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    As the high-frequency analogue to field-scale earthquakes, acoustic emissions (AEs) provide a valuable complement to study rock deformation mechanisms. During the load-stepping creep experiments with CO2-saturated water injection into a basaltic sample from Carbfix site in Iceland, 8791 AE events are detected by at least one of the seven piezoelectric sensors. Here, we apply a cross-correlation-based source imaging method, called geometric-mean reverse-time migration (GmRTM) to locate those AE events. Besides the attractive picking-free feature shared with other waveform-based methods (e.g. time-reversal imaging), GmRTM is advantageous in generating high-resolution source images with reduced imaging artefacts, especially for experiments with relatively sparse receivers. In general, the imaged AE locations are found to be scattered across the sample, suggesting a complicated fracture network rather than a well-defined major shear fracture plane, in agreement with X-ray computed tomography imaging results after retrieval of samples from the deformation apparatus. Clustering the events in space and time using the nearest-neighbour approach revealed a group of ‘repeaters’, which are spatially co-located over an elongated period of time and likely indicate crack, or shear band growth. Furthermore, we select 2196 AE events with high signal-to-noise-ratio (SNR) and conduct moment tensor estimation using the adjoint (backpropagated) strain tensor fields at the locations of AE sources. The resulting AE locations and focal mechanisms support our previously assertion that creep of basalt at the experimental conditions is accommodated dominantly by distributed microcracking.

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  2. Abstract. Geological carbon sequestration provides permanentCO2 storage to mitigate the current high concentration of CO2 inthe atmosphere. CO2 mineralization in basalts has been proven to be oneof the most secure storage options. For successful implementation and futureimprovements of this technology, the time-dependent deformation behavior ofreservoir rocks in the presence of reactive fluids needs to be studied indetail. We conducted load-stepping creep experiments on basalts from theCarbFix site (Iceland) under several pore fluid conditions (dry,H2O saturated and H2O + CO2 saturated) at temperature,T≈80 ∘C and effective pressure, Peff=50 MPa,during which we collected mechanical, acoustic and pore fluid chemistrydata. We observed transient creep at stresses as low as 11 % of thefailure strength. Acoustic emissions (AEs) correlated strongly with strainaccumulation, indicating that the creep deformation was a brittle process inagreement with microstructural observations. The rate and magnitude of AEswere higher in fluid-saturated experiments than in dry conditions. We inferthat the predominant mechanism governing creep deformation is time- andstress-dependent subcritical dilatant cracking. Our results suggest thatthe presence of aqueous fluids exerts first-order control on creepdeformation of basaltic rocks, while the composition of the fluids playsonly a secondary role under the studied conditions. 
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  3. Abstract

    We conducted experiments to investigate the influence of pore fluid pressure on the frictional strength and slip behavior of gouge bearing faults. Saw cut porous sandstone samples with a layer of gouge powders placed between the precut surfaces were deformed in the conventional triaxial loading configuration. A series of velocity‐step tests were performed to measure the response of the friction coefficient to variations in sliding velocity. Pore volume changes were monitored during shearing of the gouge. Our results demonstrate that under constant effective pressure, increasing pore pressure stabilizes the frictional slip of faults with all four gouge materials including antigorite, olivine, quartz, and chrysotile. The stabilizing effect is the strongest in antigorite gouge, which shows an evolution of friction parameters from velocity‐weakening toward velocity‐strengthening behavior with increasing pore pressure. Experiments with controlled pore volume show that the pore volume reduction diminishes under high pore fluid pressures, implying an increasing dilation component at these conditions. The dilatant hardening mechanism can explain the observed strengthening. These results provide a possible explanation to the observed spatial correlation between slow slip events and high pore pressure in many subduction zones.

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