Ambient Noise Sources Research Articles

Imaging near-surface heterogeneities by natural migration of backscattered surface waves

S U M M A R Y We present a migration method that does not require a velocity model to migrate backscattered surface waves to their projected locations on the surface. This migration method, denoted as natural migration, uses recorded Green’s functions along the surface instead of simulated Green’s functions. The key assumptions are that the scattering bodies are within the depth interrogated by the surface waves, and the Green’s functions are recorded with dense receiver sampling along the free surface. This natural migration takes into account all orders of multiples, mode conversions and non-linear effects of surface waves in the data. The natural imaging formulae are derived for both active source and ambient-noise data, and computer simulations show that natural migration can effectively image near-surface heterogeneities with typical ambient-noise sources and geophone distributions.

Theoretical model and simulation of ship underwater radiated noise

Ship underwater radiated noise is one of the most important ocean ambient noise sources, and building a reasonable model for the ship underwater radiated noise is helpful for understanding the physical mechanism and reducing research cost of ship underwater radiated noise. The quasi-periodic random sound pulse sequence signals act well in explaining the rhythm and the power spectrum variation of the ship underwater radiated noise, and reveal that there are not any real sinusoidal components in ship radiated noise signals, which come from the non-linear transformation of the signals, and the analysis of some representative experimental data of ship radiated noise also supports this idea. Based on this, the explosion-type cosine pulses are used as the units of quasi-periodic random sound pulse sequences. This model can generate the power spectrum with a peak, and the peak location can change with ship velocity or ship type. The power spectrum variation characteristics of quasi-periodic random sound pulse sequences consisting of the explosion-type cosine pulses are in good agreement with the measured ship underwater radiated noise data, which shows that this model is of important practical value.

Cross-correlation imaging of ambient noise sources

This research was supported by the Swiss National Supercomputing Center (CSCS) in the form of the GeoScale and CH1 projects, by the Swiss National Science Foundation (SNF) under grant 200021 149143 and by the Netherlands Organisation for Scientific Research (VIDI grant 864.11.008). Iber-Array data are a contribution of the Team Consolider-Ingenio 2010 TOPO-IBERIA (CSD2006-00041).

Feasibility of virtual source reflection seismology using interferometry for mineral exploration: A test study in the Lalor Lake volcanogenic massive sulphide mining area, Manitoba, Canada

ABSTRACTApproximately 300 hours of ambient noise data were recorded on a grid of receivers covering an area of 4 km2 over the Lalor Mine, Canada, to test the capability of seismic interferometry to image ore deposits in the crystalline rock environment. Underground mining activities create the main source of ambient noise in the area. Alongside the ambient noise survey, a larger three‐dimensional active‐source seismic survey was also acquired and used to evaluate the interferometry results. Power spectral density calculations show random ambient noise with a frequency range of 2 Hz–35 Hz. A beamforming analysis identified body waves arriving from the west–northwest (pointing towards the mine) and surface waves propagating from the northeast. The calculated virtual shot gathers retrieved by cross‐correlating ambient noise at all receivers were processed following both two‐dimensional and three‐dimensional approaches using a sequence similar to the one applied to the active‐source three‐dimensional data. The dip‐moveout stacked section reveals a number of events similar to those observed on the processed active seismic sections. In particular, the passive seismic interferometry method is capable to partly image shallowly dipping reflections but did not produce convincing images of steeply dipping reflections. Dip‐moveout stacked sections obtained with different cross‐correlation time windows indicate that the strength and number of reflections generally increase with longer noise records. However, a few reflections at depth show reduced coherency with longer noise time windows. The passive seismic interferometry results over the Lalor mining area are encouraging, but image quality of the passive survey is lower than the acquired active three‐dimensional survey at the area. Future ambient noise surveys with longer offsets, shorter receiver spacing, and wider azimuth distribution are needed in crystalline rock environment to address the potential of the method for mineral exploration.

Effects of seasonal changes in ambient noise sources on monitoring temporal variations in crustal properties

Continuous data recorded at 39 broadband stations near the Longmen Shan Fault operated by the China Earthquake Administration from 1 January 2008 to 30 September 2010 are used to study temporal variability in direct surface wave arrivals extracted from ambient noise. We use a cross-correlation technique to compute Empirical green functions (EGFs) for all available station pairs at the frequency range of 0.1 to 0.5Hz. Delay times are measured by cross-correlating reference empirical green functions and moving 60-day stacks of EGFs. By comparing the temporal changes with and without the correction for seasonal variations, our results show that for some station pairs temporal variations were strongly affected by the seasonal variation. After correction for seasonal variations, we measure a 0.5-% maximum velocity drop after the 2008 Ms8.0 earthquake in Sichuan, China. We find that the Sichuan Basin exhibited a larger relative velocity drop than the Tibetan plateau area. Our results suggest that correction for seasonal variation is an important procedure for monitoring temporal variations in crustal properties using the direct arrival surface waves extracted from ambient noise.

Phase-sensitive noise suppression in a photoacoustic sensor based on acoustic circular membrane modes

A photoacoustic (PA) sensor based on higher order acoustic modes is demonstrated. The PA sensor is designed to enhance the gas-detection performance and simultaneously suppress ambient noise sources (e.g. flow noise, electrical noise and external acoustic noise). Two microphones are used and positioned such that the PA signals are ($\pi$) out of phase. Ambient acoustic noise are approximately in the same phase and will be subtracted and thus improve the SNR. In addition, by placing the gas in- and outlets so that the gas flows through the node of the first higher order membrane mode the coupling of flow noise is approximately 20 dB lower compared with flow through the fundamental mode at 5 L/min. The noise reduction and thus the increase in sensitivity is demonstrated by measuring vibrational lines of methanol and methane using a broadband interband cascade laser emitting radiation at 3.38 $\mu$m. A signal-to-noise improvement of 20 (26 dB) using higher order modes are demonstrated compared with the fundamental mode. The maximum normalized noise equivalent absorption coefficient is 1.1 $\times 10^{-6}$ W cm$^{-1}$ Hz$^{1/2}$ for a flow of 5 L/min. It is anticipated that the noise cancelation strategy may find use in many practical industrial and environmental PA sensors, where ambient noise sources plays a crucial role for the absolute sensitive.

Comparison of managed care and wild walrus source characteristics

Male Pacific walruses perform acoustic displays while in rut. The purpose of these displays is unknown but are hypothesized to be for territory defense or mate advertisement. Understanding source characteristics will allow the estimation of perceptibility by conspecifics. The displays occur in the Bering Sea in late winter where direct human observation is difficult. Working with an animal in managed care provided the ability to make direct observations of a male producing breeding vocalizations and the direct calculation of source level. Source characteristics from recordings of managed care and wild walruses were analyzed. The mean peak source level of the impulsive knocks produced by the managed care male was 183 dB (re: 1 μPa). The mean peak source level from the wild recordings was 177 dB (re: 1 μPa). For both wild and managed care vocalizations, a significant relationship between ambient noise level and source level was identified. An increase of approximately 5 dB in source level was found for an i...

Impulsive sound localization using time-domain beamformer

This paper presents a beamforming technique for locating impulsive sound source in low signal to noise ratio (SNR). Impulsive sound generates a high peak-pressure for a short duration in time-domain. However, the measured signal of the sensor are always embedded in the interference noise sources and measurement noise, therefore, it is difficult to estimate the direction of arrival of impulse sound in low SNR environment. In contrast to a frequency-domain beamformer, it has been reported that a time-domain beamformer can be better suited for transient signals. We proposes peak and crest factor as alternative directional estimators to enhance the performance of a time-domain beamformer in view of the fact that impulsive sound has high peak sound pressure as well as short duration time compared to the ambient noise sources. The performance of three directional estimators, the peak, RMS, and crest factor of output values, are investigated and compared with the incoherent measurement noise embedded in multiple microphone signals. The proposed formula is verified via experiments in an anechoic chamber using a uniform linear array, and the results show that the crest factor estimation of beamformer output determines the direction in a low SNR condition in which interference sources are dominants.

Simulation of GPR passive interferometry using cross-correlation for LNAPL model monitoring application

Passive interferometry technology is based on the relation between the reflection and the transmission responses of the subsurface. The transmission response can be received at surface in the presence of the ambient noise source in the subsurface with the cross-correlation (CC) or multidimensional deconvolution methods. We investigate the feasibility of electromagnetic (EM) wave passive interferometry with CC method. We design a 2-D finite-difference time domain (FDTD) algorithm to simulate the long-duration ground penetrating radar (GPR) measurements with random distribution of passive EM sources. The noise sources have random duration time, waveform and spatial distribution. We test the FDTD GPR passive interferometry code with above source characteristics and apply the method to light non-aqueous phase liquid (LNAPL) monitoring. Based on the model simulation data, by using common midpoint velocity analysis and normal move out correction to process the interferometry retrieve record, we can accurately obtain the dynamic changing characteristics of the target's permittivity. The LNAPL dynamic leakage model can be imaged as well. The synthetic results demonstrate that the GPR passive interferometry is feasible in subsurface LNAPL monitoring. Our work provides a foundation for a passive interferometry field application using GPR.

A note on elastic noise source localization

The problem of reconstructing the spatial support of ambient noise sources from elastic wavefield boundary measurements using cross-correlation techniques is dealt with. It is demystified that the cross-correlation-based standard source localization functional in elastic media does not provide optimal refocusing due to different pressure and shear wave speeds. Then, a weighted functional is proposed to rectify the coupling artifacts. A numerical experiment is presented to substantiate the appositeness of the proposed functional.

Semiannual velocity variations around the 2008 Mw 7.9 Wenchuan Earthquake fault zone revealed by ambient noise and ACROSS active source data

We continuously monitor the long-term seismic velocity variation of one of the major ruptured faults of the devastating 2008 Mw 7.9 Wenchuan earthquake in China from July 2009 to January 2012, jointly using accurately controlled routinely operated signal system active source and seismic noise-based monitoring technique. Our measurements show that the temporal velocity change is not homogeneous and highly localized in the damaged fault zone and the adjacent areas. Velocity variations from the active and passive methods are quite consistent, which both are characterized by ±0.2 % seasonal variation, with peak and trough at winter and summer, respectively. The periodic velocity variation within fault zone exhibits remarkably positive correlation with barometric pressure with stress sensitivity in the order of 10−6 Pa−1, suggesting that the plausible mechanism might be the crack density variation of the shallow subsurface medium of the damaged fault zone in response to the cyclic barometric pressure loading.

Offshore Marine Ambient Noise Measurement Study Based on Laser Interferometer Hydrophone

Sound pressure is an important parameter of marine background noise monitoring. A laser interferometric hydrophone is designed based on Michelson interferometry. By tracking compensation method, immunity from interference of the hydrophone is improved. Error analysis shows that the vibrating membrane is the main source of the sound pressure error, which can be eliminated to a certain extent by a vibrating membrane materials selection. Offshore data show that hydrophone pressure measurement of high precision, and with a good frequency response. By comparison with the theoretical model to some extent also verified the accuracy of data. In coastal waters, wind noise and ship radiate noise is the main noise source of marine ambient noise. Here, marine ambient noise spectrum of three wind speed were calculated, and with Kundsen curves were compared. Results show that measured data and Kundsen curve fitting is better. In addition, the measurement of vessel radiation noise is mainly concentrated in frequency range of 350~500Hz.

Mode analysis of distant shipping noise in the SPICE experiment

A major source of ambient noise for frequencies below 100 Hz is due to distant shipping (Wenz, 1962). Shipping noise is trapped in the SOFAR channel and propagates long distances. Since this part of ambient noise excites the low normal modes, it decays with increasing depth. A modal analysis of shipping noise provides valuable insights about its depth dependence. SPICEX offered a unique opportunity to study the ambient noise in the Northeastern Pacific using two vertical line arrays an axial array and a deep array. A 40-hydrophone axial array spanned the SOFAR channel and recorded ambient noise at regular intervals over a year. The axial array data makes it possible to estimate the wavenumber-frequency spectra of ambient noise. We can estimate the mode powers associated with the angle-limited noise in the wavenumber-frequency spectra using a least squares fit. Given the estimated mode powers, depth-dependence of the shipping noise component is predictable. This talk compares the predicted noise levels as a function of depth with data measured by the deep SPICEX line array.

Towed body measurements of flow noise from a turbulent boundary layer under sea conditions.

Results from an underwater experiment under sea conditions on flow noise beneath a flat-plate turbulent boundary layer are presented. The measurements were performed with a towed body at towing speeds U=2.3,…,6.1 m/s and depths h=-150,…,-100 m. Flow noise is measured with a linear array of equally spaced hydrophones (Δx=70 mm) that is orientated in streamwise direction and embedded within a laterally attached flat plate. In order to separate flow noise from ocean ambient noise and other acoustical noise sources wavenumber-frequency filtering is applied. The (nondimensionalized) spectral power density of flow noise Φ(ω) ⟨U∞⟩/ (⟨δ(∗)⟩ (1/2ρ ⟨U∞⟩)(2)) is found to scale like (ω⟨δ(∗)⟩/⟨U∞⟩)(-4.3) in a wide frequency range at higher towing speeds. Here, ω, ⟨δ(∗)⟩, and ⟨U∞⟩ denote frequency, boundary layer displacement thickness, and potential flow velocity in the array region, respectively. Potential flow velocity is estimated from numerical simulations around a symmetrical, two-dimensional body with a semi-elliptical nose. Evidence is given that a χ(2)-(Tsallis) superstatistics provides a reasonable representation of the probability distribution function of flow noise at higher towing speeds.

Resolution enhancement from scattering in passive sensor imaging with cross correlations

It was shown in [Garnier et al., SIAM J. Imaging Sciences 2 (2009), 396] that it is possible to image reflectors by backpropagating cross correlations of signals generated by ambient noise sources and recorded at passive sensor arrays. The resolution of the image depends on the directional diversity of the noise signals relative to the locations of the sensor array and the reflector. When directional diversity is limited it is possible to enhance it by exploiting the scattering properties of the medium since scatterers will act as secondary noise sources. However, scattering increases the fluctuation level of the cross correlations and therefore tends to destabilize the image by reducing its signal-to-noise ratio. In this paper we study the trade-off in passive, correlation-based imaging between resolution enhancement and signal-to-noise ratio reduction that is due to scattering.

Shallow crustal structures of the Tehran basin in Iran resolved by ambient noise tomography

In this study, we present an application of the ambient noise tomography (ANT) to study the near-surface geological structures of the metropolitan Tehran/Iran region. Short-period fundamental mode Rayleigh wave Green's functions were estimated using cross-correlations of the vertical component of the ambient noise from 2009 October to 2011 May using a variety of seismic sensors, for example, accelerometers and seismometers, deployed in the Tehran area. Standard common low frequency processing procedures were applied to the cross-correlations, and shorter time-windows comprising 10-min segments were used in the processing step to enhance the time resolution of the signal in the frequency range of interest (1–10 s). Stacking was also conducted using the rms of the estimated empirical Green's functions. Our results demonstrate that ambient seismic noise tomography is a viable technique at periods of 1–10 s in length, even when different sensor types are present. Analysis of the empirical Green's functions indicates that the dominant sources of ambient seismic noise originated from the same origin, and no significant seasonal or spatial variations in the ambient noise sources were observed. Multiple-filter analysis was used to extract the group velocities from the estimated empirical Green's functions, which were then inverted to image the spatially varying dispersion at periods of lengths between 1 and 7 s using tomographic inversion of the traveltimes estimated for each frequency. The resulting group velocity maps show high correlations with known geological and tectonic features of the study region. In general, most of the Tehran basin, with certain exceptions, could be clearly resolved with low group velocities, whereas the mountain ranges were found to be correlated with high group velocities. In the Tehran basin, for 2 and 3 s periods, the low-velocity zone deepens towards the south–southwest, which reflects thicker sediments in the southern part of the basin than in the north. This feature has also been observed in other geological studies. The Vs models also show that bedrock depth varies between 400 and 1400 m from north to south within the Tehran basin. At longer periods main faults are associated with abrupt transitions between regions of high- to low-velocity anomalies. In general, our results indicate that ANT can be a flexible and effective approach for studying near-surface heterogeneity using short-period surface wave data.

Ground Motion in the Presence of Complex Topography: Earthquake and Ambient Noise Sources

To study the influence of topography on ground motion, eight seismic recorders were deployed for a period of one year over Poverty Ridge on the east side of the San Francisco Bay Area, California. This location is desirable because of its proximity to local earthquake sources and the significant topographic relief of the array (439 m). Topographic amplification is evaluated as a function offrequency using a variety of methods, including reference-site-based spectral ratios and single-station horizontal-to-vertical spectral ratios using both shear waves from earthquakes and ambient noise. Field observations are compared with the predicted ground motion from an accurate digital model of the topography and a 3D local velocity model. Amplification factors from the theoretical calculations are consistent with observa- tions. The fundamental resonance of the ridge is prominently observed in the spectra of data and synthetics; however, higher-frequency peaks are also seen primarily for sources in line with the major axis of the ridge, perhaps indicating higher resonant modes. Excitations of lateral ribs off of the main ridge are also seen at frequencies consistent with their dimensions. The favored directions of resonance are shown to be transverse to the major axes of the topographic features.

Nonlinear time series analysis of snapping shrimp sounds

Snapping shrimp produce sounds through cavitation bubbles generated by the rapid closing of their claws. These sounds are a primary source of ambient noise in sub-tropical, shallow water environments. Though seasonal and daily variations of snapping shrimp sound levels have been reported, comprehensive studies of short time series have been limited. We investigate the respective spectral characteristics from acoustic arrays recordings at various locations in Oahu, HI, over hourly, daily and weekly periods. Nonlinear time series analysis methods encompassing recurrence plots are used to investigate the acoustic time series together with bivariate and cross recurrence plot analysis with that of tidal and sunlight cycles. Signal entropy and determinism are investigated and transient acoustic ship sounds are located

Noise annoyance due to construction worksites.

Noise pollution can contribute to adverse health effects in humans. Noise annoyance and related problems, caused by noise emission during the progress of building construction, have become increasingly important. These problems can influence the exposed workers, as well as vicinity residents. The goal of the present study was to assess the noise annoyance due to noise from construction worksites among residents of Hamadan City (west of Iran). In this cross-sectional study, 20 construction worksites and 140 near vicinity residents were selected. The main sources of construction worksite noise were diesel power generators, cutting and welding processes, heavy machinery (such as trucks) and transport of materials. Ambient noise levels were measured using a calibrated sound level meter, at each home of the residents included in the study. A noise annoyance questionnaire was employed for annoyance assessment. The majority of subjects had very high annoyance (87.1%). The mean and SD of ambient levels were 74.57±7.12 dB (A) which exceeded the acceptable recommended level for residential areas. The most common problems among the participants were disturbance in sleep, difficulty in reading and distraction. Results showed significant relationship between noise annoyance and some factors including residing which floors had highest annoyance (F=13.22, P<0.001) and ambient noise (F=11.313, P<0.001). High levels of noise annoyance among near vicinity residents who are affected by construction activities. This means that construction activities and other noise related sources should be regarded as the major source of ambient noise leading to noise annoyance.

Acoustic detection and long-term monitoring of pygmy blue whales over the continental slope in southwest Australia

A 9-yr dataset of continuous sea noise recording made at the Cape Leeuwin station of the Comprehensive Nuclear-Test-Ban Treaty hydroacoustic network in 2002-2010 was processed to detect calls from pygmy blue whales and to analyze diurnal, seasonal, and interannual variations in their vocal activity. Because the conventional spectrogram correlation method for recognizing whale calls in sea noise resulted in a too high false detection rate, alternative algorithms were tested and the most robust one applied to the multi-year dataset. The detection method was based on multivariate classification using two spectrogram features of transients in sea noise and Fisher's linear discriminant, which provided a misclassification rate of approximately 1% for missed and false detections at moderate sensitivity settings. An analysis of the detection results revealed a consistent seasonal pattern in the whale presence and considerable interannual changes with a steady increase in the number of calls detected in 2002-2006. An apparent diurnal pattern of whales' vocal activity was also observed. The acoustic detection range for pygmy blue whales was estimated to vary from about 50 km to nearly 200 km from the Cape Leeuwin station, depending on the ambient noise level, source level, and azimuth to a vocalizing whale.