Migrating around in circles

Given the identified and picked primary reflection arrivals on a CMP stack section (or a zero-offset section), it is well known (assuming a constant macro-velocity model) that the depth-migrated subsurface image of each point on the picked reflection time curve is a half circle in the depth domain. As a consequence, the poststack (or zero-offset) depth-migrated subsurface image of the reflection-time curve (upper half of Figure 1) is the envelope of all half circles (lower half of Figure 1). These half circles, also known as isochrons or aplanats, are explained in all texts about seismic migration and are quite familiar to geophysicists.