Measurements of anisotropic poroelastic moduli in Westerly granite

We measured both drained (dry and saturated) and undrained elastic and poro-elastic moduli in a sample of thermally cracked Westerly granite. Measurements were made at nominal effective confining pressures of 30 MPa and 100 MPa. Differential stress was also applied to the sample in order to induce increasing anisotropy (transverse isotropy). Radial and axial stress were independently cycled at increasing levels of differential stress to enable the measurement of anisotropic moduli in a sample with increasing anisotropy. Measurements were first conducted under dry conditions in order to obtain the drained dynamic moduli before the sample was saturated with water. We used newly developed, miniature differential pressure transducers were located directly around the sample surface, which allowed for direct measurement of Skempton's radial coefficient Bx and axial coefficient Bz. Wavespeeds measured on the dry sample were inverted to obtain the compliances Cijkl. The coefficients Bx and Bz were then calculated indirectly from the compliances using poro-elasticy theory for comparison with the direct measurements. Our results show that the values of Bz calculated from the compliances compare well with those measured directly, but that the calculated values of Bx significantly overestimate those measured directly, particularly at an effective pressure of 30 MPa. The discrepancy between the direct measurements and those obtained from drained moduli is likely due to nonlinear elastic effects, such as crack closure and opening, which occur during the small but finite pressure and stress steps.