Density contrast is difficult to determine from AVO

Laurence R. Lines

A recent analysis of seismic amplitude variation with offset (AVO) by Fatti et al. (1994) recasts the AVO equations of Aki and Richards (1979) into a form that is preferable for analysis of rock properties. Fatti et al. showed that the AVO equation could be expressed in terms of P-wave reflectivity, S-wave reflectivity and fractional density change. In an interesting paper, Goodway et al. (1997) then utilized Fatti's equation to deal with Lam parameters, and such parameters were used to effectively detect hydrocarbons. In this short note, I use sensitivity analysis of Fatti's equation to formally show that direct detection of rock density is difficult for limited apertures and typical seismic velocities. Sensitivity analysis allows us to estimate fractional changes of P-wave and S-wave impedance, but shows that AVO effects are not sensitive to density changes for normal seismic apertures.