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.
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.
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 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 small target is a target which is far enough to a detector, and its image on FPA can’t be large enough to show its shape and size. In this situation, when a small target is detected by an infrared imaging single-band detector, we can only analyze it by the dispersion point or the subpixel image caused by it. The target discrimination can be impossible when it meets a smaller-sized target with higher temperature and a larger-sized target with lower temperature, because their image on FPA can be quite similar when they’re far enough. However, with the dual-band detection, we can figure out the temperature via dual-band ratio easily, without the information of distance and target size. Equivalent area can be also figure out during this calculation. The target discrimination can be achieved with the temperature and equivalent area known. And according to some priori knowledge, can we make those target recognized in a particular scene. This article briefly show the benefit of dual-band detection compared to single band detection.