Abstract：The conversion of VSP data into offset depth domain can be achieved by either VSP CDP stacking or migration techniques. But conventional seismic migration techniques can hardly bring desirable VSP migration result because VSP data usually appear as common-shotpoint gather and contain less data volume. kirchhoff integral algorithm is extendedly applied to variant-velocity prestack migration of VSP data. The key to applying kirchhoff integral algorithm to VSP data lies in determining kirchhoff locus,which is equivalent to determining an isochronous locus whose foci are at the source point and the receiving point.This locus is different from the elliptic locus formed in constant velocity kirehhoff integral migration of conventional surface seismic data. The variant-velocity kirchhoff locus is determined using the ray equation. This migration method includes the same steps as usual prestack migration methods. The main steps are the determination of imaging time, record wavefield extrapolation using an approximate kirehhoff algorithm, summation for imaging, etc.
