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The fiber optic sensors also called as optical fiber sensors use optical fiber or sensing element. These Sensors can measure a large variety of parameters, such as temperature, pressure, strain, refractive index, vibrations, displacements, bending, loading, and liquid level or concentration of chemical species.
Over the past decade, optical fiber sensors have gained widespread acceptance within the oil and gas industry due to their reliability, flexibility and low operating costs, as well as the benefits brought by their multipoint and distributed sensing capabilities. More recently, extensive interest has been shown in the development and commercialization of fiber optic seismic and acoustic sensing arrays – land and underwater – for oil and gas exploration, pipeline surveillance, geophysical monitoring, reservoir monitoring and management, geothermal monitoring, and structural monitoring of offshore platforms and oil tankers. Distributed acoustic sensing (DAS) enables detection, discrimination and location of acoustic events on an optical fiber over tens of kilometers.
Distributed optical fiber sensors (DOFS) offer unprecedented features, the most unique one of which is the ability of monitoring variations of the physical and chemical parameters with spatial continuity along the fiber. Distributed optical fiber sensors are receiving an ever-increasing interest as they offer the unique and unparalleled ability of mapping diverse physical fields along the area span by an optical fiber. Up to a million of sensing points can be interrogated with a single distributed optical fiber sensor, covering distances than can range from some tens of meters to a few hundreds of kilometers, with a spatial resolution of meters down to millimeters.
Distributed optical fiber sensors (DOFS) can measure lots of sensing parameters distributedly along the fiber under test (FUT) such as strain, stress, vibration, temperature, 3D shape, flow rate, refractive index, magnetic field, radiation, gas, etc, which are very useful for many sensing applications. Presently, most of the research groups have moved their focus towards distributed sensors, capable of detecting various dynamic parameters including dynamic strains, electromagnetic fields and sound waves.
One extremely beneficial use for fiber laser sensors is in the development of seismic sensor arrays that can operate in underwater conditions, making it useful in earthquake detection, borehole monitoring, and ocean bottom cabling systems. Acting like a hydrophone, it can detect seismic signals reliably and therefore is also useful in towed array sonar systems, which can typically be found towed behind submarines or surface ships. This is useful for many industries, especially security, military, and oilfield-exploration.
Several DOFS technologies have been developed based on the measurement of intrinsic backscattering, including Raman, Brillouin and Rayleigh scattering, which are based on optical time domain reflectometry (OTDR) and optical frequency domain reflectometry (OFDR). In OTDR, the input pulse generated is launched into the fiber under test (FUT). As the input pulse propagates along the FUT, the light from the pulse that gets back-reflected light either via Rayleigh backscattering or Fresnel reflection is then measured by using a photo-detector. The sensing range to the point of reflection is dependent on the time delay between the input light pulse and the corresponding back-reflected light.
Among all the distributed sensing techniques, OFDR has been given tremendous attention because of its high spatial resolution and large dynamic range. A basic OFDR configuration consists of a tunable laser source (TLS) which optical frequencies can be tuned linearly in time without any mode hops and an interferometer that comprises a test path and a reference path. The reference path is considered as a local (LO) oscillator whereas the FUT is connected to the test path.
Interferences are generated between the LO signals and back-reflected light in FUT coming from the test path that contains Rayleigh backscattering and Fresnel reflection. The beat frequencies are obtained by a Fourier transform applying to the interferences signals. If the tuning rate of the TLS is a constant, the beat frequencies are proportional to the length of FUT
Infrastructure monitoring
Technology using fiber sensors has been used for structural health monitoring of large-scale infrastructures such as bridges, dams, and tunnels, as well as the monitoring of the efficiency and the performance of the equipment being used.
Fiber-optic sensors could soon be deployed commercially to help cut theft from overhead power lines, after a successful demonstration project by UK firm Bandweaver. The company’s distributed acoustic sensing (DAS) system, which exploits back-scattering effects to detect disturbances along an optical fiber, showed it could detect any intrusion and disruption of the line. Bandweaver says that power line tampering is a major problem worldwide, and costs the industry billions of dollars every year in electricity theft alone.
As well as the theft costs, tampering can create power supply disruption and operational losses for supply companies and national grids. Current methods for tackling the problem are said to be time-consuming, inefficient and expensive. “In the majority of cases, detecting and identifying the occurrence of theft events is the main obstacle,” says Bandweaver.
This is also useful for monitoring conditions in other places, such as airplane wings, wind turbines, oil wells, pipelines, and even spacecraft, where the fiber laser sensors can sense shape determination and spacecraft qualification testing. The benefit here is that the sensors can be used both as surface-mounted sensors and embedded sensors within the structure.
According to Bandweaver’s managing director Richard Kluth, the sensor system uses a standard fiber-optic cable connected to the poles being monitored. “By constantly analyzing just one fiber in the cable, the entire route can be monitored for threats 24/7,” he said. Jesús Muñoz from SSS Group added: “We installed the system and filtered out surrounding environmental noises such that any underlying disturbances could be detected. Edesur employees initiated various disturbances at random locations – each were seen and located by the Horizon DAS system.”
Bandweaver believes that the demonstration proved the ability of Horizon DAS to identify the precise location of the disturbance, before passing the tampering information to security systems and sending alerts to company personnel. The hardware, based around a 1.55 µm source and a singlemode fiber, offers a range of up to 100 km per unit, delivering positional accuracy to 5 meters and a response time of 2-10 seconds.
Underwater telecom cables make superb seismic network
Fiber-optic cables that constitute a global undersea telecommunications network could one day help scientists study offshore earthquakes and the geologic structures hidden deep beneath the ocean surface.
In a paper appearing this week in the journal Science, researchers from the University of California, Berkeley, Lawrence Berkeley National Laboratory (Berkeley Lab), Monterey Bay Aquarium Research Institute (MBARI) and Rice University describe an experiment that turned 20 kilometers of undersea fiber-optic cable into the equivalent of 10,000 seismic stations along the ocean floor. During their four-day experiment in Monterey Bay, they recorded a 3.5 magnitude quake and seismic scattering from underwater fault zones.
Their technique, which they had previously tested with fiber-optic cables on land, could provide much-needed data on quakes that occur under the sea, where few seismic stations exist, leaving 70% of Earth’s surface without earthquake detectors. “There is a huge need for seafloor seismology. Any instrumentation you get out into the ocean, even if it is only for the first 50 kilometers from shore, will be very useful,” said Nate Lindsey, a UC Berkeley graduate student and lead author of the paper.
Lindsey and Jonathan Ajo-Franklin, a geophysics professor at Rice University in Houston and a visiting faculty scientist at Berkeley Lab, led the experiment with the assistance of Craig Dawe of MBARI, which owns the fiber-optic cable. The cable stretches 52 kilometers offshore to the first seismic station ever placed on the floor of the Pacific Ocean, put there 17 years ago by MBARI and Barbara Romanowicz, a UC Berkeley Professor of the Graduate School in the Department of Earth and Planetary Science. A permanent cable to the Monterey Accelerated Research System (MARS) node was laid in 2009, 20 kilometers of which were used in this test while off-line for yearly maintenance in March 2018.
“This is really a study on the frontier of seismology, the first time anyone has used offshore fiber-optic cables for looking at these types of oceanographic signals or for imaging fault structures,” said Ajo-Franklin. “One of the blank spots in the seismographic network worldwide is in the oceans.”
The ultimate goal of the researchers’ efforts, he said, is to use the dense fiber-optic networks around the world — probably more than 10 million kilometers in all, on both land and under the sea — as sensitive measures of Earth’s movement, allowing earthquake monitoring in regions that don’t have expensive ground stations like those that dot much of earthquake-prone California and the Pacific Coast. “The existing seismic network tends to have high-precision instruments, but is relatively sparse, whereas this gives you access to a much denser array,” said Ajo-Franklin.
Photonic seismology
The technique the researchers use is Distributed Acoustic Sensing, which employs a photonic device that sends short pulses of laser light down the cable and detects the backscattering created by strain in the cable that is caused by stretching. With interferometry, they can measure the backscatter every 2 meters (6 feet), effectively turning a 20-kilometer cable into 10,000 individual motion sensors.
“These systems are sensitive to changes of nanometers to hundreds of picometers for every meter of length,” Ajo-Franklin said. “That is a one-part-in-a-billion change.” Earlier this year, they reported the results of a six-month trial on land using 22 kilometers of cable near Sacramento emplaced by the Department of Energy as part of its 13,000-mile ESnet Dark Fiber Testbed. Dark fiber refers to optical cables laid underground, but unused or leased out for short-term use, in contrast to the actively used “lit” internet. The researchers were able to monitor seismic activity and environmental noise and obtain subsurface images at a higher resolution and larger scale than would have been possible with a traditional sensor network.
“The beauty of fiber-optic seismology is that you can use existing telecommunications cables without having to put out 10,000 seismometers,” Lindsey said. “You just walk out to the site and connect the instrument to the end of the fiber.”
During the underwater test, they were able to measure a broad range of frequencies of seismic waves from a magnitude 3.4 earthquake that occurred 45 kilometers inland near Gilroy, California, and map multiple known and previously unmapped submarine fault zones, part of the San Gregorio Fault system. They also were able to detect steady-state ocean waves — so-called ocean microseisms — as well as storm waves, all of which matched buoy and land seismic measurements.
“We have huge knowledge gaps about processes on the ocean floor and the structure of the oceanic crust because it is challenging to put instruments like seismometers at the bottom of the sea,” said Michael Manga, a UC Berkeley professor of earth and planetary science. “This research shows the promise of using existing fiber-optic cables as arrays of sensors to image in new ways. Here, they’ve identified previously hypothesized waves that had not been detected before.”
According to Lindsey, there’s rising interest among seismologists to record Earth’s ambient noise field caused by interactions between the ocean and the continental land: essentially, waves sloshing around near coastlines. “By using these coastal fiber optic cables, we can basically watch the waves we are used to seeing from shore mapped onto the seafloor, and the way these ocean waves couple into the Earth to create seismic waves,” he said.
To make use of the world’s lit fiber-optic cables, Lindsey and Ajo-Franklin need to show that they can ping laser pulses through one channel without interfering with other channels in the fiber that carry independent data packets. They’re conducting experiments now with lit fibers, while also planning fiber-optic monitoring of seismic events in a geothermal area south of Southern California’s Salton Sea, in the Brawley seismic zone.
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