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Results of examination of experimental data on non-linear elasticity of rocks\udusing experimentally determined pressure dependences of P- and S-wave\udvelocities from various literature sources are presented. Overall, over 90\udrock samples are considered. Interpretation of the data is performed using an\udeffective-medium description in which cracks are considered as compliant\uddefects with explicitly introduced shear and normal compliances without\udspecifying a particular crack model with an a priori given ratio of the\udcompliances. Comparison with the experimental data indicated abundance\ud(∼ 80 %) of cracks with the normal-to-shear compliance ratios that\udsignificantly exceed the values typical of conventionally used crack models\ud(such as penny-shaped cuts or thin ellipsoidal cracks). Correspondingly,\udrocks with such cracks demonstrate a strongly decreased Poisson ratio\udincluding a significant (∼ 45 %) portion of rocks exhibiting negative\udPoisson ratios at lower pressures, for which the concentration of not yet\udclosed cracks is maximal. The obtained results indicate the necessity for\udfurther development of crack models to account for the revealed numerous\udexamples of cracks with strong domination of normal compliance. Discovering\udsuch a significant number of naturally auxetic rocks is in contrast to the\udconventional viewpoint that occurrence of a negative Poisson ratio is an\udexotic fact that is mostly discussed for artificial structures

nonlinear processes in geophysics

Extracting real-crack properties from non-linear elastic behaviour of rocks: abundance of cracks with dominating normal compliance and rocks with negative Poisson ratios