Distributed Fiber Optic Acoustic Sensing System DAS

Fiber optic temperature sensor, Intelligent monitoring system, Distributed fiber optic manufacturer in China

Definition of Distributed Fiber Optic Acoustic Sensing

Distributed Acoustic Sensing System (DAS) is a fiber optic optoelectronic device that measures acoustic interactions along the length of the fiber optic sensing cable.

 

The unique feature of a distributed acoustic sensing system is that it provides a continuous (or distributed) temperature distribution along the length of the sensing cable, rather than at discrete sensing points.

Distributed acoustic sensing technology

Usually, DAS technology uses standard telecommunications fiber optic cables, and specialized fiber optic cables are only required at high temperatures (greater than 100 ° C). Sensing fibers are usually based on single-mode fibers, although some specialized applications use multi-mode sensing fibers.

 

The range of a DAS system is usually up to 50km per sensing fiber, and each inquiry unit usually has 1 or 2 channels that can be operated simultaneously. For example, DAS can measure up to 100km, and 2-channel units can measure 50km in any direction.

 

measuring principle

The distributed acoustic sensor interrogation unit transmits laser pulses to the fiber optic. When this type of light pulse propagates along the fiber, the interaction inside the fiber causes light reflection called backscattering, which is determined by small strain (or vibration) events inside the fiber, which are caused by local sound energy. This backscattered light propagates upwards along the fiber optic to the interrogation unit, where it is sampled at Rayleigh frequency. The time required for laser pulses allows for accurate mapping of backscatter events to the fiber distance – this is known as an optical time-domain reflectometer.

 

Most distributed acoustic sensing systems on the market today are based on a principle called Coherent Optical Time Domain Reflectometer (COTDR).

Spatial resolution and spatial sampling period

The spatial resolution is mainly determined by the duration of the emitted pulse, and a resolution of 10m given by a 100ns pulse is 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. In order to improve the maximum range, it is hoped to use longer pulse lengths to increase the level of reflected light, but this leads to greater spatial resolution. Typically, the spatial resolution of most systems is 5-10 meters.

 

Comparison between DAS and other fiber optic distributed sensing systems

There are many other distributed fiber optic sensing technologies that rely on different scattering mechanisms and can be used to measure other parameters.

 

Brillouin based systems are commonly used to measure distributed strain and temperature.
Brillouin scattering is much weaker than Rayleigh scattering, so reflections from multiple pulses must be added together to enable measurement. Therefore, the maximum frequency for measuring changes using Brillouin scattering is usually several tens of Hz, while Rayleigh based COTDR DAS systems have kHz sensitivity.

Raman based systems are commonly used for temperature measurement, while DTS systems are typically based on Raman technology. The intensity of Raman scattering is even lower than that of Brillouin scattering, so it usually takes an average of many seconds or even a few minutes to obtain reasonable results. Therefore, Raman based systems are only suitable for measuring slowly changing temperatures.

 

Data collection, signal processing, and visualization

Due to the large amount of data generated by distributed acoustic sensing systems, it is crucial to have a strategy for management, processing, and data visualization. These systems collect data at speeds above 10 Khz at up to 20 sensing points. This is equivalent to the rate at which terabyte drives can be filled within a few days.

 

Usually, the inquiry unit is connected to the processing unit (industrial PC or server) that manages data storage and processing. Usually, there is a scrolling buffer used to store raw data because there is very little content stored beyond this.

 

The processing unit is programmed using a series of intelligent algorithms to interpret raw data and analyze whether it matches pre-defined events, such as intrusion events or pipeline leaks. The fiber optic sensing cable will be divided into multiple areas, where specific selected algorithms will be selected and alerts will be assigned within each area.

 

There are many ways to visualize these events. One approach is to use DTS specific visualization software, such as displaying the path of optical fibers based on site maps or charts, and if there are events, it will highlight the location of the events and display alarms. Another approach is for the DAS software interface to be integrated with existing SCADA, control, or security software packages. In this case, the event will highlight the software of the parties involved in 3.

DAS measurement principle:

 

Please add a link to describe that DAS is a distributed fiber optic sensor based on coherent Rayleigh scattering. It utilizes the sensitivity of optical fibers to sound (vibration). When external vibrations act on the sensing optical fiber, due to the elastic optical effect, the refractive index and length of the optical fiber will undergo slight changes, resulting in a phase change of the transmitted signal inside the optical fiber and a change in light intensity.

 

The phase change caused by sound waves is very small, so DAS systems usually use highly coherent pulse light sources. Interference occurs between Rayleigh scattering signals within the pulse width area. When external vibration causes a phase change, the intensity of the coherent Rayleigh scattering signal at that point will change. By detecting the intensity change of the Rayleigh scattering light signal before and after vibration (differential signal), vibration event detection can be achieved, and multiple vibration events can be accurately located simultaneously.

 

DAS technology advantages:

 

Continuous distributed measurement of temperature and vibration without measurement blind spots

 

Simultaneous detection and accurate localization of multiple events

 

Fiber optic is a sensor that combines transmission and sensing

 

60 kilometers of ultra long measurement distance, rich measurement information

 

Fast response speed, alarm within 1 second

 

Optical signal transmission, completely electrically insulated, resistant to electromagnetic interference

 

Intrinsic safety, suitable for long-term operation in flammable and explosive environments

 

Stable and reliable measurement with low false alarm rate

 

Long service life of optical fibers, up to 30 years maintenance free

 

DAS performance characteristics:

 

Long temperature distance: 50km

 

Fast response time: typical 1 second

 

High positioning accuracy: 2-50m

 

High sensitivity: can perceive vibrations within 40m around the optical cable

 

Simultaneous monitoring of vibration and temperature

 

Online monitoring function for fiber optic faults

Perceiving all things is an important technological support for building a smart earth, smart city, and smart ocean. Distributed Fiber Optic Acoustic Sensing (DAS) technology is a new type of sensing technology that can achieve continuous distributed detection of vibration and sound fields. It utilizes the highly sensitive characteristics of coherent Rayleigh scattering induced by narrow linewidth single frequency laser in optical fibers, combined with the principle of reflectometer, to perceive environmental vibration and sound field information interacting with optical fibers over long distances and with high spatiotemporal accuracy. This unique information perception ability has attracted widespread attention from both academia and industry for DAS technology. The performance of DAS technology is constantly improving, and its applications are developing rapidly. It has demonstrated its unique technological advantages and potential in perimeter intrusion detection, online monitoring of railway safety, geophysical exploration, and other areas.

Due to its unique advantages, DAS has attracted more and more experts from various fields to seek industry breakthroughs, while also placing increasing demands on the improvement of DAS technology.

After more than a decade of development, DAS has played an irreplaceable role in multiple fields, especially in the application scenarios of long-distance, large-scale, and spatiotemporal dense detection, including perimeter security, transportation, geophysical exploration, structural health monitoring, and other fields. Researchers are also continuously improving DAS technology to meet the personalized application needs of various fields.

In the field of perimeter security, compared to conventional methods, DAS has advantages such as strong environmental adaptability, high concealment, large monitoring range, and distributed blind spots. However, how to determine what kind of disturbance and intrusion occurred along the fiber optic cable based on the large number of complex signals detected by DAS is a technical challenge.
In the field of railway transportation, DAS technology uses passive optical fibers as sensing and transmission devices, which can achieve spatial continuous sensing of disturbance signals along the fiber optic line. It has the characteristics of anti electromagnetic interference, long-distance distributed measurement, low cost per unit distance, and no need for on-site power supply. It can effectively compensate for the shortcomings of existing point electromagnetic sensing technology, meet the application needs of railway transportation, and can be quickly integrated into existing railway lines. It has been widely applied.

 

Oil and gas resource exploration is also an important application of DAS technology. The conventional oil and gas resource exploration technology uses point type electronic detectors, which have drawbacks such as low deployment efficiency and long large-scale experimental time. DAS uses conventional communication optical fibers as sensor components, which are low-cost and can play a role throughout the entire life cycle of drilling, completion, production, etc., with significant advantages.
In addition, due to the small size and light weight of optical fibers, they are easy to embed into structures such as aerospace composite materials, building materials, soil media, etc. DAS can easily obtain acoustic emission signals inside the materials, achieving permanent online monitoring of materials and structures.

Future development trends and challenges

DAS technology has been continuously maturing, the application market is expanding, and the prospects are thriving. Recently, foreign scholars have proposed using existing underground communication optical fibers to build a large-scale monitoring network for geological analysis and major natural disaster (earthquake) detection. This development direction can tap into the advantages of DAS’s large-scale spatial continuous perception, reactivate all redundant communication fiber optic resources underground worldwide, and has very high market value and development potential.
Although DAS technology has made significant progress, it is not yet fully mature and there are still important technical bottlenecks that need to be addressed, mainly including sensitivity improvement, multi-dimensional detection, and new data processing paradigms.
The sensitivity of DAS technology is relatively high compared to distributed sensing technology. However, compared to conventional point sensing technology, there is still a significant gap. To apply DAS technology on a large scale, it is necessary to significantly improve the sensitivity of this technology, making it close to the level of existing point sensing devices, in order to truly replace existing technological means in various application fields.
At the same time, the existing detection capability of DAS is still limited by the one-dimensional axial structure of optical fibers, and it is difficult to achieve three-dimensional positioning of disturbance sources and multi-component detection of signals, which to some extent limits the technical performance and application scope of DAS. Distributed 2D/3D positioning detection and drone countermeasures based on images

In addition, the long-distance, spatially dense sampling, and time-domain dense sampling features of DAS generate a huge amount of sensing data. How to convert the huge amount of raw data into useful sensing signals in real time requires the development of new data processing methods and algorithms.

In summary, DAS technology provides a revolutionary technological means for the perception of the physical world, which is of great significance for promoting scientific research and the intelligent development of human society.