Elastic wave velocities as indicators of lithology-based geomechanical behaviour of sedimentary rocks: an overview

Different geomechanical properties such as elastic modulus/deformation modulus, Poisson's ratio (upsilon), uniaxial compressive strength, shear strength properties, tensile strength and point load index are widely used for rock mass characterisation in geological and geotechnical engineering. However, there are no such direct methods by which these properties can be obtained in the laboratory or in situ, without following time-consuming and a laborious procedure. Thus, ultrasonic technique, an indirect method, was found reliable to determine these properties in rocks. Many researchers have studied the correlation between the compressional wave velocity (Vp) and the geomechanical properties of sedimentary rocks such as sandstones and carbonates. However, these correlations have not considered the change in rock mineralogy, porosity and saturation conditions when deriving relationships. Hence, in this study, the authors attempt to understand the variability of Vp with the help of a fragmented analysis of major mineral constituents, porosity variation and saturation conditions for rock mass classification based on Vp. A review of the existing studies on the relationship between the rock properties and Vp has been used to perform this analysis. The resulting template can be a basis for interpreting more realistic lithology-based geomechanical behaviours and thus highlights the importance of an integrated study involving geological, petrophysical and engineering data. The results derived from fragmented analysis indicate that with an increase in quartz content and a subsequent decrease in feldspar content in sandstones, Young's modulus (E) and Vp increase. With an increase in porosity from 2 to 40%, there is a decrease in E and Vp values for all types of saturation scenarios (water, gas, brine and oil saturated) for sandstones and carbonates.