Highly resolved detection and selective focusing in a waveguide using the DORT method

The D.O.R.T. method (French acronym for Decomposition of the Time Reversal Operator) is a scattering analysis technique using an array of transducers. The method is effective to achieve detection and selective focusing on pointlike scatterers through inhomogeneous media [J. Acoust. Sec. Am. 99, 2067-2076 (1996)]. Laboratory measurements in a water waveguide are presented. Taking advantage of the multiple reflections at the interfaces of the guide, high resolution is achieved with the D.O.R.T. method. The separation of two scatterers and the selective focusing are obtained with a transverse resolution at least nine times better than the free-space limit prediction. The detection of a scatterer from the water/air interface of the guide is also achieved with high resolution (1/20 of the free space diffraction spot). The effect on the D.O.R.T, method of surface waves produced at one interface of the guide is measured. Finally, the impulse response function of each scatterer to the array is computed as a combination of the eigenvectors of the time-reversal operator obtained at each frequency. Using these impulse Green's functions, selective focusing with high temporal and spatial compression is performed. (C) 1999 Acoustical Society of America. [S0001-4966(99)00505-6].