Converted-wave reflectivity is zero at normal incidence (zero offset), but has a sinusoidal-type dependence on offset. Conventional NMO and stacking correct the P-S events to zero-offset traveltime and then sum the offset-variant reflections. Thus, the P-S zero-offset section is an average of the P-S AVO response across the set of source-receiver offsets. Variable offsets in the stack, from location to location, may lead to amplitude variations in the sections. Thus, in survey design we should try to have not just smooth fold but a consistent offset distribution. Weighting the offset P-S data in attempt to produce a zero-offset, pure-shear value is another approach to try to smooth out offset variability. Synthetic and field examples from the Blackfoot 3C-3D survey indicate that there is significant variation in the P-S reflectivity with offset.
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