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Abstract—Using a mathematical statistics approach, we review the procedure for type classification of acoustic emission (AE) events into shear, tension, and collapse, proposed by Zang et al. (1998). The procedure is based on counting the signs of first pulses of waves arriving at acoustic sensors and is widely used in rock physics experiments. Under the assumption that the determination errors of first-pulse signs at sensors have uniform and independent distribution , the statistical significance and power of the type separation test are evaluated for a given number of sensors used. We consider and compare three methods of the construction of a statistical test based on the P-value approach and symmetric and asymmetric statistical hypothesis tests. Considering the results of the statistical study, we propose some practical recommendations for selecting a threshold to classify AE event types in experimental studies. [ABSTRACT FROM AUTHOR]

Abstract—The acoustic emission (AE) patterns observed in laboratory conditions at the initiation of fracture in rock samples by fluids of different viscosities are revealed. A series of experiments with sandstones and granites of order-of-magnitude different porosities and fluids of viscosities ranging within two orders of magnitude have been conducted. The effects of fluid injection into dry samples and pore pressure increase in saturated samples are examined. Pore pressure was varied both in abrupt steps and in smooth increase-and-decrease cycles. In the case of fluid injection into dry samples, the time delay of AE activation corresponds to the fluid front propagation time calculated in the piston-type model of air displacement by a fluid in a pore space. In the case of fracture initiation by steps in pore fluid pressure in saturated samples, the time delay of the response is substantially longer than predicted by the linear piezo conductivity model with constant hydraulic diffusivity and significantly shorter than the time of fluid front propagation in a dry sample. The experiments with smooth variations in the pore pressure in saturated samples revealed characteristic changes in AE activity: the minimum b-values (slope of magnitude–frequency relationship) fall in the intervals of the maxima in acoustic responses, and the maximum b-values occur during the rise and fall phases of pore pressure. The results of the experiments on the rock samples of different porosities with fluids of different viscosities can be useful in the analysis of field data both in the regions of anthropogenic seismicity associated with reservoir operation and fluid injection in wells and in the interpretation of the seismicity patterns due to tectonic and volcanic activity in the subduction zones. [ABSTRACT FROM AUTHOR]

Abstract—Experiments on sandstone, basalt and granite were carried out on a controlled hydraulic press under triaxial loading conditions at confining pressure of 5, 10 and 15 MPa. The tests on sandstone and basalt were conducted with the injection of a fluid (water) and subsequent modulation of pore-fluid pressure, and the tests on granite were performed without fluid injection. In the experiments, both the acoustic emission (AE) signals arising during fracture of the material and the ultrasonic (U/S) sounding signals were recorded in 16 directions. Based on the U/S data, a change in the power spectra of recorded U/S signals is revealed at different stages of sample testing. It is shown that the changes in the median of the power spectrum of U/S signals (median frequency fmed) can reach more than 100% depending on the degree of fracture in a rock; besides, compared to elastic velocities, the changes in the spectral content of the U/S signals more dynamically reflect fracture development in samples. The dependence of median frequency on applied axial load is established. It is shown that at fluid injection into a dry sample, fmed shifts strongly towards the low frequencies. The shift of individual spectral components of the U/S signals can be considered as an indicator of changes occurring in the sample. A decline in fmed testifies to the increasing degree of fracture of the material. The effect is most pronounced in highly porous sandstones. [ABSTRACT FROM AUTHOR]

Laboratory experiments on studying the aftershock regime are carried out on sandstone specimens at different levels of axial loading and uniform compression and at constant pore pressure. The aftershock sequences are modeled by the scenario of stepwise increasing axial loading of a specimen with strain control, which ensures the regular generation of aftershock sequences. The experiments are conducted on intact specimens and on those with preliminarily formed shear macrofractures simulating natural faults. The multichannel recording of the signals of acoustic emission (AE) during the experiments allowed locating the AE sources. Several types of the dependence of the parameters of relaxation of the acoustic activity—parameters p and c of the modified Omori law and the Gutenberg–Richter b-value—on the level of acting stresses are revealed. The b-value decreases with the growth of axial stresses at all levels of uniform compression. In the case of a fracture on the preexisting fault, the Omori relaxation parameter p increases with the growth of axial stresses; parameter c—the time delay before the onset of relaxation—decreases with the growth of axial stresses and increases with the rise of the level of uniform compression. In the case of a fracture of an undamaged specimen, parameter p remains unchanged with the growth of axial stresses, whereas parameter c increases slightly. Parameter variations in the case of a complex stress state with both varying deviatoric (differential stresses) and spherical parts (effective pressure) of the stress tensor take on a unified form when expressed in terms of Coulomb stresses. It is hypothesized that the time delay of the relaxation of the aftershock activity is determined by the kinetics of a fracture in accordance with the kinetic concept of strength in solids. This hypothesis is supported by the exponential dependences of parameter c on stresses and the effective strength of the medium which are revealed in the experiments. Under this hypothesis, based on Zhurkov's formula for the durability of materials, it is possible to unify the dependences of parameter c on the Coulomb stresses at different effective strength values. The obtained parameter estimates for the dependence of c on strength and stresses suggest that the c value is determined by the difference of the strength and the acting stresses, thus indicating how far the stress state of the medium is from critically corresponding to the ultimate strength. [ABSTRACT FROM AUTHOR]