Abstract: AVO and Anisotropy from Logs and Walkaways

Scott W. Leaney, William E. Borland

AVO (amplitude versus offset) is the seismic technique used for mapping lithology, and modeling is an important step for successful AVO interpretations. Shear velocity measurements are essential, since AVO attempts to exploit the elastic (as opposed to acoustic) nature of seismic wave propagation. A property of seismic wave propagation not often considered is anisotropy. This is probably because the magnitude of the anisotropy has been difficult to measure, and its effect on AVO is not widely known.

New technology is helping to improve AVO modeling. Dipole source shear logging tools can now measure very slow shear velocities, increasing the range of applicability of AVO, and new borehole seismic techniques can measure anisotropy. When integrated, these new measurements provide more detailed information about the elastic moduli that govern wave propagation, and bring the possibility for greater reliability in AVO interpretation.

Two examples of ray-trace AVO synthetics are shown to illustrate typical AVO responses and the importance of shear velocity information. An overview of anisotropy is given and a new technique to estimate anisotropy from walkaway surveys (multi-offset VSPs) is described. The log and walkaway measurements are then integrated to study the impact of anisotropy on AVO. First, the effect of anisotropy on Ostrander's shale-sand model is investigated, and second, isotropic and anisotropic ray-trace synthetics are compared.

The effect of anisotropy on AVO is found to be significant and may be lead to misinterpretations of AVO anomalies.

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994