  Passive imaging is a new technics which has been proved to be very efficient,
for example in seismology: the correlation of the noisy fields between
different points is strongly related to the Green function of the wave
propagation. The aim of this paper is to provide a mathematical context for
this approach and to show, in particular, how the methods of semi-classical
analysis can be be used in order to find the asymptotic behaviour of the
correlations.
