Amplitude Vibration Measurement by Harmonic Frequency Analysis of a Distributed Acoustic Sensor

Distributed acoustic sensors (DAS) based in coherent optical time-domain reflectometry (C-OTDR) provide a cost-effective solution for intrusion monitoring of large civil infrastructures like pipelines, railways or motorways. Although detection of events is well demonstrated, an estimation of the amplitude of these events is difficult to achieve. We propose a new method to recover the amplitude of the vibration from the conventional C-OTDR backscattered power traces. It is based on the FFT analysis of the DAS signals. Using a discrete accelerometer as a reference, we have calibrated the response of an optical fiber DAS using known stimuli. A correlation between the amplitude of the vibration and the ratios of the amplitudes of harmonics to the fundamental of the DAS signals is demonstrated. This analysis overcomes the main issues of the amplitude measurement, related to the interaction between the stimulus and the interference pattern.