Short answer
Subsea fiber-optic sensing uses Distributed Acoustic Sensing (DAS) to turn a seabed optical cable into a continuous hydrophone array with thousands of channels. A single interrogator at the landing station detects and locates anchor drag, trawl strikes, pipeline leaks, divers and vessel propellers, with no electronics in the water. DuyuSense develops DAS-based solutions for underwater diver and vessel detection and welcomes pilot projects.
What can DAS monitor under water?
DAS monitors both the cable itself and the acoustic environment around it. Every few metres of fiber acts as an independent acoustic channel, so vibration, pressure waves and mechanical contact are recorded in real time and located along cable runs that typically reach tens of kilometres.
One fiber asset can serve several purposes: power and telecom cable protection, port security, pipeline surveillance and marine acoustics. What decides the design is where and how the cable is laid, and which events matter most.
- Anchor drag, trawling and fishing-gear strikes on subsea power and telecom cables
- Cable burial status and exposure caused by currents or seabed mobility
- Divers, swimmer delivery vehicles, ROVs and small vessels near ports, piers and coastal facilities
- Leaks and third-party interference on offshore pipelines
- Vessel traffic and marine mammal acoustics as a secondary use
How does DAS protect subsea cables from anchors and trawlers?
DAS recognises a dragging anchor or an approaching trawl by the vibration and noise it produces before it reaches the cable. Industry data attributes the majority of subsea cable faults to fishing and anchoring, so early warning has a direct effect on repair costs and outage time.
Because events are located to within metres, operators can contact the vessel in the area quickly or correlate the alarm with AIS data to identify it. If a fault does occur, the same fiber narrows down the fault location rapidly, shortening the time a repair ship spends searching.
How are divers and vessels detected at ports and coastal facilities?
A fiber laid on the seabed forms an invisible acoustic fence across a harbour entrance or in front of a facility. Diver breathing and fin noise, the thrusters of swimmer delivery vehicles and ROVs, and the propeller signature of small boats each appear as distinct patterns across DAS channels.
The signal-processing layer separates these patterns from background noise such as waves, currents and routine harbour traffic. DuyuSense's patented DAS signal processing targets improved spatial resolution and signal-to-noise ratio for higher event-detection accuracy. Naval installations, commercial ports and critical coastal infrastructure are the main candidates for this application.
- Continuous coverage along the cable, without sonar shadows or blind sectors
- Passive operation: emits no sound and does not reveal its presence
- Alarm verification through integration with cameras, radar and sonar
How does DAS detect offshore pipeline leaks?
A fiber laid along or near a pipeline picks up the acoustic signature and pressure transient of escaping pressurised fluid. The same system reports third-party interference such as anchors, dredging or excavation approaching the line.
Where strain or temperature must be tracked precisely at specific points (risers, connections, free spans), Fiber Bragg Grating (FBG) sensors complement DAS. Near 1550 nm an FBG typically shifts about 1.2 pm per microstrain and about 10 pm/°C, giving a direct measure of structural load.
Buried or seabed-laid cable: which works better?
A buried cable is better protected mechanically and less affected by flow noise, while a seabed-laid cable couples directly to the water column and is more sensitive to acoustic events. The right choice depends on the target events and the seabed conditions.
Cable protection projects usually reuse an existing buried cable, and gradual exposure of that cable becomes a monitored parameter in itself. For diver and vessel detection, a short purpose-laid cable, weighted where needed, delivers higher acoustic sensitivity.
- Buried: lower anchor risk and flow noise, weaker acoustic coupling
- Seabed-laid: strong acoustic coupling and simple installation, more exposed to external damage
- Hybrid: a laid section at the critical zone, the rest of the route buried
What matters when deploying subsea DAS?
The key advantage of subsea DAS is that there are no wet electronics: the only active unit, the interrogator, sits at the landing station or inside the facility. This largely removes the maintenance, power-feed and watertightness issues of in-water sensors.
Cable selection is critical, as armouring, fiber type and the loose-tube construction all affect acoustic sensitivity. Spare (dark) fibers in existing telecom cables can often be used directly. DuyuSense works as a project partner through sensor and system design, prototyping, integration and training support.
- Single-channel range is typically tens of kilometres, often cited around 40-50 km
- Spatial resolution is typically a few metres
- Onshore: uninterruptible power, data link and control-room integration
Can the same fiber support marine acoustic monitoring?
Yes, a DAS installation built for security or cable protection can also monitor marine mammals and vessel traffic. Whale and dolphin vocalisations, ship noise levels and earthquake signals have all been recorded with DAS in published research.
This secondary use adds environmental and scientific value to a single fiber investment. Wideband acoustic analysis, however, produces large data volumes, so storage and processing need to be planned from the start.
| Feature | DAS fiber | Hydrophone array / sonar |
|---|---|---|
| Coverage | Continuous along the cable, typically tens of km | Discrete points or a sector limited by sensor range |
| Wet electronics | None, interrogator onshore | Requires in-water electronics and power feed |
| Operating mode | Passive, emits no sound | Hydrophones passive; active sonar emits sound and can be detected |
| Localisation | Within metres along the cable | Depends on array geometry and bearing estimation |
| Sensitivity | Lower than a single hydrophone, offset by many channels | High acoustic sensitivity at each point |
| Maintenance | Passive cable, servicing at the onshore unit | Periodic subsea servicing and replacement |
| Use of existing assets | Dark fibers can be reused | Usually a new installation |
Frequently asked questions
Can an existing subsea telecom cable be used for DAS?
Often, yes. A spare (dark) fiber in the cable can become an acoustic sensor when connected to a DAS interrogator at the landing station. Suitability depends on fiber type, cable construction, burial depth and whether optical repeaters sit along the route, which limit the reachable span.
How far away can DAS detect a diver?
Detection distance depends on whether the cable is buried or laid, seabed conditions, background noise and the diver's equipment. Rather than quoting a fixed figure, a site-specific acoustic survey and pilot are recommended. Short, purpose-laid cables generally give the highest sensitivity for diver detection.
Does subsea DAS replace sonar?
It usually complements it. DAS provides passive, continuous early warning along a long line, while sonar and cameras classify and confirm the target. Used together, they reduce blind zones and cut false alarms, and DAS alarms can cue the sonar to the right sector.
Can anchor drag be detected before it damages a cable?
In most scenarios, yes. A dragging anchor and chain generate clear vibration on the seabed before reaching the cable. DAS reports the approach with its location, and operators can identify and warn the vessel using AIS. The actual warning time depends on vessel speed and cable depth.
Why does having no wet electronics matter?
In-water electronics need watertight housings, a power feed and periodic servicing, and any failure calls for divers or a vessel. With DAS the cable is fully passive and the only active device, the interrogator, is onshore. This substantially lowers operating cost and failure risk.
What role does FBG play in subsea applications?
FBG sensors measure strain, temperature and pressure precisely at chosen points. On pipeline risers, connections and cable landing zones they complement the wide-area acoustic coverage of DAS. Both technologies can be planned on the same fiber infrastructure and read by interrogators located onshore.
What does DuyuSense offer for subsea sensing?
DuyuSense develops DAS-based solutions for underwater diver and vessel detection and supports signal processing, system design, prototyping and integration. It is open to pilot projects with ports, coastal facilities and cable operators. Contact: contact@duyusense.com, +90 212 285 6987.