Interpretative Seismic Imaging With the Wavenumber-Structure Monitoring of Velocity Models

Despite a record of great success, seismic imaging is regarded to be far from solved because of a big challenge in velocity building where potential failures are particularly salient for salt-related structures and fault-karst belts that pose strong velocity contrasts and dipping angles. The challenge results from the traveltime equivalence of velocity distortion and structure anamorphose during the imaging. It is closely related to two different wavenumber contents implicit in velocity models and seismic structures, respectively. For salt-related structures, the initial velocity model obtained by current industry techniques is characteristic of a low-wavenumber (LW) distortion, which partially destroys the topological invariance of subsurface structures implicitly present in surface seismic data and yields erroneous high-wavenumber (HW) reflectivities correspondingly. For fault-karst belts, the estimated initial velocity is too low in resolution to satisfactorily image the high-frequency part in seismic data. The fundamental question regarding how to couple distorted LW and HW components in velocity building remains largely unaddressed. We propose a general scheme by integrating LW velocity models, HW seismic structures (achieved from automatic interpretation of migrated volumes), and logging salt velocities to achieve interpretative velocity building. For assistance monitoring analysis, we introduce a slowness accumulation method to measure the wavenumber content of velocity fields. It provides a quick and easy way to monitor the change of wavenumber contents during the velocity updating. We also address a tentative scheme to assess the complexity of seismic imaging by a statistical description of velocity contrasts and dipping angles that dominate the wavenumber content of velocity models. Numerical experiments and case studies with overthrust salt structures from western China and fault-karst belts from northwest China demonstrate the applicability of interpretative seismic imaging.