We introduce the concepts of waveform correlations, and the way in which they can be exploited to extract information from seismograms.We show how correlation procedures can be used to determine time and phase delays. We then consider the closely related topic of transfer functions between aspects of the wavefield, and this leads into a discussion of the ways by which seismograms can be compared – a topic of importance in the comparison of observations and simulations. We also consider the nature of receiver functions and the correlation of teleseismic signals at a receiver to yield information on local structure.

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.