Theory and practice of refraction interferometry

The theory and practice of refraction interferometry is presented. We show that the crosscorrelation between a nearby pair of refraction traces yields a head wave event kinematically equivalent to one generated by a source at position x along the refractor. This redatumed source location x is independent of the surface source location, so that all head waves arrive at the same time in a redatumed common geophone-pair gather (CPG). Thus, the traces in a redatumed CPG can be stacked together to yield an N-fold refraction trace, and combining different trace pairs yields N-fold refraction shot gathers. The benefits are that traveltime picking errors can be greatly reduced for noisy head wave arrivals such as far offset refraction events or secondary head-wave arrivals. We also show that head-wave events in a redatumed common geophonepair gather will follow a flat trajectory in offset-time coordinates compared to the curved trajectory of a diving wave event. Thus, head waves can be distinguished from diving waves using redatumed CPGs, with the possibility of estimating the type of velocity gradient along an interface. This might be important for determining the lithology associated with important interfaces such as the Moho or an oil/gas bearing boundary.