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In contrast, seismic wavelengths are typically tens and hundreds of metres, which means that we can model a fractured zone as a seismically anisotropic solid. Fracture intensity is less than 0.75 fractures/m for very sparse sets, and may exceed 10 fractures/m for very crowded sets. The fracture aperture size is typically in the ballpark of hundredths to tenths of a millimetre, and the distance between adjacent fractures is much greater than the fracture aperture.

Put simply, subsurface fractures usually occur in sets and are locally aligned in one dominant orientation. HTI (Horizontal Transverse Isotropy): a set of vertical fractures, for example. TTI (Tilted Transverse Isotropy): dipping shales, for example. VTI (Vertical Transverse Isotropy): VTI is the special case where the bedding-plane normal is vertical and the beds are horizontal (a simple flat layer cake type model). For the simple forms addressed in this article, the anisotropy can be described in terms of the symmetry axis. Seismic Anisotropy: velocity varies with direction of travel. In seismically isotropic material, velocity has no directional dependence. We then summarize how the anisotropy parameters output by each technique can be compared and formulated in terms of the three rock physics frameworks.ġ S-wave amplitude differences may also be indicators of anisotropy this method is not discussed here.Interested readers please see Reasnor (2001).

Azimuthal Fourier Coefficients Elastic Inversion (Az FC Inv): a new technique potentially integrating all of the above methods.

We review the following azimuthal techniques for quantifying seismic anisotropy: The discussion begins with a brief review of the nature and scale of real-world fractured rock systems, and then considers three simplified frameworks to represent this complicated reality mathematically. (The Azimuthal Fourier Coefficients approach resolves this: read on!)Īll techniques deliver estimates of anisotropy, but to understand these in terms of fracture properties you have to assume a physical model and the measurements mean something different in each model. The traditional near offset Rüger-style reflectivity AzAVO technique is limited by an inherent ambiguity in the fracture direction it estimates. S-wave Splitting and VVAz relate more to trends, while reflectivity AzAVO captures local deviations. The techniques ‘see’ different scales, or resolutions.

Reflectivity Azimuthal AVO methods measure interface properties: they ‘see’ how the anisotropic layer contrasts to the overlying layer.Travel time methods – Shear-wave Splitting and Velocity Variation with Azimuth (VVAz) – measure layer properties: they see how the seismic wavefront is affected by traveling through the anisotropic zone.