An Overview of Distributed Acoustic Sensing (DAS) Technology

Distributed Acoustic Sensing (DAS) is a technology that enables continuous, real-time measurements along the entire length of a fiber optic cable. It uses fiber optic cables as sensing elements to detect acoustic signals over long distances and in harsh environments. DAS has garnered significant attention due to its diverse range of applications in several sectors such as pipeline monitoring, seismic sensing, and perimeter intrusion monitoring.

In this article, we will cover the following:

• Applications of DAS technology.
• Rayleigh Scattering based DAS.
• Coherent Optical Time Domain Reflectometry (C-OTDR) based DAS.
• Conclusion.

Applications of DAS Technology

DAS technology has become an increasingly important tool in many industrial sectors, and this is largely due to its excellent sensing capabilities. The technology finds diverse applications in several sectors, including pipeline monitoring, seismic sensing, and perimeter intrusion monitoring.

• Pipeline Monitoring: Pipeline monitoring is one of the most popular DAS applications. This technology detects leaks, flow irregularities, and unwanted interference. With DAS sensors, pipeline leak detection becomes easier and faster, reducing the amount of damage leakage may cause. Also, DAS sensing cables can be fixed to power cables to enable 24/7 monitoring of pipeline insulation. This will prevent failure and reduce the risks associated with elevated temperatures.
• Seismic Sensing: Seismic sensing is another DAS application. It can monitor ground stability and monitor seismic activity. DAS sensors can detect minute acoustic interactions that generate sound waves through the ground. DAS technology offers spatial resolutions of up to 10 meters. This makes it possible to identify small changes in the subsurface.
• Perimeter Intrusion Monitoring: Perimeter intrusion monitoring is the use of sensors to identify abnormal sounds or intrusion activities. DAS technology can detect perimeter intrusions by capturing unusual sounds before physical breaches occur. The technology can also detect low-frequency vibrations that signify uninvited human or vehicle activity.

The list above is far from exhaustive; other applications of DAS technology include monitoring carbon dioxide injection for enhanced geothermal systems (EGS), carbon capture and storage (CCS) projects, traffic monitoring, and many others.

In the following sections, we will delve deeper into the two most common types of DAS technology, Rayleigh Scattering based DAS and Coherent Optical Time Domain Reflectometry (C-OTDR) based DAS

Rayleigh Scattering based DAS

Rayleigh Scattering based DAS uses fiber optic cables to provide distributed strain sensing. The system is sensitive to strain and temperature variations, and measurements can be made simultaneously at all sections of the fiber. The system allows acoustic frequency strain signals to be detected over long distances and in rugged environments.

When DAS technology works on fiber optic cables, the fiber strands experience a small amount of strain that causes changes in the velocity of light throughout the fiber. Rayleigh back-scattering, an interaction between light and matter, is responsible for detecting these changes.

Optical fibers that are used for standard telecoms are suitable for distributed acoustic sensing. The fibers have a very long length and are often unused. Therefore, it makes sense to capitalize on the existing fiber optic cables in place.

Distributed Acoustic Sensing that works based on Rayleigh Scattering usually has a low signal-to-noise ratio, which can be altered by employing multiple sensing cables.

Coherent Optical Time Domain Reflectometry (C-OTDR) based DAS

C-OTDR-based DAS systems transmit coherent optical signals along the fiber cable while monitoring backscattered light simultaneously. The variation in the backscattered signal provides information about the location of the disturbance and the intensity of the signal.

With the use of C-OTDR technology, DAS can be used to localize faults, detect pipeline leaks, and measure their lengths. Also, it is possible to distinguish the complex acoustic fields that result from various events on the sensing cable. C-OTDR uses scattered light to achieve high measurement quality and is witnessing widespread use in pipeline monitoring and seismic sensing applications.

Distributed Acoustic Sensing (DAS) technology offers significant potential in several industrial sectors, such as pipeline monitoring, seismic sensing, and perimeter intrusion monitoring. The versatility of this technology makes it adaptable to various use cases and can offer a real-time detection of acoustic interactions throughout a fiber optic cable.

C-OTDR and Rayleigh Scattering-based DAS, are the two standard techniques in DAS/distributed vibration sensing (DVS) technology. C-OTDR is an excellent solution where high measurement quality is necessary, whereas Rayleigh scattering is a cost-effective choice where real-time data on vibration or strain is the primary need. As the technology continues to evolve and improve, it will become an even more important tool for managing environmental and industrial risks.