Ambient Noise Spectrum Research Articles

A Passive-Source Ocean-Bottom Seismic Experiment in the South China Sea: Evaluations of Data Quality and Instrument Performance

Abstract This study presents a passive-source ocean-bottom seismograph (OBS) experiment conducted in the southwest subbasin of the South China Sea (SCS) aimed at exploring the geodynamic processes shaping this short-lived oceanic basin. The successful deployment and recovery of 24 passive-source OBS units, including 14 I-7C types and 10 Pankun OBS units, resulted in the acquisition of one of the most extensive passive-source seismic data sets ever recorded in the SCS. We provide comprehensive details of the experiment, with a focus on evaluating the data quality and performance of the Pankun OBS units. This evaluation includes assessments of horizontal orientation determination, leveling system effectiveness, timing accuracy, and ambient noise spectrum. In addition, we compared the waveforms recorded by the Pankun OBS units with those from land stations and I-7C units, as well as the noise spectra between Pankun and global OBSs using the same seismometer. Although these comparisons suggest that the Pankun OBS achieves satisfactory performance, we identified a few inadequacies with this new instrument, such as issues with the differential pressure gauge, clock shifts, and sensor package calibration.

Evolution of Sliding Along a Heterogenous Fault. A Large-Scale Laboratory Experiment.

A laboratory setup was constructed in IDG RAS to investigate the process of shearing the contact of rock blocks of one-meter scale. It was used to investigate deformation processes in a fault with a heterogenous structure of the sliding interface, which contained strong contact patches – analogs of the “asperity” in the model of Hiroo Kanamori. It is shown that when a large slip occurs, the rupture, that starts in the zone of maximal deficit of interblock displacement, cuts the segments of the fault with lower effective strength, the latter being decreased in previous deformation events. Those previous events may be “slow” slips with low seismic efficiency. In nature the events that “prepare” the fault interface for a large slip may be smaller earthquakes – foreshocks, or they can be either low frequency earthquakes or slow slip events, both can hardly be detected in seismic records. Thereupon a promising diagnostic indication is the shift of the spectrum of ambient seismic noise to lower frequencies caused by the decrease of fault stiffness.

Boothless audiometry: Ambient noise considerations

Ambient noise in the test environment will impact signal detection during hearing threshold measurements due to psychoacoustic masking effects. Technical standards specify the maximum permissible ambient noise levels (MPANLs) for use during audiometric testing. MPANLs are dependent on several factors, including transducer characteristics (supra-aural, circumaural, type of ear cushions or earphone enclosures, and insert earphones), the nature of the hearing test being performed (air conduction vs bone conduction and threshold test vs screen at a suprathreshold level), and measurement instrumentation. The nature of the ambient noise (spectrum and constant vs variable) at the test site must be determined and continually accounted for during the boothless hearing test procedure. Ambient noise monitoring procedures are reviewed and examples of ambient noise characteristics in real-world settings, where hearing testing might be performed outside of a sound-treated environment, are provided. Practical considerations are presented, including examples of available tools for ambient noise monitoring, selection of test locations, and transducer attenuation. These are discussed in the context of calculating MPANLs and how best to ensure that ambient noise levels are not negatively impacting the validity of hearing thresholds.

Improved characterization of ambient sound levels in a coastal environment via use of an unmanned Wave Glider

In this project, we use an unmanned Liquid Robotics SV3 wave glider as a tool for basic research studying ambient acoustics in shallower, coastal waters off the coast of East-Central Florida. The primary objective is to characterize sound levels in varying conditions via collection of high-resolution environmental data. The SV3 is used to systematically characterize the acoustic environment in coastal waters, while simultaneously measuring oceanographic parameters such as wind speed, wind direction, and wave height. Quantifying the influence of these fundamental oceanographic influences on shallow water acoustics will provide improved characterization of surface noise and comparison with traditional ambient noise spectra. Ambient noise spectra will be presented for various sea states and wind scenarios, as measured at 25 foot depth using a towfish attached to the SV3. Some of these measurements are also compared to fixed station bottom recordings. Finally, this effort also includes characterization of unique sources of biological sounds recorded by the SV3, including not previously documented loud fish chorusing in this region.

Performance of a Bayes factor (BF) based broadband active acoustic system

We explore the performance of a Bayes Factor (BF) active acoustic inference approach under various refraction and rough surface scattering regimes that typify diverse ocean waveguides. The BF processor, a time varying quadratic form over beam-angle observations, enables various user risk minimization strategies. The nature of the BF quadratic form is shaped by the expected ambient noise, reverberation and target angle-delay spectra. The structure of the BF processor provides intuition regarding the role of prior environmental and bathymetric information on detection in shallow water ocean waveguides with rough boundary scattering. The distribution of the BF under the composite null hypothesis when target and reverberation are well approximated as uncorrelated is shown to be a superposition of heteroskedastic gamma densities with the degree of freedom being the dimension of the target hypothesis scattering sub space. The distribution of the BF under the composite alternative is a superposition of heteroskedastic scaled non-central chi-square distributions. These distributions do not admit simple closed forms but can be well approximated. We consider a number of refractive and scattering regimes and relate the probability of detection as a function of probability of false alarm under conventional fixed threshold and cost rules. [This work supported by the Office of Naval Research.]

Improved characterization of ambient sound levels in a coastal environment via use of an unmanned wave glider

In this project, we use an unmanned Liquid Robotics SV3 wave glider as a tool for basic research studying ambient acoustics in shallower, coastal waters off the coast of East-Central Florida. The primary objective is to characterize sound levels in varying conditions via collection of high-resolution environmental data. The SV3 is used to systematically characterize the acoustic environment in coastal waters, while simultaneously measuring oceanographic parameters such as wind speed, wind direction, and wave height. Quantifying the influence of these fundamental oceanographic influences on shallow water acoustics will provide improved characterization of surface noise and comparison with traditional ambient noise spectra. Ambient noise spectra will be presented for various sea states and wind scenarios, as measured at 25 foot depth using a towfish attached to the SV3. Some of these measurements are also compared to fixed station bottom recordings. Finally, this effort also includes characterization of unique sources of biological sounds recorded by the SV3, including not previously documented loud fish chorusing in this region.

Analysis of Ambient Noise Spectrum Level Correlation Characteristics in the China Sea

The ambient noise field in the ocean is formed by a mixture of many source types, such as seismic noise, marine noise, shipping noise, and wind-generated noise. The correlation matrices of the ambient noise spectrum levels offer a promising approach for identifying and isolating the active source mechanisms in ambient noise datasets. This study attempts to use frequency correlation matrices to examine inter-frequency relationships and identify frequency bands that are dominated by specific sources present in samples of ambient noise. Deep-ocean ambient noise recorded by hydroacoustic monitoring systems in the East China Sea and South China Sea was used to analyze the frequency correlation characteristics over a broadband frequency range, with a focus on the contribution of wind-related and shipping-related source mechanisms to the overall ambient noise field at various frequencies. The primary sources observed in both the South China Sea and East China Sea were shipping sources and wind sources. During the observation window, wind was responsible for a significant amount of the ambient noise energy above 477 Hz in the East China Sea, while the corresponding frequency threshold was shifted up to 575 Hz in the South China Sea. Another interesting feature was also observed in the East China Sea: a higher degree of correlation was found between the levels at the wind noise dominated frequencies and mixture noise (mainly consisting of wind noise and shipping noise) dominated frequencies at the upper receiver than at the lower receiver. Data analysis results show that the cause of this effect is that shipping activity contributes more to the soundscape at the lower receiver than at the upper receiver.

Seismic Noise in Central Alaska and Influences From Rivers, Wind, and Sedimentary Basins

AbstractAmbient noise is useful for characterizing frequency‐dependent noise levels and for assessing data quality for seismic stations. We use 4 years of ambient noise spectra from 16 stations in central Alaska to examine environmental and structural influences on seismic stations. The region contains a major river (Tanana River) that is ice covered for half the year and is underlain by a sedimentary basin (Nenana basin) that strongly influences the seismic wavefield. Nenana basin amplifies ambient seismic noise by 12–16 dB at 0.1–0.7 Hz and 17–30 dB at 0.7–3 Hz. A meteorological station and river gauge at Nenana provide environmental data for comparison with seismic stations. During the summer, the Tanana River produces noise levels elevated by 30–40 dB at frequencies near 10 Hz, as recorded by all five stations within 100 m of the main river channel. The Tanana River sediment is predominantly silt and sand in this region; therefore, we attribute the 10‐Hz river signal to turbulence within the water and to unsteady shearing on the river bottom. The influence of wind is apparent on seismic noise at low (<0.05 Hz) frequencies, due to atmospheric‐induced tilting, and at high (>2.0 Hz) frequencies, due to unsteady shearing and turbulence near the ground. Our empirical findings motivate future studies, such as how flow from air or water couples to the ground and how deep sedimentary basins influence the ambient noise wavefield. Our results have implications for seismic site selection, environmental monitoring, and detection and characterization of earthquakes.

Separable Estimation of Ambient Noise Spectrum in Synchrophasor Measurements in the Presence of Forced Oscillations

The ambient noise in synchrophasor measurements carries core dynamic signatures of a power system and its spectrum can strongly reflect the dynamic properties and operating conditions of the system and its loads. Typically, the ambient noise spectrum is estimated from a window of measurements captured during steady state operating conditions of the system. The presence of any Forced Oscillation (FO) in the measurement window severely biases the estimated ambient noise spectrum. In this article, an algorithm is proposed for separable estimation of the ambient noise spectrum in the presence of periodic FOs in synchrophasor measurements. The algorithm utilizes the Thomson's multitaper spectral estimation and harmonic analysis techniques and is capable of effectively reducing the bias in estimating the ambient noise spectrum due to the presence of FOs in the measurements. The performance of the estimator is analyzed theoretically and the theoretical results are verified experimentally using simulation studies. Application of the algorithm to field measured data illustrates that the algorithm is capable of effectively reducing the bias introduced by FOs in estimating the ambient noise spectrum from the synchrophasor measurements containing both ambient noise and FOs.

Detection of Periodic Forced Oscillations in Power Systems Using Multitaper Approach

An algorithm based on Thomson's multitaper spectral estimation and harmonic analysis techniques is proposed for the detection and frequency estimation of periodic forced oscillations in a power system. Unlike traditional periodogram-based techniques, the method does not rely on true ambient noise spectrum of the system, and can operate continuously in an online environment without requiring any system knowledge except synchrophasor measurements. It compares a test statistic derived from only measurements against a threshold, and the threshold is established from general expressions of the test statistic's distribution and the probability of false alarm. Performance of the detector is explicitly expressed using probability of detection expression, and evaluated using simulation studies. Results upon application of this algorithm to field measured synchrophasor data suggest usability of the technique for forced oscillation detection in practical monitoring of the power system.

Estimating structure and geoacoustic parameters of sub-bottom by using spatial characteristics of ocean ambient noise in shallow water

The field of ocean ambient noise contains numerous information about the water column, especially the information about the sub-bottom. The geoacoustics parameters of sub-bottom are very important factors influencing the spatial characteristics of ocean ambient noise field. For different layered structures of the sub-bottom, the bottom-loss shows different critical angles according to sound speed of each layer, while the structure of interference fringe is dependent on the thickness of the sediment. Flux theory of ocean ambient noise proposed by Harrison is used in this paper. Using this theory, the curve of bottom-loss can be extracted by computing the ratio between the energy of the upward wave and the downward wave. From the ideal reflection coefficient, the influence of sound speed, density and attenuation coefficient on reflection coefficient are discussed in the situation of the sub-bottom of acoustic half space, while the reflection coefficient of 1 layer of sediment is simplified. Initially, the reflection coefficient is the sum of sound waves reflect from the sub-bottom transmitted from the same source at the same angle. Only the first two terms are reserved, so that the mechanism of the interference fringe can be easily discussed. The structure of interference fringe can be explained which is affected by the thickness of the sediment. The curve of bottom-loss oscillates periodically with the increase of the thickness of the sub-bottom. Also by the reciprocity principle, the interference fringe of the reflection coefficient can be explained by considering the sound transmitted from two point sources at the surface of the sea. In this way the same result can be obtained as that from the method of simplification. The result of the experiment in China Yellow Sea shows that the information about the reflection coefficient of the sub-bottom can be extracted by the vertical azimuth spectrum of ocean ambient noise. In this way, the critical angle can be obtained, so that the sound speed of the sub-bottom can be estimated by using Snell law. The structure of the interference fringe is also contained in the bottom-loss curve estimated by ocean ambient noise. Therefore the layered structure, sound speed and the thickness of the layer of the sub-bottom can be estimated.

Insights into sediment acoustics from analytic solutions of forward problems

Closed-form analytic solutions of wave equations are available for a rather limited subset of geoacoustic models of practical interest. Apart from elucidating the challenging underlying physics of wave propagation, the analytic solutions prove to be useful in sediment acoustics when these predict distinct, identifiable features of measured acoustic fields. Theoretical predictions can be used then to identify or constrain the type of sediment stratification, lead to quick, low-parameter inversions, guide application of statistical inversion methods, and help with interpretation of their results. In this paper, application of analytic solutions of forward problems to acoustic remote sensing of marine sediments will be illustrated by results obtained in a few experiments, which involve observations of slow interface waves near the seafloor and/or resonance peaks in either bottom-reflected energy or in the power spectrum of ambient noise. Coupling between shear and compressional waves in the stratified ocean bottom plays a key role in both examples. [Work supported, in part, by ONR.]

Measurements of underwater noise north of Spitsbergen using deep-sea recorders

This paper presents results from year-long passive acoustic recordings in the seasonally ice-covered ocean north of Spitsbergen (Svalbard archipelago) from July 2016 to June 2017. Two moorings were deployed by FFI, each with an AMAR Ultra Deep acoustic recorder (JASCO Applied Sciences) equipped with a single hydrophone. The moorings were deployed and retrieved during open-water conditions and remained during periods of partially to near fully ice-covered conditions. We present results from analysis of the acoustic data for ambient noise spectra and statistics, and discuss characteristics of the spectra in relation to environmental factors including ice cover, wind and ocean waves. Seasonal noise spectra are compared with historic measurements from the eastern Arctic. Components of the sound field including transients due to ice, marine mammals, and anthropogenic noise will also be discussed.

The persistent signature of tropical cyclones in ambient seismic noise

The spectrum of ambient seismic noise shows strong signals associated with tropical cyclones, yet a detailed understanding of these signals and the relationship between them and the storms is currently lacking. Through the analysis of more than a decade of seismic data recorded at several stations located in and adjacent to the northwest Pacific Ocean, here we show that there is a persistent and frequency-dependent signature of tropical cyclones in ambient seismic noise that depends on characteristics of the storm and on the detailed location of the station relative to the storm. An adaptive statistical model shows that the spectral amplitude of ambient seismic noise, and notably of the short-period secondary microseisms, has a strong relationship with tropical cyclone intensity and can be employed to extract information on the tropical cyclones.

Vector Ocean Ambient Noise Spectrum Simulation Based on Parabolic Equation Model in Shallow Water

Ocean ambient noise spectrum is one of the most important characteristics of ambient noise. An ocean vector ambient noise field model was built up based on parabolic equation in this paper. Then the spectra of sound pressure, horizontal particle velocity and vertical particle velocity were calculated applying the model considering noise sources well distributed on the surface with typical summer sound speed profile in South China Sea. The simulation results showed that spectra of sound pressure, horizontal particle velocity and vertical particle velocity were obviously not varied with depth. Then, the simulated results were compared with the experiment results at the receiving depth of a trail in South China Sea in July 2012. Compared with the experimental results, the simulation results are consistent well with the experimental one of sound pressure and horizontal particle velocity in the trend. But the simulation values at low frequency band below 500[Formula: see text]Hz, are not consistent with the experimental one very well, in the band the simulation results are lower than the experimental by about 3–5[Formula: see text]dB. But the simulation result of vertical particle velocity was not consistent with the experimental one, illustrating that the precision of the model might not be enough in the vertical direction.

Macroscopic superpositions and gravimetry with quantum magnetomechanics.

Precision measurements of gravity can provide tests of fundamental physics and are of broad practical interest for metrology. We propose a scheme for absolute gravimetry using a quantum magnetomechanical system consisting of a magnetically trapped superconducting resonator whose motion is controlled and measured by a nearby RF-SQUID or flux qubit. By driving the mechanical massive resonator to be in a macroscopic superposition of two different heights our we predict that our interferometry protocol could, subject to systematic errors, achieve a gravimetric sensitivity of Δg/g ~ 2.2 × 10−10 Hz−1/2, with a spatial resolution of a few nanometres. This sensitivity and spatial resolution exceeds the precision of current state of the art atom-interferometric and corner-cube gravimeters by more than an order of magnitude, and unlike classical superconducting interferometers produces an absolute rather than relative measurement of gravity. In addition, our scheme takes measurements at ~10 kHz, a region where the ambient vibrational noise spectrum is heavily suppressed compared the ~10 Hz region relevant for current cold atom gravimeters.

Comparison of scattering mechanisms for excitation of axial modes by surface sources

The vertical spectrum of low-frequency ambient noise in deep water contains significant contributions from distant shipping. Distant noise is concentrated at angles around broadside that are associated with the low order modes. Near-surface sources cannot excite the low modes directly. Dashen and Munk suggest three mechanisms for transferring energy from high modes to low modes: internal wave (IW) scattering, downslope conversion, and excitation at high latitudes where the sound channel intersects the surface. Dashen and Munk conclude that slope conversion is the most likely due to the weakness of IW scattering and the lack of shipping traffic at high latitudes. Commenting on their IW analysis, Dashen and Munk note that they are not totally convinced by their theoretical results and admit that IW scattering may have a significant effect. This talk compares the IW scattering and downslope conversion mechanisms for transferring surface source energy to the low modes. IW scattering is modeled using transport theory [Colosi and Morozov, JASA 2009] and compared to parabolic equation and coupled mode simulations. Results show that IW scattering from a large number of ships can produce vertical spectra comparable to those obtained through downslope conversion. [Work supported by ONR.]

Wind-induced noise in a convective boundary layer

Wind-induced noise, as sensed by a screened microphone in the atmosphere, produces a highly varying noise floor. At low frequencies, this noise source may dominate the ambient noise spectrum and present a challenge for signal detection. The sensed noise is affected by several factors in the atmospheric convective boundary layer: the mean flow at the height of the microphone, turbulent eddy interaction with the windscreen, and pressure fluctuations within the turbulent flow. Recently, van den Berg (2006) compared measured wind noise to modeled spectra. In that study, several meteorological variables were inferred from fits of modeled wind noise spectra rather than from meteorological observations. If additional meteorological observations are collected, then it may be possible to conduct model validation. In addition to temperature, pressure, and wind velocity, the surface roughness length for momentum and the vertical temperature flux must be observed. Recent models of turbulent wind velocity, in the convective boundary layer, suggest that the planetary boundary layer height may also be an important variable for wind-induced noise. This study analyzes data collected in the Southwestern United States during a 2007 long-range sound propagation experiment. Wind noise data are compared against concurrent meteorological data and several wind-induced noise models.

Spatial variation of ocean ambient noise spectrum in offshore China

Ocean ambient noise always presents spatial difference because of the diversity of the noise source and the propagation condition. This paper analyzes the noise spectrum level (NL) of two stations measured in offshore China, and finds that the difference of the two NL is more than 10 dB with different variation for per octave. Through analyze the NL and the transmission loss level (TL) from the noise source to the measuring station, a correlation is found between the noise level difference (NLD) and transmission loss difference (TLD), with a correlation coefficient of 0.78 at the band of 50 Hz ~500 Hz. This is a strong proof to connect the spatial variation of NL with the varied propagation channel, and also suggests that at this band the ambient noise is closely related to the ship noise sources. In addition, the difference of the ship noise source level coincides with the number of vessels before and after the fishing moratorium period in China, which further explains the difference between the NL of two experimental stations are highly related to ships.

Global examination of the wind-dependence of very low frequency underwater ambient noise.

Ocean surface winds play a key role in underwater ambient noise generation. One particular frequency band of interest is the infrasonic or very low frequency (VLF) band from 1 to 20 Hz. In this spectral band, wind generated ocean surface waves interact non-linearly to produce acoustic waves, which couple into the seafloor to generate microseisms, as explained by the theory developed by Longuet-Higgins. This study examines long term data sets in the VLF portion of the ambient noise spectrum, collected by the hydroacoustic systems of the Comprehensive Nuclear-Test Ban Treaty Organization in the Atlantic, Pacific, and Indian Oceans. Three properties of the noise field were examined: (a) the behavior of the acoustic spectrum slope from 1 to 5 Hz, (b) correlation of noise levels and wind speeds, and (c) the autocorrelation behavior of both the noise field and the wind. Analysis results indicate the spectrum slope is site dependent, and for both correlation methods, a high correlation between wind and the noise field in the 1-5 Hz band.