Decomposition of Structural Amplitude Eect and AVO Attributes: Application to a Gas-water Contact YANGHUA WANG, 1 MICHAEL H.WORTHINGTON, 2 andR.GERHARD PRATT 3 Abstract ÐThe structural amplitude eect, associated with focusing and defocusing due to the re¯ector curvature, importantly contributes to re¯ection seismic amplitudes. This paper develops a conciliatory approach for estimating the structural amplitude eect and the attributes of amplitude variation versus oset AVO). The AVO attributes are extracted from raw amplitudes, in which the structuraleectistakenintoaccountexplicitlybasedonastructuralmodelreconstructedfromtravel-time inversion.OneofthegoalsistoconducttheAVOanalysisnotjustlocallyperCDP)butalsohorizontally toseetheglobalvariationalongthere¯ection.ThelateralvariationsofAVOattributesaredecomposedby theChebyshevexpansion.Themethodisdemonstratedwithanexampleofweakshallowgas-watercontact appearing on a 2-D seismic pro®le of a site survey in the North Sea. Key words: AVO attributes, amplitude inversion, Chebyshev decomposition, gas-water contact, seismic inversion. 1. Introduction TheanalysisofamplitudevariationversusosetAVO)hasbeenusedextensively for gas prediction e.g., OSTRANDER, 1984). In most cases, one assumes that the structural eect in amplitude data has been removed, and that the amplitude variationcharacterisesonlythere¯ectioncoecientatatargetre¯ector.Inthispaper we attempt to extend the conventional AVO analysis to the application on raw amplitude data in which the structural amplitude eect, including the focusing and defocusing of curved re¯ectors, is compensated explicitly to the AVO attributes. This work follows the research on re¯ection tomographic inversion using synthetic amplitude data WANG and HOUSEMAN, 1994, 1995; WANG and PRATT, 1997,2000).Inre¯ectionseismictomography,amplitudedataareusedseparatelyor jointly with travel-time data to reconstruct the interface geometry and the velocity 1 Robertson Research International Ltd., Horizon House, Azalea Drive, Swanley, Kent BR8 8JR, England. E-mail: yanghua@geo.robresint.co.uk 2 Department of Earth Sciences and Engineering, Imperial College of Science, Technology and Medicine, London SW7 2BP, England. 3 Department of Geological Sciences, Queen's University, Kingston, Ontario, Canada K7L 3N6. Pure appl. geophys. 159 2002) 1305±1320 0033±4553/02/061305±16$1.50+0.20/0 Ó Birkha È user Verlag, Basel, 2002 Pure and Applied Geophysics