Empirical Investigations of the Instrument Response for Distributed Acoustic Sensing (DAS) across 17 Octaves

Abstract

With the potential of high temporal and spatial sampling and the capability of utilizing existing fiber-optic infrastructure, distributed acoustic sensing (DAS) is in the process of revolutionizing geophysical ground-motion measurements, especially in remote and urban areas, where conventional seismic networks may be difficult to deploy. Yet, for DAS to become an established method, we must ensure that accurate amplitude and phase information can be obtained. Furthermore, as DAS is spreading into many different application domains, we need to understand the extent to which the instrument response depends on the local environmental properties. Based on recent DAS response research, we present a general workflow to empirically quantify the quality of DAS measurements based on the transfer function between true ground motion and observed DAS waveforms. With a variety of DAS data and reference measurements, we adapt existing instrument-response workflows typically in the frequency band from 0.01 to 10 Hz to different experiments, with signal frequencies ranging from 1/3000 to 60 Hz. These experiments include earthquake recordings in an underground rock laboratory, hydraulic injection experiments in granite, active seismics in agricultural soil, and icequake recordings in snow on a glacier. The results show that the average standard deviations of both amplitude and phase responses within the analyzed frequency ranges are in the order of 4 dB and 0.167π radians, respectively, among all experiments. Possible explanations for variations in the instrument responses include the violation of the assumption of constant phase velocities within the workflow due to dispersion and incorrect ground-motion observations from reference measurements. The results encourage further integration of DAS-based strain measurements into methods that exploit complete waveforms and not merely travel times, such as full-waveform inversion. Ultimately, our developments are intended to provide a quantitative assessment of site- and frequency-dependent DAS data that may help establish best practices for upcoming DAS surveys.

Rotation, Strain, and Translation Sensors Performance Tests with Active Seismic Sources

“…In order to characterize the instrument response of fiber-optic cables used for distributed acoustic sensing, Wang et al [ 46 ] and Lindsey et al [ 47 ] co-located fiber-optic DAS-arrays with conventional seismometers and compared the direct strain measurement to the strain that was obtained by finite differencing of two seismometer waveforms. In these studies, the observations match well for signal periods from 10 s to 120 s. Paitz et al [ 48 ] extended the frequency range for this kind of comparison and found a good match of observations for frequencies from 1/3000 Hz to 60 Hz.…”

On the Detection Capabilities of Underwater Distributed Acoustic Sensing

“…The recorded DAS noise arises from several natural sources, including ocean‐solid earth interactions, which produce surface gravity waves and microseisms, which are recorded at frequencies where DAS response is flat (e.g., Lindsey et al., 2020; Paitz et al., 2020). The natural noise amplitude can be affected by local seismic amplification effects.…”

“…To further investigate the response of underwater DAS to transient ground deformations, DAS signals are compared to the ground motion measurements recorded by nearby seismometers. The ability to convert DAS records to ground motions was investigated and demonstrated by several previous studies (e.g., Daley et al., 2016; Lindsey et al., 2020; Paitz et al., 2020; Wang et al., 2018). To convert strain‐rate records to ground motions, phase velocities need to be determined.…”

“…Since DAS is mostly sensitive to deformation along optical fibers, i.e., extension and compression (e.g., Ajo‐Franklin et al., 2019; Kuvshinov, 2016; Mateeva et al., 2014; Papp et al., 2017; Yu et al., 2018), only single component measurements are obtained, oriented along the fiber. Previous studies demonstrate the broadband response of DAS by comparing DAS and co‐located broadband seismometers (Jousset et al., 2018; Lindsey et al., 2020; Paitz et al., 2020). These studies show that reliable deformation measurements can be obtained for both low (e.g., ambient noise, large earthquakes) and high (e.g., active sources, small earthquakes) frequency signals.…”

Seismic Applications of Downhole DAS

“…In general, the instrument response of DAS is broader than that of traditional receivers. However, it requires careful treatment and calibration as the fiber properties, casing, and installation method can influence it [31,[48][49][50]. A virtual DAS receiver is a single-axis measurement sensor, which is sensiti strain (or strain-rate) along the direction of the fiber.…”