Experimental study on attenuation characteristics of elastic wavespropagation in layered coal rock

The micro-seismic technology based on elastic wave propagation has been applied to the monitoring of hydraulic fracturing in coal mine and the early warning of dynamic disasters. As a typical stratified medium,the propagation of elastic waves in the stratified coal rocks is affected by the heterogeneity of the media and the reflection and refraction of the layers. Therefore,the study on the propagation law and attenuation characteristics of the elastic waves still needs to be improved. The attenuation characteristics of elastic wave propagating in layered coal,pure coal and pure sandstone are studied by using DS2 series full information acoustic emission signal analyzer. The results show that under the same test conditions,the attenuation of amplitude of elastic wave signals generated by different groups of coal rocks decreases with the increase of source distance and source excitation frequency. In layered coal rock,the attenuation of the amplitude of elastic wave signal is first fast and then slow,and the attenuation gradient in the coal seam is larger than that in the rock layer,showing a periodic attenuation characteristic. The centroid frequency of the elastic wave signal is shifted,and the peak frequency is graded. The main factor affecting the attenuation of elastic wave is the degree of tightness between the coal seam and rock layer,the higher the degree of tightness,the smaller the attenuation; secondly,the higher the hardness of the coal seam,the greater the initial amplitude of the elastic wave signal will be. In the low frequency range (FREQ≤4 MHz),the elastic wave amplitude in the layered soft coal seam decayed faster than the hard one; in layered coal rock,the source excitation frequency and elastic wave propagation distance are negatively correlated and dependent,the low-frequency signals in the short range have a farther propagation distance and higher amplitude attenuation gradient than the high-frequency signals; thirdly,the attenuation gradient of the elastic wave amplitude gradually decreases as the quality factor increases,and the quality factor from small to large is soft coal,hard coal,and sandstone. The research results can provide a theoretical basis for the optimization of the arrangement method of the micro-seismic pickup.