TECHNOLOGY

INTRODUCTION

Cobalt is a revolutionary optical fiber sensor system, capable of simultaneous and independent distributed sensing and location of multiple acoustic disturbances along a length of standard single-mode optical fiber cable.

Utilising a novel OTDR (optical time domain reflectometry) method this system offers single-ended operation over a range of up to 40km [25 miles] in a single span, with a location accuracy and sensing resolution of 10m [33 ft].

• 40km [25 miles] Range
• 10m [33 ft] Spatial Resolution
• Single-Ended Operation
• Simultaneously and Unambiguously Resolves Multiple Disturbances
• Raw System Output Is An Acoustic Signal For Every 10m [33 ft]

The primary applications for which this system is designed are perimeter and border security. A standard single-mode fiber cable, typically buried at a depth between 150mm to 1.2m [6" to 4 ft] can protect a long perimeter or border from intrusion. The system can categorize a disturbance as either personnel or vehicle by analysing its acoustic signature. Cobalt is inherently stealthy because the sense fiber radiates no energy, and the buried non-metallic cable is unlocatable by conventional means.

Due to its single ended operation, the system is extremely robust. Even if the cable were cut, intrusion detection continues to operate normally up to the point of damage, and the system raises a Cable Cut alarm (which reports the exact location of the cut). When access to both ends of the fiber is possible, as is normally the case in perimeter or border security applications, it is possible to configure a system that will simultaneously interrogate the remaining cut cable from the opposite end. The perimeter can therefore be always protected, even if the fiber is cut.

Other applications for which this system are suitable include:

• Intrusion Detection
  • Buildings, Perimeter fences, International borders, etc
• Utility Protection
  • Cables, Pipelines, Power lines, etc.
• Surveillance
  • Buildings, Perimeters, Anywhere
• Structural monitoring
  • Buildings, Bridges, Radio masts, Pylons, etc.
• Vibration detection/monitoring
  • Machinery, Aircraft , Ships, Railways, Bridges, etc.

SYSTEM PROCESSING

The raw output of the sensor is a real-time acoustic signal for each 10m [33 ft] resolution "bin" along the whole optical fiber cable. Several layers of processing are applied before presentation to the user. The signal processing and alarm classification criteria for each individual 10m [33 ft] resolution bin are independent, and different criteria can be applied to each and every 10m [33 ft] section of fiber.

Consider, for example the application of perimeter security and intrusion detection.

The low-level measure of a threat is the magnitude of the acoustic signal (after application of a number of system-defined filters). The system automatically learns the history of this signal as a function of both time and position along the perimeter. Using this historical data a threat threshold level is defined. If at any time a disturbance exceeds this alarm threshold, several further processing steps can occur.

The system may be configured simply to raise an alarm without further analysis: This may be the best approach if the installation whose perimeter is being monitored is in a location where there is very little background acoustic disturbance, which may indeed be the case if the sensor is employed in the "sterile zone" of a military installation.

In most cases, at least part of the protected perimeter will be in an area where frequent background acoustic disturbances are expected, for example at the entrance gate to the facility. In this situation, the simplest approach may be to apply a higher threshold in the noisy sections, but Cobalt provides additional layers of intelligent processing to minimize the false alarm rate. For example, the system can be configured to only raise an alarm if the acoustic signature at that location matches a known threat type - such as a person walking or running, or a vehicle crossing the buried cable.

The system is also capable of correlating the disturbances over several discrete bins to provide intelligent threat reporting. Consider the situation where a single fiber is installed around a perimeter such that it is first looped outside the perimeter fence, and then continues around a second loop laid within the perimeter fence. To ensure an alarm is raised only when the fence is breached, the system can be configured to ignore disturbances unless (1) a certain acoustic signature is detected first on the perimeter's outside fiber and (2) a short time later, that same acoustic signature is detected in the same vicinity on the perimeter's inside fiber.

USER INTERFACE

The Cobalt system sends alarm data over a TCP/IP enabled network to a terminal PC acting as the Graphical User Interface (GUI). The figure below shows Cobalt's user interface in a plant protection application. An aerial photo is overlaid with a virtual fiber route, which indicates the state of alarm at any given position around the installation. Along with alarm annunciation the software maintains a database of alarm activity and status. A histogram of the magnitude of the acoustic disturbance along the fiber route is updated in real-time, and the user can listen to the sound detected at any given location.

SYSTEM OUTPUT

The following recordings show Cobalt's response to a number of disturbances, including: personnel, vehicles, cable intrusion and digging.

PERSONNEL: Walking

The images below show the spectrogram and time-domain plot of Cobalt's acoustic output as a person walked at a normal rate laterally across the cable. This recording was made using a cable at a depth of 150mm [6"], direct-buried in grass-covered loamy soil. The output has been 20Hz High-Pass filtered to give a frequency response typical of Geophones. The spectrogram clearly shows more than 20 individual footfalls before the cable is crossed.

However, the RAW (unfiltered) system output contains further information, as is shown in the time-domain plot below. Note that the more-distant footfalls show no DC component in the response, and exhibit a frequency content consistent with the natural resonance of the ground. As the walker approaches the cable, the frequency spectrum of the recorded footfalls broadens, showing both DC and higher harmonic content. This indicates that Cobalt detects the ground actually being compressed, which demonstrates that the sensor output is stable, and DC coupled over this time period.

VEHICLE: 4-Stroke Single Cylinder Motorbike

The spectrograms below show the acoustic signature recorded as a motorbike was ridden in the vicinity of the fiber. This recording was made using a cable at a depth of 150mm [6"], direct-buried in grass-covered loamy soil.

The Spectrograms reveal a great deal of information about the movement and character of the vehicle. Engine harmonics and wheel movement are clearly visible. Further information, such as the ignition of the engine and the impulse created as the bike pulls away, is also evident.

CABLE INTRUSION: Manhole intrusion

The images below show the spectrogram and time-domain plot of Cobalt's acoustic output as a manhole is compromised. The spectrogram clearly shows several discrete activities. Firstly the manhole lid is repeatedly struck with the pulling spike. The lid is then lifted and the cable itself gently tapped. Finally the cable is very lightly scuffed.

PLANT PROTECTION: Backhoe or Digger operating near buried cable

A Backhoe/Digger operates in the vicinity of the buried fiber cable. The cable was in a 25mm [1"] duct at a depth of 1.2m [4 ft] in dry loamy soil. You can clearly see the vehicle approaching the cable, and the outriggers being deployed. The engine harmonics during this time are clearly visible. The bucket is then banged on the ground several times before a short trench is excavated (0.6m [2 ft] to the side of the cable). The engine is then switched off for a while to demonstrate Cobalt's excellent Signal to Noise Ratio (SNR). The engine is then restarted, and the trench refilled. As you can see, this activity would be clearly identified, possibly allowing time to warn the operator of the backhoe - or at least re-route essential communications traffic - before critical damage occurred.

PLANT PROTECTION: Post-hole digger

A technician uses a postholer in the vicinity of a buried fiber cable. The cable was in a 25mm [1"] duct at a depth of 1.2m [4 ft] in dry loamy soil. The postholer can be easily observed at distances greater than 60ft lateral offset from the buried cable. As the distance from the cable increases, the higher frequency components are more attenuated by the ground, leaving only the low frequency impulse to be observed.