Algorithm providing breakthroughs for subsalt imaging

CGGVeritas recently provided an update of their algorithm aimed at shedding greater light on subsalt structures obtained from wide- and multi-azimuth acquisitions. Its Reverse Time Migration incorporating Tilted Transverse Isotropy, known as TTI RTM, is becoming the algorithm of choice for complex salt bodies around the world.

TTI anisotropic migration is a method to image structures lying beneath dipping anisotropic overburdens. Until recently, it has been limited to ray-based algorithms, such as isotropic or Vertical Transvers Isotropy (VTI) implementations. However, CGGVeritas’ TTI RTM takes data acquired from wide-azimuth techniques to derive reliable and accurate TTI anisotropic parameters that enhance image quality, the company states.

A TTI model is able to flatten image gathers regardless of azimuth, unlike VTI anisotropic velocity models. This results in velocity models that produce more geographically viable subsalt structures with better focusing, illumination, and positioning. In addition, the TTI RTM provides more accurate depth and amplitude control beneath complex salt bodies.

CGGVeritas routinely uses TTI RTM for production imaging as part of a subsalt imaging process that includes wide-azimuth acquisition and true 3D processing. The company has acquired multiclient and proprietary data for several hundred Gulf of Mexico lease blocks using wide-azimuth techniques, and reportedly has seen significant subsalt imaging improvements for several recent major discoveries, such as the Chevron-operated Jack field and BP’s Tiber.

Isotropic RTM (left) and TTI anisotropic RTM (right) of a line near the Jack discovery. TTI RTM produces more continuous subsalt images than isotropic RTM does.

The algorithm has provided similar imaging improvements for narrow-azimuth surveys such as those going on in the Santos Basin, offshore Brazil. The dip angles of some deep basins in the area can reach more than 50 degrees, too severe for VTI models to accurately process but within TTI RTM’s range of applicability. Implementing the algorithm also ensures that anisotropic effects are not ignored, which avoids incorrectly positioning salt flanks and distorting the base of salt and presalt structures.

The company states that the use of TTI RTM has significantly improved the focusing and positioning of data from west Africa and the North Sea as well, and is expected to be of benefit anywhere where imaging is required beneath dipping sedimentary layers.

TTI RTM is part of the range of advanced imaging and true 3D wide-azimuth processing algorithms which make up CGGVeritas’ geovation platform, seismic data-processing and imaging technologies that aim to serve as the industry reference for geophysical technology, advanced interactive features, and efficient processing of ever-increasing wide-azimuth data volumes.

More information can be found at the CGGVeritas website.