Thus it is a convolution in both time and x . More precisely, it is the Huygens secondary wave source that was exemplified by ocean waves entering a gap through a storm barrier. Adding up the response of multiple gaps in the barrier would be convolution over x .
Migration is more than downward continuation of wave fields.
In practice, the downward continuation is done in the frequency domain. Recall to find the zero frequency component we sum over all time t . A nuisance of using Fourier transforms in migration and modeling is that spaces become periodic. This is demonstrated in Figure 5 . Anywhere an event exits the frame at a side, top, or bottom boundary, the event immediately emerges on the opposite side.