245 8. Seismic anisotropy applied to geothermal prospection 8.3 Amplitude versus Azimuth (AVAz): an inversion methodology Amplitudes are related to impedance contrasts rather than impedance itself. Consequently, it is preferred, for Amplitude variation versus Azimuth (AVAZ) methodology, to perform a series of seismic inversions (Al‐Kandari et al., 2009), to evaluate the anisotropy of a key elastic property of the media, the P-impedance, on each azimuthal stack. The method is the one described in the previous chapter, applied to azimuthal stacks. These stacks must contain information from large offsets to be able to detect anisotropy. In addition, as the amplitude is compared from one azimuthal stack to another, the events should be properly aligned before applying the processes. To avoid introducing any bias related to the different azimuthal sectors, the key parameters should be defined using the full-stack full-azimuth seismic data: • Unique optimal wavelet: initial shape, phase rotation, energy. • Uniform well-to-seismic calibration: the wells are tied the same way to the seismic data. The synthetic at well does not model anisotropy. • Unique prior model. • Homogeneous inversion parameter set: the parameters are the same to ensure the same level of convergence of the algorithm. At the end of the process, as many inverted P-impedance models as azimuthal stacks are obtained, from which the only difference comes from the signal itself. 8.4 Ellipse fitting on properties to estimate the anisotropy Either for the velocity (VVAZ) or the impedance (AVAZ), the ellipse fitting allows to capture the variability of the property according to the azimuth (Adelinet et al., 2013). In polar coordinates, each sector response (for each cell, in 3D) is plotted as a point, for which the radius corresponds to the magnitude of the property, and the angle to the average azimuth angle, as displayed Figure 8.2. An isotropic response, corresponding to the same magnitude for all angles, will result in a circle, while a different response will be approximated by an ellipse. This ellipse has two main parameters: • The orientation of the major axis: corresponding to the orientation associated with the major magnitude of the property. • The ratio of the axis: 1 for a circle, greater than 1 for anisotropy detection.
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