Fibre optics monitoring
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Principle
Fibre optics are known to be excellent telecommunications cables. In other words, when a light signal is emitted from one end of them, it propagates very quickly over long distances, with virtually no loss.


Any light signal passing through a fiber necessarily interacts with its constituent elements. These interactions result in very small, successive reflections of light – truly very small, but nonetheless measurable.

To simplify, these reflections are proportional to a function of the fiber's temperature and its length variation along the fiber's axis, where the interactions occur. Thus, it is sufficient to determine – using devices called « optoelectronic interrogators » – the characteristics of the emitted signal and the successively reflected signals to transform the fiber into a series of distributed longitudinal thermometers and extensometers, capable of continuously measuring the temperature and the fiber's elongation/contraction along its entire length.

Indeed, the area near the end of the fibre where the transmitter is located reflects the first received echo, then the one emitted immediately after, and so on, until the echo emitted by the other end of the fiber.

In practice, with the best industrial optoelectronic interrogators currently available on the market, accuracies of 0.1°C per meter every 10 minutes, and 2×10⁻⁶ per meter every 10 minutes – that is, 2 µm over 1 meter along the length of the fiber (within the limits of deformations greater than 10⁻², which break the fibers) – can be achieved every 10 minutes. By combining these measurements with high-frequency acquisitions, the fibre can be transformed into an acoustic sensor.

The fibers of these « fibre optics monitoring systems » can be buried in structures or bonded to their surface. They can therefore be used, for example, to measure temperature, deformation and acoustic or seismic noise present on virtually any type of structure (dikes, dams, tunnels, bridges, roads, pipelines, high-voltage pylons, buildings, etc.).


This type of distributed sensor has the advantage of being virtually insensitive to electromagnetic noise and radiation. It is also stable over time – since we now have access to fibres installed more than 50 years ago. Finally, it is economically competitive as long as the area to be monitored is on the order of a few kilometers in size (given that the cost of fibres adapted to these applications is generally on the order of a few euros to a few tens of euros per meter, while the instrumentation costs from a few tens to a few hundred thousand euros, and operation is very simple, therefore inexpensive).
Advantages of fibre optics monitoring
While traditional monitoring methods (visual inspections, piezometers, etc.) are usually sufficient to prevent risks, this is not always the case.

For example, the Teton Dam in the United States, 93 meters high, broke on June 5, 1976, due to internal erosion. Only 3.5 hours elapsed between the appearance of the first visible pipe from the outside and the complete collapse of the structure. The operator responsible for monitoring did not detect the pipe's appearance early enough to secure the dam.

In such a case, a monitoring system providing access to higher spatial and temporal sampling (on the order of meters and hours, respectively) would have been beneficial.

Temperature and deformation have long been known to be good indicators of structural problems.

Continuous temperature monitoring within the structure, in all areas potentially intersecting leak zones, would have allowed for the detection of the first signs of erosion, even before any visible damage appeared from the outside.

Fibre optics monitoring enables precisely such monitoring, at a reasonable cost for long distances. This is achieved by burying a fibre optics cable at the base of the structure or within the drains installed inside it, and the recorded thermal anomalies can reasonably be considered as indications of leaks.


When the fibre optics cable contains electrical wires, it is possible to locally heat the optical fiber before thermal measurements simply by injecting an electrical current prior to the optical measurements. This allows for the detection of any leak, including those that, unfortunately, happen to carry fluids that are at the same temperature as the surrounding material. Indeed, by measuring the cooling of the fibre optics immediately after the current injection stops, leak zones appear where cooling is most rapid, precisely when the leaks are drained by the fluids carried by the leaks.



This technology can therefore be deployed directly behind the geomembrane, where the temperature is uniform. It then allows for the immediate localization, as soon as they appear, of the tears causing the leaks detected in the drains – and thus avoids having to spend colossal sums in terms of lost revenue to drain the system in order to reveal the tear(s).

Of course, the same fibre optics cable can be used to detect micro-deformations or acoustic noises that are precursors to other problems (sinking, etc.).

These monitoring systems also allow – and sometimes primarily – for a different approach to preventive and long-term maintenance of the structure. Indeed, by continuously recording everything that happens within the structure throughout its lifespan, they make it possible to identify, during rehabilitation projects, where areas need reinforcement and, conversely, where no problems have ever appeared and therefore nothing needs to be rebuilt (whereas, all too often, the entire length is rebuilt, even if it remains largely in good condition).


Similarly, data via fibre optics can guide routine maintenance operations (such as the order in which visual inspections should be carried out, etc.).
Key features of fibre optics monitoring systems
1. A well-considered design, accompanied by quantitative simulations demonstrating the system's ability to perform satisfactorily under real-world operating conditions

A fibre optics cable does not have an infinite range : beyond a certain distance, it becomes ineffective and can no longer detect the anomaly being sought. Installing fibres everywhere would not be a solution, as it would unnecessarily increase the project's cost.

Therefore, the first step when embarking on a design project is to precisely define the quantitative objectives.

This step is carried out with the project owner, in consultation with the pool of specialists involved (structural safety expert, site geologist, structural engineer, environmental engineer, legal department – ​​not forgetting, of course, the monitoring team, responsible for establishing the feasibility of a monitoring system designed to meet the identified needs).

Next, depending on the hazard the monitoring system will have to track (and the quantitative value of the anomaly to be detected), it is necessary to ensure that the anticipated system will indeed be capable, under real-world conditions, of detecting the targeted minimum anomaly.

This step most often requires realistically simulating the structure and ensuring that, when confronted with the targeted hazard, the proposed system actually detects the targeted anomaly.

Then, the technical feasibility of the proposed system must be guaranteed, and its investment and operating costs must be determined to ensure that it can be financed and will remain operational throughout its anticipated lifespan.

This step is usually carried out with the operator's finance department, the only ones qualified to put a price on reliability, in conjunction with the design team, which is best positioned to determine the costs of the monitoring system (provided it has been properly selected and is fully competent in its field).

Finally, when the installation of the new monitoring system is planned as part of a project already decided upon and justified by other considerations, it is essential to ensure that the addition of new tasks related to the implementation of said monitoring system will not, under any circumstances, delay the project.

This step most often requires the involvement of recognized specialists in the implementation of the technology, who are the only ones capable of anticipating potential site problems and implementing effective and recognized preventative measures.



A well-executed design often makes it possible to drastically reduce both investment and operating costs.



2. Proven supervision of the installation of the monitoring system

Adding fibre optics to a civil engineering project is often a source of concern, given the inherent fragility of fibres and the fact that everything is buried at the end of the project (meaning all the underground data necessary for operation must be collected before the project's completion).

Proven methods now guarantee that the installation will be compliant and delivered on time, even if the construction companies selected by the tender committee lack experience in fibre optics and are likely to use fibre optics subcontractors from the telecommunications sector (where adhering to a highly optimized optical budget is usually not a concern).

This step requires the involvement of recognized fiber optic testing specialists (who are currently extremely rare in the market).



3. Acceptance of the monitoring system according to best practices

Civil engineering project managers rarely have experience with fibre optics monitoring system installations. Therefore, it is not easy for them to determine whether the delivered project will actually meet specifications.

Similarly, structure owners rarely have experience with fibre optics monitoring systems. Consequently, it is generally not easy for them to identify the key points to monitor during the handover process.

This stage requires the support of acknowleged fibre optics monitoring specialists, most often the design team, which is generally best positioned to identify the key points that must be closely monitored during the handover.

To ensure that the design team cannot impose its will on the client during the operational phase, it is vital that the acceptance process includes the collection of all documentation related to the monitoring system. This includes design data, installation plans, documentation in local language for the instruments used and supplier contact information, the data formats, the sources of the data formatting codes, and, of course, the site data files collected during the construction phase, particularly as-built drawings.

This step must be clearly and comprehensively described in the Special Technical Specifications document of the tender prepared by the design team.



4. The first year of operation must be entrusted to the design team, to ensure proper determination of the detection thresholds to be used during the operational phase.

The local response of a site cannot always be determined with sufficient accuracy during the design phase.

Therefore, monitoring devices are most often equipped with calibration systems that simulate the targeted anomaly within the structure itself (for example, by injecting water into the structure to simulate a leak, in the case of a dam where leaks are a concern).

These devices are designed to determine the site's response, without which it is impossible to quantify the chosen precursor signal of the problem once the structure's model parameters have been correctly calibrated.

In practice, this step can only be performed by the design team.



5a. A robust operating strategy, ensuring that the infrastructure remains protected from malicious attacks and is effectively monitored during times of crisis

A fiber optic monitoring system requires minimal human resources during operation.

However, it is important to remember that during crises, monitoring teams are generally overwhelmed by emergencies (such as, for example, providing regular reports to the authorities, assisting users of the facility, etc.).

It is precisely during these times that hackers most often emerge, hoping to drive up the price of their attacks to avoid carrying out their threats.

Finally, it is during these times that the facility is under the greatest strain, making it crucial to thoroughly analyze its monitoring data, which reveals the facility's behavior in extreme situations and therefore its actual safety level.

The surveillance organization put in place must ensure that not only is the system operated by skilled personnel and its access permanently secured, but also that it is subject to guaranteed monitoring and, if possible, reinforced monitoring during times of crisis.



5b. A robust operating strategy, ensuring that the infrastructure remains protected from malicious attacks and is effectively monitored during times of crisis

Although highly reliable, fibre optics monitoring systems requires regular maintenance, if only to retrieve supplementary data from the monitoring system (most often required for data analysis), keep the instruments operational, and ensure proper integration and continuous backup of the latest acquired data in the site's database.

This step is crucial for the long-term return on investment of the monitoring system. It requires the structures owner to maintain their operating budgets over time.

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