Solving Short-Wavelength Velocity Variations with High-Resolution Hybrid Grid Tomography

Abstract

Short wavelength velocity variations are observed within the Cretaceous chalk in some areas of the Southern North Sea. Depth imaging is required because of the strong lateral velocity variations but building a suitable high-resolution velocity model is not easy. Here we present a case history of a pre-stack depth migration project where five overlapping surveys with varying acquisition azimuths covering some 2000 sq km were input. The workflow uses hybrid grid tomography with dense multi-offset picks as input. Great care is taken to ensure the computed velocity variations are real. A side-effect of short wavelength velocity heterogeneity is that datasets with different azimuths may show differing moveout. However the tomography is able to make use of this and build a more robust model by making use of the additional raypaths. Without solving for the velocity anomalies the image at target is substantially degraded. Migrating with a high-resolution velocity model produced excellent imaging at the target reservoir level in the Rotliegend and Westphalian.

Full-azimuth towed-streamer seismic: An exploration tool for pre-salt hydrocarbon exploration offshore Brazil

“…With full-azimuth acquisition, multiazimuth tomographic methods can be used for velocity model updating. Dazley et al (2007) showed how the introduction of additional information from multiple azimuths in reflection tomography reduces uncertainty and gives more confidence in more detailed velocity model updates. In our case, the data were split into three azimuth volumes for multiazimuth tomography (0-60°, 60-120° and 120-180°, and their reciprocal azimuths).…”