RTM for Improved Salt Imaging

Reverse time migration (RTM) is a powerful imaging tool. It has the ability to account for rapid spatial variations in the velocity model and to utilize all wavefront information, producing superior images of the most complex structures. This is why RTM is frequently used to interpret salt structures in regions known to have complex salt geometries like the Gulf of Mexico. With the application of recent advancements such as Vector Offset Output and 3D angle gathers, the imaging capability of RTM is enhanced even further.

RTM is a pre-stack two-way wave equation depth migration, recognized as being able to produce superior pre-salt images compared to other migration algorithms such as Kirchhoff, control beam migration (CBM) or one-way wave equation migration. Given the high accuracy of RTM, the pre-salt image quality is now limited by the accuracy of the velocity model which, in turn, is very sensitive to the definition of the salt body. For this reason, RTM is often employed not only for final migration but also for salt interpretation in regions where the salt geometry is complex and difficult to image, such as the GoM.

An RTM image allows a more conclusive determination of the salt structure, and thereby improves the reliability of the velocity model and the final image. In the case of the Santos Basin in offshore Brazil, RTM is typically used for final migration but is rarely used for salt interpretation. CBM is used instead because the salt geometry in the Santos Basin is known to be mostly tabular and less complex than in areas like the Gulf of Mexico. A study by CGG Veritas shows that even in the Santos Basin, there are areas where a CBM image does not allow conclusive determination of the salt structure. RTM technology can greatly improve the image in such areas leading to more reliable salt interpretation which is crucial to the quality of the final pre-salt image, and therefore it may be a key factor in determining pre-salt reservoir locations.