Temperature-Induced Nonlinear Elastic Behavior in Berea Sandstone Explained by a Modified Sheared Contacts Model

Rocks commonly display a reduction in elastic modulus during an acoustic perturbation followed by a logarithmic recovery of the modulus to its original value. This nonlinear elastic behavior can also be induced by changes in temperature of the rock, although experimental data for this are sparse. In this paper, we utilize temperature perturbations to broaden our understanding of the causes and mechanisms of nonlinear elasticity in rocks. We perform laboratory experiments tracking the nonlinear response of a Berea sandstone under vacuum both during and following 10 & DEG;C changes in temperature. Ultrasonic velocity decreases by up to 0.30 & PLUSMN; 0.02% during both temperature increases and decreases, before recovering in the following days. Continuous monitoring over a 2-month period also reveals a longer-term 0.78 & PLUSMN; 0.04% increase in velocity. To explain these observations, we modify a recent model for nonlinear elasticity based on the shearing and subsequent recreation of internal microscopic contacts. Application of this model to our data suggests that internal stresses from differential thermal expansion/contraction of mineral grains break contacts, inducing nonlinear weakening. The degree of weakening depends on the temperature gradient. Slow dynamics recovery in the & SIM;10 hr following a temperature change likely results from the recreation of broken contacts due to nanoscopic adhesive forces. In contrast, the long-term velocity increase results from evaporation of bound water from clay minerals. In addition to furthering our understanding of nonlinear elasticity in rocks, our results imply that sudden changes of temperature in shallow crustal environments will induce nonlinear weakening that persists for hours to days.