The application of absolutely calibrated piezoelectric (PZT) sensors is increasingly used to help interpret the information carried by radiated elastic waves of laboratory/in situs acoustic emissions (AEs) in nondestructive evaluation. In this paper, we present the methodology based on the finite element method (FEM) to characterize PZT sensors. The FEM-based modelling tool is used to numerically compute the true Green’s function between a ball impact source and an array of PZT sensors to map active source to theoretical ground motion. Physical-based boundary conditions are adopted to better constrain the problem of body wave propagation, reflection and transmission in/on the elastic medium. The modelling methodology is first validated against the reference approach (generalized ray theory) and is then extended down to 1 kHz where body wave reflection and transmission along different types of boundaries are explored. We find the Green’s functions calculated using physical-based boundaries have distinct differences between commonly employed idealized boundary conditions, especially around the anti-resonant and resonant frequencies. Unlike traditional methods that use singular ball drops, we find that each ball drop is only partially reliable over specific frequency bands. We demonstrate, by adding spectral constraints, that the individual instrumental responses are accurately cropped and linked together over 1 kHz to 1 MHz after which they overlap with little amplitude shift. This study finds that ball impacts with a broad range of diameters as well as the corresponding valid frequency bandwidth, are necessary to characterize broadband PZT sensors from 1 kHz to 1 MHz.

We performed laboratory-scale experiments on Barre granite specimen with a single pre-existing flaw to study the microscopic processes that occur during the deformation of a brittle material such as granite at different stress levels from crack initiation to the failure of the specimen. Here, we focus on the evolution of the tensile and shear cracks as a function of stress under unconfined compression. Acoustic emission technique (AET) in combination with the two dimensional (2-D) digital image correlation (DIC) technique have been used to track the changes in the source mechanisms of the registered AE events, along with the development of strains around the flaw tips of a uniaxially loaded prismatic Barre granite specimen. The parametric analysis along with the moment tensor inversion of the AE signals were used to discuss the cracking levels and the cracking mechanisms. In particular, the microcracks observed through AE monitoring prior to specimen failure were presented in terms of their spatio-temporal evolution and linked with the changes in the inelastic strain component measured through the 2D-DIC along the localized area. The mode of deformation computed from the image based strain profiles, enabled direct comparison of the nucleation, growth and interaction of the microcracks with the AE monitoring technique.

While hydraulic fracturing is a widely employed process, the underlying fracturing processes are not clearly understood. Scaled laboratory hydraulic fracturing experiments with seismic monitoring can help with better understanding of the relationship between the generated hydraulic fracture network and the induced micro-seismicity while taking into account the effect of different HF parameters (injection fluid type and rate, stress conditions). In this study, hydraulic fracturing experiments were performed on true-triaxially loaded Barre granite cubes, with real-time micro-seismic monitoring, to identify and characterize the stimulation processes associated with the viscosity and toughness dominated hydraulic fracturing propagation regimes. Water and gear oil were used as the fracturing fluids. Moment tensor inversion technique was employed to determine the fracture mechanisms (tensile, shear, or mixed-mode). Viscosity propagation regime experiments involved higher breakdown pressures and larger injection fluid volumes relative to toughness propagation regime experiments. The micro-seismicity from toughness propagation regime experiments resulted in relatively larger b-value (2.35 compared to 1.62), indicating dominance of small magnitude events. Overall, tensile fractures were dominant in both propagation regimes (ranging from 52% to 58%), which can be attributed to the very low permeability of the granite rock. These results indicate that even for a relatively impermeable rock, theoretical assumptions of mode-I tensile fracturing and the scaling analysis may only be applicable to the near borehole region and as the fracture propagates away from the borehole, the fracturing pattern varies depending on the locally encountered conditions.

While hydraulic fracturing (HF) is a widely employed process, the underlying fracturing processes are not clearly understood. Laboratory HF experiments with seismic monitoring can help with better understanding of the relationship between the generated HF network and the induced microseismicity while taking into account the effect of different HF parameters (injection fluid type and rate, stress conditions). In this study, HF experiments were performed on true-triaxially loaded Barre granite cubes, with real-time microseismic monitoring, to identify and characterize the stimulation processes associated with the viscosity and toughness dominated HF propagation regimes. Water and gear oil were used as the fracturing fluids. Moment tensor inversion technique was employed to determine the fracture mechanisms (tensile, shear, or mixed-mode). Viscosity propagation regime experiments involved higher breakdown pressures and larger injection fluid volumes relative to toughness propagation regime experiments. The microseismicity from toughness propagation regime experiments resulted in slightly larger b-value (2.25 compared to 2), indicating higher percentage of small magnitude events. The spatio-temporal evolution of fracture mechanisms indicated very dominant tensile fracturing (82-85%) during fracture initiation phase surrounding the injection region. As the fracture propagated away from the injection borehole, the number of shear and mixed-mode fracturing events increased. Overall, tensile fractures were dominant in both propagation regimes (ranging from 52% to 58%), which can be attributed maily to the absence of significant pre-existing faults/discontinuities in the very low permeability granite rock.