Fading-Induced Phase-Fluctuation Reduction and SNR Enhancement in ϕ-OTDR Using Video Block-Matching 3-D Collaborative Processing

Abstract

Coherent fading and random background noise are two major factors that degrade phase stability and weak-vibration detection in distributed acoustic sensing (DAS) systems. Coherent fading induces local amplitude fading and phase-demodulation instability, whereas random background noise obscures vibration responses and lowers the signal-to-noise ratio (SNR), especially in long-range sensing scenarios. Operating on phase-demodulated DAS data, this article proposes a DAS-oriented video block-matching 3-D collaborative processing method (VBM3D) to mitigate fading-induced phase fluctuations, suppress random background noise, and enhance SNR. The phase-demodulated DAS data are reorganized into equivalent frame sequences, allowing correlations among neighboring spatial sensing channels, within-frame temporal continuity, and inter-frame redundancy to be jointly exploited. Cross-frame predictive search, noise-parameter estimation, and frame-selection strategies are incorporated to adapt VBM3D to DAS spatiotemporal data. Through two-stage 3-D collaborative filtering, fading-induced phase fluctuations and random background noise are jointly reduced while vibration-related spatiotemporal structures are preserved. Experimental results show that VBM3D improves the SNR of low-frequency vibration signals and reduces fading-induced phase fluctuations in nondisturbance regions to 1.133 and 1.436 rad for 0.1 and 0.05 Hz vibrations, respectively. It also recovers the far-end vibration response over a 50.2 km sensing fiber with an SNR of 35.883 dB and demonstrates practical effectiveness in field intrusion scenarios with improved phase stability and background-noise reduction.