A model-based AVAZ inversion for azimuthal anisotropy

Faranak Mahmoudian, Josh Curtis, HuShun Zhou and Kristof De Meersman

ABSTRACT

AVAZ inversion can extract natural fracture network properties from 3D pre-stack seismic amplitudes, hence providing important information for unconventional reservoir characterization and hydraulic fracturing stimulation. The commonly used AVAZ inversion is a time sample-by-sample procedure to invert for the HTI symmetry axis direction, analog to dominant fracture orientation, and the anisotropic gradient B ani , analog to fracture density. The technique is based on the Rüger equation for PP reflectioncoefficients from a boundary of two HTI media. The near angle Rüger approximation results in a 90 degree ambiguity on the orientation estimate and relative, band-limited, B ani values. We present a model-based AVAZ inversion to invert for interval B ani . The method is coded using the Earth Signal Ltd processing system. Our method is, an extension of the isotropic model-based inversion by Hampson and Russell (2013) and simultaneously invert for isotropic elastic properties and B ani . The orientation direction is input to our model-based AVAZ, estimated by a sample-by-sample procedure in which the 90 degree ambiguity is removed by constraining the results to the stress information provided by the major stress direction or well control. We have tested the proposed model-based AVAZ inversion on synthetic azimuthal gathers, and the inversion for interval B ani was successful. This facilitates a meaningful interpretation of azimuthal anisotropy values, extracted from pre-stack seismic amplitudes.

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