Ambient Noise Method Research Articles

Imaging high-resolution 3D shallow crustal structure by ambient noise method in the Shiziyang Waterway area, Guangzhou, South China

The Shiziyang Waterway, located in the northern continental margin of the South China Sea, has developed a complex fault system owing to the coupling action of the oceanic crust and continental crust. However, the geological structure in the shallow crust beneath the Shiziyang Waterway and the relationship between faults and waterway development are still unclear, resulting in difficulty in assessing the earthquake risk to the surrounding cities. In this study, based on a high-density short-period seismic array that was recently deployed around the Shiziyang Waterway, a high-resolution shear wave velocity model is inverted to as shallow as 4 km by the ambient noise tomography method. The results showed that at depths ranging from 0.2 to 4 km, three sets of low-velocity zones (LVZs), trending east–west, northeast, and northwest, are revealed, which are speculated to be the Shougouling fault (SGLF), Xinhui-Shiqiao fault (XSF), and Shiziyang fault bundle (SZYFb), respectively. The shear-wave velocity anomalies illustrate that the SGLF stopped extending east at the north endpoint of the Wenchong-Zhujiangkou fault (WZF). The two-stage differential vertical movements of the WZF, Nangang-Humen fault (NHF), and Hualong-Haioudao fault (HHF) dominated the opening of the Shiziyang Waterway. This study provides new seismological evidence of the opening process for the Shiziyang Waterway and reveals that internal fault activity dominated the development and evolution of the Pearl River Delta.

Urban High-density 3D Ambient Noise Refracted Wave Extraction

Active-source seismic detection is trapped in complex urban environmental noise and dense housing facilities, so this method is not suitable for urban environment, but the ambient noise method can obtain urban underground medium information at low cost and environmental protection. At the same time, in order to have a more comprehensive understanding of the underground situation, it is necessary to conduct three-dimensional detection of underground structures, arrange three-dimensional measurement lines, and high-density spatial sampling can obtain high-resolution detection results. In terms of ambient noise body wave extraction, the extraction of refracted waves is easier. Therefore, in this experiment, high-density 3D ambient noise refracted wave extraction is carried out for urban environment. In this paper, the refracted wave signal is extracted by cross-correlation of the collected 20-day ambient noise data.

Seismic interferometry ambient noise tomography—a study in the north-central part of Iran

Seismic interferometry is one of the noteworthy new subsurface imaging methods that have been used in recent years in many tomographic studies. In this study, the interstation estimated Green functions extracted from the Rayleigh and Love wave ambient noise cross-correlations are used to obtain the Love and Rayleigh wave group velocity maps in different periods across the study area in order to determine the velocity anomalies and their corresponding geological structures within the crust. In the ambient noise method, the quality of the cross-correlations increases when longer time series of data are used, so we used 22 months of continuous data from the 17 broadband seismic stations as the primary database. After performing the initial corrections of the ambient noise data and obtaining the 3-month cross-correlation time series, the longer time series were obtained by stacking the 3-month time series with acceptable quality. In the next step, by using the time-frequency analysis and the estimated Green functions, the Love and Rayleigh wave dispersion curves were determined. Then, using the 2D tomography method, the Love and Rayleigh wave group velocity maps for the periods 10–25 s were produced. In the short periods, a low-velocity zone is observed in the Love and Rayleigh wave group velocity maps from the south Caspian basin toward central Iran, which is likely due to the effect of the thick alluvial deposits and the intermountain sediments in Alborz. At the longer periods, the Rayleigh and Love wave group velocity maps show a low-velocity zone in central Alborz, while in the eastern and western parts of Alborz, they show higher velocities, which is consistent with the prior studies. The dense path coverage permits to produce images that have substantially higher lateral resolution than is currently available from global and regional group velocity studies. Tomographic maps at high frequencies are well correlated with the upper crust structure and especially with sediment layer thickness.

Shear wave structure of southern Sweden from precise phase-velocity measurements of ambient-noise data

SUMMARY Rayleigh-wave phase-velocity tomography of southern Sweden is presented using ambient seismic noise at 36 stations (630 station pairs) of the Swedish National Seismic Network. We analyse 1 yr (2012) of continuous recordings to get the first crustal image based on the ambient-noise method in the area. Time-domain cross-correlations of the vertical component between the stations are computed. Phase-velocity dispersion curves are measured in the frequency domain by matching zero crossings of the real spectra of cross-correlations to the zero crossings of the zeroth-order Bessel function of the first kind. We analyse the effect of uneven source distributions on the phase-velocity dispersion curves and correct for the estimated velocity bias before tomography. To estimate the azimuthal source distribution to determine the bias, we perform inversions of amplitudes of cross-correlation envelopes in a number of period ranges. Then, we invert the measured and bias-corrected dispersion curves for phase-velocity maps at periods between 3 and 30 s. In addition, we investigate the effects of phase-velocity bias corrections on the inverted tomographic maps. The difference between bias-corrected and -uncorrected phase-velocity maps is small (<1.2 per cent), but the correction significantly reduces the residual data variance at long periods where the bias is biggest. To obtain a shear velocity model, we invert for a 1-D velocity profile at each geographical node. The results show some correlation with surface geology, regional seismicity and gravity anomalies in the upper crust. Below the upper crust, the results agree well with results from other seismological methods.

Overview of seismic methods for urban underground space

The development of urban underground space (UUS) has become an effective way to solve the problem of urban space. Seismic methods have achieved good results in underground engineering construction, active fault detection, urban geologic structure detection, and other fields. Active source seismic methods in UUS include reflection methods, refraction methods, surface wave methods, andvertical seismic profile methods. Interest in the ambient noise method is also rising gradually, with it becoming an important method of urban underground space detection in the future because it overcomes the problem that the active source method is interfered with by urban noise. We evaluate these methods and develop application examples of UUS.

Site response evaluation through measuring the ambient noise (case study: Iran, Babol City)

One major step in evaluating the site response is estimation of the shear wave velocity and fundamental frequency. Various methods are available for evaluating the site effect, of which the ambient noise measurement method is a prevalent one. The method is quick with low cost; it is nondestructive and gives precise results. In the present research, to assess the site response in Babol City, measurements of ambient noise were performed at 60 points and the data were analyzed by Nakamura method. Also, at 8 sites, measurements of the ambient noise were done by applying SPAC method and the data were analyzed to obtain the shear wave velocity. In addition, equivalent-linear method was performed using the shear wave velocity as well as geotechnical and geological data of those 8 sites. Furthermore, the artificial neural network (ANN), with different input data, was trained and then was utilized to predict the fundamental frequency. The fundamental frequencies acquired from ambient noise, ANN and equivalent-linear method were compared to each other to investigate reliability of the results. The results showed that most parts of the study area have a fundamental frequency less than 1 Hz, while higher frequencies could be observed in some northwestern and southern parts of the city. Finally, it is shown that, the ambient noise measurements were highly useful for evaluating the site effect and estimating important seismic parameters (fundamental frequency and shear wave velocity).

Application of Passive Seismic in Determining Overburden Thickness: North West Zambia

There are several ways to estimate overburden cover thickness. One of the non-invasive and inexpensive ways to rapidly estimate the cover thickness is the Horizontal-to-Vertical Spectral Ratio (HVSR) of the ambient seismic noise method. This approach utilises a broadband three-component sensitive seismometer to record ambient noise (or microtremor) induced by the wind, ocean waves and several anthropogenic activities. These microtremors are mainly composed of Rayleigh Waves which propagate in the surface layer. The Tromino® seismometer, which works on the HVSR principle, is a very light and portable instrument that records seismic noise in the frequency range of 0.1 to 1024 Hz, and is capable of estimating overburden cover up to 100’s of metres depth, depending on the ambient noise strength and geological setting of the survey area. The ratio of the horizontal-to-vertical (H/V) component of the shear wave (Vs) spectrum is used to calculate the peak resonance frequency at a particular survey station, which is used to estimate the overburden thickness by using one or more existing drill holes in the area, 1D modelling, or local geological knowledge about the overburden to get velocity information for calculating depth. This paper discusses different methods used to calculate the overburden thickness, which includes calculations using a hybrid approach and a regression equation. This paper shows the results of a Tromino® HVSR survey in North West Zambia and comparison of estimated overburden thickness using different methods. The results were further compared with those determined from Audio-MagnetoTellurics (AMT) and drilling data. Tromino® successfully estimated the overburden thickness and mapped the bedrock topography with reasonable accuracy.

Velocity-measurement bias of the ambient noise method due to source directivity: a case study for the Swedish National Seismic Network

The bias of velocity measurements from ambient-noise covariograms due to an anisotropic distribution of noise sources is studied assuming that the noise field consists of planar surface waves from ...

Extracting Stable Seismic Core Phases from Ambient Seismic Noise

Abstract Ambient seismic‐noise correlation is a powerful tool for extracting the seismic core phases that propagate through the interior of the Earth. In this study, we present and refine the root‐mean‐square‐stacking method to extract stable core phases (e.g., PcP , ScS , PcS / ScP , and PKiKP ) from within the Central Alborz region, Iran, using empirical Green’s functions. Our studies on the extracted core phases using empirical Green’s functions indicated that the ambient seismic noise method is independent from global seismicity ( M ≥5.5). We also show that, by dividing ambient seismic records into shorter (i.e., 2700 s) and overlapping (70%) time windows, before the cross‐correlation procedure, we can improve the quality and stability of the empirical Green’s functions generated. Consequently, 73 days (equivalent to 22% of the total time period for the dataset) of nonconsecutive ambient seismic‐noise time windows have been used to retrieve core‐phase empirical Green’s functions in a 5–10 s period band.

Synchronizing Intercontinental Seismic Networks Using the 26 s Persistent Localized Microseismic Source

Accurate instrument clocks are essential for quantitative studies in seismology. Recent studies demonstrate that ambient seismic noise can be used to detect clock time drift for ocean‐bottom seismometers (OBSs) or inland seismometers with internal hardware or software problems, but the short‐period (<20 s) interstation Rayleigh waves extracted from ambient seismic noise are too weak for intercontinental seismic station‐pair correlations and thus are less effective for synchronizing sparse global seismic networks. The 26 s persistent localized microseismic source in the Gulf of Guinea radiates strong seismic signals that can be recorded on global stations, thus providing an alternative approach for synchronizing seismometers at large interstation distances. In this study, we test the feasibility of synchronizing seismic stations in Africa, North America, and Europe using the 26 s signals in the noise cross‐correlation functions. We find that the clocks at the TAM and OBN stations are relatively stable during the years from 2002 to 2009; an ∼1 s time shift occurred at the CCM station from 2006 to 2009, and the clock drift was not constant. We also detected about 150 s time shifts at the KOWA station in October 2012 and up to 500 s in January 2013. We validate our results via comparison with the ambient noise method by Stehly et al. (2007) and teleseismic P ‐wave arrival‐time residuals. The method proposed in this study can be used to calibrate the OBSs in the north Atlantic by synchronizing their clocks with the precisely calibrated inland seismometers.

Vulnerability Assessment of Existing Low-Rise Reinforced Concrete School Buildings in Low Seismic Region Using Ambient Noise Method

Ground movements triggered by the Bukit Tinggi earthquakes in 2007 to 2009 are believed to be the possible cause of several structural damages on a secondary school building of SMK Bukit Tinggi, in the state of Pahang, Malaysia. This paper describes the ambient noise study conducted on the damaged building (a 4-storey reinforced concrete frame laboratory building) and the adjacent buildings using tri-axial 1 Hz seismometer sensors. Fourier amplitude spectra (FAS) analysis was applied to determine the buildings natural frequencies for vulnerability assessment of the damaged structure in both longitudinal and transverse axes. Significant multiple peaks of FAS curves used for natural frequencies determination of the buildings show values between 4.18 to 4.34 Hz, 5.04 to 5.23 Hz, 6.07 to 6.54 Hz and 8.17 to 8.81 Hz, indicating the existence of translational and torsional vibration modes acting on the buildings. Differences in dynamic behaviour between the laboratory and the adjacent buildings may be responsible for the structural damages due to the independent structural response and excessive torsional effect during the Bukit Tinggi earthquake tremors.

Determining the orientations of ocean bottom seismometers using ambient noise correlation

The cross‐correlation of multicomponent ambient seismic noise can reveal both the velocity and polarization of surface waves propagating between pairs of stations. We explore this property to develop a novel method for determining the horizontal orientation of ocean bottom seismometers (OBS) by analyzing the polarization of Rayleigh waves retrieved from ambient noise cross‐correlation. We demonstrate that the sensor orientations can be estimated through maximizing the correlation between the radial‐vertical component and the phase‐shifted vertical‐vertical component of the empirical Green's tensor. We apply this new method to the ELSC (Eastern Lau Spreading Center) OBS experiment data set and illustrate its robustness by comparing the obtained orientations with results from a conventional method utilizing teleseismic P and Rayleigh wave polarizations. When applied to a large OBS array, the ambient noise method provides a larger number of orientation estimates and better azimuthal coverage than typically is possible with traditional methods.

Stationary phase approximation in the ambient noise method revisited

The method of extracting Green’s function between stations from cross correlation has proven to be effective theoretically and experimentally. It has been widely applied to surface wave tomography of the crust and upmost mantle. However, there are still controversies about why this method works. Snieder employed stationary phase approximation in evaluating contribution to cross correlation function from scatterers in the whole space, and concluded that it is the constructive interference of waves emitted by the scatterers near the receiver line that leads to the emergence of Green’s function. His derivation demonstrates that cross correlation function is just the convolution of noise power spectrum and the Green’s function. However, his derivation ignores influence from the two stationary points at infinities, therefore it may fail when attenuation is absent. In order to obtain accurate noise-correlation function due to scatters over the whole space, we compute the total contribution with numerical integration in polar coordinates. Our numerical computation of cross correlation function indicates that the incomplete stationary phase approximation introduces remarkable errors to the cross correlation function, in both amplitude and phase, when the frequency is low with reasonable quality factor Q. Our results argue that the distance between stations has to be beyond several wavelengths in order to reduce the influence of this inaccuracy on the applications of ambient noise method, and only the station pairs whose distances are above several (>5) wavelengths can be used.

Surface wave dispersion measurements from ambient seismic noise analysis in Italy

SUMMARY We present the surface wave dispersion results of the application of the ambient noise method to broad-band data recorded at 114 stations from the Istituto Nazionale di Geofisica e Vulcanologia (INGV) national broad-band network, some stations of the Mediterranean Very Broadband Seismographic Network (MedNet) and of the Austrian Central Institute for Meteorology and Geodynamics (ZAMG). Vertical-component ambient noise data from 2005 October to 2007 March have been cross-correlated for station-pairs to estimate fundamental mode Rayleigh wave Green's functions. Cross-correlations are calculated in 1-hr segments, stacked over periods varying between 3 months and 1.5 yr. Rayleigh wave group dispersion curves at periods from 8 to 44 s were determined using the multiple-filter analysis technique. The study region was divided into a 0.2°× 0.2° grid to invert for group velocity distributions. Checkerboard tests were first carried out, and the lateral resolution was estimated to be about 0.6°. The resulting group velocity maps from 8 to 36 s show the significant difference of the crustal structure and good correlations with known geological and tectonic features in the study region. The Po Plain and the Southern Alps evidence lower group velocities due to soft alluvial deposits, and thick terrigenous sediments. Our results also clearly showed that the Tyrrhenian Sea is characterized with much higher velocities below 8 km than the Italian peninsula and the Adriatic Sea which indicates a thin oceanic crust beneath the Tyrrhenian Sea.

Ambient noise Rayleigh wave tomography in western Sichuan and eastern Tibet

We perform Rayleigh wave tomography in western Sichuan and eastern Tibet by applying the ambient noise method to broadband data recorded at 49 stations from the Sichuan digital seismic network. Cross-correlations of vertical-component ambient noise data are computed in one-hour segments and stacked over 7 months from June to December 2007. Then Rayleigh wave group dispersion curves from 6 to 48 s were measured using the multiple-filter analysis method. Group velocity maps at periods from 8 to 44 s were inverted. The resulting group velocity maps generally show good correlations with tectonic features. The Sichuan Basin is clearly resolved with lower group velocities at the short periods (8 to 20 s) due to thick sedimentary layers, and a prominent low-velocity zone exhibits in the middle crust beneath the eastern margin of the Tibetan Plateau. The Longmen Shan thrust belt marks not only the rapid lateral velocity variation between the topographically high Tibetan Plateau and low-elevation Sichuan Basin, but also the transition from the thick crust beneath the Tibetan Plateau to the normal continental crust beneath the Sichuan Basin.

Eikonal tomography: surface wave tomography by phase front tracking across a regional broad-band seismic array

SUMMARY We present a new method of surface wave tomography based on applying the eikonal equation to observed phase traveltime surfaces computed from seismic ambient noise. The source–receiver reciprocity in the ambient noise method implies that each station can be considered to be an effective source and the phase traveltime between that source and all other stations is used to track the phase front and construct the phase traveltime surface. Assuming that the amplitude of the waveform varies smoothly, the eikonal equation states that the gradient of the phase traveltime surface can be used to estimate both the local phase speed and the direction of wave propagation. For each location, we statistically summarize the distribution of azimuthally dependent phase speed measurements based on the phase traveltime surfaces centred on different effective source locations to estimate both the isotropic and azimuthally anisotropic phase speeds and their uncertainties. Examples are presented for the 12 and 24 s Rayleigh waves for the EarthScope/USArray Transportable Array stations in the western USA. We show that (1) the major resulting tomographic features are consistent with traditional inversion methods, (2) reliable uncertainties can be estimated for both the isotropic and anisotropic phase speeds, (3) ‘resolution’ can be approximated by the coherence length of the phase speed measurements and is about equal to the station spacing, (4) no explicit regularization is required in the inversion process and (5) azimuthally dependent phase speed anisotropy can be observed directly without assuming its functional form.

Determining sediment absorption using ambient noise in shallow water

The ambient noise field in shallow water interacts strongly with the underlying seabed. As a consequence, the vertical directionality of the noise is largely determined from the characteristics of the seabed. In previous research, this property has been utilized to perform inversions for the compressional and shear wave speeds in hard seabeds. Soft seabeds predominate along continental shelf regions. In these cases, the absorption of compressional waves in the sediment is an important determinant of the noise field statistics which creates a measurable asymmetry in the vertical directionality. This asymmetry has been used as the basis of an inversion methodology, whereby the compressional wave absorption is estimated from measurements of the noise. The technique uses a broadband measurement from a vertically separated hydrophone pair at a fixed location. The ambient noise method yields an absorption estimate representing an average over range and depth. The technique represents a new means of estimating seabed absorption which leaves the sediment undisturbed. Absorption estimates obtained from inversions of ocean noise data taken above various soft seabeds will be presented. [Work supported by ONR.]