Continental Shelf-Scale Passive Acoustic Detection and Characterization of Diesel-Electric Ships Using a Coherent Hydrophone Array

Abstract

The passive ocean acoustic waveguide remote sensing (POAWRS) technique is employed to detect and characterize the underwater sound radiated from three scientific research and fishing vessels received at long ranges on a large-aperture densely-sampled horizontal coherent hydrophone array. The sounds radiated from the research vessel (RV) Delaware II in the Gulf of Maine, and the RV Johan Hjort and the fishing vessel (FV) Artus in the Norwegian Sea are found to be dominated by distinct narrowband tonals and cyclostationary signals in the 150 Hz to 2000 Hz frequency range. The source levels of these signals are estimated by correcting the received pressure levels for transmission losses modeled using a calibrated parabolic equation-based acoustic propagation model for random range-dependent ocean waveguides. The probability of the detection region for the most prominent signal radiated by each ship is estimated and shown to extend over areas spanning roughly 200 km in diameter when employing a coherent hydrophone array. The current standard procedure for quantifying ship-radiated sound source levels via one-third octave bandwidth intensity averaging smoothes over the prominent tonals radiated by a ship that can stand 10 to 30 dB above the local broadband level, which may lead to inaccurate or incorrect assessments of the impact of ship-radiated sound.

1. Introduction

The remote monitoring of ocean vehicles over instantaneous wide areas from their sounds radiated underwater is essential in maritime surveillance and defence [1,2,3,4,5,6,7,8,9]. The sound generated by ocean vehicles contributes to environmental ambient noise [10] which often limits detection ranges in passive and active sonar systems for a wide range of ocean remote sensing applications [1,9,11,12,13,14], as well as in ocean acoustic communication [15,16]. It may also impact the behavior and communication of marine organisms [17,18,19,20], such as fish [21,22,23,24] and marine mammals [25,26,27]. Ship-radiated sound may also impact fish stock abundance estimates based on conventional ultrasonic echosounding surveys due to potential avoidance reactions by fish populations to the approaching survey vessel [22,23,24,28,29,30,31].

Here we employ a large-aperture densely-sampled horizontal coherent hydrophone array consisting of 160 hydrophone elements, providing roughly two orders of magnitude higher array gain than a single hydrophone, to detect underwater sound radiated by surface ships over wide areas using the passive ocean acoustic waveguide remote sensing (POAWRS) technique. POAWRS was previously applied to detect, localize and classify the vocalization signals from multiple baleen whale and toothed whale species [13,32,33,34,35] simultaneously over continental shelf-scale regions of approximately 100,000 km${}^2$. Here the POAWRS approach is employed to detect and characterize the underwater sound radiated from three known surface ships which were received after long-range propagation on the coherent hydrophone array. The ships investigated here are the US research vessel (RV) Delaware II, as well as the Norwegian RV Johan Hjort and fishing vessel (FV) Artus, with accurately known locations obtained from global positioning systems (GPS) at ranges varying between 4 km to 32 km from the coherent hydrophone array.

Passive acoustic methods have been previously used to examine, characterize and quantify the sound radiated from surface ships, with most measurements in the published literature made using a single hydrophone [36,37] or a small number of widely-separated hydrophones [3,4,38] located from within a few hundred meters to a few kilometers away from a ship. Ship-radiated sound has also been measured using a 128-element vertical hydrophone array [39] and bottom-mounted hydrophones [40]. The mechanism of sound generation by ships is described in [9,28,41,42,43,44,45,46]. The main sources of ship radiated sound are: (1) machinery noise generated by propulsion and auxiliary machinery such as engines, main motors and gears; (2) propeller noise generated by cavitation at or near the propeller and propeller-induced resonant hull excitation; (3) hydrodynamic noise from radiated flow noise, resonant excitation of cavities, plates, and appendages; and (4) cavitation at struts and appendages. The passive acoustic spectra of ship-radiated sound is extremely dynamic, containing both broadband signals and narrowband tonals at discrete frequencies [23,41] with source levels that can vary depending on ship conditions [42,47] such as ship speed [48,49,50], orientation [39] and maneuvers [51]. Ship noise has been previously found to be dominated by propeller cavitation, propeller singing due to physical excitation at the trailing edges of the blades, and propulsion or other reciprocating machinery [2,8,18,28,45,46]. The nonlinear analysis of marine propeller noise is provided in [52,53].

We show that the underwater sounds radiated from the three surface ships investigated here and received at long ranges (4 km to 32 km) by the coherent hydrophone array are dominated by narrowband discrete tonals and cyclostationary signals. We estimate the source levels of the prominent tonals and cyclostationary signals radiated from each ship by correcting the received pressure levels for transmission losses modeled using a calibrated [54,55,56,57] parabolic equation-based acoustic propagation model [58,59] for random range-dependent ocean waveguides. The estimated source levels of the prominent ship-radiated tonals and cyclostationary signals are then applied to determine the probabilty of detection regions for each of the three surface ships, the RV Delaware II in the Gulf of Maine, and the RV Johan Hjort and the FV Artus in the Norwegian Sea. Here we show that by employing coherent beamforming of the horizontal hydrophone array data, the signals radiated by ships can be detected with high signal-to-noise ratios (SNR) leading to significantly enhanced detection regions compared to those of a single hydrophone.

The term ‘signature’ is commonly used to denote the levels [37] of underwater radiated noise versus frequency for an underwater vehicle spanning frequency ranges from roughly 10 Hz to 100 kHz. To enable comparison of ship-radiated sound across different ships, most ship noise ranging reports contain signatures in the form of averaged curves derived from measurements usually made in a one-third octave bandwidth, and then converted to a 1 Hz band [22,26,27,28]. Here we show that this ’signature’ analysis smooths or averages out the dominant discrete tones radiated by a ship that are the key signals enabling ship detection at long ranges, and as a result can lead to significant underestimation of a ship’s detection range and region or zone of acoustic interference or influence. This one-third octave bandwidth intensity averaging is also problematic for studies of the impact of ship noise on fish behavior. This is because the dominant narrowband tonal signals radiated from a ship that typically stand 10 dB to 30 dB above the local broadband levels are significantly reduced or disappear after one-third octave bandwidth intensity averaging. This tonal reduction or disappearance can become more severe with increasing frequency since the one-third octave bandwidths get progressively larger by a factor of $2^{1/3}=1.26$ with increasing frequency. While the overall one-third octave bandwidth intensity averaged ship sound source levels may be below the International Council for the Exploration of the Sea (ICES)-recommended Cooperative Research Report No. 209 (CRR 209, Mitson, 1995) levels , the individual tonal components can have levels that typically exceed the recommended levels by over 10 to 30 dB. The dominant high-level tonal signals radiated by a ship may be the primary sound field the fish and marine mammals are responding to, and must be included in studies of fish behavioral responses to ship-radiated sounds.

2. Materials and Methods

2.1. Measurement of Ship-Radiated Sound Using a Coherent Hydrophone Array

The underwater recordings of ship-radiated sound analyzed here are drawn from two separate experiments, the US Gulf of Maine 2006 Experiment (GOME06) where the RV Delaware II was present (Figure 1A), and the Norwegian Sea 2014 Experiment (NorEx14) where the FV Artus (Figure 1B) and the RV Johan Hjort (Figure 1C) were present. Both these experiments were conducted by a collaborative team from the Massachusetts Institute of Technology, Northeastern University, the NOAA-Northeast Fisheries Science Center, the Naval Research Laboratory, Penn State University, and the Woods Hole Oceanographic Institution in the US, as well as the Institute of Marine Research-Bergen in Norway.

The GOME06 [12,13,32,33,56,57,60,61,62] was conducted from September 19–October 6, 2006, in conjunction with the US National Marine Fisheries Services (NMFS) annual Atlantic herring acoustic survey [63,64] of the Gulf of Maine and Georges Bank. The Atlantic herring areal population densities were monitored over instantaneous wide areas using ocean acoustic waveguide remote sensing (OAWRS) imaging [12,13,33,56,62] and calibrated with coincident conventional ultrasonic fisheries echosounding measurements [65]. The RV Delaware II conducted the ultrasonic fisheries echosounding survey and trawl sampling to enable fish species identification and fish physiological parameters to be extracted from the trawl samples collected over the course of the experiment [65]. The NorEx14 was conducted from February 18– March 7, 2014, in conjunction with Norway’s Institute of Marine Research (IMR) survey of the fish populations of the Norwegian Sea. Both the RV Johan Hjort and the FV Artus conducted ultrasonic fisheries echo sounding and trawl surveys of the fish populations.

In both GOME06 and NorEx14, recordings of underwater sound were acquired using a large-aperture densely-sampled horizontal coherent hydrophone array [66] towed along designated tracks for 8 to 24 h per day. To minimize the effect of tow ship noise on the recorded acoustic data, the coherent hydrophone array was towed approximately 375–405 m and 282–329 m behind the research vessel during GOME06 and NorEx14, respectively, so as to confine this noise to the forward end-fire direction of the array. The tow ship noise in directions away from the forward end-fire was negligible after coherent beamforming. The multiple nested sub-apertures of the array contain a total of 160 hydrophones spanning a frequency range from below 50 Hz to 3750 Hz for spatially unaliased sensing. A fixed sampling frequency of 8000 Hz was used so that acoustic signals with frequency contents up to 4000 Hz were recorded without temporal aliasing. Three linear nested sub-apertures of the array, the ultra low-frequency (ULF), mid-frequency (MF) and high-frequency (HF) sub-apertures, each consisting of 64 equally spaced hydrophones with inter-element spacings of 3 m, 0.75 m, and 0.375 m respectively, were used to analyze the ship-radiated sound with frequency content below 2000 Hz. The angular resolution or equivalent beamwidth $\beta (\varphi ,f_c)$ of each uniformly-spaced subaperture of the horizontal receiver array is $\beta (\varphi ,f_c)=1.44(\lambda /Lcos\varphi )$ for broadside $(\varphi =0)$ through angles near endfire $(\varphi =\pi /2)$, where $\lambda =c/f_c$ is the acoustic wavelength, c is the sound speed, $f_c$ is the center frequency, and L is the array subaperture length [33,56]. At endfire, the angular resolution is $\beta (\varphi ,f_c)\approx 2.8\sqrt{\lambda /L}$ [67]. The angular resolution of the receiver array is tabulated in Table 1 of [56] at selected frequencies for each uniformly-spaced sub-aperture. Note that the equivalent beamwidth is the angular width of a rectangular or uniform function that has the same receptivity as the array integrated over all azimuths. The water depth ranged from 180 to 250 m at the array locations and the array tow depth was roughly 105 m in GOME06. The water depth ranged from 100 to 300 m at the array locations and the array tow depth was roughly 45–70 m in NorEx2014.

Physical oceanography was monitored in both experiments by sampling water-column temperature and salinity with expendable bathythermographs (XBTs) and conductivity-temperature-depth (CTD) sensors at regular hourly intervals. The water-column sound speed profile was found to be relatively constant in space and time over the experimental duration in GOME06, as shown by the compilation of over roughly 200 samples taken during the experiment in Figure 3 of [56] and Figure 2 of [57]. The sound speed profiles measured in the Norwegian Sea are provided in Appendix F Figures F2–F6 from [68].

The detection of long-range propagated sounds is significantly enhanced by spatial beamforming and spectrogram analysis which filters the background noise that is outside of the beam and frequency band of the ship-radiated sound. The high gain [69,70] of the large aperture densely-sampled coherent hydrophone array used here, roughly $10{\log }_{10}n=$ 18 dB gain with $n=$ 64 hydrophones for each sub-aperture, enabled detection of continuous tonal noises either two orders of magnitude more distant in range or lower in SNR than a single hydrophone which has no array gain (see Figure 2).

The POAWRS coherent hydrophone array employed in GOME06 and NorEx14 detected significant sounds from a wide range of underwater acoustic sources including marine mammal vocalizations from diverse baleen and toothed whale species in the frequency range from 10 Hz up to 4 kHz [13,32,33,35]. Here the analysis is focused on the detection and characterization of the long range propagated sound radiated from the three known ships in the 10 Hz to 2000 Hz frequency range including overall broadband levels and narrowband tonals at discrete frequencies, so as to provide a calibration of the POAWRS approach for passive acoustic monitoring of surface ships.

2.2. Ship-Radiated Sound Detection, Bearing Estimation and Characterization

Acoustic pressure time series measured by sensors across the receiver array were converted to two-dimensional beam-time series by beamforming [70]. A total of 64 beams were formed spanning 360-degree horizontal azimuth about the receiver array for data from each subaperture. Each beam-time series was converted to a beamformed spectrogram by short-time Fourier transform (sampling frequency = 8000 Hz, frame = 2048 (ULF), 1024 (MF) and 516 (HF) samples, overlap = 15/16, Hann window). Significant sounds present in the beamformed spectrograms were automatically detected by first applying a pixel intensity threshold detector followed by pixel clustering, and verified by visual inspection. Beamformed spectrogram pixels with local intensity values that are 5.6 dB above the background are grouped using a clustering algorithm according to a nearest-neighbour criteria that determines if the pixels can be grouped into one or more significant sound signals. Each detected signal is next characterized by its pitch track representing the time variation of the fundamental frequencies. The pitch track is estimated using a time-frequency peak detector from a signal’s detected and clustered pixel intensity values in the beamformed spectrogram.

The horizontal azimuthal direction or bearing $\widehat{\beta }$ of each detected signal, measured from the array broadside, is estimated using beamforming technique [70], by selecting the bearing in which the beamformed, bass-pass filtered pressure-time series contained maximum energy during the time duration of the signal and in the same frequency band. The estimated relative bearings $\widehat{\beta }$ measured with respect to array broadside are then converted to absolute bearings, measured from the array center with respect to true North. The true bearings versus time trajectories of all of the signal detections for a given day of data collection are compared with the true bearings of the known research or fishing vessels present in the area, calculated from the GPS locations of each ship and the hydrophone array. Signal detections with bearing-time trajectories that coincide well with the bearing-time trajectory of each ship are further extracted and associated to that ship. The time-frequency characteristics of these extracted signals, determined from their pitch-tracks, are quantified for each ship analyzed here.

The pitch track for a signal contains a time series $t=(t_1,t_2,\dots ,t_i)$, a frequency series $f=(f_1,f_2,\dots ,f_i)$, and an amplitude series $A=(A_1,A_2,\dots ,A_i)$ describing the time-variation of the fundamental frequency in the signal [71,72]. Ten features are extracted from each signal. They are: (1) minimum frequency (Hz), $f_{min}$; (2) maximum frequency (Hz), $f_{max}$; (3) signal bandwidth (Hz), $\overline{B}=f_{max}-f_{min}$; (4) amplitude-weighted average slope (octaves per second), $\beta$; (5) amplitude-weighted average frequency (${\log }_2$[Hz]), $\overline{f}$; (6) time variation (second), $〈t〉$; (7) frequency variation ($lo{g}_2\left[Hz\right]$), $〈f〉$; (8) duration (second), $\tau =t_i-t_1$; (9) slope from first order polynomial fit (degrees), ${\beta }_{poly}$; and (10) curvature from second-order polynomial fit, $\nu$. The four amplitude-weighted features (4)–(7) are calculated using the formulation provided in Table 1 of [71]. As explained in [71], an advantage of characterizing signals with the amplitude-weighted attributes is that it provides a more consistent representation of a signal when data is noise-limited.

2.3. Source Level Estimation for Ship Tonal and Cyclostationary Signals

The source levels of the ship-radiated prominent narrowband tonals and cyclostationary signals at specific frequencies are determined following the approach described in Section 2.5 of [35] and Section 3.5 of [33]. The source level $SL({\mathbf{r}}_{\mathbf{0}})$ of each signal is estimated from its received pressure level $RL(\mathbf{r})$ via the passive sonar equation [9,33,35,73],

$$SL({\mathbf{r}}_{\mathbf{0}})=RL(\mathbf{r})+TL(|\mathbf{r}-{\mathbf{r}}_{\mathbf{0}}|).$$

(1)

The received pressure level is estimated as the peak value of the instantaneous time-domain signal bandpass filtered within a bandwidth $(f_U-f_L)$ that is 3-dB down on both sides of the spectral peak for a given prominent tonal or cyclostationary signal. The corresponding one-way acoustic transmission loss $TL(|\mathbf{r}-{\mathbf{r}}_{\mathbf{0}}|)$ from the ship location to the center of the POAWRS receiver array is calculated using a calibrated [56,57] parabolic equation-based range-dependent acoustic propagation model (RAM) [58,59] via [54,55,56,57],

$$\mathrm{TL}(|\mathbf{r}-{\mathbf{r}}_{\mathbf{0}}|)=-10{\log }_{10}\left({\int }_{f_L}^{f_U}Q(f)\langle |G(\mathbf{r}|{\mathbf{r}}_{\mathbf{0}},f){|}^2\rangle df\right),$$

(2)

where $G(\mathbf{r}|{\mathbf{r}}_{\mathbf{0}},f)$ is the waveguide Green function at frequency f for ship located at ${\mathbf{r}}_{\mathbf{0}}$ and receiver array center at $\mathbf{r}$, $Q(f)$ is the normalized spectra of ship tonal or cyclostationary signal, and $f_U$ and $f_L$ are the upper and lower frequencies used in the bandpass filter. The model takes into account the environmental parameters such as the range-dependent water depth and sound speed profiles to stochastically compute the propagated acoustic intensities via Monte-Carlo simulations following the approach of [54,55,56]. The range- and depth-dependent acoustic transmission loss is plotted for transects in the Gulf of Maine in Figure 4 of [57] and in the Norwegian Sea in Figures 5 and 6 of [68]. The mean magnitude-squared waveguide Green function is obtained by averaging over multiple depths for each ship from the sea surface to the ship draft depth or draught $z_{draft}$ and over multiple Monte-Carlo simulations to account for waveguide fluctuations. The draught used here are $z_{draft}=4.5$ m for RV Delaware II, $z_{draft}=6.5$ m for RV Johan Hjort, and $z_{draft}=6$ m for FV Artus.

The stochastic broadband transmission loss model calculations have been extensively calibrated and verified with: (1) thousands of one-way transmission loss measurements made during the same GOME06 experiment discussed here at the same time and at the same location [56,57]; (2) thousands of two-way transmission loss measurements made from herring shoal returns and verified by conventional fish finding sonar and ground truth trawl surveys during the same GOME06 experiment discussed here at the same time and at the same location [12,56,64]; (3) roughly 100 two-way transmission loss measurements made from calibrated targets with known scattering properties during the same GOME06 experiment discussed here at the same time and at the same location [61]; and (4) thousands of one-way transmission loss measurements made during a past POAWRS experiment conducted in a similar continental shelf environment [54]. Calibration of the stochastic broadband transmission loss model for the Norwegians Sea is provided in [68]. The source level results are provided in units of dB re 1 $\mathsf{\mu }$Pa/Hz at 1 m so that they can be compared to the results in the literature.

2.4. Probability of Detection Regions for Prominent Ship-Radiated Signals

The probability of detection (POD) regions for the prominent tonals or cyclostationary signals radiated from each of the research and fishing vessels considered here and received on the coherent hydrophone array are calculated using the approach provided in Appendix A. We model the POD regions for RV Delaware II in the Gulf of Maine, and RV Johan Hjort and FV Artus in the Norwegian Sea.

3. Results

Here we first associate the acoustic signal detections in the 10 Hz to 2000 Hz frequency range measured by the coherent hydrophone array with the known ships since they occur in bearing-time trajectories that coincide with those of the ships the RV Delaware II in the Gulf of Maine, and the FV Artus and RV Johan Hjort in the Norwegian Sea. We then provide a statistical time-frequency characterization of the detected signals associated with each ship. We find that the sounds radiated by the ships and received on the coherent hydrophone array after long-range propagation (4 km to 32 km range) are dominated by distinct narrowband tonals and/or cyclostationary signals in the 100 to 2000 Hz frequency range. We estimate the source level distributions of the prominent tonal and cyclostationary signals radiated by each ship. These results are then applied to estimate the probability of detection regions for the ship-radiated signals in the ocean environment where the measurements were made for each ship. Finally, we investigate the dependence on ship speed of the ship-radiated sound spectra and source level.

3.1. RV Delaware II

The bearing and time of all detected acoustic narrowband signals in the 200–1000 Hz frequency range, that stand at least 5.6 dB above the local ambient background noise, in the beamformed spectrograms of the coherent hydrophone array on October 3, 2006 during GOME06 when the RV Delaware II was present are shown in Figure 3A. The bearings of RV Delaware II as a function of time are also overlain for comparison. Detected signals in bearing-time trajectories that exactly coincide with the bearing-time trajectory of RV Delaware II (see Figure 3) are extracted and associated with RV Delaware II. The same procedure is applied to extract the signal detections associated with RV Delaware II in other frequency subbands, 10–200 Hz and 1000–2000 Hz.

The tow tracks of the coherent hydrophone array and the corresponding locations of RV Delaware II on October 3, 2006 in the Gulf of Maine during GOME06 based on GPS measurements are shown in Figure 4A. The bearing-time trajectories of detected signals measured by the coherent hydrophone array that are associated with RV Delaware II in the three frequency subbands: 10–200 Hz, 200–1000 Hz and 1000–2000 Hz are plotted in Figure 4B–D.

The time-frequency characteristics of the detected signals associated with the RV Delaware II in the three frequency subbands are shown in Figure 5. The sound radiated by the RV Delaware II is dominated by two distinct narrowband tonals at roughly 176 Hz and 573 Hz Examples of beamformed spectrograms containing these two distinct tonals, as well as other broadband and narrowband signals associated with RV Delaware II are shown in Figure 5A–C. The ensemble of pitch tracks and the histogram of mean frequency weighted by signal duration for the ship-associated detections in the three frequency subbands are shown in Figure 5D–I.

The source level distributions for the ship-radiated tonals at 176 Hz and 573 Hz are determined from a subset of the measured signals (see Figure 6) on the coherent hydrophone array. The mean and standard deviation of the source level in units of dB re 1 $\mathsf{\mu }$Pa/Hz at 1 m for the tones at 176 Hz and 573 Hz are 148.7 ± 5.2 and 162.3 ± 6.0, respectively, with 3 dB-down instantaneous bandwidths of 3.5 Hz and 0.8 Hz, respectively.

The POD regions for the tonal sound at 176 Hz and 573 Hz radiated by RV Delaware II and received by the coherent hydrophone array in the Gulf of Maine environment are shown in Figure 7. The 50% POD region extends over an area measuring between 100 km to 200 km diameter. The POD region is significantly larger for the 573-Hz tonal sound radiated by the RV Delaware II than for the 176 Hz tonal. This is because the 573-Hz tone is radiated with a roughly 14-dB higher source level than the 176-Hz tone and the background ambient noise level is lower at 573 Hz than at 176 Hz by 5.2 dB. The 176 Hz tone has a slightly larger bandwidth of 3.5 Hz compared to 0.8 Hz for the 573 Hz tone. In contrast, the 50% POD region for tonal detection using a single hydrophone at each of the center locations of the coherent hydrophone array is significantly smaller (see dashed curve in Figure 7).

3.2. RV Johan Hjort

The tow tracks of the coherent hydrophone array and the corresponding locations of RV Johan Hjort on March 3, 2014 in the Norwegian Sea during NorEx14 based on GPS measurements are shown in Figure 8A. The bearing-time trajectories of detected signals measured by the coherent hydrophone array that are associated with the RV Johan Hjort in the three frequency subbands: 10–200 Hz, 200–1000 Hz and 1000–2000 Hz are plotted in Figure 8B–D.

The time-frequency characteristics of the detected signals associated with the RV Johan Hjort in the three frequency subbands are shown in Figure 9. The sound radiated by RV Johan Hjort is dominated by three distinct narrowband tonals centered at $168.5$ Hz, 694 Hz and 808 Hz. Examples of beamformed spectrograms containing these tonal signals, as well as other broadband and narrowband signals associated with the RV Johan Hjort are shown in Figure 9A–C. The ensemble of pitch tracks and the histogram of mean frequency weighted by signal duration for the ship-associated detections in the three frequency subbands are shown in Figure 9D–I.

The source level distributions for the ship-radiated tonal signals centered at $168.5$ Hz, 694 Hz, and 808 Hz are determined from a subset of the measured signals (see Figure 10) on the coherent hydrophone array. The mean and standard deviation of the source level in units of dB re 1 $\mathsf{\mu }$Pa/Hz at 1 m for the tonal signals centered at $168.5$ Hz, 694 Hz and 808 Hz are 168.4 ± 7.2, 147.6 ± 4.3 and 148.5 ± 4.0 dB, respectively, with three dB-down instantaneous bandwidths of 0.8 Hz, 3 Hz and 2 Hz, respectively.

The POD regions for the tonal signals centered at $168.5$ Hz, 694 Hz and 808 Hz radiated by the RV Johan Hjort in the Norwegian Sea environment are shown in Figure 11. The 50% POD region extends over an area with a 100 km to 200 km diameter. The POD region is the largest for the 168.5 Hz tonal signal in comparison to that at the other frequencies.

3.3. FV Artus

The tow tracks of the coherent hydrophone array and the corresponding locations of the FV Artus on February 21, 2014 in the Norwegian Sea during NorEx14 based on GPS measurements are shown in Figure 12A. The bearing-time trajectories of signal detections measured by the coherent hydrophone array that are associated with the FV Artus in the three frequency subbands 10–200 Hz, 200–1000 Hz and 1000–2000 Hz are plotted in Figure 12B–D.

The time-frequency characteristics of the detected signals associated with FV Artus in the three frequency subbands are shown in Figure 13. The sound radiated by FV Artus is dominated by four distinct cyclostationary signals centered at roughly 170 Hz, 716 Hz, 1584 Hz and 1791 Hz. Examples of beamformed spectrograms containing these four cyclostationary signals, as well as other broadband and narrowband signals associated with the FV Artus are shown in Figure 13A–C. The ensemble of pitch tracks and the histogram of mean frequency weighted by signal duration for the ship-associated detections in the three frequency subbands are shown in Figure 13D–I.

The source level distributions for the ship-radiated cyclostationary signals centered at 170 Hz, 716 Hz, 1584 Hz, and 1791 Hz are determined from a subset of the measured signals (see Figure 14) on the coherent hydrophone array. The mean and standard deviation of the source level in units of dB re 1 $\mathsf{\mu }$Pa/Hz at 1 m for the cyclostationary signals centered at 170 Hz, 716 Hz, 1584 Hz and 1791 Hz are 148.7 ± 2.1, 143.5 ± 5.8, 133.5 ± 6.0, and 130.0 ± 5.2 respectively, with 3 dB-down instantaneous bandwidths of 1.3 Hz, 2.5 Hz, 3.4 Hz, and 4 Hz, respectively.

The POD regions for the cyclostationary signals centered at 170 Hz, 716 Hz, and 1584 Hz radiated by the FV Artus and received by the coherent hydrophone array in the Norwegian Sea environment are shown in Figure 15. The 50% POD region extends over an area with between 100 km to 200 km diameter. The POD region is the largest for the 716 Hz cyclostationary signal in comparison to that at the other frequencies.

The prominent signals radiated by FV Artus have frequencies that are periodically oscillating about a mean value (see Figure 13). These signals can be modelled as cyclostationary random processes [74]. The frequency variation with time of the prominent cyclostationary signals can be approximated as,

$$f=f_0+B\sin \left[2\pi {\nu }_1(t-t_0)\right]$$

(3)

where $f_0$ is the mean frequency, B is the amplitude of the frequency oscillation, and ${\nu }_1=1/{\tau }_1$, where ${\tau }_1$ is the period of the frequency oscillations. Estimates of $f_0$, ${\nu }_1$ and B for the three cyclostationary signals centered at 716 Hz, 1584 Hz and 1791 Hz radiated by FV Artus are provided in

Table 1. Parameters used in Equation (3) to model the frequency oscillations of the narrowband cyclostationary acoustic signals radiated by the FV Artus.

${\mathit{f}}_0$	B	${\mathit{\nu }}_1$
(Hz)	(Hz)	(Hz)
715.6	9.3	0.15
1583.8	8.0	0.16
1790.9	8.3	0.164

and plotted in Figure 16. The period of the frequency oscillations is roughly ${\tau }_1\approx$ 6.3 s.

3.4. Dependence of Ship-Radiated Sound on Ship Speed

The histogram of ship speed over the analysis time period for each of the three vessels are plotted in Figure 17. The ship speed is broadly distributed with multiple local peaks spanning stationary 0 m/s to fast moving at 18 m/s for the RV Delaware II. The ship speed is relatively consistent and centered at roughly 4 m/s for FV Artus. The speed of the RV Johan Hjort is bimodal and centered at roughly 1 m/s and 5 m/s.

Here we examine the source level and spectra of signal detections for the RV Johan Hjort as a function of two ship speed ranges, ≤2 m/s and 4–6 m/s. At low ship speed ≤2 m/s, the mean frequency of detected signals is more broadly distributed (Figure 18A) with the dominant tonal at $168.5$ Hz emitted with more consistent source level, since its source level distribution is peakier with smaller standard deviation (Figure 19). Furthermore, the higher frequency tonals at 694 Hz and 808 Hz are negligible. At higher ship speed 4–6 m/s, the sound emitted is dominant by multiple tones, including the ones $168.5$ Hz, 694 Hz and 808 Hz. The dominant tonal at $168.5$ is emitted with more broadly distributed source level (Figure 19).

4. Discussion Including Implications for Studies of Vessel-Induced Fish Behavior

The ship-radiated sound received on the coherent hydrophone array after long range propagation (4–32 km) from all three research and fisheries survey vessels investigated here: the RV Delaware II, the RV Johan Hjort and the FV Artus; are dominated by narrowband tonals or cyclostationary signals that occur in the frequency range from 150 Hz to 2000 Hz. The lower frequency tonals around 170 Hz radiated from each ship are probably excited by internal machinery of the ship, such as motor and gear assembly, and then radiated through the hull [41]. The higher frequency tonals and cyclostationary signals in the 500 Hz to 2000 Hz range may be “propeller singing” resulting from fluid–structure interaction at the trailing edges of the propeller blades [75] (see Figure 1 of [41]).

The published spectra of ship-radiated sound from RV Johan Hjort are presented as one-third octave bandwidth intensity-averaged levels in the 10 Hz to 10 kHz frequency range in [23,28]. In [23], the results of which are reproduced in [22,30], the one-third octave bandwidth intensity averaged source levels of the RV Johan Hjort in units of dB re 1 $\mathsf{\mu }$Pa/Hz at 1 m in the 100 Hz to 1000 Hz frequency range at ship speed of 11 knots lie in the 134 to 142 dB range (see Figure 2 of [23]). In the same reference [23], it is also shown that the underwater sound radiated from RV Johan Hjort is highly dynamic where the one-third octave bandwidth intensity averaged source level can increase by 20 dB with changes in ship conditions such as ship speed and machinery propeller pitch and shaft speed. In [28], the one-third octave bandwidth-averaged source levels of RV Johan Hjort in units of dB re 1 $\mathsf{\mu }$Pa/Hz at 1 m in the 100 Hz to 1000 Hz frequency range from 125 dB to 150 dB with local peaks near 170 Hz, 320 Hz and 500 Hz with levels of 150 dB, 142 dB and 138 dB, respectively (see Figure 13 of [28]).

The source level of the dominant tonals found here for RV Johan Hjort based on measurements with the coherent hydrophone array in 2014 are 168.4 ± 6 at 168.5 ± 0.8 Hz, 147.6 ± 6 at 694 ± 3 Hz and 148.5 ± 6 at 808 ± 2 Hz in units of dB re 1 $\mathsf{\mu }$Pa/Hz at 1 m. The 168.5 ± 0.8 Hz tonal found here lies within the one-third octave band with corner frequencies of 140.3 Hz and 176.8 Hz, and bandwidth of 36.5 Hz. This tonal contribution can get reduced by up to $10{\log }_{10}(36.5/0.8)\approx 16.5$ dB after on third-octave bandwidth averaging, providing a closer match to the 170 Hz level of 150 dB re 1 $\mathsf{\mu }$Pa/Hz at 1 m found in [28]. The 694 ± 3 Hz tonal found here lies within the one-third octave band with corner frequencies of 561.2 Hz and 707.1 Hz, and bandwidth of 145.9 Hz. This tonal contribution can get reduced by up to $10{\log }_{10}(145.9/3)\approx 16.8$ dB after one-third octave bandwidth averaging. For both frequency bands, the overall one-third octave bandwidth averaged level will include intensity contributions from broadband signals present in the bandwidth, in addition to the tonals. In conclusion, the sound radiated by a ship underwater and received at long ranges can vary significantly with ship conditions such as ship speed and state of various machinery present in the vessel [23,28], as well as ocean environmental propagation conditions, which can enhance measurement of the dominant tonal frequency components of sound radiated from a ship.

When the high source levels of the narrowband tonal or cyclostationary signal radiated from a ship are not appropriately accounted for, it can lead to incorrect estimates of the spatial extent of the ship-radiated sound field. For instance, if the POD region calculations for the 168.5 Hz and 808 Hz tonals radiated by the RV John Hjort and received on the coherent hydrophone array were based on the one-third octave bandwidth intensity averaged source level values at these frequencies, provided in Figure 2 of Ref. [23], the estimated POD regions would be extremely small (see cyan lines in Figure 11)) and could not explain the observed data.

The measurements of ship-radiated underwater sound have been applied to study the potential avoidance behavior of fish to ships and other underwater vehicles employed for fish echosounder and trawl sampling surveys [22,23,24,28,29,30,31]. It is believed that the ’vessel-induced fish behavior’ may bias survey estimates of fish stock abundance. Previous studies have compared overall one-third octave bandwidth intensity averaged broadband ship-radiated sound source level for different ships or vehicles used for fish assessment surveys with the ICES-recommended one-third octave bandwidth-averaged source levels (CRR 209) for silent research vessels. These studies found no significant differences in response of fish to ships with contrasting one-third octave bandwidth averaged broadband ship-radiated source level that are above or below the ICES CRR 209 levels [22,30,31]. Here we find that the sound field radiated from an underwater vehicle is significantly dominated by their narrowband tonals with bandwidths that range from less than 1 Hz to about 3 Hz and source levels that may stand 10 dB to 30 dB above the one-third-octave bandwidth-averaged levels. Since the bandwidth of the dominant tonals are small (roughly 1 Hz), their levels get averaged out, and so is not accurately represented by one-third octave bandwidth averaging. The fish populations are more likely to sense the high level tonal signals than the lower level broadband sound. According to [22], the vessel stimulus to fish behavior investigated in that study were based on the one-third octave bandwidth-averaged levels from RV Johan Hjort, and found to be an inappropriate parameter to quantify ship sound. Here we propose that the full unaveraged spectra should be used for investigation of fish behavior to properly account for the unusually high levels of tonal sounds from ships that the fish would be most sensitive to. Our findings are consistent with recommendations of Mitson in [23] regarding ship-radiated sound since the “smoothing effect of averaging can disguise very strong tones present in the true signature occurring within the hearing spectrum of fish”.

In the fishery research community there has long been a focus on building noise-reduced research vessels according to recommendations provided by ICES [28]. Since then (1995) many research vessels have been built meeting these requirements. However, there is no reason to believe that these specially designed vessels do not suffer from some of the same extreme tonal sound levels as observed from conventionally designed vessels in our study. Therefore, some of these vessels should also be studied with our method to potentially explain the unexpected impacts from noise-reduced vessels like those described by Ref. [22].

5. Conclusions

The sound radiated underwater by three research and fisheries survey vessels have been detected and characterized from long ranges (4–32 km) with a large-aperture densely-sampled coherent hydrophone array using the passive ocean acoustic waveguide remote sensing (POAWRS) technique. The detected underwater sounds radiated from each of the three research and fishing vessels are found to be dominated by a number of narrowband tonals or cyclostationary signals in the 150 Hz to 2000 Hz frequency range. The temporal-spectral characteristics and source levels of the prominent tonals or cyclostationary signals have been quantified for each ship. The probability of detection regions for the most dominant signals of the RV Delaware II in the Gulf of Maine, and the RV Johan Hjort and the FV Artus in the Norwegian Sea are estimated and shown to extend over areas spanning roughly 200 km in diameter when employing a large-aperture densely-sampled coherent hydrophone array. The current standard procedure for quantifying ship-radiated sound source levels via one-third octave bandwidth intensity averaging smoothes over the prominent tonals radiated by a ship that can stand 10 dB to 30 dB above the local broadband level, which may lead to incorrect or inaccurate assessments of the impact of ship-radiated sound.