PreSDM Reprocessing of Dense WAZ Data - A Processing Team Delivers

Abstract

Abstract We present an example of how pre-stack depth migration (PreSDM) reprocessing of dense wide-azimuth (WAZ) data can improve imaging quality in areas with complex geology. The main challenges were the multiples hampering picking of proper primary signals, combined with a complex salt geometry. We will show that the experience gained over the years in using dense WAZ data for velocity model building and applying the latest technologies can significantly improve the imaging quality when compared to legacy processing. Since 2010, the experience of working with dense WAZ data has built up especially on key processing steps like data preconditioning and velocity model updating. In our case, there were two processing steps that were key to successful data preconditioning and residual moveout (RMO) picking: –Multiple removal by 3D dip filtering using local linear event extraction based on dip discrimination.–Using narrow-azimuth (NAZ) sectors instead of full WAZ data as input for picking. For the velocity model building workflow, the most important aspects were: –The use of multi-layer non-linear slope tomography.–Salt scenario testing in areas with poor top salt visibility.–Intra-salt tomography. RMO pick quality is critical for successful tomography for PreSDM. Most of the high amplitude train of multiples in the post-salt sediments could be removed using a dip-based demultiple algorithm. This allowed us to pick more stable and reliable RMO and dip information, which is a big improvement over the legacy PreSDM. Going back to azimuth sectors instead of using full WAZ data was mainly driven by experience. It was found that thorough and effective QC of RMO picks was easier and more accurate using MAZ data as opposed to WAZ data. As a consequence, more reliable and consistent pick information was used for tomography. Using multi-layer tomography as an alternative to a layer stripping approach for velocity model building reduced the number of tomographic updates from five in the legacy PreSDM to three in the current project and produced more geologically consistent and accurate velocity models for PreSDM. Salt scenario testing improved the position of top salt and imaging below in areas with poor top salt visibility. Intra-salt tomography updated the smooth velocity within the salt and provided detailed velocity for intra-salt layers. Altogether this led to significant improvements in terms of velocity model building and imaging. In this case study we present the results of the first PreSDM reprocessing of the first dense WAZ data acquired in the Sultanate of Oman. We explain in detail the methods used for data preconditioning and velocity model building and we show examples of the improvements achieved compared to legacy processing in terms of velocity model and imaging.