  When averaged over sources or disorder, cross-correlation of diffuse fields
yield the Green's function between two passive sensors. This technique is
applied to elastic ultrasonic waves in an open scattering slab mimicking
seismic waves in the Earth's crust. It appears that the Rayleigh wave
reconstruction depends on the scattering properties of the elastic slab.
Special attention is paid to the specific role of bulk to Rayleigh wave
coupling, which may result in unexpected phenomena like a persistent
time-asymmetry in the diffuse regime.
