Transient Electromagnetic Forward Modeling Of Coal Seam Goaf Based On CN-FDTD Method

As a highly effective geophysical prospecting method,transient electromagnetic method(TEM)is widely used in geological exploration.Its employment in the exploration of coal seam distribution exhibits high theoretical and practical application.Finite-difference time domain(FDTD)algorithm is a main method to calculate transient electromagnetic three-dimensional forward modeling method,but it also has its own advantages.In other words,the method is restrained by the stability conditions of Courant-Friedrich-Levy(CFL),namely,its selection of time step is related to space step,in the simulation of the fine structures,the traditional FDTD method tends to be super time-consuming.In order to solve the above-mentioned problems,Crank-Nicolson finite-difference time domain(CN-FDTD)algorithm was used as the calculation method for transient electromagnetic three-dimensional forward modeling.It can do without meeting the stability conditions of CFL and enjoys a good calculation accuracy.Based on the analysis of the CN-FDTD algorithm,this paper adopts the CN-FDTD as the calculation method to perform the transient electromagnetic three-dimensional forward modelling for the uniform half-space model,and analyzes the influence of the changes of parameters(resistivity,current amplitude,turn-off time)on the simulation results.On this basis,forward modelling was run for the typical two-layer geoelectric model including type D geoelectric model and type G geoelectric model,and the typical three-layer geoelectric model including type A geoelectric model,type H geoelectric model,type K geoelectric model and type Q geoelectric model,and compared with the uniform half-space model.Then the law of transient electromagnetic response curves for corresponding geoelectric model was obtained;In addition,CN-FDTD transient electromagnetic forward modelling was applied to the simulation analysis of coal seam goaf.The transient electromagnetic response rules of one-layer low-resistance model,one-layer high-resistance model,two-layer low-resistance model,two-layer high-resistance model,two-layer high-resistance model,two-layer high-and low-resistance model and two-layer high-and low-resistance model were established and analyzed.The simulation results were compared and analyzed.The influence of the changes of water-filling conditions in goaf on transient electromagnetic simulation results is also discussed.In the above-mentioned analysis,the parallel method MPI(Message Passing Interface)was introduced,and simulation was carried out.The simulation results show that the CN-FDTD method proves desired applicability in coal seam goaf,and MPI method can effectively shorten the simulation calculation time and lift the calculation efficiency.