Monitoring and early detection of internal erosion: Distributed sensing and processing

International audienceEarly detection of leakages in hydraulic infrastructures is important to ensure their safety and security. Significant flow of water through the dike can be an indicator of internal erosion and results in a thermal anomaly. Temperature measurements are therefore capable of revealing information linked to leakage. Optical fiber-based distributed temperature sensors present an economically viable and reliable solution for recording spatio-temporal temperature data over long distances, with spatial and temperature resolutions of 1m and 0.05 C, respectively. The acquired data are influenced by several factors, among them water leakages, heat transfer through the above soil depth, seasonal thermal variations, and the geomechanical environment. Soil properties such as permeability alter the acquired signal locally. This article presents leakage detection methods based on signal processing of the raw temperature data from optical fiber sensors. The first approach based on source separation identifies leakages by separating them from the non-relevant information. The second approach presents a potential alarm system based on the analysis of daily temperature variations. Successful detection results for simulated as well as real experimental setups of Electricité de France are presented

Source separation and distributed sensing: The key of an efficient monitoring

International audienceAs a complement to classical sensors, Distributed Optical Fiber Sensors now play a prominent role in several engineering fields and act as an antenna array. Depending of the devices used (Raman, Rayleigh, or Brillouin scattering), measurement records depend on temperature, strain, or pressure profile. As the wanted signal is often hidden by noise and other undesired sources, we can express the problem as a source separation problem. In this paper, we show that with the help of recent techniques based on data decomposition and source separation (PCA, ICA, NMF techniques) from the virtual antenna, we can accurately identify water leakages from a noisy Raman spectra or a strain profile from Brillouin spectra with a spatial resolution of 1cm instead of 1 meter for classical devices

De l'importance des traitements préalables à l'application de l'Analyse en Composantes Indépendantes en spectroscopie Raman

La spectroscopie Raman est un outil puissant d'analyse de la composition moléculaire d'échantillons biologiques. La complexité de certains tissus rend l'analyse des spectres tributaire de méthodes d'extraction des informations pertinentes. L'Analyse en Composantes Indépendantes est montrée comme un outil efficace de séparation des spectres à condition que des prétraitements adaptés et développés dans ce papier soient appliqués afin de linéariser le modèle génératif des spectres déformés par l'instrumentation

Séparation de sources non négatives parcimonieuses appliquée aux spectres Brillouin acquis par capteur à fibre optique

National audienceStructure Health Monitoring (SHM) is an important issue in EDF. An optical fiber senor enables to acquire distributed strain measurements with a spatial sampling of 40cm over several kilometers. For every sensing point, a Brillouin spectrum is acquired centered on the Brillouin frequency sensitive to temperature and strain into the optical fiber. If the strain is non uniform within the integration base, then the spectrum will be distorted. A method of Non negative Matrix Factorization (NMF) enables to decompose it into several spectral components, centered on local Brillouin frequencies. A method enables to link their amplitudes to their positions within the integration base. Thus, the presented methodology enables to estimate a centimeter strain profile. It is successfully tested on experimental controlled data set. The comparison with a reference measurement enables to validate the presented methodology which permits a better sampling resolution and strain sensitivity.La surveillance d'ouvrage de grande ampleur est une problématique importante au sein d'EDF (Electricité de France). Un capteur à fibre optique permet de mesurer la déformation de manière répartie avec un pas de mesure de 40cm sur plusieurs kilomètres. A chaque distance interrogée, un spectre Brillouin est acquis. Ce spectre est centré sur une fréquence Brillouin sensible à la température et à la déformation imposée par l'ouvrage à la fibre optique. Si la déformation est non uniforme, alors le spectre est distordu. Nous proposons une méthode de Factorisation de Matrices Non négatives (FMN) afin de décomposer le spectre sur une base de spectres Brillouin élémentaires. Un algorithme basé sur leurs amplitudes relatives permet d'estimer la déformation avec une nouvelle résolution spatiale de l'ordre d'un centimètre. Cette méthodologie est testée sur des données expérimentales acquises en milieu contrôlé. La comparaison avec une mesure de référence montre la validité de cette approche

Nonnegative unmixing methodology applied on Brillouin Optical Fiber Sensor

International audienceAs a complement to conventional sensors, Distributed Optical Fiber Sensors (DOFS) have gradually played a prominent role in Structural Health Monitoring (SHM) for the last decade. The distributed Brillouin sensor enables to measure strain along kilometers of cable with a spatial resolution of 1 meter. The challenge is to have a centimeter spatial resolution to improve structure defaults detection and localization. A numerical model, based on the sensor physic, is first proposed in order to study Brillouin spectra distortion depending on strain distribution within spatial resolution. Then, based on nonnegative least squares (NNLS) problem, Brillouin spectra are decomposed into several elementary spectra. The estimated central frequencies and maxima permit to estimate a centimeter frequency distribution within the spatial resolution. It has been verified with numerical and experimental data: that method enables to enhance the accuracy and spatial resolution of the sensor from meter to centimeter

Quantification of sub-millimeter displacements caused by sinkholes, using distributed optical fiber sensors

International audience—The estimation of sinkhole-induced ground displacement is an important issue for monitoring soil structures. Distributed optical fiber sensors composed of an interrogator based on scattering effects in an optical fiber cable sensing element can be used to assess ground displacement. These sensors provide longitudinal strain measurements of the soil structure. This article proposes a methodology that enables estimation of displacement fields in the soil structure when a sinkhole appears. It also exposes an experiment which was carried out to create an artificial sinkhole instrumented by optical fiber sensors. This is the first time that those sensors are used to provide sub-millimeter vertical displacements. The first step of the methodology is to model the ground displacement under two-dimensional conditions. The longitudinal strain measured by a distributed optical fiber sensor can thus be linked to the displacement of the structure. This model is described by those parameters: the spatial extent of the displacement signature; a coefficient that depends on the interface between the optical fiber cable and the soil; the depth of the sinkhole; and the maximal vertical displacement. The second step consists of the estimation of each parameter independently. The spatial extension is given by fitting the measured strain signature with the empirical model. The depth of the sinkhole can be determined by measurement of the spatial extension of the ground-displacement profile at several observation depths in the structure. Finally, the vertical maximal displacement is furnished with high precision

Enhancement of an Optical Fiber Sensor: Source Separation Based on Brillouin Spectrum

International audienceDistributed optical fiber sensors have gained an increasingly prominent role in structural-health monitoring. These are composed of an optical fiber cable in which a light impulse is launched by an opto-electronic device. The scattered light is of interest in the spectral domain: the spontaneous Brillouin spectrum is centered on the Brillouin frequency, which is related to the local strain and temperature changes in the optical fiber. When coupled with an industrial Brillouin optical time-domain analyzer (B-OTDA), an optical fiber cable can provide distributed measurements of strain and/or temperature, with a spatial resolution over kilometers of 40 cm. This paper focuses on the functioning of a B-OTDA device, where we address the problem of the improvement of spatial resolution. We model a Brillouin spectrum measured within an integration base of 1 m as the superposition of the elementary spectra contained in the base. Then, the spectral distortion phenomenon can be mathematically explained: if the strain is not constant within the integration base, the Brillouin spectrum is composed of several elementary spectra that are centered on different local Brillouin frequencies. We propose a source separation methodology approach to decompose a measured Brillouin spectrum into its spectral components. The local Brillouin frequencies and amplitudes are related to a portion of the integration base where the strain is constant. A layout algorithm allows the estimation of a strain profile with new spatial resolution chosen by the user. Numerical tests enable the finding of the optimal parameters, which provides a reduction to 1 cm of the 40-cm spatial resolution of the B-OTDA device. These parameters are highlighted during a comparison with a reference strain profile acquired by a 5-cm-resolution Rayleigh scatter analyzer under controlled conditions. In comparison with the B-OTDA strain profile, our estimated strain profile has better accuracy, with centimeter spatial resolut ion

Parametric Inversion of Brillouin spectra using L-curve criterion to enhance the accuracy of distributed strain measurement

International audienceTo ensure stability and durability of engineering structure in natural soil, optical fiber sensors have gained interest over last decade. In addition to conventional geophysical sensors, Brillouin spectra based sensor enables to perform distributed strain measurement. Its algorithm performs a strain measurement with a 40cm spatial sampling over several kilometers. The monitoring of engineering installations needs a centimeter spatial sampling and a better strain accuracy. Previous works highlighted that the industrialized algorithm has great limitation for the exploitation of the local information contained into Brillouin spectra. Indeed, based on its asymmetry and broadening, it is possible to estimate local Brillouin frequencies with a better strain accuracy. We propose here to apply a parametric inverse method using L-curve criterion to estimate the strain with a 5cm spatial sampling. To validate this method, a one-to-one scale experiment has been implemented by optical fiber cable at several depths. Comparing the distributed strain provided by the Brillouin based sensor and our algorithm with a reference strain sensor, the proposed algorithm successfully fulfills the combination of a 5cm spatial sampling over kilometers and a high strain accuracy

Parametric Inversion of Brillouin Spectra to Enhance the Accuracy of Distributed Strain Measurement

International audienceTo ensure stability and durability of engineering structure in natural soil, optical fiber sensors have gained interest over last decade. In addition to conventional geophysical sensors, Brillouin spectra based sensor enables to perform distributed strain measurement. Its algorithm performs a strain measurement with a 40cm spatial sampling over several kilometers. The monitoring of engineering installations needs a centimeter spatial sampling and a better strain accuracy. Previous works highlighted that the industrialized algorithm has great limitation for the exploitation of the local information contained into Brillouin spectra. Indeed, based on its asymmetry and broadening, it is possible to estimate local Brillouin frequencies with a better strain accuracy. We propose here to apply a parametric inverse method using L-curve criterion to estimate the strain with a 5cm spatial sampling. To validate this method, a one-to-one scale experiment has been implemented by optical fiber cable at several depths. Comparing the distributed strain provided by the Brillouin based sensor and our algorithm with a reference strain sensor, the proposed algorithm successfully fulfills the combination of a 5cm spatial sampling over kilometers and a high strain accuracy