Inhomogeneous anisotropy versus depth: when the direction of anisotropy axes versus depth varies, and/or if a heterogeneity is present between the emitter and closest receiver R1 of the dipole sonic tool, the measured birefringence relates ONLY to the medium facing the receiver array, (medium-2), independently from the prior S-wave propagation in medium-1 (ref. Figs. 13, 14 of Sondergeld and Rai [24]).