An underwater acoustic barrier based upon forward scattering in a Time-Reversal Mirror (TRM) was experimentally demonstrated for the first time in 2000 by Song et al.. The barrier consisted of a TRM, a vertical receive array (VRA) and a co-located probe source working at 3,500 Hz in an ocean waveguide near the western coast of Italy. In April 2003 further barrier tests were performed by applying for the first time at sea a new method that provided the capability to focus at different depths by using only the TRM transducers without the complication of additional probe sources. An echo repeater towed by R/V Alliance crossed the barrier, emulating the field forward scattered by a possible intruder insonified by the TRM. The presence of a target between the time reversal mirror and the focus can be detected if it significantly disturbs the quiescent region. A normal mode code is used to model the sound propagation in a waveguide. This is applied to predict the unperturbed and perturbed focused acoustic field measurements conducted at sea. Model-data comparison suggests that target detection performance is reasonably predictable using a numerical propagation model.
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