Numerical Simulation Of Finite Difference Forward Modeling Of Mine Transmission Transient Electromagnetic Using Magnetic Source

Coal mining face often encounters structure and mine water inrush during the mining process,which seriously affects the production schedule and the safety of personnel and equipment.The current geophysical methods for detecting geological anomalies in the working face have their own limitations.Due to the limited detection depth of reflected transient electromagnetics,it is often impossible to effectively detect the geological structure in the working face.The transmission transient electromagnetic method has many advantages such as accurate positioning compared to the reflection transient electromagnetic method.In this paper,numerical simulation is used to study the transmission transient electromagnetic technology of the magnetic source.First,based on Maxwell's equations and finite-difference theory in time domain,three-dimensional transient electromagnetic finite-difference forward calculation is implemented.Magnetic dipole is used as the emission source.CPML boundary is used to reduce the reflection in the calculation space and boundary.On this basis,numerical simulations were conducted on the uniform full-space model,coal seam and roadway model,high-resistance body model in working face and low-resistance body model in working face,and the transmission response characteristics of each model were analyzed and summarized,and It is compared with the reflection response characteristics.The results of the study show that:(1)The "smoke ring" of the magnetic dipole in a uniform whole space always spreads out from the center of the current ring on the plane of the equivalent current-carrying current loop of the magnetic dipole,which is different from the diffusion law in half space.(2)Numerical simulation results show that the transmission transient electromagnetic method has a larger detection range than the reflection transient electrical method in theory,and the effective transmission distance can be further increased in high-resistance surrounding rocks.(3)The presence of coal seams and roadways has little effect on the reflected transient electromagnetic response,but has a greater effect on the transmitted transient electromagnetic response.(4)The transmission transient electromagnetic method is the same as the reflection transient electromagnetic method.It does not reflect the high-resistance body obviously,but the low-resistance body obviously.The low-resistance body in the working surface will attract the center of the electromagnetic field from the emission position to the center of the low-resistance body,and reduce the overall attenuation speed of the reflected and transmitted magnetic fields,and reduce the peak value of the transmission induced electromotive force curve.(5)When the size,electrical parameters and center position of the low-resistance body in the working surface are the same,compared with the low-resistance body running perpendicular to the working surface,the reflection and transmission response characteristics of the low-resistance body parallel to the working surface are more obvious.