From subsurface damage to surface instability: Integrating InSAR, UAV and microseismic observations of deep longwall mining beneath thick loess

Deep underground, the earth is not a static block of stone but a living, shifting system that reacts to the removal of material. When miners extract coal in long, continuous strips, the rock layers above them begin to bend, crack, and settle. This process, known as mining subsidence, does not always happen all at once. Instead, it often begins as invisible damage deep below, slowly accumulating until the ground above can no longer hold together. In some places, this leads to sudden, dangerous collapses; in others, it creates slow, creeping cracks that ruin the landscape. Understanding exactly how this damage travels from the deep rock to the surface is crucial for keeping communities safe, especially in regions where the ground is covered by thick, loose soil that behaves differently than solid rock.

Scientists have long used satellites to watch the ground move, measuring how much the earth sinks over time. They have also used small, high-flying cameras to take pictures of the surface and see where cracks appear. Separately, they have listened to the earth with sensitive instruments that detect tiny tremors caused by rocks breaking underground. For a long time, these three ways of watching the earth—satellites, cameras, and listening devices—were used as separate tools. Each gave a piece of the puzzle, but no one had successfully put them together to see the full story of how underground damage turns into surface disaster. This is particularly important in the thick loess regions of northwestern China, where a massive layer of wind-blown soil sits on top of the coal seams, acting like a sponge that can hide the early signs of trouble.

A team of researchers decided to bring these three methods together to watch a deep coal mine in the Zhengning coalfield. They focused on an area where the coal lies nearly a kilometer underground, covered by about 200 meters of loose, yellow loess soil. The team used a satellite to track the slow sinking of the ground over two years, a drone to map the exact location of new cracks and holes, and a network of underground sensors to listen to the rocks breaking. By combining these views, they could see not just where the ground was moving, but how the movement started deep down, traveled upward, and finally broke the surface.

The researchers discovered that the ground did not simply sink in a smooth, even bowl. Instead, the process happened in distinct stages. At first, the ground above the active mining panels sank in separate, isolated areas. As the mining continued, these separate areas of sinking began to grow toward each other. Eventually, they merged into one large, complex basin. The most surprising finding was that the surface did not break in the center of the deepest sinking. Instead, the dangerous cracks and holes appeared along the edges where the ground was changing shape most rapidly. The satellite data showed that the ground was stretching and compressing most intensely in these transition zones, and the drone photos confirmed that this is exactly where the earth tore open.

Perhaps the most significant discovery was the timing of these events. The sensors deep underground detected a steady accumulation of tiny rock fractures weeks before the surface began to move dangerously fast. The researchers found that the underground damage, measured by the energy of these tiny tremors, preceded the acceleration of surface sinking by about 17 days. This suggests that the earth gives a warning signal from below before the surface shows signs of stress. However, the relationship is not perfect. While the slow buildup of damage usually leads to faster surface movement, there are also moments when a single, massive underground failure happens, causing the surface to react almost instantly. This means there are two different ways the ground can fail: a slow, creeping process that gives a warning, and a sudden, violent break that happens without much delay.

The study also clarified why the thick layer of loess soil changes the way mining damage looks. Because this soil is weak and full of air pockets, it tends to delay the movement from below and concentrate the stress at the edges of the sinking area. This is why the surface cracks form in specific bands rather than spreading out evenly. The researchers concluded that to truly understand mining hazards, one cannot just look at how much the ground has sunk. One must also watch how fast it is moving, how the speed is changing, and listen for the quiet cracking happening deep underground. By combining these different views, they created a new way to see the hidden life of the earth, offering a clearer picture of how deep mining transforms the landscape above.