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.