Ambient Noise Cross-correlation Research Articles

Using multicomponent ambient seismic noise cross-correlations to identify higher mode Rayleigh waves and improve dispersion measurements

SUMMARYAmbient seismic noise cross-correlation with three-component sensors yields a nine-component empirical Green's tensor, in which four components of the radial–vertical plane contain Rayleigh waves. We exploit the retrograde elliptical nature of particle motion of the fundamental mode Rayleigh wave to correct the phase of the four radial–vertical components and stack them to obtain an average fundamental mode Rayleigh-wave time-series. This technique can suppress incoherent noise and wave packets that do not follow the targeted elliptical particle motion. The same technique can be used to isolate the first higher mode Rayleigh wave that follows prograde elliptical particle motion. We first demonstrate the effectiveness of the method on synthetic waveforms and then apply it on noise cross-correlations computed in Central California. Using this method, we isolate 1st higher mode Rayleigh waves on noise cross-correlations in the Great Valley, California, which provides new phase velocity constraints for estimating velocity structure in the sedimentary basin. We also obtain improved estimates of fundamental mode Rayleigh-wave dispersion for surface-wave tomography. The waveforms stacked assuming retrograde particle motion return at least ∼20 per cent more group velocity dispersion measurements satisfying a minimum signal-to-noise ratio (SNR) criterion than the individual components for periods ∼4–18 s. For equivalent group velocity measurements, SNR for the stacked estimate of the fundamental mode Rayleigh wave is on average 40 per cent greater than that measured on the individual components at periods less than 10 s. The technique also provides an easy way to detect large errors in sensor orientation.

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.

Structure of the Ecuadorian forearc from the joint inversion of receiver functions and ambient noise surface waves

SUMMARYThe Ecuadorian forearc is a complex region of accreted terranes with a history of large megathrust earthquakes. Most recently, a Mw 7.8 megathrust earthquake ruptured the plate boundary offshore of Pedernales, Ecuador on 16 April 2016. Following this event, an international collaboration arranged by the Instituto Geofisico at the Escuela Politécnica Nacional mobilized a rapid deployment of 65 seismic instruments along the Ecuadorian forearc. We combine this new seismic data set with 14 permanent stations from the Ecuadorian national network to better understand how variations in crustal structure relate to regional seismic hazards along the margin. Here, we present receiver function adaptive common conversion point stacks and a shear velocity model derived from the joint inversion of receiver functions and surface wave dispersion data obtained through ambient noise cross-correlations for the upper 50 km of the forearc. Beneath the forearc crust, we observe an eastward dipping slow velocity anomaly we interpret as subducting oceanic crust, which shallows near the projected centre of the subducting Carnegie Ridge. We also observe a strong shallow positive conversion in the Ecuadorian forearc near the Borbon Basin indicating a major discontinuity at a depth of ∼7 km. This conversion is not ubiquitous and may be the top of the accreted terranes. We also observe significant north–south changes in shear wave velocity. The velocity changes indicate variations in the accreted terranes and may indicate an increased amount of hydration beneath the Manabí Basin. This change in structure also correlates geographically with the southern rupture limit of multiple high magnitude megathrust earthquakes. The earthquake record along the Ecuadorian trench shows that no event with a Mw >7.4 has ruptured south of ∼0.5°S in southern Ecuador or northern Peru. Our observations, along with previous studies, suggest that variations in the forearc crustal structure and subducting oceanic crust may influance the occurrence and spatial distribution of high magnitude seismicity in the region.

Improving cross-correlations of ambient noise using an rms-ratio selection stacking method

SUMMARY Stacking of ambient noise correlations is a crucial step to extract empirical Green's functions (EGFs) between station pairs. The traditional method is to linearly stack all short-duration cross-correlation functions (CCFs) over a long period of time to obtain final stacks. It requires at least several months of ambient noise data to obtain reliable phase velocities at periods of several to tens of seconds from CCFs. In this study, we develop a new stacking method named root-mean-square ratio selection stacking (RMSR_SS) to reduce the time duration required for the recovery of EGFs from ambient noise. In our RMSR_SS method, rather than stacking all short-duration CCFs, we first judge if each of the short-duration CCF constructively contributes to the recovery of EGFs or not. Then, we only stack those CCFs which constructively contribute to the convergence of EGFs. By applying our method to synthetic noise data, we demonstrate how our method works in enhancing the signal-to-noise ratio of CCFs by rejecting noise sources which do not positively contribute to the recovery of EGFs. Then, we apply our method to real noise data recorded in western USA. We show that reliable and accurate phase velocities can be measured from 15-d long ambient noise data using our RMSR_SS method. By applying our method to ambient noise tomography (ANT), we can reduce the deployment duration of seismic stations from several months or years to a few tens of days, significantly improving the efficiency of ANT in imaging crust and upper-mantle structures.

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.

Finite‐Frequency Sensitivity Kernels for Seismic Noise Interferometry Based on Differential Time Measurements

AbstractFull waveform adjoint tomography has achieved great success in applications from global Earth structure using earthquakes to exploration seismology using active sources. When combined with ambient seismic noise data, however, the ambient noise cross correlations are subject to strong variability and bias due to nonstationary and unevenly distributed noise sources. As a result, the shape of the structure sensitivity kernel can deviate significantly from the classic banana‐doughnut kernel of a point source, which has been used in previous studies as an inference from empirical Green's function and contains bias. In this study I calculate the sensitivity kernel for ambient noise cross correlation by introducing an additional station to the classic two‐station setting. I compute sensitivity kernels for differential traveltime measurements. These differential sensitivity kernels show promise for canceling the overlapping part of the original source and structure kernels for pairs of stations in interferometry, thus significantly reducing the effect of nonisotropically distributed and nonstationary noise sources. I apply the ambient noise differential sensitivity kernel to synthetic data examples based on 2‐D membrane waves, though the approach will apply for 3‐D as well. Our results for multiple station pairs show promise for velocity tomography based on seismic noise interferometry when perfect information on noise source distribution is not available.

Evaluating Uncertainties of Phase Velocity Measurements from Cross-Correlations of Ambient Seismic Noise

Abstract Ambient-noise tomography (ANT) has become a well-established method to image the crust and uppermost mantle structures in the past 15 yr. Having a good estimate of uncertainties of phase velocity dispersion measurements in ANT is critical as they can guide the level of data fitting in tomography. However, to date, there are still no systemic studies to evaluate these uncertainties. In this study, we obtain cross correlations with different stacking durations from 17 yr of ambient-noise data recorded at 120 stations in the United States. We analyze the variations of signal-to-noise ratio (SNR) and phase velocities of cross correlations. We find that the uncertainties of phase velocities are affected by SNRs, interstation distances, and stacking durations. However, none of those three variables can be solely used as a proxy to estimate the uncertainties of phase velocity measurements. Based on our analysis, we graphically present empirical relations of uncertainties of phase velocity measurements as a function of SNR, interstation distance, and stacking duration. These relations can be employed as a guide to estimate phase velocity uncertainties in applications of ANT, assisting in evaluating the reliability of resulting models from ANT.

Crustal and uppermost mantle shear wave velocity structure beneath the Middle East from surface wave tomography

SUMMARY We have constructed a 3-D shear wave velocity (Vs) model for the crust and uppermost mantle beneath the Middle East using Rayleigh wave records obtained from ambient-noise cross-correlations and regional earthquakes. We combined one decade of data collected from 852 permanent and temporary broad-band stations in the region to calculate group-velocity dispersion curves. A compilation of >54 000 ray paths provides reliable group-velocity measurements for periods between 2 and 150 s. Path-averaged group velocities calculated at different periods were inverted for 2-D group-velocity maps. To overcome the problem of heterogeneous ray coverage, we used an adaptive grid parametrization for the group-velocity tomographic inversion. We then sample the period-dependent group-velocity field at each cell of a predefined grid to generate 1-D group-velocity dispersion curves, which are subsequently inverted for 1-D Vs models beneath each cell and combined to approximate the 3-D Vs structure of the area. The Vs model shows low velocities at shallow depths (5–10 km) beneath the Mesopotamian foredeep, South Caspian Basin, eastern Mediterranean and the Black Sea, in coincidence with deep sedimentary basins. Shallow high-velocity anomalies are observed in regions such as the Arabian Shield, Anatolian Plateau and Central Iran, which are dominated by widespread magmatic exposures. In the 10–20 km depth range, we find evidence for a band of high velocities (>4.0 km s–1) along the southern Red Sea and Arabian Shield, indicating the presence of upper mantle rocks. Our 3-D velocity model exhibits high velocities in the depth range of 30–50 km beneath western Arabia, eastern Mediterranean, Central Iranian Block, South Caspian Basin and the Black Sea, possibly indicating a relatively thin crust. In contrast, the Zagros mountain range, the Sanandaj-Sirjan metamorphic zone in western central Iran, the easternmost Anatolian plateau and Lesser Caucasus are characterized by low velocities at these depths. Some of these anomalies may be related to thick crustal roots that support the high topography of these regions. In the upper mantle depth range, high-velocity anomalies are obtained beneath the Arabian Platform, southern Zagros, Persian Gulf and the eastern Mediterranean, in contrast to low velocities beneath the Red Sea, Arabian Shield, Afar depression, eastern Turkey and Lut Block in eastern Iran. Our Vs model may be used as a new reference crustal model for the Middle East in a broad range of future studies.

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.

The 2018 Geothermal Reservoir Stimulation in Espoo/Helsinki, Southern Finland: Seismic Network Anatomy and Data Features

AbstractA seismic network was installed in Helsinki, Finland to monitor the response to an ∼6-kilometer-deep geothermal stimulation experiment in 2018. We present initial results of multiple induced earthquake seismogram and ambient wavefield analyses. The used data are from parts of the borehole network deployed by the operating St1 Deep Heat Company, from surface broadband sensors and 100 geophones installed by the Institute of Seismology, University of Helsinki, and from Finnish National Seismic Network stations. Records collected in the urban environment contain many signals associated with anthropogenic activity. This results in time- and frequency-dependent variations of the signal-to-noise ratio of earthquake records from a 260-meter-deep borehole sensor compared to the combined signals of 24 collocated surface array sensors. Manual relocations of ∼500 events indicate three distinct zones of induced earthquake activity that are consistent with the three clusters of seismicity identified by the company. The fault-plane solutions of 14 selected ML 0.6–1.8 events indicate a dominant reverse-faulting style, and the associated SH radiation patterns appear to control the first-order features of the macroseismic report distribution. Beamforming of earthquake data from six arrays suggests heterogeneous medium properties, in particular between the injection site and two arrays to the west and southwest. Ambient-noise cross-correlation functions reconstruct regional surface-wave propagation and path-dependent body-wave propagation. A 1D inversion of the weakly dispersive surface waves reveals average shear-wave velocities around 3.3 km/s below 20 m depth. Consistent features observed in relative velocity change time series and in temporal variations of a proxy for wavefield partitioning likely reflect the medium response to the stimulation. The resolution properties of the obtained data can inform future monitoring strategies and network designs around natural laboratories.

Application of temporal reweighting to ambient noise cross-correlation for improved seismic Green's function

SUMMARY Due to the partly diffuse character of ambient noise, the retrieval of amplitude information and attenuation from noise cross-correlations has been difficult. Here, we apply the temporal reweighting method proposed by Weaver & Yoritomo to seismic data from the USArray in the central-midwest US. The results show considerable improvements in retrieved Green's functions in both symmetry and causality. The reweighting is able to make the effective incident noise field more isotropic (though not yet truly isotropic). It produces more robust amplitude measurements and also makes both the causal and anticausal parts usable. This suggests that it could be widely applicable for retrieval of Green's functions from ambient noise for attenuation study. The results also suggest an alternative measure of signal-to-noise ratio that complements the conventional one.

An adapted eigenvalue-based filter for ocean ambient noise processing

Accurate approximations of Green’s functions retrieved from the correlations of ambient noise require a homogeneous distribution of random and uncorrelated noise sources. In the real world, the existence of highly coherent, strong directional noise generated by ships, earthquakes, and other human activities can result in biases in the ambient-noise crosscorrelations (NCCs). We have developed an adapted eigenvalue-based filter to attenuate the interference of strong directional sources. The filter is based on the statistical model of the sample covariance matrix and can separate different components of the data covariance matrix in the eigenvalue spectrum. To improve the effectiveness and make it adaptable for different data sets, a weight is introduced to the filter. Then, the NCCs can be calculated directly from the filtered data covariance matrix. This approach is applied to a 1.02 h data set of ambient noise recorded by a permanent reservoir monitoring receiver array installed on the seabed. The power spectral density indicates that the noise recordings were contaminated by strong directional noise over nearly half of the whole observation period. Beamforming and crosscorrelation results indicate that the interference still exists even after applying traditional temporal and spectral normalization techniques, whereas the adapted eigenvalue-based filter can significantly attenuate it and help to obtain improved crosscorrelations. The approach makes it possible to retrieve reliable approximations of Green’s functions over a much shorter recording time.

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.

Monitoring of velocity changes based on seismic ambient noise: A brief review and perspective

Over the past two decades, the development of the ambient noise cross-correlation technology has spawned the exploration of underground structures. In addition, ambient noise-based monitoring has emerged because of the feasibility of reconstructing the continuous Green’s functions. Investigating the physical properties of a subsurface medium by tracking changes in seismic wave velocity that do not depend on the occurrence of earthquakes or the continuity of artificial sources dramatically increases the possibility of researching the evolution of crustal deformation. In this article, we outline some state-of-the-art techniques for noise-based monitoring, including moving-window cross-spectral analysis, the stretching method, dynamic time wrapping, wavelet cross-spectrum analysis, and a combination of these measurement methods, with either a Bayesian least-squares inversion or the Bayesian Markov chain Monte Carlo method. We briefly state the principles underlying the different methods and their pros and cons. By elaborating on some typical noise-based monitoring applications, we show how this technique can be widely applied in different scenarios and adapted to multiples scales. We list classical applications, such as following earthquake-related co- and postseismic velocity changes, forecasting volcanic eruptions, and tracking external environmental forcing-generated transient changes. By monitoring cases having different targets at different scales, we point out the applicability of this technology for disaster prediction and early warning of small-scale reservoirs, landslides, and so forth. Finally, we conclude with some possible developments of noise-based monitoring at present and summarize some prospective research directions. To improve the temporal and spatial resolution of passive-source noise monitoring, we propose integrating different methods and seismic sources. Further interdisciplinary collaboration is indispensable for comprehensively interpreting the observed changes.

Arrival-angle effects on two-receiver measurements of phase velocity

SUMMARYWe compile a data set of Rayleigh-wave phase velocities between pairs of stations, based on teleseismic events located on the same great circle as the two stations. We validate our observations against dispersion estimates based on ambient-noise cross correlations at the same station pairs. Discrepancies between the results of the two methods can in principle be explained by deviations in the wave propagation path between earthquake and receivers, due to lateral heterogeneity in the Earth’s structure, but the latter effect has, so far, not been precisely quantified nor corrected for. We implement an algorithm to measure the arrival angle of earthquake-generated surface waves and correct the dispersion measurements accordingly. Application to a data set from the Central-Western Mediterranean shows that the arrival-angle correction almost entirely accounts for the discrepancy in question, decreasing significantly the velocity bias for a wide range of periods.

Shear wave velocity changes induced by earthquakes and rainfall at the Rotokawa and Ngatamariki geothermal fields, Taupō Volcanic Zone, New Zealand

SUMMARY Fluid injection for geothermal production has the potential to produce subsidence and microseismicity that can incur heavy financial cost or hazard. Due to this, novel ways to monitor subsurface deformation to supplement existing methods are highly sought after. We use seismic ambient noise to obtain time-dependent measurements of shear velocity within the geothermal reservoirs of Rotokawa and Ngatamariki, two producing geothermal fields in the Taupō Volcanic Zone located in the central North Island of New Zealand and operated by Mercury Energy. We investigate the relationship between shear wave velocity changes and geothermal injection by selecting time periods at the fields when injection and production volumes were significantly altered: 2009–2010 at Rotokawa, when geothermal injection was quadrupled due to the start-up of a new power station, and 2012–2013 at Ngatamariki, the beginning of geothermal injection for electricity production at that field. Shear wave velocity changes are computed from the ambient noise cross-correlation coda using the Moving Window Cross-Spectral (MWCS) technique, with a reference stack encompassing all data prior to the change in injection rate and moving stacks of 10–50 d. Gradual positive and negative shear velocity changes with a periodicity of approximately 12 months were observed at both sites, with maximum amplitude of 0.06 ± 0.04 and –0.08 ± 0.03 per cent at Rotokawa and 0.07 ± 0.03 and –0.06 ± 0.02 per cent at Ngatamariki. We hypothesize that these changes are due to seasonal rainfall, as seismic velocities computed by ambient noise are sensitive to the filling and emptying of near-surface pore space. In addition to these gradual responses, we found several sharp negative changes in velocity that reach minimum values over a few days and then gradually equilibrate to prior values over a few weeks. The amplitude of these responses is between –0.03 and –0.07 per cent and coincides with regional and local earthquakes. We hypothesize that these responses are primarily produced by the creation of new fractures, the same mechanism that produces gradual groundwater level decreases at regional distances from earthquake epicentres. We analyse a periodic signal within the time-delay measurements and determine that it is at least in part caused by the MWCS window size smoothing the cross-coherence of the ambient seismic signal. We do not observe shear wave velocity changes coinciding with geothermal injection, which may suggest that the signal has lower amplitude compared to the seasonal and seismic responses. We use bandstop filters and polynomial curve fitting to remove the contribution of the seasonal signal, but see no evidence of a shear wave velocity response due to geothermal fluid injection.

Temporal changes of seismic velocities in the San Jacinto Fault zone associated with the 2016Mw 5.2 Borrego Springs earthquake

SUMMARYWe study temporal changes of seismic velocities associated with the 10 June 2016 Mw 5.2 Borrego Springs earthquake in the San Jacinto fault zone, using nine component Green's function estimates reconstructed from daily cross correlations of ambient noise. The analysed data are recorded by stations in two dense linear arrays, at Dry Wash (DW) and Jackass Flat (JF), crossing the fault surface trace ∼3 km northwest and southeast of the event epicentre. The two arrays have 9 and 12 stations each with instrument spacing of 25–100 m. Relative velocity changes (δv/v) are estimated from arrival time changes in the daily correlation coda waveforms compared to a reference stack. The obtained array-average δv/v time-series exhibit changes associated with the Borrego Springs event, superposed with seasonal variations. The earthquake-related changes are characterized by a rapid coseismic velocity drop followed by a gradual recovery. This is consistently observed at both arrays using time- and frequency-domain δv/v analyses with data from different components in various frequency bands. Larger changes at lower frequencies imply that the variations are not limited to the near surface material. A decreasing coseismic velocity reduction with coda wave lapse time indicates larger coseismic structural perturbations in the fault zone and near-fault environment compared to the surrounding rock. Observed larger changes at the DW array compared to the JF array possibly reflect the northwestward rupture directivity of the Borrego Springs earthquake.

1-D velocity model for the North Korean Peninsula from Rayleigh wave dispersion of ambient noise cross-correlations

Monitoring seismic activity in the north Korean Peninsula (NKP) is important not only for understanding the characteristics of tectonic earthquakes but also for monitoring anthropogenic seismic events. To more effectively investigate seismic properties, reliable seismic velocity models are essential. However, the seismic velocity structures of the region have not been well constrained due to a lack of available seismic data. This study presents 1-D velocity models for both the inland and offshore (western East Sea) of the NKP. We constrained the models based on the results of a Bayesian inversion process using Rayleigh wave dispersion data, which were measured from ambient noise cross-correlations between stations in the southern Korean Peninsula and northeast China. The proposed models were evaluated by performing full moment tensor inversion for the 2013 Democratic People’s Republic of Korea (DPRK) nuclear test. Using the composite model consisting of both inland and offshore models resulted in consistently higher goodness of fit to observed waveforms than previous models. This indicates that seismic monitoring can be improved by using the proposed models, which resolve propagation effects along different paths in the NKP region.

Rayleigh wave group velocity dispersion tomography of West Africa using regional earthquakes and ambient seismic noise

West Africa could teach us much about the early tectonic history of Earth, but current seismic models of the regional crustal and lithospheric structure lack the resolution required to answer all but the most basic research questions. We have improved the resolution of group velocity maps of the West African Craton by complementing the uneven path distribution of earthquake-generated surface waves with surface waves reconstructed from ambient noise cross-correlations. Our joint dataset provides good spatial coverage of group velocity measurements from 20- to 100-s period, enabling us to reduce artifacts in our group velocity maps and improve their resolution. Our maps correlate well with regional geological features. At short periods, they highlight differences in crustal thickness, recent tectonic activity, and thick sediments. At long periods, we found lower velocities due to hot, thin lithosphere under the Pan-African mobile belt and faster velocities due to cold, thick lithosphere under the Man-Leo and Reguibat shields. Our higher resolution maps advance us a step towards revealing the detailed lithospheric structure and tectonic processes of West Africa.

Passive guided wave tomography for structural health monitoring.

In this paper the authors present a baseline-free quantitative method for imaging corrosion flaws in thin plates. It only requires an embedded guided wave sensor network used in a fully passive way, i.e., without active emission of waves. This method is called passive guided wave tomography. The aim of this development is the use of this method for the structural health monitoring of critical structures with heavy limitations on both sensor's intrusiveness and diagnostic's reliability because it allows the use of sensors that cannot emit elastic waves such as fiber Bragg gratings, which are less intrusive than piezoelectric transducers. The idea consists in using passive methods in order to retrieve the impulse response from elastic diffuse fields-naturally present in structures-measured simultaneously between the sensors. In this paper, two passive methods are studied: the ambient noise cross-correlation and the passive inverse filter. Once all the impulse responses between the sensors are retrieved, they are used as input data to perform guided wave tomography.