Microwave frequency OTDR for high strain-rate sensing

Distributed acoustic sensing (DAS) systems have attracted significant attention from seismology thanks to their ability to perform spatially resolved measurements of the seismic wavefield over tens of kilometers. Unfortunately, conventional DAS systems are limited to measuring small strain rates (typically 10s of nanostrains shot-to-shot), which limits their use to the measurement of relatively small events (M<4 at 10 km distance from the source). This makes DAS systems appealing for scientific studies but not for early warning systems which are intended for alerting the population. In this paper, a novel DAS system based on microwave frequency OTDR with high saturation strain sensing ability is formulated and demonstrated. The system employs radiofrequency modulation on the probe pulse injected into the fiber. Large external mechanical perturbations cause localized phase changes on the backscatter signal beating at the microwave frequency component. From the demodulated phases, the strain signals are recovered efficiently, even for large strain-rate signals. A proof of principle experiment with an external perturbation of a few microstrain amplitude and 400 Hz frequency over a distance of 60 meter is well recovered over 4 km fiber. Hence, the new system is proved to broaden significantly the upper measurement limit of typical DAS, bringing a significant application prospect of extensive, real-time large strain sensing.