Dhe open-cast lignite mines in the Rheinische Revier between Cologne and Aachen are among the largest in the world. Although they are not the deepest open mining areas, they are one of the largest areas. The Hambach opencast mine extends over an area of more than 43 square kilometers. This includes areas in which the lignite is mined, but also large areas in which the overburden, i.e. the removed soil, as well as the geological layers overlying the coal seams are accumulated for later recultivation.
In some of the open-cast mines, the ground subsidence is considerable. In addition, the slopes along the pits become unstable over time and can slide, which is dangerous for people and buildings. In order to identify the risks early and to be able to take measures in good time, the mining areas must be continuously monitored. A German-Chinese research group has now developed a new method with which both the vertical ground movements in an open-cast mine and the stability of the flanks of the pits can be recorded from space with earth observation satellites.
Radar satellites keep an eye on ground movements
In order to be able to measure the ground movements associated with mining in the Rhenish Revier, the researchers led by Mahdi Motagh from the Geo Research Center in Potsdam used the data from, among other things, the German radar satellite „Terrasar-X“ and the satellite duo „Sentinel 1A und 1B“ of the European Copernicus program. The three scouts circle the earth at altitudes between 500 and 700 kilometers. They continuously send out radar beams and register the portion that is reflected from the earth’s surface.
For many years now, scientists have been using such radar measurements to measure ground movements that occur in active volcanoes or as a result of earthquakes or are caused by pumping out large amounts of groundwater. The radar signals that the satellites have measured in two successive orbits from exactly the same area of the earth’s surface are superimposed.
Motagh’s researchers have now refined this so-called radar interferometry in such a way that ground movements, such as those that occur in open-cast mining, can be recorded with a very high resolution. With their method, called “Synthetic Aperture Radar Inferometry” (InSAR), they evaluated radar measurements from 2017 and 2018. According to Motagh and his colleagues im „International Journal of Applied Earth Observation and Geoinformation“ the floors in Hambach have sagged by up to 50 centimeters per year, in the Garzweiler pit by 38 centimeters. In the third open-cast mine investigated in the Rhenish lignite mining area, the Inden mine, there were subsidence of 31 centimeters as well as uplifts of up to eight centimeters per year.
First embankment, then lowering
In all three pits, the greatest subsidence occurred in the areas where the spoil is stored. In Hambach alone, more than 250 million tons of overburden are produced each year, some of which is deposited on the Sophienhöhe heap in the northwest of the opencast mine. This hill now towers over the surrounding landscape by 200 meters. However, the overburden is only heaped up. Over time, the loose material thickens. The ground sags, sometimes by up to 50 centimeters. With the new measurement method, these subsidence could now be monitored in the long term even after recultivation, according to the scientists.
The method makes it possible to determine the stability of the flanks of an open pit. In opencast mines, there are always significant landslides along the flanks. The largest landslide to date took place in the open in 2013 Kupferbergwerk Bingham Canyon held near Salt Lake City. At that time, almost 70 million cubic meters of rock slipped onto the floor of the open pit and shut the mine down for months. Because the landslide could be predicted in good time, it was possible to evacuate all miners so that no one was harmed. On the other hand, it was different with a landslide in July 2018 in the Set Mu mine in Myanmar, where jade is extracted. At that time, 27 miners were killed.
The radar measurements of the Rhenish lignite mine have shown that there is no acute danger from landslides there. In the Hambach mine, movements of the up to 250 meters high, sometimes quite steep slopes of around twelve centimeters were recorded per year. According to the scientists, this movement is a direct result of subsidence and mainly affects loose material. However, it does not pose a risk, but rather contributes to the consolidation of the slopes and thus to their greater stability.