Lunar Penetrating Radar Data Research Articles

Research on lunar regolith of the Chang'E-4 landing site: An automated analysis method based on deep learning framework

On January 3, 2019, the Chang'E-4 lander successfully landed within the Von Kármán crater, located in the South P ole-Aitken Basin (SPA) on the farside of the Moon (45.5°S, 177.6°E), marking the first soft landing on the lunar farside. The lander, equipped with the Lunar Penetrating Radar (LPR) system, aimed to provide insights into the structure and evolution of the Moon. Previous research often relied on manually identifying hyperbolic features to analyze the lunar shallow subsurface properties. This inefficient approach may lead to subjective biases, resulting in unstable outcomes. This research constructed an automatic analysis framework by integrating the Swin Transformer with a 3D velocity spectrum, which is then applied to analyze the properties of the Chang'E-4 LPR data. The experimental results indicate that the framework achieved a precision of 98.9 % and a recall of 96.7 % in hyperbolic feature identification, with an F1 of 0.9782 and AP of 94.8 %. Additionally, it has been experimentally validated that the framework can accurately invert hyperbolic features' two-way travel time and velocity. Finally, the framework is applied to analyze the lunar shallow subsurface structure and properties within the landing area of the Chang'E-4 mission.

Pattern Analyses of Lunar Penetrating Radar Images at Chang'E‐3, 4, and 5 Landing Sites: A New Insight Into the Evolution of Lunar Regolith

AbstractAn “atlas” of the Lunar penetrating radar (LPR) patterns is organized and a comprehensive interpretation of the lunar subsurface layers in the areas of the Chang'E (CE) mission is proposed. There are three layers inferred by the LPR high‐frequency profiles of the CE‐3 and CE‐4. The first two layers (i.e., the fine regolith and the ejecta layer) at CE‐3 and CE‐4 sites have the same strata type but differ in internal structure and thickness. The third layer at the CE‐3 site is inferred as the paleoregolith, while at the CE‐4 site, it is inferred as the alternating layer with fine materials, ejecta, and fragments. Due to the limited penetration depth of the CE‐5 LPR, the data disclose only the fine regolith, with support from the empirical ejecta distribution model. The thicknesses, physical properties, and equivalent growth rates of lunar regolith at three landing sites are estimated and discussed in detail. The lunar regolith thicknesses inferred by the CE‐3 and CE‐5 LPR data are both approximately 1.0 m. In contrast, the thickness of the lunar regolith at the CE‐4 site is 10.0 m, which is greater than that at the other two landing sites. However, the equivalent growth rate of the regolith at the CE‐3 and CE‐5 landing sites is four times greater than that at the CE‐4 landing site. This illustrates that with the increase in lunar regolith thickness, the influence of small impact gardening events on the lunar regolith reconstruction gradually weakened, and the self‐buffering effect avoids the fast growth of lunar regolith.

Exploring the Lunar Regolith's Thickness and Dielectric Properties Using Band‐Limited Impedance at Chang’E‐4 Landing Site

AbstractThe Yutu‐2 rover onboard China's Chang’E‐4 was the first to land in the Von Kármán crater on the lunar farside, where structural and dielectric properties may provide previously unknown clues to lunar formation and evolution. Based on the Lunar Penetrating Radar (LPR) with 500 MHz onboard the Yutu‐2 rover, we proposed a band‐limited impedance (BLIMP) inversion method to generate continuous dielectric 2‐D profiles with the help of dielectric constants retrieved from point reflectors (e.g., buried rocks), which provide low‐frequency information that may be lost or distorted in the LPR data pre‐processing. The dielectric constants retrieved from point reflectors were transformed from dielectric constant fitting curves, where these discrete dielectric constants were calculated using the hyperbolic fitting method. We estimated that the top fine‐grained regolith thickness along the rover path varies from ∼5.3 to ∼15.0 m in the first 27 lunar days of operation. The thickness variation could most likely be attributed to changes in ancient surface topography buried underground and ejecta from nearby craters. Compared to methods based on a singular dielectric constant, the estimated 2‐D dielectric profile in this study can reduce the uncertainties in lunar regolith thickness estimation, especially in the shallow layers. The BLIMP method and the estimated regolith thickness can improve our understanding of lunar subsurface structure and formation.

Simulation of Lunar Comprehensive Substructure With Fracture and Imaging of Later LPR Data From Chang’e-4 Mission

As one of the most important geophysical methods for detecting the lunar underground structure of the Moon, the lunar penetrating radar (LPR) is applied to the Chang’e-4 (CE-4) mission to explore the Von Kármán crater on the far side of the Moon. With the Yutu-2 rover continues to move towards the Zhinyu crater at the west, radar will likely detect the fractures created by the impact that formed the Zhinyu Crater. Therefore, the establishment of a comprehensive lunar subsurface structure model with fracture, random media and fractal terrain is crucial for the LPR data processing and the understanding of the lunar geological impact process. In addition, the method that can image the radar data with fractures well has great significance to the interpretation of the LPR data. In this paper, we establish a comprehensive lunar subsurface structure model firstly, the considering factors including fractures, random media, fractal terrain, and the radar response are calculated through forward modeling. Secondly, according to the high resolution and the sensitive to inhomogeneities of LPR data, we design a set of processing flow including plane-wave destruction, velocity analysis based on the focusing analysis method and migration imaging based on the velocity continuation. Finally, the results including simulation data and Antarctic fracture data are used to verify the effectiveness of imaging method.

Study on the Influencing Factors of the Permittivity Estimation Method Considering Antenna Layout Based on Lunar Penetrating Radar

The permittivity of lunar regolith is crucial for further processing and interpretation of radar data. The conventional hyperbolic fitting method ignores the antenna height and spacing and has a significant error at a shallow depth. For the new method that considers the layout of the antenna, the influencing factors have not been studied. In this paper, we studied the influence of the position of the hyperbola peak and time zero on the new method for permittivity derivation. The simulation results show that when the input errors of time zero, abscissa and ordinate of the hyperbolic peak are ±2 ns, ±0.02 m and ±0.2 ns respectively, the average errors of the calculated results by points within 1 m from the hyperbolic peak are 10.0%, 16.7% and 38.2%, respectively. To improve the accuracy, we used the average results by points that are horizontally more than 1 m away from the hyperbola peak. Hence, we calculated the permittivity of the lunar regolith by the new method based on Lunar Penetrating Radar data. The average permittivity of the lunar regolith is estimated to be 3.3 ± 1.2.

Sub-surface stratification and dielectric permittivity distribution at the Chang’E-4 landing site revealed by the lunar penetrating radar

Context.In 2019, China’s Chang’E-4 (CE-4) probe landed on the far side of the Moon: a first in lunar exploration. The Lunar Penetrating Radar (LPR) mounted on the Yutu-2 rover allows the mapping of the near-surface structure and the dielectric permittivity of the landing area. The dielectric properties of the lunar soil affect the propagation of the LPR signals, which can be used to infer the depth of sub-surface boundaries and derive the composition of the component materials.Aims.Our objectives are to estimate the fine-resolution spatial distribution of relative permittivity and to improve the interpretation of the geological processes combined with the radargram of the CE-4 landing area.Methods.We used a modified method that combines the F-K migration and the minimum entropy of the ground penetrating radar (GPR) signals to estimate the velocity and permittivity values; this has the advantage of obtaining the appropriate velocity and permittivity, even with the incomplete or unnoticeable hyperbolic curves in the radar imageResults.The sub-surface stratification of the CE-4 landing area is seen in the first 31 lunar days of the LPR data. A fine-resolution dielectric permittivity profile ranging from ~2.3 to ~6.3 is obtained with our method, and the actual depths of the observed prominent sub-surface interfaces are determined, giving a maximum average depth of ~38 m. The thickness of the regolith layer is in the range of ~5.7–15.6 m, with an average of 11.8 m. The permittivity of the near-surface regolith (<30 cm) is ~2.78 ± 0.01, the bulk density is 1.57 ± 0.01 g cm−3, which is close to the results of ~1.61 g cm−3at the Apollo 15 landing area. The permittivity map is consistent with the radargram; the regolith and the paleo-regolith layer have relatively low permittivity and low echo strengths, while the rock debris has high permittivity and shows strong echos in the radargram. Two buried craters of different diameters beneath the navigation sites 4–11 and 16–31 are revealed in the radar profile. The permittivity distribution map can show detailed variations of material properties both inside and outside craters.

Yutu-2 Radar Sounding Evidence of a Buried Crater at Chang’E-4 Landing Site

Buried craters within tens of meters of lunar regolith are rarely studied but are significant for understanding the evolution of surface processes on the Moon. Here, we first report the evidence of an intact buried crater within the layered strata at Chang’E-4 (CE-4) landing site revealed by the lunar penetrating radar (LPR). The time–frequency comparative analysis method based on the variational mode decomposition (VMD) and the rock quantitative analysis method based on the local unit correlation (LUC) are proposed and applied to the processing and analysis of LPR data within 15 lunar days. The results presented by the two methods provide evidence of a buried crater at the CE-4 landing site and simultaneously reveal the rock-concentrated structure within the buried crater. According to the results, it is considered that the filling materials within the buried crater have survived the impaction and gardening during the formation of the overlying fine-grained regolith. Recent works have proposed that the near-surface material at the CE-4 landing site is mainly the lunar mantle materials excavated from the nearby Finsen crater. Therefore, the buried crater probably preserves the initial lunar mantle materials.

Diffraction Separation With Plane-Wave Destruction Filer for the Lunar Penetrating Radar Sensor Data

China transported the lunar rovers to the lunar surface via the Chang’E rockets for lunar exploration. An important detection tool in the lunar rover is the lunar penetrating radar (LPR). LPR can obtain the lunar surface structure information by transmitting and receiving high-frequency electromagnetic waves. There are many types of waves received by LPR sensors, among which diffracted waves are often ignored. The diffractions in the LPR data contain a lot of underground structure information, especially for faults and small targets with high resolution. A major technical problem in effectively using the information is how to separate diffractions from different types of wave field information. This paper focuses on the diffraction issue and employs the plane wave decomposition filter method to separate the diffractions in LPR data sets. First, the records are converted into a dip field, and then the non-diffraction waves in the dip field is filtered to achieve the purpose of separating the diffractions. Three numerical models will be used to verify the effectiveness of the diffraction separation method.

Dielectric Properties of Lunar Materials at the Chang’e-4 Landing Site

On January 3rd 2019, the Chang’e-4 mission successfully landed in the Von Kármán Crater inside the South Pole-Aitken (SPA) basin and achieved the first soft landing on the farside of the Moon. Lunar penetrating radar (LPR) equipped on the rover measured the shallow subsurface structure along the motion path for more than 700 m. LPR data could be used to obtain the dielectric properties of the materials beneath the exploration area, providing important clues as to the composition and source of the materials. Although the properties of the upper fine-grained regolith have been studied using various methods, the underlying coarse-grained materials still lack investigation. Therefore, this paper intends to estimate the loss tangent of the coarse-grained materials at depth ranges of ~12 and ~28 m. Stochastic media models with different rock distributions for the LPR finite-difference time-domain (FDTD) simulation are built to evaluate the feasibility of the estimation method. Our results show that the average loss tangent value of coarse-grained materials is 0.0104±0.0027, and the abundance of FeOT+TiO2 is 20.08 wt.%, which is much higher than the overlying fine-grained regolith, indicating different sources.

Self-Organization Characteristics of Lunar Regolith Inferred by Yutu-2 Lunar Penetrating Radar

Most previous studies tend to simplify the lunar regolith as a homogeneous medium. However, the lunar regolith is not completely homogeneous, because there are weak reflections from the lunar regolith layer. In this study, we examined the weak heterogeneity of the lunar regolith layer using a self-organization model by matching the reflection pattern of both the lunar regolith layer and the top of the ejecta layer. After a series of numerical experiments, synthetic results show great consistency with the observed Chang’E-4 lunar penetrating radar data and provide some constraints on the range of controlling parameters of the exponential self-organization model. The root mean square permittivity perturbation is estimated to be about 3% and the correlation distance is about 5–10 cm. Additionally, the upper layer of ejecta has about 1–2 rocks per square meter, and the rock diameter is about 20–30 cm. These parameters are helpful for further study of structural characteristics and the evolution process of the lunar regolith. The relatively small correlation distance and root mean square perturbation in the regolith indicate that the regolith is mature. The weak reflections within the regolith are more likely to be due to structural changes rather than material composition changes.

Intermittent volcanic activity detected in the Von Kármán crater on the farside of the Moon

Sensitive to permittivity differences, lunar penetrating radar (LPR) can detect stratification due to material property differences in a medium. We examined the subsurface structures of Von Kármán crater within the South Pole-Aitken (SPA) basin on the farside of the Moon using LPR data obtained by the Chang'E-4 mission. Various distinct strata with depth are clearly recognized, consistent with multiple periods of intermittent lava flows, paleo-regolith, and crater ejecta. Numerous concave and arc shape structures observed at various depths are consistent with buried craters and broken rocks, indicating that the ancient lunar surfaces which are currently buried by up to 30 m deep in Von Kármán, as well as the current surface, suffered numerous impacting events. An imaged subsurface thick paleo-regolith infers a quiescent period with low volcanic activity within the Von Kármán crater. Following the quiescent period (shallower in the profile), a relatively young period of volcanism in the Von Kármán crater is inferred. The mapped stratigraphy and chronology of these events present a self-consistent history of the Von Kármán crater within the SPA, which suggests connections to events on the lunar nearside.

Properties of Lunar Regolith on the Moon's Farside Unveiled by Chang'E‐4 Lunar Penetrating Radar

AbstractThe complex thermal history of the Moon leads to an unequal distribution of volcanic products between the lunar nearside and the farside. So far, no lunar materials have been sampled from the Moon's farside and no detailed properties of lunar regolith on the farside have been detected before. On January 3, 2019, Chang'E‐4 (CE‐4) touched down onto the Von Kármán crater on the Moon's farside. CE‐4 Lunar Penetrating Radar (LPR) onboard the Yutu‐2 rover is the first surface radar on the Moon's farside. Here we show the subsurface structure and properties of regolith materials at the landing region with LPR data during the first five lunar days. The thickness of lunar regolith is constrained as ∼12 m, much thicker than that at Chang'E‐3 (CE‐3) landing site, which is expected since CE‐3 landed on lunar maria. The relative permittivity of lunar surface (<30 cm) at CE‐4 landing region is identified to be 2.35 0.20. The loss tangent and TiO2 + FeO content of the regolith materials layer (0–∼12 m) are constrained to be and wt.%, respectively, much lower than those at CE‐3 landing site. It indicates that local surface materials possess less attenuation for radiowave, in accordance with the greater penetrating depth of CE‐4 LPR than that of CE‐3 LPR. Furthermore, the results also prove that the growth rate of lunar weathered regolith successively declines over time and the growth rate of lunar regolith on the Moon's farside may well be higher than that on the nearside due to the more frequent meteorite impacts.

Rock abundance and evolution of the shallow stratum on Chang'e-4 landing site unveiled by lunar penetrating radar data

The Yutu-2 rover, part of the Chang'e-4 (CE-4) mission to the lunar farside, has been exploring the Von Kármán crater (VK) within the South Pole-Aitken (SPA) gathering data on its geology. The Lunar Penetrating Radar's (LPR) 500 MHz antenna reveals the subsurface structure up to 40 m in depth, which helps to construct models on the evolution of the shallow stratum. This work presents the analysis and interpretation of the data collected during the first twenty lunar days, from January 2019 to July 2020. Below an 11±4-meter surface regolith layer, the radar image reveals well-defined reflectors that are interpreted as discrete depositional layers. These marked density boundaries could indicate a geologically brief time interval between each arrival, which prevented the materials to undergo a deeper homogenization through space weathering. To investigate further, we generated a subsurface rock distribution map using the correlation of dual-channel antennas data from the same channel. Clusters of rocks located at the bottom of each ejecta stratum are revealed, probably sorted by size due to the statistically less frequent occurrence of larger impact events admixing the deeper-seated materials. The bowl-shaped outline discovered in both rock and LPR energy distribution maps is interpreted to represent a 150±30 m wide and 10±5 m deep paleo-crater that was excavated within three thin ejecta layers. The CE-4 LPR reveals a complex geological history within the Von Kármán crater with no fewer than four impact events delivering meters thick ejecta at the CE-4 landing area following the end of the infilling phase.

Velocity Analysis Using Separated Diffractions for Lunar Penetrating Radar Obtained by Yutu-2 Rover

The high-frequency channel of lunar penetrating radar (LPR) onboard Yutu-2 rover successfully collected high quality data on the far side of the Moon, which provide a chance for us to detect the shallow subsurface structures and thickness of lunar regolith. However, traditional methods cannot obtain reliable dielectric permittivity model, especially in the presence of high mix between diffractions and reflections, which is essential for understanding and interpreting the composition of lunar subsurface materials. In this paper, we introduce an effective method to construct a reliable velocity model by separating diffractions from reflections and perform focusing analysis using separated diffractions. We first used the plane-wave destruction method to extract weak-energy diffractions interfered by strong reflections, and the LPR data are separated into two parts: diffractions and reflections. Then, we construct a macro-velocity model of lunar subsurface by focusing analysis on separated diffractions. Both the synthetic ground penetrating radar (GPR) and LPR data shows that the migration results of separated reflections have much clearer subsurface structures, compared with the migration results of un-separated data. Our results produce accurate velocity estimation, which is vital for high-precision migration; additionally, the accurate velocity estimation directly provides solid constraints on the dielectric permittivity at different depth.

Application of Denoising CNN for Noise Suppression and Weak Signal Extraction of Lunar Penetrating Radar Data

As one of the main payloads mounted on the Yutu-2 rover of Chang’E-4 probe, lunar penetrating radar (LPR) aims to map the subsurface structure in the Von Kármán crater. The field LPR data are generally masked by clutters and noises of large quantities. To solve the noise interference, dozens of filtering methods have been applied to LPR data. However, these methods have their limitations, so noise suppression is still a tough issue worth studying. In this article, the denoising convolutional neural network (CNN) framework is applied to the noise suppression and weak signal extraction of 500 MHz LPR data. The results verify that the low-frequency clutters embedded in the LPR data mainly came from the instrument system of the Yutu rover. Besides, compared with the classic band-pass filter and the mean filter, the CNN filter has better performance when dealing with noise interference and weak signal extraction; compared with Kirchhoff migration, it can provide original high-quality radargram with diffraction information. Based on the high-quality radargram provided by the CNN filter, the subsurface sandwich structure is revealed and the weak signals from three sub-layers within the paleo-regolith are extracted.

Stratification of Lunar Regolith Based on Attribute Analysis

On December 14, 2013, China’s Chang ’e-3 missions landed on the lunar surface, providing valuable information for better study of the moon. LPR (Lunar penetrating radar) is a very important and effective detection method. The lunar surface shallow radar data we obtained through the lunar exploration radar can be used to understand the geological structure of the lunar surface and the thickness of the lunar regolith. This paper analyzes the processed LPR data of “Chang’e-3” by using instantaneous attributes, and obtains the instantaneous amplitude attributes, instantaneous phase attributes, and instantaneous frequency attributes of the radar profile. According to the results of the instantaneous attribute analysis, the lunar regolith structure of the radar profile is divided, and it is found that it can be well matched with the previous research results.

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

The Lunar Penetrating Radar (LPR) onboard the Yutu-2 rover from China’s Chang’E-4 (CE-4) mission is used to probe the subsurface structure and the near-surface stratigraphic structure of the lunar regolith on the farside of the Moon. Structural analysis of regolith could provide abundant information on the formation and evolution of the Moon, in which the rock location and property analysis are the key procedures during the interpretation of LPR data. The subsurface velocity of electromagnetic waves is a vital parameter for stratigraphic division, rock location estimates, and calculating the rock properties in the interpretation of LPR data. In this paper, we propose a procedure that combines the regolith rock extraction technique based on local correlation between the two sets of LPR high-frequency channel data and the common offset semblance analysis to determine the velocity from LPR diffraction hyperbola. We consider the heterogeneity of the regolith and derive the relative permittivity distribution based on the rock extraction and semblance analysis. The numerical simulation results show that the procedure is able to obtain the high-precision position and properties of the rock. Furthermore, we apply this procedure to CE-4 LPR data and obtain preferable estimations of the rock locations and the properties of the lunar subsurface regolith.

Dielectric Properties of Lunar Subsurface Materials

AbstractThe dielectric permittivity is one of the most significant properties of lunar material. For the past decades, researchers have assumed or used an indirect density versus depth relation to constrain the subsurface permittivity on the Moon. On 3 January 2019, Chang'E‐4 (CE‐4) rover‐borne lunar penetrating radar (LPR) successfully landed on the Von Kármán crater on the Moon's farside. CE‐4 LPR is the first surface radar on the Moon's farside for exploring regolith thickness and geological structure. Numerous parabolic‐shaped diffractions (PSD), which indicate subsurface objects, are identified in LPR data. We estimate the permittivity of the first ~50 m at the landing region using these PSDs based on a hyperbolic fitting method. A best‐fit permittivity/density versus depth relation is obtained, and this relation could be used at different areas on the Moon.

Stratigraphy of the Von Kármán Crater Based on Chang'E‐4 Lunar Penetrating Radar Data

AbstractThe Chang'E‐4 (CE‐4) achieved the first successful soft landing on the floor of the Von Kármán crater located in the northeast of the South Pole‐Aitken (SPA) basin on the Moon. Here, through analysis and processing of the dual‐channel lunar penetrating radar (LPR) data, the shallow regolith structure and deep geological strata below the landing site are exposed. The high‐frequency data (CH‐2, 500 MHz) show that the Von Kármán crater underwent remodeling because of ejecta from multiple craters within a thickness of 40 m. Under the surface fine‐grained regolith, superpositions among these crater materials and mare basalt units establish shallow regolith stratigraphic sequences. The low‐frequency data (CH‐1, 60 MHz) show a deep stratigraphic structure to the depth of 360 m that indicates the previous geological evolution history of the Von Kármán crater. The LPR results provide an important scientific basis for understanding the geological history of the far side of the Moon.

Estimated Lunar Regolith Structure Based on the Least-Squares Kirchhoff Migration of CE-3 Lunar Penetrating Radar Data

The lunar penetrating radar (LPR) carried on the Chang E-3 Yutu Rover observations at 500 MHz successfully reveals the thickness and geological structure of the shallow lunar regolith. Previous work used the calculated permittivity and the migration image to estimate the layer interface and local target. In this letter, a least-squares Kirchhoff migration (LSM) algorithm is presented to reduce the migration artifacts (i.e., recording footprints) due to incomplete data. Both synthetic and CE-3 LPR data are used to test the effectiveness of the LSM method. The numerical results show that LSM can achieve a much superior image quality than Kirchhoff migration and frequency-wavenumber (F-K) migration results. Based on the LSM result of CE-3 LPR data, we can find evident strata reflection and continuous distinct layers in the shallow lunar subsurface below the CE-3 site. It provides a more reliable and robust way to interpret the lunar regolith structure.