Complex 3D velocity-depth model with RTM from top-down

Salt geometry model building in some salt environments in GOM is very challenging due to complex overhangs, steeply dipping salt flanks, Christmas tree-like salt flanks, and other wave propagation effects. Ray-based and one-way wave equation algorithms have limitations to handle such complexities. Ray-based algorithms cannot easily handle prism wave propagation and one often needs to smooth the velocity model. One-way wave equation migration has steep dip limitations and cannot handle prism waves. As a consequence, in this study we use reverse time migration (RTM) algorithm in our velocity model building workflow to define the salt geometry.

A key factor in this model building loop is to use overhang and steep dips saltflood model as early as possible. In sediment flood we take advantage of the turning seismic waves to image very steep dips and overhangs, while during saltfloods we take advantage of prism waves to image some areas of the salt walls as well as base of salt. This presentation will show examples of the inabilities of one-way wave equation and Kirchhoff methods to handle prism waves.

Another key factor in this study is the ability to use multi-Z surface interpretation in most stages of the model building. The benefits are: it reduces the number of salt-related migration runs; reduces the salt building cycle time; simplifies the 3D visualization of the salt body; and it allows inclusion of complex features such as Christmas-trees in the saltbody.

The results in this survey shows that a careful use of multi-Z surface interpretation and RTM algorithm takes into account most of the recorded wavefield in WAZ dataset. In specific areas acoustic forward modeling was used to support interpretation. The forward modeling exercise boost our confidence in interpreting areas of weak top and base of salt.