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Current migration methods face limitations in the presence of complex, steeply dipping reflectors such as those found on salt flanks. Reverse time migration (RTM) overcomes these constraints, enabling structures with dips greater than 90 degrees to be properly imaged.
Standard wave equation techniques utilize mathematical approximations which assume that wavefields propagate in only one direction – down for the source wavefield and up for the receiver (or scattered) wavefield. The integrity of these wave equation approximations breaks down as dip increases; for dips greater than 70 degrees, wave equation techniques cease to be applicable. In these circumstances, geophysicists are forced to revert to the Kirchhoff technique, introducing another series of constraints and compromises.
 RTM provides an alternative approach to migration with fewer compromises. RTM works by running the wave equation forward in time for the source and backwards in time for the receiver. RTM properly propagates the wavefields through the most complex velocity regimes, including sub salt, for structures having dips in excess of 70 degrees, and in the presence of reflection boundaries that may generate internal multiples.
Although RTM is not a new concept, its application has been limited due to lack of computational power needed to run the RTM algorithms cost effectively and in a timely manner. ION's GX Technology Imaging Solutions (GXT) group has enhanced the RTM approach to improve its efficiency and has applied the technique on over 60 projects worldwide, including several in the Gulf of Mexico, North Sea and offshore West Africa.
As noted in the example below, the improvements to image quality can be significant. The application of GXT’s RTM demonstrates the uplift in image quality of the subsalt sediments, as well as the salt boundary.
| Typical PreStack Depth Imaging |
GXT RTM |
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