Challenges and Case Studies in Sub-Salt Imaging

Rietveld, Walter; Poole, Alan J.¹; and Ray, Amal
¹[email protected]

SUMMARY

Ever since the first subsalt discovery in the Gulf of Mexico in 1993, it has been clear that the use of seismic in a subsalt environment brings along its own set of challenges. Almost all developments in depth imaging made since, have been driven by attempts to improve the image subsalt. More recent developments in marine acquisition technology were also driven by our inability to produce good quality images of subsalt reservoirs. With recent sub- and presalt discoveries in Angola, and presalt discoveries in Brazil, the subsalt imaging challenge has gone worldwide. This paper will review the challenges in subsalt imaging and illustrate the impact of some of the available solutions on our ability to see through salt.

SEISMIC ACQUISITION CHALLENGES FOR SUBSALT IMAGING

A revolution in seismic acquisition started with the first WATS (Wide Azimuth Towed Streamer) experiment over Mad Dog (2005) and has resulted in the Deepwater GoM now being covered almost completely by wide-azimuth data. The success of this has also resulted in the first WATS surveys outside of the GoM in Angola (2008) and more recently in the Red Sea and elsewhere. These wide-azimuth methods have also opened up new processing solutions and challenges and have helped us with the processing challenges discussed below.

SEISMIC PROCESSING CHALLENGES FOR SUBSALT IMAGING

The main seismic processing challenges can be categorized as follows:

- Removal of noise

- Derivations of the velocity model for imaging

- Imaging through complex overburdens

The removal of noise is essential, as the subsalt primary energy is relatively weak due to the large contrast of salt and sediment velocities. The main source of coherent noise are surface multiples. Removing those without removing primaries is essential. Velocities are crucial for imaging. Due to the large velocity contrasts the accurate mapping of the salt structure is very important. However, especially in geological complex areas, recent work has shown that the inclusion of anisotropy, as well as "dirty salt" is required.

More recently RTM (Reverse Time Migration) methods have proven to be the tool to image complex structures below complex overburdens. However, these methods are still expensive.

CONCLUSIONS

What this indicates is that although the challenges for subsalt imaging are still many and tough, we have continuously been improving acquisition and processing, with as direct result images which five years ago we could only dream of.

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013