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The technology behind Distributed Acoustic Sensing

FOTAS is a fibre optic based distributed acoustic sensor. SAMM Technologies, the company behind FOTAS describes how this technology works: the solution is a combination of optics, electronics, and computer science, which together make this device as effective as it is.

Fibre optics

Fibre optics is a study field that deals with the optical phenomenon related to optical fibres. The fibre optic cable, situated underground along the designated security line, ensures that the laser beams sent by the interrogator circulate through the entire system that needs protection. Although similar to standard electrical cables, optical fibre has a very thin structure made of pure glass that enables high-speed data circulation.

It consists of two main parts, the core and the cladding, which have different refractive indexes. The cladding not only protects the fibre from impacts but also contributes to the guidance of optical waves moving in the fibre. Polymer coatings to provide protection surround the parts that make up the optical cable.

Distributed acoustic sensors

Distributed acoustic sensors (DAS) are optoelectronic devices that can sense acoustic signals. Sound waves are the most common example of acoustic waves. As an example, vibration is seen as propagating acoustic waves; hence, the system can sense vibrations.

There are active and passive sensors. DAS have active units that can be stored anywhere. In sensing location there is no active component, the interrogation occurs through the fibre optic cables. The system can detect almost any intrusions along the fibre optic line.

Acoustic signals are measured at any point of the fibre cable. Unlike sensors that measure at determining fixed points, the fibre cable is used as a sensor in the DAS system. In other words, distributed acoustic sensor system turns fibre cable into vibration sensors.

Optical time-domain reflectometry (OTDR)

FOTAS uses phase sensitive OTDR technology to measure the differences of backscattered light pulses and determines if there is an acoustic effect. This combined with artificial intelligence algorithms deliver accurate information of the location of a break or other event. It is worth noting that the fibre must be calibrated to the environment it is deployed. A system designed for pipeline security, for example, will not work as well for areas with lower signal rates.

OTDR measures time and location (domain). It sends light pulses and measures backscattered light amplitude. From the time difference, OTDR can calculate the fibre length and from the amplitude loss, OTDR measures general losses, breaking points, and other effects on the fibre optic cable.

Backscattered light amplitude always declines through the fibre optic cable. This is because of the impurities and light-particle interaction in the fibre optics. By this declining amplitude, OTDR can determine the loss coefficient. This principle helps the AI to determine if the fibre is broken or under any loss-causing effect.

FOTAS is based on phase sensitive OTDR. The light is sent to the fibre optic by the interrogator with the help of a laser and the signal reflects back from many points in accordance with the theory called Rayleigh Backscatter. The time reversal of these signals is recorded, and the time signals taken in the fibre cable are modulated. Since OTDR measures fibre length, the shortness of the optical pulse permits the software to determine which part of the cable causes the back reflection. As a result, threats can be detected with OTDR technology, as the pulses occurring in the region disrupt the routine movement of the light circulating in the fibre.

Integrating ground and air

While DAS systems provide accurate data about breaks or intrusions around the fibre optic cable, the response to these events can be a problem because of the length of perimeters or pipelines etc. Sending a response vehicle to the location of an event can take too long, allowing the criminals to escape. In addition, the responders do not know what they are dealing with.

Drones or unmanned aerial vehicles (UAV) are a good fit with DAS. They can fly in a direct and optimal route to the breach area, analyse the situation through surveillance video, and assist the response team with data such as the number of trespassers and if they are armed, avoiding potential injuries or fatalities. Another significant advantage is the ability to scare off intruders if the drone is outfitted with sirens or speakers.

Although the points mentioned above make for a very appealing integrated solution, there are a few factors to consider when successfully implementing an UAV on-site. We must find a good ratio of maximum response time before the damage is done, as well as where to base the drone’s launch pad for optimal intervention. Next, we must consider how long it takes to launch the UAV, its flying speed, and its range. For an efficient set-up and operation, all these elements must be optimally computed.

SAMM Technologies is currently developing solutions to pair DAS and UAV technologies for fast intervention.

For more information, go to https://fotas.samm.com/ or contact the local distributor XtraVision at +27 11 450 1056, [email protected], www.xtravision.co.za

This article is a shortened combination of two articles by SAMM Technologies. They can be found at https://fotas.samm.com/technology/ (short link: www.securitysa.com/*xv1) and https://fotas.samm.com/the-integration-of-unmanned-aerial-vehicles-with-fiber-optic-sensing/ (short link: www.securitysa.com/*xv2).

XtraVision integrates DAS and Arteco

XtraVision (a distributor of SAMM FOTAS DAS fibre intrusion detection technology) and Arteco Video Even Management Software in the South African, African, and South American regions, has recently facilitated the integration between the two manufacturers.

There was a need in many of the projects in the different regions for the integration, via Arteco Open Connector, of the two products. This process was quick since the two international companies are large, professional and well-established companies - both with in-house R&D; departments.

The benefits of the integrations are as follows:

• Integration takes place via Internet Protocol communication network.

• All alarms generated can be displayed on either product in a graphical user interface.

• All cameras on site can be used to support visual verification of alarms on DAS fibre detection routes with a resolution of around

5 metres of human activity.

• Cameras can be directed to a GPS location or pre-shot.

• All events can now be recorded on both systems.

• The SAMM FOTAS display and maps can be pulled through to the Arteco system as a video stream.

This integration allows both systems to run independently, however the information is shared and displayed on either platform. “We feel that this is a great integration, which satisfies requests by both system integrators and end users alike,” says Nick Grange, director of XtraVision. “We feel confident that integration opens a large amount of perimeter protection projects in the industry.”

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