Pushing the limits of seismic imaging .1. Prestack migration in terms of double dynamic focusing

In this paper, the author proposes to extend the synthesis of areal sources (controlled emission) to the synthesis of areal detectors (controlled detection) such that the concept of numerical focusing can be formulated as a special version of target-oriented synthesis. As a consequence, the insight in the complex prestack migration process can be improved significantly by making use of the concepts ''focusing in emission'' and ''focusing in detection.'' Focusing in emission transforms shot records into so-called common focus-point (CFP) gathers. Focusing in detection transforms CFP gathers into the prestack migration result. If structural information is sought, the focus point in emission is chosen equal to the focus point in detection: confocal version of CFP migration. If rock and pore information is required as well, the focus point in emission is chosen different from the focus point in detection: bifocal version of CFP migration. Errors in the underlying macro velocity model can be better analysed than before by using CFP gathers as an intermediate migration output. The error analysis involves a comparison between each CFP gather and its related focusing operator. The quality (amplitude accuracy, noise content, resolution) of prestack migration results can be evaluated effectively at each subsurface grid point by analysing the two focused beams involved (pre-evaluation) and by analyzing the so-called grid-point gather (post-evaluation). The proposed pre-evaluation method may lead to an improved way of coping with ''acquisition footprints'' of relatively sparse source and receiver coverage.