SOME EFFECTS OF VELOCITY VARIATION ON AVO AND ITS INTERPRETATION

In recent years interest has increased in the interpretation of the am plitude variation of reflected signals as a function of offset (AVO). A more meaningful relationship for interpreting reflection coefficient s at the target horizon is amplitude variation with incident angle (AV A). The challenge is to convert from AVO to AVA. The effects of veloci ty variation in the overburden on amplitude variation with offset (AVO ) and on the final inversion of AVO data into velocity, density, and P oisson's ratio can be significant. Examples are given here for subsurf ace medium with a vertical velocity gradient range of -0.2 s-1 to 0.8 s-1. When the medium is treated as homogeneous in the conversion from AVO to AVA, this velocity variation causes significant errors (about 1 0 percent) in both the gradient of AVA and in the normal incident refl ection coefficient. Such errors produce errors of similar magnitude in the inversion of AVA data into the elastic parameters of velocity, Po isson's ratio, and density. The errors depend on the velocity gradient , the offset range, the elastic parameter contrast across the interfac e, and the interface depth.