PRESTACK AMPLITUDE ANALYSIS METHODOLOGY AND APPLICATION TO SEISMIC BRIGHT SPOTS IN THE PO VALLEY, ITALY

The amplitude-versus-offset (AVO) characteristics of three separate bright spots on the same seismic section are analyzed. One of the bright spots results from a water-bearing gravel layer, and the others correspond to gas-saturated sandy beds. The amplitude analysis includes reflections from the entire range of incidence angles available from the survey; for the shallower amplitude anomaly, these angles reached values up to 66°. Extension of the analysis to longer offsets is aimed at detecting possible critical-angle phenomena in order to reduce the uncertainty when the zero-offset reflection's polarity is unknown. The reflection from the gravel layer has this property. Its amplitude exhibits an initial decrease followed by a sudden rise in the AVO trend due to critical reflection and head waves. The gas-related anomalies have a much different AVO characteristic, one in which the amplitude increases with offset distance. -from Author