The virtual refraction: useful spurious energy in seismic interferometry

Limitations in the source energy distribution in seismic interferometry lead to spurious energy in estimates of the Green's function between receivers. Instead of attempting to suppress spurious waves, we use one such spurious wave -- we call it the virtual refraction -- to infer subsurface parameters. Numerical and real data examples illustrate how the slope of the virtual refraction defines the velocity of the faster medium and the stationary phase point in the correlation gather provides an estimate of the critical offset. We illustrate the conditions for improvement in signal-to-noise of the virtual refraction, compared to conventional refraction analysis. This is due to the stacking of sources inherent in the method of seismic interferometry. Just as conventional refraction analysis, applications of this virtual refraction range from the near-surface (for example, mapping depth to the water table), to exploration seismology (aiding in the computation of static corrections), to mapping depth of crustal basins.