Distributed Fiber Optic Sensing (DFOS) refers to a range of technologies that enable the measurement of physical variations along a fiber optic cable. The systems rely on backscattering phenomena within the fiber optic core. At each location along a fiber cable, a small portion of light is reflected towards the original source. Changes in the strain (elongation or shortening of the fiber), temperature, or vibration within the fiber optic cable can be detected in changes to the backscattered light. The DFOS systems are broadly divided by the parameters they are sensitive to and the associated backscattering phenomena that they rely upon:
- Distributed Acoustic Sensing (DAS), Rayleigh Scattering – sensitive to vibrations within the fiber
- Distributed Strain and Temperature Sensing (DSTS), Brillouin Scattering – sensitive to absolute strain within the fiber, which can be divided into mechanical and thermal strain (temperature)
- Distributed Temperature Sensing (DTS), Raman Scattering – sensitive to temperature within the fiber
Unlike conventional point sensors which only generate measurements at the location of the instrument, DFOS can generate measurements anywhere the fiber optic cable is installed. This can provide significant benefits over point-based sensors for many applications, including monitoring of long, linear assets such as railways, highways, bridges, tunnels, and levees; monitoring of diffuse hazards such as sinkholes, landslides, structural distress, or fire within a tunnel; and capturing the real behavior of an asset rather than an idealized version necessitated by interpolating between limited sensing points.
A DFOS system is comprised of two components: the fiber optic analyzer which generates the laser signal and captures the response, and the fiber itself. Each of these components must be carefully selected for both the monitoring application and their compatibility with the other. Regardless of which DFOS system you choose, there are some key factors to success to keep in mind:
Match the analyzer to your monitoring goals: The commercial DFOS analyzer market is mature and has many options across the sensing types. Each analyzer will have a range of advertised performance parameters; however, it is important to recognize that it is rarely possible to get the peak performance across all parameters at the same time. For instance, a system may advertise a very low spatial resolution (the length of fiber over which each individual measurement point represents), but this may only be available for short lengths of fiber, for example, under 300 feet. It is therefore important to understand the monitoring goals of the application and what performance can be expected from the analyzer, given the specific combination of sensing parameters. The main parameters that must be balanced are fiber length, spatial resolution, measurement output spacing (how often along the fiber a measurement point is output), reading duration/achievable reading frequency, and measurement accuracy/precision/repeatability.
Selecting the correct fiber: Each type of DFOS sensing requires choosing a type of fiber optic cable that will best capture the measurement parameter. For instance, when measuring strain with a Brillouin system, it is necessary to choose a tight-buffered fiber optic cable where any exterior mechanical strain is transferred without slippage to the fiber optic core within. Deploying a loose-tube or standard telecoms fiber for a strain application would produce poor strain data, however without recognition of this, the data may be treated as valid and used to inform engineering or operational decisions. Each type of fiber optic sensing requires a specific type of fiber optic cable, and choosing the correct one for each application is a critical step in the preparation for a successful monitoring project.
Proper installation technique: Even with the correct analyzer and fiber optic cable, the system performance may be limited by how and where the fiber optic cable is installed. Common pitfalls can range from placing the fiber in the wrong location (for example, on the horizontal axis of a pipeline when trying to capture vertical bending) or installing it using the wrong materials (for example, using a flexible epoxy to attach a fiber for strain monitoring). The successful design and installation of a fiber optic system requires careful selection of the installation technique and location to ensure that the monitoring variable (strain, temperature, vibration) is being transferred to the fiber core to be accurately measured.
Data management and analysis: One of the benefits of DFOS can also be a downside – generating huge amounts of measurement data. Fiber optic systems can often measure fibers that are tens of miles long, with output spacing as little as every foot. This means generating tens of thousands of data points for each reading, with systems that can take measurements every few minutes to dynamic sampling in the kilohertz range. It is therefore necessary to have a well-designed system to process the data, interpret any points of interest, and output a clear-to-understand metric. This can range from single or aggregated readings in engineering units to operational outputs and life-safety alarms. In short, the data from a DFOS system is only as good as the understanding that it can offer to its users and stakeholders.
The Geocomp team has extensive experience working with clients to select, design, install, and operate a range of DFOS systems to address unique project challenges. This includes fiber optic monitoring of levee health in Louisiana, sinkhole detection under active rail lines in Europe and the Middle East, aerial settlement monitoring in California, and foundation integrity and load testing across the US. We work closely with our clients to best understand their project requirements and assess when monitoring applications could directly benefit from the use of fiber optic sensing. If you have any specific questions or would like to discuss whether DFOS may be a suitable solution for your project, please contact our experts.
Post by: Andrew Yeskoo, PhD, PE, Senior Engineer for Massachusetts Consulting and Monitoring Divisions. Andrew completed his PhD at UC Berkely with research primarily focused on geotechnical and geostructural applications of distributed fiber optic sensing (DFOS).
