The emergence of spurious arrivals in Green’s function extraction and passive imaging

Abstract

Cross-correlations of ambient noise at two receivers can extract the two-point Green’s function, given that the wave-field is spatially uniform. The presence of scatterers that act as uncorrelated secondary sources can destroy this condition. We retrieve the Green’s function in one-sided noise with a scatterer in an aeroacoustic experiment, using a pair of microphones (separation 1 m) parallel to the beach which generate one-sided surf noise. The scatterer is a 20-cm-radius polyvinyl chloride pipe 2.5 m to the landside of the microphone pair. We cross correlate 400–2000 Hz noise to retrieve the Green’s function. The results show that spurious scattered arrival emerges in the cross-correlation functions when the source distribution is limited. This causes the generalized optical theorem to break down, and thus, there is no guarantee that the spurious scattered arrival cancels. However, the spurious waves have a geometric interpretation, which are useful for inversion. By combining the travel times of the spurious and physical scattered waves, only a pair of receivers is sufficient to locate the scatterer passively.