  In time reversal acoustics experiments, a signal is emitted from a localized
source, recorded at an array of receivers-transducers, time reversed, and
finally re-emitted into the medium. A celebrated feature of time reversal
experiments is that the refocusing of the re-emitted signals at the location of
the initial source is improved when the medium is heterogeneous. Contrary to
intuition, multiple scattering enhances the spatial resolution of the refocused
signal and allows one to beat the diffraction limit obtained in homogeneous
media. This paper presents a quantitative explanation of time reversal and
other more general refocusing phenomena for general classical waves in
heterogeneous media. The theory is based on the asymptotic analysis of the
Wigner transform of wave fields in the high frequency limit. Numerical
experiments complement the theory.
