This study revealed that the presence of vertically aligned fractures in reservoir rock could potentially be detected through analysis of P-wave reflectivity variations (AVO) with azimuth. The azimuthal AVO responses measured at an interface separating overburden anisotropy (a VTI medium) and fractured rock is shown to depend on several parameters. These parameters can be separated into the overall properties of the background rock, the fracture parameters and the fracture fill. Shapes of AVO gradients resemble ellipses with a semi-major axis parallel to the fracture direction. In addition, if the sealing rock is anisotropic (the VTI medium), the AVO signature may be significantly distorted. The contribution of the VTI cap rock is azimuthally invariant and reduces to an angles of incidence dependent effect. The VTI overburden tends to modify the slopes of the AVO gradients. This suggests that an assumption of overburden isotropy may result in a large error in predicting the fluid type and saturation.
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