We demonstrate a novel type of distributed optical fiber acoustic sensor, with the ability to detect and retrieve actual temporal waveforms of multiple vibration events that occur simultaneously at different positions along the fiber. The system is realized via a dual-pulse phase-sensitive optical time-domain reflectometry, and the actual waveform is retrieved by heterodyne phase demodulation. Experimental results show that the system has a background noise level as low as 8.91×10^-4 rad/√Hz with a demodulation signal-to-noise ratio of 49.17 dB at 1 kHz, and can achieve a dynamic range of ∼60  dB at 1 kHz (0.1 to 104 rad) for phase demodulation, as well as a detection frequency range from 20 Hz to 25 kHz.

We report the first distributed optical fibre trace-gas detection system based on photothermal interferometry (PTI) in a hollow-core photonic bandgap fibre (HC-PBF). Absorption of a modulated pump propagating in the gas-filled HC-PBF generates distributed phase modulation along the fibre, which is detected by a dual-pulse heterodyne phase-sensitive optical time-domain reflectometry (OTDR) system. Quasi-distributed sensing experiment with two 28-meter-long HC-PBF sensing sections connected by single-mode transmission fibres demonstrated a limit of detection (LOD) of ∼10 ppb acetylene with a pump power level of 55 mW and an effective noise bandwidth (ENBW) of 0.01 Hz, corresponding to a normalized detection limit of 5.5ppb⋅W/Hz. Distributed sensing experiment over a 200-meter-long sensing cable made of serially connected HC-PBFs demonstrated a LOD of ∼ 5 ppm with 62.5 mW peak pump power and 11.8 Hz ENBW, or a normalized detection limit of 312ppb⋅W/Hz. The spatial resolution of the current distributed detection system is limited to ∼ 30 m, but it is possible to reduce down to 1 meter or smaller by optimizing the phase detection system.

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Microseismic monitoring is of importance for several geoscience research aspects and for applications in oil and gas industry. For signals generated by the ultra-weak microseismic events, conventional moving-coil geophone systems have reached their limit in detection sensitivity especially at high frequency range. Here we for the first time present a specially tailored fiber-optic sensing system targeting at downhole microseismic monitoring. The system contains 30 individual interferometric accelerometers and 2 reference sensors, which are time-division multiplexed into a 12-level vector seismic sensor array. The multiplexed accelerometers can achieve ∼50 ng/ √ Hz noise equivalent acceleration, which is superior to the commercial available moving-coil geophone systems at frequencies above 200 Hz. The measured sensitivity of the accelerometers can reach ∼200 rad/g from 10 Hz to 1 kHz. The dynamic range is above 134 dB over the same frequency range and is higher than its electronic counterpart in the low frequency band. Moreover, the sensors can function properly under the harsh condition of 120 • C temperature and 40 MPa pressure over the 4-hour test duration. The sensor array along with the interrogator has been running uninterruptedly over 3 weeks in a multi-stage hydraulic fracturing stimulation field test. On-site results show that our system can clearly resolve the vector nature of both compressional and shear waves generated by the microseismic events. INDEX TERMS Fiber-optic accelerometer, Seismic sensor, Time-division multiplexing, Microseismic monitoring II. PRINCIPLE OF THE FIBER-OPTIC SEISMIC SENSOR UNIT A. THE SEISMIC SENSOR UNIT This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/ACCESS.2020.3003374, IEEE Access Fei Liu et al.: Downhole microseismic monitoring using time-division multiplexed fiber-optic accelerometer array

Understanding signal fading effect is essential for the application of Rayleigh-scattering-based distributed acoustic fibre sensors (DASs) due to the nature of coherent beam interference within the pulse length. Statistical properties for the intensity of the Rayleigh backscattered light (i.e. intensity fading) and its impact on the sensitivity of DAS systems have been intensely studied over the last decades. Here we for the first time establish an analytical model for the phase signal retrieved from the dual-pulse heterodyne demodulated DAS system, which can be exploited to investigate the phase fading effect in this system. The developed model reveals that the phase fading phenomenon mainly originates from the randomness in the phase retardant of the Rayleigh scatters. The quantitatively resolved statistical features of the phase fading is confirmed by experimental results. Based on the analytical model, a noise figure is defined to characterize the global fading-induced noise level via taking into account contributions from all channels along the sensing fiber. The model also reproduces the anti-correlation relation between the power spectrum density of retrieved phase at the heterodyne frequency and the phase fading noise level. Following the analysis and the definition of the noise figure, an optimized real-time weighted-channel stack algorithm is developed to efficiently suppress the fading noise. Experimental results show that the algorithm can achieve a maximum noise figure reduction of 15.8 dB without increasing the system complexity.

Common-mode noises (CMNs) are frequent in the fiber optic accelerometer, and their suppression is extremely important, particularly in the ultra-weak signal detection application, e.g., micro-seismic monitoring. This Letter proposes a 3-component (3C) low-reflectivity fiber Bragg gratings accelerometer for CMN self-suppression. When compared with the traditional CMN suppression method, the proposed 3C accelerometer is able to improve the CMN suppression effect by an average value larger than 4.5 dB in three axes, as well as double the effective signal amplitude due to the push–pull structure, which brings an enhanced signal-to-noise ratio. Besides, the proposed 3C accelerometer does not need an additional reference interferometer to achieve such a CMN suppression effect; hence, it largely reduces the volume and cost of the sensing system, which shows huge advantages, particularly in the large-scale quasi-distributed array. The proposed 3C accelerometer provides a promising candidate for the weak vector vibration detection.