Ground Imaging through Telecommunications Cables Technology: MIT Ph.D. student Hilary Chang has successfully used the university’s fiber optic cable network to image the ground beneath campus using distributed acoustic sensing, a method that can efficiently and effectively understand ground composition and assess seismic hazard.
How Telecommunications Cables Can Image the Ground Beneath Us
A New Method for Site Characterization
MIT Department of Earth, Atmospheric and Planetary Sciences (EAPS) Ph.D. student Hilary Chang recently used the MIT fiber optic cable network to successfully image the ground underneath campus using a method known as distributed acoustic sensing (DAS). By utilizing existing infrastructure, DAS can be an efficient and effective way to understand ground composition, a critical component for assessing the seismic hazard of areas.
The Dark Fibers
The MIT campus fiber optic system, installed from 2000 to 2003, services internal data transport between labs and buildings as well as external transport, such as the campus internet (MITNet). There are three major cable hubs on campus from which lines branch out into buildings and underground. Some of these cables are ‘dark fibers,’ or cables that are not actively transporting information.
Imaging the Ground
DAS can use existing telecommunication cables and ambient wavefields to extract information about the materials they pass through, making it a valuable tool for places like cities or the ocean floor, where conventional sensors cannot be deployed. Chang decided to try out DAS on the MIT campus, using the dark fibers in the fiber optic network.
Distributed Acoustic Sensing (DAS) is a technology that utilizes optical fibers to detect and analyze acoustic signals.
This method converts light signals into sound waves, allowing for the detection of even slight movements or vibrations along the fiber's length.
DAS has various applications in fields such as pipeline monitoring, seismic exploration, and oil and gas production.
Its accuracy and high-resolution capabilities make it an essential tool for real-time monitoring and early warning systems.
The Experiment
To get access to the fiber optic network for the experiment, Chang reached out to John Morgante, a manager of infrastructure project engineering with MIT Information Systems and Technology (IS&T). They decided on a path starting from a hub in Building 24, because it was the longest running path that was entirely underground. The path ran from east to west, beginning in Building 24, traveling under a section of Massachusetts Ave., along parts of Amherst and Vassar streets, and ending at Building W92.
Locating the Cables
After renting an interrogator, a device that sends laser pulses to sense ambient vibrations along the fiber optic cables, Chang and a group of volunteers were given special access to connect it to the hub in Building 24. They let it run for five days. To validate the route and make sure that the interrogator was working, Chang conducted a tap test.
Analyzing the Data
Once Chang collected the data, she was able to see plenty of environmental activity in the waveforms, including the passing of cars, bikes, and even when the train that runs along the northern edge of campus made its nightly passes. After identifying the noise sources, Chang and Nakata extracted coherent surface waves from the ambient noises and used the wave speeds associated with different frequencies to understand the properties of the ground the cables passed through.
Implications for Seismic Hazards
Information like this is critical for regions that are susceptible to destructive earthquakes and other seismic hazards. Areas of Boston and Cambridge, characterized by artificial fill during rapid urbanization, are especially at risk due to its subsurface structure being more likely to amplify seismic frequencies and damage buildings. This non-intrusive method for site characterization can help ensure that buildings meet code for the correct seismic hazard level.
Seismic hazards refer to the potential damage and risk caused by earthquakes.
These hazards can be categorized into three types: ground shaking, soil liquefaction, and landslide triggered by earthquakes.
Ground shaking is the most common type of seismic hazard, causing structural damage to buildings and infrastructure.
Soil liquefaction occurs when water-saturated soils lose strength during an earthquake, leading to instability and collapse.
Landslides are also triggered by earthquakes, particularly in areas with steep terrain or unstable geological conditions.
Conclusion
Chang’s experiment demonstrates the potential of DAS in imaging the ground beneath us. By utilizing existing infrastructure, this method can provide valuable insights into ground composition and help mitigate seismic hazards. As Chang notes, ‘Destructive seismic events do happen, and we need to be prepared.‘
