A Chinese research team came from Shanghai Institute of Optics and Fine Mechanics (SIOM) of the Chinese Academy of Sciences (CAS), has developed a new method to enhance the practical performance of DAS for marine seismic detection. The result was published in Journal of Lightwave Technology on Oct.1, 2021.

DAS has many apparent advantages, including large coverage, high spatial-and-temporal resolution, and strong ambient adaptability, and it is widely applied in many fields, especially in marine seismic detection, ocean acoustic monitoring, etc. Thanks to the unique capability to form large scale and synchronous detecting array, DAS can make up for the lack of existing detection techniques.

The main challenges are insufficient signal-noise ratio (SNR) and reliability, which are originated from the inherent sensing mechanism of DAS. The former is due to the in-pulse interference of DAS sensing principle and the inhomogeneity of the refractive index of the fiber, while the latter is mainly limited by the extremely weak Rayleigh scattering coefficient of the fiber.

These problems greatly limit the reliability of DAS in existing application fields and are not good for DAS to be widely used in weak signal monitoring.

The research team proposes a multi-channel data fusion method based on distributed sensing paradigm, which breaks through the inherent physical limitations of DAS and realizes the practical performance enhancement.

Since 2017, the team is based on the distributed densely detection characteristics of DAS unique research work, in-depth cognition and mining disturbance source location information, and the internal connection of distributed sensing data spatial correlation, successively from positive and negative association thinking, realize and give the DAS distributed stereotaxic and directional listening ability. The academic results have been published in Optics Letters [2019, 44(7):1690-1693; 2020, 45(20):5672-5675] and have been reported by international mainstream media such as Laser Focus World [2020, 11, 16-17].

Based on the previous work, the research team proposed a new paradigm of distributed sensing, and attempted to use this correlation to solve the inherent physical limitations of DAS and achieve performance upgrades and application implementation. Based on the spatial diversity idea, the channel independence of signal fading is utilized to solve the problem of signal fading by using the method of equal gain synthesis. The SNR and sensitivity of the system can be improved by using multi-channel beamforming based on the idea of spatial reuse and channel independence of system noise. This result successfully applied the new paradigm of distributed sensing to DAS technology and realized the suppression of signal fading and the improvement of sensitivity. It is expected to solve the problems of poor reliability and low detection ability of weak signals in practical applications and promote the large-scale application of DAS, especially in marine seismic detection, ocean acoustic monitoring, etc.

This work was supported by National Key Research and Development Program of China, National Natural Science Foundation of China, Strategic Priority Research Program of Chinese Academy of Sciences, and Natural Science Foundation of Shanghai.

Figure. Experiment results of distributed multi-channel signal integration. (a) Results of signal fading suppression; (b) Results of system sensitivity enhancement. (Image by SIOM)