Ambient Noise Cross-correlation Research Articles

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.

The transition zone between the Eastern Alps and the Pannonian basin imaged by ambient noise tomography

The aim of this study was to establish a 3D shear-wave velocity model of the transition zone between the Eastern Alps and the Pannonian basin by means of ambient noise tomography. Datasets from permanent networks as well as past and recently deployed temporary networks of broadband seismic stations have been processed. Empirical Green's functions were estimated via cross-correlation of vertical component ambient seismic noise and further processed to obtain Rayleigh-wave group-velocity dispersion curves. They were directly inverted for shear velocities through a wavelet-based sparsity-constrained tomography method avoiding the intermediate step of constructing 2D group velocity maps. A horizontal resolution of 0.2° in the upper crust and 0.3°–0.5° in the lower crust was achieved. Our upper crustal shear velocity model revealed low velocities in the sedimentary depocenters and high velocities below the surrounding mountains. However, at greater depths (24–29 km), high shear velocities beneath the basins suggest crustal thinning accompanied by mantle updoming. An extremely deep low velocity anomaly was mapped beneath the Vienna basin, which we argue is caused by either sediment transfer to the lower crust; ductile deformation suggested by seismic anisotropy; or the presence of fluids.

Multifrequency inversion of global ambient seismic sources

SUMMARYWe develop and apply a method to constrain the space- and frequency-dependent location of ambient noise sources. This is based on ambient noise cross-correlation inversion using numerical wavefield simulations, which honour 3-D crustal and mantle structure, ocean loading and finite-frequency effects. In the frequency range from 3 to 20 mHz, our results constrain the global source distribution of the Earth’s hum, averaged over the Southern Hemisphere winter season of 9 yr. During Southern Hemisphere winter, the dominant sources are largely confined to the Southern Hemisphere, the most prominent exception being the Izu-Bonin-Mariana arc, which is the most active source region between 12 and 20 mHz. Generally, strong hum sources seem to be associated with either coastlines or bathymetric highs. In contrast, deep ocean basins are devoid of hum sources. While being based on the relatively small number of STS-1 broad-band stations that have been recording continuously from 2004 to 2013, our results demonstrate the practical feasibility of a frequency-dependent noise source inversion that accounts for the complexities of 3-D wave propagation. It may thereby improve full-waveform ambient noise inversions and our understanding of the physics of noise generation.

The azimuthal dependence of Rayleigh wave ellipticity in a slightly anisotropic medium

SUMMARYA recent study analysed the Rayleigh wave ellipticity obtained by ambient noise cross-correlation in periods of 8–20 s, and observed the Rayleigh wave ellipticity is backazimuth-dependent with a 180° periodicity in the contiguous United States. However, the azimuthal anisotropic parameters have not been inverted to depths, and the comparison with other seismic results has not been possible so far, partially due to the lack of related theoretical investigations. Here, we first derive explicit formulation to relate the period-dependent backazimuthal Rayleigh wave ellipticity with the depth-dependent azimuthal wave speed variation in a slightly anisotropic medium based on the variational principle; by carefully examining relations among different parametrizations of a horizontally transverse isotropic medium, we then express the final formulation in terms of Crampin’s notation. The formulation is verified by comparison with the results of anisotropic propagator matrix technique. Tests show the backazimuth-dependent Rayleigh wave ellipticity provides complementary information on anisotropic parameters in addition to the widely used phase velocity. A simple application of the derived formulation to real data in North America is also provided. Our formulation can be regarded as an extension of the classic work on azimuthal-dependent phase velocity, and helps to quantitatively explain the backazimuth-dependent Rayleigh wave ellipticity.

Time Correction of Ocean-Bottom Seismometers Using Improved Ambient Noise Cross Correlation of Multicomponents and Dual-Frequency Bands

Abstract An effective approach was developed for identifying and correcting ocean-bottom seismometer (OBS) time errors through improving ambient noise cross-correlation function (NCCF) analysis and combination with other methods. Significant improvements were illustrated through analyzing data from a passive-source seismic experiment in the southwestern sub-basin of the South China Sea. A novel method was first developed that can effectively identify errors in the sampling frequency of the OBS instruments. The traditional NCCF method was then expanded by increasing the analyzed data spectrum from a single-frequency band to dual-frequency band pairs, thus doubling the number of available data points and substantially improving the time correction quality. For data with relatively low signal-to-noise ratios, the average time errors were reduced from the original average values of 60–80 ms by the conventional methods to <40 ms using the improved approaches. The new multistep procedure developed in this study has general applicability to analysis of other OBS experiments. The demonstrated significant improvements in the data quality are critical for advancing seismic tomography and other modern marine geophysical studies that require high accuracy in the OBS data.

Hydrothermal systems characterization of the Stromboli volcano using spatial and temporal changes of the seismic velocities

Geophysical and geochemical networks are routinely deployed on active volcanoes to observe and monitor unrest periods. However, changes of the physical parameters with time are related to either magma/fluid movement or variations occurring in the hydrothermal systems hosted in the volcanic edifice.Here we use four years of ambient seismic noise recorded at the Stromboli volcano to reconstruct its shallow structure using the ambient noise cross-correlation method. At the same time, we detect temporal variations of the seismic velocities associated with the volcanic activity. We correlated the most reliable variations both in space and in time with other seismic and geochemical information in order to identify lapses of time of general unrest. We also implemented a method to enhance their spatial visualization for the interpretation. We found that the spatial mapping of the seismic velocity temporal variations obtained processing the data at high frequency (1.5–2.5 Hz) match with the location of the main hydrothermal reservoirs of the volcano.This suggests that there is a relationship between volcanic activity and hydrothermal areas that can be detected and monitored for a better understanding of the volcanic phenomena.

Laterally varying scattering properties in the North Anatolian Fault Zone from ambient noise cross-correlations

SUMMARY Intrinsic absorption and scattering properties provide us with information about the physical state and heterogeneity of the Earth’s crust. These properties are usually obtained by observing the energy decay of naturally occurring earthquakes, leading to sparse spatial sampling and therefore average scattering values over a large region. This study uses ambient noise cross-correlations to analyse the energy decay and scattering properties over a part of the North Anatolian Fault (NAF; Turkey) from the continuous records of the 73 stations of the DANA temporary array in the frequency band 0.1–0.5 Hz. The region covered by the stations has rapidly varying geological characteristics and is highly faulted around the northern strand of the NAF. We measured in the noise correlations the space–time evolution of the energy of the coda waves. We first perform measurements in separate subregions. The local scattering and attenuation properties are obtained by global optimization of a 2-D solution of the radiative transfer equation for surface waves. We found that the mean free path and attenuation coefficient are considerably varying laterally with strong scattering observed in the region lying along the northern strand of NAF. The optimization provides well-constrained values for the scattering mean free path on the order of 10 km in the fault region. The mean free path is much larger (>100 km) in the neighbouring regions. We compare our global observations with a phonon based Monte Carlo simulation of scattered energy in a laterally variable scattering model. These simulations confirm the large contrast of heterogeneity between NAF and the surrounding crust and provide further constraints on the lateral extent of NAF. When sources are located inside the fault zone, we find a signature of the actual non-uniform scattering properties, observed as a concentration of energy in the fault zone for a limited amount of time. This in turn suggests that lateral variations of scattering properties should be taken into account in future monitoring studies.

Imaging the structure of the Sun Pyramid (Teotihuacán, Mexico) from passive seismic methods

The Sun Pyramid in Teotihuacán, Mexico, belongs to the cultural heritage of Mexico and the world. Built nearly a millennium ago, the Pyramid is poorly known in terms of its mechanical properties. In order to assess these properties, we measured and studied the ambient seismic noise which has become a popular tool to explore non-invasively geological and human-made structures. This research is aimed to contribute to the structural diagnosis and seismic vulnerability assessment of this emblematic Mexican pre-Hispanic monument. We analyzed the horizontal-to-vertical spectral ratio and the cross-correlations of seismic ambient noise recorded during four hours by 15 broadband stations, distributed on different levels of the structure, and propose a three-dimensional velocity model for the Sun Pyramid in Teotihuacán. The processed data includes the stacking of cross-correlations. We identified arrivals corresponding to a rich mixture of body and surface waves. Group velocity dispersion curves were extracted from the symmetrized cross-correlation of vertical displacements between station pairs. The retrieved travel times between station pairs allowed a tomographic analysis based on the Fast Marching Method (FMM). Besides, simultaneous inversions of the local dispersion curves (obtained from the group velocity tomography) and the HVSR mitigate non-uniqueness of the retrieved P- and S-wave velocities. Our model reveals two main features of the Sun Pyramid: i) The low-velocity anomalies in the cover layer correlate well with the presence of support walls or counterforts that, apparently, were part of the construction technique seeking to provide support to weak parts of the Sun Pyramid. ii) The core of the structure, with soft material subjected to the action of infiltrated water, is susceptible to causing wall failures. These results could be useful to design conservation strategies for this emblematic Mexican pre-Hispanic monument.

The influences of large earthquake signals on the recovery of surface waves from ambient noise cross-correlation functions*

Ambient noise tomography (ANT) has been widely used to image crust and upmost mantle structures. ANT assumes that sources of ambient noise are diffuse and evenly distributed in space and the energy of different modes is equipartitioned. At present, the sources of the primary and the secondary microseisms are well studied, but there are only a few on the studies of long-period ambient noise sources. In this study, we study the effects of large earthquake signals on the recovery of surface waves from seismic ambient noise data recorded by seismic stations from the US permanent networks and Global Seismographic Network (GSN). Our results show that large earthquake signals play an important role on the recovery of long-period surface waves from ambient noise cross-correlation functions. Our results are consistent with previous studies that suggest the contribution of earthquake signals to the recovery of surface waves from cross-correlations of ambient noise is dominant at periods larger than 20–40 s.

3-D crustal VS model of western France and the surrounding regions using Monte-Carlo inversion of seismic noise cross-correlation dispersion diagrams

Summary Due to a too sparse permanent seismic coverage during the last decades, the crustal structure of western France and the surrounding regions is poorly known. In this study, we present a 3-D seismic tomographic model of this area obtained from the analysis of 2-year continuous data recorded from 55 broad-band seismometers. An unconventional approach is used to convert Rayleigh wave dispersion diagrams obtained from ambient noise cross-correlations into posterior distributions of 1-D VS models integrated along each station pair. It allows to avoid the group velocity map construction step (which means dispersion curve extraction) while providing meaningful VS posterior uncertainties. VS models are described by a self-adapting and parsimonious parameterization using cubic Bézier splines. 1268 separately inverted 1-D VS profiles are combined together using a regionalization scheme, to build the 3-D VS model with a lateral resolution of 75 km over western France. The shallower part of the model (horizontal cross-section at 4 km depth) correlates well with the known main geological features. The crystalline Variscan basement is clearly associated with positive VS perturbations while negative heterogeneities match the Mesocenozoic sedimentary basins. At greater depths, the Bay of Biscay exhibits positive VS perturbations,which eastern and southern boundaries can be interpreted as the ocean-continent transition. The overall crustal structure below the Armorican Massif appears to be heterogenous at the subregional scale, and tends to support that both the South-Armorican Shear Zone and the Paris Basin Magnetic Anomaly are major crustal discontinuities that separate distinct domains.

Ambient noise correlation analysis of S-net records: extracting surface wave signals below instrument noise levels

SUMMARY We applied ambient noise cross-correlation analysis to the cabled ocean bottom seismic network offshore northeast Japan (Seafloor observation network for earthquakes and tsunamis along the Japan Trench: S-net) to extract surface waves propagating in the ocean area of the forearc region. We found two types of peculiar pulses in the cross-correlation functions (CCFs) of ambient seismic noise records: periodic pulses mainly every minute and sharp pulses around the lag time zero. These pulses strongly contaminate the surface wave signals in the CCFs at frequencies below ∼0.1 Hz. The periodic pulses originate from periodic instrument noises, while the zero-lag pulses originate from random instrument noises which are coherent within station pairs. By developing solutions to remove the periodic and zero-lag pulses based on the characteristics of the pulses, we succeeded in extracting Rayleigh and Love wave signals from the S-net records at 0.03–0.3 Hz, while the surface wave signals at 0.03–0.1 Hz were not visible without the application of these solutions. These solutions widen the frequency range of analysis, and may be applicable to other seismic networks, particularly to recent dense but non-broad-band networks. We identified the fundamental and first higher modes of Rayleigh waves and the fundamental mode of the Love wave. The extracted surface wave signals can constrain the shear wave velocity structure from the sediment to seismogenic zone around the megathrust plate boundary in the forearc region.

Crustal structures beneath the Eastern and Southern Alps from ambient noise tomography

Abstract. We study the crustal structure under the Eastern and Southern Alps using ambient noise tomography. We use cross-correlations of ambient seismic noise between pairs of 71 permanent stations and 19 stations of the Eastern Alpine Seismic Investigation (EASI) profile to derive new 3D shear velocity models for the crust. Continuous records from 2014 and 2015 are cross-correlated to estimate Green's functions of Rayleigh and Love waves propagating between the station pairs. Group velocities extracted from the cross-correlations are inverted to obtain isotropic 3D Rayleigh- and Love-wave shear-wave velocity models. Our models image several velocity anomalies and contrasts and reveal details of the crustal structure. Velocity variations at short periods correlate very closely with the lithologies of tectonic units at the surface and projected to depth. Low-velocity zones, associated with the Po and Molasse sedimentary basins, are imaged well to the south and north of the Alps, respectively. We find large high-velocity zones associated with the crystalline basement that forms the core of the Tauern Window. Small-scale velocity anomalies are also aligned with geological units of the Austroalpine nappes. Clear velocity contrasts in the Tauern Window along vertical cross sections of the velocity model show the depth extent of the tectonic units and their bounding faults. A mid-crustal velocity contrast is interpreted as a manifestation of intracrustal decoupling in the Eastern Alps that accommodated eastward escape of the Alcapa block.

Systematic recovery of instrumental timing and phase errors using interferometric surface-waves retrieved from large-N seismic arrays

SUMMARYInstrumental timing and phase errors are a notorious problem in seismic data acquisition and processing. These can be frequency independent, for example due to clock drift, but may also be frequency dependent, for example due to imperfectly known instrument responses. A technique is presented that allows both types of errors to be recovered in a systematic fashion. The methodology relies on the time-symmetry usually inherent in time-averaged cross-correlations of ambient seismic noise: the difference between the arrival time of the direct surface-wave at positive time and the arrival time of the direct surface-wave at negative time is quantified. Doing this for all eligible receiver–receiver pairs of a large-N seismic array, including one or more receivers devoid of instrumental timing errors, the instrumental timing errors of all incorrectly timed receivers can be determined uniquely. Most notably, this is accomplished by means of a weighted least-squares inversion. The weights are based on the receiver–receiver distances and decrease the adverse effect of inhomogeneities in the noise illumination pattern on the recovered instrumental timing errors. Inversion results are furthermore optimized by limiting the inversion to receiver couples that (i) exceed a specific receiver–receiver distance threshold and (ii) whose time-averaged cross-correlations exceed a specific signal-to-noise ratio threshold. Potential frequency dependence of the timing errors is incorporated by means of an iterative, frequency-dependent approach. The proposed methodology is validated using synthetic recordings of ambient seismic surface-wave noise due to an arbitrary non-uniform illumination pattern. The methodology is successfully applied to time-averaged cross-correlations of field recordings of ambient seismic noise on and around the Reykjanes peninsula, SW Iceland.

Normal modes separation by use of ambient noise cross-correlation function in shallow sea

The possibility to extract the time-domain Green's function from ambient noise cross-correlation function was demonstrated by a number of authors. This approach has a weak point: the accumulation time of the noise signal. The accumulation time is significantly reduced when using mode signals and separating normal modes. Both numerical study of the spectrogram of the noise cross-correlation function and experiment in Arctic shelf demonstrated that dispersion curves observed in the spectrogram in the case of large distances between hydrophones, with a decrease in this distance are transformed into separate maxima localized both in frequency and in time delay. These maxima arise due to the presence of a stationary phase point near the minimum of the group velocities of the modes, and also due to modes interference, which is more efficient at small distances between hydrophones. At small distances, working with a signal in the frequency band where only the lowest hydroacoustic mode is excited has a number of advantages, the main of which is a decrease in the accumulation time. We demonstrate the change in the regime of modes separation with a change in the distance between hydrophones and discuss the optimal conditions for the applicability of the method.

Peculiarities of the Green's function reconstruction from the ambient noise cross-correlation function based on the mode approach

Peculiarities of the Green's function reconstruction from the ambient noise cross-correlation function based on the mode approach

SeisNoise.jl: Ambient Seismic Noise Cross Correlation on the CPU and GPU in Julia

AbstractWe introduce SeisNoise.jl, a library for high-performance ambient seismic noise cross correlation, written entirely in the computing language Julia. Julia is a new language, with syntax and a learning curve similar to MATLAB (see Data and Resources), R, or Python and performance close to Fortran or C. SeisNoise.jl is compatible with high-performance computing resources, using both the central processing unit and the graphic processing unit. SeisNoise.jl is a modular toolbox, giving researchers common tools and data structures to design custom ambient seismic cross-correlation workflows in Julia.

Time‐Lapse Monitoring of Seismic Velocity Associated With 2011 Shinmoe‐Dake Eruption Using Seismic Interferometry: An Extended Kalman Filter Approach

AbstractSeismic interferometry is a powerful tool to monitor the seismic velocity change associated with volcanic eruptions. For the monitoring, changes in seismic velocity with environmental origins (such as precipitation) are problematic. In order to model the environmental effects, we propose a new technique based on a state‐space model. An extended Kalman filter estimates seismic velocity changes as state variables, with a first‐order approximation of the stretching method. We apply this technique to three‐component seismic records in order to detect the seismic velocity change associated with the Shinmoe‐dake eruptions in 2011 and 2018. First, ambient noise cross correlations were calculated from May 2010 to April 2018. We also modeled seismic velocity changes resulting from precipitation and the 2016 Kumamoto earthquake, with exponential type responses. Most of the results show no significant changes associated with the eruptions, although gradual inflation of the magma reservoir preceded the 2011 eruption by 1 year. The observed low sensitivity to static stress changes suggests that the fraction of geofluid and crack density at about 1‐km depth is small, and the crack shapes could be circular. Only one station pair west of the crater shows the significant drop associated with the eruption in 2011. The gradual drop of seismic velocity up to 0.05% preceded the eruption by 1 month. When the gradual drop began, volcanic tremors were activated at about 2‐km depth. These observations suggest that the drop could be caused by damage accumulation due to vertical magma migration beneath the summit.

Upper Mantle Shear Wave Velocity Structure of Southeastern China: Seismic Evidence for Magma Activities in the Late Mesozoic to the Cenozoic

AbstractThe Fujian‐Taiwan Strait‐Taiwan region, as the active eastern margin of Eurasia since the Mesozoic, has experienced the subduction and retreat of the Paleo‐Pacific plate from the Middle Mesozoic, the breakup of the continental margin in the early Cenozoic, and then the collision with the Philippine Sea plate in the late Cenozoic. To better understand the tectonic evolution process, a high‐resolution 3‐D shear wave velocity model of the crust and upper mantle was obtained from surface wave tomography through a direct inversion method. Intermediate‐ to long‐period Rayleigh wave dispersion curves, extracted from teleseismic surface wave with the traditional two‐station method, were combined with the short‐period dispersion curves obtained from ambient noise cross correlation to obtain broader band surface wave data. Two groups of large‐scale low‐velocity bodies are found to be located in the upper mantle of Fujian and the Taiwan Strait and are inferred to be closely related to the magmatic activities caused by the Mesozoic subduction and Cenozoic extension, respectively. The low‐velocity anomalies in Fujian are concentrated in the west Cathaysia block in deep regions and split into small bodies in the uppermost mantle, while the low‐velocity body in the strait is divided into northern and southern parts, corresponding to the two‐stage Cenozoic extension. The velocity gradient was also calculated to estimate the lithospheric thickness, which is only 60–80 km, and the lithosphere‐asthenosphere boundary also features strongly horizontal variations and discontinuities. Our results provide important seismological constraints to investigate the geological and tectonic processes of this region.

Ambient-Noise Tomography of the Baiyun Gold Deposit in Liaoning, China

Abstract Ambient-noise tomography (ANT) has become an effective method for determining the fine velocity structure of the shallow crust. However, studies on metal mines using this method are rarely reported. To investigate the tectonic background and prospecting of the deep mine in the Baiyun gold deposit (BYGD) of eastern Liaoning Province, China, we use ANT to determine a 3D S-wave velocity structure model of the BYGD. A total of 21 broadband seismic stations were installed in an area of 15×14 km, centered at the BYGD. Continuous observations for approximately three months were made. After single-station preprocessing, cross correlation of ambient noise, and phase-weighted stacking, the empirical Green’s function for the Rayleigh waves between stations was recovered. Next, group-velocity dispersion with 0.8–3 s periods was measured. A direct inversion method of surface-wave dispersion based on raytracing was then adopted to determine a 3D S-wave velocity structure of the BYGD from the ground surface to a depth of 1.8 km. The results show that the distribution of S-wave velocities in the study area well reflected the geological characteristics of the surface. The velocities were significantly low within the “ore field” and the regional ore-controlling Jianshanzi fault. Combining this with the fact that a large number of magmatic veins were visible inside both structures, it was deduced that both structures had experienced large-scale magmatic intrusion activities, thus confirming that BYGD was a magmatic hydrothermal deposit. The significantly low S-wave velocities beneath the gold deposit extended to a depth of 1.8 km. This might imply the occurrence of blind ore bodies at that depth. The fine velocity structure of the BYGD reconstructed by this study provided a direction for subsequent prospecting of deep regions and demonstrated that ANT has good potential in metal mine exploration.

Passive geoacoustic inversion in the Mid-Atlantic Bight in the presence of strong water column variability.

Empirical Green's functions are obtained for 31 paths in a highly dynamic coastal ocean by cross-correlation of ambient and shipping noise recorded in the Shallow Water 2006 experiment on a horizontal line array and a single hydrophone about 3600 m from the array. Using time warping, group speeds of three low-order normal modes are passively measured in the 10-110 Hz frequency band and inverted for geoacoustic parameters of the seabed. It is demonstrated that, despite very strong sound speed variations caused by nonlinear internal waves, noise interferometry can be successfully used to acoustically characterize the seafloor on a continental shelf.