MR41D-2685
Slow Dynamics in Berea Sandstone: a Non-logarithmic Relaxation at Early Times Revealed by Dynamic Acousto-Elasticity.
Thursday, 17 December 2015
Poster Hall (Moscone South)
Jacques Vincent Riviere1, Parisa Shokouhi2, Chris Marone3, Derek Elsworth4, Robert A Guyer1,5 and Paul A Johnson1, (1)Los Alamos National Laboratory, Earth and Environmental Sciences, Los Alamos, NM, United States, (2)Pennsylvania State University, Civil and Environmental Engineering, University Park, PA, United States, (3)Pennsylvania State University, University Park, PA, United States, (4)Pennsylvania State University Main Campus, Energy and Mineral Engineering and Geosciences, EMS Energy Institute, G3, University Park, PA, United States, (5)University of Nevada Reno, Physics Department, Reno, NV, United States
Abstract:
We study nonlinear elastic/acoustic phenomena in rocks at the laboratory scale, with the goal of understanding observations at crustal scales, for instance during strong ground motion and earthquake slip processes. In particular, a long-term goal is to relate microstructure of rocks/gouge to nonlinear acoustic properties. A dynamic perturbation with modest (i.e. acoustic) strain amplitude (10-6 < ε < 10-5) in rocks typically leads to a transient elastic softening (elastic modulus decrease) followed by a log(t)-relaxation back to the initial elastic modulus as soon as the excitation is turned off. The relaxation typically lasts from minutes to hours depending on rock type and amplitude/duration/frequency of perturbation. This log(t)-recovery implies that no characteristic time or rate can be extracted, i.e., the relaxation spectrum is flat. In this study, we use Dynamic Acousto-Elasticity (DAE) to probe the relaxation characteristics of a sample of Berea sandstone to explore short-term relaxation, down to 10-4s (DAE is the dynamic equivalent of measuring acoustic velocity as a function of applied pressure). We find that early recovery does not follow a logarithmic law, while some earlier studies based on resonance techniques and at times larger than 1s do exhibit log(t)-recovery. From this non-log(t) dataset, we extract a spectrum of relaxation rates and discuss the potential relation between characteristic rates and rock microstructure. We also discuss the possible links between transient elastic softening and transient increase in permeability due to dynamic perturbation.
