Experimental research on effects of water absorption on acoustic velocity anisotropy in coal rock
Zhao Yu1,2, Zhang Yugui2, Yu Hongyi2
1. School of Civil Engineering, Henan Polytechnic University, Jiaozuo, Henan 454000, China; 2. College of Safety Sciences and Engineering, Henan Polytechnic University, Jiaozuo, Henan 454000, China
Abstract:In the research of coal rock ultrasonic wave response characteristics,the influences of water absorption and anisotropy on longitudinal wave and transverse wave velocity are lack of comprehensive consideration.However,these issues have great significance for ultrasonic technology in coal physical characteristics tests.In our experiment,J16-17 seams in the Pingdingshan No.8 Mine are selected,the vertical-bedding samples (Z-direction),the parallel-bedding and vertical-face cleat samples (Y-direction),the parallel-bedding and vertical-butt cleat samples (X-direction) are used for water absorption and ultrasonic experiments.The following experimental observations are obtained:①Under the same conditions the water absorption for the same coal seam in different directions are anisotropic,the Y-direction samples are greater than the X-direction and Z-direction samples; ②The anisotropy of P-wave and S-wave velocity in coal layers is obvious,the maximum velocity is in the Z direction,and the minimum velocity is in the Y direction; ③P-wave velocity shows approximately exponential variation with water content increase.In early stages,when the water saturation is less than 60%,the change of water absorption in coal is ignored,which may minimally affect P-wave velocity.When the water saturation is higher than 60% (corresponding to WX=1.3%,WY=2.65%,WZ=0.72%),P-wave velocity increases greatly in perpendicular to bedding and parallel bedding coal; ④S-wave velocity increases with coal natural water absorption.But this change is much less than P-wave.The velocity ratio of P-/S-waves in coal samples decreases first and then increases with water absorption rate; ⑤The influence mechanism of pore fluid on ultrasonic parameters is analyzed with the space average model and the time average model,P-wave is very sensitive to pore fluid in coals,but S-wave velocity can be ignored.This property can be used to detect the presence and movement of fluid in coal.
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