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Abstract

In rocks and other consolidated geomaterials, dynamic excitation leads to a fast softening of the material, followed by a slower recovery process where the material recovers part of its initial stiffness as a logarithmic function of time. This requires us to exit the convenient framework of time-independent elastic properties, linear or not, and investigate non-classical, non-linear elastic behavior. These phenomena can be observed during seismic events in affected infrastructure as well as in the subsurface. Since the transient material changes are not restricted to elastic parameters but also affect hydraulic and electric parameters as well as material strength, as documented for instance by long lasting changes in landslide rates, it is of major interest to characterize the softening and recovery phases. To further characterize this behavior in a controlled environment, we perform experiments on Bentheim sandstone in a Materials Testing System triaxial cell with pore pressure and confining pressure control. Our sample is subjected to various low-strain loading cycles in both dry and water-saturated conditions, while an active acoustic measurement setup allows us to perform Coda Wave Interferometry to continuously monitor the changes in modulus in the rock.Our transducer array allows us to observe the dynamic softening as well as the recovery processes in the sample during repeated loading phases of various time lengths. Our observations indicate high spatial, frequency content and lapse-time sensitivity of the observed velocity changes, indicating a rich landscape of concurrent effects and physical phenomena affecting our sample during these simple experiments.