Forward Modeling Of Electromagnetic Characteristics Of Abnormal Body In Coal Seam With Space Stepping Frequency Ground Penetrating Radar

At present,coal mines still occupy a dominant position in China's energy system.The unfavorable geological anomaly structures such as mine goafs,water-rich areas,and collapse columns in the coal body in front of mining and tunneling have brought great safety hazards to coal mining.As a safe and efficient non-destructive detection and exploration technology,ground penetrating radar is widely used in advanced detection of geological structures in front of underground tunneling.Aiming at the problems of low detection resolution and:small amount of acquired data in traditional underground single-transmit and single-receive detection radars,it is proposed to use the spatial stepped frequency radar under the multiple-input multiple-output system for underground detection.This article mainly focuses on the propagation of the spatial stepped frequency signal in the coal seam structure in the direction of driving.Firstly,using the central difference theory,the finite-difference time-domain algorithm is derived,and the stability conditions and absorption boundary conditions of the algorithm are studied.Then,aiming at the problem of abnormal structure in the coal seam in the heading direction of coal mining,the abnormal body detection model of coal seam was established.The FDTD algorithm was used to carry out forward simulation on the established model to explore the propagation law of spatial stepping frequency signal in the coal seam in the heading direction.According to the small structural environment of underground coal mine,the forward modeling is carried out for the three situations of different shapes of abnormal bodies in coal seam,different media and different burial depths.The experimental results show that with the increase of relative dielectric constant of abnormal body in coal seam,the amplitude of reflected echo increases.When the signal propagates in coal seam and enters water and rock,the phase of echo changes in reverse,while when it enters air,it remains the original phase.With the increase of depth in the same medium,the amplitude of reflected wave decreases.According to the stratified geological environment,the model of coal seam goaf,water-rich area and two kinds of rock area is established.The forward modeling results show that the reflection echoes from the front and back surface can be observed in both the goaf and the rock layer,and only one reflection echo from the upper surface can be observed in the water-rich area.The simulation proves that the spatial stepping frequency signal can detect the abnormal body of coal seam.In the process of forward data preprocessing,in view of the direct coupling wave in the simulation result,the method of reducing the air-to-air signal is used to remove it;in view of the direct wave problem on the coal surface,the SVD average cancellation fusion algorithm is proposed to filter the direct wave.In addition,its effectiveness is verified through experiments;the method of gain adjustment is used to enhance the echo signal of abnormal bodies inside the coal seam.After the data is preprocessed,the interference between the direct coupling wave and the direct wave is reduced,and the echo of the abnormal body is clear and distinguishable.Through the use of forward simulation results to verify the model thickness parameters and achieve effective imaging of abnormal body regions.