On the Broadband Instrument Response of Fiber‐Optic DAS Arrays

Abstract

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.

Sensing earth and environment dynamics by telecommunication fiber-optic sensors: An urban experiment in Pennsylvania USA

“…To verify that this DAS system (interrogator, fiber, and cable in conduit) records a useful metric of ground motion, we convert our DAS recordings to particle velocity and compare to the nearest broadband seismometer. This calibration procedure is based on several previous studies (Daley et al, 2016;Wang et al, 2018;Lindsey et al, 2020a). To ensure this paper is self-contained, we briefly review this procedure.…”

“…We assume refractive index changes linearly with strain rate ( ∆n n = (ζ −1) ∆x x ). The scalar multiplicative factor (ζ) is determined by the material properties: ζ = 0.735 for single-mode fiber glass with light propagating inside (Lindsey et al, 2020a). Hence the phase changes at the same 4 https://doi.org/10.5194/se-2020-103 Preprint.…”

“…, where σ is the threshold to avoid instabilities in division by zero frequency and wavenumber values (Lindsey et al, 2020a). Finally we apply inverse Fourier transform to V (ω, k) to obtain particle velocity in the time domain.…”

“…Here we report observations of earthquakes and thunderquakes from Penn State FORESEE array. We further include a comparison of the array to standard seismometers at both low and high frequencies, a necessary step to verify the broadband response of any new DAS array (Lindsey et al, 2020a). Figure 6 shows DAS recordings of four earthquakes (Peru M8.0 earthquake on May 26 2019, Ridgcrest M7.1 earthquake on July 06 2019, Tennessee M3.8 earthquake on January 20 2020, and PA M1.1 earthquake on August 27 2019.…”

Very Broadband Strain-Rate Measurements Along a Submarine Fiber-Optic Cable Off Cape Muroto, Nankai Subduction Zone, Japan

“…Wang et al (2018) have demonstrated the similarity between DAS and seismograph strain measurements, and confirmed that the observed signal is axial strain along a fiber-optic cable in the ordinary frequency range for seismic observations (i.e., several Hertz). At lower frequencies, Ajo-Franklin et al (2019) observed the surface waves from large teleseismic earthquakes, and Lindsey et al (2020) demonstrated how to convert DAS records into velocity seismograms via a comparative analysis of DAS and broadband seismometer signals. Further applications for subsurface structural investigations have been proposed, such as receiver function (Yu et al, 2019) and H/V spectral ratio (Spica et al, 2020a) analyses.…”