Imaging sand/shale sequences is problematic when the impedance contrast between them is very small, a problem commonly encountered in basins with Tertiary clastics. Impedance trends for sand and shale in these basins display higher impedances for shales than for sands at shallow depths; however, sands exhibit higher impedances than shales at greater depths. That implies a crossover between the trends at some depth and that it is going to be difficult to image sands near the crossover depth with traditional seismic surveys.

Sometimes stacking far-offset data is a solution because a strong AVO gradient can generate significant reflections at high...

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