Use sound and vibration signals to understand the subsoil

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Editors Vox is a blog of the publications service of AGU.

Distributed Acoustic Sensing (DAS) is a technology that uses short pulses of laser light and fiber optic cable to measure acoustic signals. Since DAS is capable of providing high resolution, continuous and real-time measurements, it has been widely adopted in different industries ranging from civil engineering to risk mitigation. A new book in AGU’s geophysical monograph series, Distributed acoustic detection in geophysics: methods and applications, examines the many applications of DAS in geophysics. We asked the editors of the book a few questions about DAS and what readers can expect.

How would you explain “distributed acoustic detection” (DAS) to a non-expert?

A DAS system is an optoelectronic device with a laser light source and an optical detector measuring the coherent Rayleigh backscattering generated by defects in the fiber. By quickly sampling this scattered light, the DAS can measure small stress changes in the material around the fiber with fine spatial resolution and a wide frequency range. Using this approach, a single DAS unit can obtain seismic measurements at thousands of points spaced a few meters apart, serving as an ultra-dense seismic network.

Illustration explaining the operating principle of distributed acoustic detection. Credit: Li et al. [2021], Figure 1.1

What are the advantages of DAS over other detection techniques?

Traditionally, seismic waves are recorded using a seismometer, geophone, or accelerometer, all of which are discrete sensors. DAS offers advantages over conventional sensors in terms of large number of sampled locations, high spatial density and high bandwidth. DAS systems can also be used with fibers installed for other purposes, including existing or discontinued telecommunications cables, significantly reducing costs.

Fiber optic cables are quite durable if they are designed and packaged correctly. They can also withstand extreme environments, especially high temperatures and pressures that would destroy conventional seismic sensors. This makes the DAS an attractive tool for measurements in locations ranging from the freezing cold of the High Arctic to the sweltering heat of geothermal and volcanic systems.

What are the real applications of DAS?

Anything that generates a small strain or vibration can be monitored using DAS systems. It can be used to measure and monitor activities such as digging or vehicle traffic, as well as the movement of animals. In civil engineering, DAS is used to continuously monitor infrastructure such as pipelines, dams, bridges, tunnels, railways, highways, power plants and wind farms. It plays a role in ensuring safety and security ranging from tunnel fire detection and gas leak detection to perimeter security systems and monitoring of critical facilities.

How has DAS been applied in different fields of geophysics?

DAS has found many applications in geophysics in both active source imaging and passive acquisition, including microseismic monitoring.

A fiber optic cable waiting to be installed. Credit: disoculated (CC BY-SA 2.0)

In the energy industry, DAS has been used for 3D (three-dimensional) imaging and monitoring of oil and gas reservoirs and for geothermal monitoring. DAS in boreholes have been used for vertical seismic profiling (DAS-VSP) both on land and in marine environments. In these scenarios, the DAS is used to measure the reflectivity, velocity and attenuation of the subsoil, thus enabling imaging of subsoil structures.

DAS can also be used for passive surveillance and is a powerful tool for detecting and locating microseismic events generated by hydraulic fracturing operations. Repeated DAS-VSP readings are now commonly used for 4D (four dimensional) monitoring of fluid movement, including CO2 injection in the context of geological carbon storage.

The low frequency components of the DAS can also be used to measure strain such as slow strain changes associated with fracture propagation, surface subsidence or even natural cycles such as land tides.

DAS is increasingly used for geohazard mitigation, including monitoring for local earthquakes, volcanoes, faults and landslides. The same detection capability makes DAS a valuable tool for monitoring the built environment, including the health of critical infrastructure and facilities, including pipelines, power plants, dams, bridges, tunnels, railroad tracks, highways and skyscrapers.

Near-surface geophysicists can also use DAS to image shallow structures to characterize hazards, monitor variations in the aquifer, and characterize seismic noise generated by human activity or natural systems such as rivers.

What have been the most exciting developments in DAS instrumentation and applications in recent years?

An exciting area of ​​development is the active experimentation of new fiber optic cables and network designs to improve wide response and (theoretically) enable DAS 3C measurements. Improvements in DAS interrogators, lasers, and the development of enhanced scattering fibers have also served to improve the signal-to-noise ratio (SNR) of DAS, in some cases greatly expanding potential applications.

Fiber optic cables after being installed in a trench. Image Credit: Jonathan Ajo-Franklin

Multi-parameter fiber optic measurements are another exciting trend with cables combining DAS with other techniques such as DTS and DSS. Interrogator manufacturers are also pushing the boundaries of spatial resolution and incorporating adjustable gauge lengths to allow optimization of tradeoffs between SNR and channel density, satisfying a wider range of applications. Finally, the development of hybrid wire / DAS interventions has enabled DAS readings without the permanent installation of fiber cables in the shafts.

Another interesting recent advance has been the use of DAS systems in the marine environment. In the context of offshore field monitoring, advances in submarine wet tree wells and umbilical connections have enabled DAS-VSP acquisition over the life of the field. The use of offshore telecommunication cables for DAS acquisition has also greatly expanded the possibilities for monitoring offshore faults and oceanographic processes.

How do you see DAS technologies and applications evolving over the next decade?

We are really excited to see the development potential of DAS in terms of applications and use.

We predict that the cost of DAS interrogators and installation of fiber in boreholes, on land and on the seabed will decrease dramatically with a dramatic increase in the number of users and rapid growth of DAS applications. More DAS networks will be established for the dual role of education and scientific studies, especially in monitoring earthquakes, land subsidence, shallow structures, city traffic, infrastructure, landslides , faults on the surface and at the bottom of the sea and the detection of groundwater.

A perspective of fiber optic cables from a trench. Credit: Jonathan Ajo-Franklin

DAS 3C measurement approaches will mature and will be used to study the mechanisms and anisotropy of seismic sources as well as 3D vector migration and imagery. At the same time, DAS 4D techniques will be enhanced to monitor CO2 injection, oil and gas production, fracture growth, and well activity and integrity in real time.

In addition to the use of existing fiber cables, an appropriate geometric design of fiber installations using optimized DAS networks at the borehole surface will facilitate the application of network algorithms, better detection of distant events and imaging. Simultaneous measurement of multiple parameters such as vibration, temperature, stress and possibly electromagnetic (EM) fields using different fibers in the same fiber cable will be common for scientific and industrial purposes.

Finally, efficient DAS data management and convenient sharing of big DAS data from dark fibers on land and seabed, pipelines, campus DAS networks, and oil, gas, geothermal and mining fields will enable us to ” make the best use of these precious resources to develop clean energy, monitor the environment, mitigate geohazards and create smart cities.

Who will your book be useful to?

This book is a comprehensive manual for anyone interested in learning DAS principles and applying this rapidly developing new technology to their field related to geosciences. This can include geophysicists, seismologists, geologists, hydrologists, geoscientists, and environmental scientists.

It is also useful for people working in industry: engineers, people providing and selling DAS services, software developers, AI experts, data scientists managing DAS big data.

Finally, it will be of interest to graduate and undergraduate students in these respective fields.

Distributed acoustic detection in geophysics: methods and applications, 2022, ISBN: 978-1-119-52179-2, list price, $ 199.95 (hardcover), $ 159.99 (eBook)

—Yingping Li ([email protected]; 0000-0002-9661-5012), BlueSkyDas; Martin Karrenbach ( 0000-0003-2190-8598), OptaSense Inc .; and Jonathan Ajo-Franklin ( 0000-0002-6666-4702), Rice University and Lawrence Berkeley National Laboratory

Editor’s Note: It is AGU Publications ‘policy to invite authors or editors of recently published books to write an abstract for Eos Editors’ Vox.

Quote: Li, Y., M. Karrenbach and J. Ajo-Franklin (2022), Using sound and vibration signals to understand the subsoil, Éos, 103 years old, https://doi.org/10.1029/2022EO2150001. Posted on January 6, 2022.
This article does not represent the opinion of AGU, Eos, or one of its affiliates. This is solely the opinion of the author.


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