Path Loss Estimation of Wireless Sensor Networks in Coal Mine Collapsed Zone

Wireless underground sensor network (WUSN) has potential application value in environmental monitoring, disaster detection, and personnel rescue in the coal mine collapsed zone. Due to the absorption and reflection of rocks and minerals, the attenuation law of the electromagnetic signal in the coal mine collapsed area is complicated and the path loss is large. Understanding the signal attenuation characteristics and estimating path loss is the basis for WUSN design, deployment, and optimization. In this article, we conduct an extensive survey on the geological structures and materials of coal mine collapsed areas. We propose a path loss model for wireless communications in coal mine collapsed areas. The signal attenuation under different geological and communication conditions is discussed. Based on the model results, the basic design suggestions of WUSN in coal mine collapsed areas are put forward. When the material in the collapsed area is shale or mudstone, the WUSN design should focus on solving the problem of large signal path loss. When the collapsed area is sandstone or limestone, the WUSN design should focus on solving the multipath fading problem. The signal with operating frequency of 300–600 MHz has a small path loss in the mine collapsed area, but the signal power fluctuates greatly. Signal fluctuation can be reduced by shortening the communication distance, placing the wireless node near the roof or the floor of the collapsed zone, and using vertically polarized antennas.