Elastic anisotropy of Opalinus Clay under variable saturation and triaxial stress

A novel experimental method is introduced to estimate the Thomsen's elastic anisotropy parameters ε and δ of a transversely isotropic shale under variable stress and saturation conditions.The method consists in recording P-wave velocities along numerous paths on a cylindrical specimen using miniature ultrasonic transducers.Such an overdetermined set of measurements is specifically designed to reduce the uncertainty associated with the determination of Thomsen's δ parameter compared to the classical method for which a single off-axis measurement is used (usually at 45 • to the specimen's axis).This method is applied to a specimen of Opalinus Clay recovered from the Mont-Terri Underground Research Laboratory in Switzerland.The specimen is first saturated with brine at low effective pressure and then subjected to an effective pressure cycle up to 40 MPa, followed by a triaxial loading up to failure.During saturation and deformation, the evolution of P-wave velocities along a maximum of 240 ray paths is monitored and Thomsen's parameters α, ε and δ are computed by fitting Thomsen's weak anisotropy model to the data.The values of ε and δ obtained at the highest confining pressures reached during the experiment are comparable with those predicted from X-ray diffraction texture analysis and modelling for Opalinus Clay reported in the literature.These models neglect the effect of soft-porosity on elastic properties, but become relevant when soft porosity is closed at high effective pressure.