| With the development of science and technology,human understanding of the moon has developed from obtaining all the topography,composition,and structure of the moon to quantitatively analyzing the problems of lunar geology in situ,and correctly understanding the evolution of the moon based on scientific analysis.Lunar penetrating radar(LPR)is a positioning detection method carried out along the route for the shallow stratum of the landing area by carrying the ground-penetrating radar on the lunar inspector.The data obtained by Chang'E-3(CE-3)lunar penetrating radar has the problems of the small amount of data and low signal-to-noise ratio,which affects the quality and efficiency of interpretation.To make better use of lunar penetrating radar data and study the formation and evolution of lunar shallow strata in detail,it is very necessary to carry out the analysis and processing of lunar penetrating radar data.Because the signals received by the three receiving antennas of CE-3 Lunar Penetrating Radar show different interference characteristics,this paper uses different analysis and processing methods: firstly,collect the existing data,establish a geological model more suitable for the acquisition environment,and calculate the near lunar surface electromagnetic response of the Lunar Penetrating Radar;Then,starting with the most reliable CH-2B data,the complex signal analysis method is used to quickly and effectively divide the lunar soil horizon,and the existing understanding of lunar soil is supplemented and verified;Then,the Shearlet method is used to further analyze and process the CH-2A and CH-2B data in the Shearlet subdomain of different scales and angles.Referring to the CH-2B data processing results,the two-channel data are fused to obtain a more balanced profile.The profile accurately marks the distribution of stones in the ancient lunar soil layer and reconstructs the interface between lunar soil and bedrock;Finally,for CH-1 data with a great dispute over data quality,the CEEMD method is used to decompose the data and analyze the time-frequency attributes.The effective signal and noise are judged according to the actual physical significance,and the components containing the effective signal are synthesized.The reliability is proved by comparing the shallow and high-frequency data.Because the processed profile can not directly reflect the formation information,Therefore,the interface identification method is used to help distinguish the stratum,and then discuss the rationality and geological significance of the stratum and carry out comprehensive interpretation.Through the research on data processing of lunar radar,the following understanding is obtained:The complex signal analysis method is not only suitable for GPR data processing but also can give play to the advantage of multi-parameter joint analysis in lunar radar data processing to avoid the deviation caused by the interpretation of a single radar electromagnetic wave profile.In the instantaneous attributes calculated by Wavelet transform,the instantaneous amplitude spectrum and instantaneous frequency spectrum can determine the approximate location of anomaly or stratification,and the instantaneous phase spectrum can accurately determine the anomaly location and stratification contour.Shearlet transform is an effective multi-scale and multi-directional method.Compared with Wavelet transform and Curvelet transforms,Shearlet transform shows obvious advantages in sparsity and mathematical structure.Shearlet variation based on a strictly compact framework can characterize high-dimensional signals according to direction and scale in data reflecting anisotropy.The horizontal transverse noise found in CH-2A and CH-2B data can be accurately identified and separated according to scale and angle in the Shearlet domain.After the Shearlet transform,the lunar soil structure of the CE-3 landing site can be reconstructed by data fusion by dividing sparse domain coefficients into low-scale and high-scale according to the frequency band.The two sets of data have overall advantages,and the reconstructed results are more balanced and have high reliability.The sparse transform method is effective in processing high-dimensional signals:Shearlet transform has good performance in signal representation(high redundancy and small structural error),but the amount of calculation is large;The CE-4 mission will collect long-term data on the back of the moon,and the amount of data will increase exponentially.When there is a lack of sufficient hardware support,Curvelet transform is a better choice to balance the processing effect and computing cost.The signal artifact generated by electromagnetic coupling is the primary problem of CE-3 CH-1 data processing,which is related to the accuracy of stratigraphic interpretation.Compared with short-time Fourier transform and Wavelet transform,the traditional HHT method has more advantages in lunar radar signal processing with complex waveform,wide frequency band,and strong interference.However,the HHT method based on EMD has inevitable mode aliasing,which leads to serious deviation of signal time-frequency distribution,At the same time,the components of each order IMF also lose their clear physical significance,which is not conducive to the signal analysis of lunar radar.CEEMD method not only suppresses mode aliasing but also can accurately reconstruct the original signal,which is more suitable for analyzing lunar radar signals.The CH-1 data is decomposed into seven IMF by the CEEMD method.It is found that IMF1 is a high-frequency noise component and IMF4 is a low-frequency noise component.There are two energy anomalies of 3500 ns and 5800 ns in IMF5.The IMF5 decomposed by track also observed obvious energy anomalies at the same frequency,It is considered to be the artifact caused by the electromagnetic interaction between the radar system and the metal detection vehicle.IMF5,IMF6,and IMF7 are considered low-frequency noise components.IMF2 and IMF3 are considered useful signals because their component distribution is similar to the transmission frequency of a low-frequency antenna.After stacking IMF2 and IMF3,the first 200 ns section shows that the shallow stratification results are consistent with the fused high-frequency data section and the recognized interface.Therefore,it is considered that the stacking results of IMF2 and IMF3 can accurately reflect the shallow stratum information of the moon and are reliable data.The interface recognition method is effective in the image processing of the high-frequency data section of lunar exploration radar.More noise is observed in the shallow part of the calculation results of the Log operator and Canny operator.This is because the two methods are relatively flexible.Different thresholds can be selected according to the signal-to-noise ratio decreasing with depth to obtain more formation information.Canny operator extracts the direct wave edge to form a thin line with a single pixel,which can locate the stratum more accurately,but it is greatly affected by noise.The results of low-frequency data image processing show that the effect of Sobel operator in processing actual data is not ideal,because the detection template of Sobel operator is single,and there is only horizontal and vertical detection,which limits the accuracy of complex image processing;Lo G Operator can adjust the standard deviation according to the image information,has relatively accurate recognition ability at all depths,and can determine the approximate contour of horizon interface in lunar radar profile;Canny operator has the highest accuracy,but it is also most affected by interference,which is used to accurately describe horizons.Through the comprehensive interpretation of lunar radar data,the following understanding is obtained:The subsurface of the CE-4 landing area can be divided into the strongly weathered layer(0-150ns),weakly weathered layer(150-250ns),sedimentary layer(250-450ns),and basalt basement(> 450ns).The strongly weathered layer is relatively uniform as a whole,with the sporadic reflection of some large rocks.The overall reflection on the radar profile is weak,but there are more strong reflections on the lunar surface and sporadic strong reflections inside;The energy shown on the instantaneous amplitude attribute profile is also weak.The weakly weathered layer is mainly distributed with a large number of highly reflective rock blocks,the upper part is randomly distributed with more uniform rock blocks,and the lower part is uneven in size and distribution.The number and shape of rocks in the sedimentary layer are smaller than the upper layer,and there are several electrical interfaces and a convex mirror with more uniform and finer particles.Therefore,it is considered that the composition and structure of the convex mirror are the same as that of bedrock.The shallow strata of the CE-3 landing area can be divided into sputter layer(0-3m),paleo-regolith layer(3-9m),Eratosonian basalt(9-43m),Imbrian basalt(43-131m),and layered volcanic rock(> 131m).Some impact craters near the CE-3 landing site were formed in the Copernican period,and the sputter produced by the impact covered the surrounding lunar soil layer to form a ? 3m thick sputter layer.In the late Eratosonian and early Copernican,various meteorites hit the basalt cover on the lunar surface and formed a? 6m thick paleo-regolith layer.With the thickening of the ancient lunar soil layer,the usual small meteors will not be able to crush the deep lunar soil,which leads to the uneven distribution of rock fragments and meteorite fragments with the increase of depth.The Ericksonian basalt was produced by a long-term intense volcanic activity.About 2.96 Ga ago,the lava flow covered the Yuhai formation from the southwest to form the basement.The Imbrian basalt may have been emplaced from the north direction from 3.3Ga,during which three large melt flow events and one impact event occurred.The formation time of layered volcanic rocks exceeds 3.6Ga,which may be caused by intermittent lava flow and pyroclastic rocks. |
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