Discriminating physical and non-physical energy in wavefield interferometry

Source-receiver interferometry is a technique that allows the Green's functions between sources and receivers to be estimated by means of convolution and cross-correlation of other recorded wavefields. Source-receiver interferometry has been observed to work surprisingly well in practical applications when theoretical requirements (e.g. closed surrounding boundaries of other sources and receivers) are contravened: this paper contributes to explain why this may be true. Commonly-used inter-receiver interferometry requires wavefields to be generated around specific stationary points on the boundaries which are controlled purely by medium heterogeneity and receiver locations. By contrast, we show that source-receiver interferometry constructs at least kinematically correct physically scattered waves between a source and receiver by convolution of scattered data from and to any and all points on the boundary. This reduces the ambiguity in interpreting wavefields generated using source-receiver interferometry with only partial boundaries (as is standard in practical applications), as it allows spurious or non-physical events in the constructed Green's function to be identified and either interpreted or ignored.