The primary zero-offset reflection of a point source from a smooth reflector within a laterally inhomogeneous velocity earth model is (within the framework of ray theory) defined by parameters pertaining to the normal-incidence ray. The geometrical-spreading factor--usually computed along the ray by dynamic-ray tracing in a forward-modeling approach--can, in this case, be recovered from traveltime measurements at the surface. As a consequence, zero-offset reflections can be time migrated such that the geometrical-spreading factor for the normal-incidence ray is removed. This leads to a so-called 'true-amplitude time migration.' In this work, true-amplitude time-migrated reflections are obtained by nothing more than a simple diffraction stack essentially followed by a time derivative of the diffraction-stack traces. For small transmission losses of primary zero-offset reflections through intermediate-layer boundaries, the true-amplitude time-migrated reflection provides a direct measure of the reflection coefficient at the reflecting lower end of the normal-incidence ray. The time-migrated field can be easily transformed into a depth-migrated field with the help of image rays.

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