Beyond the Basics: Exploring the Advanced Applications of DFOS in Tunnel Monitoring

Introduction

As the Silkyara tunnel collapse in India unfolds, it underscores the urgent need for more advanced monitoring systems in tunnelling projects — systems capable of continuous, real-time, remote, and precise monitoring. This tragic event has brought into sharp focus the critical role of innovative technologies in ensuring structural safety.

Following our previous LinkedIn article on Distributed Fiber Optic Sensing (DFOS), there has been a notable surge in interest and curiosity. Many readers reached out, seeking a deeper understanding of this cutting-edge technology and its applications in areas other than tunnelling. Others inquired about its real-world applications in tunnel projects, and some were curious about how DFOS differentiates itself from traditional point-based measurement systems.

In response, this article aims to shed light on the intricate workings of DFOS, its distinct advantages over conventional methods and its versatile applications. While we will briefly introduce a tunnelling case study, a more comprehensive exploration of this application will be the subject of a subsequent article.

Let's delve into the world of DFOS, a technology that's reshaping our approach to structural monitoring and safety.

Acknowledgement: The cover image at the top of this article has been taken from the paper by Lienhart et al., 2019 (see "References" at the end of this article).

The Versatility of DFOS: Beyond Tunnelling

Distributed Fiber Optic Sensing (DFOS) technology offers a wide range of applications, extending its utility far beyond tunnel monitoring. This technology transforms standard optical fibers into highly sensitive 'nerves' that can be deployed on various structures for continuous monitoring. Here are some of the key areas where DFOS is making a significant impact:

1. Geotechnical and Structural Engineering: DFOS is crucial in monitoring complex civil engineering projects like piles, tunnels, embankments, slopes, landfills, dams, and dykes. It detects small changes early, continuously, and over thousands of measurement points, remotely and over the long term. This technology is widely used for structural health monitoring and in smart and intelligent structures. For instance, in concrete curing, DFOS takes the guesswork out of monitoring the temperature of concrete as it cures in large structures, critical for energy and cost-efficient builds. It also plays a vital role in pile load testing, optimizing foundation design, and reducing material requirements and CO2 emissions.
2. Renewable Energy: In the renewable energy sector, DFOS is used for monitoring wind turbines, solar farms, and other renewable energy infrastructures. It helps in ensuring the structural integrity and efficient operation of these installations.
3. Pipeline and Boreholes: DFOS technology is particularly effective in pipeline monitoring, detecting leaks, temperature changes, and potential structural weaknesses in pipelines. It's also used in borehole monitoring, providing valuable data for the exploration and extraction of natural resources.
4. Mining and Nuclear Waste Management: In mining operations and nuclear waste management, DFOS provides essential data for ensuring the safety and efficiency of these high-risk environments. It monitors the structural integrity of mines and storage facilities, detecting potential hazards early.
5. Power Cables: For power cable monitoring, DFOS technology offers a reliable solution for detecting faults and ensuring the efficient operation of power grids. It's particularly useful in monitoring the condition of underground and underwater power cables.
6. Research and Development: DFOS is also a valuable tool in research and development, aiding in the advancement of new technologies and methodologies in various fields, including civil engineering, environmental science, and energy.

Each of these applications underscores the adaptability and versatility of DFOS technology. By providing continuous, real-time data, DFOS is not just a monitoring tool but a critical component in ensuring the safety, efficiency, and sustainability of various structures and systems.

In-Depth Look at DFOS Technology

Recent advancements in fibre-optic sensor technology have made them highly effective for monitoring extensive linear structures such as tunnels, bridges, railway tracks, pipelines, dams, slopes, and embankments. Modern Distributed Fibre Optic Sensing (DFOS) systems are capable of accurately detecting strain and temperature variations over long distances, often spanning several kilometres. This capability is crucial for the early identification of localized issues like cracks and leakages. Additionally, these systems are instrumental in tracking long-term strain patterns, like deformations in tunnels, which aids in implementing condition-based maintenance strategies.

DFOS systems function by utilizing the natural back reflection properties of an optical fibre. When light of a specific wavelength, typically around 1550 nm, is introduced into the fibre, it generates linear (Rayleigh) and non-linear (Brillouin and Raman) backscatter effects (see Figure below). Raman backscattering is exclusively responsive to temperature changes, while Rayleigh and Brillouin backscattering are sensitive to both temperature and strain. Brillouin instruments can capture measurements over very long ranges (up to several tenths of kilometres) with a typical spatial resolution of 0.5–2.0 m, whereas Rayleigh strain instruments can measure only up to several tenths of metres but with a better spatial resolution in the range of a few millimetres.

Brillouin instruments are, therefore, considered the best for the purpose. This is at the heart of the patented Brillouin Optical Frequency Domain Analysis (BOFDA) for high-resolution loop measurements, and the patent-pending Brillouin Optical Frequency Domain Reflectometry (BOFDR), enabling measurement configurations with access to only one fibre end.

How it works

Optical fibres are inherently responsive to specific physical aspects of their surroundings Two of these are temperature and strain.

When light is transmitted through an optical fibre, external factors modify the characteristics of the light signal as it propagates along the fibre. A system comprising an interrogator (a read-out unit) and a sensing cable transforms a standard optical fibre into a continuous, long-range strain and temperature sensors. The optical fibre itself, usually in a protective cable, is the sensor.

These cables are then integrated into or attached to the structure that requires monitoring.

The interrogator unit, along with analysis software, processes the signal to extract the measurement data. Brillouin-based sensing read-out units provide a continuous profile of the strain and temperature distributions along the entire length of the sensing cable.

This setup allows for the monitoring of several tens of kilometres of sensing cable, achieving a spatial resolution of less than one meter, and can even reach a finer resolution of 20 cm over shorter distances. Under optimal conditions, the measurement accuracy leads to a repeatability of 2 μm/m for strain and 0.1°C for temperature.

A unique aspect of this technology involves injecting two light waves into the optical fibre from both ends. These waves intersect and cause the fibre to 'shiver', generating an acoustic wave within it. This acoustic wave alters the light that returns to the instrument, providing information about the fibre's speed of sound and, consequently, its strain and temperature.

The key to achieving this sensitivity, while addressing common issues of fibre attenuation, lies in the interaction between pump and probe lightwaves with the fibre's molecular vibrations (phonons). This interaction allows for the detection and localization of small strain and temperature events.

To ensure effective and adaptable implementation in various environments, including earth and rigid structures, application-specific fibre optic sensing cables are used. These specialized cables are designed for reliability and versatility in different monitoring applications.

Typical installation

• Loop configuration can be used for temperature compensation
• Fibre-optic switches for channel multiplexing
• Data transfer to control room by Ethernet or wireless connection
• Cloud-based data storage service

Fibre optic sensing cables for Distributed Sensing

A fibrisTerre system is capable of measuring both strain and temperature, with the distinction between these measurements being determined by the design of the sensing cable. For strain measurement, a specialized strain sensing cable is used. This cable incorporates a tight-buffered fibre, which is designed to translate the mechanical deformation of the structure or material directly into strain on the sensing fibre.

For temperature measurement, a dedicated temperature sensing cable is employed. This cable features a loose-tube optical fibre, which moves freely and includes extra fibre length. This design ensures that the fibre is mechanically decoupled from external strains. As a result, the optical fibre primarily responds to changes in environmental temperature, as heat is conducted through the protective layers and components of the cable.

Separating strain and temperature

The choice of cables and system configuration ensures strain and temperature events are detected, as shown below:

Performance Parameters

Reliability

Thanks to the Brillouin frequency-based sensing technique, once the instrument is calibrated, the measurements provide excellent long-term stability.

Robustness

The high signal-to-noise ratio means that optical signals can be detected, even when the fibre condition is challenging.

Accuracy and Sensitivity

Brillouin sensing is extremely sensitive. With calibrated sensing fibres, it provides measurement accuracy of better than 2 με (micro-meter per meter) for strain and better than 0.1°C for temperature.

Spatial resolution

Defines the size of the smallest change or event the system can detect. It is specified for a fibre as the minimum distance between two-step transitions of the fibre’s strain/temperature condition. A higher spatial resolution resolves more details.

Imagine being able to pinpoint a strain or temperature event to within 50 cm at a distance 25 km from where you are sitting or standing right now. The system we are talking about can do just that – and at 50 km distance, the resolution will be 1 m.

Resolution enhancement

Besides the selectable spatial resolution and spatial accuracy, there are instruments that offer a unique selectable post-processing technique to significantly increase the system’s ability to detect small strain and temperature events.

With this enhancement, a spatial resolution of 20 cm can be achieved for fibres up to 2 km length – ideal for geotechnical monitoring or civil engineering projects.

Distance range

Quite simply, how far do you want to measure? More than 50 km can be monitored with a spatial resolution of 1 m.

For very long distances or where you need highly accurate measurements, additional interrogator units can be connected. Up to 32 channels can be monitored by one interrogator using a switch.

Now just add to this performance an interrogator unit that weighs just 7 kg.

The instrument’s performance criteria are classified using the following key parameters (defined in EC 61757-2-2:2016 Fibre optic sensors – Part 2-2: “Temperature measurement – Distributed sensing”). Specific parameters for your application or project can be made available.

Acquisition time

The time it takes to obtain and process a measurement. For example, 2 km of fibre sensor is profiled in 1 minute. The measurements are continuous. This means that an event happening 2 km away from the interrogator will be ‘seen’ by the user or trigger an alarm 2 minutes later. Events can be tracked as they evolve.

Attenuation budget

The sensing cable has performance parameters too, which affect distributed sensing. The most common is attenuation, defined as “The decrease in optical power of a signal, or light wave, from interaction with the propagation medium, for example, absorption, reflection, diffusion, scattering, deflection, dispersion or resistance.” (Standard Terminology Relating to Optical Fiber Sensing Systems Designation: F3092 − 14).

Thanks to Brillouin Optical Frequency Domain Analysis developed and patented by fibrisTerre, their fTB 5020 range of instruments overcome a level of attenuation which would leave other measurement techniques in the dark.

Instrument reliability

The fibrisTerre unit is fan-less, fully sealed, and dust-proof, suitable for 19’’ instrument racks and on-site assignments.

Field friendly

Transportable, low power. Weighing only 7 kg and consuming 40 W, (the computer uses around 150 W), the fTB 5020 range is energy-efficient and transportable.

Advantages of DFOS: A Comparative Analysis

Distributed Fiber Optic Sensing (DFOS) technology offers a range of advantages over traditional monitoring methods. This section delves into a comparative analysis, highlighting how DFOS stands out in terms of precision, coverage, and long-term benefits.

1. Comprehensive Coverage: Unlike traditional point-based sensors that provide data from specific, discrete locations, DFOS offers continuous monitoring along the entire length of the fibre. This comprehensive coverage ensures that no critical changes are missed, providing a more complete picture of structural health.
2. Real-Time Data and Early Detection: DFOS systems offer real-time monitoring capabilities. This immediacy is crucial for early detection of potential issues, allowing for timely interventions. Traditional methods often involve periodic manual inspections, which can lead to delays in detecting problems.
3. High Sensitivity and Accuracy: DFOS technology is highly sensitive and accurate, capable of detecting minute changes in strain and temperature. This level of precision is often beyond the reach of conventional sensors, making DFOS ideal for applications where detailed monitoring is essential.
4. Durability and Longevity: Fibre optic sensors are inherently resistant to corrosion, electromagnetic interference, and harsh environmental conditions. This durability translates into a longer lifespan compared to traditional sensors, reducing the need for frequent replacements and maintenance.
5. Cost-Effectiveness Over Time: While the initial investment in DFOS might be higher than traditional methods, its long-term cost-effectiveness is notable. The reduced need for maintenance, combined with the longevity and comprehensive data collection, results in a more cost-effective solution over time.
6. Versatility in Application: DFOS is versatile and can be adapted to various environments and structures. This adaptability is a significant advantage over traditional sensors, which may have limitations in certain applications or environments.
7. Enhanced Safety and Risk Management: The comprehensive and accurate data provided by DFOS enhances safety and risk management. It allows for a more informed decision-making process, which is crucial in high-risk industries like mining, construction, and energy.
8. Integration with Digital Platforms: DFOS technology integrates seamlessly with digital platforms, including AI and IoT systems. This integration allows for advanced data analysis and predictive modelling, further enhancing the monitoring capabilities beyond what traditional methods can offer.

In summary, DFOS technology represents a significant advancement in monitoring and structural health assessment. Its comprehensive coverage, real-time data, high sensitivity, and integration with digital technologies make it a superior choice compared to traditional monitoring methods. As industries continue to evolve, the adoption of DFOS is set to increase, driven by its clear advantages in ensuring structural integrity and safety.

Application of DFOS in Tunnelling – a Case Study

In the next article, we will describe an example of the application of DFOS monitoring in Semmering Base Tunnel in Austria. However, a summary focussing on the key aspects of this case-study is given below:

1. Background and Tunnel Construction: The Semmering Base Tunnel, part of a historic railway line recognized by UNESCO, began construction in 2012. It's a complex project due to its geological and hydrogeological challenges. The tunnel features a two single-track tube structure, excavated using both drill and blast, and tunnel boring methods.
2. Strain Monitoring and Geotechnical Monitoring: DFOS was used for strain monitoring during excavation and construction to assess structural integrity and ensure safe operation. Dedicated fibre optic cables, embedded during construction, provided precise detection of strain and temperature changes, ideal for the harsh tunnel construction environment.
3. Application of DFOS in Tunnelling: DFOS provided displacement measurements during construction and continues to monitor the condition of tunnel and shaft linings, a large earth retaining structure, and relocated pipelines. The fibrisTerre system, based on Brillouin Optical Frequency Domain Analysis (BOFDA), monitored deformation in the shotcrete tunnel and shaft linings.
4. Installation and Data Analysis: Fibre optic cables were installed on the supporting wire meshes of shotcrete layers and their exact locations were recorded for precise monitoring. A novel proprietary shape sensing software was used to provide curvature profiles, comparing preconstruction models with actual data.
5. Results and Observations: Autonomous measurements began immediately after shotcrete installation, showing strain profiles within acceptable limits. The data confirmed the appropriateness of materials and techniques used, allowing safe continuation of construction.
6. Shaft Monitoring and Challenges: The project also involved monitoring two 240 m shafts, a challenging task due to complex structures and water ingress. DFOS was crucial in detecting early degradation of the shafts’ stability, overcoming challenges faced by other technologies in such environments.
7. Long-term Monitoring and Maintenance: The measurement campaign will continue until the end of construction, estimated at eight years, and will inform condition-based maintenance during the tunnel's 150-year operational lifetime.

This case study demonstrates the effectiveness of DFOS in providing critical, real-time data for tunnel construction and maintenance, ensuring structural integrity and safety.

Conclusion: Embracing the Future with DFOS

As we've explored throughout this article, Distributed Fibre Optic Sensing (DFOS) technology stands at the forefront of a new era in structural monitoring and safety. Its unparalleled precision, comprehensive coverage, and adaptability across various sectors underscore its significant advantages over traditional monitoring methods.

The versatility of DFOS, as evidenced in applications ranging from geotechnical and structural engineering to renewable energy and beyond, highlights its potential to revolutionize how we approach safety and efficiency in numerous industries. Its integration with digital technologies like AI and IoT paves the way for even more sophisticated monitoring and predictive analysis, further enhancing its value.

As we continue to witness the evolution of this technology, it's clear that DFOS is not just a tool for today but a foundation for tomorrow's advancements in structural health monitoring. Its role in enhancing safety, reducing risks, and improving decision-making processes is invaluable.

Looking ahead, our next article will delve into a specific case study of DFOS in tunnelling. This will provide a real-world example of how DFOS technology is applied in complex tunnelling projects, demonstrating its effectiveness in ensuring the safety and integrity of these critical structures.

In conclusion, the journey into the world of DFOS is just beginning. Its potential to reshape industries and improve safety standards is immense, and we are just scratching the surface of what this technology can achieve. Stay tuned for our next article, where we will explore the practical application of DFOS in tunnelling, further illustrating its impact and importance.

References

Lienhart W, Buchmayer F, Klug F and Monsberger CM (2019): Distributed fibre-optic sensing applications at the Semmering Base Tunnel, Austria. Proceedings of the Institution of Civil Engineers – Smart Infrastructure and Construction 172(4): 148–159.

Krohn D, McDougall T and Mendez A (2014): Fiber Optic Sensors: Fundamentals and Applications, 4th edn. SPIE Press, Bellingham, WA, USA.

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