Velocity analysis and quality control from sub-stacks

We present a robust and accurate method of velocity analysis based on measuring any difference in timing between near and far offset stacks. Unlike semblance-based analysis this method identifies the correct moveout velocity of reflectors showing a class 2 AVO response with polarity reversal. If so desired, it can be run as a quality control in conjunction with semblance-based analysis and can flag the occurrence of a polarity reversal. Time shifts are measured by an optimally accurate maximum likelihood method. A criterion is established for detecting two special cases when the amplitude on either the near stack or the far stack is close to zero. A simple remedy is then to halve the substack fold. By including only the nearest or farthest offsets, the sub-stack sums only amplitudes dominated by the same polarity.