Distributed Acoustic Sensing
Updated: Nov 17, 2020
Distributed acoustic sensing (DAS) is a fiber-incorporated technology that uses coherent Rayleigh backscattering in an optical fiber. It is an advanced sensing technology that continuously monitors the acoustic data and signals along the entire length of an optic cable. It can monitor multiple dynamic events in real-time along the entire path of a fiber optic cable. DAS uses photonic sensing technology, which converts communications fiber into discrete vibration sensors. This technology is utilized in various places that generate distinct acoustic characteristics such as hot-tapping pipelines, perimeter intrusions, moving vehicles, industrial operations, failing mechanical components, etc. It also helps in detecting leaks and external interferences.
They are also being used for monitoring and hydraulic-fracture profiling of shale gas and tight-sand reservoirs. Shale gas is natural gas that is trapped in fine-grained sedimentary rocks, which are known to be a rich source of natural gas. Tight sand reservoirs generally produce dry natural gas. This DAS-based technology accurately monitors the in-well activities during the fracture-stimulation treatment, which is one reason it continues to maintain a rapidly growing share of the fiber-optic sensor market.
An acoustic field interacts with the backscattered light (Source)
Principle of Operation
Fiber optic detectors have been around for a long time, but they are typically quasi- distributed, so individual detectors are mostly isolated. Quasi- distributed implies that multiple sensors are incorporated along the multiple sensing regions. In a single-point sensing scheme, there is only one sensing element across the entire fiber; in a distributed sensing scheme, the entire fiber is the continuous sensing element.
Multiple sensing regions must be created along the optical fiber line to achieve a quasi-distributed sensing scheme. When light pulses enter the sensors, some pulses are reflected, which causes the digital information to arrive discretely at different detectors. Inhomogeneities in the optical fiber cable cause to light scatter, which is known as Rayleigh Scattering (explained later). Each of these inhomogeneities scatters light back to the source, where the light is captured. Whenever there is a disturbance in the fiber, the distance between the inhomogeneities varies slightly, causing a change in optical path length, and this change can be detected with an interrogator unit.
The intelligent distributed acoustic sensor (iDAS) helps digitally record the acoustic signals at every position along a continuous length of optical fiber. Fig. 1 shows the iDAS operation principle, where an acoustic field interacts with the backscattered light generated along a continuous length of optical fiber. By analyzing the backscattered light and measuring the time between the laser pulse being launched and the signal being received, the iDAS can measure the acoustic signal at all points along with the fiber with lengths extending into tens of kilometers. In short, DAS technology looks over these vibrations, and if DAS is supported by the appropriate software, it precisely detects, classifies, and reports on the vibration events.
Rayleigh scattering is a dispersion of electromagnetic radiation by particles with radiuses of approximately one-tenth of the radiation's wavelength. It is a linear scattering of light by particles that are much smaller than the light's wavelength, and only a small portion of the scattered light is reflected back such that the light is again guided in the fiber core.
Rayleigh Backscattering (Source)
Advantages and Applications
Out of the numerous advantages of this technology, a few important ones are listed below:
There is no electrical energy required in the cable
It has a straightforward design
One fiber optic cable can become a 50 KM long contiguous sensor
It enables monitoring on a truly distributed basis
It is immune to the radiations such as EMI (Electromagnetic Induction) & ESD (Electrostatic Discharge)
The cable is compatible with harsh environments (dust, temperature, harmful gases).
Distributed acoustic sensing is extremely important when remote monitoring is required. It has various other applications such as:
Process temperature monitoring
Pipe and tank monitoring
Monitoring failing mechanical components
Power cable temperature monitoring
Ground movement detection
Early detection of overheating
Increasing energy demand, the ability to function in harsh conditions, enhancing oil & gas operations, and the surge in the reliability of distributed fiber optic sensing equipment are some factors responsible for the market's high growth.
DAS has been used in the border and security industry, owing to real-time intrusion detection and monitoring. DAS is heavily used in the oilfield services for in-well monitoring and other applications.
Top players in this market have the largest number of patents. Companies such as Halliburton Energy Services, Baker Hughes, General Electric, IBM, Schlumberger Technology, Robert Bosch, and Samsung Electronics all have secured patents with Halliburton Energy Services having the most at 423 patent families.
Top 10 Market Players
DAS is used in various industries such as oil and gas, power and utility, security and surveillance, environment and infrastructure sectors, the transportation sector, and others. The top three industries that are utilizing distributed acoustic sensing technologies are listed below.
Oil and Gas
A distributed acoustic sensing system enables its user to monitor the acoustic field at every point along the fiber optic cable, several kilometers in length. The fiber optic cable is placed in a well, where the system monitors its condition and registers production-relevant events.
Power and Utility
Distributed acoustic sensing systems can detect intrusion and encroachment of overhead power lines, reducing potential thefts and costs. Power line theft is a significant problem worldwide, costing the industry billions of dollars every year. Distributed acoustic sensing technology identifies the exact point of the disturbance or location being tampered with and passes the tampering information onto security systems for immediate action.
Security and Surveillance
Distributed acoustic sensors can be used in modern surveillance technology, for example, when it is deployed in international border areas or other regions of high concern. The device sends optical signals and receives it back with changes introduced by the suspicious movement's intrusion signals. These signals are then processed, and an alarm is generated, alerting the concerned authorities.
Industrial Importance of DAS
Today, the US leads in terms of the total count of patent families for DAS. The other jurisdictions, such as CN, EP, and JP, also own significant numbers of patent families, which is seen in the graphs depicted below.
Top 10 Markets in Distributed Acoustic Sensing
The Future of DAS Applications
DAS is a relatively reliable sensor technology for the railway sector, and many innovative applications are possible. One of its unique features is that it can support multiple applications using a single sensor. We can monitor the condition of the track and the rolling stock, as well as monitor the train and detect obstacles simultaneously, which may open the door for autonomous driving in the future. DAS uses fiber optic cables along the track to sense acoustic signals produced by noise and vibrations. DAS behaves as a massive set of microphones along the railway track, which catches a train's sound, covering a length of 40 kilometers with a single sensor.
This technology is growing at a precipitous pace, covering a wide range of applications in the oil and gas sector, power and utility, security, and surveillance, and many more. With no need for electrical power in the cable, its simple design, and the ability to work in a harsh environment, DAS is likely to gain the market quickly. Its operation has become necessary for remote monitoring such as pipe and tank monitoring (for detection of leakage), power cable temperature monitoring, and industrial operations.
Associate at Lumenci
Kumar Harshavardhana is an Associate at Lumenci. He holds a Bachelor's degree in Electrical and Electronics Engineering from NIT Jamshedpur. Harsh spends his off-work hours reading and writing technical pieces.