| My country's coal seams are affected by multiple periods of structural history,the hidden hazards such as faults,collapse columns,stress concentration areas and gas in the coal seams have seriously affected the safe and efficient mining of coal mines.In order to reduce the occurrence of disasters and accidents during coal mining,exploration the stress state of the coal seam and the occurrence of gas are particularly important.Ultrasonic detection technology has the characteristics of good directionality,strong penetrability,long propagation distance and not easy attenuation,and is very suitable for the field of coal seam geophysical exploration.In order to obtain a clear coal seam ultrasonic geophysical inversion law,this article is based on the relevant theories of ultrasonic dynamics,rock mechanics and seepage mechanics,and conducts research from three aspects:laboratory analysis,mechanism research and field test verification.The main research contents of the thesis are as follows:(1)The NM-4B non-metal ultrasonic detection analyzer,low-frequency rock physics measuring device,and other instruments were used to carry out coal ultrasonic experiments under average temperature and pressure,stress loading,and adsorption and desorption conditions.The corresponding relationship between the ultrasonic kinematic characteristics(P-wave and W-wave velocity)and coal type,stress loading state,and adsorption and desorption state based on the consideration of coal anisotropy is obtained.And the corresponding relationship between dynamic characteristics(quality factor)and coal structure type,stress,and strain.(2)Combined with a typical coal body's stress-strain curve under uniaxial loading,the change mechanism of coal body ultrasonic velocity and quality factor under different pressure conditions is studied.Simultaneously,under the premise of only considering the attenuation of ultrasonic scattering,the theory of fracture mechanics is further used to analyze the crack development law of coal under load,and the theoretical calculation model of coal quality factor under uniaxial action is deduced.A comparative analysis of the ratio of the quality factors obtained from the experiment and theory shows that the two's trends are in good agreement.(3)By analyzing the change of ultrasonic velocity of coal under the gas adsorption experiment's conditions,it is obtained that the sonic velocity is affected by the combined effect of the positive effect of gas adsorption expansion and the negative effect of gas pressure compression.1.5 MPa is the critical gas pressure for the change of ultrasonic wave velocity of coal.When the gas pressure is less than 1.5 MPa,the positive effect of gas adsorption and expansion is dominant,manifested as an increase in wave velocity.When the gas pressure is more significant than 1.5 MPa,the negative effect of gas pressure compressing the matrix is dominant,manifested as a reduction in wave velocity.From the perspective of the proportion of coal porosity,the expression of the ultrasonic velocity of gas-containing coal under different gas pressure conditions is derived.(4)The improved ultrasonic detection device was used to carry out an ultrasonic test on the 310205 working face of Xinyuan Coal Co.,Ltd.It was obtained that there is a good correspondence between the coal P-wave velocity and the supporting pressure in front of the working face.The P-wave velocity has an excellent positive correlation with the regional gas content,which provides a new method for using ultrasonic detection technology to determine the degree of coal seam gas enrichment. |
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