Development of a New Empirical Relation to Assess P-wave Velocity Anisotropy of Rocks.

Several physical parameters and anisotropy related to rock textural arrangements, schistosity and weakness planes such as cracks and joints affect the P-wave velocity (V_P). First, V_P anisotropy of quartz-mica schist as a common type of widespread metamorphic rock was compared with V_P anisotropy of jointed homogeneous limestone specimens to clarify effect of these two different types of anisotropies. The results showed that the V_P anisotropy of quartz-mica schist texture is stronger than the V_P anisotropy of jointed limestone, because all body of quartz-mica schist specimens have V_P anisotropy behavior. Many rocks are anisotropic and degree of anisotropy varies from one to another. Various investigations have been carried out on V_P anisotropy but there is not a unique comprehensive relation to represent the influence of different degrees of anisotropy on the V_P for different rocks. The relation between V_P and angle (θ) between the axis of symmetry (perpendicular to weakness planes) with the wave propagation direction was analyzed for a wide range of anisotropy degrees using the results of nine different types of rocks including: Angouran quartz-mica schist, Golgohar mica schist, amphibole schist, mica-quart schist, Marcellus shale, Withby shale WUK47B, WUK70 and WUK2, and Veroia-Polymylos gneiss. A new simple empirical relation fitted to all groups of results was obtained to assess V_P for different degrees of anisotropies with a good correlation of determination (R² = 0.937), low RMSE (RMSE = 320 m/s) and low CV (CV = 7.0%). P wave velocity anisotropy can simply be predicted by the developed relation using only two parameters of V_P0 and V_P90 or ε that is the percentage change of V_P0 with respect to V_P90. A V_P anisotropy classification diagram was also developed based on the different values of ε. [ABSTRACT FROM AUTHOR]