Properties Analysis of Lunar Regolith at Chang’E-4 Landing Site Based on 3D Velocity Spectrum of Lunar Penetrating Radar

Abstract

The Chinese Chang’E-4 mission for moon exploration has been successfully completed. The Chang’E-4 probe achieved the first-ever soft landing on the floor of Von Kármán crater (177.59°E, 45.46°S) of the South Pole-Aitken (SPA) basin on January 3, 2019. Yutu-2 rover is mounted with several scientific instruments including a lunar penetrating radar (LPR), which is an effective instrument to detect the lunar subsurface structure. During the interpretation of LPR data, subsurface velocity of electromagnetic waves is a vital parameter necessary for stratigraphic division and computing other properties. However, the methods in previous research on Chang’E-3 cannot perform velocity analysis automatically and objectively. In this paper, the 3D velocity spectrum is applied to property analysis of LPR data from Chang’E-4. The result shows that 3D velocity spectrum can automatically search for hyperbolas; the maximum value at velocity axis with a soft threshold function can provide the horizontal position, two-way reflected time and velocity of each hyperbola; the average maximum relative error of velocity is estimated to be 7.99%. Based on the estimated velocities of 30 hyperbolas, the structures of subsurface properties are obtained, including velocity, relative permittivity, density, and content of FeO and TiO2.

Rock Location and Property Analysis of Lunar Regolith at Chang’E-4 Landing Site Based on Local Correlation and Semblance Analysis

“…The vertex positions of the diffractions are shown in the upper right corner of The forward simulation results of CH-2A with the offset of 0.16 m and CH-2B with the offset of 0.32 m are illustrated in Figure 3b,c, respectively. The double diffraction hyperbolas were generated due to the upper and bottom surfaces of the rocks [16]. We focused on the upper diffraction hyperbola to reduce the error of velocity estimation.…”

“…Furthermore, we demonstrate the results by applying the local correlation based on the frequency wavenumber (FK) fan filtered data in Figure 3d, where the red-cross indicates the vertex position of the diffractions. Dong et al determined the vertex positions of the diffractions by directly applying a 3D velocity spectrum for the whole data [16], which would cause a relatively large error because the maximum value position of semblance analysis would be significantly influenced by the noise. The vertex position of the diffraction determinations based on local correlation would have higher accuracy than the direct 3D velocity spectrum.…”

“…The other properties of lunar regolith including bulk density and FeO and TiO 2 abundance can be calculated based on the relative permittivity. A procedure that combined the local correlation and semblance analysis has an advantage over traditional methods [12][13][14][15][16] in terms of accuracy and robustness in the interpretation of the high-frequency LPR data.…”

“…Hu et al proposed an adaptive rock extraction method based on local similarity constraints to achieve the rock location and quantitative analysis for regolith [15]. Dong et al analyzed the regolith properties from the LPR data of Chang'E-4 based on the 3D velocity spectrum [16]. However, a complex subsurface structure and interference of noise always result in incomplete, interlaced, and amplitude-varying hyperbolas which cause the inaccurate velocity analysis of field LPR data [16], and the previous studies cannot reach a satisfied accuracy.…”

“…Dong et al analyzed the regolith properties from the LPR data of Chang'E-4 based on the 3D velocity spectrum [16]. However, a complex subsurface structure and interference of noise always result in incomplete, interlaced, and amplitude-varying hyperbolas which cause the inaccurate velocity analysis of field LPR data [16], and the previous studies cannot reach a satisfied accuracy. In addition, the determination of the rock location or the vertex position of the diffractions is the key procedure to obtain a high-precision diffraction velocity analysis spectrum.…”