Salt imaging techniques
Salt bodies, including salt domes and salt diapirs, are found in the subsurface all over the world. Subsalt imaging in the Gulf of Mexico has grown increasingly important since it has over 500 salt domes. Other large salt dome regions are located in Brazil, Germany, Iran, and Iraq. Seismic imaging of these salt domes is important because salt domes can be sources of sulfur, sources of salt, and, most importantly, petroleum reservoirs. [1]
Contents
Pitfalls and Problems with imaging salt bodies
Salt bodies in the subsurface can cause a wide array of problems with seismic imaging. These pitfalls end up distorting the seismic image of the subsurface. These pitfalls include, but are not limited to:
- The varied composition of salt bodies makes creating a velocity model difficult
- Salt bodies can cause poor illumination (dim amplitudes)
- Time migrations can cause poorly imaged reflections near the salt body
- False reflections can be produced [2]
Subsalt vs Presalt plays
Salt plays can either be categorized as either subsalt plays or presalt plays.
Subsalt
Subsalt plays are underneath a layer of salt that was not formed in the region, while also being positioned above stratigraphically younger rock. [3]
Presalt
Presalt plays are underneath a layer of salt that was formed in the region, while also being positioned above stratigraphically older rock. [3]
Techniques
Due to the fact that imaging salt in the subsurface can be difficult, multiple imaging techniques have been created to make salt imaging easier. These techniques include:
Wide Azimuth acquisition (WAZ)
Wide Azimuth acquisition can be useful when acquiring data for subsalt plays. A variety of short to long offset source positions are used for a wide range of source-receiver azimuths. 3-D seismic surveys apply wide azimuth acquisition techniques using a source area that is much larger than the receiver area (cross-spread shooting template). If wide azimuth acquisition techniques are used in a marine environment, a main source vessel, a streamer vessel, and at least one other additional source vessel are used to acquire the necessary seismic data. [6]
Reverse Time Migration (RTM)
RTM is a two-way wave equation that computes numerical solutions. This technique is useful when trying to image areas of complex structure and velocity models, like salt bodies. RTM is also helpful when Kirchhoff migration provides insufficient results. [7] Since RTM extrapolates data from time instead of depth, wavefields have no dip limitations. The reflectivity of the earth is approximated by using subsurface and reflected wavefields. [8]
Least-squares Migration (LSM)
LSM can be used as an iterative inversion salt imaging technique. LSM obtains subsurface images by repeatedly minimizing the differences between the demigrated data and the field data. LSM is useful when dealing with poor illumination in seismic data. For this technique, every inversion iteration will have at least one migration and at least one demigration. [5] Even if velocity model errors are present, LSM can give good amplitude information. [9]
References
External Links
- CGG - Subsalt Imaging
- Schlumberger - Exploring the Subsalt
- IonGeo - Subsalt Imaging Case Study
