Analysis of acoustic wave transmission characteristics in underground coal mine drill pipe

The acoustic wave transmission of MWD data is an important means to realize the high-speed and accurate transmission of borehole data in coal mine, which is conducive to changing the situation that most of the underground gas extraction in coal mine is in “blind drilling”. In order to verify the reliability of acoustic wave transmission in drill pipe, the acoustic wave propagation characteristics of drill pipe pillar composed of mining drill pipe are investigated. According to the basic theory of sound wave transmission in solid, the sound wave transmission channel is equivalent to a periodic structure composed of drill pipe and its joint, and the transmission characteristics of sound wave in drill pipe are analyzed by using the propagation equation of sound wave in periodic structure. Through numerical simulation, the propagation of sound waves in drill pipe is analyzed, and the effects of different drill pipe diameters, knock positions and sand injection conditions on the propagation characteristics and attenuation law of sound waves are compared. The results show that the larger outer diameter of drill pipe and the lower knock point are beneficial to signal transmission. The water injection signal inside the drill pipe attenuates obviously, and the amplitude of the signal attenuates exponentially, while the influence of the external sand environment on the signal intensity can be ignored. The excitation signal, acoustic output and attenuation characteristics obtained by the test platform agree with the basic trend of the simulation results.