Researches On Electromagnetic Imaging Technique Of Reverse-Time Migration And Bloch-Floquet Periodic Boundary Condition In Arbitrary Anisotropic FDTD

This paper focuses on two research aspects:electromagnetic reverse-time migration imaging technology and Bloch-Floquet periodic boundary conditions under Finite-Difference Time-Domain(FDTD)method with arbitrary anisotropy.In view of the advantages of electromagnetic FDTD method,such as high computational efficiency,small memory and easy access to approximate numerical solutions,the program codes in two research topics are based on electromagnetic FDTD method as the underlying basis,and further research is carried out.In the framework of reverse-time migration imaging technology,each unit of the transmitting array transmits the signal to the area to be detected in turn,and then the receiving unit records the reflected and scattered signals from this area.This rich information of reflected and scattered waves can be applied to obtain these images according to the reciprocity and wave field correlation between transmitted and received signals in the estimation model.Reverse-time migration imaging technology has been gradually applied to geological analysis,seismic research and biomedical imaging.On the premise of the unknown distribution in the subsurface structure,the electromagnetic reverse-time migration imaging technology adopts the multi-station ultra-wideband antenna to transmit electromagnetic pulse in turn,and processes the transmitted/reflected signal measured in the experimental process,effectively realizes the subsurface imaging research in the simulated lunar soil environment structure,and can roughly predict the location of buried objects.To optimize the conventional reverse-time migration imaging technology,the following three steps are made:code parallel processing,sparse storage of time-domain boundary field and high-order optimization of the underlying algorithm.After thousands of repetitive validations in the previous three years,the high-order electromagnetic reverse-time migration technology described in this paper has successfully achieved great significance for Chang' E-5 to take the lead in realizing high-resolution subsurface three-dimensional electromagnetic imaging with the measured data in the earth laboratory.On the other hand,in this paper,we also implement the second research topic,which is mainly about the realization of Bloch-Floquet periodic boundary conditions under the arbitrary anisotropic FDTD.The electromagnetic properties of anisotropic media have been discussed and validated in many electromagnetic numerical methods.Unfortunately,most of these methods are limited to two-dimensional electromagnetic problems or partially anisotropic special media,and most of them are lack of practical application.Based on the Maxwell's curl equation of the arbitrary anisotropic medium,dielectric tensor,permeability tensor,conductivity tensor and magnetic loss tensor are introduced into the electromagnetic FDTD method.The numerical method is realized through anisotropic medium in time-domain electromagnetics.Combining with Bloch-Floquet periodic boundary condition theory,stratified anisotropic medium model is established for the obliquely incident electromagnetic waves.The proposed numerical method can be varified by transmission energy analysis.Compared with the commerical software COMSOL,the proposed method can show the good agreement and save more computational resources.