Scientists track hidden ground sinking beneath a 10-mile subway corridor

A subway line can look perfectly normal from above even as the ground beneath it slowly sinks, making the danger difficult to spot until cracks, structural damage, or other warning signs appear. Catching this movement early is especially difficult because ground settlement (when soil slowly sinks or loses support) can occur across large areas while the conditions causing it remain hidden underground. Now, researchers at Seoul National University of Science and Technology have developed a way to hunt for these hidden warning signs. Their forensic multi-scale remote sensing (MSRS) framework combines satellite radar, laser scanning, and ground-penetrating radar to find suspicious areas, investigate them in detail, and probe what may be happening underground. From a 10-mile corridor to one suspicious shaft The problem with relying on one monitoring method is that each technology sees only part of the picture. Satellite-based InSAR, for example, can track tiny changes in ground elevation across wide areas, but it cannot by itself explain what is causing the movement. Ground surveys can provide more detailed information at a particular site, but covering an entire subway corridor that way would be difficult. Researchers have previously paired technologies such as InSAR with GPR or laser scanning, but the new approach links three methods in a staged investigation. “Our approach allows problems associated with excavation, inadequate compaction, and other construction defects to be characterized in greater detail than would be possible by using any single technology,” Taeyong Park, lead researcher, said. They tested MSRS along more than 16 kilometers of the Seoul Metropolitan Subway Bundang Line corridor between Suseo Station and Cheongnyangni Station. The first step was InSAR (interferometric synthetic aperture radar). Satellites repeatedly send radar signals toward Earth’s surface and compare the returning signals over time. Changes in those signals can reveal whether the ground is moving up or down. The team analyzed the corridor over the long term and used seasonal-trend decomposition using LOESS, a statistical method that helped separate seasonal fluctuations from longer-term settlement. This wide-area scan identified a ventilation shaft with a distinct settlement signal. Rather than investigating every location along the route in equal detail, the researchers could now focus their attention on this potential trouble spot. The satellite analysis found settlement of up to about 5 millimeters across the study area, while the selected ventilation shaft and its surrounding area showed maximum cumulative settlement of about 15 millimeters. The settlement was progressive, meaning the ground continued to sink over time. Zooming in on the suspicious spot That raised another question—was the satellite signal showing a genuine structural problem, or simply an artifact of remote sensing? To find out, the researchers moved to the second layer: laser scanning (L/S). They surveyed the selected ventilation shaft in the field, where visual inspection had already revealed multiple cracks and signs of previous repairs on the ceiling. Laser scanning allowed the researchers to measure the ceiling’s shape and settlement in much greater detail. The results showed that the deformation became stronger toward the section of the ceiling directly beneath the roadside above the shaft, similar to how modern LiDAR systems can generate detailed three-dimensional measurements. The third layer looked beneath this road. The researchers used ground-penetrating radar (GPR), which sends high-frequency electromagnetic pulses into the ground and detects changes caused by buried structures or differences in soil. The GPR survey found signals consistent with void-like structures near the shaft, along with reduced continuity in underground layer boundaries. Together, these findings pointed to uneven conditions beneath the road. As the three methods provided consistent evidence at different scales, the researchers had stronger grounds for treating the settlement anomaly as a genuine signal rather than relying on a single dataset. Turning warning signs into prevention The significance of MSRS is not simply that it combines three technologies. It creates a chain of evidence. For instance, InSAR finds where something is changing, laser scanning shows how a structure is deforming, and GPR investigates what may be happening underneath it. The framework can therefore connect a broad settlement warning with increasingly detailed evidence about the affected site, rather than relying on one measurement alone. That could give infrastructure managers a more practical way to prioritize inspections. Instead of waiting for visible damage or a sudden failure, they could use wide-area satellite monitoring to identify high-risk locations and then deploy more detailed tools where they are most needed, an approach that could complement other efforts to monitor critical infrastructure. The approach does not automatically establish the exact cause of every settlement anomaly, and underground conditions can be complex. However, the researchers see a broader role for the system. “Our research could help shift the paradigm of urban disaster management from reactive response to proactive prevention. In the long term, the MSRS system will support timely maintenance and targeted investigations of high-risk areas, thereby reducing the risk of sudden sinkholes, structural damage, and infrastructure failure,” Park explained. Their broader goal is to move urban disaster management away from reacting after damage occurs toward proactive prevention—using multiple layers of evidence to investigate trouble before it becomes an emergency, much as underground engineering projects must account for hidden conditions before and during construction. The study is published in the journal Tunnelling and Underground Space Technology. Get the latest in engineering, tech, space & science - delivered daily to your inbox.Rupendra Brahambhatt is an experienced writer, researcher, journalist, and filmmaker. With a B.Sc (Hons.) in Science and PGJMC in Mass Communications, he has been actively working with some of the most innovative brands, news agencies, digital magazines, documentary filmmakers, and nonprofits from different parts of the globe. As an author, he works with a vision to bring forward the right information and encourage a constructive mindset among the masses.