Vibration-Induced Beat Frequency Offset Compensation in Distributed Acoustic Sensing Based on Optical Frequency Domain Reflectometry

We investigate the impact of the frequency modulation induced by vibration occurring at a preceding distance in distributed acoustic sensing (DAS) based on optical frequency domain reflectometry (OFDR), which analyzes a vibration waveform by calculating the spectrum shift of Rayleigh backscattered light in the same manner as a fiber Bragg grating. When the measurement time of a beat signal is shorter than the period of the vibration at the preceding distance, the vibration-induced frequency modulation can be treated as a frequency offset for the backscattered light. The frequency offset becomes a distance offset through the distance beat frequency allocation of OFDR, and the distance offset forces us to interrogate the Rayleigh backscattered light spectra at unintentional distances for each measurement since the distance offset is time-varying. Since the spectra at unintentional distances are uncorrelated, the spectrum shift calculated with an uncorrelated spectrum results in a spurious vibration, which is a measured waveform that is different from the actual vibration waveform and that constitutes a measurement error in a DAS measurement. We propose a technique to compensate for the spurious vibration. The technique estimates the vibration-induced distance offset by calculating the cross-correlation between Rayleigh backscattered light waveforms and shifts the spectrum analysis range by the estimated offset in order to track the consistent spectrum. We perform a DAS measurement on a sensing fiber that experiences simultaneous vibrations at different distances and confirm the validity of the proposed technique.