NTT has announced that it has verified a method for estimating underground cavity formation using high-precision "Distributed Acoustic Sensing (DAS)" technology. This approach captures weak underground vibrations by using existing optical fibers that support internet networks as sensors. While optical fibers have already been installed for DAS technology utilization in plant maintenance and security purposes, using established optical fiber networks makes it possible to maintain communication networks. It also enables geological assessments of the subsurface and continuous monitoring of changes over time. If practical applications progress, there is the potential to conduct disaster prevention, disaster mitigation, and infrastructure maintenance at low cost over a wide area.
Provided by NTT
Measuring where and what is happening from changes in the returning scattered light
DAS stands for Distributed Acoustic Sensing. According to Senior Research Engineer Daisuke Iida at NTT Access Network Service Systems Laboratories, who is engaged in research on optical fiber transmission path testing technology and optical measurement technology, when pulsed light is sent into an optical fiber, the light travels forward while some scattered light returns backward. Weak vibrations from social activities on the ground, such as cars passing by, are transmitted through the surface, causing the optical fiber to bend and stretch by several tens of nanometers to approximately one micrometer per meter. DAS technology uses a receiver to capture changes in the scattered light that returns after light is sent into the optical fiber, measuring what kind of vibration (acoustic) is occurring at which point along the fiber.
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From plant and fiber network maintenance to earthquake measurement and ground evaluation
Research and developments into DAS have progressed globally since around 2010. Since laying optical fibers along pipelines enables constant monitoring of pipe deterioration without the need for hammering tests, it has been utilized at oil and gas plants. There are also examples of high-security sites as military facilities installing DAS optical fiber networks to detect intrusions by people or objects.
Domestically in Japan, universities and other institutions conduct earthquake observation research using existing optical fiber networks. At NTT, research has progressed to not only use DAS for maintenance and management of optical fiber networks themselves, but to explore whether it could be used for ground evaluation and expanded applications.
Technology that can obtain ground characteristics at depths of approximately 3 to 30 meters
To verify ground evaluation, NTT jointly conducted underground cavity detection with the National Institute of Advanced Industrial Science and Technology (AIST) in Tsukuba City, Ibaraki Prefecture and Soka City, Saitama Prefecture from July to September 2025. NTT and AIST simultaneously performed DAS optical fiber sensing and microtremor array surveys, with which AIST has extensive experience.
In microtremor array surveys, multiple microtremor meters are arranged on the ground in fixed vertical and horizontal configurations, each simultaneously recording and analyzing slight movements. While this approach has the advantage of being able to easily estimate conditions deep underground, equipment needs to be transported for each survey. Furthermore, surveying beneath roads necessitates temporary road closures, making it unrealistic to conduct ground monitoring over wide areas at high frequency.
On the other hand, DAS makes use of existing optical fiber used for communications, meaning work can be significantly reduced. If the data from microtremor array surveys and DAS are equivalent, it can be assumed that DAS is also obtaining the necessary data for ground monitoring and estimating cavity formation.
When the results of approximately 20 minutes of microtremor array survey were compared with approximately one day of DAS results the measurement data from both were consistent at frequencies indicating a range from approximately 3 meters to 30 meters of underground depth. This demonstrates the feasibility of examining soil softness with DAS and that remote monitoring can be performed once a day. Through observing temporal changes in ground characteristics, it becomes possible to estimate signs of progressing underground cavity formation.
Provided by NTT
The results were exhibited at the NTT R&D Forum 2025, held from November 19 to 26. In urban areas, road collapse accidents caused by underground cavity formation due to soil flowing into aging water supply and sewage systems have become a social problem. These can cause traffic disruptions and severing of lifelines, having a major impact on local communities. Going forward, the company will advance the sophistication of analysis algorithms and other aspects with the aim of providing infrastructure monitoring and disaster prevention systems. Simultaneously, they will carry out successive verification experiments in real-life urban environments with the cooperation of local governments and water supply and sewage operators.
Expectations for low-cost, frequent ground monitoring
NTT has reportedly installed approximately 2.4 million kilometers of optical cables across Japan. In this verification experiment, optical fibers within "conduits" —pipes for communication cables installed under roads and elsewhere—were used. There are 620,000 kilometers of conduits nationwide. If ground monitoring over this range can be conducted more frequently at lower cost than before, it is hoped that accidents like the road collapse that occurred in Yashio City, Saitama Prefecture in January 2025 can be prevented from happening again.
(NAGASAKI Midoriko / Science Portal Editorial Office)
Original article was provided by the Science Portal and has been translated by Science Japan.