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Distributed Fibre Optic Sensing for Monitoring and Testing of Industrial and Civil Infrastructures
(2018)
Fibre optic sensors enable the gathering of sensory information in environments where conventional sensors fail or simply can not be utilised. Their small size enables their use in confined spaces as well as their embedding or integration in components, structures or composite materials while offering, depending on the approach, high sensitivity, very high spatial resolution and, especially, the possibility to detect or measure physical quantities in a distributed way, meaning spatially continuous sensing over very long distances.
Those distributed fibre optic sensors (DFOS) provide access to sensing information from different locations along the sensing fibre without the need for technologically challenging multiplexing of many point sensors and the subsequent sensor data fusion. In this way, DFOS are excellently suited as smart sensors for monitoring or testing purposes as the spatially distributed sensing data make available spatially resolved measurand profiles without a gap. Furthermore, the possible embedding or integration of DFOS could facilitate the development of smart structures or smart components. In particular distributed vibration sensing (DVS) and distributed temperature sensing (DTS) are also ideally suited for condition monitoring of machinery, industrial installations or extensive infrastructure, also, on-line real-time assembly or component testing in production lines, e.g. for quality control, is possible.
In this talk, we will shortly outline the underlying principles of some fibre optic sensing approaches, explain their fundamental differences in mode of operation and performance, and discuss their applicability and limitations. Moreover, we will report on a subset of current and previous applied research projects at BAM, investigating different areas of use for distributed fibre optic sensors in particular.
Anwendungsfelder von verteilter faseroptischer akustischer Sensorik (DAS) werden anhand von früheren, gegenwärtigen und zukünftigen Projekten an der BAM vorgestellt und die Leistungsfähigkeit von DAS demonstriert. Die dargestellten Anwendungen sind Monitoring von Rohrleitungen, von Unterseekabeln, von Zügen und Gleisen, von Brücken, von Strassen und Verkehr, von Erdbebenauswirkungen in urbanen Gebieten, von Bohrlöchern im Kontext Erdgasspeicher und für Exploration und Monitoring von Untergründen für Geothermie-Anwendungen.
Die Eigenschaften von optischen Fasern und faseroptischen Sensoren werden beschrieben und die Funktionsweise von verteilter faseroptischer akustischer Sensorik (DAS) erklärt. Die Verwendung von DAS für Zustandsmonitoring vor allem von Infrastruktur und Anlagen wird motiviert. Im Folgenden wird die Einsetzbarkeit von DAS für verschiedenste Anwendungen anhand einiger Beispiele (vergangene und gegenwärtige Projekte der BAM zum Thema DAS für Infrastrukturmonitoring) gezeigt. Diese betreffen das Monitoring von Rohrleitungen/Pipelines, von Energiekabeln, von Gleisanlagen und Zügen, von Brücken und von Strassen und Strassenverkehr.
The feasibility of traffic monitoring along a major urban road using Distributed Acoustic Sensing (DAS) is demonstrated. We present measurement results of conventional intensity-based DAS along a dark fiber in a tubed fiber optic cable buried roadside. With only minimal postprocessing different classes of traffic participants can be distinguished (cars, bikes, pedestrians). Different methods for pre-processing raw data are compared, especially with regard to providing suitable inputs for pattern recognition algorithms. Furthermore, the various challenges for automatized vehicle detection and classification related to varying sensitivity and inhomogeneous signal propagation are discussed. Sensitivity fluctuations and variations are in part inherent to the measurement technology and originate in part in ground conditions. The measurement quality and usefulness for traffic monitoring of intensity-based DAS is compared to that of state-of-the-art phase-resolved DAS, allowing quantitative evaluation of vibration signals.
Division 8.6's competencies and work focus areas are shortly outlined and investigations related to pipeline and submarine power cable monitoring using different distributed fiber optic sensing techniques are presented with some results. Furthermore, BAM 8.6 expertise in the field of sensor application, embedding and integration is shown.
The Alpine Fault at the West Coast of the South Island (New Zealand) is a major plate boundary that is expected to rupture in the next 50 years, likely as a magnitude 8 earthquake. The Deep Fault Drilling Project (DFDP) aimed to deliver insight into the geological structure of this fault zone and its evolution by drilling and sampling the Alpine Fault at depth. Here we present results from a seismic survey around the DFDP-2 drill site in the Whataroa Valley where the drillhole almost reached the fault plane. This unique 3D seismic survey includes several 2D lines and a 3D array at the surface as well as borehole recordings. Within the borehole, the unique option to compare two measurement systems is used: conventional three-component borehole geophones and a fibre optic cable (heterodyne Distributed Vibration Sensing system (hDVS)). Both systems show coherent signals but only the hDVS system allowed a recording along the complete length of the borehole.
Despite the challenging conditions for seismic imaging within a glacial valley filled with sediments and steeply dipping valley flanks, several structures related to the valley itself as well as the tectonic fault system are imaged. The pre-processing of the seismic data also includes wavefield separation for the zero-offset borehole data. Seismic images are obtained by prestack depth migration approaches.
Within the glacial valley, particularly steep valley flanks are imaged directly and correlate well with results from the P-wave velocity model obtained by first arrival travel-time tomography. Additionally, a glacially over-deepened trough with nearly horizontally layered sediments is identified about 0.5 km south of the DFDP-2B borehole.
With regard to the expected Alpine fault zone, a set of several reflectors dipping 40-56° to the southeast are identified in a ~600 m wide zone between depths of 0.2 and 1.2 km that is interpreted to be the minimum extent of the damage zone. Different approaches image one distinct reflector dipping at 40°, which is interpreted to be the main Alpine Fault reflector. This reflector is only ~100 m ahead from the lower end of the borehole. At shallower depths (z<0.5 km), additional reflectors are identified as fault segments and generally have steeper dips up to 56°. About 1 km south of the drill site, a major fault is identified at a depth of 0.1-0.5 km that might be caused by the regional tectonics interacting with local valley structures. A good correlation is observed among the separate seismic data sets and with geological results such as the borehole stratigraphy and the expected surface trace of the fault.
In conclusion, several structural details of the fault zone and its environment are seismically imaged and show the complexity of the Alpine Fault at the Whataroa Valley. Thus, a detailed seismic characterization clarifies the subsurface structures, which is crucial to understand the transpressive fault’s tectonic processes.
Fiber optic Distributed Acoustic Sensing (DAS) is an emerging method for many different monitoring purposes, enabling a spatially and temporally resolved collection of acoustic and vibration information over many kilometers. DAS, thus being a "dynamic" sensing technique, allows for online condition monitoring and the detection and localization of threats or hazards in real time via characteristic acoustic/vibration states and their changes or via occurring anomalous signals, respectively.
At BAM, we have employed this technology for a number of different applications of monitoring of large infrastructures, e.g., bridges, pipelines, submarine power cables or railway tracks. Currently, we are investigating the use of DAS for further innovative uses, which aim at facilitating the energy transition, enabling "smart" infrastructure and providing the basis for comprehensive hazard monitoring and warning systems, respectively. Our research fields include fiber optic borehole monitoring in the context of hydrogen storage caverns, traffic and road monitoring, using DAS for earthquake monitoring in urban areas for disaster management and long-term monitoring of large-scale subsidence caused by mining activities. Another new topic is structural health monitoring in concrete structures in the framework of the FSP Security.
In this talk, we will first briefly present the basics and capabilities of DAS. Then, we will portray our previous and current works related to this technology and show some attractive results. Finally, we will discuss our upcoming projects on exciting new applications of DAS.
Die verteilte faseroptische akustische Sensorik (DAS) wird vorgestellt, hinsichtlich ihrer Leistungsfähigkeit beschrieben und verschiedenste Anwendungsfelder, z.B. für das Zustandsmonitoring oder für seismische Messungen, beispielhaft gezeigt. Zudem werden mögliche Anwendungsfelder von DAS für den Kontext Kerntechnische Entsorgung (KTE) und Rückbau aufgezeigt.
After describing the basics of distributed fiber optic sensing in general and distributed fiber optic acoustic sensing (DAS) in particular, several currently and previously investigated fields of application of DASin infrastructure monitoring and seismic monitoring are presented. A focus is placed on seismic measurements and DAS measurements determining the dynamic responses of concrete structures to certain stimuli.