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Distributed Fiber Optic Sensing for Homeland Security

This ground-based seismic sensing system can be used to protect pipelines, power distribution, and transportation.

To address the need to protect our borders and critical infrastructure, fiber optic sensing technology developed for antisubmarine warfare applications has been adapted to homeland security applications. Ground-based seismic sensing applications have significantly different requirements than traditional underwater acoustic applications. As a result, new optical interrogation and signal processing techniques are needed. Border and critical infrastructure sensor systems must be able to monitor long lengths (several km to several 10s of km) with reasonable spatial resolution (5 to 100 m), and have sufficient seismic sensitivity to detect targets of interest. A fiber optic distributed seismic sensor system was developed that is capable of meeting these requirements.

The buried Fiber Optic Distributed Seismic Sensor border monitoring concept.
The system concept for a border monitoring application is shown in the figure. The sensor is a commercial off-the-shelf (COTS) optical fiber cable buried in the ground along the perimeter to be monitored. Seismic activity in the ground couples to the buried cable and induces a strain in the optical fiber within the cable. The optical interrogation system simultaneously monitors the entire fiber length (currently up to 5 km) for timevarying changes in strain. The interrogation system segments the optical fiber into sequential spatial channels of a fixed length (currently 10 m), as shown in the figure. With this spatial aperture, the system is capable of detecting very low signal- level-induced strains in the fiber. One of the goals of these field tests was to equate the strain resolution capabilities of the optical system with real-world seismically induced strain levels observed in a buried optical cable.

The interrogation and signal processing system is connected to one end of the optical cable, which would typically be placed in a secure location. The interrogation system houses the electrooptics components that optically interrogate the sensor cable and demodulate the return signal to recover the seismically induced strain. The signal processing system monitors activity along the entire perimeter, and includes a realtime display and data archival functions.

The system has been tested at two different locations in the southwestern United States using optical cables buried from less than one foot deep to as much as four feet deep. Several fiber optic cable designs, with different fiber protection schemes and cable armoring, have been tested. As might be expected, different burial depths and cable designs yielded different levels of sensor sensitivity, but in general, all tested configurations performed satisfactorily.

Many test scenarios were run, including individuals walking and running, digging, and a variety of vehicles both on-road and off-road. In one of the walking test scenarios, an individual started at the buried cable and walked perpendicular to the cable. When he reached the end point, he stomped his foot ten times to mark the end of the test. The cadence of the walker lowers in frequency as the walker slowed down near the end of the test.

The fiber optic system is capable of transforming a standard, single-mode optical fiber in a cable into a sensitive seismic sensor. The achieved sensitivities and detection capabilities are in line with the requirements for trip wire border monitoring and homeland security applications.

This work was done by C.K. Kirkendall and R. Bartolo of the Naval Research Laboratory, and J. Salzano and K. Daley of SFA Inc. NRL-0054

Topics:
Cables Fiber optics Fibers Optics Parts Physical Sciences Sensors and actuators
This Brief includes a Technical Support Package (TSP).
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Distributed Fiber Optic Sensing for Homeland Security

(reference NRL-0054) is currently available for download from the TSP library.

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    Defense Tech Briefs Magazine

    This article first appeared in the June, 2012 issue of Defense Tech Briefs Magazine (Vol. 6 No. 3).

    Read more articles from this issue here.

    Read more articles from the archives here.

    Overview

    The document discusses the development and field testing of a fiber optic distributed seismic sensor system designed for homeland security applications, particularly for monitoring borders and critical infrastructure. This technology, originally created for antisubmarine warfare, has been adapted to meet the unique requirements of ground-based seismic sensing.

    The system utilizes standard commercial off-the-shelf (COTS) optical fiber cables buried along the perimeter to be monitored. Seismic activity in the ground induces strain in the optical fiber, which is then detected and analyzed by an optical interrogation system. This system can monitor up to 5 kilometers of fiber length, segmenting it into 10-meter spatial channels to detect very low signal-level induced strains. The goal of the field tests was to assess the system's sensitivity and resolution capabilities in real-world scenarios.

    Field tests were conducted in various locations in the southwestern United States, with optical cables buried at depths ranging from less than one foot to four feet. Different cable designs and burial depths were tested, yielding varying levels of sensor sensitivity, but all configurations performed satisfactorily. The tests included various activities such as individuals walking, running, and vehicles moving both on and off-road. The results demonstrated the system's ability to detect seismic events, with clear visual representations of the data collected, including time series plots and spectrograms.

    The document emphasizes the system's capability to transform a standard optical fiber into a sensitive seismic sensor, achieving detection sensitivities that align with the requirements for border monitoring and security applications. The technology provides a unique sensing capability that enhances the protection of borders and critical infrastructure by enabling real-time monitoring of seismic activity.

    In conclusion, the fiber optic distributed seismic sensor system represents a significant advancement in homeland security technology, offering a reliable and effective means of monitoring extensive perimeters for potential threats. The successful field tests validate its operational effectiveness, making it a valuable tool for safeguarding vital national resources.

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