Very Low Frequency Propagation Characteristics Analysis in Coal Mines

Electromagnetic wave penetration coal-rock communication is a significant part for the smart mine communication technology, there are great challenges for electromagnetic wave is rapidly attenuated by factors such as geology and coal seam structure. In order to provide a theory basis for wireless communication technology development in coal rock, based on the Maxwell's equations, this paper establishes a physical model of wireless communication under the conditions of coal seam. To characterize the performance of the electromagnetic wave propagation, we use the Maxwell's equations to derive the exact expression of attenuation coefficient. Also, the parameters of coal and other factors affecting the electromagnetic wave propagation are analyzed and discussed. To further obtain more insights, the attenuation coefficient and skin depth of the very low-frequency (VLF) electromagnetic wave in coal medium with different degrees of metamorphism are studied, as well as the influence of resistivity. This provides scientific theoretical support for the application of VLF communication in coal mines. Finally, our theoretical analyses are verified by computer simulation, and the simulated numerical results show that: 1) The range of frequency of electromagnetic wave suitable for coal medium propagation is 3~3KHz (VLF); 2) The order of the electromagnetic waves attenuation coefficient in coal with different degrees of metamorphism is: anthracite> lignite > fat coal > coking coal > lean coal, and the order of skin depth is: lean coal > coking coal > fat coal> lignite > anthracite; 3) The resistivity of coal has little effect on the attenuation of electromagnetic wave when VLF is used for communication.