Abstract

Distributed acoustic sensing (DAS) measures local strains along a sensing fiber using as an array of dense virtual strain sensors. One related application of DAS is a seismic wave sensing. Due to the densely deployed virtual sensors, DAS can detect seismic wave propagation along the sensing fiber, which is useful for underground structure imaging. Thus, DAS has been used as an alternative to the conventional sparsely deployed seismometers for investigating geophysical phenomena. DAS for seismic wave sensing uses optical time domain reflectometry (OTDR), which measures a differential phase caused by elongation between two locations. The bandwidth of a probe pulse light defines the gauge length and allowable differential phase or strain rate. Although OTDR has shown to be effective, the nature of the log-scale amplitude of earthquakes may require a broad amplitude dynamic range for DAS. To broaden the bandwidth, we utilize optical frequency domain reflectometry (OFDR), which launches a frequency-swept probe light and measures optical frequency shift proportional to the strain. Using an existing optical fiber in a telecommunication network as a sensing fiber, we demonstrate sensing for two seismic waves with amplitude scales of 0 and 3. Because the spatial resolution is higher than that of OTDR, we can finely visualize underground facilities along the existing telecommunication optical fiber. We also discuss an inherent limitation of OFDR that becomes evident when measuring a long wavelength vibration such as a seismic wave.