|Fig. 1: A schematic DAS. A fiber optic is attached to an interrogator unit. The interrogator unit probes the cable with laser pulses, and energy is backscattered where the fiber is vibrated. It processes backscattered energy into information about vibrations along the cable. (Source: E. Martin - after Mateeva et al. )|
As energy companies continue to produce from reservoirs using new enhanced oil recovery techniques, a growing need has developed for frequent low-cost reservoir monitoring solutions. One such technology that has seen rapid development over the past decade is Distributed Acoustic Sensing (DAS), a system that repurposes a fiber optic cable so that it can detect vibrations along the length of the cable. 
The use of DAS in monitoring reservoirs has exploded for three reasons: (i) Only a single power source at the surface is needed, so there is no worry about electronics failures in more extreme conditions down wells. (ii) The cost per sensor is low relative to traditional vibration sensors. (iii) Fiber optics are small and flexible, thus easy to install so that they cover an entire well at all times.  By decreasing the cost of continuous monitoring throughout production, energy companies can not only make more economical decisions, but also improve their decision-making with regards to environmental health and safety at fields where new enhanced oil recovery techniques are being used.
As seen in Fig. 1, an interrogator unit with a laser is attached to a fiber optic cable that has some small variations in its index of refraction. If there are seismic vibrations around the fiber, this will stretch the fiber. A short laser pulse is sent down the fiber (typically around one meter), and some of the light undergoes Rayleigh backscattering and bounces off parts of the fiber that are being stretched by the vibrations.  With DAS we can say where vibrations occur and how big those vibrations are. Depending on how long the backscattered light takes to return to the interrogator unit, we can say that that signal came from a particular location along the fiber. We assume that fiber segments associated with more backscattered energy are in regions with stronger vibrations. 
|Fig. 2: In a helical fiber optic cable, the thin fiber optic cable is wrapped around a central tube or another thick cable. (Source: E Martin - after Den Boer et al. )|
Note that a straight fiber optic cable is only sensitive to waves propagating along the length of the fiber, so vibrations coming in at a perpendicular angle to a straight fiber optic cable would not be detected well. To process the backscattered energy, the interrogator unit looks at differences between signals from two segments of fiber that are each a pulse length (typically 1 m) long, and that are a gauge length (typically 10 m) apart.  As a consequence DAS has much worse sensitivity (proportional to cos2 θ ) to waves coming in at an angle, θ , to the fiber than a traditional vibration sensor (proportional to cos θ ). [3,4] Compounding this issue, DAS looks at strain rate rather than strain, which boosts high frequency noise.
|Fig. 3: A DAS system is installed with a horizontal fiber optic cable over a reservoir. A seismic survey is conducted with a vibration source at the surface, and much of the energy that hits the reservoir below reflects back at an angle nearly perpendicular to the cable. New helical cables must be used in seismic surveys with this setup. (Source: E. Martin)|
One recent advance is the use of fibers that are wound in a helix as shown in Fig. 2.  Because these helical fibers can be stretched in any direction, they can detect vibrations coming in from any angle. This partially overcomes the limitations of more common straight fiber optic cables. One example of the improvement would be if a straight fiber and a helical fiber were installed horizontally above a reservoir as in Fig. 3. The straight fiber would not detect much reflection energy carrying information about the reservoir. However, the helical cable could detect this energy and thus help monitor changes in the reservoir.
DAS is a rapidly developing technology that is helping keep costs down while greatly increasing monitoring capabilities throughout the production of oil fields, which is particularly important when enhanced oil recovery techniques are used. New advances including the use of helical fibers are helping overcome the early limitations of DAS, thus expanding its adoption in industry.
© Eileen Martin. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 T. M. Daley et al., "Field Testing of Fiber-Optic Distributed Acoustic Sensing (DAS) for Subsurface Seismic Monitoring," The Leading Edge, 32, 966 (2013).
 R. Posey, Jr. , G.A. Johnson, and S.T. Vohra, "Strain Sensing Based on Coherent Rayleigh Scattering in an Optical Fibre," Electron. Lett. 36, 1688 (2000).
 A. Mateeva et al. "Advances in Distributed Acoustic Sensing (DAS) for VSP," Society of Exploration Geophysicists, segam2012-0739.1, SEG Technical Program Expanded Abstracts 2012.
 J. J. Den Boer et al., "Detecting Broadside Acoustic Signals With a Fiber Optical Distributed Acoustic Sensing (DAS) Assembly," U.S. Patent Application US 2014/0345388 A1, 27 Nov 14 [PCT/US2012/069464, 13 Dec 12].