Derivation of seismic depth sections

The purpose of the seismic processing step "migration" generally is to present a reliable section of the subsurface with respect to the correct spatial location of reflecting elements. The procedure is usually performed in the time or frequency domain. The transformation to the depth domain requires the knowledge of the underlying velocity model. A simple depth conversion of the time scale is a very limited procedure and fails completely in the presence of dipping overburden layers. Substantial lateral velocity variation already falsifies the result of time migration as refraction of rays is normally not considered in this process. The error depends on the amount of refraction and the depth interval between the refracting and the reflecting interface. If steep and/or conflicting dips are involved in the data a special dip moveout (DMO) processing is required to improve the stacked data for migration. Wave theoretical depth migration takes the effect of the refraction of rays into account by incorporating the so-called thin-lens term in the migration algorithm. This technique solves both the imaging and lateral positioning problems. For evaluation of a proper depth dependent velocity field an interactive procedure is suggested by applying a horizon migration based on a ray tracing method. The resulting velocity distribution is then used for the wave equation migration of seismic data leading to more reliable depth sections. The effectiveness of the method is illustrated by a sequence of field examples