Salt interpretation enabled by reverse-time migration

Abstract

Many production targets in greenfield exploration are found in salt provinces, which have highly complex structures as a result of salt formation over geologic time. Difficult geologic settings, steep dips, and other wave-propagation effects make reverse-time migration (RTM) the migration method of choice, rather than Kirchhoff migration or other (by definition approximate) one-way equation methods. Imaging of the subsurface using any depth-migration algorithm can be done successfully only when the quality of the prior velocity model is sufficient. The (velocity) model-building loop is an iterative procedure for improving the velocity model. This is done by obtaining certain measurements (residual moveout) on image gathers generated during the migration procedure; those measurements then are input into tomographic updating. Commonly RTM is applied around salt bodies, where building the velocity model fails essentially because tomography is ray-trace based. Our idea is to apply RTM directly inside the model-building loop but to do so without using the image gathers. Although the process is costly, we migrate the full frequency content of the data to create a high-quality stack. This enhances the interpretation of top and bottom salt significantly and enables us to include the resulting salt geometry in the velocity model properly. We demonstrate our idea on a 2D West Africa seismic line. After several model-building iterations, the result is a dramatically improved velocity model. With such a good model as input, the final RTM confirms the geometry of the salt bodies and basically the salt interpretation, and yields a compelling image of the subsurface.

Adaptive variable-grid least-squares reverse-time migration

“…RTM has been widely studied and developed by many scholars because of its advantage in providing high-accuracy subsurface images (Sun and McMechan, 2001;Rocha et al, 2016;Du et al, 2017). However, RTM images usually suffer from artifacts (Zhang and Sun, 2009), incomplete illumination (Buur and Kühnel, 2008) and low-frequency noise (Díaz and Sava, 2016) because conventional RTM algorithm uses the adjoint of the linearized wave equation rather than its inverse (Nemeth et al, 1999). Inverse theory-based least-squares migration (LSM) (Lailly and Bednar, 1983) aims to obtain images with fewer artifacts and acquisition marks by approximating the exact inverse of the wave equation modeling operator (Lambaré et al, 1992;Kühl and Sacchi, 2003;Hu et al, 2016).…”

A non-reflecting wave equation through directional wave-field suppression and its finite difference implementation

“…One of these acoustical imaging methods is known as reverse time migration (RTM) 1 , 2 . RTM is comprised of two modeling phases, a forward modeling of source data and a backward modeling of time-reversed receiver data.…”

RTM technology for improved salt imaging in the Santos Basin, Brazil

“…Given the high accuracy of RTM, the sub-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 (Reasnor, 2007;Buur and Kuhnel, 2008). 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.…”