Distributed acoustic sensing (DAS) is a technique that measures strain changes along an optical fiber to distances of ∼100 km with a spatial sensitivity of tens of meters. In November 2021, 4 days of DAS data were collected on two cables of the Ocean Observatories Initiative Regional Cabled Array extending offshore central Oregon. Numerous 20 Hz fin whale calls, northeast Pacific blue whale A and B calls, and ship noises were recorded, highlighting the potential of DAS for monitoring the ocean. The data are publicly available to support studies to understand the sensitivity of submarine DAS for low-frequency acoustic monitoring.

Low frequency sound within the oceans is generated by a variety of physical, biological, and anthropogenic sources (e.g., Hildebrand, 2009; Wilcock et al., 2014). These include the wind, deformation of sea ice and icebergs, earthquakes, volcanic activity, baleen whale and fish vocalizations, ship propellers and machinery, seismic airguns, and pile driving. Passive acoustic monitoring of the ocean soundscape is thus a useful tool to study the environment and understand the impacts of anthropogenic activities and changing climate on the ocean (e.g., Duarte et al., 2021). Since sustained hydro-acoustic observations are challenging and expensive to obtain offshore, there is motivation to explore new technologies that might enhance our ability to record and characterize sounds within the oceans.

Distributed acoustic sensing (DAS) is a new observational technique that interrogates an optical fiber with repeated laser pulses and applies interferometry to the Rayleigh backscattered light to measure changes in strain along the fiber (Hartog, 2017). The method can work to distances of up to ∼100 km, has a spatial resolution of meters, and a broad frequency sensitivity from <0.001 Hz to >1 kHz depending on the configuration (Lindsey and Martin, 2021). A DAS fiber optic cable behaves similarly to a long line of single-axis broadband seismometers spaced meters to tens of meters apart and oriented in the direction of the fiber, although DAS measures the spatial derivative of ground velocity (i.e., rate of change of strain) rather than ground velocity (Hartog, 2017).

DAS measurements are associated with an averaging length scale, termed the gauge length, that is controlled by the duration of the laser pulse and length of time over which each interferometric measurement is averaged (Hartog, 2017). DAS data are collected as a set of time series on channels that correspond to the phase of backscattered light averaged over uniformly spaced segments of the fiber. The spacing between these segments, or channel spacing, ranges from about equal to, down to much smaller than the gauge length. Increasing the gauge length decreases spatial resolution and the sensitivity to short wavelength strain signals but improves the signal to noise ratio and thus allows measurements to greater distance from which the backscattered light is more attenuated. The temporal resolution is limited by the two-way travel time of light along the fiber because there should be no more than one light pulse in the fiber (Hartog, 2017). For example, for a 100 km–long fiber, the maximum laser interrogation rate is ∼1000 Hz. If the sampling rate is at least a factor of 2 lower than the maximum laser interrogation rate, then successive interrogations can be combined to increase signal to noise ratio.

Within the hydrocarbon industry, DAS has been used for a decade (Mateeva et al., 2014). Within academia, DAS has various applications including studies of earthquakes, seismic structure, glacier deformation, and anthropogenic noise (Lindsey and Martin, 2021). On land, DAS observations can take advantage of the network of spare (dark) fibers that has been laid in urban areas and along transportation corridors to provide growth capacity for telecommunications. In the oceans, DAS experiments are more challenging because submarine telecommunications cables do not always include dark fibers.

In 2019, three studies documented the utility of submarine DAS for recording earthquakes and oceanographic signals using data from short tests on the Monterey Accelerated Research System (MARS) cabled observatory in Monterey Bay (Lindsey et al., 2019), the Mediterranean Eurocentre for Underwater Sciences and Technologies (MEUST) cabled observatory in the Mediterranean (Sladen et al., 2019), and a cable in the North Sea (Williams et al., 2019). This work has spurred a rapid growth in interest in submarine DAS including applications to acoustics.

Rivet et al. (2021) used DAS to track a tanker passing over the MEUST cable at water depths of 85 m and 2000 m. Matsumoto et al. (2021) compared DAS and hydrophone recordings of airguns using a cable extending off Japan to >3000 m water depth. Both systems were sensitive to airgun signals from 0.1 to tens of Hertz although the DAS had lower signal to noise ratios above a few Hertz. In a shallow Fjord in Norway, airguns were recorded with similar noise levels using DAS on a cable at 100–400 m water depth and a towed hydrophone streamer (Taweesintananon et al., 2021). Using the same DAS dataset, Bouffaut et al. (2022) and Landrø et al. (2022) present recordings of Baleen whales and ships at frequencies up to nearly 100 Hz and demonstrate tracking near the cable.

In this paper, we present an overview of a 4-day public-domain submarine DAS experiment that was conducted on two cables extending offshore central Oregon, demonstrate the capabilities of DAS for recording hydro-acoustic signals from fin whale (Balaenoptera physalus) calls, blue whale (Balaenoptera musculus) calls and ship noises, and discuss the preliminary results and opportunities for future research with these acoustic signals.

The Ocean Observatories Initiative Regional Cabled Array (RCA) (Fig. 1, inset) operates two submarine cables that land at Pacific City, Oregon (Smith et al., 2018). Each cable includes a twisted pair of optical fibers that support 10 Gbps ethernet to the submarine observatory infrastructure.

FIG. 1.

Bathymetric map showing the nearshore portion of the two OOI RCA cables (red lines), the shore station (red circle), and the first optical repeaters (red squares). Also, shown are the fin whale call locations obtained for the data shown in Figs. 2(c) and 2(f) (numbered yellow triangles), and the northward track of the cargo ship for which data are shown in Fig. 4 (bold green dashed line). Contours are labeled in meters and are unevenly spaced (50– 200 m depth, 100– 500 m depth, and 500 m at larger depths). The inset map shows the geometry of the complete RCA cable with primary nodes on the cable as red squares, the area of the main figure as a black box, and the base of the continental slope as a thin black line.

FIG. 1.

Bathymetric map showing the nearshore portion of the two OOI RCA cables (red lines), the shore station (red circle), and the first optical repeaters (red squares). Also, shown are the fin whale call locations obtained for the data shown in Figs. 2(c) and 2(f) (numbered yellow triangles), and the northward track of the cargo ship for which data are shown in Fig. 4 (bold green dashed line). Contours are labeled in meters and are unevenly spaced (50– 200 m depth, 100– 500 m depth, and 500 m at larger depths). The inset map shows the geometry of the complete RCA cable with primary nodes on the cable as red squares, the area of the main figure as a black box, and the base of the continental slope as a thin black line.

Close modal

From November 1–5, 2021, a maintenance shutdown of the RCA allowed a 4-day community fiber sensing experiment to interrogate the fibers in each cable extending out to the first optical repeaters, located at 1600 m depth, 95 km along the south cable and at 600 m depth, 65 km along the north cable (Fig. 1). These nearshore sections of the cables are buried to a nominal depth of 1.5 m below the seafloor. On the south cable, DAS data were collected on both fibers using Optasense QuantX and Silixa iDASv3 interrogators. On the north cable, DAS data were collected on one fiber with a second Optasense QuantX interrogator while a Silixa ULTIMA SM distributed temperature sensor was deployed on the other fiber. The primary goal of the experiment was to collect a public domain dataset that could be explored by the scientific community to understand the utility of submarine DAS for seismic, acoustic, and oceanographic purposes. The data has a total volume of 26 TB and can be accessed through a data repository hosted by the University of Washington, along with information about the experiment configuration and data format (Wilcox and Ocean Observatories Initiative, 2023).

Some data were recorded with sample rates up to 1000 Hz and gauge lengths down to 3 m, but most were collected at 200 Hz with gauge lengths of 30–50 m to ensure sufficient signal to noise ratio for recording near the far ends of the fibers. (See the supplementary material for summary of DAS recording parameters in Table S1).1

The unfiltered DAS data [Fig. 2(a)] are dominated by the long period signals from ocean surface waves (primary microseisms) in shallow water and secondary microseisms in deeper water (Sladen et al., 2019; Williams et al., 2019) but acoustic signals are readily apparent when the records are filtered above ∼10 Hz [Fig. 2(b)]. Acoustic signals can be further enhanced by applying an f-k filter (Zhou, 2014) to remove signals propagating along the cable at less than the speed of sound [Fig. 2(c)].

FIG. 2.

Example of fin whale recordings. (a) Recording beginning on November 4, 2021, 02:00:27 UT, showing 30 s of unfiltered data recorded by the Optasense interrogator on the north cable. Distance from the interrogator is plotted on the horizontal axis and time is plotted on the vertical axis with the amplitude envelope shown by logarithmically scaled shading after normalizing each trace to its median amplitude. (b) The same as (a) except after the application of a 15–27 Hz bandpass filter (17–25 Hz passband, cosine tapers from 13–17 Hz, and 252–9 Hz). (c) The same as (b) but with a f-k filter to remove energy with an apparent velocity along the cable <1.4 km/s (cosine taper from 1.35–1.45 km/s). Manual picks of the fin whale arrivals (red solid line) and model times (bold blue dashed line) for a uniform velocity of 1.48 km/s are shown offset 2 s from the fin whale calls and are numbered to indicate the corresponding whale location in Fig. 1. (d) Spectrogram beginning on November 2, 2021, 18:15:40 UT, for the Silixa interrogator on the south cable computed with 200 sample Hamming windows and 95% overlap, averaged over 100 channels, showing six notes in a fin whale doublet song. (e) Recording beginning on November 2, 2021, 18:16:54 UT, for the Silixa interrogator showing channels within ∼1 km of the closest point to a fin whale call. (f) The same as (c) but for the Optasense interrogator on the south cable.

FIG. 2.

Example of fin whale recordings. (a) Recording beginning on November 4, 2021, 02:00:27 UT, showing 30 s of unfiltered data recorded by the Optasense interrogator on the north cable. Distance from the interrogator is plotted on the horizontal axis and time is plotted on the vertical axis with the amplitude envelope shown by logarithmically scaled shading after normalizing each trace to its median amplitude. (b) The same as (a) except after the application of a 15–27 Hz bandpass filter (17–25 Hz passband, cosine tapers from 13–17 Hz, and 252–9 Hz). (c) The same as (b) but with a f-k filter to remove energy with an apparent velocity along the cable <1.4 km/s (cosine taper from 1.35–1.45 km/s). Manual picks of the fin whale arrivals (red solid line) and model times (bold blue dashed line) for a uniform velocity of 1.48 km/s are shown offset 2 s from the fin whale calls and are numbered to indicate the corresponding whale location in Fig. 1. (d) Spectrogram beginning on November 2, 2021, 18:15:40 UT, for the Silixa interrogator on the south cable computed with 200 sample Hamming windows and 95% overlap, averaged over 100 channels, showing six notes in a fin whale doublet song. (e) Recording beginning on November 2, 2021, 18:16:54 UT, for the Silixa interrogator showing channels within ∼1 km of the closest point to a fin whale call. (f) The same as (c) but for the Optasense interrogator on the south cable.

Close modal

The experiment occurred during the breeding season for fin whales and songs of the stereotypical 1 s–long 20 Hz fin whale chirp are recorded everywhere except within 10 km of the coast. Calls are observed out to distances of tens of kilometers, forming a characteristic “V” shape in the record sections [Figs. 2(b), 2(c), and 2(f)]. Most songs are characterized by a doublet pattern of alternating lower and higher frequency notes [Fig. 2(d)] (Weirathmueller et al., 2017). The recorded amplitudes are low at the location on the cable closest to the whale [Fig. 2(e)], as would be expected for a measurement that is sensitive to strain along rather than across the cable.

The fin whale calls can be localized using time difference of arrival. Figures 2(c) and 2(f) show an example where vocalizations from five whales are localized at distances from 25– 75 km offshore and within no more than a few kilometers of one cable (Fig. 1).

The calls of the Northeast Pacific blue whale are much less common, but several sequences of the A and B calls are observed (Fig. 3) with the first three harmonics of the B call apparent. In contrast to fin whales, blue whale calls are only recorded out to distances of ∼10 km.

FIG. 3.

Example of Northeast Pacific blue whale recordings on the south cable. (a) Recording beginning on November 2, 2021, 10:36:09 UT for the Optasense interrogator, showing an example of an A call with the closest location on the cable at a distance of 38 km. The A call is overlain by several higher amplitude fin whale calls. The data have been filtered with a 10.5–18 Hz bandpass filter (11–17 Hz passband, cosine tapers from 10–11 Hz, and 17–19 Hz), and an f-k filter as in Fig. 2(c). (b) The same as (a) but showing a B call with recording beginning, on November 2, 2021, 10:33:36 UT. Frequency filtering has removed all but the first harmonic. (c) Spectrogram beginning on November 2, 2021, 10:32:24 UT for the Silixa interrogator, averaged over 100 channels, showing an A call followed by a B call.

FIG. 3.

Example of Northeast Pacific blue whale recordings on the south cable. (a) Recording beginning on November 2, 2021, 10:36:09 UT for the Optasense interrogator, showing an example of an A call with the closest location on the cable at a distance of 38 km. The A call is overlain by several higher amplitude fin whale calls. The data have been filtered with a 10.5–18 Hz bandpass filter (11–17 Hz passband, cosine tapers from 10–11 Hz, and 17–19 Hz), and an f-k filter as in Fig. 2(c). (b) The same as (a) but showing a B call with recording beginning, on November 2, 2021, 10:33:36 UT. Frequency filtering has removed all but the first harmonic. (c) Spectrogram beginning on November 2, 2021, 10:32:24 UT for the Silixa interrogator, averaged over 100 channels, showing an A call followed by a B call.

Close modal

Figure 4 shows an example of ship noise recorded by the Optasense interrogator on north and south cables. Automatic identification system (AIS) data show that a 180 m–long cargo ship  passed above the cable at a speed of ∼13.2 knots.

FIG. 4.

Example of ship sound recording beginning on November 3, 2021, 01:57:31 UT, traveling at 13.2 knots over both cables recorded by the Optasense interrogator with a sample rate of 200 Hz and gauge length of 50 m. (a) Recording showing 60 s of a cargo ship sound with the closest location on the south cable at a distance of 50.3 km. The data have been filtered with a 10–90 Hz 8th order bandpass Butterworth filter and an f-k filter as in Fig. 2(c). (b) Recording showing channels within ∼5 km of the closest point to the ship. (c) Spectrogram computed with 200 sample Hann windows and 95% overlap averaged over 100 channels, showing acoustic energy between 10 and 60 Hz. (d) Plane-wave beamformer output for the signal shown in (a) using a sub-array of the fiber optic cable consisting of 150 channels starting at 49.7 km.

FIG. 4.

Example of ship sound recording beginning on November 3, 2021, 01:57:31 UT, traveling at 13.2 knots over both cables recorded by the Optasense interrogator with a sample rate of 200 Hz and gauge length of 50 m. (a) Recording showing 60 s of a cargo ship sound with the closest location on the south cable at a distance of 50.3 km. The data have been filtered with a 10–90 Hz 8th order bandpass Butterworth filter and an f-k filter as in Fig. 2(c). (b) Recording showing channels within ∼5 km of the closest point to the ship. (c) Spectrogram computed with 200 sample Hann windows and 95% overlap averaged over 100 channels, showing acoustic energy between 10 and 60 Hz. (d) Plane-wave beamformer output for the signal shown in (a) using a sub-array of the fiber optic cable consisting of 150 channels starting at 49.7 km.

Close modal

Compared to fin whale and blue whale calls, ship noises are recorded over a shorter distance (∼5 km). The multipath interferences are noticeable in Fig. 4(b) which could be affected by the ship's motion over the cable, varying coupling of the fiber, different bathymetry along the cable, and fiber curvature. Similar to Fig. 2(e), the recorded amplitudes are low at the location on the cable closest to the ship (at a distance of 50 km).

Plane-wave beamforming (Jensen et al., 1994) is used to calculate the bearing of the vessel relative to a 150-channel sub-array between 49.7 and 50 km, away from the closest approach where amplitudes are low. The beamforming output is maximal at 29.6° which is consistent with the bearing of 26° calculated using the ship location from the AIS data.

The OOI community DAS experiment confirms earlier work that shows that buried submarine telecommunication cables can record low frequency acoustic signals (Rivet et al., 2021; Matsumoto et al., 2021; Taweesintananon et al., 2021; Landrø et al., 2022; Bouffaut et al., 2022). Numerous fin whale calls, blue whale calls, and ship noises were recorded to distances of up to ∼40, 10,  and 5 km, respectively.

An important question is why these detection distances differ. Studies suggest that the source levels for fin and blue whales are similar. In the northeast Pacific, average source levels of 171–189 dB re 1 μPa at 1 m (Charif et al., 2002; Watkins et al., 1987; Weirathmueller et al., 2013) and 180–186 dB (Thode et al., 2000; McDonald et al., 2001) have been estimated for fin whales and blue whales, respectively. While the uncertainties in the estimates do not discount fin whale calls being somewhat louder, such an explanation for the difference in the maximum detection distance for DAS would be inconsistent with work using ocean bottom seismometers and hydrophones where blue whale B calls are detected to larger ranges (e.g., Wilcock and Hilmo, 2021).

The differences may be related to the frequency sensitivity of DAS. First, the optical fiber within an armored buried submarine cable may couple better to acoustical strain at lower frequencies. Second, DAS observations average strain changes over the gauge length and when this length approaches or exceeds the signal wavelength, the summed strain change measurements will experience aliasing, reducing the recorded amplitude. The blue whale B call has a significant amount of energy in the 3rd harmonic at 40–45 Hz (Thode et al., 2000). The 35 m wavelength of this harmonic is similar to the 30–50 m gauge length so that it may be less well recorded.

The source levels of commercial ships vary from 177–188 dB re 1 μPa at 1 m (McKenna et al., 2012; MacGillivray and de Jong, 2021) which suggests that ships have similar or slightly lower source levels than fin and blue whales. However, ships radiate acoustic energy in a broad frequency range that can go as high as 1000 Hz with most ships having significant energy to ≥100 Hz. (McKenna et al., 2012). The ship noises recorded in the OOI DAS experiment do not show acoustic energy above 60 Hz which would be consistent with reduced sensitivity at higher frequencies as an explanation for the lower detection range.

Another potential explanation for differences in detection range could be the source depth. Ship propellers are located close to the surface while fin and blue whales vocalize at depths of up to a few tens of meters (e.g., Stimpert et al., 2015). With warming ocean surface temperature, the mode excitation depths move deeper than the typical ship source depths and this can cause a reduction in the ship noise band spectral level (Dahl et al, 2021). The Lloyd's mirror effects could also explain the differences in detection range between ships and marine mammals. Additional work is needed to understand the impact of source depth on the detection range of DAS.

The DAS sensitivity, as expected, is strongly directional with the recorded amplitudes of both whales [Fig. 2(c)] and ships [Fig. 4(d)] very low at the position of closest approach where the propagation direction is perpendicular to the cable. This effect is understood to be due to the cable-longitudinal strain rates being insensitive to plane acoustic waves at normal incidence. It also appears from the fin whale localizations that whales are only clearly detected on both cables when the curvature of the cables results in the call propagating sub-parallel to both cables [e.g., locations 3 and 4 in Fig. 1 and the corresponding detections in Figs. 2(c) and 2(f)].

The OOI DAS experiment recorded tens of thousands of fin whale calls. These provide a remarkable dataset to investigate the directional and water depth dependent acoustic sensitivity of DAS near 20 Hz and characterize the spatial distribution, depth of calling, and behavior of vocalizing fin whales offshore central Oregon. One of the challenges of DAS is determining the location of each channel, given uncertainties in the path of the fiber and the speed of light in the fiber. A joint inversion for the location of fin whale calls and DAS channels would serve as an analog to the tap tests used to locate fibers on land (Lindsey and Martin, 2021). The fin whale can also be exploited to study low frequency sound propagation with water column velocity structure potential including this as an unknown in inversions. Finally, beamforming approaches should be used to explore whether the DAS data can be used to detect fin whales at azimuths and ranges where they are not apparent in the filtered plots.

The acoustic signals from ships and whales recorded by the OOI DAS experiment with gauge lengths of 3, 10, 30, and 50 m (Table S1) can be used to understand frequency sensitivity of DAS at lower frequencies and its dependence on gauge length. Such work should motivate future experiments that deploy hydrophones near cables to ground truth recordings and that explore the utility of DAS to detect high-amplitude higher-frequency signals, such as those from humpback or sperm whales.

DAS often generates large datasets; extrapolating the OOI DAS experiment to continuous recordings would generate 0–2 PB/yr of data. Managing such data volumes will require a variety of approaches. Further work is required to determine optimal channel spacing and to determine whether the 2 m channel spacing used in the OOI experiment is justified. Triggered acquisition could leverage smart, potentially machine learning–based algorithms run in an edge-computing topology to find and only record signals of potential interest at high spatial and temporal sampling rates, while still defaulting to a lower rate data that would enable studies of the ambient field. Both lossy and lossless real-time data compression should be considered, and recent results on this topic have shown promise (Dong et al., 2022). The public domain OOI DAS data provide a resource to support the development of such approaches.

We thank the technical staff of the OOI RCA and field data acquisition team. The data collection was supported by National Science Foundation Grant No. OCE-2141047.

1

See supplementary material at https://www.scitation.org/doi/suppl/10.1121/10.0017104 for a summary of DAS recording parameters in Table S1.

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