DFS Sensor and System
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DFS is the short form for distributed fibre sensing (or distributed fibre optics sensor). It is the ultra long sensing distance type of fiber optic sensors. Most of the time ordinary optical fiber (single mode fiber) may be used to sense for heat changes in existing infrastructure such as smart nation applications. Distributed Temperature Sensing (DTS) systems are the most commercial available DFS system in the market today. Their typical sensing spatial resolution is 1 meter with 1 degree Celsius accuracy in very ideal conditions. Wealthy end user such as those in oil and gas industry use Distributed Acoustic Sensing (DAS) system for acoustic sensing. In practical use cases, DAS are greatly affected by environment (wind, thunder, etc.) and human activities (machinery alike disturbance, false intrusion, etc.) if present.
Sensing Principle and Information
Distributed Fibre Sensing (DFS) is an intrinsic sensor that is able to determine parameter(s) at every point along the length of the optical fiber.
Three Light Scattering Processes
- Raman - Intensity is intrinsic dependence on the temperature of the optical fiber
- Brillouin - Frequency is intrinsic dependence on the density of the optical fiber
- Rayleigh (elastic) - intrinsically independent of almost any external physical field for a wide range of conditions
- Propagation effects (negligible in Raman and Brillouin) include attenuation, phase interference and polarization variation
The processes in general
- Laser pulse is send into the optical fiber and the imperfection in optical fiber backscatters small portion of the light back where its phase is measured
- Small variation as a function of time infer information on the surrounding environment along the optical fiber
- The sensing range and pitch is limited by the optical components used to control the pulse and phase detection
Distributed Sensor Type
- Highly similar to Fiber Bragg Grating (FBG) sensors in terms of sensing principle, although one uses time or frequency domain and other other as a function of wavelength
- Some literature differentiates DFS as fully distributed sensor and FBG as quasi distributed (multiplexed) sensor
- Measurand resolution
- Spatial resolution
- Sensing range
- Time require to achieve resolution
Type of Sensing
- Vibration/ intrusion
Distributed acoustic sensing is a type of OTDR.
Advantages of DAS System
- Single strand of optical fiber covering typically up to 50 Km
- Proven track record for pipeline due to wealthy end user / customers in the oil and gas industry
- Deployed for large scale oil pipeline and seismic monitoring
- Residential security and small scale monitoring are very limited due to its high overall life cycle cost
- Inherent properties of the robust performance optical fiber sensor technologies
- Temperature resistant, for example up to 700 °C +
Disadvantages of DAS System
- In practical cases, lower spatial resolution (larger pitch or sensing zones) to cover longer total distance,
- In other words, lower accuracy for long distance sensing
- Signal to Noise Ratio (SNR) deteriorates when the range between disturbance and localised sensor increases in pipelines, moreover lower cost internal components tend to have high instrumental noise than sensed signal
- Large overall signal attenuation along the length of the optical fiber
- Lack of directionality.
- More sensitive in the axial direction compared to radial direction
- Requires additional optical fiber or complex layout which is not commercially practical
- Interrogator maintenance required highly trained and skilled workers and spare parts for component repairs are costly
- High power laser (NEA Class 3b/ Class 4) typically used pose health issue when optical fiber broken or interrogator malfunction
- Poor fiber–medium coupling
- Need for large-scale data processing for real-time applications
- Large cooling system requires to cool the optical components inside the DAS interrogator
- Internal optical components have smaller supply chain compared to FBG sensing system
Typical Sensing Capabilities ^
- Sensing distance from 0 meter to 50,000 meters
- Small class: 10 Km
- Medium class: 20 Km
- Large class: 50Km
- Sensing pitch (spatial resolution) 10 meters, and 1 meter (on conditions)
^ Reference with Optasense OLA2.1 and ODH-4
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- Maria-Teresa Hussels, Sebastian Chruscicki, Detlef Arndt, Swen Scheider, Jens Prager, Tobias Homann and Abdel Karim Habib, " Localization of Transient Events Threatening Pipeline Integrity by Fiber-Optic Distributed Acoustic Sensing," Sensors, vol. 19, no. 15, pp. , Jul. 2019. DOI: 10.3390/s19153322
- Abstract: Pipe integrity is a central concern regarding technical safety, availability, and environmental compliance of industrial plants and pipelines. A condition monitoring system that detects and localizes threats in pipes prior to occurrence of actual structural failure, e.g., leakages, especially needs to target transient events such as impacts on the pipe wall or pressure waves travelling through the medium. In the present work, it is shown that fiber-optic distributed acoustic sensing (DAS) in conjunction with a suitable application geometry of the optical fiber sensor allows to track propagating acoustic waves in the pipeline wall on a fast time-scale. Therefore, short impacts on the pipe may be localized with high fidelity. Moreover, different acoustic modes are identified, and their respective group velocities are in good agreement with theoretical predications. In another set of experiments modeling realistic damage scenarios, we demonstrate that pressure waves following explosions of different gas mixtures in pipes can be observed. Velocities are verified by local piezoelectric pressure transducers. Due to the fully distributed nature of the fiber-optic sensing system, it is possible to record accelerated motions in detail. Therefore, in addition to detection and localization of threatening events for infrastructure monitoring, DAS may provide a powerful tool to study the development of gas explosions in pipes, e.g., investigation of deflagration-to-detonation-transitions (DDT)
- Robert J. Mellors, Christopher Sherman, Pengcheng Fu, John McLennan, Joseph Morris, Frederick Ryerson, and Christina Morency, " Potential Use of Distributed Acoustic Sensors to Monitor Fractures and Microseismicity at the FORGE EGS site," in Proc. of 44th Workshop on Geothermal Reservoir Engineering, SGP-TR-214, pp. 1-6, 2019
- Abstract: Distributed fiber optic acoustic sensors (DAS) installed in boreholes have provided a new and data-rich perspective on fracturing processes and microseismicity created by stimulation. The use of fiber optic sensors is increasing in the oil and gas industry but less so at geothermal sites. These sensors measure strain (or strain rate) with high spatial resolution (~ 1 m) along the fiber and can survive extreme conditions. Here, we explore the information that these sensors may reveal in an Enhanced Geothermal System (EGS) system, which includes both low-frequency signals associated with fracture opening and high-frequency microseismic signals. As a test case, we use parameters from the FORGE site in Milford, Utah, which is expected to create a reservoir at a depth of roughly 2 km in a crystalline granite formation with temperatures of more than 175°C. The subsurface signals are simulated in two ways: 1) a massively parallel multi-physics code that is capable of modeling hydraulic stimulation of a reservoir with a pre-existing discrete fracture network, and 2) a parallelized seismic wave propagation code for high-frequency seismic signals created by microseismic activity. The objective is to understand how fracture geometry could be constrained with fiber optic sensors located both in the main borehole and in nearby monitoring boreholes, and how well microseismic events could be characterized in terms of locations and moment tensors.
- Khalid Miah, and David K. Potter, " A Review of Hybrid Fiber-Optic Distributed Simultaneous Vibration and Temperature Sensing Technology and Its Geophysical Applications," Sensors, vol. 2018, no. 3897873, pp. 1-16 , May 2018 . DOI: 10.3390/s17112511
- Abstract: Distributed sensing systems can transform an optical fiber cable into an array of sensors, allowing users to detect and monitor multiple physical parameters such as temperature, vibration and strain with fine spatial and temporal resolution over a long distance. Fiber-optic distributed acoustic sensing (DAS) and distributed temperature sensing (DTS) systems have been developed for various applications with varied spatial resolution, and spectral and sensing range. Rayleigh scattering-based phase optical time domain reflectometry (OTDR) for vibration and Raman/Brillouin scattering-based OTDR for temperature and strain measurements have been developed over the past two decades. The key challenge has been to find a methodology that would enable the physical parameters to be determined at any point along the sensing fiber with high sensitivity and spatial resolution, yet within acceptable frequency range for dynamic vibration, and temperature detection. There are many applications, especially in geophysical and mining engineering where simultaneous measurements of vibration and temperature are essential. In this article, recent developments of different hybrid systems for simultaneous vibration, temperature and strain measurements are analyzed based on their operation principles and performance. Then, challenges and limitations of the systems are highlighted for geophysical applications.
- Luca Palmieri, and Luca Schenato, "Distributed Optical Fiber Sensing Based on Rayleigh Scattering, " The Open Optics Journal, vol. 7, pp. 104-127, Dec. 2013. DOI: 10.2174/1874328501307010104 [ResearchGate]
- .n.d. (n.d.). Non-Ionising Radiation. Retrieved Apr. 04, 2020 from National Environment Agency
Tags: #DFS #OTDR #OFDR #PSOTDR #POTDR #sensor #i2r #singapore #limjunlong