Depth dependence of noise resulting from ship traffic and wind

Under conditions of distantly generated noise, the noise level is found to decrease with depth in the mid-northeastern Pacific. These data show a decrease in noise level greater than 25 dB between critical depth and the ocean bottom. A result of this decrease is that locally wind-generated noise can be detected on near-bottom receivers for wind speeds less than 10 kn. It is shown that the noise level generated form local sources such as wind and nearby shipping is almost independent of receiver depth. The differences in spectra shape between the distant shipping noise and wind-generated noise and the low noise levels detected near the ocean bottom allow the measurement in the frequency band at 200-500 Hz of local wind noise level for wind speeds less than 10 kn     

Ship sources of ambient noise

The rapid increase in world shipping results in an increase in low-frequency ambient noise at an average rate of about 1/2 dB per year. During the past 10 years there has been a virtual revolution in the sizes and speeds of merchant ships, resulting in significant increases in the noise radiated by the average ship. This trend is continuing. In this paper, the trends in world merchant shipping will be presented, including important changes in propulsion plants as well as in numbers and sizes of ships. The need for radiated noise measurements of these new ship types will be stressed. Ambient noise is also dependent on the geographical distribution of shipping. The LRAPP-sponsored program to establish standard shipping distributions for the Northern Hemisphere will be discussed, and the reliability of current information will be assessed.     

Bubbles as sources of ambient noise

It has been shown that the main mechanism which produces the Knudsen region of the ambient noise spectrum is the free oscillations of bubbles. Some experimental results which seem to confirm these facts and to refute various alternative theories involving spray impacts and turbulent forcing of bubble oscillations are described. The results show that the mechanism which excites the bubbles is their formation at the surface; once a bubble has been formed and has radiated the excess energy resulting from its formation, it is more or less silent. It is possible for extremely violent conditions to re-excite bubbles by breaking them into smaller fractions, but it is not clear how important this process would be in the ocean. How the entrainment process imparts energy to the bubble is discussed     

Acoustic vector-sensor correlations in ambient noise

Most array-processing methods require knowledge of the correlation structure of the noise. While such information may sometimes be obtained from measurements made when no sources are present, this may not always be possible. Furthermore, measurements made in-situ can hardly be used to analyze system performance before deployment. The development of models of the correlation structure under various environmental assumptions is therefore very important. In this paper, we obtain integral and closed form expressions for the auto- and cross-correlations between the components of an acoustic vector sensor (AVS) for a wideband-noise field, under the following assumptions concerning its spatial distribution: 1) azimuthal independence; 2) azimuthal independence and elevational symmetry; and 3) spherical isotropy. We also derive expressions for the cross-covariances between all components of two spatially displaced AVSs in a narrowband-noise field under the same assumptions. These results can be used to determine the noise-covariance matrix of an array of acoustic vector sensors in ambient noise. We apply them to a uniform linear AVS array to asses its beamforming capabilities and localization accuracy, via the Cramer-Rao bound, in isotropic and anisotropic noise     

Matched field processing of deep-water ambient noise

Matched-field ambiguity surfaces produced by deep-water ambient-noise data are discussed and quantified in terms of power levels and correlation values. Two processors were implemented (Bartlett and minimum variance) using data at 35 and 95 Hz with similar but distinct vertical angular distributions. In general, the ambiguity surfaces have both diffuse and discrete components. The diffuse distribution extends across the sound channel with correlation values increasing with distance. The discrete sources have higher correlation values and are distributed in a convergence-zone sidelobe structure. Local wind conditions appear to affect the received power but not the correlation values or the processor output power     

船舶噪声对浅海海洋环境噪声空间相关的影响

通过对2001年亚洲海国际声学实验(ASIAEX)得到的海洋环境噪声数据进行处理,计算了噪声场空间相关系数、功率谱密度和垂直指向性,分别研究了受到远距离和近距离船舶辐射噪声影响的海洋环境噪声空间相关性.结果表明:船舶距离较远(10 km)时其辐射噪声会导致500 Hz以下的海洋环境噪声的空间相关性提高,而对500 Hz以上噪声的空间相关性没有影响;船舶距离较近(≤10 km)时,船舶辐射噪声会导致500 Hz以下的海洋环境噪声相关系数振荡变化,噪声空间相关性曲线在噪声信号声程差相差四分之一波长时出现第一个零点,对500 Hz以上的海洋环境噪声则会引起噪声能量升高,噪声的空间相关性也相对提高.基于现有理论,分别讨论了远距离船舶和近距离船舶产生的噪声场声学特性,对两种噪声场的海洋环境噪声空间相关性进行了合理的解释.     

Using ocean ambient noise for array self-localization and self-synchronization

Estimates of the travel times between the elements of a bottom hydrophone array can be extracted from the time-averaged ambient noise cross-correlation function (NCF). This is confirmed using 11-min-long data blocks of ambient noise recordings that were collected in May 1995 near the southern California coast at an average depth of 21 m in the 150-700 Hz frequency range. Coherent horizontal wavefronts emerging from the time derivative of the NCF are obtained across the array's aperture and are related to the direct arrival time of the time-domain Green's function (TDGF). These coherent wavefronts are used for array element self-localization (AESL) and array element self-synchronization (AESS). The estimated array element locations are used to beamform on a towed source.     

Measurements of ambient noise spectra in the south Norwegian Sea

Measurements have been made of ambient sea noise north of the Shetland Islands in the southern part of the Norwegian Sea. Shore-terminated, omnidirectional hydrophones, bottomed in 240 and 350 fathoms, were used. Twenty minute samples of single hydrophone outputs were recorded during daylight hours about every 3 days over a 4-year period from the summer of 1957 to 1961. Amplitude levels were measured at selected frequencies in the band 30 cps to 1 kc. Matching observations of wind and weather were coordinated with the noise data. The noise spectrum was found to be relatively flat compared with the spectrum measured in deep ocean areas of the western North Atlantic. Also, the pressure levels at frequencies above 100 cps are considerably higher. Seasonal changes were observed in the spectrum below 400 cps. The annual excursion in level from winter high to midsummer low was about 5 db at 50 cps and 3.5 db at 100 cps. Although no seasonal changes occurred in the upper spectrum, day-to-day variations in level at the higher frequencies generally followed patterns of change in local weather. There was good correlation between wind velocity and ambient noise at 800 and at 1000 cps. The character of sea-noise in the area seems to be shaped to two factors. First, a combination of thermal structure and a very gradual bottom slope result in high propagation losses. This in turn restricts the acoustic access at a bottomed hydrophone to a localized field. Second, except for a few summer calms, weather in the islands is severe, and strong winds and high sea states are sustained throughout the year.     

海洋环境噪声垂直分布测试和分析

采用船舷法对某海域海洋环境噪声垂直分布进行了测量.数据处理与分析结果表明,在6种接收深度下,当地的海面风生破碎波浪噪声对环境噪声有显著贡献.给出了所测海域环境噪声在0.1～20.0kHz频段的宽带声级和接收深度以及多种频率谱级与风速的对数之间的关系.1.0～4.0kHz频段的谱级与风速的对数呈良好的线性关系,且基本上不受接收深度的影响.     

An ambient noise model for the northeast Pacific Ocean basin

An ambient noise model for the Northeast Pacific Ocean Basin is presented. This model possesses the capability of synthesizing the noise field, with resolution in the vertical and horizontal finer than 1/spl deg/. Simulation results utilizing the synthesized field are shown to be in excellent agreement with measured horizontal directionality, vertical directionality, and depth dependence data for frequencies from 12.5 to 250 Hz. An important difference between this model and other models is the consideration of the SOFAR channel component, which is the dominant noise at these low frequencies. It is shown that only when this component of the noise is included can the simulation results be expected to agree with measured data.     

High-frequency ambient noise and its impact on underwater tracking ranges

A theoretical model for the vertical directionality and depth dependence of high frequency (8 to 50 kHz) ambient noise in the deep ocean is developed. The anisotropic noise field at a variety of depths and frequencies is evaluated and displayed. It was found that at high frequencies and deep depths, a bottom-mounted hydrophone receives the maximum noise energy from overhead rather than from the horizontal. This leads to the consideration of an oblate hydrophone receiving response pattern for underwater tracking ranges that would provide a constant signal-to-noise ratio (SNR) for an acoustic source located anywhere in a circular area centered above the hydrophone. Two of the desirable characteristics of this type of pattern are the increase in receiving range of a bottom-mounted sensor and the decrease of the dynamic range of signals that a signal processor must handle.     

星载微波散射计海面风场与海洋环境噪声的相关特性分析

根据海洋环境噪声机理及风关噪声已有的研究成果,提出利用星载微波散射计反演的海面风场数据进行海洋环境噪声分析,并对HY-2A和ASCAT数据与噪声谱级的相关性进行了对比分析。选取南海海域作为研究区,利用潜标测量系统获取的噪声数据和多源散射计风场数据开展了相关实验,并采用NCEP海面风场数据进行对比分析。结果表明,ASCAT数据与噪声的相关性优于HY-2A,散射计数据优于NCEP数据,散射计风场更适合海洋环境噪声的分析研究。该研究内容拓展了微波散射计风场数据的应用领域,并为海洋环境噪声研究提供了更好的技术手段。     

基于潜标测量的海洋环境噪声谱特性分析

利用海洋环境噪声测量潜标系统对南海典型海域开展了为期3个月的海洋环境噪声测量,16通道海洋环境噪声测量系统每小时测量两分钟噪声信号。数据处理结果表明,800~5 000Hz范围内,噪声谱与风速相关性最好,且风速越大相关性越好,噪声谱与风速的相关性好于与浪高的相关性。风关噪声谱级在海水中部基本不随接收深度发生变化,但由于测量水听器阵长度未能覆盖整个水深,因此未给出海面和海底处谱级变化规律。在400Hz以上的高频段整个风速范围内噪声谱级都随风速发生变化,且噪声谱级与对数风速具有很好的线性关系。     

Effects of sediment properties on surface-generated ambient noise in a shallow ocean

Surface-generalized ambient noise in a shallow ocean waveguide with a sediment layer possessing a specific class of density and sound speed distributions capable of describing a realistic seabed environment is considered in this analysis. This class of non-uniform sediment layer has the density and sound speed distributions varying with respect to depth as a generalized-exponential and an inverse-square function, respectively. The study invokes a formulation developed by Kuperman and Ingenito (Kuperman, W. A., Ingenito, F., 1980. Spatial correlation of surface-generated noise in a stratified ocean. J. Acoust. Soc. Am., 67, 1988-1996.) for surface noise generation, in conjunction with the analytical solutions for the Helmholtz equation corresponding to the sediment layer, to arrive at an analytical expression convenient for numerical implementation. The intensity and spatial correlation of the noise sound field are analyzed with respect to the variation of the system parameters, including frequency, sediment layer thickness, sound speed gradient, with emphasis on the effects of sediment properties on the ambient noise field. The results have demonstrated that the intensity of the noise field is relatively sensitive to the variation of the parameters, but the spatial correlation is affected to a less extent, suggesting that the energy distribution, rather than the spatial structure, of the noise field is more susceptible to the environmental variations.     

Ice-ocean acoustic energy transfer:: ambient noise in the ice-edge region

The ambient noise produced by large areas of ice floes, such as the Odden and the marginal ice zone, was recorded and analysed. A model based on the collision, compression and shearing of floes driven by ocean swell was constructed. This model allows the level and variance of the ambient noise to be predicted provided the conditions of the ice and the sea state are known. The frequency spectrum of the ambient noise in the range 100 Hz to 100 kHz is explained in terms of acoustic propagation and absorption by sea water. The probability distribution of ambient noise intensity, obtained from a statistical analysis of collision frequencies, is a form of K distribution and agrees with observations.     

Variation of underwater ambient noise observed at IORS station as a pilot study

The Ieodo Ocean Research Station(IORS) is an integrated meteorological and oceanographic observation base which was constructed on the Ieodo underwater rock located at a distance of about 150 km to the south-west of the Mara-do, the southernmost island in Korea. The underwater ambient noise level observed at the IORS was similar to the results of the shallow water surrounding the Korean Peninsula (Choi et al. 2003) and was higher than that of deep ocean (Wenz 1962). The wind dependence of ambient noise was dominant at frequencies of a few kHz. The surface current dependence of ambient noise showed good correlation with the ambient noise in the frequency of 10 kHz. Especially, the shrimp sound was estimated through investigations of waveform and spectrum and its main acoustic energy was about 40 dB larger than ambient noise level at 5 kHz.     

The Seafloor Borehole Array Seismic System (SEABASS) and VLF ambient noise

The Seafloor Borehole Array Seismic System (SEABASS) has been developed to measure the pressure and threedimensional particle velocity of the VLF sound field (2–50 Hz) below the seafloor in the deep ocean. The system consists of four three-component borehole seismometers (with an optional hydrophone). a borehole digitizing unit, and a seafloor control and recording package. The system can be deployed using a wireline re-entry capability from a conventional research vessel in Deep Sea Drilling Project (DSDP) and Ocean Drilling Project (ODP) boreholes. Data from below the seafloor are acquired either onboard the research vessel via coaxial tether or remotely on the seafloor in a self-contained package. If necessary the data module from the seafloor package can be released independently and recovered on the surface. This paper describes the engineering specifications of SEABASS, the tests that were carried out, and preliminary results from an actual deep sea deployment. VLF ambient noise levels beneath the seafloor acquired on the Low Frequency Acoustic-Seismic Experiment (LFASE) are within 20 dB of levels from previous seafloor borehole seismic experiments and from land borehole measurements. The ambient noise observed on LFASE decreases by up to 12 dB in the upper 100 m of the seafloor in a sedimentary environment.     

利用海底地震仪数据分析台风对海底环境噪音的影响

在海底布设的海底地震仪（OBS）能比较清晰地记录到海底的环境噪音,而台风可以直接或间接的产生在海底传播的弹性波,从而影响海底的环境噪音,并在较大程度上影响OBS的数据记录。本文通过分析台风对工作区的整个影响过程中OBS记录数据的振幅变化,再选择合适的滤波方式,首次发现台风产生的风浪及涌浪在短周期海底地震仪的记录数据上有良好的表现特征,指出了台风对海底环境噪音的另一种可能的影响方式,并由此得出：1）台风产生的风浪和涌浪对海底环境噪音的影响模式不同;2）风浪和涌浪所加强的海底环境噪音的范围和程度不同;3）短周期OBS可以比较清晰的记录涌浪信息,其周期主要是6—8 s,且能量稳定（简称“8秒现象”）。这三点结论为后期的海洋地震研究和海洋学其他研究提供经验与借鉴。     

Hotspots in ambient noise caused by ice-edge eddies in the Greenland and Barents Seas

Mesoscale eddies are frequently observed in the Greenland and Barents Seas' marginal ice zone (MIZ). The objective of this study was to investigate the hypothesis that acoustic hotspots along the ice-edge region are due to mesoscale eddy currents interacting with the broken-up ice floes in the MIZ. To test this hypothesis, ambient-noise case studies were carried out during the MIZEX 85-87 and SIZEX 89 field experiments. In each experiment, ice-edge eddies were localized visually from aircraft and by use of satellite remote-sensing data obtained in near real time. Sonobuoys were, thereafter, deployed by fix-wing aircraft and helicopters in selected eddy areas. Ambient-noise data, recorded over several hours by aircraft, were analyzed estimating averaged ambient-noise levels at four selected frequencies: 40, 100, 315, and 1000 Hz. The analysis showed high mean levels and large gradients in ambient noise near ice-edge eddies and during strong wind and wave forcing against the ice edge. The conclusion of this study is that mapping of the ice edge and localization of mesoscale ocean processes using remote sensing from space will provide important input to ambient-noise prediction in the MIZ.