Ground settlement, the downward movement of the ground surface or soil, is caused by changes in underlying soil or rock. With the continuing development of underground spaces in urban areas, the frequency of ground surface settlement has increased, posing considerable concern for infrastructure stability and public safety. It can particularly threaten the safety of critical infrastructure like subway systems. Since ground settlement is difficult to predict in advance, continuous monitoring over a wide area is essential.
Conventional techniques, however, relying only on a single mode of monitoring, are limited in providing reliable monitoring over large areas. For example, Global Positioning System (GPS)-based monitoring is not suitable for measuring deformation in underground structures because GPS signals cannot penetrate competent ground. Satellite-based Interferometric Synthetic Aperture Radar (InSAR), on the other hand, can quantify ground deformation over large areas and over long periods, but its observations can be affected by factors such as observation geometry and atmospheric delays, which can complicate interpretation. Combining complementary monitoring techniques can therefore provide a more comprehensive assessment of settlement.
In a new study, a research team led by master's student Tae-Yong Park from the Department of Civil Engineering at Seoul National University of Science and Technology has developed a forensic multi-scale remote sensing (MSRS) framework integrating InSAR, laser scanning (L/S), and ground penetrating radar (GPR). "This innovative fusion of technologies enables not only the detection of ground settlement but also the investigation of its underlying causes," explains Mr. Park. "Particularly, 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." Their study was made available online on April 25, 2026 and published in Volume 174 of Tunnelling and Underground Space Technology on August 01, 2026.
The proposed MSRS system enables wide area monitoring, site-specific validation, and near-surface condition assessment. For testing, the approach was applied over the Seoul Metropolitan Subway Bundang Line corridor between Suseo Station and Cheongnyangni Station with a length of over 16 kilometers. The system has three main components.
First, long-term settlement is measured along the entire corridor using satellite InSAR time-series analysis. InSAR estimates surface displacement by comparing radar signals acquired repeatedly over the same area. In the study, the team also applied a technique called "seasonal-trend decomposition using LOESS" to exclude the seasonal fluctuations in settlement. InSAR analysis of the subway corridor revealed a ventilation shaft with a distinct settlement signal, which was then selected for subsequent detailed analysis. Further analysis showed a progressive long-term settlement.
Next, field L/S was conducted at the selected shaft. This method utilizes LiDAR to create digital point-cloud representations of the target area, allowing quantification of the shaft structure. Since visual inspection revealed multiple cracks and signs of repair on the ceiling, L/S was used to analyze the settlement tendency of the ceiling. The results demonstrated a settlement pattern that became more pronounced towards the part of the ceiling directly beneath the roadside above the shaft.
Based on this result, GPR was then carried out along the road section above the shaft. GPR is a non-destructive technique that uses high-frequency electromagnetic pulses to investigate subsurface features. The GPR survey showed signals representing void-like structures near the shaft and reduced continuity of layer boundaries, indicating non-uniform subsurface conditions.
Finally, an integrated visualization of the three results enabled cross-interpretation and showed that the detected settlement anomaly was not an artifact from any one technique but a reliable result obtained from a multi-scale analysis. This indicates that with the proposed system, interpretational uncertainty associated with relying on a single dataset can be reduced.
"Our research could help shift the paradigm of urban disaster management from reactive response to proactive prevention," remarks Mr. Park. "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."
Overall, the proposed system facilitates early detection of potential ground-related risks across extensive urban areas and identification of warning signs before they develop into serious hazards.
