Passive distributed temperature sensing (DTS) helps to improve the measurement of moisture content and temperature increase. The DTS technology has been already tested in China to prove the opportunities provided by this technique. Therefore, distributed temperature sensors are very promising for agriculture.
Several land conditions include bare soil, plastic mulch, plastic mulch covered with potatoes, and plastic much covered with maize. Land moisture plays a crucial role in influencing grain yield. Nevertheless, distributed temperature sensing is not suitable for the last type of land.
The thing is that several techniques allow for determining soil moisture content. Distributed sensing systems include point, ground, and regional types. Herewith, point fiber optic sensors consist of “oven-drying, neutron scattering, electrical impedance, time-domain reflectometry (TDR), frequency domain reflectometry (FDR), tensiometer, and thermal probe techniques.”
Conventional fiber sensors offer benefits of high precision and reliability but most of them are limited to a specific area. This is the main reason why it is difficult to determine accurately the overall ground moisture condition, especially in places where soil moisture distribution is discontinuous.
Thus, scientists have suggested applying distributed temperature sensing (DTS). DTS technology allows for estimating agricultural soil moisture, herewith, the distance of measurement can achieve several tens of kilometers. The operating principle of distributed temperature sensors is based on the heating of optical cables. When the temperature increases, the soil moisture can be estimated.
The DTS technology is not new and it quickly found the application in moisture measurement. According to this technique, it is possible to measure in situ soil moisture by employing analytical or semi-empirical models. Nevertheless, firstly it is necessary to calculate the thermal conductivity of the soil.
Compared to traditional techniques that have limitations, distributed temperature sensing allows for overcoming them. DTS estimates the temperature response of soil, therefore, measures soil moisture based on the data obtained by fiber optic sensors. Nonetheless, the proposed technique has several disadvantages.
Various weather conditions can influence the heat transfer way in the soil and the data obtained by distributed temperature sensors will be inaccurate. Additionally, boundary conditions and even time change periods also influence the precision. Finally, the standard algorithm is very difficult for application in practice.
The technology of distributed temperature sensing (DTS) is based on the application of Raman scattering from a laser beam light through optical fibers to detect temperature parameters along a fiber optic cable. The thing is that temperature resolution plays a crucial role. Herewith, this feature makes it possible to efficiently use DTS systems in oceanography.
Even though oceanographic applications of distributed temperature sensing are not new but such observations are not often. The reason is the serious challenges of deployment, calibration, and operation in oceanographic environment conditions. Nevertheless, researchers have tested the DTS system to overcome oceanographic configuration, calibration, and data processing difficulties.
It should be noted that they also evaluate temperature errors of DTS for several common scenarios. Difficult conditions influence the whole process, thus, the researchers look for alternative calibration, analysis, and deployment methods for distributed temperature sensing.
Therefore, these errors will be reduced and the successful application of DTS systems will be increased in dynamic ocean conditions. The thing is that DTS technology allows for “continuously sampling at a relatively high temporal and spatial resolution for significant duration over broad spatial scales.”
Despite distributed temperature sensing is widely applied in environmental applications, the oceanographic area remains challenging and still relatively rare. The main purpose of new DTS development is the solution to common problems present in oceanographic deployments.
To be more precise, the researchers use 2 various DTS systems, 3 fiber optic cables, and 24 thermistors. All of them help to test cables and different calibration configurations and perform distributed temperature sensing. Test results enable them to improve future oceanographic deployments. Moreover, they aid to achieve the best possible temperature signal in difficult deployment and operational environments.
DTS technology is a relatively new oceanographic tool. It allows for detecting temperature across wide spatial and temporal scales. Herewith, the application of such a fragile DTS system in remote and dynamically complex conditions remains difficult. Moreover, sometimes it is impossible to perform distributed temperature sensing at all.
Additionally, DTS systems face challenges during the detection of air/sea boundary. The reason is the change of water level, for instance, tides, waves, surge, etc., when the fiber optic cable can be exposed. Finally, the new DTS has succeeded to detect the temperature variance between the air-sea interface.
Almost all pipelines suffer from numerous leaks during their operation, therefore, they require systems for fiber optic pipeline leak detection. Despite the fact that there are various techniques for leak detection, distributed temperature sensing systems (DTS) are considered to be an ideal option for the purpose.
Distributed temperature sensing is a technique that has been applied for more than two decades. DTS systems are regarded as the best option when a leak leads to a temperature differential between the ambient air and the escaping liquid or gas. The thing is that “temperature differentials generally occur when the pipeline product is at high pressure, high temperature or low temperature, all relative to ambient, which is characteristic of numerous pipelines.”
The operating principle of DTS is based on fiber optic sensing systems that operate as a sensor and measure temperatures along the entire length of optical fibers. Herewith, the optical fiber is put along the outside of the pipeline within the protective coating. It should be noted that the accurate installation location depends on the relative area(s) of the anticipated temperature differential caused by a leak, and on other reasons such as available mounting space.
To be more precise, DTS systems allow fastly identifying and precisely locating slow leaks at weld points, pipeline fittings, and herewith, sudden leaks. Fiber optic pipeline leak detection system enables detecting the precise location of leaks, often overcoming other distributed sensing technology. The fact is that even a tiny leak leads to a crucial temperature change, one that can be recorded by the DTS system. Most DTS measures temperatures with a precision of a few degrees, more than sufficient for leak detection.
For instance, a modern leading distributed temperature sensing technology allows measuring temperatures at a distance of 6 km, totaling 6000 points of measurement. The fiber optic sensing system’s transceiver measures “temperatures for 6 km both upstream and downstream of its installation point, for a total coverage of 12 km per each transceiver.” It is possible to employ several transceivers with accompanying fiber optic cables to offer coverage for long pipelines, totaling hundreds or thousands of kilometers in distance.
DTS technology acts as a semi-automatic leak detection system, obtaining data information to enable operators to take action before automation and/or safety system activation. It should be mentioned that a semi-automatic system means that the leak detection occurs automatically, resulting in an alarm signal in a continuously staffed control room.
Dams applied for hydropower, irrigation or mining play a crucial role in human life, herewith, they evoke significant human, economic, and environmental consequences when they fail. Nevertheless, distributed fiber optic sensing increases dam safety by offering early alerts of potential problems.
To be more precise, modern distributed sensing systems are considered to have high accuracy for monitoring promoting a continuous understanding of dam conditions, taking dam safety to a higher level. For instance, distributed temperature sensing (DTS technology) uses high spatial resolution temperature data from distributed temperature sensors to record tiny seepage flow changes and to estimate seepage rates in a dam structure.
It should be noted that seepage happens in most embankment and earth dams as the impounded water looks for the path of least resistance through the dam and its foundations. Therefore, excessive seepage presents a threat while high-tech sensing systems enable to detect and analyze subsurface processes and prevent erosion. Distributed fiber optic sensing is a promising technology that can be employed to control critical geophysical parameters, for instance, temperature and strain with a sub-meter resolution over several km.
Additionally, distributed sensing systems provide the benefits of cost-effective high spatial monitoring coverage. The thing is that optical fiber acts as the sensing system along the full length of the fiber optic cable allowing operators to obtain detailed data information along the entire dam. Distributed temperature sensors can catch tiny, localized changes in the seepage flow rates that would otherwise remain unnoticed. “They deliver temperature readings with the accuracy of point sensors with the indisputable benefit of fiber optics: the highest possible spatial coverage. ”
Moreover, the distributed temperature sensing does not need specialized optical fibers resulting in relatively low-cost installation. The thing is that measurements based on DTS systems provide data along the entire dam with high spatial resolution and high-temperature precision. Herewith, distributed temperature sensors have already been used in tailings dams. One of the main elements of the increasing number of permanent tools is the ever-increasing performance of the DTS systems. Modern fiber optic sensing systems achieve the world’s most accurate measurements, with sampling resolutions of 12cm (over 5km) and with temperature resolution as low as 0.01 C.
Finally, seepage detection used distributed temperature sensing is regarded as a crucial technology and has prominently improved the monitoring capabilities of dam operators. The application of optical fiber networks provides additional benefits like the ability of distributed sensing systems develops further.
Temperature is a key safety indicator in any industry. The technology of distributed temperature sensors using optical fiberallows measuring the temperature at any point in the fiber, with an interval of 1 meter, resulting in the detailed temperature dependence of all required areas. The data obtained by this technique makes it possible to develop intelligent warning systems based on it, therefore, replacing outdated point-based monitoring systems.
The operating principle is based on the reflectivity of stimulated Raman scattering of light (Raman effect). A semiconductor laser is also used to determine the location of temperature changes in a fiber optic cable. The fact is that the structure of the optical fiber changes when the temperature changes.
When laser beam light from the laser system enters the area of temperature change, it interacts with the changed structure of theoptical fiber, and in addition to direct light scattering, reflected light appears.
Benefits of temperature sensing systems
The main advantages of fiber optic sensors in comparison with classical analogs are the following:
Very fast response to parameter changes in the environment;
Resistance to chemicals and aggressive environments;
DTS is not affected by electromagnetic disturbances;
The sensitive part of thefiber sensor does not require connection to power lines.
The processing unit measures the propagation speed and power of both direct and reflected light and determines where the temperature changes. For instance, at a wavelength of 1550 nm, a pulsed generation mode is used with a laser power limit of 10 mW.
Types of sensors for temperature measurement
There are several types of optical fibers, each of which meets certain requirements for its properties, depending on the application due to the fact that the properties of the optical fibercan be varied over a wide range.
Physical effects on the optical fiber, such as pressure, deformation, temperature change, affect the properties of the fiber at the point of exposure and it is possible to measure the environmental parameters by measuring the change in the properties of the fiber at a given point.
In general, afiber optic sensorconsists of two concentric layers: fiber core and optical coating. The fiber optic light guide part can be protected by a layer of acrylate, plastic, reinforced sheath, etc., depending on the application of thisfiber cable.
Thus, distributed fiber optic sensors are perfect for industries related to combustible and explosive materials, such as coal, oil and gas production, etc. for use in fire alarm systems of various structures.
Detecting a fire in an industrial environment is not an easy task because of the large number of disturbing factors, many of which can be considered by detectors as carriers of fire signs. In addition, dust deposited on the DTS‘ sensitive elements makes it difficult to operate and it can disable them.
It is also necessary to take into account the possible smoldering of the deposited dust, which can also lead to false alarms. The presence of fumes and aerosols makes it impossible to operate smoke optical-electronic fire detectors. The presence of carbon monoxide will trigger gas fire detectors.
Industrial facilities and production are characterized by large volumes of premises, high ceilings, the presence of long tunnels, collectors, mines, inaccessible areas, and premises with a complex configuration and geometry. And in these conditions, it is certainly possible to protect using traditional fire alarm systems, but this involves the use of a large number of detectors, and therefore they have high costs, including installation and maintenance of alarm systems and automation.
It is difficult to select detectors for explosive zones, especially for use in underground operations and mines. Aggressive media are often present in chemical industries. There are also objects of sea and river transport, characterized by the aggressive salt fog.
Oil and gas application
The use of non-electric sensing devices, the use of fiber optic cable allows the DTSto be applied in enterprises of the oil and gas complex, mines, underground operations, chemical industries (including those with aggressive environments), and metallurgy and energy enterprises.
As for oil companies, the active development of high-viscosity oil fields, which imposes strict requirements on the production equipment, and the severe depletion of most oil and gas fields require mining organizations to conduct prospecting and exploration operations, change production technologies and control the technical condition of wells.
The main task for mining companies to increase the well’s production capacity in real-time is to track information about the processes occurring in wells and fields. Solutions based on standard temperature sensors suggest well logging using point measuring instruments, which leads to the inaccuracy of the data obtained.
The disadvantages of suchsensing devices include the inability to fix the distribution of one of the most important parameters of the well – the temperature profile in real-time, as well as the need for power supply, the impact on the measurement results of external electromagnetic fields, labor and time costs required for the departure of the team and performing various operations, including the immersion of thefiber sensorelement and its movement along the well, data processing, etc.
The fiber optic cable is resistant to mechanical damage. Additionalfiber optic cableprotection is not required during descent and lifting operations, but the protection of the fiber cable from mechanical damage during descent and lifting operations can be provided by the use of protective coatings.
Distributed temperature system provides continuous underground power lines monitoring of temperatures, detecting hot spots, delivering operational status, condition assessment, and power circuit rating data. This helps operators to optimize the transmission and distribution networks, and reduce the cost of operation and capital.
Usually, the DTS systems can detect the temperature to a spatial resolution of 1 m with precision to within ±1°C at a resolution of 0.01°C. Measurement distances of greater than 30 km can be monitored and some specialized systems can provide even tighter spatial resolutions. The advantages of working with Optromix:
Our DTS system has the superior quality, however, its price is one of the lowest in the market;
Optromix is ready to develop DTS systems based on customer’s specifications.
A new distributed temperature sensor (DTS) system has been developed to perform optimization of the temperature precision with the enhanced temperature sensitivity of backscattered spontaneous Raman scattering. The DTS system is based on the difference in sensitive-temperature compensation.
Distributed temperature sensors apply the dual-demodulation, self-demodulation, and double-end configuration principles. The DTS system has been already tested and demonstrates great results: the temperature precision is considered to be 12.54 °C, 8.53 °C, and 15.00 °C along the 10.8 km under the traditional R-DTS systems, respectively.
It is possible to use the sensing system with difference sensitive-temperature compensation for the dual-demodulation, self-demodulation, and double-end configuration R-DTS, herewith, this fiber optic sensing technology enables to make the temperature precision better than 1 °C for these three demodulation systems.
The operating principle of Raman Distributed Temperature Sensor is based on “specific optical effects along the sensing optical fiber to obtain a spatially distributed temperature profile”. Compared to traditional discrete sensing techniques, R-DTS systems provide unique attributes and capabilities.
It should be noted that spontaneous Raman scattering of distributed temperature sensors uses the energy exchange in the optical fiber, therefore, when the pulsed light quantum and fiber optic material molecule leads to an inelastic collision in optical fiber, this will create an anti-Stokes light.
The thing is that the anti-Stokes light is regarded to be very sensitive to the surrounding temperature, and it allows modulating the environmental temperature using the principle of Raman scattering. Nowadays, such DTS systems find their application in the temperature safety monitoring thanks to the benefits of distributed measurement, long-distance, and high spatial resolution, as well as in transport infrastructure, smart grid and gas pipeline, etc.
It is necessary to pay on the following parameters when you choose distributed temperature sensors with high-performance: temperature precision, temperature resolution, and spatial resolution. DTS systems can be used as an industrial temperature measurement system, for instance, the carrier density in the power cable can be measured by employing a specific temperature. Additionally, distributed temperature sensors allow locating the position of pipeline leakage.
Tests demonstrate that the temperature demodulation system based on distributed temperature sensing offers higher temperature precision and resolution of the self-demodulation than the dual-demodulation system due to the signal-to-noise ratio. Moreover, the double-ended configuration for DTS systems allows avoiding the measurement error based on the change of local external attenuation.
The fiber Bragg grating distributed temperature sensing on gas-insulated line spacer influences surfaces charge accumulation. Nevertheless, it is very difficult to perform the DTS measurement because the traditional electric sensors have huge dimensions, they are difficult to multiplex and highly sensitive to electromagnetic interference.
Thus, a distributed temperature sensing system of the GIL spacer based on the technology of the optical frequency domain reflectometry was designed to solve the challenges. The operation of the DTS system is based on the ultra-weak fiber Bragg grating or FBG technology to change single-mode optical fiber because of its higher signal-noise ratio.
It should be noted that the distributed temperature sensing also used the demodulation technique to compensate for the nonlinear frequency tuning errors caused by the unstable tunable laser. Therefore, the DTS system allows determining the connection between the space temperature and the wavelength shift during the calibration test.
Additionally, the distributed temperature sensing includes the application of the data processing technique for 3D surface temperature on a cone-type spacer. Finally, the DTS system based on FBG technology enables to obtain efficiently high-resolution temperature measurements on the spacer surface with the help of the optical frequency domain reflectometry, which is virtually impossible to perform with the traditional electric temperature sensing systems.
Nowadays the technology of direct current transmission with the gas-insulated line is an effective way due to its low electrical losses compared with, for example, AC transmission. That is why the distributed temperature sensing is considered to be a key factor that should be known first.
To be more precise, the disadvantages of traditional electric sensing systems such as dimensions, multiplexing, and sensitivity to EMI are solved by optical fiber sensing in electrical engineering. Fiber Bragg grating technology is a precise optical temperature sensing technique widely applied. But there are some limits in the multiplexing ability of conventional FBG sensors.
The new distributed temperature sensing system includes ultra-weak fiber Bragg gratings providing a reflectivity of only 0.1% compared to the conventional FBG sensors. Compared to the optical frequency domain reflectometry based on the Rayleigh technology, “ultra-weak FBG can achieve higher signal-noise ratio since the reflection of ultra-weak FBG sensor is much higher than the backscattering in the optical fiber”.
Traditional in situ observations of meteorological variables are limited by surface levels, herein, it is possible to carry out the lowest observation around just 1-m height. Therefore, observation results of both shallow fog, and the initial growth stage of thicker fog layers can be missed in this case. Nevertheless, the use of distributed temperature sensing or DTS technology allows measuring temperature and humidity parameters at centimeter resolution in the lowest 7 m.
It should be noted that it is very important to obtain high-resolution observation for radiation fog, and DTS sensors solve the problem. Two techniques are applied to make tests in the near-surface layer at a higher resolution than the traditional sensing devices.
Distributed temperature sensing is ideal for the measurement temperature and relative humidity parameters. DTS technology offers a high spatial and temporal resolution. DTS application includes detection of surface temperature and soil heat fluxes, the radioactive skin effect at the surface of water bodies, the Bowen ratio, near-surface turbulent fluxes under varying stability, and wind speed.
The combination of distributed temperature sensing with unmanned aerial vehicle provides the observation of the morning boundary-layer transition from stable to unstable conditions. Compared to traditional sensing techniques, DTS technology has a great advantage for studies of the stable boundary layer that is the resolution of steep gradients.
DTS sensors are able to detect shallow cold pools at high resolution that is the mark of radiation fog formation. The thing is that the fog presence causes elevated DTS temperatures of up to 0.7 ℃ when compared to traditional temperature parameters. However, the technology of distributed temperature sensing is required to be further tested to provide its reliability under stable, foggy conditions.
DTS devices enable to measure temperature characteristic along with optical fiber cables that are based on the backscattered signal of a laser pulse. The DTS sensors were tested, herewith, the optical fiber includes two multi-mode cores, while a simple single-ended (non-duplexed) configuration is applied for the measurements.
Finally, even in the conditions of fog formation absence, compared to DTS technology traditional sensing devices are not able to measure the strong temperature inversions in the lowest 1 m of air. Distributed temperature sensing provides an efficient solution to the problem.
The application of DTS systems is not limited by fog detection, but the broader near-surface (stable) boundary layer. Additionally, DTS sensors offer a better physical understanding of such processes as the collapse of turbulence at the onset of the stable boundary layer, intermittent turbulence within the stable boundary layer, and the transition between different boundary-layer regimes due to the ability of distributed temperature sensing to catch steep gradients in both temperature and relative humidity parameters.
According to a recent study, the technology of distributed temperature sensing allows demonstrating the mechanical properties of fiber optics under harsh conditions. The fact is that numerous land and undersea oil procedures depend heavily on distributed temperature sensing for providing safety and functionality in severe environments.
Usually, the manufacturers use silica-based fiber optics for distributed temperature sensing and distributed acoustic sensing, where temperature and acoustic signals are transmitted and recorded continuously along the length of fiber sensor cable. Such fiber optic solution for FBG interrogator of 15 km length enables well and pipeline operators to apply fiber-based distributed sensing technology to measure the whole wellbore or pipeline span with a resolution of 1 m or less virtually in real-time.
For example, the technology of distributed temperature sensing with fiber optics is used in the operation of steam-assisted gravity drainage or SAGD technique, in which the main goal is the production of heavy crude oil and bitumen materials. Nevertheless, it is only one example, while distributed temperature sensing has numerous fiber optic applications.
It should be noted that the distributed temperature sensing system monitors the following extreme environments while optical fiber operation: high temperatures and pressures, ionizing radiation, and aggressive chemicals in the environment. However, in order to be applied in such severe conditions, fiber optics have to be highly reliable, while transmitting optical power with a minimum of added signal loss.
Most of the fiber sensor cables used in fiber optic applications contain a silica-based core and cladding because of the silica benefits that include high optical fiber transmission, superior thermal stability, and mechanical reliability. Herewith, the use of a polymer coating in fiber cables provides the mechanical protection of fiber optics and minimization of bend-induced optical attenuation.
The technology of distributed temperature sensing also enables to detect the factors limiting the performance of fiber optic cables at elevated temperatures and/or in aggressive conditions. The most frequent failures for fiber optics are related to added attenuation or loss of mechanical strength. Herewith, failure criteria differ from each other and depend on the type of fiber optic application.
Today operators in highly fractured carbonate reservoirs use running wireline gradiometric surveys as a traditional way of oil-rim movement monitoring. Nevertheless, the method is not ideal because it does not allow providing the necessary information in a manner timely enough to influence operations since the surveys are conducted just a few times a year. Distributed temperature sensing (DTS) system is an alternative solution that overcomes these drawbacks.
Distributed temperature sensing offers real-time monitoring of the oil rim in carbonate reservoirs. DTS technology is based on the use of optical-fiber Bragg gratings (FBGs) and is able to measure pressure in extreme environmental conditions. It should be noted that the DTS system does not require electronics, herewith it provides long service term even at elevated downhole temperatures. Also, distributed temperature sensing is quite stable, it can determine even small pressure changes.
Moreover, dozens of distributed temperature sensors can be implemented on one fiber optic cable connected to the FBG interrogator. The principle of operation includes a diaphragm pressure transducer that converts changes in the hydrostatic pressure of its environment into the deformation of a pressure-sensitive diaphragm, and then into the strain, within an optical fiber attached to the diaphragm. The thing is that optical fibers offer numerous advantages for oil and gas industry application, containing the following:
high transmission efficiency that means the FBG interrogator can be installed several kilometers away from the downhole sensors;
high data quality and accuracy due to data encoding in the light wavelength;
data immunity to the electromagnetic influence by machines;
At present time the information about pressure and temperature measurements, as well as system-health and self-diagnostic information are transmitted from the FBG interrogator into a database at 3-hour intervals. If fresh data sets are obtained, an algorithm processes them into gas/oil-contact and oil/water-contact locations. Also, it is possible to visualize pressure, temperature, and fluid-level data to see them in real-time.
Further improvement of the DTS system will allow the application of the system for the production monitoring in ultrahigh-temperature thermal-recovery wells, where fluid temperatures up to 280 °C can be reached. Finally, distributed temperature sensing provides the reduction of a production delay, the reduction of operational costs, health, safety, and environmental risk reduction, and the possibility of system expansion.