MAP MIGRATION OF THE PANDORA DIAPIR IN THE NORDKAPP BASIN, BARENTS SEA
Master thesis
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http://hdl.handle.net/11250/239558Utgivelsesdato
2011Metadata
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Sammendrag
Seismic imaging in the Nordkapp Basin in the Barents Sea involves many problems, which are not easily solved with conventional imaging methods. The basin has complex geometry as it consists of several salt structures. The conditions are favorable to generate strong multiples, as the structures are located just beneath the seabed and cause much energy generated from the seismic source to be reflected overcritical. Since the water depth is rather shallow in this area and the absorption rate in water is low the overcritical energy will generate strong multiples which will disturb the data enormously. The seabed and top salt is rugose and will set up multiple diffractions which are difficult to handle with conventional multiple removal algorithms. In addition the contrast in acoustic impedance between the salt and its surrounding sediments are low, and will cause the salt-sediment interface to be recorded as a weak signal. The phenomena described above will set up a shadow zone around the salt structure and its shape will not be observable.
This work, focused around the Pandora Diapir, investigates if map-migration of horizons interpreted on unmigrated seismic data will be able to give new information of the salts shape. The map-migration will be performed by a ray tracing algorithm down to the horizons of interest where interpreted time maps and interval velocities will act as input. The velocities are determined from available well logs and check shots. In the migration one assumes no lateral velocity changes in the velocity zones. The effect of the existing lateral velocity gradient will be demonstrated by increasing the velocity in all zones with a factor corresponding to this gradient. This is crucial as the horizons hold significant dips towards the salt structure.
The results conclude that map-migration will provide new information of the salt structure. Compared with existing Kirchhoff time-migrated data the salt-sediment interface is located at a different position. To get a better and more correct result a more sophisticated velocity model, which properly considers the lateral velocity changes towards the salt, should be used.