Theoretical Channel Model and Characteristics Analysis of EM-MWD in the Underground Coal Mine

The measurement while drilling (MWD) system in underground coal mine currently uses wire for signal transmission. However, it has shortcomings such as strict technical requirements for cable drill pipe processing and high cost of use. Electromagnetic measurement while drilling (EM-MWD) is a new technology has a high rate of information transmission with fast measurement speed and low cost and is not affected by the drilling media. Hence, it is necessary to investigate a reliable electromagnetic transmission channel which is suitable for underground coal mine. This study selects the excitation method through comparison and conducts theoretical research on this basis. Based on the classical electromagnetic theory, the equivalent transmission line method is adopted to calculate and analyze the channel characteristics, while the electrical properties of the coal seam and the surrounding rock are considered. The results demonstrate that the formation resistivity, drill pipe resistivity, and the emission frequency are the main factors affecting signal transmission. On the basis of the uniform equivalent transmission line model, the theoretical model is established by combining the segmented uniform transmission line theory. The layered equivalent transmission line model can be better used to evaluate the regional applicability of the EM-MWD system.