## 3 Results  
Figure 5 shows the entire unprocessed profile of GPR data with only the corrected topography and two cut sections from/to the drone base station. The meticulously planned flight path, strategically designed to optimize the capture of subsurface features, is depicted in Figure 3 (green lines with numbers that are the same waypoints as in Figure 5). The drone commenced its survey from the hilltop (the purple circle in Figure 3), where the natural rock layer is about at the Earth's surface, and followed a perpendicular direction to the dip of the hill. The flight was organized into distinct sections. The initial section, from west to east (waypoints 3 to 9), aimed to circumvent urban obstacles such as 4 m trees, a 1.5 m fence, and 4.5 m streetlights while maintaining a low altitude of 5 m above the ground. \begin{table}
\begin{tabular}{c c} \hline \hline  & **Filter Type** \\ \hline
1 & Data Editing \\
2 & Dewow \\
3 & Static Correction \\
4 & Background Removal \\
5 & X-Interpolation by GPS Coordinates \\
6 & Median Filter \\
7 & Automatic Gain Control \\
8 & Stolt (f-k domain) Migration \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The list of filters applied to GPR raw data. The second section, traversing from northwest to waypoints 10 to 17 (Figures 3 and 5), mirrored the first but in the opposite direction, mapping the sub-rock natural surface to the hills' natural surface. The third section examined underground variations in the east-west direction, investigating whether the slope exhibited drastic changes or maintained a consistent formation beneath the surface (waypoints 18 to 25, Figures 3 and 5). As detailed above, the minimum penetration depth for the aerial survey was established at 20 m, slightly more profound than the desired depth of the investigation (15 m). This parameter ensured comprehensive subsurface data collection, allowing for a thorough exploration of the geological structure beneath the clayey soil. Figure 6 presents the zoomed-in part of the processed radargram from the first survey section (waypoints 7-8, as illustrated in Figures 3 and 5) offers detailed insight into subsurface structures. The horizontal and vertical axes denote path and depth distances in meters. Figure 5: The unprocessed radargram with topography correction. Figure 6: The zoomed-in part of the processed radargram from the first survey section (waypoints 7–8, as illustrated in Figures 3 and 5) offers detailed insight into subsurface structures. The horizontal and vertical axes denote path and depth distances in meters. (**a**) The processed radargram with clearly seen underground structures (the nearly horizontal reflections and amplification at the bottom of the radargram), (**b**) The dashed and full lines show the underground boundaries between layers discussed in the text. The radargram's distinct features were identified by carefully interpreting the processed data (Figure 6a). The dashed black line representing the surface was determined by calibrating the radar signal with known surface markers. Variations in the distributed soils, marked (for example) by the yellow dashed line, were discerned based on their differing dielectric properties, which cause variations in radar signal reflections. Note the existence of several such layers above and below the yellow dashed line. At approximately 5 m deep, the red dashed line indicates a saturated water layer, identified through the reflection patterns common in water-saturated zones. The purple dashed line representing the top of the weathered/erosive rock at approximately 8 m depth was distinguished by a significant change in signal amplitude and continuity. In comparison, the continuous purple line highlighting the natural hard rock surface was identified around 10 m below the ground level by the robust amplification of reflected signals. Additionally, the hyperbolic reflections from fences and trees were recognized based on their distinctive typical shapes and adjusting their slope to the wave velocity in the air.