Near-Surface Complexity Could Masquerade As Anisotropy

Zhu, Xianhuai (ConocoPhillips) | Shaw, Simon (ConocoPhillips) | Roy, Baishali (ConocoPhillips) | Hall, Matt (ConocoPhillips) | Anno, Phil (ConocoPhillips) | Whitmore, Dan (ConocoPhillips) | Gurch, Michael (ConocoPhillips)



Near-surface velocities could vary with azimuth, impacting seismic data processing and interpretation. In this study, we developed a methodology to investigate the variations of near-surface velocities with azimuth, using 3D turning-ray tomography. The input data are the first arrivals selected from pre-defined azimuth sectors in terms of shot-receiverpair directions. The output velocities from tomography correspond to the selected azimuth sectors. A near-surface tomography study based on seismic data from a shallow heavy-oil reservoir in Canada has suggested that the observed azimuthal traveltime variations are not necessarily related to azimuthal (HTI) anisotropy induced by the stress field or fractures. It could also be caused by the nearsurface heterogeneity or acquisition footprint. Near-surface complexity could masquerade as anisotropy. Potentially this can influence statics and prestack imaging.