The USA researchers started a number of experiments that aim to analyze the first data about seafloor under Arctic sea ice with the usage of a new method. The research team was able to connect a distributed acoustic sensing (DAS) system with a fiber optic cable. The cable vibrations can record the data 24/7. That helped the scientists to get all the activities and changes within the ocean all day long. This was the first time in history when a DAS system was used on the seafloor of the Arctic or Antarctic oceans.
The appliance looks like an electronic box that is attached to the fiber optic cable on land. It uses a laser to send thousands of short pulses of light. The small amount of the light is reflected back. And the reflected light helps the appliance to monitor events along with the fiber and store the data on hard drives.
According to the research, the DAS technology showed the icequakes, different climate signals, and marine life. The researchers are expecting to note other climate signals like ocean wave height, timing, and distribution of sea ice breakup, and ice thickness, etc. The usage of the distributed acoustic sensing system has the potential to record a variety of Arctic phenomena so the scientists can better see the climate change effects and sea life. Moreover, the DAS system makes it cost-effective and safe in comparison with the other methods. The scientists have already recorded a number of events that the traditional hydrophone or ocean bottom seismometers couldn’t even detect. With the help of the DAS system, the scientists hope to also record whale songs.
However, the research team has to face challenges during the first week of the tests. And the most difficult one was the harsh climate. It was really cold, most of the territory is tundra. It’s snowing most of the time and it’s getting dark really early. No wonder, that the team should find new creative ways of data fixing like DAS technology to get everything working.
That’s why the researchers chose a distributed acoustic sensing system to cope with the weather conditions. Fiber optic cable is double-armored with copper and steel. All the network components are created to hold the extreme Arctic environment. They have no need in sending a boat to plant monitors, moving over the sea ice to install the sensors. This fiber optic cable can exist for years or decades without replacing it.
This project of watching the Arctic ocean with the usage of the distributed acoustic sensing system is going to last over the next two years. The research team will collect the data. And the next third year will be spent on its analysis.
According to the researchers, the production of fiber optic systems based on the DAS technology can be easily automated. However, there is still a space for developing and finding new ways of optimization.
Optromix is a DAS system manufacturer that provides top of the line distributed acoustic sensing systems suitable for monitoring commerce networks. If you have any questions or would like to buy a DAS system, please contact us at firstname.lastname@example.org
Scientists are looking for new ways of employment for distributed acoustic sensors (DAS). The fiber optic system contains a mandrel with a wounded with sensitized optical fiber. It is the acoustic sensor for a heterodyne that is protected for underwater use.
Nowadays, the DAS system can be used both for military purposes as well as for peaceful life. In the military, they are mostly utilized for submarine locations. While they are also in active use for monitoring sea animals’ life or finding and exploring marine mineral sources. The fiber optic systems based on the DAS technology have more advantages over the other items. First of all, they are thin and reliable. Secondly, there are no underwater electrical devices. And finally, the systems with acoustic sensors are immune to electromagnetic interference.
These devices usually contain an array of DAS sensors along with the fiber. Herewith, the arrays are up to a hundred or even fewer acoustic sensors because of the technical restrictions. The spacing between these acoustic sensors is fixed. That’s why there are some limitations in marine acoustic detection, for example.
The recent researches from China exploited a distributed acoustic sensing based on heterodyne coherent detection and demonstrated its field-testing. The optical cable contains a supporting mandrel, special optical fiber, and cable sheath. Acoustic signals from the fiber optic system disturb the mandrel and the fiber. That all causes phase changes which are the desired signal. The whole model was created to analyze the equivalence and specific character of the acoustic wave response.
With the array signal processing, the DAS device can easily find underwater acoustic signal sources and track motion trajectories. Moreover, the results of the experiments are highly accurate.
There are also obvious benefits of distributed acoustic sensors (DAS) in various industries. Most of which are elements of longer-term goals.
Some of the potential advantages of a distributed acoustic sensing are numbered below:
a low-cost acquisition system;
a simple design;
no electrical energy required in the fiber optic cable;
the fiber optic cable is suitable for harsh environments (dust, temperature, harmful gases);
the fiber optic cable is immune to the radiations such as EMI (Electromagnetic Induction) & ESD (Electrostatic Discharge);
the fiber optic system can transform several kilometers long sensor that enables it to monitor on a truly distributed basis.
According to the researchers, the production of fiber optic systems based on the DAS technology can be easily automated. However, there is still a space for developing and finding new ways of optimization.
Nowadays scientists pay more attention to the process of Alpine microseismicity. The thing is that the sensing of seismic activity is important for studying landscape-shaping processes and predict dangerous mass movements. Nonetheless, the amount of modern fiber optic sensors is still low in Alpine regions. Therefore, distributed acoustic sensing (DAS) promotes solving the problem.
The application of advantageous fiber cables makes DAS technology very promising in seismic monitoring of glacier movements and natural hazards. Additionally, DAS technology has significantly changed the portability of seismic devices. This is why the performance of seismic monitoring in difficult-to-reach areas becomes more and more accessible.
It should be noted that the purpose of distributed acoustic sensing “focus on processes near the Earth’s surface rather than on traditional seismology subjects like the deeper crust and mantle.” DAS applies fiber optic sensors into which an interrogator enters a sequence of laser beam pulses. Herewith, fiber optic systems as distributed acoustic sensing are widely used in geophone chain deployments.
Fiber optic systems allow for sensing local earthquake signals, which are too weak to be recorded by conventional seismometers. Moreover, DAS enables the record of anthropogenic noise. Even though individual channels of distributed acoustic sensing have some disadvantages as a lower signal-to-noise ratio, they overcome seismometers. The thing is that there are a lot of unused fiber optic systems with unprecedented sensor coverage and density.
Novel DAS technology performs the records of microseismic signals and ambient noise in glacier areas. Compared to standard fiber optic systems, new distributed acoustic sensing offers important improvements in stick-slip event location and determines weak seismic waves. Herewith, the potential and utility of DAS systems are doubtless for sensing glacial processes.
According to scientists, DAS technology performs measuring seismogenic glacier flow and even small Alpine mass movements, for example, rockfalls. The benefits of distributed acoustic sensing include better limitation of static and dynamic properties of the glacier and its surroundings. The most significant thing is DAS technology provides precise arrival time measurements despite spatial averaging.
Finally, the advantages of DAS channels increase the location quality of stick-slip events significantly. The density of distributed acoustic sensing detects numerous reflections and extremely refracted waves. Herewith, spaced seismometer networks can not carry out it.
The thing is that human hearing is actually limited. Nonetheless, the technology of Distributed Acoustic Sensing (DAS) allows for detecting ground vibrations that are difficult to hear. Therefore, DAS systems perform analysis of seismic signals produced by people, animals, or even vehicles, etc.
To be more precise, DAS technology collects acoustic data with the help of fiber optic cables. Herewith, distributed acoustic sensing records signals and the backscatter pattern that can be analyzed later. For instance, distributed acoustic sensors can detect various environmental conditions – earthquake and hydrological activity, wind and weather events, and more.
It should be noted that virtually any fiber cable can be used as an acoustic sensor thanks to DAS technology. The thing is that it needs limited power, and the DAS is immune to electromagnetic and radiofrequency interference. This is the reason why distributed acoustic sensors can be applied for long-term, persistent monitoring in difficult-to-reach places.
The most popular DAS applications include the oil and gas industry where acoustic sensors are used to monitor pipeline leakage and seismic activities. Additionally, DAS systems perform perimeter security and smart city applications and control traffic and assets.
A company-manufacturer of fiber optic solutions from the U.S. has presented a new project for existing fiber cables. They test DAS technology to improve its sensitivity and resolution. Thus, new DAS applications would appear to offer a greater understanding of acoustic signals.
Thus, DAS helps to detect cattle movement in an agricultural experimental station. The existing fiber optic cables record four main signal types: cattle movement, earthquake and wind activities, and human traffic. Herewith, the researchers claim that it is very easy to turn existing cables into acoustic sensors of high efficiency.
In the nearest future, distributed acoustic sensors can be used for the detection of geological, hydrological, meteorological, and biological (human and animal) activity. During the tests, DAS systems have provided a greater understanding of the movements of cattle and people in the land.
Distributed acoustic sensing demonstrates a high level of efficiency. “DAS using fiber optic cables could prove to be an effective solution for monitoring the movement of livestock and wildlife and weather events such as high winds, storms, and lightning in remote locations.” Moreover, the same technique can be useful for other applications mentioned above. Even though DAS technology still needs to be improved, it is planned to be used to test new analytical techniques and devices.
Nowadays cases, when people meet polar bears, have dramatically increased especially in arctic areas because the environment continues rapidly transforming. Thus, there is a need for a structural health monitoring system that allows detecting bears to decrease the number of such meetings. Distributed acoustic sensing (DAS) is an ideal technology to perform this task.
DAS systems are used as an intrusion detection system that is able to operate in environments where temperatures fall to -70 C. Usually, the installation of the distributed acoustic sensing system takes place on the ground, that is why DAS system performance requires its testing in the snow. The DAS has been already tested at similar conditions (deep snow and extreme cold), herewith, people helped to imitate polar bears walking near the distributed acoustic sensing system.
It should be noted that standard DAS system consists of the following components:
a sensing fiber optic cable that can be stretched over long distances;
a laser central processing unit (DAS).
The operating principle of fiber optic technology is based on mechanical vibrations that undertake fiber impingement, leading to laser beam backscatter and therefore, allowing researchers to measure the signal required.
The researchers planed to test several opportunities provided by the DAS system: the ability of optical fiber to maintain extreme temperatures, the suitability of distributed acoustic sensing to snow coupling. The performance of the DAS system has been tested at the temperature of the -70C in the MTS environmental control chamber, and it demonstrated good results.
“Launch boxes with 2200m of spooled fiber optic cable were applied on either side of the 150m of distributed acoustic sensing cable to imitate a field deployment of 4.5km.” The researchers pay careful attention to four separate test temperature ranges provided by the DAS system, they even calculate shoe surface area combined with the weight of the participants in order to learn human foot pressure and connect it with polar bear foot pressure to classify bears of different sizes.
Additionally, DAS also allows differentiating humans from polar bears. The thing is that polar bears generally walk with 3 points of ground contact, while people need only one point. Even though the feet of polar bears are quite large, researchers can easily offer similar foot pressures.
Finally, the test results of distributed acoustic sensing demonstrate that fiber optic cables can maintain extreme cold temperatures in the arctic regions, where the temperature is required not to disturb optical fiber performance. The DAS system detects signals at depths of at least 0.65m in the unprocessed data.
The red palm weevil is regarded as a serious problem for the cultivation of date palms leading to massive economic losses worldwide. Although curative techniques are not challenging, early detection is still difficult to be performed. Fiber optic technology allows overcoming the threat and offers reliable detection of RPW by a distributed acoustic sensor (DAS).
Modern DAS systems enable to detect feeding sound created by larvae as young as 12 days, in an infected tree. Compared to traditional methods, the DAS technique provides a cost-effective and non-invasive alternative that can perform 24-7, real-time monitoring of 1,000 palm trees, or even more. Moreover, distributed sensors allow controlling temperature, a crucial characteristic to monitor farm fires, one more important problem for the cultivation of palm trees around the world.
Nowadays there are various techniques to detect the weevils. For example, it is possible to use trained dogs to smell the odor, however, such sensing is not precise and has low efficiency. That is why distributed acoustic sensors are considered to be the most promising early detection techniques. Current sensing technologies use sound probes to install them right into the tree trunk but such acoustic sensors damage plants and create a nest for other insects.
The novel distributed acoustic sensor combined with a signal processing algorithm offer a reliable solution for the early detection of red palm weevils. The design of the DAS system is based on the use of the laser and photodetector installed within a single unit, while only the optical fiber is wound around the palm trees to create an optical network. The developed DAS technique has been already tested on two palm trees (one healthy and one infested with ~12 days old larvae.).
The thing is that the novel distributed acoustic sensors are “uniquely non-invasive, providing 24-7 monitoring, at relatively low cost, and offering wide coverage of the farming area, using only a single fiber optic cable.”
To be more precise, all the optical/electronic components applied to design acoustic sensors are put into a sensing unit, which is linked to an optical fiber that is extended throughout the palm-trees farm. Herewith, the fiber circles each tree trunk, from the ground up to a ~1 m height because the probability of RPW is extremely high there. Also, the fiber optic cable between trees can be either put on the ground buried in the soil providing real-time monitoring that promotes precise identification of locations of the infected and healthy trees.
The design of DAS systems includes the use of phase-sensitive optical time-domain reflectometry (Φ-OTDR) that has numerous potential applications in the oil and gas industries as well as for real-time structural health monitoring. The operating principle of distributed acoustic sensors is based on “launching a train of optical pulses generated by a narrow linewidth laser into a single-mode fiber.”
A technique of noise suppression used f-x deconvolution and the wavelet transform allows reducing jump edges in the phase noise of distributed acoustic sensing. The DAS system has been already tested and demonstrated great results of high performance in terms of increasing the quality level of seismic waves. Distributed acoustic sensing provides relatively stable phase sensitivity as well as better the signal-to-noise ratio, leading to more remarkable features of seismic wave signals.
To be more precise, distributed fiber optic sensing based on Rayleigh scattering is not new and used in various applications to control current assets and employ new resources. It is possible to apply such DAS systems to provide efficient and comparatively inexpensive development of perimeter security systems and produce defensive redundancy.
Different methods have been used to apply Rayleigh scattering in the development of distributed fiber optic sensors. For instance, a distributed sensing technology of optical time-domain reflectometry (OTDR) was used for the first time over three decades ago. Nowadays a noise suppression technique based on f-x deconvolution and the wavelet transform is considered to be the most efficient for distributed acoustic sensors.
The thing is that f-x deconvolution is regarded as a traditional technique of seismic data processing. The application includes the treatment of seismic signals by fiber optic sensors as a two-dimensional (2D) image in terms of time and channel. The DAS system enables us to extract the signal characteristics of each channel with a comparatively stable noise distribution between channels.
Usually, such techniques as deconvolution or the wavelet transform are applied separately in traditional seismic noise processing but here these techniques are combined in distributed sensing technology to minimize the number of wavelets transform layers and reach effective low-frequency noise suppression. The DAS system offers such benefits as comparatively stable phase sensitivity over the whole sensing range, the reduction of jump edges in phase noise, and more evident features of seismic wave signals.
The operating principle of distributed acoustic sensing used f-x deconvolution is based on “the assumption that desired signals are continuous and predictable whereas random noise is incoherent and unpredictable.” DAS sensors are actively used for oil and gas exploration. Thus, the noise compression technique improves the phase noise performance of the DAS system. Moreover, the jump edges from the noise of fiber optic sensors are reduced with the same peaks and valleys compared to the original signals which play a crucial role in calculating the time delay of peaks and valleys between various channels.
Specially developed algorithms allow converting a measurable backscatter signal trace (signature) into valuable information, for example, about moving rolling stock, about people moving along or near tracks, or other actions, such as earthmoving operations.
A common application of acoustic sensors
The application of DAS-based systems becomes widespread, for instance, in the oil and gas industry, as well as in the border protection due to these technical capabilities.
Since the majority of railway tracks already have fiber optic cables, the above-mentioned possibilities of DAS application in the railway infrastructure can be performed to a large extent using existing resources.
If an existingfiber optic cable already laid close to the railway infrastructure is used, it is possible to monitor trains, auxiliary rolling stock, track crews, strangers near tracks, or natural influences on the infrastructure.
Accordingly, DAS technology can find application in tracking systems for the movement of trains, monitoring the track and rolling stock, as well as in the protection of railway infrastructure.
Only one set of DAS systems enables to monitor processes and components on and off track for 40 km. It is possible to combine many such units into a common sensing systemto cover extensive networks of railway tracks. At the same time, the DAS system operates with an accuracy of 10 m and provides information about the location of the recorded event on the site and GPS coordinates.
Opportunities in acoustic sensing
Several acoustic sensing sets can be combined into a single system to control longer tracks. The possibilities of the co-use of distributed acoustic sensingand wheel hole registration systems were already studied to meet the requirements of the railroads, taking into account the considered limitations.
The application of advanced axis counters is caused by the need to detect individual axes and the train location on a particular track in compliance with safety conditions.
Unlike track circuits, directly establishing the free or occupation part of the track, the axis counting system operates indirectly. If the track part was free in the initial period, and then the number of wheelsets entering and leaving coincided, the part is registered as free from railroad rolling stock. If this condition is not fulfilled, the part is considered occupied.
Also, DAS technology provides an even more accurate determination of the train length. Moreover, this combination of sensing systems offers the opportunity to localize events, for example, you can determine which axis has a slider. In this combination, DAS can also be used on sections of railway tracks with complex track development, where several parallel tracks are connected by operations.
The user interface system of distributed acoustic sensing displays in a convenient form both data received directly from the DAS systemand information generated using combined technical solutions, including additional axis counters and a system for registering the wheel hole of railroad rolling stock.
Conditions detected by the DAS system and a combined technical solution are carefully classified and all received information is provided in a visual form. This serves as the basis for the planning and implementation of activities arising from the detection results.
Besides, the data collected by distributed acoustic sensingcan be redirected directly to mobile end-user devices. Nevertheless, they can also be transmitted, for example, to unmanned aerial vehicles, which are sent to the appropriate location using available GPS data. Thus, the DAS systemallows you to quickly respond to a variety of events.
The interference of both signals will make it possible to more accurately correlate information about the state of train components with a specific location in the future, for example, it will be easy to establish which axis has the slider on it. New possibilities are opened up for using DAS technology in complex railway tracks, where several parallel tracks are connected by operations.
Abilities presented by DAS systems
The system of distributed acoustic sensingallows both monitorings of rolling stock and state track components: the DAS systemcompletely controls the railways and the area around them. Even unforeseen events that are difficult to detect are recognized reliably.
This also applies to fractures of rails, which represent one of the main risks on the railway. This sensing systemalso detects electrical discharges on air-track lines due to overload, floods, stone falling, falling of trees, and mudflows. Fiber optic acoustic sensors can significantly reduce the number of costly violations of the usual operation in railway transport.
With an increase in the accuracy of event localization, conditions are created for the application of DAS technology in areas with complex track development.
The signals (signatures) recorded during the movement of trains by the axis counters and the acoustic sensors are brought together to determine the exact location.
– the state of free/occupation of a track particular section;
– number of axes on the track section;
– direction of the traffic;
Level of safety and security
An important aspect of railway operation is safety. Security has many indications and affects numerous different areas. DAS provides a comprehensive solution to cope with several tasks – from labor protections to the protection against vandalism.
Distributed acoustic sensing offers railway operators a single solution for the protection of infrastructure and the safety of railway workers, with an extended range and high efficiency.
DAS converts measured signals (signatures) into valuable information, for example, about moving vehicles and individuals. Based on this information, messages are generated about the presence of objects or people, which can be more accurately classified due to the high sensitivity of thesensing system. It also allows for directly recognizing certain actions, for example, earthmoving on the way, and displaying the corresponding alarm messages.
The use of a distributed acoustic sensing systems for the railway industry opens up wide applications for monitoring the movement of trains, monitoring the condition of equipment, protecting infrastructure, and ensuring the safety of people in real-time.
Advantages of acoustic sensors
Moreover, recent advances in DAS make the sensing systemscost-effective, highly precise, herewith, these acoustic sensors do not require accurate alignment resulting in tuning vibration measurement to a particular point in the optical ﬁber. Thus, new DAS systems promote the speed of measurement beyond the previously established theoretical limit set by the sensing distance. The technology of new fiber optic acoustic sensors is based on the application of “colored” probe pulses or linear frequency multiplexing.
It should be noted that DAS is a highly reliable technology because it continues its operation even after it has been cut. DAS has the biggest influence in the signaling area, for example, distributed sensing helps to manage trains by control of their accurate position and motion in real-time. The technology enables to reduce journey times while increasing rail capacity and improving safety.
The operational principle of distributed acoustic sensing or fiber optic DAS is based on coherent Rayleigh backscattering in an optical fiber. Today the technology of acoustic sensing is regarded as a common technique for structural health monitoring of various dynamic actions in real-time. DAS applications in safety, security, and integrity monitoring systems promote the steady growth of the fiber optic DAS market.
Fiber optic acoustic sensors offer the opportunity to measure various changes in environmental parameters provoked by numerous events over long distances. The applications of DAS technology include transportation, oil and gas, and process control systems, herewith, they continue increasing. Additionally, distributed acoustic sensing allows performing measurements of both slowly changing (for instance, temperature or static strain) and fast-changing parameters (dynamic strain or vibration) providing fast and precise monitoring in real-time.
Therefore, DAS systems for the mentioned measurements are required to pay careful attention. Despite numerous developments that have been made in distributed acoustic sensing to increase the measurement speed over short distances with high spatial resolution, measurements at long distances remain considerably slow. Nevertheless, fiber optic acoustic sensors provide interesting alternatives for fast distributed measurements over long distances.
Such developments in distributed acoustic sensing as “use of high ER pulses to reduce coherence noise, fast denoising in the optical domain using optical pulse coding technique, generation of high ER pulses using nonlinear Kerr eﬀect, and the identiﬁcation of pulse shapes robust against modulation instability” enable to enhance the performance of fiber optic acoustic sensors.
Moreover, recent advances in DAS make the sensing systems cost-effective, highly precise, herewith, these acoustic sensors do not require accurate alignment resulting in tuning vibration measurement to a particular point in the optical ﬁber. Thus, new DAS systems promote the speed of measurement beyond the previously established theoretical limit set by the sensing distance. The technology of new fiber optic acoustic sensors is based on the application of “colored” probe pulses or linear frequency multiplexing.
Finally, the improved distributed acoustic sensors have higher spatial resolution due to the use of tweaking of the conventional set up to make the optical noise lower, and more accurate quantitative measurement of an external impact thanks to frequency shift measurements and direct phase demodulation techniques.
The sensing system is a fundamental device that presents data information about the features of the surrounding environmental conditions to electronic tools. The information obtained through distributed fiber sensing is used for analytical purposes or processed and employed to take specific actions. Herewith, today distributed sensors find widespread application since they are applied in most of the human daily used items.
For instance, distributed optical fiber sensors apply light to probe a kilometer-length optical fiber employed as the sensing system. Thus, distributed sensors allow for the detection of strain or temperature variations along the fiber length. The principle of distributed optical fiber sensor operation is based on “scattering processes happening along with the optical fiber, either Rayleigh stimulated Brillouin or Raman scattering”.
The characteristics of various scattering processes offer different applications to distributed sensors. For example, distributed optical fiber sensors based on Raman technology are regarded as highly efficient temperature sensing devices. Recent developments in sensing technology enable us to reach a better resolution, higher bandwidth, or longer-range operation.
Nowadays new sensing technique to interrogate an optical fiber applying the Rayleigh backscattering process is considered to be very advanced. Such distributed sensors are based on phase-sensitive optical time-domain reflectometry (OTDR) technology, herein, they use a train of linearly-chirped optical pulses resulting in a quite simple conventionally used methodology. Additionally, distributed acoustic sensing for phase-sensitive OTDR technology provides amazing robustness against laser phase noise and a record measured sensitivity.
The technology of distributed acoustic sensing for phase-sensitive OTDR has been experimentally demonstrated a couple of years ago, based on the application of a chirped-pulse as a probe, in an otherwise direct-detection-based standard setup: chirped-pulse (CP-)ΦOTDR. The benefits of DAS systems include intrinsic immunity to fading points and use of direct detection, therefore, distributed acoustic sensing offers reliable high sensitivity measurements.
Finally, DAS technology for OTDR finds its use in diverse applications that include seismology or civil engineering (monitoring of pipelines, train rails, etc.), and new applications based on distributed acoustic or temperature sensing appear everyday. Such a distributed optical fiber sensor can operate in up to 100 km with a low cost-setup, showing performances close to the attainable limits for a given set of signal parameters.