Directionality Of Ambient Noise Research Articles

Observation and Modelling on the Shipping Noise in Shallow Waters with Complex Islands and Reefs of the East China Sea

The impact of the noise radiated from merchant ships on marine life has become an active area of research. In this paper, a methodology integrating observation at a single location and modelling the whole noise field in shallow waters is presented. Specifically, underwater radiated noise data of opportunistic merchant ships in the waters of Zhoushan Archipelago were collected at least one day in each month from January 2015 to November 2016. The noise data were analyzed and a modified empirical spectral source level (SSL) model of merchant ships was proposed inspired by the RANDI-3 model (Research Ambient Noise Directionality) methodology. Then combining the modified model with the realistic geoacoustic parameters and AIS data of observed merchant ships, the noise mappings in this area were performed with N×2D of Normal Mode calculations, in which the SSL of each ship was estimated using the modified model. The sound propagation at different receiving positions is different due to the shielding effect of islands and bottom topography. The methodology proposed in this paper may provide a reference for modelling shipping noise in shallow waters with islands and reefs.

Estimation of seafloor properties in shallow oceans using minimally processed vertical directivity of surface-generated ambient noise

Typically, data from an array with vertical aperture will be beam-formed to provide the level and variation of spectrally averaged ocean ambient noise as a function of vertical angle. This paper considers the potential for such measurements to provide information about the acoustical properties and layering of a seafloor in a shallow ocean. First, with the assumption of dipole surface sources, it is feasible to describe the ambient noise in the near-horizontal in terms of the Weston α parameter (Weston, J. Sound Vib. 18, 271–287, 1971). Furthermore, measurements of ambient noise in the positive and negative vertical directions may be used to provide a spectrally averaged determination of bottom loss. With the presence of a surficial seafloor layer, each of the near-horizontal and near-vertical noise intensities will exhibit spectral variation. Techniques are described by which such relative variations in noise directivity, over a frequency span of some octaves, enable a reasonable estimation of, first, the thickness of a seafloor layer, and second, the acoustical properties of the layer and the seafloor basement.

Estimation of seabed properties from ocean ambient noise in shallow water: A review

Ocean ambient noise is the sound persisting in the ocean, which includes contributions from natural and anthropogenic sources, such as rain, wind, shipping, and marine mammals. Multiple interactions of the sound with the surface and bottom in shallow water affect the spatial characteristics of ambient noise. Thus, the spatial noise properties, coherence, and directionality of ambient noise depend on the characteristics of the shallow water environment, such as depth, sound speed profile, and seabed sediment properties. Acoustic properties of seafloor sediments can be extracted from ambient noise based on these noise properties using various types of noise data and geo-acoustic inversion methods. This paper presents a review of literature concerning investigations into available methods, the implementations of these methods, and their verification and validation. The efficacy and the applicability of these inversion methods for a wide range of ocean noise data are also examined in this review.

The Effect of Directional Ambient Noise on an Underwater Acoustic Link in Shallow Environments

To evaluate the performance of underwater acoustic communication systems, it is typically assumed that the noise at the receiver is uncorrelated spatially and temporally. This assumption underestimates the impact of acoustic ocean ambient noise on the performance of communication systems. In this article, the impact of ocean ambient noise on a coherent acoustic communication system is analyzed. The communication performance is assessed in narrowband conditions at a center frequency of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\text{2.048}\;\text{kHz}}$</tex-math></inline-formula> using the noise measurements from two different experiments—DalComm1 and the shallow water Canada Basin acoustic propagation experiment (SW CANAPE). DalComm1 focuses on characterizing the acoustic channel over ranges of 1–10 km on the Nova Scotian littoral, while the spatio-temporal variability of noise propagation in shallow and deep water environments was characterized during the CANAPE experiments. The ambient noise coherence and directionality in both environments were also measured. Two distinct noise modeling methodologies are presented to represent realistic synthetic ambient noise with defined directionality. Further, the synthetic noise is validated against measured ambient noise. The impact of ambient noise characteristics on an optimum space-time filter is characterized. A frame structure with an optimum training duration is also defined for the adaptive filter. It is observed that the bit-error rate of the space-time filter depends on optimizing the training and payload duration in the received signal.

A Linear Inversion Approach to Measuring the Composition and Directionality of the Seismic Noise Field

We develop a linear inversion technique for measuring the modal composition and directionality of ambient seismic noise. The technique draws from similar techniques used in astrophysics and gravitational-wave physics, and relies on measuring cross-correlations between different seismometer channels in a seismometer array. We characterize the sensitivity and the angular resolution of this technique using a series of simulations and real-world tests. We then apply the technique to data acquired by the three-dimensional seismometer array at the Homestake mine in Lead, SD, to estimate the composition and directionality of the seismic noise at microseism frequencies. We show that, at times of low-microseism amplitudes, noise is dominated by body waves (P and S), while at high-microseism times, the noise is dominated by surface Rayleigh waves.

Temporal and spatial characteristics of the Beaufort Sea ambient noise environment.

Underwater ambient noise from the Beaufort Sea, collected in March 2016, is analyzed to investigate the effect that environmental changes in the region have on the spatial and temporal characteristics of the ice-generated ambient noise. In particular, the influx of warm Pacific water, so called the Beaufort Lens, has dramatically altered the level and vertical directionality of ambient noise, creating a low noise zone in the ∼75-250 m depth interval with a noise notch at low grazing angles. It is also demonstrated how the observed noise vertical directionality is consistent with ice-mechanical activity along an active pressure ridge ∼30-50 km from the recording array, as shown by satellite imagery. The discrete ranges to this ice activity explain the peak arrival angles between -10 to -15 degrees in noise vertical directionality, as well as the horizontal noise notch. Transient noise events associated with the ice activity are analyzed using an image processing approach with hierarchical clustering applied to the recorded spectrograms. The observed events are grouped into three categories-short-time-broadband, long-time-narrowband, and long-time-broadband-each likely generated through a different mechanism by the ice cover. The spectral and temporal distribution of these transients are discussed.

Directionality of ambient noise in the Mississippi embayment

SUMMARYCross-correlations of ambient seismic noise from 277 broad-band stations within the Mississippi embayment (ME) with at least 1 month of recording time between 1990 and 2018 are used to estimate source locations of primary and secondary microseisms. We investigate source locations by analysing the azimuthal distribution of the signal-to-noise ratios (SNRs) and positive/negative amplitude differences. We use 84 stations with 1 yr of continuous recordings to explore seasonal variations of SNRs and amplitude differences. We also investigate the seasonal ambient noise ground motions using 2-D frequency–wavenumber (FK) analysis of a 50-station array. We observe that: (1) two major azimuths can be identified in the azimuthal distribution of SNRs and amplitude differences. We also observe two minor azimuths in the seasonal variation of SNRs, amplitude differences and 2-D FK power spectra. Monthly 2-D FK power spectra reveal that two energy sources are active in the Northern Hemisphere winter and two relatively weak sources are active in summer. (2) Backprojection suggests that primary microseisms originate along the coasts of Australia or New Zealand, Canada and Alaska, Newfoundland or Greenland and South America. (3) Secondary microseisms are generated in the deep water of the northern and southern Pacific Ocean, along the coasts of Canada and Alaska associated with near-shore reflections and in the deep water of south of Greenland. (4) Weak energy is observed in the third quadrant of the azimuthal distribution of amplitude differences of sedimentary Rayleigh and Love waves in the period band of 1–5 s and correlates with the direction of widening of the basin.

Ambient seismic noise wavefield in Japan characterized by polarization analysis of Hi-net records

The polarization of seismic waves can be used to infer the directionality of the ambient noise wavefield. We demonstrate a simple relationship between the vertical–horizontal cross-spectra at a single station and the azimuthal energy distribution of incident waves. The imaginary parts of the cross-spectra are related to the azimuthal energy distribution of elliptically polarized waves (Rayleigh waves), and the real parts are related to linearly polarized waves (P waves). The relationship can be used to estimate the dominant backazimuth and directional intensity of the incident waves. We apply our polarization method to records of length 1 yr observed by Hi-net to investigate the characteristics of the ambient noise wavefield in Japan. Rayleigh waves in secondary microseisms in the period range 4–8 s are sensitive to ocean wave activity in the adjacent sea, showing seasonal variations in directionality that are related to a pressure source region estimated by an ocean wave model. Compared with Rayleigh waves, the sources of the P wave energy are distant. The relative contribution of P wave energy in ambient noise increases when the adjacent sea is calm. At periods of 2–4 s, the directionality of the ambient noise wavefield seems to correspond to major tectonic boundaries. In addition to spatial variations of directionality due to the source distribution of microseisms in 2–4 s, strong crustal heterogeneities along tectonic boundaries may produce the abrupt change in the directionality. Trapping and scattering in thick sediments and around those boundaries may reduce the ambient noise directionality originating from their source distribution. The backazimuths at 1–2 s periods point towards the nearest coasts from each station without remarkable seasonal variations, which is attributed to strong path effects, wave scattering and attenuation. As polarization analysis is applicable through a wide frequency range our technique provides a means of characterizing the broad-band ambient seismic wavefield.

Spatial Vertical Directionality and Correlation of Low-Frequency Ambient Noise in Deep Ocean Direct-Arrival Zones.

Wind-driven and distant shipping noise sources contribute to the total noise field in the deep ocean direct-arrival zones. Wind-driven and distant shipping noise sources may significantly and simultaneously affect the spatial characteristics of the total noise field to some extent. In this work, a ray approach and parabolic equation solution method were jointly utilized to model the low-frequency ambient noise field in a range-dependent deep ocean environment by considering their calculation accuracy and efficiency in near-field wind-driven and far-field distant shipping noise fields. The reanalysis databases of National Center of Environment Prediction (NCEP) and Volunteer Observation System (VOS) were used to model the ambient noise source intensity and distribution. Spatial vertical directionality and correlation were analyzed in three scenarios that correspond to three wind speed conditions. The noise field was dominated by distant shipping noise sources when the wind speed was less than 3 m/s, and then the spatial vertical directionality and vertical correlation of the total noise field were nearly consistent with those of distant shipping noise field. The total noise field was completely dominated by near field wind generated noise sources when the wind speed was greater than 12 m/s at 150 Hz, and then the spatial vertical correlation coefficient and directionality pattern of the total noise field was approximately consistent with that of the wind-driven noise field. The spatial characteristics of the total noise field for wind speeds between 3 m/s and 12 m/s were the weighted results of wind-driven and distant shipping noise fields. Furthermore, the spatial characteristics of low-frequency ambient noise field were compared with the classical Cron/Sherman deep water noise field coherence function. Simulation results with the described modeling method showed good agreement with the experimental measurement results based on the vertical line array deployed near the bottom in deep ocean direct-arrival zones.

Estimation of Seabed Properties Using Ambient Noise from Shallow Water Sites of the Indian Continental Shelf

ABSTRACTIn this study an attempt has been made to extract sediment geoacoustic properties using ambient noise measured from a vertical hydrophone array. Time series noise data recorded from three shallow water sites (Chennai, Cuddalore and Cochin) along the Indian continental shelf were used for the analysis. The compressional sound speed of sediment for all the sites was estimated from the vertical directionality of ambient noise. Using the value of the compressional sound speed remaining wave properties and material properties were deduced from the Grain-Shearing (G-S) theory of wave propagation in saturated granular media. The type of sediment extracted from the G-S theory correlates well with the results obtained from sieve and particle size analysis of grab samples, collected from all the sites. The study clearly shows the application of ambient noise in extracting environmental information in shallow water, and further applying it to improve sonar performance modeling.

Two-element acoustic array gives insight into ice-ocean interactions in Hornsund Fjord, Spitsbergen

AbstractGlacierized fjords are dynamic regions, with variable oceanographic conditions and complex ice-ocean interactions, which are still poorly understood. Recent studies have shown that passive underwater acoustics offers new promising tools in this branch of polar research. Here, we present results from two field campaigns, conducted in summer 2013 and spring 2014. Several recordings with a bespoke two-hydrophone acoustic buoy were made in different parts of Hornsund Fjord, Spitsbergen in the vicinity of tidewater glaciers to study the directionality of underwater ambient noise. Representative segments of the data are used to illustrate the analyses, and determine the directions of sound sources by using the time differences of arrivals between two horizontally aligned, broadband hydrophones. The results reveal that low frequency noise (&lt; 3 kHz) is radiated mostly from the ice cliffs, while high-frequency (&gt; 3 kHz) noise directionality strongly depends on the distribution of floating glacial ice throughout the fjord. Changing rates of iceberg production as seen for example in field photographs and logs are, in turn, most likely linked to signal amplitudes for relevant directions. These findings demonstrate the potential offered by passive acoustics to study the dynamics of individual tidewater glaciers.

Directionality of ambient noise on the Juan de Fuca plate: implications for source locations of the primary and secondary microseisms

Based on cross-correlations of ambient seismic noise computed using 61 ocean bottom seismometers (OBSs) within the Juan de Fuca (JdF) plate from the Cascadia Initiative experiment and 42 continental stations near the coast of the western United States, we investigate the locations of generation of the primary (11–20 s period) and secondary (5–10 s period) microseisms in the northern Pacific Ocean by analysing the directionality and seasonality of the microseism (Rayleigh wave) signals received in this region. We conclude that (1) the ambient noise observed across the array is much different in the primary and secondary microseism bands, both in its azimuthal content and seasonal variation. (2) The principal secondary microseism signals propagate towards the east, consistent with their generation in deep waters of the North Pacific, perhaps coincident both with the region of observed body wave excitation and the predicted wave–wave interaction region from recent studies. (3) The primary microseism, as indicated by observations of the azimuthal dependence of the fundamental mode Rayleigh wave as well as observations of precursory arrivals, derives significantly from the shallow waters of the eastern Pacific near to the JdF plate but also has a component generated at greater distance of unknown origin. (4) These observations suggest different physical mechanisms for generating the two microseisms: the secondary microseisms are likely to be generated by non-linear wave–wave interaction over the deep Pacific Ocean, while the primary microseism may couple directly into the solid earth locally in shallow waters from ocean gravity waves. (5) Above 5 s period, high quality empirical Green's functions are observed from cross-correlations between deep water OBSs and continental stations, which illustrates that microseisms propagate efficiently from either deep or shallow water source regions onto the continent and are well recorded by continental seismic stations.

Directionality of the ambient noise field in an Arctic, glacial bay.

The directionality of ambient noise in an Arctic tidewater glacier bay was measured using two horizontally spaced, broadband hydrophones. Segments of noise were divided into two frequency bands and analyzed for arrival angle. These data show that different classes of source radiate noise in distinct spectral bands and are spatially diverse. A previously unidentified source, the interaction of surface gravity waves with underside of ice ledges at the periphery of icebergs, is described. The generation of noise by ice-wave interaction suggests that surface waves should be measured if ambient noise is to be used to monitor ice dynamics in glacial fjords.

Effect of Guide’s Acoustic Parameters on Vertical Directivity of the Marine Ambient Noise

The vertical directivity characteristic of ambient noise is one inherent characteristic of the ocean in shallow water. And it includes the information of guide’s acoustic characteristic information. The marine guide is composed of sea water; seabed and surface boundary, there into, the acoustic parameters of seabed are hardly obtained exactly. In this paper, the model of vertical directivity for ambient noise is established. Based on the ray theory of sound propagation, the influence of guide’s acoustic parameters which include sound speed, density and attenuation coefficient on vertical directivity of marine ambient noise is simulated. The results are propitious to analysis and command the characteristics of ambient noise, and valuable to accelerate the exertion of acoustic equipment performance.

The effect of seawater attenuation on the directionality and spatial coherence of surface-generated ambient noise

Acoustic attenuation in seawater is sufficiently weak that it usually has little effect on the spatial characteristics of ambient noise. However, at frequencies above 10 kHz and depths below 6 km, seawater attenuation may influence the directionality of surface-generated ambient noise. In a semi-infinite, homogeneous ocean, all the noise is downward-traveling, since there are no bottom reflections, and, in the absence of attenuation, the directional density function varies as the cosine of the polar angle measured from the zenith. When attenuation is present, the angular width of this noise lobe becomes narrower, because sound from distant surface sources is attenuated more than acoustic arrivals from overhead. This narrowing effect increases as frequency rises, since the attenuation is a rapidly increasing function of frequency. The spatial coherence of the noise is also modified by the attenuation. For a pair of horizontally (vertically) aligned sensors, the zeroes in the spatial coherence function are shifted to higher (lower) frequencies. These effects of attenuation on the noise field are sufficiently large to be detectable using the recently developed, deep diving instrument platform Deep Sound, which is capable of measuring broadband ambient noise on multiple hydrophones to depths of 11 km. [Research supported by ONR.]

Directionality of ambient noise measurements in Barrow Strait of the Canadian Arctic

In August 2012, a field trial was carried out in Barrow Strait south of Gascoyne Inlet in the vicinity of 74.630 N 91.340 W. Underwater acoustic data was collected using a JASCO Autonomous Multichannel Acoustic Recorder (AMAR) and in-house designed sensor systems called Starfish Cubes. The Starfish Cubes were deployed twice, at different locations, each for one week duration and at depths of approximately 110 m. The Cubes consist of seven hydrophones with 1 m spacing and geometrically configured as three cross-dipoles with a central hydrophone, and have an operational frequency range of 5—750 Hz. During the trial 400 and 500 Hz tones were transmitted from discrete locations at various ranges. By using a beamforming method the tones were used to determine the orientation of the Starfish Cubes during their data collection periods. This enables investigation of the horizontal and vertical directionality of ambient noise. Unique localized sources contributing to the ambient noise are discussed such as a nearby grounded iceberg and a low frequency wandering tonal.

Measuring the effect of ambient noise directionality and split-beam processing on the convergence of the cross-correlation function

Measurements of ambient noise have been used to infer information about the ocean acoustic environment. In recent years the correlation of ambient noise has been shown to give estimates of the travel time of acoustic paths between the sensors recording the noise. A number of issues affect the results of the noise correlation. This paper presents the results of noise correlation of the two horizontally separated arrays of sensors in the 2010 ambient noise experiment. Using the experimental data, the effects on the convergence of the noise correlation are examined with respect to the size and shape of the arrays, the length of time used, and the directionality of the noise field.

Theory of the directionality and spatial coherence of wind-driven ambient noise in a deep ocean with attenuation

Acoustic attenuation in seawater usually has little effect on the spatial statistics of ambient noise in the ocean. This expectation does not hold, however, at higher frequencies, above 10 kHz, and extreme depths, in excess of 6 km, an operating regime that is within the capabilities of the most recently developed acoustic instrument platforms. To quantify the effects of attenuation, theoretical models for the vertical directionality and the spatial coherence of wind-generated ambient noise are developed in this paper, based on a uniform distribution of surface sources above a semi-infinite, homogeneous ocean. Since there are no bottom reflections, all the noise is downward traveling; and the angular width of the directional density function becomes progressively narrower with increasing frequency because sound from the more distant sources experiences greater attenuation than acoustic arrivals from overhead. This narrowing of the noise lobe modifies the spatial coherence, shifting the zeros in the horizontal (vertical) coherence function to higher (lower) frequencies. In addition, the attenuation modifies the amplitudes of the higher-order oscillations in the horizontal and vertical coherence functions, tending to suppress the former and enhance the latter. These effects are large enough to be detectable with the latest deep-diving sensor technology.

Source directionality of ambient seismic noise inferred from three‐component beamforming

AbstractThe increased use of ambient seismic noise for seismic imaging requires better understanding of the ambient seismic noise wavefield and its source locations and mechanisms. Although the source regions and mechanisms of Rayleigh waves have been studied extensively, characterization of Love wave source processes are sparse or absent. We present here the first systematic comparison of ambient seismic noise source directions within the primary (~10–20 s period) and secondary (~5–10 s period) microseism bands for both Rayleigh and Love waves in the Southern Hemisphere using vertical‐ and horizontal‐component ambient seismic noise recordings from a dense temporary network of 68 broadband seismometers in New Zealand. Our analysis indicates that Rayleigh and Love waves within the primary microseism band appear to be mostly generated in different areas, whereas in the secondary microseism band they arrive from similar backazimuths. Furthermore, the source areas of surface waves within the secondary microseism band correlate well with modeled deep‐water and near‐coastal source regions.

Investigation of the horizontal directivity of underwater ambient noise with the circular hydrophone array

The ambient noise field in shallow water is highly variable in time and space. The horizontal directivity pattern of ambient noise is anisotropic and can be exploited for source localization in sonar signal processing. In the present work, the 12-element circular hydrophone array with radius of 1 meter was designed and used to observe the ambient noise in the shallow water experiment in the Yellow Sea. The MVDR beamformer with diagonal loading was designed to get the horizontal directivity pattern in frequency band of 1kHz to 3kHz. The conventional beamformer was used to analyze the horizontal directivity of ambient noise above 3 kHz. The noise data were sampled at 25kHz, bandpass filtered between 100Hz and 5kHz in the experiment. The horizontal directivity patterns at the experiment location at different frequencies were obtained based on above methods and the noise data. This work was supported by the National Natural Science Foundation of China(10734030)