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Arch Owen OptaSense

Cambridge, MA USA Arch.Owen@OptaSense.com

Greg Duckworth OptaSense

Cambridge, MA USA Gregory.Duckworth@OptaSense.com

Jerry Worsley OptaSense

Farnborough, UK Jerry.Worsley@OptaSense.com

Abstract— The OptaSense® Distributed Acoustic Sensing (DAS) system is an acoustic and seismic sensing capability that uses simple fibre optic communications cables as the sensor. Using existing or new cables, it can provide low-cost and high-reliability surface crossing and tunnel construction detection, with power and communications services needed only every 80-100 km. The technology has been proven in worldwide security operations at over one hundred locations in a variety of industries including oil and gas pipelines, railways, and high-value facility perimeters—a total of 100,000,000 kilometre-hours of linear asset protection. The system reliably detects a variety of border threats with very few nuisance alarms. It can work in concert with existing border surveillance technologies to provide security personnel a new value proposition for fighting trans-border crime. Its ability to detect, classify and locate activity over hundreds of kilometres and provide information in an accurate and actionable way has proven OptaSense to be a cost-effective solution for monitoring long borders. It has been scaled to cover 1500 km controlled by a single central monitoring station in pipeline applications.

I. OptaSense Distributed Acoustic Sensing (DAS) Technology

The enabling technology for our approach is the OptaSense Fibre-optic Distributed Acoustic Sensing (DAS) system. A single 5U rack-mount DAS system provides up to 4000 “virtual sensors” over 50 km that provide sensitivity to strain on commercial cables by measuring the change in length and index of refraction of the fibre induced by the acoustic or seismic waves around it. It measures the picostrain-level signatures of these signals using an interferometric approach employing only the Rayleigh1 scattering from sub-wavelength heterogeneities formed when the fibre was drawn. No special reflectors or fibre Bragg gratings are needed, and it is applicable to any unmodified fibre—even communications cables already in the ground. By using very short optical pulses we can achieve “virtual” strain sensors with element separations as small as 1m.

As shown in Fig. 1, light from these scatterers sums coherently at the detector, and a typical coherent optical time-domain reflectometer (C-OTDR) return for all channels at one instant in time is shown at the bottom. In its simplest form, as the seismic wave straining the cable changes the separations among the scatterers, this coherent sum changes and is detected in each range bin as the returned power of the C-OTDR trace. When pulsed at thousands of times per second, the time series of the detected power from each range bin set by the pulse length (shown as 10 m) follows the seismic signal in the environment around the fibre in that range bin.

backscatter from collection of Rayleigh scatterers in each range bin

OpticalAmplifier

OpticalDetector

20 μs for 2 km

50 nsec pulse of coherent

light

Single Mode Optical Fiber (9 μm core)10 m pulse length

C- OTDR Output

Rayleigh scatteringTS ~ - 80dB//m

OpticalPower

Time (microseconds)

range bin

Fig. 1: Operating principle for the current-generation commercial OptaSense DAS system. Each range bin is sampled 2000 times/second (50 km fibre) to 20000 times/sec (5 km fibre) and provides the “virtual” strain sensor output. A single system can provide 4000 channels programmable at 5, 10, or 12.5 meter spacings.

The DAS technique described above works very well for many intrusion detection applications when the fibre is buried in the ground. It is a very effective technology, and is currently used very successfully for perimeter, border, and asset intrusion protection. It can also support time-delay-of-arrival techniques for off-axis localization of threats. The next-generation OptaSense technology improves upon amplitude transduction by directly measuring the true calibrated strain of the fibre rather than a relative amplitude related to strain. This provides a lower noise floor, phase coherence, and stable sensor calibration, so that the data are suitable for phased-array beamforming processing for signal gain and more accurate localization. In this transduction method, two pulses closely spaced in wavelength are injected in the fibre, separated by the desired length of the virtual strain gauge sensors, e.g., 10 m. Their backscatter interferes at the optical detector, and the optical phase between the two scattering regions measures the net effect of the combined strain and index of refraction change in the sensor region between the two pulses. Just as above, for all range bins, we get a measure of this phase for each optical pulse pair, and thus a calibrated phase-coherent time series for each virtual sensor every 1 m down the fibre. This new product is used only for downhole seismic work at this time, but can also be used for demanding border work in the future, such as tunnel detection.

For long assets and borders, multiple 40-50 km systems can be combined to monitor thousands of km as a single unit. OptaSense works on any telecom-grade dark fibre: pre-

2012 European Intelligence and Security Informatics Conference

978-0-7695-4782-4/12 $26.00 © 2012 IEEE

DOI 10.1109/EISIC.2012.59

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existing or readily installed using standard equipment and methods. Using state-of-the-art signal processing methods, OptaSense reliably detects activities of interest to border security missions with extremely low false-alarm rates. (See Fig. 2.)

Fig. 2 OptaSense has a demonstrated high Probability-of-Detection and low false and nuisance alarm rates for detecting human foot traffic, vehicles, mechanical & manual digging, and general human activity. Indicated detection ranges are conservative, and events are often observed at greater ranges. Also shown are external interfaces for remote operators and responders, and cuing of other sensors.

Conservative detection ranges are shown in the figure. Actual detection ranges depend on soil conditions, cable burial method, and event type, e.g., walking, car, backhoe digging, etc. For example, Fig. 3 shows the nearly 10 dB SNR achieved on each and every footstep 30 m from the cable in a medium to fine sand pasture in east Texas, USA. The algorithms use the spectral and temporal characteristics of these signals to reliably differentiate threats from non-threats, such as animals.

Fig. 3: Signals from normal walking footsteps at 30 meters from the cable at CPA are seen on more than 80 meters of cable at SNR(dB) values high enough for reliable detection and localization. The channel time-delays are apparent in this figure.

The Oak Ridge National Laboratory (ORNL) has independently evaluated OptaSense DAS for combatting “border smuggling and illicit trafficking”2. They reported that “The system met the DOE spec of 90% Ps (Probability of Sense) for all activities and soil conditions tested.” This included all seasons and demanding “stealth walking” (wherein the intruder tries to walk as softly and quietly as feasible), perpendicular to the fibre (to minimize time in the detection area), and in frozen soil. (See Fig. 4.) OptaSense users report virtually no false alarms due to system noise.

Nuisance alarm rates due to threat-like clutter are site-dependent, and for all of our installations are low enough that we know of no system that has been turned off due to this factor—a common practice when the nuisance alarm rate is too high.

Fig. 4: Results from Oak Ridge National labs (ORNL) testing show DAS provides Ps (Probability of Sense) greater than 90%, with a 95% confidence, for all threat conditions evaluated. In fact, the system detected (and classified) all 142 attempts by a person to cross the fibre, no matter what the soil condition or “activity” (e.g., walk, jog, or stealth walk), and the reported number is lower due to proper statistical correction for sample size.

II. OptaSense DAS—an Effective Complement to Existing

Border Technologies The concept of using DAS for border protection has been proposed in the past.3 OptaSense has reduced DAS to practice, and is a unique complement to other traditional border surveillance technologies (e.g., radars, EO/IR, UAVs, fences/barriers), providing border security personnel with a cost-effective, rugged, and simple seismic approach. It has a number of distinctive features:

• The system is truly persistent. No preventative maintenance or downtime is required, and performance is not limited by foliage, weather, and terrain. It is a good solution for rough areas where radar/optical coverage is poor due to line-of-sight problems.

• In addition to reliably sensing traditional border threats (e.g., foot traffic, vehicular traffic) it also detects asymmetric border threats not addressed by some traditional surveillance methods. These include tunnelling, low/ ultralight aircraft, and gunfire. (Fig. 5.)

• DAS requires minimal infrastructure buildout. It requires power and communications only every 80-100 km, with two systems at each site looking in opposite directions. The sensing cable itself can provide a communications backbone. OptaSense may be used with solar power systems (approximately 250W/system), and there are no batteries in buried or hidden sensors to maintain, as with conventional unattended ground sensors (UGS).

Fig. 5: OptaSense® DAS can detect and distinguish asymmetric border threats as well as traditional threats. These are the unique time (vertical axis)-space (horizontal axis) signatures of different threats that are processed for classification.

The operational characteristics of DAS are excellent, and especially so where other systems are weak. (See Fig. 6.) This makes it an ideal complement to other sensing approaches.

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Surveillance Technology

Operational Characteristics

Performance Maintenance Availability Adaptability

Ground-based EO/IR, and radar

Very good performance when line of sight is available

Optical sensors require periodic lens maintenance.

Only available when weather and ground clutter permit, and systems are maintained.

Sensor locations are fixed.

UAV-based EO/IR, and radar

Very good performance when assets are available

Require extensive and regular maintenance

Not persistent; only suitable when weather and ground clutter permit.

Can operate over wide area, not fixed to one location.

Unattended ground sensors

Detection range not as far as some sensors

Require periodic battery replacement

Very good availability if batteries are maintained.

Cannot be scaled to long borders.

OptaSense® Distributed Acoustic Sensing

Short detection range but intrinsically dense coverage

No preventative maintenance. Rapid repair/ replacement.

Available 24/7 in all weather conditions. Applicable to many environments.

Fibre location is fixed, but many different threats can be detected.

Fig. 6: OptaSense DAS operational characteristics (performance, maintenance, availability, and adaptability) nicely complement other border security technologies. Cued assessment of DAS contacts by UAVs is especially promising. III. Examples of DAS Border Employment Concepts The most straightforward application of OptaSense for border security is a virtual fence along extended stretches of borders. OptaSense also enables new, innovative concepts for which other sensors are not well suited:

• As the backbone for an adaptive surveillance system • For monitoring of access routes to/from the border • Opportunistic sensing with existing fibre infrastructure.

A. Backbone for an Adaptive Surveillance System It is often difficult to design and build a good system in one try. An alternative approach is to monitor traffic patterns with an initial “backbone” surveillance system, and then incrementally add additional sensing as required, as threat patterns reveal themselves. In such a strategy, OptaSense is an ideal “backbone”. (Fig. 7.) Once the OptaSense system is in place, threat TTPs can be ascertained, and additional sensing (e.g., more cable, cameras, radars, etc.) added to address any weaknesses. The core backbone OptaSense system continues to work with the additional sensing layers by cueing these more specialized and narrow field-of-view sensors for assessment.

Fig. 7: OptaSense can be the initial "backbone" sensing capability in an evolutionary border surveillance system.

B. Monitor Access Routes to/from the Border Region Some regions pose natural geographic challenges to personnel and vehicular movement, enabling different surveillance strategies. In a heavily forested area, for example, travel through the woods is difficult, so some combination of roads and rivers will be used to move any significant amount of personnel and material over larger distances. These roads and rivers act as access routes to/from the border region. In very dense unpopulated forest, there will only be a small number of access routes. Illegal trans-border traffic will likely stay away from population centres or checkpoints near the border, but will move to these access routes as soon as possible. For these situations, a better strategy might be to monitor the access routes, rather than the border itself. (See Fig. 8.)

Fig. 8: OptaSense can be used to monitor for illegal trans-border traffic entering, exiting, and using a border access route (i.e., road or river). The fibre is deployed along the length of the access route, instead of just along the border. This shows two conceptual 40km fibre laydowns following Amazonian rivers in Brazil near the Colombian state of Vaupes. Infiltrators might cross anywhere along the border, but having done so, the terrain forces them to the rivers as these are the best routes away from the border. By deploying two lengths of fibre along each river, this border region can be effectively monitored, detecting personnel and boats.

C. Opportunistically Exploit Pre-Existing Fibre Optic Cable Instead of installing new cable to address border security needs, OptaSense can exploit existing cables. As an example, Fig. 9 shows a segment of existing commercially-managed network fibre near the border in a sparsely populated area west of El Paso, TX. Cables can be “dual-use”, wherein one fibre is used for OptaSense to monitor general traffic patterns in this near border area, while the others continue to carry commercial telecoms traffic. This can reduce the cost of border coverage.

Fig. 9: Pre-existing fibre optic cable, e.g., this existing telecoms fibre west of El Paso TX, can be opportunistically exploited to monitor long border stretches. (Image courtesy of FiberLocator) [1] R. Posey Jr., G.A. Johnson and S.T. Vohra, “Strain sensing based on

coherent Rayleigh scattering in an optical fibre”, Electronics Letters Vol. 36 No. 20, 28 September 2000.

[2] B. Stinson, M. Kuhn, T. Donaldson, G. D. Richardson, N. Rowe, J. Younkin, C. Pickett, “A Physical Protection Systems Test Bed for International Counter Trafficking System Development,” Oak Ridge National Laboratory, INMM 2011 Meeting Conference Proceedings.

[3] C.K. Kirkendall, R. Bartolo, J. Salzano, K. Daley, “Distributed Fiber Optic Sensing for Homeland Security”, 2007 NRL Review.

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