Distributed acoustic sensing (DAS) is an emerging sensing technology that uses the inherent properties of optical fibers to detect and locate sounds over long distances. In DAS systems, laser pulses are sent down a fiber optic cable and the system analyzes the light scattered back from imperfections along the cable. Any vibrations or acoustic waves interacting with the fiber will cause minor fluctuations in the backscattered light. By measuring these tiny fluctuations, the DAS system can determine the amplitude and location of sounds or vibrations along the entire length of the fiber. This allows DAS to effectively turn any optical fiber into a continuous array of microphones.
How Does DAS Work?
Standard DAS systems make use of a technique called coherent Rayleigh scattering. Laser pulses are sent down the fiber and most of the light continues propagating down the cable. However, a very small amount of light scatters back inelastically from density fluctuations in the glass fiber. Any acoustic waves or vibrations interacting with the fiber will cause minor changes in the density and alter the backscattered light.
The returning light is compared to a local oscillator beam from the same laser using interferometry. This allows incredibly sensitive detection of even tiny phase and amplitude changes in the backscattered signal. By measuring the interference pattern of backscattered light over time, the Distributed Acoustic Sensing (Das) system can determine the location and characteristics of any acoustic disturbances along the entire length of the fiber. The scattering process is nonlinear meaning that a single laser and receiver can monitor sound over extended distances up to ten kilometers or more using only one optical fiber.
Applications for Pipeline Monitoring
One of the most promising applications for DAS is pipeline monitoring. Oil and gas pipelines often extend over long distances through remote areas making them difficult to monitor using traditional methods. Distributed acoustic sensing systems take advantage of the thousands of kilometers of optical fiber that may already be installed alongside pipelines for communications purposes.
By installing the fiber in a loose tube directly buried alongside or attached to the pipeline, the fiber essentially acts as a distributed hydrophone. It can detect flows within the pipe, third party digging activity, movement on the ground surface above, and even leaks emanating acoustic signals. Any acoustic signals generated along the pipe will induce strain perturbations on the fiber allowing continuous monitoring along the entire length. This provides utilities with an invaluable tool for preventative leak detection and third party interference monitoring to help protect pipeline infrastructure and the environment.
Continuous Perimeter Security
In addition to industrial applications, DAS also shows potential for perimeter security and intrusion detection. Optical fiber can be installed in fields, along fence lines or buried just below the surface to act as a distributed sensor array. By listening for any vibrations through the fiber, the DAS system can continuously monitor a site for any intrusions like vehicles driving onto private land, digging, or anything that generates acoustic signals.
An alarm can immediately alert security to the exact location of an intrusion along the monitored perimeter. This provides a unique capability compared to other sensor technologies with inherently limited coverage areas. Distributed acoustic sensing systems fiber sensors could even be installed underwater for port security applications, monitoring harbors for suspicious acoustic signatures indicative of diver intrusions or explosives detonated within the monitored area. The potential applications are wide-ranging wherever long, continuous monitoring of acoustics over an extended area would provide value.
Seismic Monitoring and Underground Acoustics
Given its ability to listen through optical fibers over extended areas, DAS is also being explored for applications in seismic monitoring and underground acoustics. Fiber installed in tunnels, mines and underground infrastructure provides a means to continuously listen for signals that could indicate impending sinkholes, rockfalls or seismic activity.
Research is also investigating the potential of DAS for monitoring ground vibrations to help locate underground resources, map subsurface geology and detect man-made structures or tunnels. One study installed fiber along an abandoned railroad line and was able to detect buried drums and other objects by listening for their reverberated acoustic signatures through the ground. As the technology continues advancing, DAS opens up new opportunities for understanding our world through innovative acoustic sensing applications both on and below the earth's surface.
Distributed acoustic sensing leverages the unique properties of fiber optics to provide a new paradigm for continuous, ultra-long range monitoring of acoustics. By turning any installed fiber cable into an array of distributed microphones, DAS has enabled entirely new capabilities for critical infrastructure protection, perimeter security, and subsurface exploration. As systems improve and costs decline, the range of potential uses for this groundbreaking sensing approach will likely continue expanding across multiple industries. Distributed acoustic sensing promises to become an increasingly important tool for improving situational awareness and protection of assets wherever continuous acoustic monitoring at scale could provide value.
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