World's First Demonstration of a Method for Detecting Early Signs of Road Cave-ins Using Reflected Radio Waves from SAR Satellites

NTT announced that it has achieved the world's first successful demonstration of a method to detect early signs of road cave-ins using data from SAR satellites. This method enables the efficient and cost-effective identification of high-risk locations without on-site operations.

The technology works by detecting early cave-in signs based on the scattering2 of radio waves with multiple polarizations. The reliability of this technology has been verified through comparison with actual road cavity inspection data.

Building on this achievement, we will further improve the technology's reliability through demonstration experiments conducted in collaboration with local governments, contributing to greater safety and security in society. Under the "NTT C894" brand, we will continue to leverage satellites to address various challenges related to social infrastructure.

In recent years, the deterioration of social infrastructure has progressed, and road cave-in incidents have become a societal concern. Meanwhile, municipalities responsible for managing social infrastructure have limited budgets and personnel, making it difficult to thoroughly maintain and inspect the vast infrastructure network spread across the country.

Currently, on-site visual inspections of underground structures such as sewer systems, which can cause cave-ins, as well as ground-penetrating radar surveys from the surface, are conducted. However, these methods cover only limited areas and require substantial labor and cost, making it impractical to inspect wide areas comprehensively. In addition, cave-ins caused by underground structures begin with cavity formation and progression below the surface. Satellite-based observations have largely been limited to surface conditions, which has posed challenges for applying satellite data to road cave-in detection.

NTT has also been researching satellite-based methods for estimating soil moisture content to support landslide prediction. The research has demonstrated that radio waves penetrate the soil and that soil moisture content can be estimated with high accuracy by analyzing the degree of wave penetration6.

Through this approach, we have established a method that can directly detect early signs of road cave-ins using radio wave measurements. Verification based on comparison with inspection data of underground cavities confirmed that these cavities can be identified using satellite data alone. Traditionally, vehicle-mounted ground-penetrating radar has been used to identify underground cavities, but applying this new technology is expected to reduce costs by approximately 85 percent.

The optical fiber-based ground monitoring method7, previously announced by NTT, is a technology that monitors the progression of underground cavities deep below the surface using underground optical fibers. This method enables early detection of major cavities that may lead to road cave-ins. In contrast, the new technology utilizes satellite-based radio waves and is therefore suited for detecting cavities that form closer to the surface. By using wide-area satellite data, it becomes possible to detect high-risk cavities that have advanced near the surface. Through the complementary use of these technologies, NTT aims to more reliably detect early signs of road cave-ins.

■ Identification of high-risk areas using satellite data alone and capability for regular monitoring

This technology does not indirectly estimate potential road cave-ins by combining data from underground utilities, environmental data, and satellite data. Instead, it directly measures conditions using radio waves from SAR satellites. As a result, early signs of road cave-ins can be detected with high reliability using only satellite data obtained from satellites orbiting and observing the Earth.

The mechanisms that lead to road cave-ins are complex, and deterioration can progress suddenly, making periodic inspections conducted every few years insufficient and prone to oversight. In contrast, SAR satellites regularly orbit the Earth, making it easier to monitor conditions more frequently than traditional on-site inspections. Continuous monitoring of road conditions through this technology reduces the risk of overlooking major cavity development.

■ Analysis of signal components derived from transmitted and received radio waves

When radio waves transmitted by SAR satellites interact with the ground, they may be reflected directly back (specular reflection) or scattered in various directions depending on the surface and subsurface conditions. By analyzing the polarization components of the radio waves in transmission and reception, the three scattering components shown in Figure 2 can be identified. By focusing on the process of road cave-in formation and the associated scattering components, this technology has successfully enabled road cave-in detection.

We will work with business partners and local governments to streamline and reduce the cost of inspection operations for preventing road cave-ins. Through demonstration experiments, we aim to further enhance the reliability of this technology. While listening to the needs and challenges faced by municipalities, we will continue developing this method so it can be practically implemented in the field.

By deploying sustainable maintenance technologies to address critical issues such as infrastructure deterioration and demographic aging, we will contribute to building a safer and more resilient society.

Source: NTT media announcement