Distributed Acoustic Sensing (DAS) stands as a revolutionary technology offering real-time insights into acoustic and vibration data along the length of a fiber optic cable. Essentially, it transforms standard optical fiber or specially designed cables into a vast network of sensitive virtual microphones or vibration sensors. DAS captures an extensive array of acoustic information from these virtual microphones, all in real-time.
A DAS unit initiates the process by emitting a brief pulse of laser light along the optical fiber. This light encounters minute variations in refractive index, preset during manufacturing, prompting a portion of it to be reflected back towards the source through Rayleigh backscatter or controlled reflection methods like Fiber Bragg Gratings. Any sound waves or vibrations traversing through the glass modify this backscatter pattern, indicating activity at specific points. These alterations can then be interpreted to discern vibrational characteristics or audibly analyzed.
DAS finds application across various linear assets such as pipelines, roads, railway tracks, borders, and fences. Employing tailored algorithms, it enables the monitoring, tracking, and detection of diverse activities and events along these assets, including vehicle and human traffic, excavation, or tunneling. Moreover, it serves as a valuable tool for asset condition monitoring, facilitating the detection of incidents like pipeline leaks or damaged rails, among others.
DAS boasts several advantages, notably its ease of installation and minimal power requirements in the field. It can cover expansive ranges of up to 45 km (30 miles), with this range doubling for twin-headed devices. Furthermore, it simplifies sensing systems significantly compared to traditional point-sensing methods. In many instances, existing “dark” fibers within close proximity to the asset under surveillance can be utilized alongside established networks. Deploying new fiber optic cables comes at a relatively low cost, and surplus fibers can be repurposed for tasks such as asset communications or rural broadband provision.
In essence, Distributed Acoustic Sensing represents a groundbreaking technology offering unparalleled insights into acoustic and vibrational data, with wide-ranging applications across various industries and sectors.
Within the fiber, the light signal undergoes numerous reflections and refractions due to its interaction with the inner core and cladding. Even the tiniest movement triggers alterations in these reflections and refractions.
These alterations, when observed at the terminal using an interrogator, provide a clear indication of any activity along the line. By employing precise digitized algorithms, the interrogator determines the distance of the signal change, effectively pinpointing the location of the disturbance along the line.
Picture a laser light signal coursing through a fiber optic cable akin to water flowing through a hose. Just as a disturbance in the middle of a running water hose generates a small wave in the flow, any physical interference with a fiber optic sensing cable induces a change in the signal. This alteration travels along the fiber and manifests clearly at the endpoint, much like the flow of water out of the hose.
Fiber optic distributed acoustic sensing serves various applications including seismic sensing, border and perimeter security, pipeline integrity monitoring, and safety assurance in industrial, mining, hydropower, and transportation infrastructure. Additionally, fiber cables can detect temperature changes, although this function is distinct from acoustic sensing.
The benefits of fiber optic distributed acoustic sensing lie in its high sensitivity to microscopic physical changes in the fiber, enabling detection of seismic events, human movement, and airborne sound vibrations. Using laser light signals ensures immunity to electromagnet and Radio Frequency Interference, making optical sensing cables suitable for high-voltage and lightning-prone environments. Furthermore, fiber cables offer high flexibility, being lightweight and adaptable to various terrains without requiring termination at both ends for sensing.
FOTAS (Fiber Optic Based Distributed Acoustic Sensing) systems utilize distributed acoustic sensing for security and detection purposes, leveraging existing fiber optic infrastructure. These systems excel in detecting activities such as digging, excavations, and trespassing, with real-time monitoring of vibrations like walking, precise to 10 meters. By utilizing neglected zones, false alarms in constantly active areas are minimized, with all received alarms reported via web interface and stored for analysis.
