AVAZ inversion for fracture density and orientation: a physical modeling study

Faranak Mahmoudian and Gary F. Margrave

ABSTRACT

We present a pre-stack amplitude inversion of P-wave data for fracture orientation and intensity. We test the method on multi-azimuth multi-offset physical model reflection data acquired over a simulated fractured medium. Our simulated fractured medium is composed of phenolic material with controlled symmetry planes, and its elastic properties have already been determined using traveltime analysis; this experimental model represents an HTI layer. We follow Jenner (2002) amplitude inversion on small incident angle data to extract the fracture orientation (direction of isotropic plane of the medium), and estimate the fracture orientation with remarkable accuracy. Knowing the fracture orientation, we have modified the linear PP reflection coefficient equation by Rüger (2001) to invert for anisotropy parameters (ε(V) ; δ(V) ; γ). We incorporated some constraints on the vertical velocities and density in the inversion process. Large-offset data are required for the azimuthal amplitude inversion of the simulated fractured layer, as the material shows only slight azimuthal amplitude variations. The results for all three anisotropy parameters from AVAZ inversion compare very favorably to those obtained previously by a traveltime inversion. This result makes it possible to compute the shear-wave splitting parameter, γ, (historically determined from shear-wave data) which is directly related to fracture intensity, from a quantitative analysis of the PP data.

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