Migration Imaging Of Chang’e-4 Lunar Penetrating Radar Data And Regolith Structure Interpretation

In 2004,our country first proposed the Chang ’E lunar exploration project,with a three-step strategy of "orbiting," "landing" and "back" AS the core,carrying out the Lunar exploration mission,and planning to set up a research base on the Moon in the future.Based on the theory of highfrequency electromagnetic waves,the Lunar radar method can be used to classify the lunar soil layer structure and estimate the dielectric attribute parameters based on the difference in the lunar dielectric constant.The Chang ’e-3 spacecraft is Penetrating the first geologic profile of the Moon using lunar penetrating radar(LPR),revealing geological structures and evolution within a depth of330 meters below the landing area.In 2019,the Chang ’e-4 LPR detected the first subsurface structure on the far side of Moon.Its main scientific tasks are to explore the distribution and enrichment of material elements in the Von Kármán crater on the far side of the Moon and to detect the regolith thickness and stratification.By July 2022,the Yutu-2 rover has been working on the far side of Moon for 44 months,traveling about1,200 meters LPR data.It provides abundant scientific data for studying the evolution of lunar regolith structure and the distribution of material composition on the far side of Moon.The data processing of LPR is mainly based on the ground-penetrating radar(GPR)processing process,which includes primarily signal denoising and migration imaging to realize hierarchical structure detection,permittivity inversion and loss tangent estimation based on radar signal travel time and waveform information.Migration imaging is a vital processing technique to obtain the fine lunar regolith stratification structure.The measurement path of the LPR is a curved survey line,which is used as a straight-line acquisition method to carry out two-dimensional data migration imaging in the data processing of the LPR data.In the actual detection environment,the radar electromagnetic wave emitted by the LPR is three-dimensional radiation,and the lunar regolith is a non-uniform structure.The approximate linear 2D migration processing ignores the reflected signals of many underground structures,resulting in errors in imaging accuracy.In addition,many rocks of different sizes are in the lunar regolith structure.Due to the wave propagation effect,each subsurface scattering will be mapped to the hyperbola in the profile.There are many cross-hyperbolic curves in the section,and it is challenging to identify complex structures directly from the section.Mathematically,migration processing is a process of removing diffraction signals and focusing reflected solid energy.Currently,the primary methods,such as F-K migration,Kirchhoff migration,and two-way wave traveltime migration,all have varying degrees of error when dealing with complex structures.Due to the heterogeneity of the lunar regolith structure,diffraction waves in the radar profile can not be eliminated.To address the above issues,we present a pseudo-three-dimensional least square migration algorithm based on a curved survey line to image the 3D spatial distribution structure of the lunar regolith within the range of the curved survey line.Based on the migration structure,synthetic aperture focusing technology(SAFT)is used to focus the LPR data.The LPR imaging accuracy can be improved based on the migration structure,and the imaging results can more clearly reflect the layered regolith structure and continuous lateral change rate.Finally,based on the lunar geological data and previous research results,we explain regolith stratigraphic structures and build geological stratification within 40 m and 300 m depth of the landing site based on the dual channel LPR data.The shallow lunar regolith layer within 40 m is composed of 12 m shallow topsoil composed of ejecta from Finsen crater,and 17 m below it is composed of the sputtering layer formed by ejecta from Von Kármán Crater and its surrounding craters on basalt bedrock and basalt bedrock at the very bottom.The low-frequency channel LPR data are used to classify the stratified structure of the inner Von Kármán Basin at a depth of 450 m,which is consistent with the lunar evolution chronology.In this study,the stratigraphic division and geological interpretation of the lunar regolith layer structure are based on the migration imaging results of dual-channel LPR data,providing a reliable scientific basis for further understanding the lunar interior structure and evolution history.