Caractérisation et détection de cibles en guide d'ondes non stationnaire par Décomposition de l'Opérateur de Retournement Temporel

Abstract : Time reversal is a multi-element technique allowing for focusing in an unknown heterogeneous medium by using the time-reversal invariance of the wave equation. The matrix formulation of Time reversal gave birth to a multi-target technique, the DORT method (French acronym for Decomposition of the Time-Reversal Operator). This thesis investigates into the application of the DORT method to underwater acoustics. In order to emit the eigenvectors given by the DORT method, we propose a new method to reconstruct the temporal eigenvectors called SVP for EigenVectors Synchronization. This method is based on a numerical backpropagation in free space of the monochromatic eigenvectors needing little information on the medium. Instantaneous acquisition of the multistatic data matrix K with orthogonal codes are also investigated into. We show that the DORT method provides a mean to extract without any a priori knowledge the frequency signature of a target in a waveguide, despite the blurring caused by multipath. Indeed, the first eigenvalue is proportional to the monostatic frequency response of the target. Moreover, we show that the other eigenvalue contain additional information on the radiation pattern. Then we look into a problem specific to the application of the DORT method at sea. Due to constant fluctuation, the first principle of Time-reversal based techniques, which is a stationary medium, is often faulty. We show here that the DORT method can be used in a non stationary medium, in the case of a target moving vertically or a waveguide having sea surface waves. The study of the eigenvectors allows then to retrieve the positions of the target during the acquisition or the surface state over the target
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Submitted on : Wednesday, December 9, 2009 - 8:00:00 AM
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Franck-David Philippe. Caractérisation et détection de cibles en guide d'ondes non stationnaire par Décomposition de l'Opérateur de Retournement Temporel. Physique [physics]. Université Paris-Diderot - Paris VII, 2008. Français. ⟨pastel-00005606⟩

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