| Abstract: | We use a marine seismic dataset to examine the reflections from two gas sands, a lignitic sand and a cineritic bed, by means of their amplitude versus offset (AVO) responses. This offset-dependent signature is related to specific petrophysical and thus elastic situations or to peculiar interference patterns and may help to distinguish the nature of the amplitude anomalies on the stack sections. The prestack analysis is carried out on seismic data which have undergone an accurate true-amplitude processing. It is found that the lignitic-sand reflections exhibit a decreasing AVO while the two-gas sands show markedly increasing AVO trends. Also the reflections from the cineritic layer show increasing amplitudes with offset that may be due either to the petrophysical nature of the cinerites or to thin-layer interference or to both. In order to verify the reliability of the actual AVO responses we develop a detailed model from well data and compute a synthetic CMP seismogram. In order to account for mode conversions and thin-layer effects, the synthetic seismograms are computed using the reflectivity method. The wavelets used in the synthetics are retrieved from actual seismic and borehole data by means of wavelet processing. When finely layered structures are present, the estimation of a reliable wavelet is extremely important to get the correct synthetic AVO response. In particular, the AVO responses of the cineritic layer differ substantially if we make use in the computation of the synthetics of a Ricker wavelet or of a wavelet estimated through wavelet processing. The good match between the observed and modelled data confirms the reliability of the processing sequence and of the final AVO signatures. |
