Compared to GPR surveys conducted by helicopter, the system permits the acquisition of high-density datasets that can be used to assess and monitor internal glacier dynamics and hydrology.
GPR has been used over glaciers for several decades, but data acquisitions are typically carried out either directly on the surface of the glacier or by helicopter several tens of meters above the ice. Glacier-based surveys permit the acquisition of high-density 3D GPR data but are labor intensive and sometimes dangerous due to crevasses and other hazards. Helicopter-based surveys, on the other hand, are safe and efficient, but extremely costly and do not allow for the acquisition of such high-density measurements. A drone-based GPR system specifically designed by the ISTE research team for glacier work combines the advantages of both of these types of surveys.
The system is in part composed of a Matrice 600 drone from DJI, a custom-built and lightweight GPR and antenna, UgCS Skyhub and a radar altimeter from SPH Engineering, and an independent GPS. Using such a configuration, it becomes possible to record high-density 4D GPR data by repeating the same data acquisition over time. These 4D GPR data might offer interesting insight regarding internal glacier dynamics and hydrology, which is the overall aim of the research group’s work.
‘To record high-quality data, we need to stay as close to the ice surface as possible, but at the same time avoid these obstacles and maintain a smooth flight path. This means adjusting the altitude of the drone along the flight to roughly follow the glacier surface at a prescribed height. Using SPH Engineering’s True Terrain Following (TTF) algorithm, the GPR also remains close to the ice allowing for excellent data quality. Tests have shown that our drone-based GPR system is able to record data at least four times faster than previous surveys on foot, with an overall improvement in data quality’, Bastien Ruols, PhD Student in Applied Geophysics at ISTE, explains. ‘After some testing, we chose last summer to fly our GPR at a height of 5m and at a speed of 3 m/s. With these parameters, we were able to record more than 3 line-km of GPR data on one set of drone batteries, which lasted approximately 20 minutes. The TTF algorithm perfectly fulfills all of our requirements’.
‘In addition, UgCS is a great piece of software that offers all of the functionality needed to carry out our surveys in challenging mountain environments. The simulation mode importantly allows us to run practice flights before flying with expensive geophysical instruments. We can easily create trajectories of all types, and the software works well without a network connection’, Bastien adds. ‘As it is complicated to access all the flight data directly from the DJI drone, the fact that all of the information is recorded directly on the UgCS Skyhub datalogger is very valuable’.
In 2018, the University of Lausanne research team recorded, on foot, a high-density 3D GPR dataset over the Otemma glacier in Switzerland, which was used to locate subglacial drainage channels close to the glacier snout. In the summer of 2021, they recorded data over the same area using their drone-based system. Initial results suggest that the drone data are even higher quality than those recorded on foot, despite the need for additional processing to correct for antenna ringing and the height of the GPR above the surface.
In the fields of geophysics and glaciology, UAV-based data collection has become increasingly popular in recent years. However, development of a drone-based GPR system for 3D and 4D surveying of glaciers is definitely innovative. As of today, the ISTE research team is known to be the only group to have developed a system specifically for this purpose.