Distributed Acoustic Sensing (DAS), also knows as Distributed Fiber Optic Sensing (DFOS), is an emerging type of distributed sensing technology that uses lasers to sense sound in real time along an optical fiber cable. Simply by plugging into one end of a normal optical fiber cable, a DAS unit causes the fiber to behave like 1000s of vibration sensors, or microphones, spread out over many kilometres, with no additional electronics or hardware.
What is Distributed Acoustic Sensing (DAS)?
How Does Distributed Acoustic Sensing Work?
To achieve this remarkable coverage a Distributed Acoustic Sensing unit, or interrogator, sends thousands of laser pulses into an optical fiber every second. Although optical fiber is very good at transmitting light, there are microscopic imperfections in all fibers, and some laser light hits these imperfections and bounces back – a phenomenon known as Rayleigh backscatter.
This is the same principle that causes the sky to be blue – as light from the sun travels across the atmosphere above you, some of it is scattered down to Earth by Rayleigh scattering. Blue light is more likely to be scattered, so when you look up at the sky and see the light directed down towards you, it appears blue.
The light that scatters back from these imperfections is measured by the interrogator, recording a “fingerprint” of the fiber at the time of the laser pulse. The system then sends another pulse into the fiber a fraction of a second later, measuring another fingerprint. By comparing these two fingerprints the system can calculate how much the fiber has stretched between the two pulses. By repeating this process thousands of times per second, a DAS system measures tiny stretches of a fiber optic cable in real-time, all the time. DAS systems are incredibly sensitive, capable of resolving length change down to a few atoms.
How is DAS different from a microphone?
As the name Distributed Acoustic Sensing suggests, a DAS system is an acoustic sensor. This means it acts much like a microphone – it can record any vibration or sound nearby that stretches the optical fiber. The wide frequency range of DAS means this vibration could be as high-pitched as a leaky pipe or a squeaky bearing, or as low-pitched as an earthquake. A DAS system can detect these vibrations through the ground or through the air, from hundreds of meters away, or in the case of earthquakes, hundreds of kilometres away.
Unlike a microphone, DAS measurements are also distributed across many kilometres. This means that a DAS interrogator can sense leaky pipes or squeaky bearings at many points along the optical fiber, and then determine exactly where these sounds are coming from, within meters. This is like having a string of thousands of microphones, but without any electrical cables or batteries. DAS installations like this are much easier to install and maintain than microphone arrays used in similar applications.
Applications of Distributed Acoustic Sensing
- Pipelines: DAS can monitor leaks, pipe health, tampering, and anomalous vibration for oil & gas, water, or slurry pipelines, often leveraging existing fiber installations.
- Conveyors/rolling stock: Continuous sensing along the entire length of a conveyor allows identification of individual bearing failures without manual inspections.
- Seismic acquisition: Fiber can easily be deployed to measure seismic events, acquire geophysical exploration data, or even measure earthquakes.
- Perimeter Monitoring: DAS can be installed along fence lines, borders or in buildings to detect intrusion, tunnelling, or unauthorised movement.
- Smart cities: Unused optical fiber underneath cities allows a unique opportunity to use DAS to monitor construction, utilities, or even traffic, simultaneously on underground fiber.
- Downwell & downhole sensing: Optical fiber provides a low-maintenance, high uptime sensor for monitoring oil & gas wells, geothermal energy production, carbon capture and sequestration, and mining activities.
Advantages of DAS
- Remote awareness: DAS allows you to check on assets all the time, anywhere with an internet connection.
- Cost effective: DAS + Fiber costs far less than other large microphone or vibration sensor arrays.
- Low maintenance: no electronics required at sensing points means very few points of failure.
- Informative datasets: Terabytes of rich acoustic data can be interpreted using Terra15’s purpose-built monitoring software or integrated into existing industrial monitoring systems to inform decisions on asset health.
- Real-time data: DAS records real-time information on production assets, enabling operators to minimise downtime and optimise gains.
These examples represent just some of the proven use cases for Distributed Acoustic Sensing, and there are still many untapped possibilities. A sister technology to DAS, Distributed Temperature Sensing (DTS), was similarly developed several years ago and has since seen widespread adoption throughout many industries, all over the world. Now, widely available fiber networks, a versatile sensing technique, and various established applications mean that DAS is poised to be the sensing technology of the future.
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Author
Simon Stobie
Applications Engineer
Terra15
