Abstract

The author proposes an operator-driven prestack migration scheme that is based on the synthesis of common focus-point (CFP) gathers. Each CFP gather represents the response of a synthesized source array that aims at the illumination of one subsurface gridpoint (focus point). The involved synthesis operator is referred to as the focusing operator. If the time-reversed focusing operator and its related focus-point response have equal traveltimes, then the underlying macro velocity model is correct and the focus-point response in the CFP gather is stacked by weighted addition along the common traveltime curve (CFP-stacking), yielding the prestack migration result at the subsurface grid point under consideration. If the time-reversed focusing operator and its related focus-point response have different traveltimes, then the underlying macro velocity model is incorrect and the correct focusing operator can be derived from the two traveltime curves. A simple updating procedure is proposed.

The total CFP migration process of synthesis, updating, and stacking is repeated for all subsurface grid points of interest, leading to the prestack migration result in one-way image time together with a distribution of updated focusing operators. In a postprocessing step, all operator traveltime information can be used to derive a velocity model for the time-to-depth conversion process. Hence, in the presented “CFP technology” the author proposes to estimate the velocity model from the correct focusing operators by a global inversion process after the migration process has been carried out (“beyond depth migration”).

For each subsurface grid point, the amplitudes along the pairs of updated traveltime curves provide amplitude-versus-offset (AVO) information. In addition, by introducing the grid-point gather with the aid of an extension of the second focusing process, the author shows that this gather leads to the extraction of pre- and postcritical amplitude-versus-ray parameter (AVP) information at each grid point. Finally, just as a velocity model can be estimated from all grid-point-oriented traveltime information, a lithology model can be estimated from all grid-point-oriented amplitude information by a postimaging global inversion process.

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