Multishot prestack depth migration

The method of reverse-time migration is widely known for high-quality results as well as extremely high computational costs. However, as a result of a number of algorithmic modifications which have increased its computational efficiency by orders of magnitude, it is now routinely used for migrating exploration-scale 3-D poststack data. We present two different algorithms aimed at improving the computational efficiency of this method for migrating prestack data. In both cases, we pursue the strategy of performing migration on a number of shot gathers simultaneously. In the first case, we compute a time image matrix for each shot gather but perform migration on a seismogram obtained by summing the various shot gathers. The second algorithm is quite similar except that we use only one time image matrix obtained by exploding simultaneously all the sources along the surface of the velocity model and recording thefirst arrival time of the wave train at each grid point. Our results which include both synthetic and real 2-D data are very encouraging and point to a reduction of computational effort by a factor of about 12.5. For 3-D problems, this implies speedup by afactor approaching 156.