242 Geophysics in Geothermal Exploration Beicip-Franlab, part of IFPen group, able to extract anisotropy either for velocity or amplitude anomalies. The use of the VVAZ – AVAZ methodology is illustrated by a practical case study in geothermal prospection in Geneva basin, Switzerland (Baillet and Caudroit, 2024). 8.1 Technical background 8.1.1 The HTI and VTI models for anisotropy models A VTI media, standing for Vertical Transverse Isotropic, is characterized by horizontal layering, as evidenced in shale overburdens. The stiffening of the rock in the horizontal direction increases the P-wave velocity in this direction compared to vertical propagation. This model is suitable for lithology prediction. On the other hand, a HTI media, standing for Horizontal Transverse Isotropic, is characterized by vertical layering, such as seen in a fractured reservoir. Here the rock is stiffer along the strike of the fractures giving the fastest P-wave velocity in this direction. It is important to highlight that both AVAZ and VVAZ approaches are sensitive to both anisotropy models, represented Figure 8.1. Figure 8.1 VTI (left) and HTI (right) to simplify the anisotropy modeling. For VTI media, Thomsen introduced three variables (Thomsen, 2002), called Thomsen parameters, ε, γ, and δ. In practice, δ and ε can be derived by adjusting the hyperbola during the NMO, using an additional term in the equation. As such, the VVAZ approach as presented in this chapter is also sensitive to this VTI configuration.
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