11:00 AM - 11:15 AM
[SCG46-08] Characterizing Transient Brittle Creep by Ultrasonic Pulsing and Acoustic Events
★Invited Papers
Keywords:Brittle creep, Subcritical micro-cracking, Rock-fluid interaction
Brittle creep is a dominant deformation mechanism under upper crustal conditions leading to localized failure and the development of macroscopic faults. Time-dependent brittle creep of rocks is inferred to be controlled by subcritical micro-cracking.
To characterize the long-term deformation behavior of basalts under a range of conditions relevant for brittle creep deformation, we conducted load stepping creep experiments on basalts from the CarbFix site in Iceland and basalts from Mount Etna at temperature, T ~80 ºC and effective pressures, Peff = Pc - Pf of 50 MPa and 30 MPa.The load was held constant for 24 h before it was increased by a fixed amount during a subsequent load step. This load-stepping procedure was repeated until failure occurred. Our study shows that the strain evolution of creep deformation obeys a log-time function: ε(t) = βlog(1+t/τ); where β and τ are stress dependent constants and t is time duration of the creep deformation. During creep, we collected ultrasonic pulsing data at 15 s intervals and analyzed the change in p-wave arrival time (Δt) to get insights into the microstructural evolution. The analysis shows that a general change in Δt trend corresponds to the transition from bulk compaction to dilation. Arrival time is shortened during compaction and delayed after deformation becomes dilatant. Within one creep step, the arrival time increases immediately following the load step, then slows down entering a long-term evolution where arrival time decreases progressively. We interpret the evolution of Δt during a creep step to be a result of the competition between recovery/compaction and damage/dilation. As sample approaches failure, damage/dilation become dominant, Δt increases further signaling that a tertiary creep can be achieved. Using the dynamic time warping (DTW) method, we have identified 3 groups of acoustic events (AEs). Changes in temporal distribution of different groups is also observed to be associated with the transition from bulk compaction to dilation. Clustering analysis using DTW methods also helps us in identifying events related to crack initiation, propagation and those that are associated with the crack coalescence that could potentially lead to dynamic failure of the rock.
Overall, analysis on the ultrasonic pulsing and AEs data demonstrated that the transition from bulk compaction to dilation marks an important change during the deformation of rock. We have also identified the evolution of p-wave arrival time and the growth of AE cluster representing crack coalescence as important parameters for failure predictions.
To characterize the long-term deformation behavior of basalts under a range of conditions relevant for brittle creep deformation, we conducted load stepping creep experiments on basalts from the CarbFix site in Iceland and basalts from Mount Etna at temperature, T ~80 ºC and effective pressures, Peff = Pc - Pf of 50 MPa and 30 MPa.The load was held constant for 24 h before it was increased by a fixed amount during a subsequent load step. This load-stepping procedure was repeated until failure occurred. Our study shows that the strain evolution of creep deformation obeys a log-time function: ε(t) = βlog(1+t/τ); where β and τ are stress dependent constants and t is time duration of the creep deformation. During creep, we collected ultrasonic pulsing data at 15 s intervals and analyzed the change in p-wave arrival time (Δt) to get insights into the microstructural evolution. The analysis shows that a general change in Δt trend corresponds to the transition from bulk compaction to dilation. Arrival time is shortened during compaction and delayed after deformation becomes dilatant. Within one creep step, the arrival time increases immediately following the load step, then slows down entering a long-term evolution where arrival time decreases progressively. We interpret the evolution of Δt during a creep step to be a result of the competition between recovery/compaction and damage/dilation. As sample approaches failure, damage/dilation become dominant, Δt increases further signaling that a tertiary creep can be achieved. Using the dynamic time warping (DTW) method, we have identified 3 groups of acoustic events (AEs). Changes in temporal distribution of different groups is also observed to be associated with the transition from bulk compaction to dilation. Clustering analysis using DTW methods also helps us in identifying events related to crack initiation, propagation and those that are associated with the crack coalescence that could potentially lead to dynamic failure of the rock.
Overall, analysis on the ultrasonic pulsing and AEs data demonstrated that the transition from bulk compaction to dilation marks an important change during the deformation of rock. We have also identified the evolution of p-wave arrival time and the growth of AE cluster representing crack coalescence as important parameters for failure predictions.