Sparse seismic instrumentation in the oceans limits our understanding of deep Earth dynamics and submarine earthquakes. Distributed acoustic sensing (DAS), an emerging technology that converts optical fiber to seismic sensors, allows us to leverage pre-existing submarine telecommunication cables for seismic monitoring. Here we report observations of microseism, local surface gravity waves, and a teleseismic earthquake along a 4192-sensor ocean-bottom DAS array offshore Belgium. We observe in-situ how opposing groups of ocean surface gravity waves generate double-frequency seismic Scholte waves, as described by the Longuet-Higgins theory of microseism generation. We also extract P- and S-wave phases from the 2018-08-19 [Formula: see text] Fiji deep earthquake in the 0.01-1 Hz frequency band, though waveform fidelity is low at high frequencies. These results suggest significant potential of DAS in next-generation submarine seismic networks.

Distributed sensing of microseisms and teleseisms with submarine dark fibers.

Optical fiber sensors have attracted considerable attention for marine environment and marine structural health monitoring, owing to advantages including resistance to electromagnetic interference, durability under extreme temperature and pressures, light weight, high transmission rate, small size and flexibility. In this paper, the optical fiber sensors employed for marine environment and marine structural health monitoring are summarized for the understanding of their basic sensing principles, and their various sensing applications such as physical parameters, chemical parameters and structural health monitoring. This review paper shows the feasibility of using optical fiber sensing technology for marine application and, due to the aforementioned advantages, it is possible to envisage a widespread use in this research field in the next few years.

Optical fiber sensing for marine environment and marine structural health monitoring: A review

Distributed acoustic sensing (DAS) using coherent Rayleigh backscattering in an optical fiber has become a ubiquitous technique for monitoring multiple dynamic events in real time. It has continued to constitute a steadily increasing share of the fiber-optic sensor market, thanks to its interesting applications in many safety, security, and integrity monitoring systems. In this contribution, an overview of the recent advances of research in DAS based on phase-sensitive optical time domain reflectometry (ϕ-OTDR) is provided. Some advanced techniques used to enhance the performance of ϕ-OTDR sensors for measuring backscattering intensity changes through reduction of measurement noise are presented, in addition to methods used to increase the dynamic measurement capacity of ϕ-OTDR schemes beyond conventional limits set by the sensing distance. Recent ϕ-OTDR configurations which significantly enhance the measurement spatial resolution, including those which decouple it from the probing pulse width, are also discussed. Finally, a review of recent advances in more precise quantitative measurement of an external impact based on frequency shift and phase demodulation methods using simple direct detection ϕ-OTDR schemes is given.

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Recent Advances in Distributed Acoustic Sensing Based on Phase-Sensitive Optical Time Domain Reflectometry

This paper proposes a novel distributed fiber-optic acoustic sensor, which can solve both the tradeoff between the maximum measurable distance and the spatial resolution, and that between the measurement distance and the vibration response bandwidth. The system is based on frequency-division-multiplexing time-gated digital optical frequency domain reflectometry, which consecutively injects linear-frequency-modulated probe pulses with different frequency ranges. Undersampling method is introduced to reduce the sampling rate of the analog-to-digital converter and the data size, which can reduce the cost of the system and facilitate real-time data processing. In experiments, two simultaneous vibrations with frequency up to 9 kHz are detected over the 24.7-km-long fiber, with a sign-to-noise ratio of 30 dB and spatial resolution of 10 m.

Distributed Fiber-Optic Acoustic Sensor With Enhanced Response Bandwidth and High Signal-to-Noise Ratio

Continuous, real-time monitoring of surface seismic activity around the globe is of great interest for acquiring new insight into global tomography analyses and for recognition of seismic patterns leading to potentially hazardous situations. The already-existing telecommunication fiber optic network arises as an ideal solution for this application, owing to its ubiquity and the capacity of optical fibers to perform distributed, highly sensitive monitoring of vibrations at relatively low cost (ultra-high density of point sensors available with minimal deployment of new equipment). This perspective article discusses early approaches on the application of fiber-optic distributed acoustic sensors (DASs) for seismic activity monitoring. The benefits and potential impact of DAS technology in these kinds of applications are here illustrated with new experimental results on teleseism monitoring based on a specific approach: the so-called chirped-pulse DAS. This technology offers promising prospects for the field of seismic tomography due to its appealing properties in terms of simplicity, consistent sensitivity across sensing channels, and robustness. Furthermore, we also report on several signal processing techniques readily applicable to chirped-pulse DAS recordings for extracting relevant seismic information from ambient acoustic noise. The outcome presented here may serve as a foundation for a novel conception for ubiquitous seismic monitoring with minimal investment.

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Distributed acoustic sensing for seismic activity monitoring

For a distributed fiber-optic vibration sensor (DFVS), the vibration signal extracted from the phase of backscattering has a linear response to the applied vibration, and is more attractive than that from the intensity term. However, the large phase noise at a random weak-fading-point seriously limits the sensor's credibility. In this paper, a novel phase-detection DFVS is developed, which effectively eliminates the weak-fading-point. The relationship between phase noise and the intensity of backscattering is analyzed, and the inner-pulse frequency-division method and rotated-vector-sum method are introduced to effectively suppress phase noise. In experiments, two simultaneous vibrations along the 35-kilometer-long fiber are clearly detected by phase detection with the signal-to-noise ratio (SNR) over 26 dB. The spatial resolution approaches 5 m and the vibration response bandwidth is 1.25 kHz.

Phase-detection distributed fiber-optic vibration sensor without fading-noise based on time-gated digital OFDR

In order to solve fading problem and realize sub-meter spatial resolution in DAS, this paper proposes a novel configuration of time-gated digital optical frequency domain reflectometry (TGD-OFDR) based on optical intensity modulator (IM). IM has a large modulation bandwidth and the positive and negative harmonics can be fully used to suppress fading while the spatial resolution remains unchanged. In experiments, with fading suppressed, the spatial resolution of DAS is 0.8 m and the strain resolution is about 245.6 pe√Hz along the total 9.8-km sensing fiber. The response bandwidth of vibration is 5 kHz, only/limited by the fiber length.

High-fidelity distributed fiber-optic acoustic sensor with fading noise suppressed and sub-meter spatial resolution

The measurement distance is one of the most important parameters for distributed acoustic sensor (DAS). In this paper, we report a long-distance and high-sensitivity DAS system based on time-gated digital optical frequency domain reflectometry. The bi-directional distributed Raman amplification is adopted to realize long measurement distance. The shape of optical pulse is pre-distorted to be a standard Hanning window, which provides a theoretical peak-side lobe ratio of 46 dB to suppress the crosstalk. The interference fading and polarization fading are well suppressed, and hence phase-demodulation method is adopted, instead of intensity demodulation method. As a result, the sensitivity is enhanced and the full information (amplitude, phase, and frequency) of the vibration can be obtained. In experiments, the fiber length is about 108 km, whereas the spatial resolution is 5 m. A weak vibration with peak–peak amplitude of 14.7 n $\varepsilon$ is correctly located at the distance of 98 km with a high SNR of 30 dB. It is the first time that 220-p $\varepsilon /\surd$ Hz strain sensitivity is realized over 100-km-level fiber and the vibration waveform is retrieved linearly without harmonics.

108-km Distributed Acoustic Sensor With 220-p $\varepsilon /\surd$ Hz Strain Resolution and 5-m Spatial Resolution

This paper proposes a distributed acoustic sensor (DAS) scheme, which is immune to the fading problem and can overcome the trade-off existing in traditional ϕ-OTDR between spatial resolution and sensing distance. An optical chirped pulse and non-matched filter method are used, and hence the spatial resolution is mainly determined by the bandwidth of the chirped pulse and non-matched ratio, rather than pulse duration. The Rayleigh interference pattern method is adopted here to quantitatively demodulate strain distribution along the whole sensing fiber, so there is no fading problem, which is a serious problem in the Rayleigh phase method. In proof-of-concept experiments, a DAS with 2-m spatial resolution and 10-km distance range is demonstrated. The response bandwidth of strain is 5 kHz, only limited by the fiber length. A ne-scale strain signal is detected at the far end of fiber with a high SNR of 35 dB.

Dian Chen

Papers

Phase-detection distributed fiber-optic vibration sensor without fading-noise based on time-gated digital OFDR

Distributed Fiber-Optic Acoustic Sensor With Enhanced Response Bandwidth and High Signal-to-Noise Ratio

High-fidelity distributed fiber-optic acoustic sensor with fading noise suppressed and sub-meter spatial resolution

108-km Distributed Acoustic Sensor With 220-p $\varepsilon /\surd$ Hz Strain Resolution and 5-m Spatial Resolution

Fiber-optic distributed acoustic sensor based on a chirped pulse and a non-matched filter.