Theoretical Simulation And Inversion For Investigation Of Lunar Subsurface Structure Using Radar Sounder

THE lunar regolith layer, the uppermost layer of the lunar surface, preserves the geological history of the Moon, and knowledge of its structure, composition and distribution might provide important information concerning lunar geology and resources for future lunar exploration. Owing to the low dielectric property of the lunar regolith, microwaves at appropriate frequencies can penetrate through the lunar surface to great depth and hence provide a complementary view about subsurface geologic structure to the observations that have been obtained in the visible, infrared, and thermal infrared regimes.Spaceborne high frequency (HF) radar sounder is an effective tool for investigation of lunar subsurface structure in lunar exploration. Compared with radio waves at microwave frequencies, high frequency (HF) radar wave can penetrate much deeper into the lunar subsurface layer due to its lower frequency. Center frequency selection for the high-frequency radar sounder is discussed firstly as a compromise of penetration depth and range resolution. In the simulation for high frequency radar sounder detection of the lunar subsurface structure, the surface and subsurface areas in simulation are finite. Because of this limitation, it makes a false peak echo in the simulation result at the edge of the surface and subsurface scene. Therefore, the echoes whose range is greater than that from the edge of the scene should be cut. This simulation work on the lunar surface and subsurface area is also discussed.The primary strategy for radar sounder detection of subsurface structure is through the time delay and intensity difference of the nadir echoes from the surface and subsurface. Due to the inhomogeneous undulation of lunar surface, the weak subsurface echoes can be easily masked by the strong off-nadir surface echoes (clutter), which is the main obstacle in lunar subsurface exploration. In this study, an effective approach for subsurface echo extraction is developed on the basis of the coherent and incoherent properties of the surface nadir echoes and subsurface off-nadir echoes (clutter). By stacking and averaging the received radar sounder echo at different time series, subsurface echo can be identified under the condition that the subsurface topography variation is not too large. Taken the simulated radar sounder echoes from lunar maria and highlands area as an example, the feasibility of this approach is verified numerically. Finally, the influences on subsurface echo identification by the stacking number, surface roughness are also discussed. The approach in this study can be applied to Martian and other planetary exploration.