Background
Holtingerveld, a protected nature reserve in Drenthe, the Netherlands, features heathlands, sandy soils, and water-retaining geological layers. Our partner, Medusa Explorations, conducted a comparative survey to assess whether UAV-mounted GPR could reliably detect key subsurface formations, especially boulder clay, and, where possible, peat and thin silt layers.
The study focused on areas where ground access was limited. In accessible sections, the team employed a traditional 300 MHz ground-coupled Zond 12 system towed behind a quad. In restricted zones, where vehicles were not permitted due to environmental sensitivity and dense vegetation, aerial surveys were conducted using the 300 MHz Zond Aero LF system integrated with UgCS software and SkyHub for precise low-altitude, terrain-following flight.
Challenges
Before selecting a drone-based solution, the team encountered several limitations with traditional methods. In some parts of the Holtingerveld reserve, the quad-based setup was not permitted due to the fragile nature of the terrain, and ground contact was restricted by dense vegetation. Attempts to manually carry GPR antennas in such areas produced inconsistent results due to unstable antenna height and orientation.
The primary challenges included:
- Ecological sensitivity. Risk of damage to vegetation and alteration of natural drainage patterns by ground equipment.
- Inaccessible terrain. Dense vegetation, soft peat soils, and uneven terrain impeded the use of traditional ground-based radar methods.
- Data quality assurance. It was necessary to demonstrate that drone-collected radar data would match or closely approximate the resolution and accuracy of established ground-based methods.
Solution
Medusa Explorations implemented an aerial solution by deploying SPH Engineering’s Integrated GPR system. This setup involved mounting a lightweight 300 MHz Zond Aero LF antenna beneath a drone. The system incorporated:
- Radar altimeter for True Terrain Following.
- SkyHub flight controller for altitude maintenance at approximately 1.5 meters above ground.
- UgCS flight planning software to define accurate flight paths following topographical variations
Battery replacements were efficiently handled mid-flight, with precise resumption from the last waypoint. Parallel ground-based surveys were conducted in accessible areas to provide baseline comparative data, facilitating validation of aerial survey accuracy.
Results
The drone-mounted GPR system successfully captured subsurface data that revealed the targeted geological layer—interpreted as boulder clay—observable at depths of up to approximately 4 meters. The team also noted the presence of additional, smaller layers in the data, possibly peat or silt.
While the aerial data showed some loss of detail when compared to traditional ground-based GPR, the difference was minimal and did not compromise the overall interpretability. Profiles acquired with the drone were considered highly comparable to those collected with the ground-coupled system, validating the drone's suitability for subsurface mapping in areas where physical access is restricted.
Operational feedback included:
- Terrain-following using the radar altimeter functioned reliably, with consistent low-altitude flight maintained throughout the survey.
- Manual interruption and resumption of terrain-following within the same flight were successful.
- Battery replacements during flight did not affect data continuity, as missions resumed from the last waypoint.
- For future operations, the team is considering the use of a spotter drone to enhance situational awareness, particularly to detect the presence of people or wildlife near the survey area.
Conclusion
The drone-mounted 300 MHz Zond Aero LF GPR system demonstrated its capability for high-resolution subsurface geological mapping in environmentally sensitive and inaccessible areas. This method provided a non-intrusive alternative, with highly comparable results to traditional ground-based approaches. Medusa Explorations confirmed "excellent results," with the key geological layer clearly visible in the drone data. The team expressed strong enthusiasm for the technology and sees significant potential for its application in natural reserves, peatlands, agricultural fields, and other challenging terrains.