Common image gathers (CIGs) in the offset and surface azimuth domain are used extensively in migration velocity analysis and amplitude variation with offset (AVO) studies. If the geology is complex and the ray field becomes multipathed, the quality of the CIGs deteriorates. To overcome these problems, the CIGs are generated as a function of scattering angle and azimuth at the image point. The CIGs are generated using an algorithm based on the inverse generalized Radon transform (GRT), stacking only over migration dip angles. Including only dips in the vicinity of the geological dip, or focusing in dip, suppresses artifacts in and results in improved signal-to-noise ratio on the CIGs.
Migration velocity analysis can be based upon the differential semblance criterion. The analysis is then carried out by minimizing a functional of the derivative of the CIGs with respect to horizontal coordinates (offset/azimuth or scattering-angle/azimuth), but AVO/amplitude variation with angle (AVA) effects will degrade the performance of the velocity analysis. We overcome this problem by computing an inverse GRT modified to compensate for AVA effects. The resulting CIGs can be used for velocity analysis based upon differential semblance, while they can be stacked to produce improved images.
The algorithms are developed for inhomogeneous anisotropic elastic media, but they have so far only been tested on imaging-inversion of PP and PS reflected waves in an isotropic elastic medium. This was done on two synthetic datasets generated by finite-difference modeling and ocean-bottom seismic (OBS) data from the Valhall field. We show that by performing the imaging of the real OBS data in the angle domain, it is possible to construct a well-focused PP image of the Valhall reservoir directly beneath the “gas cloud” in the overburden.