PRACTICAL ASPECTS OF THE DETERMINATION OF 3‐D STACKING VELOCITIES*

Proper stacking of three‐dimensional seismic CDP‐data generally requires the knowledge of normal moveout velocities in all source‐receiver directions contributing to a CDP‐gather. The azimuthal variation of the stacking velocities mainly depends on the dip of the seismic interfaces. For a single dipping plane a simple relation exists between the dip and the azimuthal variation of NMO‐velocity. Varying strike and dip of subsequent reflectors, however, result in a complex dependency of the seismic parameters.

Reliable information on the spatial distribution of the normal moveout (NMO)‐velocity can be derived from a wavefront curvature estimation using a 3‐D ray‐tracing technique. These procedures require additional information, e.g. reflection time gradients or depth maps to show interval velocities between leading interfaces. Moreover, their application to an extended 3‐D data volume is restricted by high costs.

The need for a routine 3‐D procedure resulted in a special data selection to create pseudo 2‐D profiles and to apply existing velocity estimation routines to these profiles. At least three estimates in different directions are necessary to derive the full azimuthal velocity variation, characterized by the large and the small main axis and the orientation of the velocity ellipse.

Errors are estimated by means of computer models. Stacking velocities obtained by mathematical routines (least‐squares fit) and by seismic standard routines (NMO‐correction and correlation) are compared.

Finally, a general 3‐D velocity procedure using cross‐correlation of preliminarily NMO‐corrected traces is proposed.