An iterative approach to updating velocities beneath postsalt anomalies and its impact on presalt imaging

Complex geology and deepwater presalt targets offshore Brazil lend themselves to seismic imaging challenges. The postsalt sections of the Santos and Campos Basins also featured thin-bed, high-impedance layers of volcanic material and other shallow velocity anomalies. These features were often near the top of salt (TOS) and produced areas of poor presalt focusing. Because accurately modeling the postsalt was imperative for imaging the presalt, we developed a new workflow for velocity model updating. Where previous top-down methods have failed, our method embraced the challenges of imaging beneath the volcanics by incorporating additional presalt information into the updates. Our method is interpretation-dependent but provides a robust and reliable workflow for updating the postsalt velocity model in the presence of complex geology. Using a narrow azimuth data set acquired with variable-depth streamers, we evaluated the effectiveness of our iterative workflow. The result was improved presalt imaging.