## 2 Materials and Methods
### The Design of the Drone-Based GPR System  
A drone-based GPR system includes essential components, each crucial for ensuring accuracy and functionality. At the core is the GPR unit from RADAR Systems, Inc. (Riga, Latvia), which combines the radar transmitter and receiver antennas to emit and capture electromagnetic waves. Antenna selection, based on desired penetration depth and resolution, is critical for the application. Combining GPR technology with drones creates a powerful tool for foundation engineering surveys. As shown in Figure 2, the GPR unit is mounted beneath the drone, with the dipole antenna positioned for optimal ground distance, ensuring accurate data  
Figure 1: The maps of the construction site under study. (**a**) The zoomed-out map of Israel, (**b**) The map of Beer Sheva city with a superimposed red triangle indicating the location of the site under study, (**c**) The orthophoto of the Children’s Park with the community center construction site under study marked with a red line, made right before the survey, (**d**) The zoomed-in fragment of the geological map of the region [54] with a superimposed red triangle indicating the location of the site under study. acquisition. Real-time kinematic (RTK) technology provides precise georeferencing, and the SKYHUB computer (SPH Engineering, Riga, Latvia) synchronizes data between the DJI M300 drone and the GPR unit for coherent information collection. A drone-based GPR system was configured with a central frequency of 150 MHz to address the uncertainty in geological exploration and optimize subsurface penetration. The chosen operating bandwidth spans from 50 to 300 MHz, with a scan resolution of 512 to 1024 samples per scan. This GPR configuration aims to provide detailed insights into the subsurface composition, contributing to a comprehensive understanding of the geological structure near the construction site. Furthermore, the drone's capacity and flight time affect the system's overall usability. A well-balanced drone with sufficient payload capacity ensures the secure mounting of the GPR unit without compromising flight stability. Drones present a distinctive advantage in their adeptness at efficiently covering expansive areas while effortlessly navigating urban impediments such as fences, trees, bushes, and streetlights, thereby maintaining a constant and stable flight pattern around the area of interest. Given the prevalence of high trees and fences encircling the construction site at the Children's Park (Figures 1c and 3), the drone's flying altitude was restricted to 5 m above the surface. To optimize data acquisition over the undulating terrain of the park, the selected flight path commenced at the hilltop, descending around the construction site's center, aligning with the natural surface of the hills beneath the distributed soil. Leveraging SKYHUB technology, the drone's trajectory is pre-programmed to follow a designated pattern, often composed of parallel flight lines or a grid configuration tailored to the research objectives. These predefined patterns enable the GPR sensors to capture data precisely across the entire survey area. Figure 3 visually represents the specific flight pattern executed in the park around the places of interest. Figure 2: A DJI M300 drone (SZ DJI Technology Co., Ltd., Nanshan, Shenzhen, China) with a GPR 150 MHz dipole antenna at the bottom provides subsurface sensing capabilities up to 15 m below the surface. The SKYHUB computer, which is positioned on the right side, is responsible for controlling automatic flight operations. At the front of the drone, a small radar (altimeter) measures the distance from the ground, maintaining a constant flight altitude of 5 m. The survey took place around the construction site at the Children’s Park in Beer Sheva City, Israel (Figure 1c).