PUSHING THE LIMITS OF SEISMIC IMAGING .2. INTEGRATION OF PRESTACK MIGRATION, VELOCITY ESTIMATION, AND AVO ANALYSIS

The author proposes an operator-driven prestack migration scheme that is based on the synthesis of common focus-point (CFP) gathers. Each CF P gather represents the response of a synthesized source array that ai ms at the illumination of one subsurface gridpoint (focus point). The involved synthesis operator is referred to as the focusing operator, I f the time-reversed focusing operator and its related focus-point resp onse have equal traveltimes, then the underlying macro velocity model is correct and the focus-point response in the CFP gather is stacked b y weighted addition along the common traveltime curve (CFP-stacking), yielding the prestack migration result at the subsurface grid point un der consideration. If the time-reversed focusing operator and its rela ted focus-point response have different traveltimes, then the underlyi ng macro velocity model is incorrect and the correct focusing operator can be derived from the two traveltime curves. A simple updating proc edure is proposed. The total CFP migration process of synthesis, updat ing, and stacking is repeated for all subsurface grid points of intere st,leading to the prestack migration result in one-way image time toge ther with a distribution of updated focusing operators, In a postproce ssing step, all operator traveltime information can be used to derive a velocity model for the time-to-depth conversion process. Hence, in t he presented ''CFP technology'' the author proposes to estimate the ve locity model from the correct focusing operators by a global inversion process after the migration process has been carried out (''beyond de pth migration''). For each subsurface grid point, the amplitudes along the pairs of updated traveltime curves provide amplitude-versus-offse t (AVO) information, In addition. by introducing the grid-point gather with the aid of an extension of the second focusing process, the auth or shows that this gather leads to the extraction of pre- and postcrit ical amplitude-versus-ray parameter (AVP) information at each grid poi nt. Finally, just as a velocity model can be estimated from all grid-p oint-oriented traveltime information, a lithology model can be estimat ed from all grid-point-oriented amplitude information by a postimaging global inversion process.