Laboratory testing of low-frequency strain measured by distributed acoustic sensing (DAS)

Becker, Matthew W.; Ciervo, Christopher; Coleman, Thomas

doi:10.1190/segam2018-2997900.1

Cited by 16 publications

(4 citation statements)

References 4 publications

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“…Laboratory investigations are beginning to provide a bottom-up understanding of how the isolated fiber-optic, or fiber cable package act as a sensing element (Becker et al, 2018;Papp et al, 2017). A few models have been proposed to upscale these results to seismic field data (Kuvshinov, 2016;Reinsch et al, 2017).…”

Section: Ground-to-fiber Strain Transfermentioning

confidence: 99%

On the Broadband Instrument Response of Fiber‐Optic DAS Arrays

2020

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Distributed Acoustic Sensing (DAS) is a novel tool in array seismology that measures the phase of backscattered laser pulses traveling in a fiber‐optic cable, and relates this measurement to the axial strain induced on the cable by a propagating seismic wavefield. Combining DAS with telecommunications optical fiber networks has begun to address a range of earth science questions where cost and field logistics have historically hindered observations. Unlike classic inertial seismometers, DAS instrument response is presently unquantified. This topic includes a variable sensing element—the fiber, including packaging and installation—which changes between experiments. Ignoring this element, one DAS record should approximate a fixed‐length strain gauge, which exactly measures Earth's motion down to quasi‐static frequencies relevant to geodesy. In this paper, we test this hypothesis using seismological observations of teleseismic earthquakes and microseism noise spanning the 1 to 120 s period range. We use a commercial DAS interrogator unit connected to an optical fiber previously used for telecommunication and a colocated broadband seismometer to estimate the DAS transfer function. We find a 1:1 correspondence with actual ground motion from 10–120 s. At shorter periods (1–10 s), DAS amplitude response is enhanced by 3–11 dB. Phase response is flat over this range of periods. We interpret the recovered DAS response function in terms of hypothesized fiber coupling and photonic effects. We propose this calibration methodology for future DAS experiments where seismic amplitude information is desired.

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Section: Ground-to-fiber Strain Transfermentioning

confidence: 99%

On the Broadband Instrument Response of Fiber‐Optic DAS Arrays

2020

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“…Sensor coupling is critical for any seismic instrument. Laboratory tests show significantly weaker responses for loosely bonded and gel-filled fiber in metal tube sensors relative to cemented bare fiber (Papp et al 2017, Becker et al 2018), yet surface waves are routinely observed with fiber packages deployed in shallow trenches in urban areas (Dou et al 2017, Martin et al 2017a, or even in deployments on the surface simply weighed down temporarily (Spikes et al 2019). Careful DAS amplitude response calibration using colocated reference has found a 5-to 10-dB elevated amplitude response at frequencies above 0.1 Hz for many teleseismic earthquakes and nighttime microseism recording (Lindsey et al 2020), suggesting coupling must be considered for full-waveform and amplitude-based DAS studies.…”

Section: Distributed Acoustic Sensing Instrument Responsementioning

confidence: 99%

Fiber-Optic Seismology

Lindsey

Martin

2021

Annu. Rev. Earth Planet. Sci.

176

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Distributed acoustic sensing (DAS) is an emerging technology that repurposes a fiber-optic cable as a dense array of strain sensors. This technology repeatedly pings a fiber with laser pulses, measuring optical phase changes in Rayleigh backscattered light. DAS is beneficial for studies of fine-scale processes over multi-kilometer distances, long-term time-lapse monitoring, and deployment in logistically challenging areas (e.g., high temperatures, power limitations, land access barriers). These benefits have motivated a decade of applications in subsurface imaging and microseismicity monitoring for energy production and carbon sequestration. DAS arrays have recorded microearthquakes, regional earthquakes, teleseisms, and infrastructure signals. Analysis of these wavefields is enabling earthquake seismology where traditional sensors were sparse, as well as structural and near-surface seismology. These studies improved understanding of DAS instrument response through comparison with traditional seismometers. More recently, DAS has been used to study cryosphere systems, marine geophysics, geodesy, and volcanology. Further advancement of geoscience using DAS requires several community efforts related to instrument access, training, outreach, and cyberinfrastructure. ▪ DAS is a seismic acquisition technology repurposing fiber optics as arrays of dynamic strain sensors at 1- to 10-m spacing over kilometers. ▪ Easy DAS installations have availed time-lapse geophysical sensing in formerly inaccessible sites: urban, icy, and offshore areas. ▪ High-frequency wavefields recorded by DAS are analyzed with array-based methods to characterize seismic sources and image the subsurface. ▪ DAS has shown low-frequency sensitivity in the laboratory and field, for slow hydrodynamic and geodynamic processes. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 49 is May 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

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“…Even when the fiber optic cable is coupled to the formation, strain may not be perfectly transferred through the cable itself (Becker et al, 2018b; Lindsey et al, 2020). Downhole cables are generally designed to limit strain on the fiber to prevent damage.…”

Section: Introductionmentioning

confidence: 99%

“…As a result, fiber optic cable construction can have a marked impact on strain measurement. For example, Becker et al (2018b) cemented five different cables of varying design into a single PVC pipe and then strained that pipe using a stepper motor. DAS measured strains in the fibers varied by a factor of two, even though they were all subjected to identical displacement.…”

Section: Introductionmentioning

confidence: 99%

Distributed Acoustic Sensing as a Distributed Hydraulic Sensor in Fractured Bedrock

Becker

Coleman²,

Ciervo

2020

Water Resources Research

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Distributed acoustic sensing (DAS) was originally intended to measure oscillatory strain at frequencies of 1 Hz or more on a fiber optic cable. Recently, measurements at much lower frequencies have opened the possibility of using DAS as a dynamic strain sensor in boreholes. A fiber optic cable mechanically coupled to a geologic formation will strain in response to hydraulic stresses in pores and fractures. A DAS interrogator can measure dynamic strain in the borehole, which can be related to fluid pressure through the mechanical compliance properties of the formation. Because DAS makes distributed measurements, it is capable of both locating hydraulically active features and quantifying the fluid pressure in the formation. We present field experiments in which a fiber optic cable was mechanically coupled to two crystalline rock boreholes. The formation was stressed hydraulically at another well using alternating injection and pumping. The DAS instrument measured oscillating strain at the location of a fracture zone known to be hydraulically active. Rock displacements of less than 1 nm were measured. Laboratory experiments confirm that displacement is measured correctly. These results suggest that fiber optic cable embedded in geologic formations may be used to map hydraulic connections in three-dimensional fracture networks. A great advantage of this approach is that strain, an indirect measure of hydraulic stress, can be measured without beforehand knowledge of flowing fractures that intersect boreholes. The technology has obvious applications in water resources, geothermal energy, CO 2 sequestration, and remediation of groundwater in fractured bedrock. Plain Language Summary A technology developed for measuring vibrations on fiber optic cable, distributed acoustic sensing, is used to measure strain in bedrock in response to pumping and injection. This technology is extremely sensitive to dynamic strain, measuring displacements approaching the size of a water molecule. Field tests showed that fluid-induced strain in rock exhibits complex geometry. The technology can be used to better understand flow in bedrock, optimize geothermal energy extraction, and identify leakage from subsurface injection systems.

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Laboratory testing of low-frequency strain measured by distributed acoustic sensing (DAS)

Cited by 16 publications

References 4 publications

On the Broadband Instrument Response of Fiber‐Optic DAS Arrays

On the Broadband Instrument Response of Fiber‐Optic DAS Arrays

Fiber-Optic Seismology

Distributed Acoustic Sensing as a Distributed Hydraulic Sensor in Fractured Bedrock

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