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


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Distributed Fiber Optic Sensing for Homeland Security

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This article first appeared in the June, 2012 issue of Defense Tech Briefs Magazine.

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