Sediments of the Changjiang River have been found in recent studies to be enriched in cadmium (Cd). The possibility and mechanisms for evaluating total Cd concentration and its binding form using reflectance spectroscopy within the visible-near-infrared (VNIR) region (400-2500 nm) have been investigated. Bottom sediments (69 samples) in the lower reaches of the river were collected for chemical analyses and spectral measurements. Total Cd concentration in the sediments was found to be exponentially related to the spectral proxies for organic matter(spectral reflectance at 400-530 nm), clay minerals (first derivative (FD) values at the shoulders related to absorption bands near 1400, 1900, and 2200 nm), and Fe oxides (FD values at 560-760 nm). The results indicated that the spectrally featureless Cd was mostly bound to these spectrally active materials, which made it possible for Cd concentration to be determined from reflectance spectra. This conclusion was also confirmed bythe results of chemically sequential extraction of Cd. This study has demonstrated the usage and theoretical basis of reflectance spectroscopy, which is a rapid and inexpensive analytical method, for evaluating contamination by heavy metals and their binding forms in sediments.
Phase-sensitive optical time-domain reflectometry (Φ-OTDR) has attracted numerous attention due to its superior performance in detecting the weak perturbations along the fiber. Relying on the ultra-sensitivity of light phase to the tiny deformation of optical fiber, Φ-OTDR has been treated as a powerful technique with a wide range of applications. It is fundamental to extract the phase of scattering light wave accurately and the methods include coherent detection, I/Q demodulation, 3 by 3 coupler, dual probe pulses, and so on. Meanwhile, researchers have also made great efforts to improve the performance of Φ -OTDR. The frequency response range, the measurement accuracy, the sensing distance, the spatial resolution, and the accuracy of event discrimination, all have been enhanced by various techniques. Furthermore, lots of researches on the applications in various kinds of fields have been carried out, where certain modifications and techniques have been developed. Therefore, Φ-OTDR remains as a booming technique in both researches and applications.
Ultra-weak fiber Bragg grating (UWFBG) array-based phase-sensitive optical time-domain reflectometry (Φ-OTDR) utilizes the interference interaction between the reference light and the reflected light from the broadband gratings for sensing. It significantly improves the performance of the distributed acoustic sensing system (DAS) because the intensity of the reflected signal is much higher than that of the Rayleigh backscattering. This paper shows that Rayleigh backscattering (RBS) has become one of the primary noise sources in the UWFBG array-based Φ-OTDR system. We reveal the impact of the Rayleigh backscattering signal on the intensity of the reflective signal and the precision of the demodulated signal, and we suggest reducing the pulse duration to improve the demodulation accuracy. Experimental results demonstrate that using light with a 100 ns pulse duration can improve the measurement precision by three times compared with the use of a 300 ns pulse duration.
We propose a new method which can measure large dynamic strain in Φ-OTDR based on ultra-weak FBG array, where the wrongly phase wrapping is corrected by the amplitude of the signal.
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