Subsalt imaging: Sensitivity to the velocity model

Abstract

Subsalt exploration is an area of increasing interest, but imaging under salt structures is still very challenging and an increased understanding of subsalt imaging is needed. The focus of this master’s thesis is on the sensitivity of subsalt imaging to the migration velocity model. The consequences of employing an erroneous migration velocity model have been studied. The kind of errors introduced to the velocity model include; vertical mispositioning of the rugous top and base of salt, lateral mispositioning of the base and neglection of salt inclusions. Simple 2-D models of an allochthonous salt sheet with typical parameters from the Gulf of Mexico were constructed. The models include rugous top and base of salt and dirty salt with anhydrite inclusions. Synthetic data was generated by the use of acoustic finite difference modelling, while reverse time migration was employed to produce images of the subsurface. The resulting synthetic depth sections showed that subsalt imaging is very sensitive to the top of salt interpretation, and the quality of this interpretation should therefore be the main focus in execution of a depth imaging project. Errors in depth positioning of the top or base of salt in the velocity model seem to be of secondary importance compared to errors in the salt geometry, and subsalt imaging is significantly more sensitive to the external salt geometry in the velocity model compared to the internal salt geometry. Improved methods for velocity model determination might be needed for adequate subsalt imaging in complex areas with rugous top of salt surfaces.