3C-3D VSP: Normal moveout correction and VSPCDP transformation

Jitendra S. Gulati, Robert R. Stewart, Janusz Peron, John M. Parkin

In this paper, we address issues related to the normal moveout correction and VSPCDP transformation of 3C-3D VSP data. A proof is presented to show that in a horizontally layered isotropic earth the traveltime-distance (t-x) formulae for signals recorded in a VSP survey is hyperbolic. Synthetics corrected using amplitude semblance analysis show that events are corrected to normal incidence times with remarkable accuracy. Data from the Blackfoot 3C-3D VSP survey corrected to normal incidence time after amplitude semblance analysis show good correlation with that corrected using raytracing. A Dix formula is then used to estimate interval velocities below the total depth of the well from the stacking velocities obtainedduring the moveout correction.

The stacking velocities obtained during the moveout correction are used in an approximate VSPCDP transformation of the VSP data. Comparison of reflection points mapped using the approximate method with those using a raytracing methodfor synthetics show that the approximate method maps the reflections accurately for all practical purposes. The same approximate procedure applied to the Blackfoot 3C-3D VSP resulted in a 3-D image very similar to that obtained using a raytracing method. These methods developed for a horizontally layered isotropic earth are an efficientway to obtain high-resolution 2-D/3-D seismic images in simple geologies. In complex geologies, however, they could precede more accurate imaging methods like migration to give a rough idea about the geology.