SEPARATION OF INTERFERING ACOUSTIC SCATTERED SIGNALS USING THE INVARIANTS OF THE TIME-REVERSAL OPERATOR - APPLICATION TO LAMB WAVES CHARACTERIZATION

The D.O.R.T. method (in French, Decomposition de l'Operateur de Retour nement Temporel) is a scattering analysis technique using arrays of tr ansducers. The method was shown to be effective in detecting and focus ing on pointlike scatterers in Prada et al. [J. Acoust Sec. Am. 99, 20 67-2076 (1996)]. Here the D.O.R.T. method is extended to other geometr ies, applying it to an air-filled cylindrical, shell embedded in water . It is shown that the diagonalization of the time-reversal operator p ermits the various elastic components of the scattered field to be ext racted. For the considered cylinder, these components are mainly three circumferential waves (A0, A1, and S0 Lamb modes). Each Lamb mode is shown to correspond to an invariant of the time-reversal operator. The dispersion curves of these waves are calculated from the invariants. In particular, the cutoff frequency of the A1 mode is found and provid es the thickness of the shell. Finally, resonance frequencies of the s hell are deduced from the frequency dependence of the eigenvalues of t he time-reversal operator. (C) 1998 Acoustical Society of America. [S0 001-4966(98)03408-0]