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Distributed Acoustic Sensing

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Distributed Acoustic Sensing (DAS) technology is capable of detecting changes in strain and vibrations along optical fibers. This sensitive method is employed for monitoring essential infrastructure, including power lines, pipelines, and railway tracks. The fiber optic cable acts as a distributed acoustic sensor, enabling ongoing measurements throughout its entire length, thus empowering operators or automated systems to make informed decisions and implement preventive or corrective measures as needed. Machine learning and artificial intelligence-based classification algorithms are utilized to detect, identify, and locate events such as pipeline leaks, faults in power cables, and intrusion incidents.

How DAS Works

Several Distributed Acoustic Sensing technologies are available on the market, with the most prevalent being coherent optical time domain reflectometry (C-OTDR). C-OTDR makes use of Rayleigh backscattering to detect vibrations and acoustic signals over extended distances.

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Using DAS technology, laser pulses are transmitted from an interrogator through optical glass fibers. A portion of the light is reflected back due to Rayleigh backscattering, which occurs as the light waves engage with natural microscopic irregularities within the glass. These irregularities respond to environmental acoustic vibrations that impact the fiber. While the Rayleigh backscattering retains the wavelength of the initial pulse, it experiences a specific phase shift. This phase shift provides important information about the acoustic signals, which are subsequently analyzed using interferometric methods.

Strain or acoustic amplitude is measured over a chosen gauge length (such as 5 meters), reflecting the rate of strain variation over that section of fiber at each location along the entire fiber.

The preceding laser pulse requires a certain duration to traverse the entire length of the fiber and for its scattered portion to make its way back before the subsequent pulse can be sent. As a result, the maximum pulse rate is influenced by the fiber’s length. For instance, a standard 20-kilometer fiber generally permits a pulse rate of 5,000 per second. This enables acoustic signals to be examined at that frequency, with frequencies reaching up to the Nyquist frequency—half of the pulse rate—being clearly distinguishable. While higher frequency components can also be detected, they may become uncertain due to Nyquist aliasing.

As higher acoustic frequencies undergo more significant attenuation in materials, the frequencies most pertinent for detecting and classifying events are primarily in the lower range of 2 kHz. 

In the extremely low frequency range (around some Hertz and below), phase-based C-OTDR systems allow for highly sensitive detection of transient temperature changes (i.e., fluctuations in the 0.001 °C range) that occur due to the expansion or contraction of the fiber with temperature variations. This method of measurement is known as Distributed Temperature Gradient Sensing (DTGS) and is commonly utilized in the oil and gas industry.

How DAS Works

By integrating phase and amplitude measurements, Airway Security’s unique 2P Squared technology experiences less interference from the signal fading typically seen in other C-OTDR systems. The DAS system developed by Airway Security delivers consistent performance throughout the fiber, ensuring high-quality measurements. Superior raw data forms the foundation for pattern recognition and machine learning algorithms.

Key features of Airway Security’s 2P Squared DAS technology include:

  • Accurate measurement and location of the amplitude, frequency, and phase of the incident acoustic field
  • True linearity maintained across the entire distance, time, and acoustic amplitude 
  • Signal quality achieved with excellent signal to noise ratio
  • Advanced “2P Squared” optical techniques improve signal quality over long measurement ranges
  • Measurement range of over 100 km
Video: How Does DAS/C-OTDR Work?

The Role of the Optical Fiber in Distributed Acoustic Sensing

There are various types of optical fibers available in the market, each designed for different needs, particularly in telecommunication setups. Many of these fiber types can be effectively used with Distributed Acoustic Sensing (DAS), with standard single mode fibers such as G.652, G.655, or G.657 often being the most suitable option. Single mode fibers provide low signal loss over long distances, enabling the use of sensors that can extend 100 km or more, and they do not exhibit mode dispersion, which can negatively impact pulse shape and signal integrity over distance. Due to their widespread use, standard single mode fibers are also quite inexpensive. As the applications for Distributed Acoustic Sensing expand rapidly, the market is seeing an increase in fibers that are specifically optimized for various sensing needs. However, if the sensing distance reaches or exceeds 100 km, the intrinsic loss of the probing pulse and backscatter can constrain the signal-to-noise ratio (SNR) of the data collected.

Ultra Low Loss Fibers (ULL)

ULL fibers reduce fiber attenuation to around 0.15 dB/km (at 1550 nm), whereas standard fibers usually have an attenuation of about 0.2 dB/km. These ULL fibers exhibit a lower Rayleigh backscatter coefficient, which leads to decreased signal amplitude over short distances when compared to standard single mode fibers. Nevertheless, at longer distances, such as 70 km, the benefits of lower attenuation become significant beyond a certain threshold, which varies based on the fibers in use.

EF is specifically engineered to augment the backscatter signal by employing weak reflectors embedded in the fibers, created using femto-second lasers. This improvement usually amplifies the signal by 10 to 20 dB in comparison to natural Rayleigh backscatter, resulting in a backscatter signal that can be up to 100 times more potent. However, EF does come with increased attenuation, which means that the practical lengths of fibers are shorter.

Integrating ULL with EF takes advantage of both fiber types, enabling greater distances with ULL fiber while also increasing the total distance by adding enhanced fiber. A reach of 125 km has been achieved with this combination using a standard DAS interrogator, without the need for optical amplification along the sensor.

This results in enhanced and more comprehensive detection abilities, surpassing conventional discrete sensor measurement techniques.

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Distributed Acoustic Sensing (DAS) provides numerous benefits for overseeing essential infrastructure, including:

Simplicity & Robustness of the Sensor

A typical single glass fiber comprises all essential components of a sensor: it delivers energy to the sensor (via light pulses), contains the sensor itself (with the inhomogeneity of the fibers), provides precise location information due to the distributed measurement principle, and facilitates data transmission (by returning light to the instrument). Therefore, the fiber serves as an ideal medium for sensing when connected to the interrogator unit.

When embedded in durable cables, fibers are consistently safeguarded for many years during field use. These cables usually have a service life of approximately 40 years.

DAS offers constant and real-time surveillance along the full length of the optical fiber. This facilitates the early identification and pinpointing of threats to assets (such as an excavator working near a pipeline or buried power line) or deterioration of those assets, allowing for prompt action and preventive upkeep. By identifying minor changes or irregularities in infrastructure conditions, DAS systems promote proactive maintenance and help prevent expensive downtimes or failures.

AP Sensing’s Distributed Acoustic Sensing (DAS) provides remarkable location precision within a meter’s range, even across extensive distances of up to 100 km. This capability enables the system to accurately identify and locate events or disturbances along the fiber optic cable with meter-level precision. It facilitates the precise identification of problems, even within vast and intricate infrastructure networks. Events can be visually represented in relation to the asset using AP Sensing’s SmartVision software.

AP Sensing’s Distributed Acoustic Sensing (DAS) provides remarkable location precision within a meter’s range, even across extensive distances of up to 100 km. This capability enables the system to accurately identify and locate events or disturbances along the fiber optic cable with meter-level precision. It facilitates the precise identification of problems, even within vast and intricate infrastructure networks. Events can be visually represented in relation to the asset using AP Sensing’s SmartVision software.

AP Sensing’s Distributed Acoustic Sensing (DAS) provides remarkable location precision within a meter’s range, even across extensive distances of up to 100 km. This capability enables the system to accurately identify and locate events or disturbances along the fiber optic cable with meter-level precision. It facilitates the precise identification of problems, even within vast and intricate infrastructure networks. Events can be visually represented in relation to the asset using AP Sensing’s SmartVision software.

DAS systems can be overseen from a central control center, facilitating real-time data analysis, event identification, and reaction. This capability for remote monitoring allows for swift decision-making and responses to critical incidents, improving overall operational efficiency and safety..

DAS systems can be overseen from a central control center, facilitating real-time data analysis, event identification, and reaction. This capability for remote monitoring allows for swift decision-making and responses to critical incidents, improving overall operational efficiency and safety.

Fiber optic cables utilized in C-OTDR (DAS) systems are designed to withstand tough environmental conditions typical in outdoor or industrial environments. The system from AP Sensing ensures optimal reliability, even in extreme situations such as exposure to dirt, dust, corrosive chemicals, high humidity, drastic temperature fluctuations, solvent vapors, radioactivity, and explosive atmospheres caused by gas or dust (ATEX/IECEx), as well as electromagnetic interference.

Fiber optic cables are naturally resistant to electromagnetic interference (EMI), which enhances safety in settings where electrical equipment could ignite or explode. Moreover, since DAS systems can be installed over extensive distances without needing numerous access points, this minimizes the necessity for personnel to enter dangerous areas.

Use Cases of DAS systems

Distributed Acoustic Sensing technology is employed for the supervision of infrastructure and the identification and localization of events, which include:

  • Third party interference/intrusion (TPI) activities affecting power cables (onshore and offshore), pipelines, railways, and perimeter security
  • Power cable condition monitoring and network optimization
  • Power cable faults, bottlenecks, or abnormalities
  • Power cable sheath current monitoring/fiber-based current monitoring
  • Pipeline leaks
  • Pipeline inspection gauge (PIG) tracking
  • Well & reservoir monitoring
  • Seismic profiling 
  • Large-scale structural health monitoring
  • Railway monitoring

Additionally, DAS is under constant evolution, with new applications including:

  • Earthquake monitoring
  • Landslide-monitoring as a part of early-warning system
  • Integration into smart city infrastructure.
  • Mineral resource exploration
  • Further applications in the renewable energy sector

     

Benefits & Advantages of Distributed Acoustic Sensing

Distributed Acoustic Sensing (DAS) offers many advantages for monitoring critical infrastructure including:

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Airway Security’s DAS System

DAS N51/N52-Series

Enhanced performance and measurement capabilities for protecting your valuable assets and infrastructure. Our fifth generation DAS features a world-leading measurement range for true phase-based systems without requiring additional amplification. It provides improved measurement performance with enhanced usability and reduced signal artifacts such as fading.

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How Does Distributed Acoustic Sensing ‘Measure Up’ Next to Other Sensing Technologies?

Conventional Strain and Acoustic Sensors

Traditional sensors like electrical strain gauges or hydrophones have been the basis for acoustic sensing for quite some time. However, larger assets often necessitate a more intricate setup, including power sources and signal transmission lines, unlike the straightforward passive glass fiber utilized in DAS technology.

Although point sensors like strain gauges provide excellent sensitivity at certain points, they cannot continuously monitor extensive infrastructure over kilometers in real-time. In contrast, DAS systems enable ongoing monitoring over long stretches without requiring multiple sensors.

Fiber optic point sensors, like fiber Bragg gratings (FBGs), are constrained by their predetermined sensing point positions along the fiber. Strain and vibrations can only be detected at these specified sensor sites where gratings must be embedded in the fiber, rendering other fiber areas unresponsive. Typically, the quantity of sensor points is quite limited. On the other hand, DAS systems, which provide distributed sensing capabilities, deliver significantly greater spatial resolution and coverage. This makes them more appropriate for monitoring applications that necessitate detailed spatial data.

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