AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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3D velocity model building and seismic imaging combining tomography and model based approaches in the Peniche Basin


The deep water Peniche basin is located in the eastern North Atlantic offshore Portugal. This undrilled, unexplored area was recently surveyed with a high-resolution, narrow azimuth, Broadseis, 3200 km2 3D seismic survey. Since Stratigraphic features with salt-related structural component, salt-related structural traps and pinchouts against salt diapirs are present in this area; accurate imaging of the geological structures is a key factor to reduce the uncertainty of the potential prospects. Pre-stack depth migration helps to improve the seismic image around complex targets with sharp lateral velocity variations. However, high velocity salt body contrasts associated with steeply dipping complex-shaped structures, basement highs, carbonates layers, turbidites, numerous un-conformities and faults pose significant challenges for the imaging of this basin. Sediment sections show fairly large velocity variation from relatively consistent, slow, sediment basins on the south, to quite fast layers and faulted blocks on the north. The sea floor has also significant variations, associated with basement highs. Despite advances in migration algorithms, the derivation of a realistic earth model remains an important challenge, requiring tight integration of geologic interpretation and geophysical skills. While generic salt environment workflow to tackle such challenges involves several iterations of depth migration, model updating and picking of the top and base of the salt bodies; tomography methods alone fail at properly modelling deep, steeply dipping and poorly constrained complex geological structures in the salt overhangs or to accurately position high velocity carbonates contrast. Here an approach combining both tomography updates, interleaved with model based approaches is implemented, in an effort at stabilizing the deep overhangs velocity trend, to constrain the carbonate velocity contrast and to better image the deep salt and basement boundaries. The result of this approach largely improves the salt and carbonates geometries and the resolution and stability of the velocity model, thus leading to an improved final migrated image.