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The mechanical defects (i.e. cracks, microfractures, grain contacts), although representing often a minute part of the total porosity of rocks, can play a primary role in rock elasticity, especially on anisotropy. Our aim is to separate and to quantify the effect of the mechanical defects and the other causes (e.g. thin layering, mineral alignments) on anisotropy in rocks. General experimental and theoretical tools for the characterization of arbitrary anisotropy (tri-clinic) in elastic solids were previously developed in our laboratory. Here we adapt these tools in order to achieve a global characterization of the mechanical defects and of the intact rock (i.e the rock without the defects). For each state of pore and confining pressure the method leads to the identification and the orientation of the elements of symmetry, and to the replacement of the actual defects by crack/fracture systems of simpler symmetry (i.e. isotropic, transversely isotropic, orthotropic). No a priori assumption is imposed on the nature and on the properties (i.e. level of symmetry, orientation of symmetry elements) of the defects and the background medium (intact rock). The method is applied to sandstone samples. The results reported here reinforce the idea that remote acoustic testing of rock masses can provide important informations regarding the internal properties of these media (e.g. fracture density and orientation), which is the main idea behind the potential applications of anisotropy in the field. Considering the generality of the approach, the proposed method deserves a complete valadation, for instance by the comparison with independent techniques (e.g. petrographic observations). Such studies are in progress.

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