Bjorn Ursin1, Maarten de Hoop2, Sverre Brandsberg-Dahl3, Anders Sollid4, Richard A. Clarke3
(1) Norwegian University of Science and Technology, Trondheim, Norway (2) Colorado School of Mines, Golden, CO (3) BP UTG, Houston, TX (4) Statoil, N-7005 Trondheim, Norway
Common image gathers (CIGs) in the offset and surface azimuth domain are used extensively in migration velocity analysis and amplitude versus offset (AVO) studies. If the geology is complex and the ray field becomes multi-pathed, 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), but stacking only over migration dip angles. Including only dips in the vicinity of the geological dip, or focusing in dip, results in improved signal-to-noise ratio on the CIGs.
A 2.5-D algorithm for isotropic media has been applied to a synthetic dataset generated by finite-difference modeling and to recorded data from the Valhall field. 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.
A 2.5-D algorithm for TI media has been applied to OBS data from the Jotun field with almost horizontally layered sand-shale sequences in the overburden and a weak P-wave acoustic impedance contrast at the top reservoir. Partial stacks of the generated PP and PS CIGs show shows that the PS stack maps the shale-sand top of the reservoir, while the wide-angle PP stack maps the fluid (oil-brine) content.