Speaker: Alexandre Schubnel
“First, dry triaxial compression experiments performed on Carrara marble at confining pressures ranging from 10 to 100MPa revealed an important dependence of the strain rate on the confining stress. At constant shear stress, transient exponential increases in axial strain, similar to transient creep events observed on the field, were observed for small decreases of the confining pressure (1-10MPa). These experiments provide a clear experimental case of silent, slow localized failure in rocks as a result of an interplay between intragranular plasticity and microcracking: dislocation and twin accumulation is such that cracks propagation steps are small and/or slow, and thus the radiated energy release rate remains small at early stages of rupture and increases with rupture speed. However, as the rupture velocity increases beyond a threshold velocity of a few mm/s, continuous acoustic waveform recordings enabled us to illustrate the transition from aseismic to seismic faulting.
Second, a set of experiments was performed on Volterra gypsum at confining pressures ranging from 10 to 295MPa, and temperatures of 25degC, 70degC and 150degC. The deformation induced microseismicity decreases with confining pressure, but aseismic deformation is never reached, even at 295MPa confinement. In this case, the microstructure clearly highlights the interplay between cracking and mineral kinking. It is possible that mineral kinking, prevalent at high confinemnt, may radiate some acoustic energy. At 70degC, the microseismicity decreases, only to reach strain softening (at low confinement) or strain hardening (at high confinement) stick slip behavior. These stick slip events are dynamic and accompanied by low frequency Acoustic Emission events. Whether these are similar to LFEs observed in the field remains a question to debate. Finally, dehydration induced AEs were also trigered at 150degC. This last observation clearly highlights the dependence of radiated acoustic (and microseismic?) energy during crack propagation not only on the rupture propagation speed and the slip velocity but most importantly on the rock lithology and rheology.
Finally, acoustic emissions produced by a series of quartz and kaolinite mixtures submitted to various pressure and temperature conditions in a cubic multi-anvil device (MAX-80) were monitored in order to evaluate the potential of dehydration induced microseismicity. Simultaneous collection of XRD patterns of the sample using the synchrotron radiation allows matching the main acoustic events to mineralogical changes in the sample. We show the importance of the heating rate to on AE triggering and believe we might have recorded what could be a non-volcanic tremor AE during dehydration of pure kaolinite.”