Effects of attenuation and scattering on AVO measurements

The trends and variance in amplitude variation with offset (AVO) observatio ns caused by near-surface structure, attenuation, and scattering are numeri cally synthesized by pseudospectral viscoelastic 2-D modeling. Near-surface structure produces amplitude focusing and defocusing that significantly di stort AVO observations in offset windows at a scale comparable to that of t he lateral variations in the structure. Attenuation and scattering decrease absolute amplitudes at all offsets. Scattering and wave interference incre ase the variance associated with AVO measurements. Depending on the relative influence of intrinsic attenuation, apparent atte nuation associated with scattering, and geometrical focusing, a normalized AVO response can increase or decrease with offset (relative to that for the associated elastic, nonscattering, 1-D solution), and so mimic the behavio r predicted as a function of contrasts in density, velocity, porosity or Po isson's ratio. If only relative (normalized) amplitudes are available, it i s difficult to distinguish between effects of parameters whose main contrib utions are to absolute amplitude; for example, a trend of decreasing amplit ude (relative to that for an elastic flat-layered model) produced by intrin sic attenuation may be counteracted by focusing/scattering or anisotropic e ffects over wide aperture ranges. Diagnostic information on AVO effects of scattering and attenuation is lost when the noise level is sufficiently hig h. Interpretations of AVO observations based on homogeneous layered elastic models must therefore be used with caution as they are, in general, nonuni que. Lateral variations in AVO parameters are the key to detecting hydrocar bons, so lateral changes in AVO produced by lateral changes in the overburd en properties have potential for being misinterpreted, especially if the re cording aperture is small.