Multivariate Predictions of Geomechanical Properties of Rocks by Ultrasonic Velocities, Physical and Mineralogical Properties under Different Degrees of Saturation

Determination of P-wave velocity (V_p), which is closely related to intact rock, is a nondestructive, easy and less complicated procedure. Due to these advantages, there is an increasing trend to predict the geomechanical properties of rocks from V_p. In addition to mineral content, cementing material and porosity, degree of saturation (S_r) are also other factors influencing geomechanical properties of the intact rock. However, up to the present, in the studies on the prediction of geomechanical properties of rocks from V_p, the effect of S_r has not been investigated. By considering this issue, in this study, it was aimed to correlate strength properties (uniaxial compressive and tensile strengths) and Young's modulus with V_p of various rock types under different degrees of saturation. In addition, geomechanical properties of rocks were estimated from theoretical P- and S-wave (V_s) velocities, which were calculated based on their mineralogical compositions. For this purpose, fourteen different rock types were collected from several parts of Turkey and a comprehensive laboratory testing program was conducted. In the statistical analyzes, a series of different prediction relationships were developed by using independent variables such as V_p, S_r and effective clay content (ECC). Simple regression analysis results indicated that although prediction of the geomechanical properties of rocks from V_p at different degrees of saturation was possible, the equations developed would yield some under- or over-predictions. On the other hand, the multivariate equations developed have very high prediction performances and are very useful tools to estimate the geomechanical properties from the measured V_p at any S_r value. In addition to mineralogy, the variation in geomechanical properties with increasing S_r is also associated with the combined effect of porosity and clay content, which is first described in this study by the term Effective Clay Content (ECC). It is also noted that strength properties of rocks at any S_r may also be estimated using the empirical relations based on the theoretical V_p and shear-wave velocity (V_s) values, which are calculated from their mineralogical compositions.