Migration of shallow reflection data

We present an analysis of migration effects on seismic reflection images of very shallow targets. Reflection objectives in engineering, groundwater, and environmental investigations commonly occur at depths between 2 and 50 m. Because the observation surface is near these imaged reflectors, optical distortion may be so small that migration will not have any effect on the seismic image and may therefore be unnecessary. Also, because of low stacking velocities that are often encountered, vertical exaggeration of the shallowest parts of some seismic sections may make dips appear sufficiently steep to require migration. An interpreter looking at only the stacked, unmigrated section may erroneously conclude that the data should be migrated. Because shallow survey data are often processed on personal computers, unnecessary migration of a large data set can be prohibitively time-consuming and a needless use of resources. A simple set of calculations based on the “migrator’s equation” predicts whether migration of an arbitrary shallow seismic section is advisable.