Nonlinear Elasticity in Consolidated and Unconsolidated Granular Materials: Characterization of Slow Dynamics with Dynamic Acousto-Elastic Testing
Abstract
Studying the nonlinear elasticity of rocks at the laboratory scale illuminates the mechanisms behind similar observations at crustal scales, for instance, during strong ground motion and fault slip processes. A dynamic perturbation of microstrain amplitude in rocks results in a transient elastic softening followed by a log(t)-type relaxation back to the initial unperturbed elastic state after the perturbation is diminished. The relaxation may last from minutes to hours depending on the microstructure of the rock and the amplitude/duration/frequency of perturbation. This phenomenon has been observed in diverse rocks and across scales: from the lab to the Earth. An exact log(t)-recovery implies that this process has no characteristic time or rate, i.e., the relaxation time spectrum is flat. Using Dynamic Acousto-Elastic Testing (DAET), we investigate the relaxation over several time decades in different rock types (sandstone, soapstone, granite) and in glass bead packs. Multiple runs are conducted varying excitation amplitude and duration. We find that some materials exhibit a nearly perfect log(t)-relaxation (Grunnes Nidaros Soapstone), while some others recover faster at early times (Berea Sandstone/Berkeley Blue Granite). Importantly, the amplitude of excitation does not influence the shape of the relaxation time spectrum. Employing this comprehensive set of experiments, we discuss the potential relation between characteristic relaxation rates and microstructural features.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMMR41A2692S
- Keywords:
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- 5112 Microstructure;
- PHYSICAL PROPERTIES OF ROCKSDE: 7209 Earthquake dynamics;
- SEISMOLOGYDE: 8010 Fractures and faults;
- STRUCTURAL GEOLOGYDE: 8162 Rheology: mantle;
- TECTONOPHYSICS