Last month, I wrote about types of fiber optic sensors that are used in specialized applications. Most of these affect the transmission of light in the fiber to allow the physical parameter to be sensed either along the entire length of the fiber or at discrete points where sensors are connected to the fiber. Many of these sensors can be attached in series along a single fiber to connect up sensors over a large area and monitored using an instrument such as an optical time domain reflectometer.
Fiber itself is a good sensor. It is sensitive to temperature, stress and strain (vibration and acoustics), which sometimes enables regular fiber optic cables to be used as sensors. Special fibers with enhanced sensitivity are used where regular fibers have inadequate sensitivity or special physical parameters need to be measured. Fiber can also be used to deliver light to specialized sensors that can measure temperature, velocity of a medium for flow measurement, linear or angular motion and chemical composition. And of course, fiber can be used to connect regular sensors with electronics that produce digital readouts.
The oil and gas industry are big users of distributed fiber optic sensors, in exploration, extraction and distribution. Oil field exploration uses acoustic sensors to monitor the underground structure of the earth in the areas of interest, picking up vibrations from explosive charges on the surface. During the extraction of oil and gas, down-hole sensors based on fiber optics can monitor the curing of the cement liner in a drilled well, the actuation of sleeves in a hole and movement of liquids in a well, increasing extraction efficiency and safety. This type of sensing has become much more sophisticated recently, allowing already installed fiber to provide new types data.
Much of the supply of oil and gas is transported by pipelines where monitoring by fiber is very important. Fiber optic cables are installed along pipelines to monitor temperature, strain, vibration and flow, providing real-time data that can spot leaks and sometimes potential problems before they happen. Leaks are usually detected by measuring temperature along the pipeline, since a leak causes a temperature drop in the gas or liquid being transported, allowing immediate detection and location of the leak. Fiber alongside the pipeline allows video surveillance and intrusion detection, another important issue for security.
Electrical utilities extensively use fiber optic sensors’ ability to monitor temperature and vibration. Since fiber is not affected by electrical interference, a fiber optic cable can be wrapped around electric transmission lines and run around transformers to measure temperature and vibration, allowing monitoring along the entire transmission and distribution route.
Fiber can also be used to connect special sensors, such as electrical sensors that can measure high voltage and current on transmission lines, providing another set of data to correlate with the distributed temperature sensor data, allowing decisions to be made with greater confidence about the electrical utility grid. And, of course, fiber optics can connect up surveillance and monitoring systems to monitor the security of utility equipment.
Most of what I’ve discussed are applications using fiber optics that require installing special types of cables. But even regular telecommunications cables have the potential to provide useful sensor data. For years, fiber optic testing techs have known that cables installed along railways or roads could sense vibration from passing trains or vehicles. One tech I know discovered test data from aerial cables correlated with wind speed.
A researcher at Lawrence Berkeley National Laboratory was studying thousands of miles of installed telecom cable to use in geohazard awareness, monitoring landslides, permafrost slumps, sinkholes and other natural phenomena. This investigation turned up another potential application, particularly useful in California: monitoring earthquakes.
Instead of using seismometers, which are single-point monitors and require expensive installation and operation and are unfeasible in some locations such as urban areas, installed fiber optic cable can provide seismic data. There is more than 100,000 miles of cables in the United States, most of which have dark fibers (spares) that can be fitted with laser interferometers and used as earthquake monitors.
It’s not as easy as it might sound and a lot of research still needs to be done to calibrate the cables as sensors. Research is being done at University of California, Berkeley, Stanford University and Lawrence Berkeley National Laboratory at test sites in California to gather data and develop systems to analyze the data to produce useful information.