Azimuthal AVO Curvature As a Constraint on Anisotropy Orientation
Tuesday, 16 December 2014
Amplitude variation with offset (AVO) is a technique in exploration seismology that uses active-source seismic reflection amplitudes to estimate subsurface elastic parameters. Because it measures an interface property, AVO has higher resolution than transmission-based methods such as S-wave splitting and Velocity Variation with Azimuth (VVAZ), and is therefore preferred. Using the azimuthal AVO response, anisotropic elastic parameters can be estimated, providing information such as fast and slow elastic wave velocities from which dominant fracture/stress orientations can be inferred. AVO methods typically separate the effects of elastic parameters on the reflection coefficient into three terms based on the incidence angles they dominate: intercept (small angles), gradient (mid-sized angles), and curvature (large angles). The gradient is characterized by multiple independent elastic stiffness coefficients that change with azimuth, leading to nonuniqueness and a 90 degree ambiguity in the estimated anisotropy orientation. We point out that within a particular parameterization, the curvature term is only influenced by a single term proportional to the change in the squared horizontal P-wave velocity across the interface along a given azimuth. The simplicity of the curvature term provides the capability of determining anisotropy orientation without ambiguity, especially when combined with the gradient. We demonstrate the effectiveness of the curvature as a constraint on anisotropy orientation using physical modeling data of a simulated fractured medium from Mahmoudian (2013). We show that the curvature removes the 90 degree ambiguity in fracture orientation that would be present from only using the gradient.