Depth Stretching: An Alternative for Prestack Depth Migration? Experiences from
Matthias1, Jürgen Altenkirch2, Klaus Fischer3,
A. Ewan Campbell3 (1) Wintershall AG,
Moscow, Russia (2) ISI GmbH, Hannover, Germany (3) Wintershall AG,
Shallow subsurface velocity anomalies in a PrSTM 3D seismic cube in the Espirito Santo Basin, Brazil, resulted in severe velocity distortions of underlying targets. With no wells in the block, the initial depth-conversion model was derived from processing velocities only, constrained using regional well data. The initial model did not account for lateral heterogeneities in the shallow geology, hence, structural anomalies were introduced into the depth image. There were three main sources for the heterogeneities: (1) Shallow channels / canyons alternating with (2) Shallow carbonate platforms; and (3) Modern shelf edge, where the shelf deepened from 60m to almost 1000m over a distance of 5km. An iterative approach was adopted. The interval of an anomaly was defined and an initial V0k function calculated. Detailed seismic stratigraphic analysis was applied to map the extent of shallow channels and carbonates. An arbitrary interval velocity (Vint) was then applied for the channels and carbonate build-ups. The resulting time-depth pairs were then used to generate a new V0, keeping k constant. The process was repeated until the overprint of the shallow anomaly was minimized for deeper levels. A similar process was used for anomalies related to the shelf edge. Application of a wedge model, allowing for both sea-water velocities and differential compaction of slope sediments to be taken into account. A total of 8 iterations were performed. The shallow subsurface velocity model resulted in a time depth conversion showing the detailed facies geometry of an underlying carbonate platform.