Geology-Driven Depth Stretching: An Alternative for Prestack Depth Migration? Experiences from the Espirito Santo Basin, Offshore Brazil

Stollenwerk, Matthias¹, Jürgen Altenkirch², Klaus Fischer³, A. Ewan Campbell³ (1) Wintershall AG, Moscow, Russia (2) ISI GmbH, Hannover, Germany (3) Wintershall AG, Kassel, Germany

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 (V_int) 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.