Ambient Seismic Noise Analysis Research Papers (original) (raw)

Transmitting data with water as the medium is called underwater communication. Electromagnetic signals are better attenuated in water than sound. Hence it is the most reliable and versatile means of communication underwater. The acoustic signals will be affected by ocean driven noise during transmission. Ambient noise is the background noise which has to be subtracted from the acoustic signal for smooth transmission. Hence the recognition and reduction of ambient noise is the essential part of underwater signal transmission. Studies and research are being carried out extensively in this area. There are many works reported and the research is still in progress. This literature survey presents the work done on the research front of ambient noise affecting acoustic signals.

- by Michael Asten
- •
- Microtremor, Ambient Seismic Noise Analysis

Recording ambient noise at the surface is increasingly used for the assessment of site response and has become a fundamental task for seismic risk reduction in urbanized areas. Methods based on the measurement of seismic noise, which typically are fast, non invasive, and low cost, have become a very attractive option in microzonation studies. In the current work, we use the ambient noise recordings collected by single seismic stations and two‐dimensional arrays to determine the response of the near-surface soil at the two touristic cities of Hurghada and Safaga which are located at the Red Sea coast in Egypt. Based on the horizontal-to-vertical (H/V) spectral ratio Nakamura's technique, recordings of the single stations are analyzed to estimate the fundamental site frequency (f 0). Using the raw noise signals recorded by two arrays, we present a joint inversion scheme for surface wave dispersion curves. In particular, the Rayleigh wave dispersion curves and the H/V curves are combined in a joint inversion procedure. We present inversion procedure based on the computation of high-frequency correlation functions between arrays' stations. The calculation of Rayleigh wave phase velocities is based on the frequency-domain SPatial AutoCorrelation (SPAC) and frequency-wavenumber (f-k) methods. Constitutively, a tomographic inversion of the travel-times estimated for each frequency is performed, allowing the vertically varying 1-D shear wave velocity structure below the array to be retrieved. Following the relationship between resonance frequency (f 0) and thickness of sediments, the thickness of unconsolidated sediments at the investigated sites are determined.

- by abd el-aziz khairy abd el-aal
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- Ambient Seismic Noise Analysis, Hurghada

With the current technology revolution, underwater wireless sensor networks (UWSNs) find several applications such as disaster prevention, water quality monitoring, military surveillance and fish farming. Nevertheless, this kind of networks faces a number of challenges induced by the nature of the underwater environment and its influence on the network physical media. Therefore, the ultimate objective of this paper is to lay down the key aspects of the physical layer of the underwater sensor networks (UWSNs). It discusses issues related to the characteristics and challenges of the underwater communication channel, differences between terrestrial wireless sensor networks and UWSNs, and acoustic propagation models in underwater. The paper also surveys some of the underwater acoustic modems. This study is essential to better understand the challenges of designing UWSNs and alleviate their effects.

In this contribution, new relationship between the fundamental site frequency and the thickness of soft sediments is obtained for many sites in Egypt. The Horizontal-to-Vertical Spectral Ratio (BH/V^) technique (known as Nakamura technique) can be used as a robust tool to determine the thickness of soft sediments layers overlaying bedrock from observations and measurements of seismic ambient noise data. In Egypt, numerous seismic ambient noise measurements have been conducted in several areas to determine the dynamic properties of soft soil for engineering purposes. At each site in each studied area, the fundamental site frequency was accurately estimated from the main peak in the spectral ratio between the horizontal and vertical component. Consequently, an extensive database of microtremor measurements, well logging data, and shallow seismic refraction data have been configured and assembled for the studied areas. New formula between fundamental site frequency (f 0) and thickness of soft sediments (h) is established. The new formula has been validated and compared with other formulas of earlier scientists, and the results indicate that the calculated depth and geometry of the bedrock surface using new formula are in a good agreement with well logs data and previously published seismic refraction surveys in the investigated sites.

This study performs a reanalysis of the seismicity recorded during the 1996 Gjálp eruption that occurred at NW Vatnajökull, Iceland. The seismicity was recorded by the temporary HOTSPOT network consisting of 30 three-component broadband stations. In total 301 events were identified between 29 September and 12 October and their phases were manually picked. A velocity model was estimated from P-phase travel times by using VELEST. Events were first located using the algorithm NON-LINLOC in order to obtain absolute locations. Precise relative locations were obtained with HYPODD by utilizing catalog and cross-correlation differential travel times. Results show that events clustered first along the SW rim of the Bárðarbunga caldera and later along the Gjálp fissure, with most hypocentral depths located between 3 and 8 km. Waveforms of the 10 largest events that followed the Bárðarbunga earthquake were inverted in order to obtain moment tensors. For all events we found that the deviatoric moment tensor fits the data better than pure double-couple or full moment tensor solutions. Events along the Bárðarbunga caldera exhibited reverse focal mechanisms, while those at the Gjálp fissure exhibited mostly strike-slip faulting. Seismic velocity variations calculated using ambient noise interferometry, point to the possibility that a small subglacial eruption occurred at Bárðarbunga before the main earthquake. This removed melt from the magma chamber causing its roof to collapse, and also resulted in the lateral migration of magma towards the Gjálp fissure. The 2014-2015 Bárðarbunga-Holuhraun eruption shares common characteristics with the 1996 Gjálp eruption, although the size of the latter was much smaller.

- by Dimitrios Giannopoulos
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- Iceland, Seismicity, Subglacial eruptions, Ambient noise seismology

Three years of continuous waveform data recorded at 22 stations from the Corinth Rift Laboratory and
the Hellenic Unified Seismological Network are used to perform an ambient noise surface-wave
tomography of the western Corinth Rift. All available vertical component time-series were crosscorrelated
to extract empirical Rayleigh-wave Green's functions. Group velocity dispersion curves
were measured for each station-pair by applying frequency-time analysis and then inverted to build
2D group velocity maps between 1 and 6 s period. Finally, we locally inverted these velocity maps
using a neighborhood algorithm to assess the 3D shear-velocity model of the shallow crustal structure
of the western Corinth Rift. Across all studied periods the southern coast of the Corinth Gulf is
generally imaged as a region of lower velocities compared to the northern coast. At periods up to 3 s,
the spatial variation of the group velocities is correlated with the surface geology of the area. Lower
velocities are observed in areas where mostly Plio-Quaternary syn-rift sediments are present, such as
off-shore regions of the rift, the Mornos delta and the largest part of the southern coast. Higher
velocities are observed in pre-rift basement structures which are dominated mostly by carbonates. At
periods above 3 s, where Rayleigh-waves begin to sense deeper structures below the sediments within
the underlying basement, our study highlights the presence of a distinct zone of lower velocities
across the southern part of the rift with an elongation in the WNW-ESE direction. The interpretation
of this low velocity includes two arguments, the present-day active tectonic regime and the possible
involvement of fluids circulation processes at depth within a highly fractured upper crust in the
vicinity of the major faults zones. In general, the results demonstrate good agreement with the major
geological and tectonic features of the area, as well as with previous local earthquake tomography
studies and support the assumption of fluid circulations at depth. This work intends to be the base for
further investigations towards the study of the Corinth Rift structure using long-time series of ambient
noise data.

Ocean bottom seismometers (OBS) are widely in use since recent past to monitor seismicity of slow earthquakes as well as that of ordinary earthquakes. Seismic velocity structures, especially of S-waves, are essential to estimate hypocenters of them with accuracy. The horizontal to vertical spectral ratio (HVSR) method, originally proposed by Nogoshi and Igarashi (1971) and familiarized by Nakamura (1989), is a good technique to get S-wave velocity structure by inversion of the HVSR curve. However, we aim to utilize the ambient noise (possibly <0.5Hz) HVSR approach utilizing OBS data. From this perspective, here we focus on the stability of HVSR spectra of ambient noise calculated from OBS data as the first step toward future application of this method to estimate S-wave velocity structure deploying OBS stations. We had followed the Nakamura's method (1989) for obtaining HVSR spectra and then compared their shapes for stability analysis.

- by zhen guo
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- Geophysics, Microtremor, Ambient noise seismology, Ambient seismic noise levels

In this contribution, new relationship between the fundamental site frequency and the thickness of soft sediments is obtained for many sites in Egypt. The Horizontal-to-Vertical Spectral Ratio (“H/V”) technique (known as Nakamura technique) can be used as a robust tool to determine the thickness of soft sediments layers overlaying bedrock from observations and measurements of seismic ambient noise data. In Egypt, numerous seismic ambient noise measurements have been conducted in several areas to determine the dynamic properties of soft soil for engineering purposes. At each site in each studied area, the fundamental site frequency was accurately estimated from the main peak in the spectral ratio between the horizontal and vertical component. Consequently, an extensive database of microtremor measurements, well logging data, and shallow seismic refraction data have been configured and assembled for the studied areas. New formula between fundamental site frequency (f0) and thickness of soft sediments (h) is established. The new formula has been validated and compared with other formulas of earlier scientists, and the results indicate that the calculated depth and geometry of the bedrock surface using new formula are in a good agreement with well logs data and previously published seismic refraction surveys in the investigated sites.

- by abd el-aziz khairy abd el-aal
- •
- Geology, Geophysics, Seismology, Seismic Background Noise

- by Michael Asten
- •
- Comparative Study, Spatial autocorrelation, Velocity Estimation, Spectrum

SUMMARY The microtremor method (MTM) using spatial autocorrelation (SPAC) processing is a useful tool for gaining thickness and shear wave velocity (SWV) of sediments. These parameters are essential for site response modeling and regolith classification for earthquake hazard and risk assessments. Complications arise if the wave field is strongly directional (ie. insufficiently averaged in azimuth) or contains multiple Rayleigh modes. Theoretical studies compare the use of triangular, hexagonal and semi-circular arrays and show how the latter is preferable in the presence of strongly directional seismic noise, while the hexagonal array is superior in maximising the range of detectable wavelengths. Both hexagonal and semi-circular arrays can identify multiple-mode wave propagation. Observed coherency-frequency curves are inverted in coherency space (without the intermediate step of obtaining a dispersion curve from field data) to yield a profile of SWV and layer thicknesses to depths up to one hundred metres. The MTM using SPAC has achieved a precision of +-10% or better in the Vs30 zone of unconsolidated but moderately homogeneous sediments. The results show a good to strong correlation with seismic cone penetrometer tests (SCPTs) from Perth (Western Australia). Moreover, the microtremor data has an additional advantage of being capable of non-invasively detecting the base of sediments where the SCPT method fails in coarse gravels. MTM combined with SPAC has the potential to provide SWV profiles of soils and near-surface basement rocks, suitable for input into a site response model.

- by Michael Asten
- •
- Microtremor, Ambient Seismic Noise Analysis

In subduction zones, elevated pore fluid pressure, generally linked to metamorphic dehydration reactions, has a profound influence on the mechanical behavior of the plate interface and forearc crust through its control on effective stress. We use seismic noise–based monitoring to characterize seismic velocity variations following the 2012 Nicoya Peninsula, Costa Rica earthquake [Mw (moment magnitude) 7.6] that we attribute to the pres- ence of pressurized pore fluids. Our study reveals a strong velocity reduction (~0.6%) in a region where pre- vious work identified high forearc pore fluid pressure. The depth of this velocity reduction is constrained to be below 5 km and therefore not the result of near-surface damage due to strong ground motions; rather, we posit that it is caused by fracturing of the fluid-pressurized weakened crust due to dynamic stresses. Although pressurized fluids have been implicated in causing coseismic velocity reductions beneath the Japanese volcanic arc, this is the first report of a similar phenomenon in a subduction zone setting. It demonstrates the potential to identify pressurized fluids in subduction zones using temporal variations of seismic velocity inferred from ambient seismic noise correlations.

- by Esteban Chaves Sibaja
- •
- Seismology, Costa Rica, Subduction Zone Processes, Subduction Zones

Analysis of passive seismic (microtremor) array data using either beam-forming or the Spatial AutoCorrelation (SPAC) methods successfully establishes shear-wave velocity profiles at scales from a few metres to order 1 km depth. The SPAC method is enhanced by fitting the observed coherency spectrum directly with a modelled SPAC spectrum. The method yields shear-wave velocity versus depth profiles with a minimum of bias and also minimizes the number of array spacings required, compared with that required for beamforming methods. The method allows interpretation of Vs to a precision of 5-10%, facilitates recognition of higher-mode energy, and the correct interpretation of near-surface low-velocity layers. The method is demonstrated in blind and comparative studies on Holocene-Pleistocene alluvial sediments of the Santa Clara Valley, California, resolving boundaries at depths from 2 metres to 550 metres. The method is also capable of resolving a near-surface buried low-velocity layers of thickness 3-5 m (presumably silt) with Vs 100 to 150 m/sec, beneath 7 m of sands.

- by Michael Asten
- •
- Microtremor, Ambient Seismic Noise Analysis

Detailed modelling of microtremor propagation, using shear velocities acquired from seismic cone penetrometer tests (SCPT), shows that multiple peaks in the measured H/V spectral ratio of field data can be clearly correlated with maxima in particle-moti on ellipticity as computed for a chosen layered-earth regolith model. This provides additional physical understanding of why the use of peaks in H/V ratios is effective for regolith site classification. The resolution of spectral peaks is sufficient to allow the H/V ratios to constrain estimates of depth to bedrock. The use of modelled ellipticities in conjunction with observed spectral ratio data complements SCPT data, and can be used to estimate depths where the SCPT penetration stops short of bedrock, or to map laterally-changing thickness of unconsolidated sediments away from the SCPT site. In some cases the period of the modelled 1 st higher mode ellipticity maximum can be identified as a secondary maximum in H/V ratios. This wa...

- by Michael Asten
- •
- Microtremor, Water Tables, Field Data, Ambient Seismic Noise Analysis

Romania is an earthquake prone area with a few destructive earthquakes per century. The National Institute for Earth Physics carries out the seismic survey of Romania through the Romanian National Seismic Network (RNSN) consisting of 65 real-time seismic stations. Daily reports and monthly bulletins are delivered after routinely analyzing and processing the recorded data. In the present paper we applied the Seismic Network Evaluation through Simulation method for the RNSN configuration as it was in August 2011 to estimate the background noise level, assess the appropriateness of the velocity model adopted in routine location procedure, evaluate the hypocenter location uncertainty and determine the detection magnitude threshold. Areas of greater (southern Romania) and lower (Carpathians and Apuseni Mountains) background noise within the RNSN are identified by mapping the average power of noise in 1-12 Hz frequency range. The statistical study of the P and S phases residual times allow us to assess the appropriateness of the velocity model used in routine location. Both P-and S-wave velocity models can be optimized to improve the quality of the hypocenter location. As shown by our analysis, the RNSN is able to detect and locate earthquakes with M L magnitude above 2.5 anywhere on the Romanian territory, except the border areas, such as the Crisana-Maramures seismic source zone. Merging data from both sides of the border significantly improves the quality of hypocenter location in these areas.

- by Antonino D'Alessandro
- •
- Microtremor, Seismic Noise, Seismic Network, Ambient Seismic Noise Analysis

- by Michael Asten
- •
- Risk assessment, Spatial autocorrelation, Wave propagation, Western Australia

D. Giannopoulos, C. Orfanos, K. Leontarakis, A. Lois and N. Martakis show the results of an Ambient Noise Tomography performed in southwest Albania, using long time-series of ambient seismic noise
ecorded during a Passive Seismic Tomography project, to demonstrate
the efficiency and the usefulness of the Passive Seismic Interferometry technique.

- by Dimitrios Giannopoulos
- •
- Ambient Seismic Noise Analysis
- by Michael Asten
- •
- Microtremor, Ambient Seismic Noise Analysis

We present the preliminary results of an ambient seismic noise analysis performed in the western Corinth Gulf, Greece. The Corinth Gulf is a continental rift which separates the central Greek mainland from Peloponnese. The rift is approximately 120 km long and 10-20 km wide, with a WNW-ESE orientation, extending from the Gulf of Patras in the west, to the Gulf of Alkionides in the east. It is considered as one of the most active extensional intra-continental rifts in the world, with the geodetically measured rates of extension varying from ∼5 mm/yr at the eastern part, to ∼15 mm/yr at the western part. We used data from three-component broad-band seismic stations operated under the framework of the Hellenic Unified Seismological Network (HUSN) and the Corinth Rift Laboratory (CRL). After the classical processing of continuous ambient seismic noise recordings, we used both auto-correlation and cross-correlation functions of single stations and station pairs, respectively, in order to retrieve empirical Green's functions (EGFs) of surface waves and estimate relative velocity changes. For estimating the relative velocity changes we used the moving-window cross spectrum analysis (MWCS) technique. This is the first attempt to characterize the ambient seismic noise properties in the area and study the possible relation between the detected relative velocity changes and the occurrence of moderate or strong earthquakes in the study area.

- by Dimitrios Giannopoulos
- •
- Seismology, Ambient Seismic Noise Analysis

In the context of a passive seismic survey that was launched in the South Lokichar Basin, southwest of Lake Turkana, in Kenya, a seismic network consisting of 13 3-component broadband stations was deployed in the area. In this study, records of both ambient seismic noise and natural seismicity are jointly exploited, through ambient noise tomography and earthquake-based methodologies respectively, in order to obtain a reference shear-wave velocity (VS) crustal model. The applied passive seismic techniques allowed to constrain the VS distribution at depths ranging from the shallow parts of the crust down to Moho and the upper mantle. The lateral variations of the calculated VS model at depth are highly compatible with those obtained from previous experiments carried out across the Kenya rift. This agreement is a strong indication of the reliability of our results and leads us to infer that our model has the potential to serve as a reference VS crustal model for the broader region of the South Lokichar Basin.

Human activity causes vibrations that propagate into the ground as high-frequency seismic waves.Measures to mitigate the coronavirus disease 2019 (COVID-19) pandemic caused widespread changes in human activity, leading to a months-long reduction in seismic noise of up to 50%. The 2020 seismic noise quiet period is the longest and most prominent global anthropogenic seismic noise reduction on record. Although the reduction is strongest at surface seismometers in populated areas, this seismic quiescence extends for many kilometers radially and hundreds of meters in depth. This quiet period provides an opportunity to detect subtle signals from subsurface seismic sources that would have been concealed in noisier times and to benchmark sources of anthropogenic noise. A strong correlation between seismic noise and independent measurements of human mobility suggests that seismology provides an absolute, real-time estimate of human activities.

The classical approach to analysis of microtremor array data using the Spatial AutoCorrelation (SPAC) method is to estimate surface-wave velocities from the azimuthally-averaged SPAC coherency spectrum, then invert the observed surface-wave dispersion curve, fitting with a modelled dispersion curve, to obtain parameters for a layered-earth model. An alternative approach introduced four years ago is to fit the observed SPAC coherency spectrum directly with a modelled SPAC spectrum. This approach has two key advantages; firstly it reduces bias associated with phase-velocity estimates made in the presence of incoherent noise, and secondly it eliminates the uncertainty associated with multi-valued solutions of the inverse Jo Bessel function. The latter allows the use of an array of a given diameter to be extended to recognition of wavenumbers typically two to seven times larger than is possible with the classical method, which has the further advantage of reducing the number of array radii required for a given survey. The direct-fitting method of SPAC curves (MMSPAC) over multiple minima and maxima has further advantages of providing higher resolution of layer shear-velocity estimates, of facilitating recognition of higher-mode energy, and of correct interpretation of near-surface low-velocity layers. The method is demonstrated in blind and comparative studies on Holocene-Pleistocene alluvial sediments of the Santa Clara Valley, resolving boundaries at depths from 2 metres to 450 metres.

- by Michael Asten
- •
- Microtremor, Ambient Seismic Noise Analysis

The Corinth Rift separates Peloponnesus to the south from mainland Greece to the north. It is one of the most active extensional intra-continental rifts in the world, with geodetically measured rates of extension varying from ∼5 mm/yr at the eastern part to ∼15 mm/yr at the western part. This work presents a first attempt to study the crustal velocity structure of the western Corinth Rift using ambient noise recordings. We used 3 yrs (01/2012-12/2014) of continuous waveform data recorded at 24 stations from the Corinth Rift Laboratory (CRL) and the Hellenic Unified Seismological Network (HUSN). All available vertical component time-series were cross-correlated to extract Rayleigh wave Green's functions. Group velocity dispersion curves between 0.5 and 7 s period were measured for each station pair by applying frequency-time analysis and then inverted to build group velocity maps of the study area. At the studied periods, the northern coast of the Corinth Rift is generally imaged as a region of elevated seismic velocities compared to the southern coast. More specifically, low velocities are observed in areas of Plio-Quaternary syn-rift sediments such as offshore regions of the rift, the Mornos delta and a large part of the southern coast. Higher velocities are observed in pre-rift basement structures which are dominated mostly by carbonates. The preliminary results demonstrate good agreement with the major geological features of the area and agree relatively well with previous local earthquake tomography studies. This work will be the base for further investigations towards the study of the Corinth Rift structure using long time-series of ambient noise data.

The 3D S wave velocity of shallow structures, especially the quaternary sediments at 0–1 km near the surface, is an important issue of concern in urban planning and construction for seismic hazard assessment and disaster mitigation. Due to the facility and less dependence on the site environment, noise‐based technique is an ideal way to acquire the fine structure of urban sedimentary basin. Based on the dense array composed of more than 900 stations deployed in Tongzhou at a local scale of 20 × 40 km², we prove that the lateral variation of the phase velocity of multimode surface waves can be estimated directly with sufficient accuracy by beamforming seismic noise with moving subarrays. Rayleigh wave phase velocity maps, at frequencies between 0.3 and 2.5 Hz for the fundamental mode as well as 0.8 and 3.0 Hz for the first overtone, are obtained. The 3D S wave velocity model at 0–1‐km depth with lateral resolution of 1 km is then established by inverting phase velocity maps of the two modes. The sediment thickness is delineated by the impedance interface given by microtremor H/V (horizontal‐to‐vertical) spectral ratio. The proposed model is in good agreement with the distribution of tectonic units. The sediment thickness of Daxing high and two sags around Gantang and Xiadian are, respectively, 100–400 and 400–600 m, which correlates well with the 1 km/s isosurface of S wave velocity. The model presents some evidence on the extension of Daxing fault along NE direction.

- by Laiyu Lu
- •
- Seismology, Seismic data processing, Sedimentary Basins, Beamforming
- by Mostafa Toni
- •
- Ambient Seismic Noise Analysis, Hurghada
- by Michael Asten
- •
- Spatial autocorrelation, Spectrum, Microtremor, Ambient Seismic Noise Analysis
- by Michael Asten
- •
- Microtremor, Ambient Seismic Noise Analysis

We apply the Seismic Network Evaluation through Simulation (SNES) method to evaluate the performance of the Alaska Regional Seismic Network (ARSN) in locating regional earthquakes in terms of validity of the velocity models, hypocen-tral errors, and magnitude of completeness. We find that background noise levels at the sites are highest in the Aleutian region and lowest in mainland Alaska. We demonstrate that the ARSN provides the best monitoring in the south-central region, with horizontal and vertical errors of less than 1.5 and 3 km, respectively, at the completeness level of M L 2.0. At the completeness level of M L 3.0, the network is capable of locating earthquakes for the whole region down to depths of about 250 km with satisfactory errors. The lowest magnitude completeness levels, of about 1.4 and even less, are achieved with satisfactory location errors in limited areas of the south-central region that have the densest station coverage. The network does not monitor the seismogenic areas in northern, western, southeastern, and offshore Alaska at an adequate level in terms of earthquake location errors. Network upgrades in these areas could be optimized using the SNES results.

- by Antonino D'Alessandro
- •
- Earthquake, Seismicity, Seismic Noise, New Seismic Noise Models