Does AVO Inversion Really Reveal Rock Properties? NO! YES!

“ …the P-wave reflection coefficient at an interface separating two media is known to vary with angle of incidence. The manner in which it varies is strongly affected by the relative values of Poisson’s ratio of the media…” (Ostrander, W. J., 1982)

A list of parameters affecting amplitudes Geometrical spreading Source directivity and array effects Near surface velocity variation and velocity anisotropy Waveform interference, dispersion and phase distortion Propagation loss of high frequencies Processing effects (move-out stretch, timing varying scaling, etc.)

Objectives of study Estimate of a set of model parameters Estimate of uncertainties in the model parameters How sensitive is the solution to noise in the data? Is a more complicated (i.e., more model parameters) model significantly better than a simple model?

Method of study Performed generalized linear inversion to synthetic noise and noise-free AVO data of homogenous and isotropic two layer elastic media with a single horizontal interface. Used approximations of Zoeppritz plane wave equation derived by Hilterman (1990) with 6, 5, and 3 model parameters Shuey (1985) with 6 model parameters

Model AVO data Model AVO data based on the properties of carbonate-gas sandstone and carbonate wet sandstone at depth about 2.5-3.0 km.

Results Investigation of model parameters Hilterman (1990) with 6, 5, and 3 model parameters Shuey (1985) with 6 model parameters Investigation on unweighted and weighted generalized inversion Investigation on noise data Investigation on data geometry effect Investigation on wet sandstone model

Discussions and Recommendations Did AVO inversion reveal rock properties correctly? Yes/No Did we oversimplify our model? Since the inversion is only a process that can generate a lot of answers to fit the data. One may be right or none! Thus we should perform with regard of some reasonable solutions!

Conclusions Hilterman plane wave approximation with 5 model parameters are able to determined a perfect model parameter for a different Poisson’s ratio between the upper and lower media, although one of the fundamental problems of the inversion is a non-uniqueness solution. This perfect model did not depend on an initial guess.

Conclusions For noise data, random noise did not influence the model resolution, whereas a coherent noise did a little. Incorrect geometry of the AVO data did influence a great deal of model resolution. Any question!