Forward Modeling Using Finite Difference Time Domain Method For Ground Penetrating Radar And Its Experiment

Ground penetrating radar(GPR)is a high-precision,efficient and non-destructive survey technique using high-frequency electromagnetic wave.As a result of the high-frequency electromagnetic waves,its investigation depth is not high,but there is high resolution.According to the theory of electromagnetic wave propagation,high frequency electromagnetic wave propagation in the medium satisfies the Maxwell's equations.In order to study the propagation characteristics of radar signals better in the underground media,the finite difference time domain(FDTD)method is used to simulate electromagnetic wave propagation.After decades of development,the FDTD method has been maturely used in various fields.Over time,the FDTD method may intuitively show the evolution of electromagnetic wave field.In the numerical simulation,c language program is used to export data,and Matlab is used to draw simulation results.Because the resulting error will be caused by the numerical dispersion phenomena after discreting space,in theory,the smaller Yee grid is,the higher the precision is.Time step depends on the calculating resources,but also in line with the law---the smaller the time step is,the higher the accuracy is.By changing the size of dt and dx,we can explore the impact of numerical dispersion.In the forward modeling,I simulated the spread waveform using various excitation sources in various medium.According to the actual situation,I set the electromagnetic parameters of the background media and obstacles medium,and simulated the actual detection process.During the simulation,because the simulation region is limited,the electromagnetic wave propagation into the grid boundaries will inevitably result in wave reflection which affects the forward calculation results.During the simulation,the PML absorbing boundary conditions are added for the loss of electromagnetic reflection wave from the boundaries in the calculation region.In the latter part of the inversion process,this paper introduces five kinds of velocity analysis criterion which is used to estimate the velocity before migration,and this is the key of getting the optimal migration results.I mainly processed the reflected waveform from underground geologic body using the frequency-wavenumber(F-K)migration method.I processed the actual data by using this method and achieved good results,which is of great importance to accurate positioning of underground abnormal body position and size for practical engineering application.