Distributed Acoustic Sensing (DAS)

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|UPDATED: 13 FEB 2025

|OVERVIEW|

Distributed Acoustic Sensing (DAS) is an advanced sensing technology that uses standard fiber optic cables to detect acoustic, seismic, and vibrational signals over long distances.

DAS transforms fiber optic cables into a continuous array of sensors, leveraging their ability to detect minute disturbances in the environment. Underwater distributed acoustic sensing (DAS) involves the use of unused or "dark" subsea fiber optic cables to sense underwater acoustic or seismic energy. A device called an interrogator transmits pulses of light along the length of the dark fiber optic cable. When the light comes in contact with tiny imperfections in the FO cable, it is reflected back toward the interrogator (a phenomenon known as Rayleigh scattering). Acoustic or seismic energy causes strain in the FO cable, which in turn impacts the phase of the Rayleigh backscatter. This strain can then be sensed and measured by the interrogator.

By enabling real-time, large-scale monitoring with high precision, DAS could transform how maritime forces monitor and protect critical infrastructure, detect underwater threats, ensure domain awareness, and construct a common operational picture.

OPERATIONAL COMPONENTS

• Fiber Optic Infrastructure: Underwater DAS systems leverage unused ("dark") fibers within existing undersea fiber optic cables, or specially deployed cables, as a medium for signal transmission and sensing.

• Interrogator Unit: An interrogator unit sends pulses of laser light down the fiber optic cable which interact with imperfections in the fiber, creating backscattered light.

• Signal Detection: When the fiber experiences minute physical disturbances (vibrations, pressure changes, acoustic waves), the pattern of backscattered light changes, enabling the DAS system to identify the location and characteristics of the disturbance with high spatial resolution.

• Data Processing: Advanced algorithms process the backscattered signal, filtering out noise, identifying patterns, and classifying the type of disturbance.

SCIENCE AND TECHNOLOGY

• Rayleigh Scattering: Exploits micro-scale variations in the refractive index of the fiber, which reflect a portion of the light back to the source.
• Optical Interferometry: DAS uses coherent detection techniques to measure phase changes in the backscattered light, which correspond to vibrations along the fiber.
• Spatial Resolution: Modern DAS systems achieve resolutions as fine as a few meters, allowing precise localization of disturbances over tens of kilometers.
• Signal Processing: Advanced signal processing and pattern recognition algorithms provide event localization and classification.

DEFENSE APPLICATIONS

• Anti-Submarine Warfare (ASW): DAS utilizing seabed communications cables can serve as a passive, low-maintenance, wide-area acoustic array to detect and classify acoustic signatures of submarines, torpedoes, and unmanned underwater vehicles (UUVs).

• Maritime Domain Awareness (MDA): DAS can provide persistent, wide-area surveillance of surface and undersea vessel traffic for real-time situational awareness in remote or contested or denied environments.

• Seabed Infrastructure Protection:
  DAS can act as a intrusion detection system (IDS), monitoring subsea pipelines and cables, and offshore installations for potential threats, such as sabotage, natural hazards, or surveillance activity.

• Border Security and Intrusion Detection:
  Deployed along land or maritime borders, DAS systems could provide early warning of unauthorized crossings, tunneling activities, or small boat movements.

• Acoustic Communication
  Research suggests that underwater DAS could enable real-time, covert transmission of underwater data and information from offshore underwater platforms (Submarines, UUVs, seabed senaor arrays) to shore-based control stations

ADVANTAGES

• Cost-Effective: Uses existing fiber optic infrastructure, reducing the need for new deployments.
• Long Range: Can monitor events over hundreds of kilometers, with minimal power requirements.
• Scalable: Easily integrated into large-scale surveillance networks.
• Low Maintenance: Once installed, DAS systems require minimal upkeep compared to conventional sensor arrays.

LIMITATIONS

• Signal Interference: DAS systems could be affected by fiber quality as well as environmental conditions which can influence signal propagation requiring advanced signal processing for accuracy.
• Data Volume: Large-scale deployments would generate significant data, necessitating robust storage and processing capabilities.
• Localization Accuracy: Achieving precise spatial resolution depends on the quality of the fiber and the interrogator unit.

FUTURE PROSPECTS

• Enhanced Signal Processing: Improved AI and machine learning algorithms will greatly enhance event classification and anomaly detection.
• Hybrid Systems: DAS could be integrated with other sensor modalities (sonar, radar, satellite) to generate a common, multi-domain operating picture.
• Quantum-Enhanced DAS: Quantum technologies could significantly enhance DAS, generating higher sensitivity and spatial resolution in challenging, low signal-to-noise ratio environments.

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ACOMMS APPLICATIONS

Recent research suggests that underwater DAS could be used to structure an underwater acoustic communication network, with potentially significant implications for signnals intelligence (SIGINT) operations, anti-submarine warfare (ASW), and subsea/seabed warfare (SSW).

• As an ACOMMS transmitter, DAS can enable real-time, covert transmission of underwater data and information to shore-based control stations.

• DAS could be used to eavesdrop on adversary underwater communications, gathering valuable SIGINT, as well as insight into adversary acomms encryption methods, undersea wireless sensor networks, and submersible operations.

• In an ASW role, patrolling UUVs could transmit submersible detection, classification, and/or localization data to shore-based stations, which could then dispatch maritime patrol aircraft, or relay coordinates to nearby ASW frigates or destroyers to prosecute the target.

• In the case of subsea/seabed warfare, DAS could use subsea cables to relay targeting data for seabed sensor, communication, or energy infrastructure, or, conversely, act as a trip wire to intercept the acomms of malicious submersibles operating near critical underwater infrastructure (CUI).

WHALE CONSERVATION

Distributed Acoustic Sensing (DAS) is emerging as a powerful tool in whale conservation, offering continuous, high-resolution monitoring of underwater acoustic environments.

• DAS can detect and localize whale vocalizations and other sounds over extensive ocean areas, enabling researchers to monitor their presence, behavior, and movements continuously and in real-time.

• By localizing whale calls through techniques like time-domain reflectometry, DAS could help identify critical habitats such as feeding grounds, breeding areas, and migration corridors.

• DAS could help to mitigate the risk of ship strikes by providing real-time monitoring in shipping lanes high-traffic areas.

• When combined with other monitoring tools like satellite tracking or traditional hydrophone arrays, DAS could enhances our overall understanding of whale ecology and aid in the development of effective conservation strategies.

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‣ ⿻ S&T Brief: DAS for ACOMMS - 7 July 2024
‣ Hearing the Light: DAS could Revolutionize Subsea Defense - 26 January 2023

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