251 8. Seismic anisotropy applied to geothermal prospection Although acoustic inversion technique has been selected, the obtained property can be considered as a “pseudo-P-impedance”, as large offset traces will have more contribution than short offset traces in the amplitude variations, to better capture the anisotropy. The convergence for the azimuthal stacks varies from 70% to 80%, in accordance with the initial level of noise of each azimuthal stack. The noise, discarded from the synthetic seismic and observable in the residuals, hasn’t been included in the 6 optimized impedance models, in order to better estimate the AVAZ effects. 8.6.3 Results and way forward Either for the VVAZ or the AVAZ approach, the focus is based on the variation of the properties with the azimuth rather than their absolute values. In both approaches, the anisotropy has been extracted using ellipse fitting: an isotropic response would result in a circle, while a different response would be approximated by an ellipse, with two main parameters: (1) the orientation of the major axis, corresponding to the tilt associated with the major magnitude of the property (usually parallel to the fractures), (2) the ratio of the axis, greater than 1 for anisotropy detection. While comparing the results obtained in sections, Figure 8.9, it is observable that: • The anisotropy from VVAZ is more subtle than AVAZ, so that the scale has been saturated for display purposes. • The resolution of the anisotropy from AVAZ (right) seems to be better than the VVAZ (left). • Some anisotropic areas, especially around the Top Cretaceous or the Top Keuper, seem to correlate between both methods, while others don’t. • Where there is no signal, especially around the major fault at the center part of the section, no anisotropy is detected (VVAZ or AVAZ). Figure 8.9 Anisotropy in section, VVAZ (left) and AVAZ (right). At low depth, as illustrated Figure 8.10 in time slice (700 ms), both approaches are compatible with each other, focusing on objects with apparently the same size and resolution.
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