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The knowledge of the structure and dynamics of AGB-star atmospheres iscrucial to better understand the mass loss. The tomographic method, that relieson the design of spectral masks containing lines forming in given ranges ofoptical depths in the stellar atmosphere, is an ideal technique for thispurpose. It is applied to high-resolution spectro-interferometric VLTI/AMBERobservations of the Mira-type AGB star S Ori. First, the interferometricvisibilities are extracted at wavelengths contributing to the tomographic masksand fitted to those computed from a uniform disk model. This allows themeasurement of the geometrical extent of the atmospheric layer probed by thecorresponding mask. Then, we compare the observed atmospheric extension withthose measured from available 1D pulsation CODEX models and 3Dradiative-hydrodynamics CO5BOLD simulations. We found that while the averageoptical depths probed by the tomographic masks in S Ori decrease (with $<\log\tau_0> = -0.45$, $-1.45$, and $-2.45$ from the innermost to the central andoutermost layers), the angular diameters of these layers increase, from 10.59$\pm$ 0.09 mas through 11.84 $\pm$ 0.17 mas, up to 14.08 $\pm$ 0.15 mas. Asimilar behavior is observed when the tomographic method is applied to 1D and3D dynamical models. Thus, this study derives, for the first time, aquantitative relation between optical- and geometrical-depth scales whenapplied to the Mira star S Ori, or to 1D and 3D dynamical models. In thecontext of Mira-type stars, the knowledge of the link between the optical andgeometrical depths opens the way to derive the shock-wave propagation velocity,which can not be directly observed in these stars.

Tomography of cool giant and supergiant star atmospheres III. Validation of the method on VLTI/AMBER observations of the Mira star S Ori