A microseismic (MS) monitoring system in a mine can monitor the MS signals generated by coal rock rupture and blasting waves and can distinguish the two types of waves more clearly to monitor and analyze the rupture and evolution process of coal rock. According to the nonlinearity characteristics of the waveform, the fractal characteristics of a mine’s MS and blasting waves are analyzed by simple fractal and multifractal theory, and the simple fractal dimension [Formula: see text] and multifractal parameters are obtained, respectively. Results show that the simple fractal dimension [Formula: see text] reflects the complexity and frequency structure of the wave. The simple fractal dimension [Formula: see text] of a blasting wave is larger than that of a mine MS wave, which indicates that the blasting wave is relatively complex with higher frequency, while the mine MS wave is relatively simple with lower frequency. However, the simple fractal dimension [Formula: see text] can only describe the wave integrity features, not the local features. The multifractal parameters can describe the local characteristics of the wave more finely, and the multifractal spectrum describes the probability information of the singularity exponent [Formula: see text]. The singularity exponential range and multifractal spectral width [Formula: see text] of the blasting wave are smaller than those of the mine MS wave, which indicates that the probability measure of distribution unevenness and the degree of partial parameter fluctuation of the blasting wave are more severe than those of the mine MS wave. Wave signal analysis based on simple fractal and multifractal methods can not only obtain the characteristics of the wave strength and spectral structure but also other important information, such as local singularity. Therefore, it is possible to more clearly and conspicuously identify mine MS and blasting waves, so that coal rock rupture can be monitored more accurately.
The elastic wave propagating in rock has velocity dispersion characteristic that holds an imperative significance in rock engineering. This paper aims at study elastic wave velocity dispersion characteristic in limestone dynamic fracture process. For this purpose, the experiment of elastic wave propagates in the limestone when cyclic loading and unloading process is conducted, and elastic wave velocity dispersion characteristic is analyzed. Based on multifractal theory and Raiga model, the influence of fracture degree on velocity dispersion characteristic is discussed qualitatively and quantitatively with evolution characteristic of the AE event. The research results indicate that there is velocity dispersion characteristic when elastic wave propagates in the limestone sample, which presents positive correlation between velocity and frequency. The variation of wave velocity dispersion characteristic is consistent with that of AE activities and both of them show a nonlinear increase trend with an increase of loading. The reason for velocity dispersion characteristic is that the retarding effect of microcracks on elastic wave velocity with different frequencies is different, and the effect is enhanced with the microcrack scale increasing. According to the variation of multifractal parameter [Formula: see text] of the AE event energy series, the limestone sample under loading experiences a transformation process from small-scale microcrack to large-scale microcrack and small-scale microcrack dominance to multi-scale microcrack coexistence, and this complex transformation process results in nonlinear variation of velocity dispersion characteristic. Further, the multifractal parameter [Formula: see text] is introduced into Raiga model to establish the velocity dispersion quantitative model. The model can describe well elastic wave velocity dispersion characteristic, and indicates quantitatively that there is an obvious nonlinear positive correlation between the dispersion characteristic and the fracture degree. The research results have important theoretical and practical significance for understanding elastic wave velocity dispersion characteristic in limestone dynamic fracture process. It can provide important support for characterizing rock fracture process using elastic wave velocity dispersion characteristic.
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