Seismic response of fiber optic seismic sensors

Fiber-optic seismic sensors (FOSS) consist of an optical fiber connected to an optical interrogator, which is electro-optic instrumentation that injects pulses of coherent light into the fiber and receives and demodulates the returned signals. The optical fiber is usually contained within a protective cable. Experiments were performed on a barrier island setting where several kilometers of cable had been buried at sub-meter depths. It has been hypothesized that seismic disturbances strain a segment of the optical fiber thus causing its optical path length to change. An array of eight 3 axis seismometers were buried alongside the cable at the same depth, with a spacing of 0.5 m, and with one axis parallel to the cable. An electromagnetic shaker, buried at the same depth and in different configurations with respect to the array and the FOSS, was excited with a 10-cycle tone burst varying in frequency from 10 to 200 Hz. The resulting seismic waves were simultaneously recorded with the seismometers and the FOSS. Signals recorded with the seismometer array were processed to calculate the average strain along the direction of the fiber. The presentation illustrates the comparison between the computed strain outputs of the seismometers and that of the FOSS.