Prestack depth migration and wide-azimuth towed-marine acquisition provided step-change improvements in subsalt imaging during the last decade. Incremental enhancements in image quality were also provided by the development of processing algorithms like one-way wave equation (WEM), two-way wave equation (RTM) and true-azimuth 3D surface multiple attenuation (GSMP) (Dragoset et al., 2008). These compute intensive technologies are now routinely available due to cost effective compute capacity and fast algorithms.


In general the industry has made tremendous improvements in the quality of subsalt images, however there are challenging areas where the imaging is still quite poor and needs further advances in technology.

Progress has also been made in building better velocity models with azimuthal tomography, introduction of TTI anisotropy combined with other seismic and non seismic measurements such as VSP's, Well logs, MMT, Gravity, geomechanical stress fields and basin modeling. However interpreting and incorporating salt geometry into the velocity model is the weakest link in our ability to consistently image subsalt sediments.

Although the current wide azimuth data sets provide better illumination of complex salt geometry and subsalt data they still fail to illuminate in many challenging areas especially at steep dips.

The Next Chapter

The industry needs another step change in improving illumination by acquiring data at all azimuths and longer offsets instead of just wide azimuth at current offset configurations of 8 × 4 km. Current datasets are also lacking in low frequencies where the subsalt signal is strongest.

So, the next step is to raise the WAZ technology to a new level by implementing a dual-coil acquisition design. This can enable the recording of full-azimuth data with a 14 × 14-km offset range. Indeed, such a program has been identified for the Gulf of Mexico. In that program, the sources and receivers will be towed at depths appropriate to optimize low-frequency content (Moldoveanu and Kapoor, 2009).

3D finite-difference (FD) modeling of the SEG advanced modeling (SEAM) model shows the illumination improvements obtained by full-azimuth 14 × 14-km data vs. wide-azimuth 8 × 4-km data.

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