1. Introduction
Distributed acoustic sensing (DAS) in permanent monitoring installations is increasingly being used for seismic investigations [
1,
2,
3,
4,
5]. Permanent downhole fiber installations often achieve near-perfect elastic coupling with the surrounding earth (e.g., by cementing a fiber to the outside of casing), which enables some interrogator units (IUs) to acquire seismic data of a quality approaching the response of single-component geophones [
6,
7]. However, for lower-cost surface fiber deployments (e.g., fiber directly laid on the ground, passed through buried conduits, or placed in shallow trenches), it is less obvious how to effectively achieve elastic ground-fiber coupling and thereby avoid the associated loss in data quality. A related important question is how significant the deleterious effects of imperfect coupling are on DAS recordings for scenarios involving spatial and/or temporal variations in the ground-fiber coupling.
Despite the complexities involved in near-surface horizontal fiber installations, many successful DAS research examples using such deployments have emerged in the literature. For terrestrial deployments, Becker et al. presented a laboratory experiment that demonstrates the possibility of DAS-based earth-tide observations [
8]. Fernández et al. showed the promising performance of low-frequency (<1 Hz) DAS recording when compared to high-quality broadband seismometers [
9]. Spical et al. used the ambient recording from a horizontal DAS array deployed on Stanford University campus to calculate the horizontal over vertical (H/V) spectral ratio and compute interpretable near-surface imaging results [
10]. Yuan et al. investigated Rayleigh waves excited by passing cars recorded on a roadside section of the Stanford DAS-2 array and constructed a pseudo-2D shear-wave velocity profile by integrating 1D inversions [
11]. Fang et al. demonstrated the feasibility of using an existing horizontal DAS array for near-surface velocity monitoring by measuring strong time-lapse variations [
12]. Lindsey et al. analyzed and calibrated the sub-1 Hz DAS instrument response using co-located broadband seismometer records as the reference of true ground motion [
13]. Shragge et al. presented the results from a low-frequency DAS experiment that uses surface waves to constrain the shear-wave velocity profile to 0.5 km depth [
14].
There is also a growing number of examples that use optical fiber deployed in marine seafloor environments. For example, Williams et al. and Sladen et al. successfully recorded ocean microseism energy and detected regional earthquakes using ocean-bottom fiber arrays with onshore DAS IUs [
15,
16]. Lindsey et al. presented observations from four days of recording on an ocean-bottom “dark fiber” array and detected various signals such as minor earthquakes, primary and secondary microseisms, and sediment transport due to storm action [
17]. Jousset et al. demonstrated the application of 15 km of dark telecommunication fiber deployed on the Reykjanes Peninsula, Southwest Iceland to record and process high-resolution seismic waveforms that showed features such as normal faulting and volcanic dykes with unprecedented resolution [
18]. Cheng et al. constructed a near-seafloor velocity model and developed improved constraints on shallow submarine faults by inverting multimodal dispersion curves obtained from ambient DAS records acquired on 20 km of ocean-bottom cable [
19]. Ide et al. observed many earthquakes using a submarine cable located near the Nankai subduction zone [
20]. Finally, Lindsey et al. provided a review of the increasing number of long-term monitoring experiments conducted in US national labs and universities [
21].
An interesting observation is that non-elastic coupling is not commonly discussed in the aforementioned DAS literature. We postulate that this is because most IU designs either natively acquire DAS data in strain or strain-rate format that necessitates assuming a gauge length (GL) defined in hardware or applying a GL post-acquisition through digital processing to generate interpretable waveforms. Because applying a GL generally involves introducing a 1D spatial filter, this can adversely affect the wavenumber spectra of DAS records by introducing spectral notches (i.e., zeros) as well as variable spectral weighting factors [
22]. While there are digital signal processing approaches that could be used to mitigate these effects, they are also susceptible to boosting unwanted signal or noise when aiming to recover “lost” or down-weighted spectral information.
This work presented here similarly examines DAS data acquired on a near-surface horizontal fiber array; however, we use a Terra15 Treble DAS IU with a novel optical measurement design that measures a “deformation-rate” quantity [
23]. Under ideal elastic fiber-ground coupling conditions, this deformation-rate measurement acquires data that are theoretically equivalent to the single component of the ground particle-velocity vector recorded on a fiber segment oriented in the fiber axis direction. This proprietary IU design leads to a particle-velocity-equivalent quantity
that effectively represents the integral of the strain rate
from the interrogation point (at
) to point
on the fiber
where
t is time,
u is an auxiliary spatial integration variable, and the tilde symbol on
emphasizes that the measured quantity is only equivalent to the true particle velocity of ground motion when the elastic fiber-ground coupling conditions are satisfied.
To demonstrate this near equivalence, the authors of [
24] acquired DAS data on a Treble IU in deformation-rate format on a completed downhole fiber installation to show the near identicalness to the ground motion recorded on a co-located borehole geophone array. Their observations indicate that when combined with a deployed fiber elastically coupled to the borehole casing, Treble IU acquisition can achieve a geophone-like
sensitivity pattern where
is the incidence angle [
25] compared to the conventional
relationship of strain-rate DAS measurements [
26]. Consequently, the strain-rate DAS measurement suppresses waves arriving at greater incident angles, resulting, e.g., in limited sensitivity to far-offset P-wave arrivals in vertical seismic profiling (VSP) experiments. This leads to a decreased usable angular bandwidth (i.e.,
) [
27]. Comparatively, the deformation-rate format allows a broader usable angular bandwidth (i.e.,
). Importantly, Sidenko et al. demonstrated that this improved angular bandwidth can be achieved without post-processing spatial-derivative filtering to recover high-quality particle-velocity-equivalent DAS signals in the deformation-rate format, which forestalled introducing the associated adverse filtering effects (e.g., spectral notches) commonly present in DAS strain-rate observations [
24].
Motivated by these observations, this study investigates whether the advantages of the Treble IU in the native deformation-rate (i.e., particle-velocity-equivalent) acquisition format highlighted by improved angular bandwidth can be realized for superficial as opposed to downhole fiber installations, and whether different 1D and 2D filtering operations can be applied to data acquired in this format to improve the signal-to-noise ratio. Our initial deployment experiments involving imperfect elastic fiber-ground coupling scenarios (e.g., draped on the surface, deployment in conduits) acquired DAS data which exhibited low-wavenumber noise that accumulated along the length of the fiber. Unfortunately, this noise needed to be handled through post-acquisition spatial filtering operations (e.g., applying a first-derivative filter with an assumed GL) and thus offered no improvement over standard strain-rate acquisition. These initial experiments underscored the importance of elastic fiber-ground coupling when using the Treble IU for the purpose of deformation-rate DAS data acquisition.
Aiming to achieve a horizontal DAS deployment scenario that approaches near-elastic coupling, we report the findings from an experiment that used an alternate approach to establishing coupling—freezing the fiber to the ground. We describe a small-scale investigation where we deployed three parallel fiber segments of 120 m total length in a shallow trench in the frozen earth that was watered down and left to freeze in the ground overnight. Over the following day, we acquired repeat sledgehammer shots as the outside air temperature reached 6.5 °C mid-afternoon and then fell to −6.5 °C by mid-evening. In addition to a fiber–soil coupling improvement, we expect the trenched-in fiber to be better insulated from the air temperature fluctuations than the surface-deployed fiber section.
We begin by providing additional detail about the interrogator design and by describing the data acquisition, including the experimental setup to enhance the fiber–ground coupling and the presentation of raw data results. Due to unwanted residual signals in the dataset, we then discuss the 1D and 2D filtering strategies used to improve the signal-to-noise levels of the filtered DAS data panels. We then show the processed DAS data that verify the efficacy of the proposed filtering method and compare the shot gather with the data recorded from the conventional vertical-component geophones. A brief discussion section examines the amplitude fluctuations noted in the surface-deployed fiber and cautions for DAS data interpretation when acquiring data over calendar time. The last section summarizes the effects of enhanced coupling on the frozen trench and filter designs on particle-velocity DAS recording.
2. DAS Data Acquisition
To investigate the role that frozen ground can play in the ground–fiber coupling, we selected suitable test dates (15–16 February 2021) when the weather forecast for the investigated location (Arvada, CO, USA) predicted that the air temperature would remain well below freezing on the first day, rise above 0 °C by mid-morning on the second day, reach 6 °C by mid-afternoon, and then return below 0 °C by the early evening. Our deployment involved in-ground trenching of 120 m of military-grade single-mode fiber, tactical tight-buffered cable with aramid strength members, and a polyurethane jacket. We first trenched a 35 m section of fiber into the ground at approximately 10 cm depth (Section A), covered it with soil, compacted the soil by hand with a tamper tool, and then thoroughly watered it down. We then looped the fiber back along the same trench at approximately 5 cm depth (Section B), again covering the fiber, compacting the soil, and watering it down. Finally, we deployed the remaining fiber directly on the surface (Section C). The three sections allowed us to examine coupling effects at different depths with likely variable sensitivity to air-temperature fluctuations.
Figure 1a,b respectively show the overall fiber deployment geometry in both plan and cross-sectional view with the following points: (1) the interrogator is housed in a garage at point 🄋 at 0 m distance; (2) the source point is located ➀ at 20 m; (3) Section A runs between 20–55 m up to the turnaround point at ➁; (4) Section B runs between 55–90 m back to the turnaround point at ➂; and (5) Section C runs between 90–120 m up to the end of the fiber at ➃.
Figure 2 depicts the installed fiber a few minutes after watering down the section. For comparison purposes, we installed vertical component geophones 0.3 m from the fiber as a baseline for comparison. (Unfortunately, horizontal geophones that would have offered a better comparison were not available at the time of the experiment).
The DAS IU used for the seismic acquisition was a circa mid-2020 Treble IU developed by Terra15 Technologies Pty Ltd of Perth, Australia. The phase-based system has a proprietary optical design constructed to eliminate amplitude and polarization fading [
23]. The Treble IU can acquire DAS seismic data either in native deformation- or strain-rate format with the gauge length (GL) modifiable through post-processing. The IU measurement type and properties depend on the choice of the interference pair of optical signals. The Treble IU generates two pulses at a fixed time interval, and then correlates the returned time-delayed first pulse and optically delayed second pulse to natively measure a deformation-rate rather than a more typical strain-rate quantity. For more details on the optical process, we refer readers to the patent document of [
23].
We acquired roughly 3.0 Tb of DAS data in continuous mode over the two-day experiment in the deformation-rate format at 0.038 ms and 0.8 m temporal and spatial sampling intervals, respectively. To test the temporal variability of ground-fiber coupling, we used a sledgehammer and metal plate as an energy source and generated shots at approximately 30-min intervals for 12 h from 9:00 a.m. to 9:00 p.m. Because of the fiber deployment pattern (see
Figure 1a), the shot point was observed simultaneously at three effective shot locations on Sections A–C: 20 m, 90 m (back up the fiber), and 90 m (again down the fiber) from the IU, respectively.
The first processing step involved window selection where we used the noted shot times to extract 60.0 s data streams of approximately 1.0 Gb size. Because the frequencies of interest from the sledgehammer shots were lower than 150 Hz, we low-pass-filtered the extracted sections with a 150 Hz cutoff and subsampled the extracted shot windows to a more manageable data volume. We then corrected the polarity of the data acquired on Section B to compensate for the reversed fiber direction with respect to the shot location.
Figure 3a presents a shot gather recorded at 9:00 p.m. in the deformation-rate data acquisition format of the Treble IU. Surface-wave arrivals are clearly identifiable in all three fiber sections. Although high-amplitude horizontal arrivals are observed in the raw data, the corresponding moveouts are too fast to be a passing seismic wave disturbance. However, these arrivals are repeatable and thus represent coherent “unwanted signals” that should be removed through signal processing.
Figure 3b corresponds to a frequency–wavenumber (
) spectrum of the shot gather from
Figure 3a. The identified coherent noise source maps to the strong low-wavenumber components observed as two lobes with between 20–60 Hz as well as the vertical “washboard” pattern corresponding to the quasi-horizontal signals in the time–space (
) panel in
Figure 3a.
We observe that the distortion-rate recording from section A enclosed in frozen soil captures traveling surface waves well throughout the day, which is consistent with our expectations of improved fiber–soil coupling. Despite the clear arrivals presented in the distortion-rate data panel, low-wavenumber noise persists that reduces the quality of the data and ensuing processing results. Herein, we suggest that one can apply low-cut filtering such as a 1D gradient operator to eliminate low-wavenumber noise persisting in the deformation-rate data. Thus, we next examine the efficacy of several 1D and 2D filter operators to find a judicious approach for processing low-wavenumber-contaminated deformation-rate waveforms.
4. Discussion
Having now specified our preferred form of 2D filtering and interpretation, we now turn to making two additional sets of comparisons: (1) how well do the filtered DAS panels match geophone data? (2) How repeatable were the shot gathers acquired on the DAS system over the acquisition period?
We first present a comparison between the recordings acquired on the horizontal DAS fiber with those made on the adjacent vertical-component geophones.
Figure 6a,b present geophone- and DAS-acquired shot gathers, respectively. The surface wave recorded from both sensors displays comparable arrival phases and apparent moveout velocities.
Figure 6c presents the stacked magnitude spectra for both records. We observe that the dominant frequency band of the DAS and geophone records largely overlap; however, the geophone data are shifted to lower frequency and appear to have a somewhat broader response.
To examine the repeatability of the DAS shot gathers and the potential for diurnal variability, we evaluate three shots acquired at Sections A and C acquired at three different times of the day.
Figure 7a–c presents three different shot gathers recorded on fiber section A at 3:00 p.m., 5:30 p.m., and 9:00 p.m., respectively. These three shot gathers display consistent moveouts, waveforms, and amplitudes, which suggests that the shot records are largely repeatable throughout the time of the investigation.
Figure 7d–f presents the same three shot gathers as presented in
Figure 7a–c; however, these were recorded Section C of the fiber deployed on the surface. Unlike in the top three panels, the bottom three panels exhibit significant amplitude and phase variations. These observations suggest that trenching and freezing in fiber improves not just ground-fiber coupling, but also measurement repeatability.
Finally, the significant variability of the shot gathers presented in Section C are unlikely to be due to significant time-lapse variations of the bulk properties of the near-surface earth model. Rather, a more likely explanation is that as the air temperature drops from above to below freezing, one should expect refreezing of moisture in the soil and likely improved adherence of the soil to the fiber. Accordingly, the shot gather acquired at 9:00 p.m. (
Figure 7f) exhibits stronger amplitudes and higher signal-to-noise ratio compared to those acquired at 3:00 p.m. and 5:30 p.m. (
Figure 7d,e). Thus, we stress that interpreting the amplitudes (and potentially phases) of DAS data prior to removing or controlling for external environmental factors requires due caution. We encourage further examination using a hybrid system that measures strain and temperature simultaneously, such as a combined DAS and DTS (distributed temperature sensing) system or DVTS (distributed vibration and temperature sensing) [
30]. Such hybrid systems already provide solutions in various fields including oil and gas, mining, and civil engineering. For our work, future hybrid measurement will help to clarify the amplitude changes or help design a filter for the decoupling temperature effect.