Using satellites and drones to detect slow slope movements early on
"Deep mass movements have existed since the end of the last ice age, when retreating glaciers left behind valleys with very steep valley walls and the melted ice led to stress relief of the rock," Anne Hormes of the Innsbruck-based engineering firm "sky4geo," which specializes in the analysis of natural hazards, told APA. She led the VIGILANS research project, in which scientists and experts from the University of Natural Resources and Applied Life Sciences, the Federal Geological Survey, the Torrent and Avalanche Control and the Federal Forest Research Center cooperated. To conclude the project, the researchers held a workshop on 3D visualization on Friday, presenting the possibilities of satellite and drone monitoring.
Slope movements can reach depths of 100 to 300 meters
Such deep slope movements can affect entire mountain flanks and reach depths of 100 to 300 meters. Experts can easily identify such deformations in the terrain by certain landscape features such as incipient edges or crevasse formation, Hormes said. But because slopes often move only in the range of a few millimeters to centimeters per year, it is often unknown whether slope movements are active, he said.
If such movements continue for millennia, fatigue can occur in the rock and various triggers, such as changes in permafrost, more slope water or heavier precipitation, can cause faster movements that threaten localities, settlements and infrastructure. In addition, secondary processes such as landslides of partial floes, rockfalls, rockfalls, or debris flows can emerge from these deep mass movements.
"Therefore, one should be interested in how to detect such acceleration phases," Hormes explained, referring to Norway, where systematic monitoring of such slope movements has been carried out for years. The background is the country's fjord system, where one has the problem that a rockfall in a fjord can trigger tsunamis that threaten settlements further away. But this danger certainly exists in the Alps as well, the expert said, recalling the Longarone disaster: in 1963, a landslide into the reservoir of the Vajont River in northeastern Italy triggered a tidal wave that killed some 2,000 people.
Test areas in Tyrol and Salzburg
Using four test areas in northern and eastern Tyrol and in Salzburg, the scientists in the project used innovative remote sensing methods to identify and assess slow slope movements and compared the resulting movement data with results from classical surveying, terrestrial and airborne laser scanning and geological mapping.
They used data from the European Space Agency's "Sentinel-1" Earth observation satellite and the German TerraSAR-X satellite. "Sentinel-1" flies over every point in Europe every six days and uses radar to measure the distance to the Earth's surface. "This allows us to detect surface movements as small as one centimeter per year," Hormes said. One problem is that the radar signal is scattered by vegetation, so it cannot be used over forests or agricultural land, for example. Houses or roads, however, could help as reference points. The satellite data are freely available and have been since 2014. It could also be used to evaluate slope movements retrospectively, he said.
"With the satellite data, you can see at a glance in larger areas if a slope is moving quite fast and you should look at it more closely," Hormes explained. Drones, which can use 3D drone photogrammetry to register slope movements starting at about five centimeters per year, are suitable for such an operation in a limited area. In addition, they could provide a very good spatial idea of the slope deformation, such as whether there are sub-slabs, and how their boundaries run. However, flight planning for the drones is very complex and requires very accurate terrain models, control points on the ground, special drones and the global navigation satellite system.
As part of the EU's "Copernicus" Earth observation program, the European Ground Motion Service (EGMS) is scheduled to launch next year, which will record natural and anthropogenic ground motions across Europe with millimeter precision. "If we have learned anything in this project, it is that it is very difficult to process satellite data from large areas, especially in the mountains," Hormes said. Therefore, he said, it's important to use remote sensing methods like this to focus on individual areas that should be looked at more closely based on specific clues. That way, he said, you can process better and more targeted data, use better calibration points, and thus eliminate all the negative effect and uncertainties that you know from the method.