivity of acoustic signals to those variations. A particular way to accomplish that estimation (known as "travel time inversion") consists in introducing a system of linear equations, relating an apriori known vector of delays in travel time, to an unknown vector of perturbations in sound speed, through the so-called "observation matrix". Inverting the system allows to estimate the perturbations in sound speed. Travel time inversion is reviewed in this paper by introducing a plane-wave representation of the sound speed field, expanded on a set of hydrostatic orthogonal modes, which can be accurately estimated from temperature. The representation allows to develope a range-dependent regularization of the system of equations, which can be extended in a natural way in order to handle multiple hydrophones. The reliability of the method is tested through its corresponding application to acoustic data acquired during the INTIMATE'96 experiment, whose environmental scenario was strongly affected by the propagation of internal tides. Inversion results, which took into account the effects of both barotropic and baroclinic tides on signal propagation, reveal an accurate agreement between expected and actual data, when compared with temperature and sound speed measurements acquired simultaneously near the acoustic source and the system of hydrophones. The results also indicate the method's ability in achieving a high degree of range dependent tomographic resolution when applied to shallow water short-range scenarios of acoustic propagation.
ACKNOWLEDGMENT: this work was partially supported by projects INTIMATE and ATOMS from FCT (Portugal).