Research on karst collapse pillar imaging of water-rich area in coal mine

In order to solve the problem that selection of time step was limited by Courant-Friedrich-Lewy stability condition in current finite-difference time-domain(FDTD) method commonly used in advanced detection transient electromagnetic method of water-rich area in coal mine, and further improve electromagnetic calculation efficiency and imaging resolution of water-rich area, reverse time migration imaging algorithm and Crank-Nicolson finite-difference time-domain(CN-FDTD) method were applied to research of karst collapse pillar(KCP) imaging of water-rich area in coal mine. Firstly, basic principles of reverse time migration imaging algorithm and CN-FDTD method were introduced. Then a three-dimensional KCP model of water-rich area in coal mine was established. Influence of frequency and angle of excitation source coil on imaging resolution was researched, and imaging results of the KCP were obtained. Finally, computational efficiency of CN-FDTD method was analyzed. The experimental results show that when peak frequency of excitation source coil is 65 MHz and the excitation source coil is parallel to xoz plane, imaging resolution of the KCP is high. The KCP imaging of water-rich area in coal mine based on CN-FDTD reverse time migration imaging method is consistent with the actual model. Compared with traditional FDTD method, CN-FDTD method has higher computational efficiency and smaller memory proportion.