We address the nature of the correlation wavefield and its relation to the group of techniques collectively known as seismic interferometry.We establish a direct representation of the cross-correlation of the seismic signals between two stations and show how, with a suitable distribution of sources, this correlation can provide a virtual source-receiver pair whose phase properties arise from differencing.We then discuss the concept of generalised interferometry with an arbitrary distribution of sources, and illustrate the way in which processing procedures can affect the nature of correlated signals

The autocorrelation of the signals at a seismic station corresponds to the scenario with a coincident virtual source and receiver. For energy arriving from distant sources, the auto-correlation can be related directly to the structural reflectivity beneath the receiver. We show how the auto-correlation of seismic signals can provide information on reflections without conversions, and can be exploited to provide indirect imaging of heterogeneous structure. Correlations between signals at different sensors can also be exploited in reflection work to provide virtual sources that provide new ways of imaging complex structure.