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What is Distributed Acoustic Sensing (DAS)?

Acoustic sensors are designed to detect mechanical vibrations travelling through a medium, with various applications in medicine, structure monitoring, aerospace etc. While various configurations of acoustic sensors are possible, there is significant interest in optical fibre acoustic sensors due to its properties, especially the ability to create so-called distributed sensors.


Distributed sensors differ from conventional ‘point’ sensors by having multiple sensing elements exist along the path of the optical fibre cable, converting it into an ultra-sensitive sensor for continuous monitoring of the environment along its length. This is typically done by shooting precisely controlled laser light into an optical fibre and monitoring the back-reflection. For most practical applications, the optical fibre is typically buried either horizontally or vertically (see Figure 1).


Figure 1: Schematic Deployment of a Distributed Acoustic Sensor

Distributed Acoustic Sensing (DAS) systems specifically measure acoustic waves, utilizing phase-Optical Time Domain Reflectometry (Ф-OTDR). This is a dynamic technique that detects changes in the backscattered signal caused by external acoustic waves (see Figure 2).











(a) (b)

Figure 2: Graphical representation of two groups of scatterers in a section of the sensing fibre (a) before external perturbation and (b) after external perturbation

The backscattered light is then collected and processed to produce intensity profiles that are then combined into a 3D graph (see Figure 3). Here, the frequency and distance of the acoustic signal can be discerned, allowing any disturbance at any point along the optical fibre to be measured.


Figure 3: 3D plot of Fourier transformed (decomposed into sine and cosine components) output showing location and frequency of disturbance

Acoustic energy can be generated by a wide range of events, from pipeline leaks to footsteps or vehicles, all of which having a unique acoustic footprint that is recognized by the reading unit. DAS can use these footprints to not only detect and locate where an event is but even identify its nature. And thanks to the reading frequency of 200Hz, it is even possible to see the direction in which the vehicle moves or the person walks. Furthermore, direct contact between the fibre cable and source of the event is not required – footsteps can be “heard” at 10m and vehicles at 30m distance from the fibre cable. Because of this, DAS systems can be employed in a wide variety of fields e.g. seismic monitoring, gas pipe leakage detection, mineral exploration, smart city monitoring etc.


AEGiS DAS

Our collaboration with the Optoelectronics Research Centre at Southampton developed a DAS system that can be deployed relatively inexpensively in Malaysia. The device, (see Figure 4: Left) is a completely enclosed and relatively mobile device that can be deployed relatively easily in existing optical fibre infrastructure. Indeed, to show the commercial viability of the system, we have conducted two proof-of-concept demonstrations with PETRONAS Gas Bhd and Fiberail Sdn Bhd in their respective sites (see Figure 4: Right). This development should be leveraged and further enhanced to contribute to Malaysia’s effort in becoming a player in the high-tech industry.












Figure 4: (Left) AeGIS DAS system. (Right) Proof-of-Concept demonstration at Fiberail’s optical fibre patching site in

 

Author and researcher featured:

Dr. Muhammad Imran Mustafa Abdul Khudus

Copyedit: Michael Hoe Guang Jian (michaelhoe.hoe@gmail.com)


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