Study On Numerical Simulation And Imaging Of Ground Penetrating Radar In Tunnel Engineering

Ground penetrating radar(GPR)is widely used in tunnel engineering because of its high resolution,high efficiency,high anti-jamming and non-destructive characteristics.With the increasing demand for detection accuracy in engineering applications,it is particularly important to conduct theoretical research on GPR and guide its application with theory.In order to improve the detection capability of GPR and the interpretation accuracy of radar image,this paper carries out the discussion and research on imaging based on the numerical simulation of GPR.Firstly,the basic principle of ground penetrating radar(GPR)and the forward simulation principle of finite difference time domain(FDTD)method are introduced in detail.The factors affecting the detection of ground penetrating radar are numerically simulated by gprMax software.The simulation results directly reflect the influence of the depth,size,shape,electrical property,antenna central frequency of radar system and the antenna transmission and reception distance on the detection ability of ground penetrating radar,and summarize the suitable targets for ground penetrating radar detection and a set of parameters to improve the detection effect of ground penetrating radar.That is,the shallower the target is,the larger the volume is,the greater the difference between the relative dielectric constant and the surrounding medium is,the more obvious the reflected signal is,and the better the detection effect is;the setting of working parameters should take into account the resolution and depth of detection of the design,and not only follow the theoretical formula,but also choose the best value according to the actual working conditions.Secondly,based on the conclusions obtained from the discussion of influencing factors,a reasonable two-dimensional numerical simulation of typical diseases in tunnel engineering application is carried out,and the accuracy of the numerical simulation is verified by comparing with the actual GPR profile.By analyzing the results of numerical simulation of karst caves,fault fractured zones,weak interbeds,lining voids,lining voids,steel mesh and combined model in tunnel lining quality detection in advance prediction,it is concluded that karst caves and lining voids are hyperbolic reflective signals,and the coaxial is arc continuous;the fault fractured zones are strong reflective signals,and the coaxial is continuous.The amplitude changes greatly and attenuates rapidly;the weak interlayer shows clear reflection surface,continuous and smooth in-phase axis,and fast attenuation;the lining void shows obvious reflection surface,continuous in-phase axis and consistent with actual void,and hyperbolic diffraction waves at both ends;the steel bar network shows a row of regular hyperbolic reflection waves;the combined model shows the superposition of response characteristics of a single target body.There is no obvious rule to follow.The complete and systematic tunnel disease response characteristics and identification methods are summarized.Finally,further imaging processing of numerical simulation results of typical tunnel diseases is realized by MATLAB programming and MATGPR.Based on the numerical simulation of three-dimensional model,three-dimensional data imaging is realized.The image after three-dimensional imaging can reflect the spatial position,size and other information of the object in three-dimensional.Based on twodimensional finite-difference time-domain forward modeling,the principle and imaging conditions of reverse-time migration method are studied.Post-stack reversetime migration imaging based on zero-time imaging condition is realized.Reverse-time migration processing can converge the diffraction wave,locate the reflected wave and accurately reflect the position,shape and size of the target body.Through the study of numerical simulation and imaging of ground penetrating radar(GPR),the methods to improve the detection ability of GPR are summarized.The way to improve the interpretation accuracy of GPR image is further broadened from the qualitative and quantitative point of view,which is of guiding significance to the practical application of GPR in tunnel engineering.