Efficient computation of extended images by wavefield-based migration

Extended common-image-point (CIP) gathers can be constructed using wave-equation migration by preserving into the output image the non-zero spaceand timelags of the cross-correlation between the source and receiver wavefields. Correct wavefield reconstruction leads to focused events in the extended CIPs, while incorrect reconstruction leads to de-focused events that can be used for velocity model building. This approach limits the number of locations where the multi-lag crosscorrelation needs to be computed, thus reducing the computational cost of this method while preserving its capacity to indicate velocity errors. The cheaper cost is particularly important for wave-equation migration with wide-azimuth acquisition.