Distributed acoustic sensing

Last updated

Rayleigh scattering-based distributed acoustic sensing (DAS) systems use fiber optic cables to provide distributed strain sensing. In DAS, the optical fiber cable becomes the sensing element and measurements are made, and in part processed, using an attached optoelectronic device. Such a system allows acoustic frequency strain signals to be detected over large distances and in harsh environments.

Contents

Fundamentals of Rayleigh scatter-based fiber optic sensing

In Rayleigh scatter-based distributed fiber optic sensing, a coherent laser pulse is sent along an optic fiber, and scattering sites within the fiber cause the fiber to act as a distributed interferometer with a gauge length approximately equal to the pulse length. The intensity of the reflected light is measured as a function of time after transmission of the laser pulse. This is known as Coherent Rayleigh Optical Time Domain Reflectometry (COTDR). When the pulse has had time to travel the full length of the fiber and back, the next laser pulse can be sent along the fiber. Changes in the reflected intensity of successive pulses from the same region of fiber are caused by changes in the optical path length of that section of fiber. This type of system is very sensitive to both strain and temperature variations of the fiber and measurements can be made almost simultaneously at all sections of the fiber.

Capabilities of Rayleigh-based systems

Maximum range

The optical pulse is attenuated as it propagates along the fiber. For a single mode fiber operating at 1550 nm, a typical attenuation is 0.2 dB/km. [1] Since the light must make a double pass along each section of fiber, this means each 1 km causes a total loss of 0.4 dB. The maximum range of the system occurs when the amplitude of the reflected pulse becomes so low it is impossible to obtain a clear signal from it. It is not possible to counteract this effect by increasing the input power because above a certain level this will induce nonlinear optical effects which will disrupt the operation of the system. Typically the maximum range that can be measured is around 40–50 km.

Strain resolution

The maximum value of strain that can be measured depends on the carrier to noise ratio of the returning optical signal. The carrier level is largely determined by the amplitude of the optical signal while the noise is a combination of that from a variety of sources including laser noise, electronic noise and detector noise.

Spatial resolution and spatial sampling period

The spatial resolution is mainly determined by the duration of the transmitted pulse, with a 100 ns pulse giving 10 m resolution being a typical value. The amount of reflected light is proportional to the pulse length so there is a trade-off between spatial resolution and maximum range. To improve the maximum range, it would be desirable to use a longer pulse length to increase the reflected light level but this leads to a smaller spatial resolution. In order for two signals to be independent, they must be obtained from two points on the fiber that are separated by at least the spatial resolution. It is possible to obtain samples at separations less than the spatial resolution and although this produces signals that are not independent of each other, such an approach does offer advantages in some applications. The separation between the sampling points is sometimes referred to as the spatial sampling period.

Acquisition rate

Before the next laser pulse can be transmitted the previous one must have had time to travel to the far end of the fiber and for the reflections from there to return, otherwise reflections would be returning from different sections of the fiber at the same time and the system would not operate properly. For a fiber 50 km long the maximum pulse rate is just over 2 kHz. Therefore, strains can be measured which vary at frequencies up to the Nyquist frequency of 1 kHz. Shorter fibers clearly enable higher acquisition rates.

Temperature measurements

Although the system is sensitive to both temperature and strain variations these can often be separated as those due to temperature tend to occur at a lower frequency range than strain. Unlike other distributed fiber techniques such as those based on Brillouin or Raman scatter, distributed acoustic sensing is only able to detect changes in temperature rather than its absolute value.

Comparison with other fiber optic distributed sensing techniques

Distributed acoustic sensing relies on light which is Rayleigh backscattered from small variations in the refractive index of the fiber. The backscattered light has the same frequency as the transmitted light. There are a number of other distributed fiber sensing techniques that rely on different scattering mechanisms and can be used to measure other parameters. Brillouin scatter occurs due to the interaction between the light and acoustic phonons travelling in the fiber. As the light is scattered by a moving phonon, its frequency is shifted by the Doppler effect by around 10 GHz. Light is generated at both above (anti-Stokes shift) and below (Stokes shift) the original optical frequency. The intensity and frequency shifts of the two components are dependent on both temperature and strain and by measuring the shifts, absolute values of the two parameters can be calculated using a distributed temperature and strain sensing (DTSS) system. Brillouin scatter is much weaker than Rayleigh scatter and so the reflections from a number of pulses must be summed together to enable the measurements to be made. Therefore, the maximum frequency at which changes can be measured using Brillouin scatter is typically a few tens of Hz. Raman scatter occurs when light is scattered in interaction with molecular vibrations in the fiber. As with Brillouin scattering, both Stokes and anti-Stokes components are produced and these are shifted from the wavelength of the incident light by several tens of nanometers. By measuring the ratio in intensity between the Stokes and anti-Stokes components, an absolute value of temperature can be measured by a distributed temperature sensing (DTS) system. The larger wavelength shifts compared to Brillouin scatter mean that it is easier to separate the scattered Raman light from the un-shifted Rayleigh scattered component. However the intensity of the Raman scatter is lower than the Brillouin scatter and so it is normally necessary to average for many seconds or minutes in order to get reasonable results. Therefore, Raman-based systems are only suitable for measuring slowly varying temperatures.

Phase-sensitive coherent optical time-domain reflectometry

Phase-sensitive coherent optical time-domain reflectometry (ϕ-OTDR) is a technique that can provide sufficient sensitivity and resolution for these distributed acoustic sensing systems. [2] Standard optical time-domain reflectometry techniques use light sources with coherence lengths, which are shorter than pulse lengths. This can yield a sum of backscattered intensities from each scattering center, which allows monitoring splices and breaks in fiber optic cables. On the contrary, in ϕ-OTDR-based sensors, the coherence length of lasers is longer than their pulse length. An event near the fiber generates an acoustic wave that affects the optical fiber by changing the phases of the backscattering centers. An analysis of such signals can reveal their impact on the sensor and monitor acoustic sources located near fiber objects.

Applications

The sensitivity and speed of Rayleigh-based sensing allows distributed monitoring of acoustic signals over distances of more than 100 km [3] from each laser source. Typical applications include continuous monitoring of pipelines for unwanted interference and for leaks or flow irregularities; monitoring of power cables for unwanted interference and cable faults; [4] [5] monitoring traffic (roads, railways [6] [7] and trains [8] ), borders, and other sensitive perimeters [9] for unusual activity; and even oil well monitoring applications where the technology allows the state of the well all along its length to be determined in real-time. The ability of the optic fiber to operate in harsh environments makes the technology especially well suited for scenarios in which typical sensing systems are unusable or impractical due to environmental conditions. [10] The long range of the technology also allows its use in seismic sensing. One cable can provide a continuous line of regional seismic activity monitoring, and also detect earthquakes thousands of kilometers away. [11] The use of distributed acoustic sensing has also been demonstrated to monitor hydraulic stimulation in enhanced geothermal systems (EGS) [12] as well as the injection of carbon dioxide in several carbon capture and storage (CCS) projects. [13]

See also

Related Research Articles

<span class="mw-page-title-main">Optical amplifier</span> Device that amplifies an optical signal

An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. An optical amplifier may be thought of as a laser without an optical cavity, or one in which feedback from the cavity is suppressed. Optical amplifiers are important in optical communication and laser physics. They are used as optical repeaters in the long distance fiber-optic cables which carry much of the world's telecommunication links.

<span class="mw-page-title-main">Raman spectroscopy</span> Spectroscopic technique

Raman spectroscopy is a spectroscopic technique typically used to determine vibrational modes of molecules, although rotational and other low-frequency modes of systems may also be observed. Raman spectroscopy is commonly used in chemistry to provide a structural fingerprint by which molecules can be identified.

<span class="mw-page-title-main">Photonics</span> Technical applications of optics

Photonics is a branch of optics that involves the application of generation, detection, and manipulation of light in form of photons through emission, transmission, modulation, signal processing, switching, amplification, and sensing. Photonics is closely related to quantum electronics, where quantum electronics deals with the theoretical part of it while photonics deal with its engineering applications. Though covering all light's technical applications over the whole spectrum, most photonic applications are in the range of visible and near-infrared light. The term photonics developed as an outgrowth of the first practical semiconductor light emitters invented in the early 1960s and optical fibers developed in the 1970s.

Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light. Because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties.

In electromagnetism, Brillouin scattering, named after Léon Brillouin, refers to the interaction of light with the material waves in a medium. It is mediated by the refractive index dependence on the material properties of the medium; as described in optics, the index of refraction of a transparent material changes under deformation.

<span class="mw-page-title-main">Raman scattering</span> Inelastic scattering of photons by matter

In physics, Raman scattering or the Raman effect is the inelastic scattering of photons by matter, meaning that there is both an exchange of energy and a change in the light's direction. Typically this effect involves vibrational energy being gained by a molecule as incident photons from a visible laser are shifted to lower energy. This is called normal Stokes-Raman scattering.

<span class="mw-page-title-main">Backscatter</span> Reflection which reverses the direction of a wave, particle, or signal

In physics, backscatter is the reflection of waves, particles, or signals back to the direction from which they came. It is usually a diffuse reflection due to scattering, as opposed to specular reflection as from a mirror, although specular backscattering can occur at normal incidence with a surface. Backscattering has important applications in astronomy, photography, and medical ultrasonography. The opposite effect is forward scatter, e.g. when a translucent material like a cloud diffuses sunlight, giving soft light.

<span class="mw-page-title-main">Fiber Bragg grating</span> Type of distributed Bragg reflector constructed in a short segment of optical fiber

A fiber Bragg grating (FBG) is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. This is achieved by creating a periodic variation in the refractive index of the fiber core, which generates a wavelength-specific dielectric mirror. Hence a fiber Bragg grating can be used as an inline optical filter to block certain wavelengths, can be used for sensing applications, or it can be used as wavelength-specific reflector.

<span class="mw-page-title-main">Optical fiber</span> Light-conducting fiber

An optical fiber, or optical fibre, is a flexible glass or plastic fiber that can transmit light from one end to the other. Such fibers find wide usage in fiber-optic communications, where they permit transmission over longer distances and at higher bandwidths than electrical cables. Fibers are used instead of metal wires because signals travel along them with less loss and are immune to electromagnetic interference. Fibers are also used for illumination and imaging, and are often wrapped in bundles so they may be used to carry light into, or images out of confined spaces, as in the case of a fiberscope. Specially designed fibers are also used for a variety of other applications, such as fiber optic sensors and fiber lasers.

Distributed temperature sensing systems (DTS) are optoelectronic devices which measure temperatures by means of optical fibres functioning as linear sensors. Temperatures are recorded along the optical sensor cable, thus not at points, but as a continuous profile. A high accuracy of temperature determination is achieved over great distances. Typically the DTS systems can locate the temperature to a spatial resolution of 1 m with accuracy to within ±1 °C at a resolution of 0.01 °C. Measurement distances of greater than 30 km can be monitored and some specialised systems can provide even tighter spatial resolutions. Thermal changes along the optical fibre cause a local variation in the refractive index, which in turn leads to the inelastic scattering of the light propagating through it. Heat is held in the form of molecular or lattice vibrations in the material. Molecular vibrations at high frequencies (10 THz) are responsible for Raman scattering. Low frequency vibrations (10–30 GHz) cause Brillouin scattering. Energy is exchanged between the light travelling through the fibre and the material itself and cause a frequency shift in the incident light. This frequency shift can then be used to measure temperature changes along the fibre.

<span class="mw-page-title-main">Silicon photonics</span> Photonic systems which use silicon as an optical medium

Silicon photonics is the study and application of photonic systems which use silicon as an optical medium. The silicon is usually patterned with sub-micrometre precision, into microphotonic components. These operate in the infrared, most commonly at the 1.55 micrometre wavelength used by most fiber optic telecommunication systems. The silicon typically lies on top of a layer of silica in what is known as silicon on insulator (SOI).

<span class="mw-page-title-main">Fiber-optic communication</span> Transmitting information over optical fiber

Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of infrared or visible light through an optical fiber. The light is a form of carrier wave that is modulated to carry information. Fiber is preferred over electrical cabling when high bandwidth, long distance, or immunity to electromagnetic interference is required. This type of communication can transmit voice, video, and telemetry through local area networks or across long distances.

<span class="mw-page-title-main">Acousto-optics</span> The study of sound and light interaction

Acousto-optics is a branch of physics that studies the interactions between sound waves and light waves, especially the diffraction of laser light by ultrasound through an ultrasonic grating.

A fiber-optic sensor is a sensor that uses optical fiber either as the sensing element, or as a means of relaying signals from a remote sensor to the electronics that process the signals. Fibers have many uses in remote sensing. Depending on the application, fiber may be used because of its small size, or because no electrical power is needed at the remote location, or because many sensors can be multiplexed along the length of a fiber by using light wavelength shift for each sensor, or by sensing the time delay as light passes along the fiber through each sensor. Time delay can be determined using a device such as an optical time-domain reflectometer and wavelength shift can be calculated using an instrument implementing optical frequency domain reflectometry.

Pipeline leak detection is used to determine if a leak has occurred in systems which contain liquids and gases. Methods of detection include hydrostatic testing, tracer-gas leak testing, infrared, laser technology, and acoustic or sonar technologies. Some technologies are used only during initial pipeline installation and commissioning, while other technologies can be used for continuous monitoring during service.

Brillouin spectroscopy is an empirical spectroscopy technique which allows the determination of elastic moduli of materials. The technique uses inelastic scattering of light when it encounters acoustic phonons in a crystal, a process known as Brillouin scattering, to determine phonon energies and therefore interatomic potentials of a material. The scattering occurs when an electromagnetic wave interacts with a density wave, photon-phonon scattering.

Atmospheric lidar is a class of instruments that uses laser light to study atmospheric properties from the ground up to the top of the atmosphere. Such instruments have been used to study, among other, atmospheric gases, aerosols, clouds, and temperature.

Xiaoyi Bao is a Chinese Canadian physicist, recognized for her contributions to the field of fiber optics. She is a professor at the University of Ottawa, where she holds the Canada Research Chair in Fiber Optics and Photonics. Bao was awarded an honorary doctorate from the University of Lethbridge; the citation for the honour called her "a world-renowned scholar in her field."

<span class="mw-page-title-main">Tapered double-clad fiber</span>

A tapered double-clad fiber (T-DCF) is a double-clad optical fiber which is formed using a specialised fiber drawing process, in which temperature and pulling forces are controlled to form a taper along the length of the fiber. By using pre-clad fiber preforms both the fiber core and the inner and outer cladding layers vary in diameter and thickness along the full length of the fiber. This tapering of the fiber enables the combination of the characteristics of conventional 8–10 μm diameter double-clad single-mode fibers to propagate light in fundamental mode with those of larger diameter (50–100 μm) double-clad multi-mode fibers used for optical amplification and lasing. The result is improved maintenance of pulse fidelity compared to conventional consistent diameter fiber amplifiers. By virtue of the large cladding diameter T-DCF can be pumped by optical sources with very poor brightness factor such as laser diode bars or even VECSELs matrices, significantly reducing the cost of fiber lasers/amplifiers.

<span class="mw-page-title-main">Luc Thévenaz</span> Swiss physicist who specializes in fibre optics

Luc Thévenaz is a Swiss physicist who specializes in fibre optics. He is a professor of physics at EPFL and the head of the Group for Fibre Optics School of Engineering.

References

  1. ↑ "Fiber Types > Fiber-Optic Technologies".
  2. ↑ Henry F. Taylor, Chung E. Lee (March 16, 1993). "United States Patent: 5194847 - Apparatus and method for fiber optic intrusion sensing". Archived from the original on December 8, 2016. Retrieved 2016-05-06.
  3. ↑ Gregor Cedilnik; et al. (2018). "Pushing the Reach of Fiber Distributed Acoustic Sensing to 125 km Without the Use of Amplification". IEEE Sensors Letters. 3 (3): 1–4. doi:10.1109/LSENS.2019.2895249. S2CID   71149063.
  4. ↑ Gregor Cedilnik; et al. (2019). "Ultra-Long Reach Fiber Distributed Acoustic Sensing (DAS) for Power Cable Monitoring" (PDF).
  5. ↑ Rasmus Olson; et al. (2019). "Fault Localisation with Distributed Acoustic Sensing (DAS)" (PDF).
  6. ↑ "Fibre Optic Sensing for railways – Ready to use?!" (PDF). Signalling+Datacommunication / Signal+Draht. 114: 60. September 2022.
  7. ↑ Wagner, Adrian; Nash, Andrew; Michelberger, Frank; Grossberger, Hirut; Lancaster, Gavin (January 2023). "The Effectiveness of Distributed Acoustic Sensing (DAS) for Broken Rail Detection". Energies. 16 (1): 522. doi: 10.3390/en16010522 . ISSN   1996-1073.
  8. ↑ Gregor Cedilnik; et al. (2018). "Advances in Train and Rail Monitoring with DAS". 26th International Conference on Optical Fiber Sensors. pp. ThE35. doi:10.1364/OFS.2018.ThE35. ISBN   978-1-943580-50-7.
  9. ↑ Wieland Hill, John J. Williams, and Gareth Lees (2019). "Fiber-optic Sensing: Smart fiber-optic sensing systems enhance physical border walls and fences".{{cite web}}: CS1 maint: multiple names: authors list (link)
  10. ↑ Mestayer et al, Field trials of distributed acoustic sensing for geophysical monitoring, SEG Expanded Abstracts 30, 4253 (2011)
  11. ↑ Cartier, Kimberly (2019). "Unused Fiber-Optic Cables Repurposed as Seismic Sensors". Eos. 100. doi: 10.1029/2019EO118025 .
  12. ↑ Richter, Peter; Parker, Tom; Woerpel, Craig; Wu, Wenxia; Rufino, Rogelio; Farhadiroushan, Mahmoud (2019-08-10). "High-resolution distributed acoustic sensor using engineered fiber for hydraulic fracture monitoring and optimization in unconventional completions". SEG Technical Program Expanded Abstracts 2019. San Antonio, Texas: Society of Exploration Geophysicists: 4874–4878. doi:10.1190/segam2019-3215860.1. S2CID   202182897.
  13. ↑ Correa, Julia; Pevzner, Roman; Freifeld, Barry M.; Robertson, Michelle; Daley, Thomas M.; Wood, Todd; Tertyshnikov, Konstantin; Yavuz, Sinem; Glubokovskikh, Stanislav (2021-12-30), Li, Yingping; Karrenbach, Martin; Ajo-Franklin, Jonathan B. (eds.), "Continuous Downhole Seismic Monitoring Using Surface Orbital Vibrators and Distributed Acoustic Sensing at the CO2CRC Otway Project: Field Trial for Optimum Configuration", Geophysical Monograph Series (1 ed.), Wiley, pp. 175–189, doi:10.1002/9781119521808.ch13, ISBN   978-1-119-52179-2, S2CID   245049871 , retrieved 2022-10-29
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.