The free surface effect: Implications for amplitude-versus-offset inversion

David W. S. Eaton

The magnitude and direction of P-wave particle displacement at the earth's surface differs from the particle displacement within the earth due to the interaction of the wavefield with the free surface. Synthetic models of a low impedance contrast gas sand are used to investigate the significance of the free surface effect in the context of AVO analysis. The gas sand is 15 m thick, and buried to a depth of 1500 m beneath a layer having a velocity that increases with depth in a linear fashion. Inversion of the synthetic data shows that the estimated Poisson's ratio of the gas sand can exhibit very significant error if the free surface effect is not accounted for. The magnitude of the error is largest for a high velocity near surface layer. This situation may occur either when bedrock is exposed at the surface or when the ground is frozen.