GEOPHYSICS. VOL. 55. NO. 2 (FEBRUARY 1990); P. 157-166, 14 FIGS Prestack amplitude analysis methodology and application to seismic bright spots in the PO Valley, Italy Alfred0 Mazzotti* ABSTRACT 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 uncer- tainty 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 in- creases with offset distance. Two seismic events lo- cated above the bright spots were also investigated to further verify the validity of the seismic amplitude processing. The AVO trends of the three bright spots and of the two reference levels were compared with analogous trends of synthetic seismograms that were computed from models derived from borehole data. INTRODUCTION AND PROPOSED METHODOLOGY The amplitude behavior of reflections with offset (Backus et al., 1982; Ostrander, 1984; Gelfand et al., 1986) has recently been analyzed for use in seismic hydrocarbon exploration. However, whenever this methodology is ap- plied, various problems must be tackled, the most important of which relate to correct amplitude processing (Yu, 1985) and suitable petrophysical conditions (Dey-Sarkar and James, 1986). A further aspect that I believe must be considered, espe- cially in some geologic environments, is the uncertainty of reflection polarity at zero offset, due to interference, proc- essing, or source effects. If the zero-offset polarity is un- known. then the analysis of amplitudes limited to short offsets and consequently to a small range of incidence angles could lead to incorrect interpretations. There are indeed numerous cases that require determining both zero-offset reflection polarity (i.e.. knowing whether compressional- wave velocity is increasing or decreasing, assuming density to be nearly constant) and amplitude trend with offset. One of the most common cases cited in the literature is a gas-bearing sand overlain by shales that gives rise to an increasing amplitude-versus-offset (AVO) trend and a de- crease in compressional-wave velocity (VP). However, in- creasing AVO trends are not limited to situations in which V,, decreases. A common geologic situation that yields a strong increase in amplitude versus offset, but also an increase in VP. is where oyster banks occur in a siltstone sequence (Gassaway et al.. 1986). Other situations related to the presence of gas may also give rise to decreasing AVO trends and reductions in com- pressional-wave velocity. due to porosity or cementation effects. Moreover, as shown below for bright spot no. 1, even in areas where a decreasing AVO trend is by itself a negative indicator of the presence of gas, the interpreter may feel uneasy if this information is not corroborated by reflec- tion polarity. The analysis of longer offset reflection amplitudes may remove the ambiguity in reflection polarity. The extension to greater offsets and hence to wider incidence angles is nec- essary because similar amplitude trends in a limited range of incident angles could result from signals of different polari- ties. Koefoed (1955) stressed that, for a limited range of incidence angles (up to 30”), the shape of reflection coeffi- cient curves is only slightly affected by the interchange of the Presentedat the 58th Annual International Meeting, Society of Exploration Geophysicists.Manuscript received by the Editor November 30, 1988; revised manuscriptreceived July 25, 1989. *AGIP S.P.A.. Geophysical Research and Development Department, P.O. Box 12069. 20120 Milano, Italy. 0 1990Sociely of Exploration Geophysicists.All rights reserved. 157