A 3D subsalt tomography based on wave-equation migration-perturbation scans

Abstract

We have developed a simple but practical methodology for updating subsalt velocities using wave-equation, migration-perturbation scans. For the sake of economy and scalability (with respect to full source-receiver migration) and accuracy (with respect to common-azimuth migration), we use shot-profile, wave-equation migration. As input for subsalt-velocity analysis, we provide waveequation migration scans with velocity scanning limited to the subsalt sediments. Throughout the migration-scan sections, we look for the best focusing or structural positioning of characteristic seismic events. The picking on the migration stacks selects the value of the best perturbation attribute (alpha-scaling factor) along with the corresponding position and local dip for the chosen seismic events. The associated, locally coherent events are then demigrated to the base of the salt horizon. Our key observation is that this process is theoretically equivalent to performing a datuming to a base of salt followed by subsalt migration of the redatumed data perturbed-velocity profiles. Thanks to this implicit redatuming of shot profiles, no ray tracing through the salt body is required. Thus, the events picked on the subsalt-velocity scans only need to be demigrated to the base of salt. For the event demigration we use 3D specularray tracing up to the base of the salt horizon within a predefined range of reflection angles. Event demigration produces model-independent data -time and time slope -that are then kinematically migrated using the current tomographic-inversion working model. To find a finalvelocity model that will flatten best the remigrated events on common image point (CIP) angle gathers, we use the same set of demigrated observation data as the input data set for several nonlinear iterations of 3D tomographic inversion.

Enhanced prestack depth imaging of wide-azimuth data from the Gulf of Mexico: A case history

“…Once the scan set is prepared, picking of the events can begin. The picking tool for DIT scans is very similar to those originally designed for regular wave-equation migration-based velocity perturbation scan picking (Wang et al, 2006a); but instead of using a velocity scaling factor, the picked value is the time shift (such as −100 ms or þ200 ms). Both stacked sections and gathers are used for picking.…”

“…As a method for updating sediment velocities below salt, subsalt velocity perturbation scans (Wang et al, 2004(Wang et al, , 2006a can be effective, but the cost of generating migration scans is linearly proportional to the number of scans, since multiple passes of migration must be performed, one for each of the scaled velocity models. Constrained by the computation cost and run time, the number of velocity perturbation scans produced is typically limited to between seven to nine scans.…”

Practical approaches for subsalt velocity model building

“…These technical challenges have motivated geophysicists to establish alternative and innovative workflows instead of using conventional velocity-analysis tools. For example, Dirks ͑2004͒ andWang et al ͑2006͒ combined reflection tomography with wave-equation migration-perturbation scans. Furthermore, Mosher et al ͑2007͒ used various tools to build velocity models for subsalt imaging iteratively.…”

Subsalt velocity analysis by combining wave equation based redatuming and Kirchhoff based migration velocity analysis

“…Due to the geometrical complexity of the typical Gulf of Mexico (GOM) velocity models, wave equation migration is used preferentially over Kirchhoff methods for subsalt velocity model building. Although the most recent attempts at using the wave equation based migration scan techniques for subsalt velocity updating (Wang et al, 2006) are promising, the cost of generating migration scans is still very high. A migration scan is a set of PreSDM stack images that are produced from a set of locally scaled velocity models.…”