About weak layers and snow profiles
Slab avalanches are known to occur due to a bound layer ("board") above a weak layer (e.g. snow-covered surface layer) in the snowpack. The tricky thing for winter sports enthusiasts is that these weak layers cannot be recognised at the surface. This is known as the old snow problem. Apart from taking into account the information from the avalanche report, digging a snow profile was - until now - the only way to gain an insight into the snowpack. However, digging snow profiles or carrying out snowpack stability tests is not exactly without danger. Especially during times of tense avalanche situations, staying in steep terrain can be risky. In addition, snow profiles only ever provide reliable information at specific points; snow profiles are therefore not meaningful enough for an entire slope or even a terrain chamber due to the selective measurements.
However, it is essential to know the spread of the potential weak layer(s) in order to assess the risk of slab avalanches. If these are spread over a wide area, the risk of a slab avalanche increases. If the weak layers are inhomogeneous, the formation of a slab is correspondingly less likely. In order to obtain such extensive data on the snowpack structure without having to put yourself in danger, a new approach was developed in the FFG-funded research project STRATIFY.
The combination of drone and radar makes it possible
The drone can be controlled from a safe location so that human exposure in the danger zone is not necessary. The drone is equipped with several sensors to collect data: The altimeter (radar or LiDAR) ensures that the drone can maintain an approximately constant predefined distance to the snow surface. The "Skyhub" serves as an on-board computer. The georadar is mounted on the underside of the drone.
In general, a GPR emits short electromagnetic waves. These waves hit various materials in the ground, such as stones, sand or water. Depending on the material properties, the waves are reflected, i.e. bounced back, to varying degrees. The GPR receives the reflected waves and measures the propagation time of the signal. This time can be used to calculate how deep the respective material is in the ground. Applied to the snow cover, "different materials" simply means different layers of snow, which differ in their hardness, for example.
