Low frequency sound within the oceans is generated by a wide number of physical, biological, and anthropogenic sources (Wilcock et al., 2014).
These include the wind interacting with the sea-surface, the 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 a variety of processes and to understand the impacts of anthropogenic activities and changing climate on the ocean environment (Duarte et al., 2021).
Since sustained hydro-acoustic observations are challenging and expensive to obtain offshore, there is strong motivation to explore new technologies that might enhance our ability to record and characterize sounds within the oceans.
Distributed acoustic sensing (DAS) is a relatively 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 and has a spatial resolution of meters and a broad frequency sensitivity. A DAS fiber optic cable behaves similarly to a long line of closely spaced single-axis broadband seismometers 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).
The spatial resolution of DAS measurements is termed the gauge length and is controlled by both the duration of the laser pulse and length of time over which each interferometric measurement is averaged. DAS data is commonly collected with a channel spacing that is 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 of the measurement 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 at once. 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. Within industry, DAS has been used for a decade to collect vertical seismic profiles in boreholes (Mateeva et al., 2014). Within academia, DAS is now widely used for a variety of geophysical applications including earthquake studies, seismic imaging and glacier deformation, and it also has application in urban areas for anthropogenic noise sources (Zhan et al., 2019;Lindsey and Martin, 2021). On land, DAS observations can often take advantage of the extensive network of dark fibers that have 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 generally include dark fibers. Spare fibers in the nearshore portions of cables would be relatively cheap to add but they are of no use for telecommunications without the expensive optical repeaters that are necessary to transmit signals more than ~300 km.
In 2019, three studies documented the utility of submarine DAS for recording earthquakes and oceanographic signals using data from short tests of on the research infrastructure of the MARS cabled observatory in Monterey Bay (Lindsey et al., 2019), the MEUST deep sea cabled observatory in the Mediterranean off France (Sladen et al., 2019) and a cable in the North Sea off Belgium (Williams et al., 2019). This pioneering work has spurred a rapid growth in interest in submarine DAS including its applications to acoustics. Rivet et al. (2021) showed that DAS could be used to track a tanker passing over the MEUST cable at water depths of both 85 m and 2000 m. Matsumoto et al. (2021) compared DAS and hydrophone recordings of airgun signatures using cable extending offshore Japan to >3000 m water depth. They found both systems were sensitive to airgun signals from 0.1 to tens of Hz although the DAS had lower signal to noise above a few Hz. A comparison of airgun recording between DAS on cable at 100-400 m depth and a towed hydrophone streamer in a shallow Fjord in Norway (Taweesintananon et al., 2021) showed similar noise levels on both systems. Working with the same data set, Bouffaut et al. (2022) present DAS recordings of baleen whales at frequencies up to nearly 100 Hz and demonstrated tracking for animals swimming 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 (section 2), demonstrate the capabilities of DAS to recording hydro-acoustic signals (section 3) from fin whale calls (section 3.1), blue whale calls (section 3.2) and ship noises (section 3.3) and discuss the preliminary results and opportunities for future research with these acoustic signals (section 4).
The Ocean Observatories Initiative Regional Cabled Array (Figure 1, inset) operates two submarine cables that land at Pacific City, Oregon (Smith et al., 2018). The northern cable runs ~500 km west to Axial Seamount while the southern cable extends ~150 km offshore onto the Juan de Fuca plate before wrapping around to the south and east onto the continental slope and shelf off Newport, Oregon. Both cables include a single twisted pair of optical fibers that support 10 Gbps ethernet to primary nodes on the trunk cables that connect via secondary cables and junction boxes to suites of sensors on the seafloor and on moorings.
From November 1-5, 2021, a scheduled shutdown of the RCA for maintenance provided an opportunity for a 4-day community fiber sensing experiment to interrogate the fibers in each cable extending out to the first optical repeaters, which are located at 1600 m depth 95 km along the south cable and at 600 m depth 65 km along the north cable (Figure 1). These nearshore sections of the cables are buried to a nominal depth of 1.5 m depth below the seafloor. On the south cable DAS data was collected on both fibers using an Optasense QuantX interrogator and a Silixa IDASv3 system. On the north cable DAS data was 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 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 (https://oceanobservatories.org/pi-instrument/rapid-a-community-test-of-distributed-acoustic-sensing-on-the-ocean-observatories-initiativeregional-cabled-array/). Table 1 summarizes the DAS recording parameters. Although there were some intervals of recording at sample rates up to 1000 Hz and with gauge lengths down to 3 m, most of the data were collected with a sample rate of 200 Hz and gauge length of 30-50 m. These parameters were selected to ensure sufficient signal to noise to record to near the distal ends of the fibers.
The unfiltered DAS data (Fig. 2a) is 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. 2b). Acoustic signals can be further enhanced by applying an f-k filter to remove signals propagating along the cable at less than the speed of sound (Fig. 2c)
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 throughout. Fin whale calls are observed everywhere along the cables except within about 10 km of the coast. Individual calls are observed out to distances of tens of kilometers, forming a characteristic V-shape in the record sections (Fig 2b-c, f). Spectrograms show that DAS records frequency content of calls with most songs characterized by a doublet pattern of alternating lower and higher frequency notes (Fig. 2d) that now dominates songs in the northeast Pacific (Wierathumeller et al., 2017). The recorded amplitudes are low at the location on the cable closest to the whale (Fig. 2e), as would be expected for a measurement that is sensitive to strain along rather than across the cable.
The fin whale calls can be located using time difference of arrival.
Figures 2c,f show an example where vocalizations from 5 whales can be located at distances that range from 25 km to 75 km offshore and within no more than a few kilometers of one cable (Fig. 1).
The calls of the Northeast Pacific blue whale were much less common during the experiment, but several sequences of the A and B calls are observed (Fig. 3) with the first 3 harmonics of the B call well recorded. In contrast to fin whales, blue whale calls are only recorded out to distances of ~10 km. 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. 4b 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. 2e, the recorded amplitudes are low at the location on the cable closest to the ship (at a distance of 50 km) which is due to the cable sensitivity to strain along rather than across the cable.
Plane-wave beamforming (Jensen et al., 1994) is used to calculate the bearing of the vessel relative to a 150-channel sub-array between 49.7-50 km.
The beamforming output is maximum at 29.6 degree which is consistent with the bearing of 26 degree calculated using the ship location from the AIS data.
The OOI community DAS experiment confirms earlier work which shows that buried submarine telecommunication cables can record low frequency acoustic signals. Numerous fin whale calls, blue whale calls, and ship noises were recorded to distances of up to ~40km, 10 km 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. Average values of 186 (Watkins et al., 1987), 189 (Wierathmueller et al., 2013) and 171 dB re 1 μPa at 1 m (Charif et al. 2002) have been reported for fin whales in the northeast Pacific, with the last estimate likely 10-15 dB lower due to the methodology (Wierathmueller et al., 2013). For the B call of the northeast Pacific blue whale, reported values are 180 dB (Thode et al., 2000) and 186 dB re 1 μPa at 1 m (McDonald et al., 2001). While the uncertainties in these estimates are consistent with fin whale calls being somewhat louder, such an explanation for the difference in the maximum detection distance in the DAS data, 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, 2022).
The differences may be related to the frequency sensitivity of the DAS data. 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 higher order harmonics and particularly the 3 rd harmonic at 40-45 Hz (Thode et al., 2000). The 35 m wavelength of the 3 rd harmonic is similar to the 30-50 m gauge length so that at larger distances, when the call is propagating sub parallel to the cable it may be poorly recorded.
The reported 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 again 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 depth of the source. Ship propellers are located close to the surface while studies with acoustical tags show that fin and blue whales vocalize at depths of up to a few tens of meters (Oleson et al., 2007;Stimpert et al., 2015;Lewis et al., 2018). 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).
Additional work is needed to understand the impact of speed of sound profile on the detection range of different sound source recordings on DAS.
The DAS sensitivity, as expected, is strongly directional with the recorded amplitudes of both whales (Fig. 2c) and ships (Fig. 4d) 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 cablelongitudinal strain rates being insensitive to plane acoustic waves at normal incidence. It also appears from the fin localizations that whales are only clearly detected on both cables which are spaced ~10 km apart, 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 Fig. 2c,f).
The OOI DAS experiment recorded tens of thousands of fin whale calls, which provide a remarkable data set both to investigate the directional and 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 accurately 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 calls 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 (