The role of fractures, effective pressure and loading on the difference between the static and dynamic Poisson's ratio and Young's modulus of Westerly granite

Abstract

Fracture in rock is a major factor that affects the rock's elastic properties. Elastic properties can be measured statically during slow loading of a specimen, or dynamically, where the elasticity can be calculated using elastic-wave velocity. However, differences exist between the static and dynamic elastic properties. In this study, the dynamic and static elastic properties were measured under varying effective pressures and differential loads for dry and water-saturated Westerly granite that was thermally treated to 250, 450, 650 and 850 °C. Increasing the temperature produces an increased fracture density that is isotropically distributed. Increased fracture density results in a reduction in the Young's modulus and an increase in the Poisson's ratio, in both the static and dynamic cases, and is very significant for rocks treated above 450 °C. The dynamic Young's modulus is larger than the static value and the difference between them increases when water saturated. Under dry conditions, the static Young's modulus shows good correlation with dynamic values regardless of fracture density, loading or effective pressure. The static and dynamic Poisson's ratio are close to each other under both dry and saturated conditions. However, deviation between the static and dynamic Poisson's ratio occurs when the fracture density is high, differential load is high and effective pressure is low.