Nonlinear acoustoelastic constants of dry and saturated rocks

Conventional descriptions of the stress dependence of acoustic velocities in rock are often based on microcrack models. A more general, model‐independent description that ignores microstructure is third‐order elasticity theory. This theory has had much success in metals and crystals, and has recently been applied to rocks as well. We measured third‐order elastic constants in several rocks both dry and water saturated. A five‐velocity‐measurement technique was used where a rock sample is stressed either hydrostatically or uniaxially, and compressional and shear velocities are measured as functions of stress. In dry rocks we find that third‐order elasticity theory provides a good description of velocity versus stress. However, in water‐saturated rocks, agreement between theoretical predictions and measurements is significantly worse than in dry rocks. This indicates that third‐order elasticity theory does not fully describe the stress dependence of velocities in water‐saturated rock under our experimental conditions. Detailed analysis of the velocity data suggests that rock dilatancy may be responsible for the discrepancies between theory and experiment.