Abstract: Fracture Detection Using P-Wave AVO

Ronit Strahilevitz, Gerald Gardner

When fractures are present in the subsurface they will often be aligned ill the direction of maximum stress. In this case, the medium will be anisotropic and the elastic parameters will vary in different directions. The effects of anisotropy on shear waves have been well documented but little work has been published on the effects of anisotropy on P-waves. Here we show that significant variations can be present in the AVO response of lines with varying source-receiver azimuths, over gas-filled cracks. By recording 3D seismic data with a range of azimuths, AVO variations with azimuth can be determined. Comparing the field data with synthetic case studies associated with a known fracture orientation can help in predicting the orientation of fractures in a reservoir.

To predict the P-wave AVO response for different source-receiver azimuths, we use a 3D forward modeling program. We calculate the AVO response of the P-wave reflected from an anisotropic medium beneath an isotropic medium for two lines recorded perpendicular and parallel to the direction of the fractures. While the AVO response was increasing when the recorded line was parallel to the fractures a decreasing AVO response occurred when the line was perpendicular to the direction of the fractures.

One advantage of using P-waves is that they do not require special recording and therefore this method is much less expensive than using shear wave analysis which requires multi-component recording.

AAPG Search and Discovery Article #90960©1995 AAPG Southwest Section Meeting, Dallas, Texas