Nonreciprocal Green’s function retrieval by cross correlation

The cross correlation of two recordings of a diffuse acoustic wave field at different receivers yields the Green’s function between these receivers. In nearly all cases considered so far the wave equation obeys time-reversal invariance and the Green’s function obeys source-receiver reciprocity. Here the theory is extended for nonreciprocal Green’s function retrieval in a moving medium. It appears that the cross correlation result is asymmetric in time. The causal part represents the Green’s function from one receiver to the other whereas the acausal part represents the time-reversed version of the Green’s function along the reverse path.