Noise Cross-correlation Functions Research Articles

The Use of Low-Frequency Noise in Passive Seismoacoustic Tomography of the Ocean Floor

The authors have investigated the possibility of using the background seismoacoustic noise generated Scholte-type surface waves on the ocean floor in order to study the structure of the geophysical medium using passive seismoacoustic tomography. The paper presents the results of reconstructing the deep structure of the Hawaiian Islands region by processing experimental data obtained by a network of broadband ocean bottom seismographs. The solution to the inverse problem consists of two stages. First, the dispersion relations of the phase and group velocities of surface waves are estimated by analyzing the cross-correlation function of the background noise between different pairs of bottom geophones. At the second stage, the problem of inverting the estimated dispersion curves into a vertically layered elastic model of the lithosphere is solved. Comparison of the inversion results with the known velocity models of the Hawaiian Islands region demonstrates the reliability of the estimates obtained with this approach. The results also indicate the possibility of reducing the time of data acquisition in a field experiment compared to methods that use earthquake signals, which involve sounding the studied area from different directions.

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.

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.

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

Карты распределения скоростей волн Релея на территории Кавказа по данным кавказской сети CNET

Ambient noise surface wave tomography is a widely used method for determining the velocity structure of the upper layers of the Earth. It is based on the fact that the cross-correlation function (CCF) of noise at two stations, averaged over a long time interval, determines the Green's function of the surface wave. This allows us to estimate the group and phase velocities of surface waves on the paths between stations. The method was applied to the records of the vertical components of 67 seismic stations of the Caucasian network CNET network, which were obtained during 2018. The cross-correlation functions for all interstation paths were calculated. The dispersion curves of group and phase Rayleigh velocities for periods from 5 to 30 s were obtained by means of frequency-time analysis. The lateral distribution of the velocities was received for periods from 7 to 22 s, which correlate with velocity structure at depths of 5-25 km. The group and phase velocity maps for Rayleigh wave for periods 7, 10, 12, 14, 17, 22 s are presented.

Observation of a New Long‐Period (16‐s) Persistent Tremor Originating in the Gulf of Guinea

AbstractA new persistent microseismic tremor, with a narrowband period centered near 16 s, was discovered in the primary microseism spectral peak by analyzing seismic waveforms recorded by African, European, and American seismometers. This tremor may generate coherent arrivals in noise cross‐correlation functions (NCFs), which could potentially bias ambient noise tomography. Using an envelope‐stacking method, we discovered that the 16‐s tremor source is in the coastal region of the Gulf of Guinea and is probably isolated from the previously reported 26‐s tremor source. The nature of the newly identified 16‐s tremor, like the 26‐ and 27‐s tremors, is similar to a volcanic tremor radiated by the Mayotte volcanic system, thereby suggesting that the excitation of all the Gulf of Guinea tremors may be related to volcanic activities. Thus, this study implies that there may still be active volcanoes in the gulf.

Horizontally orthogonal distributed acoustic sensing array for earthquake- and ambient-noise-based multichannel analysis of surface waves

SUMMARY A 2-D orthogonal distributed acoustic sensing (DAS) array designed for seismic experiments was buried horizontally beneath the Kafadar Commons Geophysical Laboratory on the Colorado School of Mines campus at Golden, Colorado. The DAS system using straight fibre-optic cables is a cost-efficient technology that enables dense seismic array deployment for long-term seismic monitoring, favouring both earthquake-based and ambient-noise-based surface wave analysis for subsurface characterization. In our study, the horizontally orthogonal DAS array records ambient noise data for a period of about two months from November 2018 to January 2019. During this time, the array also detected seismic signals from an ML3.6 earthquake at Glenwood Springs, Colorado, which exhibit opposite signal polarities in the orthogonal DAS section recordings. We derive the transformation matrix for DAS strain measurements in horizontally orthogonal cables to retrieve both Rayleigh and Love wave dispersion information from the single-component DAS signals using the 2-D multichannel analysis of surface waves method. In addition, ambient noise interferometry is applied to long-term DAS noise recordings. Our theoretical derivation demonstrates that Rayleigh and Love wave Green's functions are coupled in the noise cross-correlation functions (NCFs) of DAS receiver pairs. Stacking NCFs over the horizontally orthogonal DAS array can constructively recover the radial Rayleigh wave component but destructively suppress the Love wave component. The multimodal Monte Carlo inversion of the earthquake-based Rayleigh wave and Love wave dispersion measurements and the noise-based Rayleigh wave measurement reveals a 1-D layered structure that agrees qualitatively with geological surveys of the site. Our study demonstrates that although straight fibre-optic cables lack broadside sensitivity, using appropriate DAS array configuration and seismic array methods can extend the seismic acquisition ability of DAS and enable its application to a broad range of scenarios.

When Clocks Are Not Working: OBS Time Correction

AbstractOcean-bottom seismographs (OBSs) are used to obtain seismic recordings offshore and are an increasingly important tool for investigating the globe. However, because OBS data cannot be time stamped using Global Positioning System (GPS) during deployment, correction for drift of the internal clock is required. This time drift is typically derived by synchronizing the clock before and after deployment. Linear correction is then applied using the timing deviation between GPS and the instrument’s internal clock at recovery, that is, the skew measurement. If synchronization measurements are missing, ambient noise cross-correlation functions (CCFs) are commonly used for time correction. When investigating recordings from a small-scale OBS network located on the Mohn’s mid-ocean ridge, we observed a remaining drift on the skew-corrected data. After recalculating the drift of the raw data using CCFs, we found that the skew-based time correction was incorrect. This was also verified with the observation of teleseismic P-wave arrivals. We describe a method to obtain properly time-corrected data and discuss the OBS timing issues in detail. The results shown were obtained using a software package that we developed for this specific purpose and made available as open-source software. Although we cannot explain the technical reason for the failure of skew correction, this study shows that skew corrections should not be trusted alone, and OBS timing should always be verified by either ambient noise correlations or P-wave arrival times.

Crustal Structure of Sri Lanka Derived From Joint Inversion of Surface Wave Dispersion and Receiver Functions Using a Bayesian Approach

AbstractWe study the crustal structure of Sri Lanka by analyzing data from a temporary seismic network deployed in 2016–2017 to shed light on the amalgamation process from a geophysical perspective. Rayleigh wave phase dispersion curves from ambient noise cross correlation and receiver functions were jointly inverted using a transdimensional Bayesian approach. The Moho depths in Sri Lanka range between 30 and 40 km, with the thickest crust (38–40 km) beneath the central Highland Complex (HC). The thinnest crust (30–35 km) is found along the west coast, which experienced crustal thinning associated with the formation of the Mannar Basin. VP/VS ratios lie within a range of 1.60–1.82 and predominantly favor a felsic to intermediate bulk crustal composition with a significant silica content of the rocks. A major intracrustal (18–27 km), slightly westward dipping (∼4.3°) interface with high VS (∼4 km/s) underneath is prominent in the central HC, continuing into the western Vijayan Complex (VC). The discontinuity might have been part of the respective units prior to the collision and could be an indicator for the proposed tilting of the Wanni Complex/HC crustal sections. It might also be related to the deep crustal HC/VC thrust contact with the VC as an indenting promontory of high VS. A low‐velocity zone in the central HC could have been caused by fluid influx generated by the thrusting process.

High-resolution 3-D crustal shear-wave velocity model reveals structural and seismicity segmentation of the central-southern Tanlu Fault zone, eastern China

The Early-Mesozoic collision between the North and South China Cratons resulted in different tectonic and geologic structures including the Tanlu Fault zone. One way to understand the geodynamic processes beneath these structures is by studying the velocity structures and spatial distribution of earthquakes in the region. We performed ambient noise tomography for the region around the central-southern Tanlu Fault zone using the vertical component of continuous ambient noise data obtained in central East China. Empirical Green's functions were retrieved from ambient noise cross-correlation functions, from which we extracted the Rayleigh wave phase velocity dispersion curves and performed quality control on the dispersion data. The direct surface wave tomography method was used to invert all phase velocity dispersion data for the 3-D shear-wave velocity model in the crust and uppermost mantle. Our results show velocity segmentations with high-velocity structures existing beneath orogenic belts and uplifts within the first 10 km depths which correspond to the High-Pressure/Ultra-High Pressure Metamorphic rocks in the area. The existence of high-velocity structures beneath orogenic belts at shallow depths implies that the exhumation of eclogites is confined to the uppermost crust. Generally, fault zones in this area are characterized by a low velocity from 0 to 5 km and relatively high velocity in the lower part of the upper crust and the middle crust. The spatial distribution of earthquakes along Tanlu Fault reveals two seismicity zones: the southern segment and the Suqian-Tancheng segment with a low-velocity transitional zone at Suqian, which suggests that the two segments respond differently to stresses. Regions of dense earthquake distribution correspond to the transition zones of high to low shear-wave velocities, which could be as a result of instability in the upper mantle and variation in compositions of crustal materials.

Using Spaced Combined Receiving Modules to Study the Scalar–Vector Characteristics of an Acoustic Field

Field experiments to measure the scalar–vector structure of an acoustic field at a Moscow State University hydroacoustic test site with an ice cover are analyzed. The anisotropy of the noise field in the water is estimated. Noise cross-correlation functions are constructed using the directional characteristics of two spaced combined receiving modules.

Studying the Phase of the Ambient Noise Cross-Correlation Function of an Oceanic Waveguide Field

Advantages and limitations of a new way of estimating the cutoff frequencies of hydroacoustic modes using data on the phase of the cross-correlation function of a shallow sea noise field are considered. The results from numerical modeling are compared to experimental data recorded in the Barents Sea.

Cross-correlation sensitivity kernels with respect to noise source distribution.

The cross correlation of the underwater noise field recorded at two receivers conveys information about the time-domain Green's function between the two locations, provided that sufficient energy is channeled into the acoustic paths connecting these. The efficiency of this procedure depends on the locations and characteristics of the receivers and noise sources, as well as on the refraction properties of the ocean sound channel. The sensitivity of the finite-frequency noise cross-correlation function with respect to the location and amplitude of the noise sources is studied here, taking into account the refractive features of the ocean environment. The sensitivity kernel describing changes in the cross-correlation envelope due to changes in the noise source distribution is used to highlight noise-source locations with maximum potential impact on the cross-correlation output.

Exploring source regions of single- and double-frequency microseisms recorded in eastern North American margin (ENAM) by cross-correlation

Summary Strongly asymmetric cross-correlation functions (CCF) of ambient noise indicate an inhomogeneous distribution of the noise sources, and can be used to locate these sources. With this premise, a grid-search procedure is applied to explore the frequency dependent source locations of the single- (SF) and double-frequency (DF) microseisms recorded in eastern North American margin (ENAM). The frequency dependent Rayleigh wave group velocities are determined based on CRUST1.0 model and the Preliminary Reference Earth model and then the theoretical travel time differences from each hypothetical source to all station pairs are calculated by the fast marching method. Then a misfit function at each hypothetical source is defined to take the difference between the calculated travel time differences and the time lags observed from CCFs. A factor of reliability is defined considering the Rayleigh wave attenuation, the correlation coefficients between the microseisms and the Wavewatch III hindcasts of ocean wave spectra, and the efficiency of conversion from DF energy in water to seismic Rayleigh waves as a function of water depth and frequency. Together with the correlation analyses of the power spectra densities of the microseisms among stations, the primary source regions of SF and DF microseisms recorded in ENAM are estimated: 1) the sources of SF microseisms are likely distributed in the continental shelf dominantly as well as adjacent deep ocean area in western North Atlantic Ocean, and 2) the long-period and short-period DF microseisms appear generated in deep ocean near the continental slope and in the continental shelf and slope of ENAM, respectively.

Characterizing the seabed in the Straits of Florida by using acoustic noise interferometry and time warping.

Interferometry of ambient and shipping noise in the ocean provides a way to estimate physical parameters of the seafloor and the water column in an environmentally friendly manner without employing any controlled sound sources. With noise interferometry, two-point cross-correlation functions of noise serve as the probing signals and replace the Green's function measured in active acoustic remote sensing. The amount of environmental information that can be obtained with passive remote sensing and the robustness of the estimates of the seafloor parameters increase when contributions of individual normal modes are resolved in the noise cross-correlation function. Using the data obtained in the 2012 noise-interferometry experiment in the Straits of Florida, dispersion curves of the first four normal modes are obtained in this paper by application of the time-warping transform to noise cross correlations. The passively measured dispersion curves are inverted for unknown geoacoustic properties of the seabed. Resulting thickness of the sediment layer and sound speed are consistent with the geoacoustic models obtained earlier by other means.

Source kernels for noise cross-correlation

A wave-theoretic tool, the source kernel, is introduced to describe the sensitivity of the finite-frequency underwater noise cross-correlation function to the location of the noise sources taking into account the refractive features of the ocean environment. The cross correlation of the noise field at two receivers conveys information about the corresponding time-domain Green’s function (TDGF), provided that sufficient energy is channeled into the acoustic paths connecting the two locations. The efficiency of the TDGF recovery is determined by the receiver and noise source locations and characteristics, as well as by the refraction properties of the ocean sound channel. The source kernel takes these characteristics into account and highlights the locations where noise sources can have maximum impact on the cross-correlation output. The distribution and coherence of the noise sources can be specified in a variety of ways to investigate a wide range of scenarios, from random noise-source distributions to coherent radiators. [Work supported by the Office of Naval Research.]

Using ambient noise to self-localize and time-synchronize independent sensors for localizing gray whales acoustic activity in Laguna San Ignacio, Mexico

Beamforming requires array elements whose data is time-synchronized and spacing is known. This typically involves either fixed sensors running off a central acquisition system, or additional equipment, such as pingers or other calibration sources. It has been previously demonstrated that for a diffuse noise field, the time-averaged ambient noise cross-correlation function between array elements can be used to infer both their spacing and relative clock-offset (Sabra et al., 2005). This allows beamforming across fully independent instruments (i.e., from different acquisition systems), whose relative positions are not precisely known, or may vary. Here, we apply this technique to pairs of bottom-mounted recorders deployed between 5 and 15 meters depth with approximately 15 m spacing. The data was collected in Laguna San Ignacio (Mexico) with the goal of localizing gray whale acoustic activity. Results show that even if the noise field is not strictly diffuse nor azimuthally symmetric, the structure of the time-averaged ambient noise cross-correlation function still allows correcting relative clock-offset and drift throughout the deployment. K. G. Sabra, P. Roux, A. M. Thode, G. L. D'Spain, W. S. Hodgkiss, and W. A. Kuperman, “Using ocean ambient noise for array self-localization and self-synchronization,” IEEE J. Oceanic Eng. 30,338–347 (2005).Beamforming requires array elements whose data is time-synchronized and spacing is known. This typically involves either fixed sensors running off a central acquisition system, or additional equipment, such as pingers or other calibration sources. It has been previously demonstrated that for a diffuse noise field, the time-averaged ambient noise cross-correlation function between array elements can be used to infer both their spacing and relative clock-offset (Sabra et al., 2005). This allows beamforming across fully independent instruments (i.e., from different acquisition systems), whose relative positions are not precisely known, or may vary. Here, we apply this technique to pairs of bottom-mounted recorders deployed between 5 and 15 meters depth with approximately 15 m spacing. The data was collected in Laguna San Ignacio (Mexico) with the goal of localizing gray whale acoustic activity. Results show that even if the noise field is not strictly diffuse nor azimuthally symmetric, the structure of the t...

Amplitude distortion of measured leak noise signals caused by instrumentation: Effects on leak detection in water pipes using the cross-correlation method

A common way to detect and locate leaks in buried water pipes is to use leak noise correlators. Vibration or acoustic signals are measured on or in the pipe using sensors placed either side of the leak, and the difference in the leak noise arrival times (time delay) at the sensors is estimated from the peak in the cross-correlation function of these signals. Over many years, much effort has been spent on improving the quality of the leak noise signals with the aim of improving the time delay estimate. In this paper it is shown that even if the signals suffer from severe amplitude distortion through either clipping or quantization, then an accurate time delay estimate can be obtained provided that the zero crossings in the noise data are preserved. This is demonstrated by using polarity co-incidence correlation on simulated and measured data. The use of random telegraph theory is also used as an approximation to allow the derivation of approximate analytical solutions for the cross-correlation function and cross spectral density of clipped noise to facilitate further insight into the effects of severe clipping.

Rayleigh-wave multicomponent cross-correlation-based source strength distribution inversion. Part 1: Theory and numerical examples

Cross-correlation-based seismic interferometry is commonly used to retrieve surface-wave Green’s functions from ambient seismic noise recordings. This approach requires that seismic sources are isotropically distributed in all directions around two receivers. However, this assumption is rarely valid in practice. Thus full-waveform inversion theory has recently been applied to seismic noise cross-correlation functions, functions that include both source and structure information. Source information (e.g. location or strength) is essential for accurate structure information estimation. In this paper, we explain physically two types of source sensitivity kernels: one derived from traveltime misfits and the other derived from waveform misfits. We use these kernels for source inversion and demonstrate the benefits of using multicomponent cross-correlations in this source estimation process.