Given a frequency f, CDP interval Δx, and upper‐surface velocity v(0), the spatial Nyquist criterion defines a range of unaliased emergence angles for seismic reflection events. For migration, events outside that range should be discarded beforehand by dip filtering, or the migration operator should be trained to ignore them. In this paper, we express the spatial Nyquist criterion as a limitation on the CDP interval, rather than on emergence angle, which allows Δx to be adjusted as a function of emergence angle, increasing with decreasing emergence angle. We exploit this fact by creating two separate migration grids: a fine grid, with trace spacing Δx, for migrating events with large emergence angles; and a coarse grid, with trace spacing 2Δx, for migrating events with small emergence angles. Combining the two migrations is straightforward. The coarse grid migration, involving one‐half the number of input traces and one‐half the number of output traces, represents a considerable time savings over the small‐emergence‐angle portion of the migration on the original fine grid. The two migrations need not be performed separately, so that overhead associated with migration setup operations need not be paid twice. In fact, a modification of this idea allows the migration to be performed with almost no increase in storage requirements.
On the effect of emergence angle on emissivity spectra: application to small bodies