Proper modeling of the way in which the speed of seismic energy propagation varies with the orientation of travel is essential to provide the best quality of results.
In the isotropic regime, we assume that propagation speed is independent of orientation. This is computationally the least demanding model. However, the Earth is anisotropic, and it is often necessary to take this into account to achieve the highest quality of results.
In simple settings, the assumption of transverse isotropy with a vertical axis of symmetry (VTI) is often used to help align the seismic image with the correct depth in the Earth, which improves the tie between the seismic interpretation and well control. VTI is also important to help align seismic events between extra-salt events, which can be affected by the anisotropic nature of the sediments, and sub-salt events that have travelled through the relatively isotropic salt.
Today transverse isotropy with a tilted axis of symmetry (TTI) is a widely used approximation for seismic wave-propagation. Making a TTI approximation can be important whenever large dips are involved. Failing to take TTI into account in these cases can result in significant movement of the flanks of large synclinal structures, which could have a large impact on oil-in-place calculations or well placements on those flanks. Another area in which TTI has a large impact is salt flank imaging and hence salt model building: failing to accurately account for TTI will result in the incorrect positioning of the salt flanks and hence an inaccurate salt model and a poor sub-salt image.
This is a TTI RTM image based on the BP 2007 TTI benchmark dataset created by Hemang Shah and is provided courtesy of BP Exploration Operation Company Limited (“BP”)
Waveseis offers all our products with isotropic, VTI or TTI two-way wave propagation to best meet your requirements.