Passive imaging is a new technique which has been proved to be very
efficient, for example in seismology: the correlation of the noisy fields,
computed from the fields recorded at 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.Comment: Invited paper to appear in NONLINEARITY; Accepted Revised versio