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.
Distributed fiber optic sensing is considered to be an advanced technology that allows changing the principle of infrastructure management. The fiber optic technology converts optical fibers into thousands of sensors and controls tens of kilometers of an asset with a single FBG interrogator. Thus, optical fiber sensors are a highly cost-effective fiber optic solution that is used by numerous industries every year.
Herewith, Distributed Acoustic Sensing or DAS technology has a wider scope of potential fiber optic applications, both scientific (seismic, mechanical) and industrial (security, integrity, operation monitoring) because DAS detects vibrations along with the optical fiber. It should be noted that the detection and analysis of sound waves remains one of the most effective techniques for sensing information.
The principle of distributed acoustic sensing operation is based on the use of a fiber optic cord that is placed in an acoustic sensing housing that picks up sounds along the length of the cord. Therefore, DAS technology makes it possible to obtain a more detailed scan of hazardous or hard-to-reach places.
Moreover, the additional DAS benefit includes the possibility to identify the strain of the fiber optic cord to help prevent damages. The fields of DAS applications include the sensing disturbances around fracking operations underground that is possible to perform during and after the process to ensure safety. Also, distributed acoustic sensing can be applied to guide the fracking operation to the right underground deposits.
Generally, the DAS system is used to offer less disturbance to places applied for oil drilling and fracking. Nevertheless, DAS technology is ideally suited for the measurement of seismic events and control of fault lines with less of an equipment footprint and impact on the surrounding area.
Distributed acoustic sensing offers the following advantages that include:
High precision due to its linear response;
Very high sensitivity;
Great coverage, exceeding 70 km;
The possibility of programming real-time applications directly on the device.
Finally, the fields of DAS applications continue expanding. At the present time, it includes perimeter intrusion detection, third-party interference detection, power cable monitoring, traffic monitoring (roads, railway, subway), seismic activity monitoring, subsea cable monitoring, asset integrity, oil, gas, and water pipelines.
Distributed acoustic sensing or DAS system is a relatively new technology that finds numerous fields of application and has several benefits such as low cost, complete coverage, and repeatability for data collection. The principle of DAS system operation is based on the use of an optical fiber cable instead of geophones in conventional systems for acoustic recording.
Compared to traditional geophones, the DAS system demonstrates more advantages during its applications. For example, DAS technology employs a slim optical cable, due to which there are no limits in a horizontal well or ultra-slim well. Also, the cost of fiber optic cables is lower than geophones’.
Moreover, the installation process of optical fiber cables is pretty easy and can include other fiber optic sensors such as distributed temperature sensing and distributed pressure sensing. Herewith, the cables offer continuous data transmission and the DAS system can be employed in the exploration well, production well, and observation well.
In spite of the mentioned benefits, nowadays distributed acoustic sensing has some limitations, for example, the low S/N ratio, channel depth uncertainty, poor transverse fiber sensibility that are an obstacle for future applications. The most frequent DAS applications include:
vertical seismic profile or VSP;
well and reservoir surveillance;
hydraulic fracturing monitoring, and diagnostics.
Despite the fact that distributed acoustic sensing technology is a highly promising prospect in the oil and gas industry, it requires several improvements for more stable operation. The most crucial problem is the development of the S/N ratio that can be solved by reducing the noise floor, using a stronger acoustic source.
It should be mentioned that distributed acoustic sensing data are connected with the strain amplitude, therefore, this factor allows to enlarge the S/N ratio by coherent stacking and correlation.
Also, removal of the random noise in distributed acoustic sensing can be made with the help of bandpass and median filters. In addition, it is very important to process a huge amount of data quickly, since DAS measurements are excessive. Herewith, DAS technology is often combined with multi-functional optical fiber sensors and the obtained measurements allow making the ambiguity lower.
Perimeter security is considered to be a significant problem for airports all over the world. For example, Denver International Airport is one of the largest airports in the USA that includes 25 airlines. Its size requires constant security operations, and it is highly challenging because the larger an airport’s size is, the bigger its perimeter, the more difficult it becomes to save integrity and police borders.
The ways for airport border security include walls, barbed-wire fencing, CCTV, guardhouse, and manned patrols. Of course, the mentioned measures are effective, however, they are mostly passive barriers that are possible to cross. Thus, the use of modern technologies such as artificial intelligence and distributed acoustic sensing or DAS system allows improving the situation with the management and security of airports.
DAS technologies offer continuous monitoring of the whole perimeter and have alerts in real-time, provide security groups the complete overview that is required for operational management, and respond to threats effectively. Moreover, DAS systems have many benefits for airport operators, when it comes to the adoption of digital technologies, their integration into perimeter security strategies.
The principle of DAS operation is based on the use of fiber optic cables that are similar to an ecosystem of highly-sensitive, vibrational sensors that creates a smart barrier around the whole perimeter. Herewith, it is possible to install a distributed acoustic sensing system both in the ground and established into fencing.
Also, it should be noted that DAS is a highly reliable technology because it continues its operation even after it has been cut. DAS system has the following advantages for the airport area:
The ability to identify and locate such potentially threatening activities as walking people, excavation and tunneling processes, fence climbing, and cutting.
Distributed acoustic sensing allows collecting data on prolonged or historical events.
DAS technology highlights long term patterns and points of potential vulnerability.
DAS offers a clear differentiation between numerous threats that might occur at the edge of an airport, herein, the technology provides security teams special alarms that enable to make decisions quickly.
Finally, DAS monitoring is the ideal solution for airport security because there are the earliest alerts for security groups. Moreover, airport operators can combine distributed acoustic sensing with other traditional techniques.