Ocean Noise Sources Research Articles

Estimation of Seismic Attenuation from Ambient Noise Coda Waves: Application to the Hellenic Subduction Zone

ABSTRACT The decay of surface-wave coda in ambient noise cross correlations can facilitate the estimation of seismic attenuation. The coda quality factor (Qc) can be measured in longer period bands using ambient noise cross correlations, which is the main advantage of using them over earthquakes. The classic model of Aki and Chouet (1975) has been applied previously to estimate coda Q in the Alps using ambient noise cross correlations. The Alps represents an ideal environment for ambient noise study, because it has very high and near-uniform station density, and the region is away from oceanic noise sources. However, many regions around the world do not have uniform station density and may be seismically more active than the Alps. One such region is the Hellenic subduction zone (HSZ), because it has a high rate of seismicity, sparser station coverage, and is surrounded by seas from outside and within. In this study, we estimate ambient seismic noise Qc in 2.5–5, 5–10, and 10–20 s period bands for the HSZ, which did not exist previously. Then, we mitigate the effects of lapse time and window length, distance, azimuth, as well as the number of stacked days on the Qc. Mapping of Qc measurements in such a geographically heterogeneous setting poses additional challenges, which we solve using a novel approach that adaptively selects paths based on their lengths and azimuthal distribution. The major tectonic zones are identified in the resulting Qc maps in the form of low Qc such as the North Anatolian fault, the Kefalonia transform zone, the Gulf of Corinth, the volcanic centers, and so on. The results also show a good correlation with large topographical features such as the Hellenides and the Thessalian plains, which have also been noticed from Qc analysis in other parts of the world.

Reduction of Low-Frequency Vessel Noise in Monterey Bay National Marine Sanctuary During the COVID-19 Pandemic

Low-frequency sound from large vessels is a major, global source of ocean noise that can interfere with acoustic communication for a variety of marine animals. Changes in vessel activity provide opportunities to quantify relationships between vessel traffic levels and soundscape conditions in biologically important habitats. Using continuous deep-sea (890 m) recordings acquired ∼20 km (closest point of approach) from offshore shipping lanes, we observed reduction of low-frequency noise within Monterey Bay National Marine Sanctuary (California, United States) associated with changes in vessel traffic during the onset of the COVID-19 pandemic. Acoustic modeling shows that the recording site receives low-frequency vessel noise primarily from the regional shipping lanes rather than via the Sound Fixing and Ranging (SOFAR) channel. Monthly geometric means and percentiles of spectrum levels in the one-third octave band centered at 63 Hz during 2020 were compared with those from the same months of 2018–2019. Spectrum levels were persistently and significantly lower during February through July 2020, although a partial rebound in ambient noise levels was indicated by July. Mean spectrum levels during 2020 were more than 1 dB re 1 μPa2 Hz–1 below those of a previous year during 4 months. The lowest spectrum levels, in June 2020, were as much as 1.9 (mean) and 2.4 (25% exceedance level) dB re 1 μPa2 Hz–1 below levels of previous years. Spectrum levels during 2020 were significantly correlated with large-vessel total gross tonnage derived from economic data, summed across all California ports (r = 0.81, p < 0.05; adjusted r2 = 0.58). They were more highly correlated with regional presence of large vessels, quantified from Automatic Identification System (AIS) vessel tracking data weighted according to vessel speed and modeled acoustic transmission loss (r = 0.92, p < 0.01; adjusted r2 = 0.81). Within the 3-year study period, February–June 2020 exhibited persistently quiet low-frequency noise and anomalously low statewide port activity and regional large-vessel presence. The results illustrate the ephemeral nature of noise pollution by documenting how it responds rapidly to changes in offshore large-vessel traffic, and how this anthropogenic imprint reaches habitat remote from major ports and shipping lanes.

Effect of COVID-19 spring lockdown on low-frequency ambient noise levels along the Brittany coast

Shipping noise is a major source of ocean noise in Brest bay, and includes cargo vessels, navy ships, fishing boats and recreational vessels with frequencies overlapping the range of marine animal sound emission. Changes in vessel activity provide unique opportunities to quantify the relationship between traffic levels and soundscape conditions in biologically important habitats. Using an autonomous recorder deployed from March 20 to July 29 2019 inside Sainte-Anne harbor in Brest area we assessed the effect of the 2020 spring COVID-19 lockdown (March 17 to May 11) on ambient noise levels. We measured the mean power spectral density levels in different third-octave bands (including Marine Strategy Framework Directive bands centered on 63 Hz and 125 Hz). The March 20–May 15 (during lockdown) levels were up to 2.6 dB re 1 μPa2/Hz below the May 15–July 28th (after lockdown) levels, corresponding to nearly a halving of acoustic power.

Bottom trawling noise: Are fishing vessels polluting to deeper acoustic habitats?

The impact of bottom trawling noise was quantified on two surrounding marine acoustic habitats using fixed mooring acoustic recorders. Noise during trawling activity is shown to be considerably louder than ambient noise and a nearby underway research vessel. Estimated source levels were above cetacean damage thresholds. Measurements at a submarine canyon indicated potential noise focussing, inferring a role for such features to enhance down slope noise propagation at continental margin sites. Modelled sound propagates more efficiently when sourced from trawling gear dragging along the seabed relative to the vessel as a surface source. Results are contextualised with respect to marine mammal harm, to other anthropogenic ocean noise sources, topography and seasons. Noise energy emitted by bottom trawling activity is a source of pollution that requires further consideration, in line with other pervasive trawling pressures on marine species and seabed habitats, especially in areas of heightened ecological susceptibility.

Quieting of low-frequency vessel noise in Monterey Bay National Marine Sanctuary during the COVID-19 pandemic

Low-frequency sound from large vessels is a major source of ocean noise, overlapping the range of marine animal communication. Changes in vessel activity provide opportunities to quantify the relationship between traffic levels and soundscape conditions in biologically important habitats. Using continuous deep-sea (890 m) recordings ∼20 km from shipping lanes, we observed quieting of low-frequency noise within Monterey Bay National Marine Sanctuary (California, USA) during the COVID-19 pandemic. Noise levels in the frequency band 31–100 Hz, which captured large vessel noise while minimizing potential biases from geological and biological sound sources, decreased during January–June 2020. At monthly resolution the 2020 quieting trend correlated with decreasing cumulative hours of vessel presence derived from Automatic Identification System (AIS) data for cargo vessels (r = 0.92; p = .01), and the sum of all positively correlated vessel types (cargo, towing, dredging, passenger, tanker; r = 0.88; p = 0.02). February–June 2020 levels were up to 2.7 dB re 1 μPa2/Hz below baseline of the two years prior. While anticipated to be short-term, at their peak these changes represent nearly a halving of acoustic power, emphasizing the significance of offshore large vessel traffic to sound levels in deeper waters in the sanctuary.

Modelling ocean ambient noise with finite ocean depth and comparisons with observations

The global International Monitoring System (IMS) network continuously detects coherent ambient infrasound noise between 0.1 and 0.5 Hz. This noise, referred to as microbaroms, is generated by the second order non-linear interaction of ocean waves, mostly during severe storms. A global and multi-year analyze of microbaroms highlights the strong influence of middle atmospheric conditions on the propagation. Various source models have been developed. Brekhovskikh et al. (1973) and Ardhuin and Herbers (2013) considered a source directivity effect in infinite depth ocean with radiative pressure depending on the wave elevation angle. Waxler and Gilbert (2006) and Waxler (2007) investigated the radiation of infrasound by ocean waves in finite depth ocean by monopolar sources. In this study, the combined effects of non-monopolar source and bathymetry on the radiation are addressed. Beyond theoretical issues, source modelisation and propagation through a realistic atmosphere are carried out. Comparing the predicted signals with the observed ones at all IMS stations shows good agreements for both directional and amplitude information. Building a global reference database of oceanic noise sources opens new perspectives for providing additional integrated constraints on middle atmosphere dynamics and disturbances.The global International Monitoring System (IMS) network continuously detects coherent ambient infrasound noise between 0.1 and 0.5 Hz. This noise, referred to as microbaroms, is generated by the second order non-linear interaction of ocean waves, mostly during severe storms. A global and multi-year analyze of microbaroms highlights the strong influence of middle atmospheric conditions on the propagation. Various source models have been developed. Brekhovskikh et al. (1973) and Ardhuin and Herbers (2013) considered a source directivity effect in infinite depth ocean with radiative pressure depending on the wave elevation angle. Waxler and Gilbert (2006) and Waxler (2007) investigated the radiation of infrasound by ocean waves in finite depth ocean by monopolar sources. In this study, the combined effects of non-monopolar source and bathymetry on the radiation are addressed. Beyond theoretical issues, source modelisation and propagation through a realistic atmosphere are carried out. Comparing the predicte...

Underwater noise from airplanes: An overlooked source of ocean noise

The effects of underwater noise pollution on marine life are of increasing concern. Research and management have focussed on the strongest underwater sound sources. Aerial sound sources have understandably been ignored as sound transmits poorly across the air-water interface. However, there might be situations when air-borne noise cannot be dismissed. Commercial passenger airplanes were recorded in a coastal underwater soundscape exhibiting broadband received levels of 84–132 dB re 1 μPa rms. Power spectral density levels of airplane noise underwater exceeded ambient levels between 12 Hz and 2 or 10 kHz (depending on site) by up to 36 dB. Underwater noise from airplanes is expected to be audible to a variety of marine fauna, including seals, manatees, and dolphins. With many of the world's airports lying close to the coast, it is cautioned that airplane noise not be ignored, in particular in the case of at-risk species in small, confined habitats.

Characterizing ocean ambient noise using infrasound network

The ability of the International Monitoring System (IMS) global infrasound network to detect atmospheric explosions and events of interest strongly depends on station specific ambient noise signatures which include both incoherent wind noise and real coherent infrasonic waves. To characterize the coherent ambient noise, broadband array processing was performed on historical continuous IMS recordings. Ocean wave interactions contribute to the atmospheric coherent ambient noise field, and we apply wave action models to model these microbarom sources. We use two-dimensional energy spectrum ocean wave products to build a global reference database of oceanic noise sources. Then, we compare observed and modeled directional microbarom amplitudes at several stations. The expected benefits of such studies concern the use complementary data to finely characterize the coupling mechanisms at the ocean-atmosphere interface. In return, better knowledge of ambient ocean noise sources opens new perspectives not only by enhancing the characterization of explosive atmospheric events, but also by providing additional integrated constraints on middle atmosphere dynamics and disturbances where data coverage is otherwise sparse.

Vertical line array measurements of ambient noise in the North Pacific.

Shipping noise and wind are the dominant sources of ocean noise in the frequency band between 20 and 500 Hz. This paper analyzes noise in that band using data from the SPICEX experiment, which took place in the North Pacific in 2004-2005, and compares the results with other North Pacific experiments. SPICEX included vertical arrays with sensors above and below the surface conjugate depth, facilitating an analysis of the depth dependence of ambient noise. The paper includes several key results. First, the 2004-05 noise levels at 50 Hz measured in SPICEX had not increased relative to levels measured by Morris [(1978). J. Acoust. Soc. Am. 64, 581-590] at a nearby North Pacific site three decades earlier, but rather were comparable to those levels. Second, at 50 Hz the noise below the conjugate depth decreases at a rate of -9.9 dB/km, which is similar to the rate measured by Morris and much less than the rate measured by Gaul, Knobles, Shooter, and Wittenborn [(2007). IEEE J. Ocean. Eng. 32, 497-512] for the CHURCH OPAL experiment. Finally, the paper describes the seasonal trends in noise over the year-long time series of the measurements.

Hearing Mechanisms and Noise Metrics Related to Auditory Masking in Bottlenose Dolphins (Tursiops truncatus).

Odontocete cetaceans are acoustic specialists that depend on sound to hunt, forage, navigate, detect predators, and communicate. Auditory masking from natural and anthropogenic sound sources may adversely affect these fitness-related capabilities. The ability to detect a tone in a broad range of natural, anthropogenic, and synthesized noise was tested with bottlenose dolphins using a psychophysical, band-widening procedure. Diverging masking patterns were found for noise bandwidths greater than the width of an auditory filter. Despite different noise types having equal-pressure spectral-density levels (95 dB re 1 μPa(2)/Hz), masked detection threshold differences were as large as 22 dB. Consecutive experiments indicated that noise types with increased levels of amplitude modulation resulted in comodulation masking release due to within-channel and across-channel auditory mechanisms. The degree to which noise types were comodulated (comodulation index) was assessed by calculating the magnitude-squared coherence between the temporal envelope from an auditory filter centered on the signal and temporal envelopes from flanking filters. Statistical models indicate that masked thresholds in a variety of noise types, at a variety of levels, can be explained with metrics related to the comodulation index in addition to the pressure spectral-density level of noise. This study suggests that predicting auditory masking from ocean noise sources depends on both spectral and temporal properties of the noise.

Seismic surveys and marine wildlife: Impacts, the lack thereof, and thoughts on managing both

Sufficient scientific data exist to conclude that seismic airguns used in geophysical exploration have a low probability of direct harm to most marine life, except at close range where physical injury is a real danger. Further, airguns in some conditions do not appear to disturb animals; however, in other conditions, they result in moderate to extreme behavioral responses and/or acoustic masking over large areas. Additionally, recent studies have reported the presence of seismic survey sound energy over ranges of ~4000 km. While the potential for effects have not even been investigated at such ranges, the presence of the signals must be taken into account when evaluating overall potential for impacts. Mitigation measures have historically focused on reducing immediate harm, but systematically measuring and understanding the full potential for impacts is an important aspect of any responsible development program. The European Union has recognized ocean noise as a pollutant and as an indicator of environmental quality under its Marine Strategy Framework Directive. Given this and the international and transboundary nature of noise from marine seismic surveys, their ubiquity, the presence of numerous other sources of ocean noise, and that incorporating acoustic disturbance into an understanding of population level consequences is progressing, a responsible path forward should focus on the creation of legally binding international commitments and standards for the management and minimization of noise.

Wind generated ocean noise in deep sea

Ocean noise is an important topic in underwater acoustics, which has been paid much attention in last decades. Ocean noise sources may consist of wind, biological sources, ships, earthquakes and so on. This paper discusses measurements of the ocean noise intensity in deep sea during strong wind periods. During the experiment, shipping density is small enough and the wind generated noise is believed to be the dominated effect in the observed frequency range. The analyses of the recoded noise data reveal that the wind generated noise source has a strong dependence on the wind speed and frequency. Based on the data, a wind generated noise source model is presented. [Work supported by National Natural Science Foundation of China, Grant No. 11125420.]

Detection of microseismic compressional (P) body waves aided by numerical modeling of oceanic noise sources

Among the different types of waves embedded in seismic noise, body waves present appealing properties but are still challenging to extract. Here we first validate recent improvements in numerical modeling of microseismic compressional (P) body waves and then show how this tool allows fast detection and location of their sources. We compute sources at ~0.2 Hz within typical P teleseismic distances (30–90°) from the Southern California Seismic Network and analyze the most significant discrete sources. The locations and relative strengths of the computed sources are validated by the good agreement with beam‐forming analysis. These 54 noise sources exhibit a highly heterogeneous distribution, and cluster along the usual storm tracks in the Pacific and Atlantic oceans. They are mostly induced in the open ocean, at or near water depths of 2800 and 5600 km, most likely within storms or where ocean waves propagating as swell meet another swell or wind sea. We then emphasize two particularly strong storms to describe how they generate noise sources in their wake. We also use these two specific noise bursts to illustrate the differences between microseismic body and surface waves in terms of source distribution and resulting recordable ground motion. The different patterns between body and surface waves result from distinctive amplification of ocean wave‐induced pressure perturbation and different seismic attenuation. Our study demonstrates the potential of numerical modeling to provide fast and accurate constraints on where and when to expect microseismic body waves, with implications for seismic imaging and climate studies.

Detection of phase‐coupled noises in the ocean

Estimates of power spectral density are being used in a variety of signal processing applications. For the precise characterization of certain noise sources in the ocean, the phase blind power spectral estimation does not yield the required result. Estimates of the bispectrum and bicoherence have been found useful in detecting non‐Gaussianity and nonlinearity, in system identification, as well as transient signals. The power spectrum, in general, fails to quantify any nonlinear interactions between the component frequencies. Such interactions induced by second‐order nonlinearities give rise to certain phase relations called quadratic phase coupling (QPC). Bispectral analysis, which is based on the third‐order cumulant sequence of the signal, can reveal the presence of as well as provide a measure to quantify such phase couplings. Bispectral analyses of various noise data records collected from different ocean noise sources have been carried out and it is seen that the bispectrum is not identically zero, implying the non Gaussian nature of the noise data. This paper presents the bispectral estimation approach for identifying the nonlinearities in the underwater noise‐generating mechanisms as well as detecting the presence of quadratic phase couplings, thus leading to the identification of the noise sources.

Extracting coherent wave fronts from acoustic ambient noise in the ocean

A method to obtain coherent acoustic wave fronts by measuring the space–time correlation function of ocean noise between two hydrophones is experimentally demonstrated. Though the sources of ocean noise are uncorrelated, the time-averaged noise correlation function exhibits deterministic waveguide arrival structure embedded in the time-domain Green’s function. A theoretical approach is derived for both volume and surface noise sources. Shipping noise is also investigated and simulated results are presented in deep or shallow water configurations. The data of opportunity used to demonstrate the extraction of wave fronts from ocean noise were taken from the synchronized vertical receive arrays used in the frame of the North Pacific Laboratory (NPAL) during time intervals when no source was transmitting.

Factors Affecting the Responses of Marine Mammals to Acoustic Disturbance

The issues surrounding marine mammals and noise cannot be managed effectively without an understanding of the effects of that noise on individual mammals and their populations. In the spring of 2003 the National Research Council released Ocean Noise and Marine Mammals, a report that reviewed sources of ocean noise (natural and anthropogenic), the effects of noise on marine mammals, patterns and long-term trends in ocean noise, and included recommendations intended to improve understanding of the sources and impacts of anthropogenic marine noise. This paper provides a brief summary of observed effects of ocean noise on marine mammals and the factors that can change the response of the animal to the noise exposure. It introduces the reader to short- and long-term behavior changes that have been observed in marine mammals in response to ocean noise, and discusses future directions for marine mammal research.

Feasibilities of long-range acoustic vision in the northwest Pacific with large aperture arrays

The feasibilities of the usage of large aperture stationary arrays for the acoustic observation of the northwest part of the Pacific ocean existing near the Kamchatka peninsula are described. Such an array has a plane aperture filled with 2400 hydrophones and allows the monitoring of ocean noise distribution in a horizontal view sector of 120 deg within a frequency range up to 1400 Hz. Reconstruction of ocean noise sources distribution over the ranges up to about 1000 km from a shore can be made by a method of emission acoustic spectrotomography. Spatial resolution can be improved by the incorporation of several arrays for observation. Acoustic large-scale vision may be applied for the monitoring of wind distribution, in particular storms.

Standard definitions for sound source levels in the ocean

Recently, increased attention has focused on the use of sound in long-term ocean-acoustic experiments. The question being the determination of begin levels or the quantification of acceptable levels. This is an important consideration due to the increased use of higher power sound sources and our increased awareness of the effects of sound on mammals. Recent discussions and papers at the 126th Meeting of the Acoustical Society illustrate the confusion which exist with respect to characterization of acoustic source levels and has resulted in erroneous and misleading reports (i.e., see information from John R. Potter reported by Malcolm B. Browne, ‘‘Science Times,’’ New York Times, 19 Oct. 1993). This paper presents some standard definitions for the comparison of decibel levels with references of peak pressure, energy, or power. Frequency-dependent characteristics of oceanic noise sources shall be discussed and the relative importance of sound levels from air guns, surface ships, marine mammals, and other sources shall be placed in perspective. Finally these sources of sound will be further examined in light of the propagation effects found in both shallow waters and the deep ocean.

The sound field near hydrothermal vents on Axial Seamount, Juan de Fuca Ridge

High‐quality acoustic noise measurements were obtained by two hydrophones located 3 m and 40 m from an active hydrothermal vent on Axial Seamount, Juan de Fuca Ridge, in an effort to determine the feasibility of monitoring hydrothermal vent activity through flow noise generation. Most of the measured noise field could be attributed to ambient ocean noise sources of microseisms, distant shipping, and weather, punctuated by local ships and biological sources. Long‐period, low‐velocity, water/rock interface waves were detected with high amplitudes which rapidly decayed with distance from the seafloor. Detection of vent signals was hampered by unexpected spatial nonstationarity due to the shadowing effects of the caldera wall. No continuous vent signals were deemed significant based on a criterion of 90% probability of detection and 5% probability of false alarm. However, a small signal near 40 Hz, with a power level of 10−4Pa2/Hz was noticed on two records taken within 3 m of the Inferno black smoker. Hie frequency of this signal is consistent with predictions, and the power level suggests the occurrence of jet noise amplification due to convected density inhomogeneities.