AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
The Impact of Imperfect Velocity Models and Seismic Artifacts on Interpretation of Louann Salt and Basement Structure in the Deepwater Gulf of Mexico
(1) Deepwater Exploration and Projects, Chevron, Houston, TX.
The application of better seismic acquisition and imaging technologies has dramatically improved our ability to characterize subsalt geometries in the Gulf of Mexico. In many areas deep Louann salt and basement reflectors are readily interpretable on pre-stack depth migrated seismic data resulting in the proposal of revised geologic models to explain observed geometries. While the extensive efforts applied to the subsalt imaging problem have resulted in significant improvements in velocity models and associated depth images, residual velocity model imperfections , persistent multiples, and over-migration in poor illumination areas contribute to interpretation uncertainty and errors in the present day geometries that define our geologic models. Fortunately, these errors and artifacts are often predictable allowing evaluation of their impact on alternative geologic models.
Imperfections in velocity models and persistent surface related and inter-bed multiples are a common source of uncertainty in deducing the most appropriate structural interpretation, particularly in areas of allochthonous salt. Minor errors in interval velocities are compounded with depth, often resulting in significant errors at the basement level. Sediment velocity estimates are typically more erroneous than estimates of salt canopy velocity creating steps in seismic images beneath abrupt changes in allochthonous salt thickness. In addition, multiples ringing to the base of the seismic volume are often found in areas with thin overburden over flat topped salt canopies and beneath strongly reflective supra-salt mini-basins, although wide-azimuth acquisition and state of the art multiple-suppression can dramatically reduce their impact on interpretation. Attempting to get beneath these persistent multiples can result in interpretation of localized structural lows or basement grabens that coincide with predictable multiple generators. Distinguishing between steps resulting from residual errors in our velocity models or persistent multiples and true steps in the base of Louann salt and basement requires an understanding of common sources of seismic artifacts. Geologists should consider the impact of predictable seismic artifacts and the implications of alternative geologic models when interpreting deep Louann salt and basement structure in the deepwater Gulf of Mexico.