Uncertainty investigation for the classification of rock micro-fracture types using acoustic emission parameters

Abstract

Classification of rock micro-fractures by acoustic emission (AE) parameters is of great significance for rock health monitoring, instability assessment, and precursor warning. But the criteria have not yet been universally formed for the most commonly used method employing rise angle (RA) and average frequency (AF). We conducted expansion rupturing experiment and elastic wave attenuation experiments to investigate the uncertainties of the RA-AF method. Results show the range of the RA-AF distribution extends with the reduction of the AE source scale. The RA-AF moves to the shear-type area in the axis for AE hits with delayed triggering. The RA increase togethering with AF decreasing when the specimen become more compact and smaller in size. It indicates that the fixed micro-fracture reference line is difficult to match potential changes in the RA-AF distribution and the classification criteria is highly dependent on the experimental environment. The uncertainty of the RA-AF method is influenced by the ray path and the source scale. The underlying reason is the uncoordinated distortion between the numerator and denominator in the RA-AF calculation caused by the attenuation and dispersion of elastic wave as well as the measurement error of characteristic parameters. It is concluded that the characteristics of RA-AF are available to understand the comparative relation of different micro-fracture types, but quantify their relations by the absolute proportion requiring consideration of a number of prerequisites. This work not only clarifies the phenomena and reasons for the uncertainty of the RA-AF method but also provides theoretical and experimental guidance for more rational improvement and engineering application.