Distributed Vibration Measurement Research Articles

Image-assisted 2D partition denoising method for distributed optical fiber vibration measurements

The traditional one-dimensional denoising method, commonly applied in vibration signal processing, would encounter issues as the calculation complexity, difficulty in maintaining spatial consistency and inability to address noise distributed spatially when measurements are from multiple points. Distributed optical fiber vibration sensing technology produces measurements densely distributed on both time and spatial axes, accompanied by inevitable noise from demodulation error, uneven packaging of sensing components and environmental interference, in which condition proposing new two-dimensional denoising method becomes particularly important. Focusing on the above spatiotemporal denoising target, this paper proposes a new image-assisted 2D partition denoising method by introducing the morphological methods, improving the traditional empirical mode decomposition, and raising the partition denoising strategy. The experimental and simulated distributed vibration measurements of a cantilever beam were carried out to verify the proposed method from the perspective of modal analysis implemented by the DATA-driven stochastic subspace identification. The analysis results indicate that the proposed method could handle the spatiotemporal noise simultaneously and is in favor of improving the continuity of mode identification results especially.

Direct detection Φ-OTDR based on UWFBG array using linear-phase-modulated double-pulse

We propose what we believe to be a novel direct detection phase-sensitive optical time-domain reflectometry (Φ-OTDR) based on ultra-weak fiber Bragg grating (UWFBG) array to achieve distributed vibration measurements with exceptional sensitivity and remarkable stability. Our system employs a pulse modulator to generate a double pulse and achieves linear phase modulation of one pulse by one cycle through a phase modulator. The phase change can be quantitatively demodulated using our proposed N-step phase-shifted demodulation algorithm. This method effectively mitigates the influence of phase noise of the laser and the pulse modulator, while also eliminating fluctuations in the half-voltage of the phase modulator. Compared with the existing phase modulation methods, our method avoids stringent requirements for the stability and precision of phase modulation. Moreover, we propose a phase-shifted approximation method, breaking the limitation of sensing length on the traditional differential approximation method and improving the accuracy significantly. The technique's effectiveness is experimentally demonstrated on a 1 km UWFBG array with a reflectivity of −40 dB to −45 dB and a spatial resolution of 10 m. Vibrations with different amplitudes are measured quantitatively with good linearity. The low-frequency self-noise is greatly suppressed and the overall self-noise is −54.3 dB rad2/Hz.

Application of Intensity-Based Coherent Optical Time Domain Reflectometry to Bridge Monitoring.

Although distributed fiber sensing techniques have been widely used in structural health monitoring, the measurement results of bridge monitoring, particularly under destructive testing, have rarely been reported. To the best of our knowledge, this paper is the first report of distributed vibration measurement results, which we obtained during a three-day destructive test on an abolished bridge. A coherent optical time domain reflectometry (COTDR) was used to acquire the vibration information while the bridge was being sawed. The obtained signal was analyzed in time and frequency domain. Some characteristics of the sawing-induced vibration were retrieved by the short-time Fourier transform; the vibration exhibited several high frequency components within the measured range up to 20 kHz and all the components appeared in the same time slot. Some unexpected signals were also detected. Thorough analysis showed that they are quite different from the sawing-induced vibration and are believed to originate from internal damage to the bridge (probably the occurrence of cracks).

Fast onboard detection of ice drift changes under stationkeeping in ice

Early detection of ice drift changes is essential for safe stationkeeping in ice-infested waters. This paper presents a conceptual system allowing onboard detection of ice drift changes in managed and unmanaged ice using distributed accelerometers installed in the hull of a stationkeeping vessel. With methods from statistical signal processing, specifically probability of exceedance analysis, a relation between the measured signals' entropy and the location along the hull where the ice action occurs is found. With field data from a stationkeeping operation in the Bay of Bothnia this relation is validated. The main contribution of this paper is a concept for ice drift estimation which only requires distributed vibration measurements along the hull and the ship's heading information.

Intruder detection trial in subway tunnel based on distributed vibration response

Identifying illegal intruders is a crucial issue in subway safety management. This paper performs an intrusion detection trial through distributed vibration measurements on a real underground engineering and the ultra-weak fiber optic Bragg grating (FBG) sensing technology was adopted to glean the strucutral vibration signal of the intruder loading. The results showed that the signal model in the distributed structural vibration can be utilized to illustrate the location and speed of the intruders. Besides, the information in both time and frequency domains in the selected experimental area demonstrated that the ultra-weak FBG-based approach is able to distinguish designed intrusion cases in terms of pattern and strength.

Multiplexable intrinsic Fabry-Pérot interferometers inscribed by femtosecond laser for vibration measurement in high temperature environments

This paper reports fabrication technique and demodulation algorithm developments to use multiplexable intrinsic Fabry-Pérot interferometer (IFPI) fiber sensor array for distributed vibration measurements at high temperatures. Using femtosecond laser direct writing scheme, IFPI array were fabricated through laser-induced Type II scattering points in single mode fiber cores. Reflection spectra of IFPI array were demodulated in real-time using modified Bunemen frequency analysis. The demodulation algorithm, which was implemented using a photodetector-array spectrometer, achieves 64-nε dynamic strain resolution at 1-kHz spectral acquisition rate and 2-kHz maximum vibration bandwidth. Performance and stabilities of IFPI sensor array were characterized from room temperatures to 800 °C. The static strain resolution was 0.6 με and the minimum detectable dynamic strain amplitude was 23 nε/√Hz at 800 °C. The multiplex performance was also tested by measuring dynamic strain in time domain through six IFPI sensor array fabricated in one fiber. This paper presents an integrated and low-cost sensing solution to perform distributed vibration measurements in harsh environments.

SNR dependence of measurement stability of heterodyne phase-sensitive optical time-domain reflectometry.

Phase-sensitive optical time-domain reflectometry (Φ-OTDR) implements distributed vibration measurements by demodulation of vibration-induced phase of Rayleigh backscattered light waves (RBLs), and suffers from measurement instability. The weak intensities of RBLs and the resulting low signal-to-noise ratios (SNRs) of intensity measurements are the dominating factors that cause the instability of vibration measurements. In this paper, dependence of the measurement stability of heterodyne Φ-OTDR on the SNR of the intensity measurement is investigated analytically and experimentally. An analytical solution of the probability density function of the demodulated phase as a function of SNR is obtained through rigorous derivation, and the dependence of the measurement stability on the SNR is investigated by analyzing the probability density function distribution of the demodulated phase. Both the theoretical predictions and experimental results reveal the impact of SNR on the measurement stability of heterodyne Φ-OTDR. This study fulfills the Φ-OTDR theory and would lead to an effective approach to stabilizing vibration quantization.

A surveillance system for urban buried pipeline subject to third-party threats based on fiber optic sensing and convolutional neural network

Third-party threats, such as construction activities and man-made sabotage, have become the main cause of pipeline accidents in recent years. This article proposes a surveillance system for protecting the buried municipal pipelines from third-party damage based on distributed fiber optic sensing and convolutional neural network (CNN). Due to the ability of detecting very small perturbation, the phase-sensitive optical time-domain reflectometry (φ-OTDR) is employed for distributed vibration measurements along the pipelines. A two-layer classifier based on CNN is developed: one layer is used to discriminate the third-party activities from the environmental disturbance; the other is to determine the specific type of the third-party events. Meanwhile, a time-space matrix is introduced to reduce the false alarm and correct possible errors by taking into account the continuity of the signals in time and space. Field tests are carried out to validate the effectiveness of the proposed surveillance system. The recognition results show that the CNN-based classifiers achieve the accuracy of over 97%, which is 14.8% higher than that of the traditional feature-based machine learning method using random forest (RF) algorithm. It also indicates that the time-space matrix can dramatically reduce the false alarm and enhance the recognition accuracy.

Distributed Fiber-Optic Dynamic-Strain Sensor With Sub-Meter Spatial Resolution and Single-Shot Measurement

A single-shot distributed dynamic strain sensor is proposed with high spatial resolution and broad detection bandwidth. Linear frequency modulated pulse is employed as the probe pulse, which not only solves the trade-off between spatial resolution and sensing distance, but also avoids the time-consuming stepped frequency tuning process. The matrix of Rayleigh backscattering (RBS) at multiple frequencies over the fiber is obtained using the first probe pulse. The strain variation is retrieved from the corresponding frequency shift of the following probe pulses using RBS signature matching. In the demonstrational experiment, distributed vibration measurement with 0.9 m spatial resolution, 224 $p\varepsilon /\surd$ Hz strain resolution, 5 kHz response bandwidth and 25.7 dB signal-to-noise radio is realized over 10-km sensing fiber.

Distributed vibration measurement based on a coherent multi-slope-assisted BOTDA with a large dynamic range.

We propose a novel technique to enhance the dynamic range of a coherent slope-assisted Brillouin optical time domain analysis. A multi-tone probe and a reference wave are launched into the fiber under test (FUT); after interacting with the pump pulse, the Brillouin gain, as well as the Brillouin phase shift of each tone, can be demodulated simultaneously. In light of this, the strain information can be determined by the Brillouin phase-gain ratio of each tone. In the experiment, a three-tone probe with a 60MHz interval is used; effective measurement frequency span larger than 180MHz is verified in a ∼2 km single-mode fiber with 2.5m spatial resolution and 1.5kHz sampling rate to strain. A vibration signal with 41Hz frequency and 2546με amplitude is successfully demodulated.

A Dual-Parameter Fusion Distributed Optical Fiber Sensor System for Oil and Gas Pipeline Monitoring

For oil and gas pipeline monitoring applications, this paper proposed a dual-parameter fusion distributed fiber optic sensor system that enables distributed temperature and distributed vibration measurements in a single fiber. Through the fiber-scattering spectrum time domain detection combined with coded pulse sequence and Raman scattering spectrum is obtained, which realizes high-resolution temperature measurement and wide-band vibrational wave measurement. The experimental results show that, on 10 km optical fiber measurement, temperature resolution up to 0.1?C and vibration response frequency range 20 Hz - 5 kHz. This sensing system achieves temperature and vibration dual-parameter measurements with fiber optics, greatly simplifying the system and facilitating installation and it can be widely used in oil and gas pipeline monitoring.

Real-Time Phase-Sensitive OTDR Based on Data Matrix Matching Method.

Phase-sensitive optical time-domain reflectometry (Φ-OTDR) is an effective technique to accomplish fully distributed vibration measurement along the entire fiber link. In this paper, a novel data matrix matching method is proposed and successfully employed in the Φ-OTDR system for real-time vibration detection and type identification. By using the novel method, the quantized response time is presented and improved to millisecond level for the first time. Meanwhile, the data can be extracted completely without packet loss, thus allowing vibration type identification to be obtained while maintaining the system simplicity. The experimental results demonstrate that the vibration signals can be detected and located with an average response time of 50.1 ms, under a data transmission speed which can go up to 77.824 Mbps. Moreover, different vibration types such as sine waves and square waves which are applied to the sensing fiber through a piezoelectric ceramic (PZT) cylinder can also be successfully identified. This method provides an efficient solution for real-time vibration location and type identification, thus exhibiting considerable application potential in many practical situations.

Optical Fiber Vibration Sensor Using Chaotic Laser

In this letter, a novel composite optical fiber sensor constructed with the combination of chaotic laser and non-balanced Sagnac/Mach-Zehnder structure is proposed and experimentally demonstrated for the distributed vibration measurement. With the help of a stable and low-coherence chaotic laser, higher order null-frequencies with accurate peaks can be successfully obtained, which reduces the location error, and thus improves the detection performance. The experimental results verify the effectiveness of the proposed sensor. It is able to detect vibration position over a total fiber length of 12.101 km, with a location error, which can be reduced to 22 m.

Distributed Fiber-Optic Vibration Sensing Based on Phase Extraction From Optical Reflectometry

Distributed fiber-optic vibration sensing based on phase extraction from optical reflectometry is discussed in this paper. The principles on the phase extraction from coherent phase-sensitive optical time-domain reflectometry and time-gated digital optical frequency-domain reflectometry are introduced, and the experimental results of distributed vibration measurements based on the two techniques are demonstrated. To have a better understanding on the performance of the techniques, the phase extraction noise in the phase extraction process is analyzed. To solve the problem of existing measurement dead zones, the influence of Rayleigh fading phenomenon is investigated, and a solution of using statistical analysis to overcome this drawback is introduced and experimentally verified. With these improvements, long-range nondead-zone distributed vibration sensing techniques with linear response to external vibration signals based on phase extraction from optical reflectometry are realized.

Retraction Note to: Fast Brillouin Optical Fiber Sensor for Distributed Dynamic Measurement Based on Differential Double-Pulse

We demonstrate a high-spatial-resolution fast Brillouin optical time-domain analysis scheme based on frequency agility and differential double-pulse for distributed dynamic measurement. The frequency-agility probe wave is obtained from the second-order sideband of modulated light by using frequency-agility microwave signal from a wideband arbitrary waveform generator. The differential double-pulse technique is proposed to improve the spatial resolution while keeping the capability of dynamic measurement. In experiment, a spatial resolution of 20 cm is achieved by using a 52/50 ns differential double-pulse, and the distributed vibration measurement is demonstrated over a 50-m Panda fiber with a maximum vibration frequency of up to 50 Hz. With only five averages, the standard deviation of the strain accuracy is of 14 μV.

Performance and comparison of fiber vibration sensing using SAC-OCDMA with direct decoding techniques

Intensity modulated fiber vibrations sensing using Khazani Syed (KS) and Modified Quadratic Congruence Code (MQC) code for quasi distributed vibration measurement have been evaluated. The systems employed Spectral Direct (SD) Decoding for Spectral Amplitude Coding-Optical Code Division Multiple Access (SAC-OCDMA). SAC-OCDMA is adopted because of its enormous impact in dealing with Multiple Access Interference (MAI). The setup was carried out for three sensor points in three consecutive measurements. The performance of the sensing system with the KS and MQC codes were compared at the same weight and number of code sequence. It was observed that the applied vibration frequencies were obtained at the receivers and the signal employing MQC are received at slightly better power than KS code. However, KS code setup employed less number of components and therefore, reduce the cost and complexity of achieving quasi-distributed vibration sensing.

Vertical seismic optical profiling on wireline logging cable

ABSTRACTVertical seismic profiles are usually acquired by deploying downhole seismic sensors below a wireline logging cable. A seismic source is triggered at surface while recording the downhole vibration via the wireline cable. In this paper, an alternative approach based on distributed vibration measurement is tested using wireline deployment for the first time.Local axial strain of a multi‐kilometre fibre optic line is measured at intervals of approximately 1 m and processed to a spatial resolution of 10 m with sub nanometer strain resolution by an optical interrogation device (distributed vibration/acoustic sensing). When deployed in a well, the optical fibre line should be mechanically coupled to the borehole wall to generate valid seismic records.A conventional vertical seismic profile was acquired with three‐component sensors in a vertical well near Bottesford, UK. The impulsive seismic source was a novel portable airgun tank. Clear seismic reflections are observed within and below the borehole, in agreement with surface seismic data. A single shot generated equivalent data with an experimental optical wireline logging cable and an adequate optical interrogator at the surface. The main difference between the two records is the presence of a strong tube wave in the optical profile, which can be easily removed with conventional velocity filter processing. Corridor stacks from both conventional and optical profiles match each other and provide a reasonable tie to a nearby surface seismic line.

RETRACTED ARTICLE: Fast Brillouin Optical Fiber Sensor for Distributed Dynamic Measurement Based on Differential Double-Pulse

We demonstrate a high-spatial-resolution fast Brillouin optical time-domain analysis scheme based on frequency agility and differential double-pulse for distributed dynamic measurement. The frequency-agility probe wave is obtained from the second-order sideband of modulated light by using frequency-agility microwave signal from a wideband arbitrary waveform generator. The differential double-pulse technique is proposed to improve the spatial resolution while keeping the capability of dynamic measurement. In experiment, a spatial resolution of 20 cm is achieved by using a 52/50 ns differential double-pulse, and the distributed vibration measurement is demonstrated over a 50-m Panda fiber with a maximum vibration frequency of up to 50 Hz. With only five averages, the standard deviation of the strain accuracy is of 14 μV.

High-Spatial-Resolution Fast BOTDA for Dynamic Strain Measurement Based on Differential Double-Pulse and Second-Order Sideband of Modulation

We demonstrate a high-spatial-resolution fast Brillouin optical time-domain analysis (BOTDA) for distributed dynamic strain measurement based on differential double-pulse and second-order sideband of modulation. The frequency-agility probe wave is obtained from the second-order sideband of the modulated light by using the microwave signal from a wideband arbitrary waveform generator (AWG), which reduces the bandwidth requirement of the AWG by half to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\sim$</tex></formula> 5.5 GHz. The differential double-pulse scheme is proposed to improve the spatial resolution while keeping the capability of dynamic measurement. In experiment, a spatial resolution of 20 cm is achieved by using a 52/50 ns differential double-pulse, and the distributed vibration measurement is demonstrated over a 50-m Panda polarization-maintaining fiber observing the vibration frequency of up to 50 Hz. With only five averages, the standard deviation of the strain accuracy is measured to be 14 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu\varepsilon$</tex></formula> .

Continuous wavelet transform for non-stationary vibration detection with phase-OTDR

We propose the continuous wavelet transform for non-stationary vibration measurement by distributed vibration sensor based on phase optical time-domain reflectometry (OTDR). The continuous wavelet transform approach can give simultaneously the frequency and time information of the vibration event. Frequency evolution is obtained by the wavelet ridge detection method from the scalogram of the continuous wavelet transform. In addition, a novel signal processing algorithm based on the global wavelet spectrum is used to determine the location of vibration. Distributed vibration measurements of 500 Hz and 500 Hz to 1 kHz sweep events over 20 cm fiber length are demonstrated using a single mode fiber.