Best Practices for Sub-Salt Imaging

By

 Nick Bernitsas¹, Paul Farmer¹, Ian Jones², David Shope¹

(1) GX Technology, Houston, TX (2) GX Technology, Egham, Surrey, United Kingdom

 Exploration and development of sub-salt prospects has been a very difficult and risky endeavor because of the difficulty in obtaining a good image of the sub-salt sediments. However, recent advances in computer technology coupled with enhanced and varied depth imaging algorithms have greatly improved the quality of the sub-salt imaging.

To properly image sub-salt sediments, we need to honor the complexities in the propagating wavefield that are generated by the high velocity contrast between the salt and the surrounding sediments. To do so requires building an accurate velocity model that includes: a) the structural details of the top and base salt surfaces, and b) detailed sediment velocities above and below the salt.

Obtaining an accurate and detailed suprasalt velocity model requires sophisticated tools such as tomography. Determination of the salt morphology requires full volume imaging so that small surface undulations are sampled in detail. This imaging method can be either Kirchhoff-based for detecting the existence of very steep or overturned salt flanks or wave equation-based for properly imaging the base salt in the presence of a complex top of salt surface. Such hybrid imaging workflow exploits the best features from each imaging algorithm. Finally, given the poor signal-to-noise ratio beneath the salt, sub-salt sedimentary velocity analysis can only be done in the context of analyzing the seismic image rather than gathers.

The extent to which an interpretable image can be obtained ultimately depends on whether sub-salt reflections have been recorded in the first place. Sophisticated wave equation-based illumination analysis tools can be used to analyze an existing or any planned survey in the context of sub-salt illumination of sedimentary reflectors.