## 4 Discussion  
Our study demonstrates the ability of drone-based ground penetrating radar (GPR) to accurately map the natural rock surface beneath a substantial layer of clayey soil at the construction site in Beer Sheva's Children's Park. The conventional challenges faced by traditional surveying methods, such as spot-wise borehole drilling, were overcome by leveraging the unique capabilities of drone-based GPR. Quantitative data supporting the efficiency claims of drone-based GPR reveal that it significantly reduces surveying time by 40% compared to traditional methods, including land-based GPR acquisition [45]. Our conclusion aligns with that of [62], highlighting that drone-based GPR is a cost-effective solution that covers large and difficult-to-access areas, significantly reducing the effort required for measurements. The practical challenges commonly associated with drone-based GPR systems, such as flight instability (sensitivity to wind gusts), reduced signal penetration compared to ground-based GPR, and clutter (artificial reflections interfering with subsurface target echoes), were thoroughly addressed in [62]. The data acquisition avoided the first issue during the day when wind gusts in our regions were rare. The calculation results presented in Section 2.3, corroborated by borehole data, confirmed that the penetration depth was sufficient for this study. The most popular method for clutter mitigation is using the Background Removal and Dewow filters, which were successfully applied (Section 2.4). Figure 8: This is a photograph of the construction site under study during the pile drilling campaign. Wet, brown clayey soil, stabilized with bentonite, is visible on the surface. Extracted yellow sandstone at 10 m corresponds accurately to GPR findings (Figure 6), affirming the reliability of aerial GPR for subsurface mapping in foundation engineering. Combining aerial platforms with subsurface sensing capabilities enabled a comprehensive understanding of the complex subsurface structure, which was crucial for foundation engineering. Our choice of a 150 MHz central frequency is a successful compromise between the subsurface dielectric properties (clayey soil in the case under study), which proved effective in penetrating and revealing the underlying natural rock formations. Specifically, using an unshielded low-frequency dipole antenna, generally not recommended in urban environments due to the effect of above-ground obstacles (e.g., fences, trees, etc. ; see Figures 2, 3 and 7a), allowed us to distinguish natural subsurface features (Figure 6) successfully. The flight path planning, illustrated in Figure 3, strategically considered urban obstacles like trees, fences, and streetlights, allowing the drone to navigate efficiently at a low altitude of 5 m above the ground. This meticulous planning and collected GPR data enabled accurate identification of the transition from clayey soil to natural rock surfaces. The validation process, involving the general geological mapping and on-site observations during foundation drilling, consistently affirmed the accuracy of our GPR-based mapping. The success in differentiating soil types and identifying the natural rock surface at varying depths showcases the potential of drone-based GPR as a transformative tool in subsurface characterization for foundation engineering applications. The successful correlation between the GPR observations and the actual subsurface structure validates the technology's efficacy and underscores its significance in overcoming challenges associated with waterlogged clayey soil during construction activities. Additionally, the decision not to use ground-based GPR was justified by the presence of too many obstacles, hindering effective data collection. The insights provided by the radargram compensate for the limitations encountered during the drilling test, offering valuable data for informed decision-making in construction planning.