**5. Conclusions**

This paper applies the 3D velocity spectrum to property analysis of lunar penetrating radar data from Chang'E-4. The result shows that 3D velocity spectrum can automatically search for hyperbolas; the maximum value along velocity axis with a soft threshold function can provide the horizontal position, two-way reflected time and velocity of each hyperbola. Based on the estimated velocities of 30 hyperbolas, the subsurface properties' structures are obtained, including velocity, relative permittivity, density, and content of FeO and TiO2. Combining these properties and the radargram, we can conclude that 0–12 m is the fine-grained regolith; 12–20 m is the coarse regolith of different sources; the thickness of the lunar regolith is more than 20 m; the surface of the shallow basalt is within 25 m in the depth. Importantly, the subsurface material and structure at the Chang'E-4 landing site vary both vertically and horizontally, so the simple horizontal stratigraphic division is not appropriate within the zone of 25–300 ns. Finally, the estimation error is discussed, the errors of horizontal position and two-way reflected time are small which can be omitted; the average maximum relative error of velocity is 7.99%. the distributions of velocities and error ranges in horizontal and vertical directions are obtained.

**Author Contributions:** Conceptualization, X.F., C.L., Z.Z. and J.L.; data curation, Z.D.; formal analysis, Z.D., H.Z., C.L., Z.Z. and W.L.; funding acquisition, X.F. and W.L.; investigation, H.Z., Z.Z. and J.L.; methodology, Z.D., X.F. and H.Z.; project administration, X.F., J.L. and W.L.; resources, C.L., Z.Z., J.L. and W.L.; software, H.Z.; supervision, C.L.; validation, Z.Z.; visualization, Z.D.; writing—original draft, Z.D. and H.Z.; writing—review and editing, X.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by Science and Technology on Near-Surface Detection Laboratory under Grant 6142414180911, and the Technology Development Program of Jilin Province: Suppression of Polarization Accompanying Interference for Full-Polarimetric Ground Penetrating Radar under Grant 20180101091JC, and the Fundamental Research Funds for the Central Universities.

**Conflicts of Interest:** The authors declare no conflict of interest.
