VELOCITY AND INTERFACE DEPTH DETERMINATION BY TOMOGRAPHY OF DEPTH MIGRATED GATHERS

A method for velocity and interface depth determination based on tomog raphy of migrated common reflecting point (CRP) gathers is presented. The method is derived from the tomographic principle that relates trav eltime change along a given ray to perturbations in slowness and layer depths. The tomographic principle is used to convert depth errors in migrated CRP gathers to time errors along a CRP ray pair and thus enab le use of conventional traveltime tomography, It is also used to affec t a very fast prestack migration and set up the tomography matrix. The velocity-depth determination method uses the available offsets of all CRPs and inverts for the parameters of all layers simultaneously, Han d picking of depth errors on CRP gathers is avoided by a method where the tomography matrix operates directly on the migrated gathers. The v elocity-depth determination method is demonstrated on a synthetic exam ple and on a field example from the North Sea.