Editors’ Vox is a blog from AGU’s Publications Department.

Distributed Acoustic Sensing (DAS) is a technology that uses short pulses of laser light and a fiber optic cable to measure acoustic signals. Because DAS is able to provide high resolution, continuous, and real-time measurements, it has been widely adopted in different sectors ranging from civil engineering to hazard mitigation. A new book in AGU’s Geophysical Monograph Series, Distributed Acoustic Sensing in Geophysics: Methods and Applications, examines the many applications of DAS in geophysics. We asked the book’s editors some questions about DAS and what readers can expect.

How would you explain “Distributed Acoustic Sensing” (DAS) to a non-expert?

A DAS system is an optoelectronic device with a laser light source and an optical detector measuring coherent Rayleigh backscattering generated by defects in the fiber. By quickly sampling this scattered light, DAS allows measurement of small strain changes in the material around the fiber with a 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 array.

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

What are the advantages of DAS compared to other sensing 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 the large number of sampled locations, the high spatial density, and broad bandwidth. DAS systems can also be used with fibers installed for other purposes including existing or abandoned telecommunications cables, which significantly reduces costs.

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

What real-world applications does DAS have?

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, railroads, highways, power stations, and wind farms. It plays a role in ensuring safety and security ranging from fire detection within tunnels and detecting gas leaks to perimeter security systems and monitoring critical facilities.

How has DAS been applied in different fields of geophysics?

DAS has found many applications in geophysics as part of 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 to 3D (three-dimension) image and monitor oil and gas reservoirs and for geothermal monitoring. DAS in boreholes have been used for vertical seismic profiling (DAS-VSP) both on-shore and in marine environments. In these scenarios, DAS is used to measure subsurface reflectivity, velocity, and attenuation allowing imaging of subsurface structures.

DAS can also be used for passive monitoring and is a powerful tool for detecting and location microseismic events generated by hydraulic fracturing operations. Repeated DAS-VSP surveys are now commonly used for 4D (four-dimension) monitoring of fluid movement including CO2 injection as part of geologic carbon storage.

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

DAS is increasingly being utilized for geo-hazard mitigation including the monitoring of local earthquakes, volcanoes, faults, and landslides. The same sensing capacity makes DAS an invaluable tool to monitor the built environment including the state-of-health of infrastructure and critical facilities, including pipelines, power stations, dams, bridges, tunnels, railroads, highways, and skyscrapers.

Near-surface geophysicists can also utilize DAS for imaging shallow structures for hazard characterization, monitoring aquifer variations, and characterizing seismic noise generated by human activity or natural systems such as rivers.

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

One area of exciting development is active experimentation with new fiber optic cables and network designs to improve broadside response and (theoretically) allow for 3C DAS measurements. Improvements in DAS interrogators, lasers, and the development of scattering enhanced fibers have also all 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 limits on spatial resolution and incorporating adjustable gauge lengths to enable optimization of trade-offs between SNR and channel density, satisfying a larger range of applications. Finally, the development of hybrid wireline/DAS interventions has allowed DAS surveys without permanent installation of fiber cables in wells.

Another exciting recent advance has been the utilization of DAS systems in the marine environment. In the context of monitoring offshore fields, advances in subsea wet tree wells and umbilical tie-backs have enabled life-of-field DAS-VSP acquisition. The use of offshore telecommunication cables for DAS acquisition has also greatly expanded opportunities for monitoring offshore faults and oceanographic processes.

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

We’re really excited to see the potential of DAS to grow in terms of its applications and usage.

We expect that the cost of DAS interrogator units and fiber installation in boreholes, on land, and on the seafloor will significantly decrease with a dramatic increase of users and rapid growth of DAS applications. More DAS networks will be established for dual roles of education and scientific studies, particularly in monitoring earthquakes, land subsidence, shallow structures, city traffic, infrastructures, landslides, faults on surface and seafloor, and underground water detection.

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

3C DAS measurement approaches will mature and be utilized for studying earthquake source mechanisms and anisotropy as well as 3D vector migration and imaging. Meanwhile, 4D DAS techniques will be improved to monitor CO2 injection, oil and gas production, fracture growth, and well activities and integrity in real-time.

In addition to using existing fiber cables, suitable geometry design of fiber installations using optimized surface-borehole DAS networks will facilitate application of array algorithms, improved detection of distant events, and imaging. Simultaneous measurement of multiple parameter measurements such as vibration, temperature, strain, and possibly electromagnetic (EM) fields using different fibers in the same fiber cable will be commonplace for both scientific and industrial purposes.

Finally, effective DAS data management and conveniently sharing of big DAS data from the dark fibers on land and seafloor, pipelines, campus DAS networks, and oil, gas, geothermal and mining fields will allow us to best utilize these valuable resources to develop clean energy, monitor the environment, mitigate geohazards, and make smarts cities. 

Who will find your book useful?

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

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

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

Distributed Acoustic Sensing in Geophysics: Methods and Applications, 2022, ISBN: 978-1-119-52179-2, list price, $199.95 (hardcover), $159.99 (e-book)

—Yingping Li (y1p2li58@gmail.com; ORCID logo 0000-0002-9661-5012), BlueSkyDas; Martin Karrenbach (ORCID logo 0000-0003-2190-8598), OptaSense Inc.; and Jonathan Ajo-Franklin (ORCID logo 0000-0002-6666-4702), Rice University and Lawrence Berkeley National Laboratory

Editor’s Note: It is the policy of AGU Publications to invite the authors or editors of newly published books to write a summary for Eos Editors’ Vox.

Citation: Li, Y., M. Karrenbach, and J. Ajo-Franklin (2022), Using Sound and Vibration Signals to Understand the Subsurface, Eos, 103, https://doi.org/10.1029/2022EO215001. Published on 6 January 2022.
This article does not represent the opinion of AGU, Eos, or any of its affiliates. It is solely the opinion of the author(s).
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