Vertical Directionality Of Ambient Noise Research Articles

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.

Spectral, spatial, and temporal characteristics of underwater ambient noise in the Beaufort Sea in 1994 and 2016.

Climate induced changes in the Arctic Ocean have severely impacted underwater acoustic communication and navigation; understanding underwater noise characteristics is critical to improving the performance of these operations. Ambient noise from the Beaufort Sea recorded in experiments more than 20 years apart (SIMI94 and ICEX16) are compared to determine differences that may be attributed to the region's rapidly changing environment. Spectral comparison shows noise within 20-350 Hz is ∼30 dB louder in 1994 than 2016; however, this is likely due to higher array self noise during SIMI94. Beamforming results show ambient noise vertical directionality is focused near the horizontal during SIMI94 but more spread in elevation during ICEX16, with a robust noise notch at the horizontal. Numerical modeling demonstrates that this difference may be attributed to ambient noise during ICEX16 being dominated by surface noise sources at discrete ranges rather than the historical assumption of a continuous and uniform distribution of sources. Temporal statistics of transient ice events show more acoustic activity during SIMI94 than ICEX16 and appear to support the new proposed surface source distribution for ICEX16.

Ambient Noise Vertical Directionality and Reflection Loss Estimates for a Shallow Tropical Site with a Fast Sound Speed Bottom

For high frequency ocean acoustic modeling applications, seabed reflection loss is a useful alternative input compared to conventional geoacoustic model parameters. Reflection loss can be estimated by comparing the noise intensity of the up and down components of the ambient noise vertical directionality pattern. The potential of this method is demonstrated with experimental data spanning one week, collected off shallow east coast of India using a 21 element vertical hydrophone array. The compact and easily operable vertical array has been designed for high frequency directionality estimation in the band 2–10 kHz. The ambient noise data are beam formed to arrive at the vertical directionality pattern. Further reflection loss values as a function of frequency and grazing angle have been estimated for 1/3 octave bands for a sandy sea bed in warm tropical waters. This has been compared with modeled reflection loss estimates using OASR reflection loss module of OASES. This will serve as inputs to propagation models for applications such as inverse techniques, ambient noise modelling, and sonar system performance prediction.

Spectral, spatial, and temporal characteristics of underwater ambient noise in the Beaufort Sea in 1994 and 2016

Climate induced changes in the Arctic Ocean have severely impacted underwater acoustic communication and navigation; understanding underwater noise characteristics is critical to improving the performance of these operations. Ambient noise from the Beaufort Sea recorded in experiments more than 20 years apart (SIMI94 and ICEX16) are compared to determine differences that may be attributed to the region's rapidly changing environment. Both datasets are collected at spring time under packed ice conditions with 32 element vertical line arrays. Spectral comparison shows noise within 20–300 Hz band is 30–40 dB louder in 1994 than 2016, suggesting the ice cover during SIMI94 was more acoustically active. Beamforming results show ambient noise vertical directionality is focused near the horizontal during SIMI94 but more spread in elevation during ICEX16, with a robust noise notch at the horizontal. Numerical modeling demonstrates that this difference may be attributed to ambient noise during ICEX16 being dominated by surface noise sources at discrete ranges rather than the historical assumption of a continuous and uniform distribution of sources. Temporal statistics of transient ice events show more transient activity during SIMI94 and appear to support the new proposed surface source distribution for ICEX16. [Work supported by ONR and DARPA STO.]

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.

The effect of internal waves on the ambient noise vertical directionality in deep ocean

Vertical array measurements of low-frequency ambient noise in the mid-latitudes indicate that distant shipping noise has a flat angular distribution concentrated around broadside, e.g., [Wales and Diachok, JASA, 1981]. Ships cannot directly excite the lowest modes (associated with broadside angles) since those modes are trapped near the sound channel axis. Dashen and Munk [JASA, 1984] considered three mechanisms that could transfer shipping noise into the low modes: (1) downslope conversion on the continental slope, (2) volume scattering due to internal waves, and (3) direct excitation of the low modes at high latitudes where the sound channel intersects the surface. Dashen and Munk conclude that the first mechanism is the most likely since the second requires impractically large propagation ranges and the third requires unrealistic shipping densities at high latitudes. A limitation of Dashen and Munk's internal wave analysis is that it does not consider the effect of bottom loss. This talk adapts the transport theory approach of Colosi and Morozov [JASA, 2009] to study the effect of internal waves on the vertical directionality of distant shipping noise in the presence of seafloor attenuation. Initial results indicate that internal waves may have a significant effect at shorter ranges than previously thought.

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.

The mode coupling and effect on ambient noise vertical directionality caused by internal waves in shallow water

Experiments show that internal waves can cause the fluctuation of the depth of thermocline in shallow water, which may bring about two results: one is the change of hit number of sound rays and sea bottom, the other is mode coupling. The former changes the sound intensity, and the latter changes the sound energy distribution. The previous studies show that internal waves cause mode coupling by changing the sound propagation paths, which can fill the noise notch. In the calculation, it is found that the notch is deepened when some solitary internal waves are present. The reason is that the sound wave excited by the distant sources which are located on the palne near the sea surface propagates through the soliton which is a single downward undulation. The mode coupling from low-order modes to higher-order modes happens first, then the reverse conversion occurs. However, the former coupling is stronger, the lower-order modes fall off and the filling ability is weakened, so the notch is deeper.

Analysis of the vertical directionality of ambient noise: SPICE04 experiment measurements and model simulations.

The SPICE04 experiment provided a unique opportunity to observe the ambient noise in the North Pacific [Worcester et al., J. Acoust. Soc. Am. 120(5), p. 3020]. Receptions on two 700-m segments of a deep vertical line array (DVLA) facilitate measurements of the vertical directionality of the noise above and below the surface conjugate depth. Several authors have proposed models for deep ocean noise, e.g., the spatial harmonic model developed by Cox [J. Acoust. Soc. Am. 54(5), pp. 1289–1301] and the normal mode model developed by Kuperman and Ingenito [J. Acoust. Soc. Am. 67(6), 1988–1996] and extended by Perkins et al. [J. Acoust. Soc. Am. 93(2), pp. 739–752]. The simulated data provided by these models can be used to assess the effects of array tilt on estimates of vertical directionality. In this talk, we first analyze the impact of tilt for the DVLA using the array element positions measured during the experiment. Then we compare the SPICE04 noise estimates for the two 700-m array segments and the relevant model output. [Work sponsored by ONR.]

Vertical Directionality of Midfrequency Surface Noise in Downward-Refracting Environments

The vertical directionality of ambient noise due to surface agitation for frequencies between 2 and 5 kHz propagated to a subsurface receiver has a characteristic shape, knowledge of which may enhance shallow-water operations. In general, the noise level is highest at upward-looking angles and attenuated at downward-looking angles depending on the nature of the bottom. In environments with a negative profile gradient, the noise level is also greatly reduced in a low-angle shadow zone or "notch" at angles around horizontal. This paper reviews the character of vertical noise directionality by examining two measured data sets and considering the underlying physical mechanisms that drive the form of the distribution. A discussion of the implications of vertical noise directionality for design and operation of receiving sonar systems is presented. In particular, the effect of mainlobe beamwidth and sidelobe suppression are considered along with the directionality of the noise field. Finally, an overview of the derivation of a vertical noise model based on the integrated mode method of propagation prediction is followed by model reproduction of measurements.

Vertical directionality of midfrequency surface noise in downward-refracting environments

The vertical directionality of ambient noise due to surface agitation in the midfrequency range (0.5 to 10 kHz) for a subsurface receiver has a characteristic shape, knowledge of which may enhance shallow-water operations. In general, the noise level is highest at upward-looking angles and attenuated at downward-looking angles depending on the nature of the bottom. In environments with a negative profile gradient, the noise level is also greatly reduced in a low-angle shadow zone or ‘‘notch’’ at angles around 0 deg. This paper reviews the character of vertical noise directionality by examining a small number of measured data sets and considering the underlying physical mechanisms that drive the form of the distribution. A discussion of the implications for receiving sonar systems is followed by model reproduction of the measured data sets.

Geoacoustic parameters inversion by ambient noise in South China Sea experiment of ASIAEX

Knowing bottom geoacoustic parameters is of great importance for using sonar systems effectively in shallow water. This study is based upon the vertical line array (VLA) in South China Sea experiment of the Asian Seas International Acoustic Experiment (ASIAEX), dated from May 3, 2001 to May 16, 2001, in which the ambient noise was also measured in shallow water. Beamforming is used to calculate the vertical directionality of ambient noise, but limited by the array element spacing, the major analysis frequency range is 100 to 400 Hz. Comparison between forward modeling (normal mode code) and experimental results show the energy ratio of up-looking and down-looking beams, which can be referred to the energy reflection coefficient of the bottom-potentially a function of angle and frequency. From the reflection coefficient, critical parameters of the sediments such as sound speed, density, and attenuation coefficient can be obtained. Finally, this result is verified by on-site chirp sonar surveys during the experiment, and also by other inversion investigation in this area using other sound source.

Measured and predicted array response to the vertical directivity of ambient noise

A great deal has been published on ambient noise. Most of this has covered (a) omni directional levels, and (b) the vertical and horizontal directivity of shipping noise at low frequencies. There is some published material on the vertical directivity of wind generated noise at lower frequencies, but very little at higher frequencies. In order to study wind generated ambient noise at higher frequencies, a small planar array from QinetiQ Bincleaves has been used to make ambient noise measurements. As well as measurements, a model called CANARY has been written to predict ambient noise vertical directivity and array responses to this noise. This paper contains some comparisons between CANARY predictions and (a) previously measured array response to ambient noise vertical directivity at 4.5 kHz, and (b) analysis of the planar array measurements at 25 kHz. The paper shows the nature of the ambient noise vertical structure and that the CANARY predictions are in good agreement with the measurements.

Geoacoustic inversion of broad-band ambient noise data using undersampled and short aperture arrays

Ocean ambient noise is generated in many ways such as from winds, rain and shipping. A technique has recently been developed [Harrison and Simons, J. Acoust. Soc. Am. 112 (2002)] that uses the vertical directionality of ambient noise to determine seabed properties. The ratio of beams steered towards the surface to those steered towards the bottom produces the bottom reflection loss curve. This technique was applied to data in the 200–1500-Hz band using a 16-m array. Extending this to higher frequencies allows the array length to be substantially shortened and greatly reduces interference from shipping. However, this limits the low end of the frequency spectrum since reduced aperture increases beam widths and the up/down beam ratio no longer produces reflection loss. Similarly, for high frequencies, if hydrophone spacing is greater than half-wavelength, the beamformed output is aliased and again the up/down ratio produces erroneous results. In general, frequencies much below the array design will suffer from large beams and frequencies above from undersampling. In this paper, we describe techniques for obtaining seabed properties from ambient noise measured on short or undersampled arrays. Results will be presented from the KauaiEx (July 2003) and ElbaEx (October 2003) experiments.

Vertical directivity of ambient noise in the presence of internal waves

Near surface sources of ambient noise, such as shipping and wind, excite only steeply traveling sound waves. After a sufficient range, the steepest are stripped away by loss mechanisms, such as bottom interaction. Consequently, the vertical directivity of surface generated noise is expected to be peaked about two (up- and down-going) angles with very little contribution near the horizontal. This expectation is not always met in measurements. A possible explanation, for the noise at the horizontal, is the random scattering of the up- and down-going waves by volume inhomogeneities caused by internal waves. The purpose of this paper is to demonstrate the validity of this hypothesis. The coupled power theory of Dozier and Tappert [J. Acoust. Soc. Am. 63, 353–365 (1978)], in an updated form, is applied to the calculation of noise due to distant near surface sources. The vertical directivity of the resultant field is shown to demonstrate that internal wave scattering can produce noise at the horizontal. Rough surface scattering is expected to have a similar effect.

Analysis on vertical directivity of shallow-water ambient noise in South China Sea

Beamforming is used to analyze the vertical directivity of the low-frequency ambient noise in shallow water. The data were measured by the vertical line array (VLA) of the Asian Seas International Acoustics Experiment (ASIAEX), from 3 May to 16 May 2001, and three frequencies, 200, 400, and 800 Hz, were studied. The results of the calculation show that the sound source of 200 and 400 Hz is from the horizontal direction and contributed by distant shipping, and the source of 800 Hz is mainly from high grazing angle that generated from surface noise. The well-known noise notch was also found in the study; the low-angle energy reflections from surface and bottom are the function of frequency, sound-speed profile, bathymetry, and the noise distribution. The time fluctuation of the noise notch is shown and correlated with oceanographic changes. The influences of internal wave and pass typhoon Cimaron during the experiment are significant too on vertical directivity of ambient noise.

Measurement of vertical noise directionality with a mixed-polarity vertical array

A novel array was designed and tested in the eastern Pacific Ocean for the measurement of the vertical directionality of ambient noise. The results of the test, conducted in 1983, illustrate the existence of a 5-dB "noise notch" for arrival angles associated only with submerged acoustic sources. A measurement system based on improvements to the design and processing is described.

Geoacoustic inversion of ambient noise

The vertical directionality of ambient noise is strongly influenced by seabed reflections. Therefore, geoacoustic parameters can be inferred by inversion of the noise. In this approach, using vertical array measurements (16 m aperture), the reflection loss is found directly by comparing the upward with the downward going noise. Theory suggests that this simple ratio is, in fact, the power reflection coefficient (a function of angle and frequency). A layer model and a search is required to find geoacoustic parameters, but no such model or search is required for reflection loss alone. Experimental data have been gathered at 11 sites, 10 in the Mediterranean, and 1 on the New Jersey Shelf during BOUNDARY2001. Site to site variations are discussed, and comparisons are made with simple layer models. Usually the vertical array is bottom-moored, but at two of the Mediterranean sites the array was allowed to drift over a few miles so that bottom properties could be surveyed. Bottom variations could indeed be seen. Simulations using the parabolic equation to mimic spatial variations of bottom properties confirm that the bottom loss inferred from this type of measurement is indeed a local one rather than a regional average.

Geoacoustic inversion of ambient noise: a simple method.

The vertical directionality of ambient noise is strongly influenced by seabed reflections. Therefore, potentially, geoacoustic parameters can be inferred by inversion of the noise. In this approach, using vertical array measurements, the reflection loss is found directly by comparing the upward- with the downward-going noise. Theory suggests that this simple ratio is, in fact, the power reflection coefficient-potentially a function of angle and frequency. Modeling and parameter searching are minimized, and the method does not require a detailed knowledge of the noise source distribution. The approach can handle stratified environments and is believed to tolerate range dependence. Experimental data from five sites, four in the Mediterranean, one on the New Jersey Shelf, are described. Most of the Mediterranean sites had temporally varying noise directionality, yet yielded the same reflection properties, as one would hope. One site was visited in conditions of very low surface noise. This paper concentrates on an experimental demonstration of the feasibility of the method and data quality issues rather than automatic search techniques for geoacoustic parameters.

Modeling the environmental influence on the vertical directionality of ambient noise in shallow water

The vertical directionality of acoustic ambient noise has been a subject of much interest in the past. It is a well-defined physical quantity that can be measured experimentally with a vertical array. It possesses certain deterministic features that can be modeled theoretically with environmental acoustic and source data. Ambient noise in shallow waters, including its vertical directionality, is not very well known and is also difficult to model/predict. This is because the acoustic environment varies with time and is location dependent. Thus arises the question, how does the vertical directionality of the ambient noise depend on the acoustic environments (found in typical coastal waters)? Due to the shallow water depth, it is noted that sound (noise) propagation can be significantly influenced by the bottom. The degree of bottom interaction will depend on the sound-speed profile in the water column: whether it is downward refractive or not. Bottom attenuation will in turn determine how far the sound will propagate in the water column. Using a modal representation, a closed-form expression is obtained which can be used to interpret and predict the distant noise vertical directionality as a function of the environmental acoustic parameters. The nearby (overhead) noise is separately modeled and is found less sensitive to environmental changes as the propagation distance is short. As the deterministic features of the noise vertical directionality is controlled by the sound propagation in the channel, it could be used as an acoustic indicator of the acoustic environment in the area. Time variations of the noise directionality will also be discussed.