Crosscorrelogram migration of inverse vertical seismic profile data

Abstract

We present the theory of crosscorrelogram migration of ghost reflections, also known as interferometric imaging, to delineate reflector geometries from inverse vertical seismic profile data. The theory includes the equations for forward modeling, migration, asymptotic inversion, and model resolution of crosscorrelgrams. Rather than using primary reflections, crosscorrelogram migration can use ghost reflections to reconstruct the reflector geometry. Other multiples can be used as well, including pegleg multiples and higher-order multiples. Its main advantages over conventional Kirchhoff migration are (1) both source location (e.g., drill-bit position) and source wavelet need not be known (such as when using a drill bit as a source in a deviated well), (2) it is insensitive to source-related static errors, and (3) it has wider subsurface illumination than conventional Kirchhoff migration of primary reflections. Crosscorrelation migration can effectively widen the source-receiver aperture by more than 50% compared to standard inverse vertical seismic profile (IVSP) migration. The primary disadvantages are (1) it uses ghost reflections for imaging, which can be weaker (or sometimes more distorted) than primary reflections; (2) crosscorrelation creates virtual multiples that can sometimes appear as coherent noise in the final image; and (3) it has poorer horizontal resolution than standard IVSP migration. Results from imaging simulated IVSP traces show that crosscorrelation migration produces reflectivity-like images that correlate well with the actual reflector geometry of a layered fault model. These images are almost completely immune to static errors at the source location and have wider subsurface illumination than conventional IVSP migration images. We also apply crosscorrelation migration to IVSP data recorded at a Friendswood, Texas, test site. Results show that the crosscorrelation image correlates better with the well log and wider subsurface illumination than a conventional Kirchhoff migration image.