The static stiffness of a rock is generally smaller than its dynamic stiffness, which, for non-dispersive rocks, is due to a difference in strain amplitude in compaction tests (quasi-static) and wave-velocity measurements (dynamic). While acoustic waves are purely elastic, quasi-static mechanical tests usually induce non-elastic deformations. A model introduced by Fjær et al., 2013 allows to quantify non-elastic contributions as a function of stress-variation amplitude and hence to determine the purely elastic stiffness from a standard rock mechanical test. However, due to experimental constraints it was not clear if the model would be valid down to strain amplitudes of acoustic waves. We have performed an experiment on undrained, water-saturated Castlegate sandstone, combining a quasi-static triaxial test with force-oscillation measurements at seismic frequency in order to investigate stress/strain amplitude-related non-elastic effects. We have found that inelastic effects are activated for strains as small as 10-6 (µstrain), and that the compliance increases linearly with increasing stress amplitude.

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