The virtual refraction: Useful spurious energy in seismic interferometry

Seismic interferometry is rapidly becoming an established technique to recover the Green's function between receivers, but practical limitations in the source-energy distribution in- evitably lead to spurious energy in the results. Instead of at- tempting to suppress all such energy, we use a spurious wave associated with the crosscorrelation of refracted energy at both receivers to infer estimates of subsurface parameters. We named this spurious event the virtual refraction. Illustrat- ed by a numerical two-layer example, we found that the slope of the virtual refraction defines the velocity of the faster me- dium and that the stationary-phase point in the correlation gather provides the critical offset. With the associated critical time derived from the real shot record, this approach includes all of the necessary information to estimate wave speeds and interface depth without the need of inferences from other wave types.