Investigation On Broadband Composite Electromagnetic Scattering And Imaging Of Layered Rough Surfaces And Buried Targets

The investigation of electromagnetic scattering and imaging of layered rough surfaces and buried targets have very important practical significance and research value in radar detection,target recognition,microwave imaging and other fields.For the scattering problem,the finite difference time domain(FDTD)method is used to model and calculate the electromagnetic scattering from layered rough surfaces and composite scattering from layered rough surfaces and buried targets in ground penetrating radar(GPR)detection.With high accuracy,FDTD method can conveniently deal with complicated medium model consists of layered rough surfaces with different roughness and targets with different shape,size,and dielectric properties.Moreover,FDTD method is a time-domain method which is easy to deal with broadband scattering.It can be well adapted to simulate the broadband electromagnetic scattering of pulse signal and the visualization processing of time-domain signal which are mainly involved in ground penetrating radar detection.For the imaging problem,a time delay calibration method and a back projection algorithm based on time-delay pre-calibration are proposed to estimate the profiles of layered rough surfaces and to imaging the target buried under rough surface respectively.The main work and achievements are as follows:1.The FDTD computation models of broadband electromagnetic scattering from single-,double-and three-layered rough surface in GPR detection are established respectively,and the B-Scan echo data in each case are obtained.The influence of root mean square(RMS)height and correlation length on the characteristics of GPR echo from single-layer rough surface,and the relationship between the echo shape and the profile of the single-layer rough surface are analyzed.The scattering characteristics of two-layered rough surfaces and three-layered rough surfaces in three cases of the same profile,different profile but same roughness,and different roughness are discussed respectively,and the influence of the upper rough surface scattering on the echo shape of the lower rough surface is investigated.2.The broadband electromagnetic scattering from simple targets and the composite scattering from buried targets and smooth horizontal interface,buried targets and single-layer rough surface,and buried targets and multi-layer rough surface are modeled and calculated using FDTD method respectively.The echo characteristics of rectangular targets,circular and elliptical targets,triangular targets and fissure targets are analyzed.The effects of shape,size,dielectric properties and placement angle of these targets on the target echo shape are discussed.The influence of single-layer rough surface scattering with different roughness on the echo shape of rectangular and circular buried targets in composite scattering is analyzed,and the composite scattering characteristics of single-layer rough surface and multiple buried targets are discussed.The influence of scattering from double-layered rough surfaces with the same roughness and different roughness on the echo characteristics of rectangular metal buried targets is analyzed.The effects of three-layered rough surfaces scattering in three cases of different RMS heights,different correlation lengths,and RMS heights and correlation lengths are all different on the echo characteristics of buried targets are discussed.3.The profile estimation method of double-layer rough surface scattering is studied.The profile estimation of the upper rough surface is realized by extracting the echo peak curve of the rough surface directly.To estimate the profile of the lower rough surface,a time-delay calibration method is proposed to calibrate the echo shape of the lower rough surface,that is,a smooth horizontal interface is artificially set to replace the original rough surface.The time-delay caused by the distance between the smooth horizontal interface and the rough surface is derived.The derived time-delay is added to the echo of the lower rough surface to calibrate the shape of the echo.The calibrated echo peak curve can be very close to the desired profile of the lower rough surface,thus realizing the profile estimation of the lower rough surface.Calibration calculation is carried out for the echo shape of the lower rough surface in the two cases of same roughness and different roughness of the upper and lower rough surfaces,and the calibration effect is verified for both the cases.4.The time-delay calibration method is extended to the profile estimation of multi-layer rough surface.The calculation steps of the profile estimation of the multi-layered rough surfaces are analyzed,and the calculation formula of the step-by-step time-delay calibration algorithm for the profile estimation of the multi-layer rough surface is deduced.Using this calculation formula,the rough surface profile of any layer can be estimated.The step-by-step multi-layer rough surface time-delay calibration algorithm is applied to the profile estimation of three-layer rough surface with different roughness under two different media parameters settings.The profiles of the second-layer and the third-layer rough surfaces are estimated to verify the profile estimation effectiveness of the proposed step-by-step time-delay calibrated algorithm.5.The back projection(BP)method is used to imaging the rectangular and circular targets with different sizes,buried positions,and dielectric properties respectively to verify the imaging effect.The calculating formulas of BP imaging method for buried targets in layered media structure are deduced,and applied to imaging the rectangular and circular targets with different sizes and buried positions in layered media under different media parameters to verify the imaging effect.A BP imaging method based on time-delay pre-calibration is proposed for imaging of buried targets in composite scattering with rough interface,and the calculation formula of the imaging method is deduced.This method is applied to electromagnetic imaging of rectangular and circular targets with different sizes and buried positions in composite scattering with rough interface,and good imaging results are achieved to verify the imaging effect.