KEYWORDS: Data acquisition, Ocean optics, Time-frequency analysis, Optical fiber cables, Digital recording, Signal processing, Sensing systems, Data processing, Optical sensing
Distributed acoustic sensing (DAS) technology holds considerable potential for marine geophysical surveys by transforming standard optic cable into dense arrays of seismo-acoustic sensors and gathering seismic data of shallow stratigraphic profiles. Here, a field trial with an abyss-class DAS system was carried out at 1423 meters depth in the South China Sea, and more than 600GB data was collected for 21 days. The vibration events of the marine integrated experimental base station were recorded, including entering, moving, landing and raising. The time and spectral domain characteristics were analyzed to reveal the station working statuses. The sea trial demonstrates the capability of the abyss-class DAS system for long-term seismic data acquisition in deep-sea environments.
Recent trends in long distance phase-sensitive optical time-domain reflectometer (φ-OTDR) have led to a proliferation of studies on nonlinear effects. We explores stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), selfphase modulation (SPM) and modulation instability (MI) in the φ-OTDR system both theoretically and experimentally. Results show that the analytical model of above nonlinear effects have significance for practice reference, except for MI in the case of long distance. The depletion of power and amplified spontaneous emission power density as the most effective consequences of modulation instability must be taken into account. Compared with theoretical analysis, numerical simulation provides more accurate results, and experiments support the existence of unreported intrinsic spectral component of MI around the signal.
KEYWORDS: Acoustics, Spatial resolution, Signal detection, Signal to noise ratio, Demodulation, Data storage, Sensing systems, Data processing, Data acquisition, Backscatter
High-resolution submarine seismic survey techniques have become necessary in many actual geological and geophysical investigations. A new type of mini-distributed acoustic sensing (DAS) module is developed for working at the bottom of the sea with several kilometers long single-mode fiber cable for tens of thousands of channels at the same time. Integrated designs of optics and electrics help to significantly reduce volume and power consumption. Compared with a common land-based DAS system, the size and power consumption of the mini-DAS module are significantly optimized. The size is 150 × 300 × 110 mm3 (width × depth × height), and the power consumption is down to 25 W. The spatial sampling resolution of ∼0.8 m is retained for high-resolution seismic profile in the deep sea survey. The upper limit of response frequency is set by 500 Hz for the channel sample rate of 1 kS / s to realize the long-term data storage. It presents a powerful signal acquisition ability with the average system noise of 4.79 × 10 − 4 rad / √Hz and the minimum detectable strain is 10.4 pε / √Hz. The innovative mini-DAS module has high enough capabilities for real-time seismic wave signal detection in deep sea.
Fiber optic distributed acoustic sensing (DAS) is a novel technology for seismic data acquisition and particular suitable for vertical seismic profiling, near surface structure imaging and natural seismic monitoring. In this paper, a series of field trials verifying the performance of DAS for detecting marine seismic data are conducted in the geological condition of lagoon. The results show DAS can detect surface waves and longitudinal waves generated by artificial earthquake, and the surface waves have higher main frequency and wider bandwidth and the wave field information of seismic data is more abundant compared to geophone under the same source and the same offset condition.
High resolution submarine seismic survey techniques have become necessary in many actual geological and geophysical investigations. A new type of mini- distributed acoustic sensing (DAS) module is developed for working at the bottom of the sea with several kilometers long single-mode fiber cable for tens of thousands channels at the same time. Integrated designs of optics and electrics help to significantly reduce volume and power consumption. Compared with common Land-based DAS system, the size and power consumption of the mini-DAS module are significantly optimized. The size is 150mm x 300mm x 110mm (Width x Length x Height), and the power consumption is down to 25W. The spatial sampling resolution of ~0.8m is retained for high resolution seismic profile in the deep sea survey. The upper limit of response frequency is set by 500Hz for the channel sample rate of 1000S/s to realize the long term data storage. It presents a powerful signal acquisition ability with the average system noise of 4.79×10-4 rad/√Hz and the minimum detectable strain is 10.4pε/√Hz. The novel mini-DAS module has high enough capabilities for real-time seismic wave signal detection in deep sea.
Fiber optic distributed acoustic sensing (DAS) based on phase-sensitive optical time-domain reflectometry (Φ-OTDR) technology has been widely used in safety monitoring areas including monitoring of oil/gas pipes, communication or power cable, perimeters and so on, however it suffers from the high nuisance alarm rate (NAR) due to the non stationarity characteristics of signal and the interference of external environment. In this paper, GMMs-HMMs is utilized to reduce nuisance alarm rate, we prove that short time signal unit of appropriate length can contain the main frequency domain characteristics of signal, GMMs-HMMs is efficient recognition method for frequency domain sequence of signal. the experience results show the average recognition accuracy rate is 88.89% for seven events.
A series of theoretical study of HWC for distributed acoustic sensing system is proposed to optimize the gaugh length. With the considition of SNR and spatial resolution, the optimal gaugh length is 0.4/(sinαcosθ) to 0.5/(sinαcosθ) times of the Ricker wavelet’s spatial wavelength, when the Ricker wavelet seismic wave travelles to the HWC with the wrapping angle α and the incident angle θ. Similar with the situation in DAS with straight fiber, the optimal gaugh length can achieve the SNR bigger than 90% of the maximum, and the deviation of detected temporal wavelength is less than 14%. Additional coefficient of 1/sinαcosθ is provided to any imping angle of the seismic wave and wrapping angle for the normal working environment.
A theoretical model combined with finite element simulation and numerical analysis is presented to design and optimize of the fiber-wrapped mandrel optic microphone based on distributed acoustic sensing (DAS). To increase the acoustic pressure sensitivity, the optimized fiber-wrapped mandrel microphone with engraved uniform grooves is fabricated and tested as the acoustic transducer. The average sensitivity is -136.915dB re:1rad/μPa between 50 Hz to 2450Hz, which is 25.306dB higher than the same fiber optic microphone without uniform grooves. The experimental results are in good agreement with the theoretical results, which shows this theoretical approach is effective to design and optimize the fiber-wrapped mandrel optic microphone based on DAS.
A real-time distributed acoustic sensing system with ten thousand channels is proposed to detect dynamic signal along the fiber. The phase-sensitive optical domain reflectometry and phase-generated carrier algorithm are used to acquire the phase information of Rayleigh backscattering along the whole fiber. The sensing length of this system could be 10km with the sample resolution of 0.4m, which means that up to 25000 channels signal processing is realized in real time with Field-Programmable Gate Array module and host computer. The working principle of coherent Rayleigh backscattering interference, phase-generated carrier algorithm and the signal processing flow are introduced, and the experimental results are given in this paper.
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