Résumé:
The decomposition of the time-reversal operator (DORT in French) is an active array detection technique.
It requires the measurement of the array response matrix K(/spl omega/) and consists in the analysis
of the eigenvalues and the eigenvectors of the time-reversal operator K*K which provides information
on the presence and localization of scatterers in the medium. It was shown that the DORT method
allows to separate and localize pointlike scatterers in a shallow water wave-guide
[J. Acoust. Soc. Am. Mordant et al. (1998) and Fotegot et al. (2003)]. Here, we extend the study to
the detection and frequency characterization of two spherical targets. Small scale ultrasonic experiments
are performed with a 3.9 MHz 24 elements transducer array and two spheres of 2 and 3 mm
diameter in a 31 mm deep wave-guide. These scatterers correspond to 15 < ka < 25 leading to a
non-isotropic scattered field. We have developed a theoretical model taking into account the wave-guide
and the acoustic properties of the spheres and using the partial waves decomposition of the scattered field.
We calculate the singular values of the array response matrix. This theoretical approach is in good agreement
with the experimental results. This 1/325/sup th/ scale ultrasonic experiment corresponds to a shallow water
experiment with a 12 kHz Vertical Linear Array (VLA).

Mots-clés: DORT method, Time-Reversal, targets detection, acoustic scattering, wave-guide