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Prestack Time Migration

We apply GXT Kirchhoff PreSTM, an amplitude-preserving routine that is ideal for imaging complex geologic conditions or velocity fields, which, however does not require PreSDM solution. The GXT methodology is not constrained by the straight ray approximation of most commercial PreSTM software packages. Instead, a turning ray approach is used that can image very steep or overturned seismic events. Capable of accounting for ray bending at angles up to 150 degrees, GXT's technique ensures there is no bias in the velocity estimates or in the positioning of steep dips. The GXT PreSTM algorithms offer superior amplitude and bandwidth preservation. The results are better event coherency, improved lateral resolution, and gathers that are ideally suited as inputs into AVO analysis, especially in areas of complex geology.

Hybrid-Gridded Tomography Approach for Velocity Model Building

The ION GXT gridded tomography iterative approach for velocity model building is particularly suitable for large-scale offshore projects. The gridded tomographic solution is the best approach in compaction driven environments or in poorly illuminated areas where picking layers is inappropriate. For the areas with stratigraphically bound rapid lateral velocity changes we offer the hybrid-gridded method.

The iterative tomographic process is characterized both by velocity updates and subsequent adjustment of seismic reflectors location, stepwise improves image quality helping to define subtle stratigraphic and structural features. The technology proved its effectiveness in the areas with gas chimneys, gas clouds, pull-up structures etc.

LARGEO in cooperation with ION GXT introduced the implemented gridded and hybrid tomography approaches on the Russian market and implemented this technology for large-scale marine projects on the Black Sea. The results demonstrate the technical ability of LARGEO to complete 3-6 iterations for 1000 sq/km for the period less the 1-2 months.

Pre-Stack Depth Migration

GX Technology’s Kirchhoff PreSDM algorithm implementation is significantly faster than other industry Kirchhoff tools. As a result, a “no compromise” approach is employed in selecting depth imaging parameters to migrate the seismic data, resulting in higher quality subsurface images in less time. The method provides amplitude preservation and can depth-image very steep or overturned seismic events and takes into account subsurface anisotropy. The input consists of a 3D interval velocity model and pre-processed time gathers. The velocity model is smoothed and travel-time tables are constructed using full 3D raytracing.

Travel times can be selected by using any of the following three criteria: highest amplitude, shortest ray path, and first arrival.

The GXT’s PreSDM Kirchhoff algorithm can be used to iteratively build 3D velocity models or to validate existing 3D velocity models through targeted output in the form of inlines, crosslines, CRP gathers, full 3D volumes, and volume “slabs”. This output is then used as the basis for further velocity analysis in our internally-developed velocity model builder tools. Once the final 3D velocity model has been validated, our Kirchhoff migration method is applied to the entire dataset to generate a full 3D prestack depth volume.

Anizotropic Migration

GXT’s PreStack time and depth migrations proved to be very effective for imaging of complex geologic conditions.  The tuned algorithms and powerful computer resources made possible implementation of compute intensive anisotropic algorithms with which improves image quality by taking anisotropy into account, allowing geoscientists to more accurately map subsurface features, identify fault and fracture patterns and optimize drilling locations.


Effect of Anisotropy on Velocity Analysis:

а) – isotropic migration, velocity analysis with muting

b) - isotropic migration, velocity analysis witouth muting

в) - anisotropic migration, velocity analysis without muting.

RTM Migration

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 compromises, enabling structures with dips greater than 80 degrees to be properly imaged. RTM properly propagates the wavefields through the most complex velocity regimes, including subsalt, for structures having dips in excess of 80 degrees. 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. GXT’s RTM was developed by matching highly sophisticated algorithms with newly available computational power, resulting in an improved and economical solution for imaging complex subsalt prospects. GXT has successfully applied advanced RTM methods for more then 30 projects, including the Gulf of Mexico, West Africa and the North Sea.


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