The cross-correlation of ambient seismic noise data can be utilized to image subsurface geological structures from ambient noise fields at local, regional, and global scales by extracting Green's functions between seismograph pairs. Precise extraction of empirical Green's functions from the cross-correlations of noise records requires that the seismic wavefield be fully diffuse. This requires the coefficients of the eigenfunction expansion of the seismic records to satisfy the statistical characteristics derived by Weaver and Lobkis (2004) in the time domain. Due to the complexities of the Earth media and noise sources, it is not feasible to obtain accurate eigenfunctions with the corresponding coefficients and adopt these statistical characteristics to evaluate actual seismic data. To resolve this issue, we derive the equivalent expressions in the frequency domain with dimensionless evaluation criteria without requiring the eigenfunctions. The evaluation of random noise, wind-induced vibrations, car- and air-traffic-excited ground motions, earthquakes, and continuous ambient seismic noise records confirms the validity of our evaluation method. We further apply the method to the widely used preprocessing procedures of ambient noise imaging techniques by examining time-domain normalization and spectral whitening operations of earthquake waveforms, thus quantitatively demonstrating how these procedures down-weight the non-diffuse component and improve the degree of waveform diffuseness (as shown in Figure 1). As an application, we select the coda wave signals generated by road traffic and earthquakes that satisfy the diffuse wavefield characteristics and extract the higher-order surface wave dispersion curves from 20-100 s seismic recordings without performing preprocessing procedures. Compared with the traditional surface wave processing process, our method is highly efficient, does not require long recording time and preprocessing such as normalization and whitening, and can be widely used in evaluating diffuse wavefield, imaging subsurface velocity and attenuation structures, and monitoring the temporal changes with high temporal resolution.

How to cite: Yang, B., Meng, H., Gu, N., Liu, X., and Chen, X.: Evaluating diffuse wavefield and its applications in seismic imaging, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4166, https://doi.org/10.5194/egusphere-egu23-4166, 2023.

The ambient seismic noise is dominated by surface waves associated with ocean microseisms that are defined by a strong acoustic/seismic coupling at the seafloor. The understanding of microseism sources not only benefits seismic passive imaging and monitoring, but can also help track ocean storms using seismic signals recorded on land. Numerical simulations show that heterogeneous seafloor morphologies and structures can significantly affect the propagation of microseism’s surface waves and therefore the accuracy in locating their origin using traditional methods, such as array beamforming. Here, we aim to investigate how the use of time-reversal imaging can help overcome those limitations. The technique has mainly been developed for acoustics but has been applied successfully in seismology for earthquake localisation. Time-reversal imaging consists on back-propagating, through a realistic model, the signal measured at a network of receivers so that it eventually refocuses back at its origin. For this study, simulations are performed using the code SPECFEM3D and a regional 3D acoustic/elastic model of the Irish offshore, with seismic receivers homogeneously distributed along the coast of Ireland. First, methodologies and stations layout are tested with synthetic data generated from forward modelling using different source distributions. Processing approaches for the time-reversed simulation results are investigated in order to optimize the recovery of the original sources. Following those tests, real time windows of seismic noise “events” are then back-propagated into the model with the aim to map the microseism sources associated with local storms in the model area. The results are compared with microseism sources derived from global ocean wave models. Overall, the use of time-reversal imaging for microseism sources localisation looks promising. Although challenging due to the diffuse distribution of sources, there is good potential for developing further our understanding of microseism sources and monitor dominant microseism generation areas in the North Atlantic region with the implementation of a larger model.

How to cite: Le Pape, F., Bean, C. J., Vijayan, A., and Fink, M.: On localising North Atlantic’s microseism sources using time reversal imaging, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8517, https://doi.org/10.5194/egusphere-egu23-8517, 2023.

Seismic interferometry gives rise to a correlation wavefield that is closely related to the Green’s function under the condition of uniformly distributed noise sources. Asymmetric correlation wavefields result from the violation of this condition and are commonly observed in field data. In the presence of an additional isolated noise source a second contribution to the correlation wavefield is introduced that emerges from the isolated source location at negative lapse time. The two wavefield contributions interfere, resulting in biased surface wave dispersion measurements. Isolated noise sources that act continuously, such as machinery or ocean microseisms, further have significant impact on the coda of the correlation wavefield. The coda can be dominated by direct waves propagating from the isolated noise source, not by multiply scattered waves originating from the master station. This fundamentally challenges the current understanding of how velocity changes detected in the coda can be measured and interpreted.

How to cite: Schippkus, S., Snieder, R., Safarkhani, M., and Hadziioannou, C.: The impact of isolated noise sources on correlation wavefields, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-124, https://doi.org/10.5194/egusphere-egu23-124, 2023.

Over the past two decades, the ambient seismic noise correlation method has revolutionized our ability to investigate the Earth's subsurface structure. Perfect reconstruction of the cross correlation wavefields towards Green's function demands some strong hypotheses about uncorrelated and spatially uniform seismic source distributions. In reality, violation of this assumption impacts the accuracy of the Green’s function estimate, which may bias applications in further studies. As this technique has become a standard method for subsurface imaging and monitoring, a methodology that identifies the effect of seismic source distributions plays an essential role in removing their contribution to achieve less-biased signals. Seismic array beamforming is commonly applied in estimating the direction of seismic waves crossing the array.

In this study, we use a new strategy based on beamforming of noise correlation signals. We consider several seismic stations surrounding the Gräfenberg array throughout Europe as virtual sources. We process two years of vertical component continuous noise recording from the Gräfenberg array in Germany and virtual sources in Poland, Italy, Portugal, France and Finland. Using the noise correlation-based beamforming method, we detect source directions for direct and coda waves for the primary (0.05-0.1 Hz) and secondary (0.1-0.4 Hz) microseism frequency bands. The source directions for the direct waves correspond to the converging and diverging part of the correlation wavefield. Throughout the coda, however, we detect the dominant noise source directions, i.e., surface waves generated by ocean microseisms in the Northern Atlantic during winter months and body waves from the Southern Pacific during summer months. This suggests that the coda of the correlation functions contains repeating direct waves from the dominant source regions, which may lead to incorrect estimates and interpretation of velocity variations if not accounted for. Knowledge of the ambient noise source origins and their spatiotemporal distribution is required to correctly interpret velocity variations.

How to cite: Safarkhani, M., Schippkus, S., and Hadziioannou, C.: Array beamforming on ambient seismic noise correlations reveals repeating direct waves in the coda, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7003, https://doi.org/10.5194/egusphere-egu23-7003, 2023.

Protection against sea submersion is a key point for the management of coastal areas. Operators need tools to monitor the aging of the sea dikes in order to reduce the risk of catastrophic events such as the Xynthia storm that occurred in 2010 (west of France). Long-term monitoring of these structures can be done using the ambient seismic noise produced by a combination of natural sources (e.g. the impact of swell on the structure, water currents, the wind force on ground-anchored constructions, etc.) and/or nearby anthropogenic sources (e.g. road/pedestrian traffic, coastal activities, etc.).

Continuous ambient noise recordings can be used for monitoring structures by detecting small variations in seismic velocities related to localized degradations that cannot be detected visually. However, this monitoring technique requires sufficient energy in the proper frequency range for the intended application (typically at wavelengths of the order of the size of the structure or less). Additionally, the distribution of the sources must be relatively stable over time in order to interpret the velocity variations explicitly. The case of sea dikes is particularly challenging as the seismic noise is highly variable due various factors, like the water height variations during the tidal cycle, the variations in tidal intensity during the year, or the effects of the climatic conditions on the direction and intensity of the swell.

This study is conducted in the framework of the SEEWALL project, which aims to develop a system for monitoring sea dikes using ambient seismic sources. A dike located on the Noirmoutier Island (France) has been instrumented with permanent accelerometers along with several geophysical and meteorological probes which have been recording continuously for about one year. This contribution focuses on the identification of the essential properties of the ambient seismic noise recorded in this setting and seeks to evaluate how these properties affect our ability to measure temporal variations of the seismic waves velocity. In other words, our objective is to identify the noise sources that contribute most favorably to the approximated empirical Green’s functions and/or that are highly repeatable over time, in order to develop methods that can be applied to various dikes and to optimize the type and amount of seismic data required to monitor such structures.

How to cite: Lehujeur, M., Kahrizi, A., Abraham, O., Michel, L., Bardainne, T., Lescoat, A., Vivin, L., Blanchais, J., Boulay, C., Devie, T., Palma-Lopes, S., Durand, O., and Gugole, G.: Characterization of ambient seismic noise sources for long term monitoring of a sea dike: preliminary results of the SEEWALL project., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4258, https://doi.org/10.5194/egusphere-egu23-4258, 2023.

Ambient noise tomography is a revolutionary technique in past twenty years, and has many far-reaching applications from exploration, regional and continental to global scales. Recently, surface wave overtones have been efficiently extracted from ambient noise cross-correlations by the frequency-Bessel (F-J) transform method. The excitations of overtones from noise cross-correlations, however, have not be investigated well. Here we collect 16-day continuous seismic data recorded by a seismic array installed in Mongolia during two super typhoons in 2011. Utilizing the F-J transform, we extract daily multimodal dispersion curves with high signal-to-noise ratios from vertical-vertical (Z-Z) component cross-correlations and transverse-transverse (T-T) component cross-correlations. For both the fundamental mode and overtones, our results show that the cross-coefficients between the wind speeds and the Z-Z dispersion amplitudes over 0.1Hz are over 0.6 in typhoon track regions, which suggests that these two typhoons excite Rayleigh waves over 0.1Hz.  For T-T surface waves, however, typhoons only excite the fundament mode over 0.1Hz, and overtones do not relate to typhoons. Our results indicate that energetic typhoons can efficiently excite multimodal Rayleigh waves while Love waves during this period may not come from the microseisms excited by typhoons, which may help us to deepen our understanding of the coupling system among the atmosphere, oceans and the solid earth.

How to cite: Feng, X. and Chen, X.: Multimodal surface waves during energetic typhoons, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16780, https://doi.org/10.5194/egusphere-egu23-16780, 2023.

The 2008 M7.9 Wenchuan Earthquake is the most devastating event in the last two decades in China. Here, we analyze 8 years (2007–2014) of seismic records to track the normal background level of the media properties, as well as the transient changes associated with tectonic activities (e.g., earthquakes). Understanding the long-term background pattern contributes to identifying transient changes. Temporal velocity variations of the surface waves show clear seasonal fluctuations and a co-seismic velocity drop after the Wenchuan mainshock in the 2-10 s period band. A comparison with meteoric data allows us to conclude that the main mechanism of the seasonal variation is the loading due to precipitation. The seasonal velocity changes exhibit spatial characteristics, where the amplitudes of the seasonal velocity changes are larger in the Tibet Plateau and smaller in the Sichuan basin. The spatial pattern is consistent with that of the tectonic deformation. The deformation is strong in the Tibet Plateau, while the Sichuan basin is relatively stable. Furthermore, there is a higher density of cracks in the Tibet Plateau than that in the basin.

Moreover, we increased the time resolution of the noise cross-correlations to twenty days to investigate changes in media associated with the 2008 mainshock by analyzing 1.5 years of data from mid 2007 to 2008 for the surface wave at 2-10 s period band. Compared with the waveforms from seismically quiet time, we observe that the waveform similarity of surface waves decreased significantly about 10 days prior to the mainshock and persisted low until the end of September 2008. To exclude that our observation is related to a changing source pattern, we analyzed the seismic activity in the Sichuan region and the frequency spectrum of the ambient noise field in the corresponding time. Our results suggest that the decorrelation may indeed indicate a regional change in the scattering properties starting 10 days prior to the Wenchuan earthquake.

How to cite: Li, J., Obermann, A., Wu, S., Sánchez-Pastor, P., and Song, X.: Temporal changes in velocity and scattering properties in the Sichuan region, China, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13164, https://doi.org/10.5194/egusphere-egu23-13164, 2023.

Ambient noise cross-correlations or auto-correlations provide near real-time information about subsurface properties. Changes in the Green’s function obtained from auto-correlations/cross-correlations inform us about velocity changes in the medium (dv/v). Numerous studies have found good correlation between dv/v and medium changes related to a large earthquake, volcanic activity, or even seasonal changes at shallow depths. Another seismic parameter which helps estimate such medium changes is the rate of decay of coda waves or the coda Quality factor (Qc). Low Qc estimates from earthquake data has been shown to represent cracks/fracture as well as fluid migration. Application of ambient noise data for Qc estimation are relatively recent (past 5 years), especially in the Alps and Japan, where good correlation was found with the regional geology. Qc measurements using ambient noise data can ensure continuous monitoring unlike those from earthquakes. In this work, we evaluate the feasibility of monitoring medium changes with Qc by using ambient noise data from Greece. We perform autocorrelation of noise data from the permanent network in Greece over a period of 2010-2021. Preliminary analysis shows a seasonal pattern in Qc at several stations when considering short period bands, which is likely related to seasonal changes at shallow depths due to precipitation. Stations with clear seasonal pattern in the dv/v are correlated with the seasonal pattern in Qc, which confirms that Qc perturbations indeed represent physical changes. Stations which show weak seasonality in dv/v have a seasonality pattern in Qc with a lag of similar number of days. These results suggest than Qc based monitoring has the potential to act as a supplementary data set to dv/v and may even provide more information about the nature of medium change based on whether Qc is related to scattering or intrinsic attenuation.

How to cite: Ranjan, P., Stehly, L., and Delouche, E.: Exploring the feasibility of seismic monitoring using ambient noise coda Q: Experiments in the Aegean (Greece), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10372, https://doi.org/10.5194/egusphere-egu23-10372, 2023.

Recently, advances in the ambient noise analysis provide new ways to detect the velocity changes in the volcanic region by measuring the time delay of the daily cross-correlation functions (CCFs). Despite abundant studies on coda waves, studies examining the direct surface waves are relatively rare because of the influence of passive noise sources. However, direct surface waves have stronger energy and carry depth information, which can be obtained by the dispersion inversion. The direct surface waves' propagation direction along the great circle path is also beneficial for conducting tomography by finding a stable passive noise source, which is key to extracting depth-dependent velocity changes. Here, we used direct surface wave to detect velocity change caused by 2018 KIlauea volcano eruption and 2019 Ridgecrest earthquake. The results show that the noise-based direct surface wave is a powerful tool to study the change of earth medium caused by geological hazards, and can accurately obtain the information of the lateral position and depth position of the change.

How to cite: Liu, Z.: Detecting seismic velocity change by noise-based direct surface wave, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10612, https://doi.org/10.5194/egusphere-egu23-10612, 2023.

How to cite: Lu, Y., Wang, Q.-Y., and Bokelmann, G.: Inferring deep soil moisture variations in Central Europe using seismic method, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7636, https://doi.org/10.5194/egusphere-egu23-7636, 2023.

The temporal variation of seismic velocity gives us information about the crust’s stress state and the relative changes associated with magmatic, tectonic, and meteorologic activity. In the present study, we analyze the temporary seismic velocity changes under Volcan de Colima to identify its relationship with volcanic and non-volcanic sources, evaluate the seasonality of the variations and describe the impact that the meteorology has on the changes. The signals of seismic velocity variation used in this work were measured using the single-station cross-component correlation technique applied to traces registered at four stations in the period 2013 – 2017. The data was analyzed in two frequency bands: 0.1 – 1 and 1 – 2 Hz.

We validate the relationship between the velocity variations and three meteorological parameters, rain, temperature, and atmospheric pressure, using wavelet coherence analysis. After fitting a linear model, we identified the environmental factors with the most impact on the seismic velocity are: 1) the rainfall-induced pore pressure, correlated negatively with the seismic velocity and causing changes close to the order of -0.5%; 2) thermoelastic strains correlated positively with the seismic velocity and causing velocity variations between -0.5 and 0.5%. Atmospheric pressure has a smaller impact, mainly of the order of 10^-3%.

How to cite: Ibarra Morales, J., De Plaen, R. S. M., Márquez Ramírez, V. H., Zúñiga Dávila-Madrid, F. R., and Arámbula Mendoza, R.: Influence of meteorology on the variations of crustal seismic velocity in Volcan de Colima, Mexico using Noise Interferometry, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7992, https://doi.org/10.5194/egusphere-egu23-7992, 2023.

To design an efficient seismic monitoring infrastructure, the characterization of the background seismic noise level of each potential seismic station installation site is one of the most important data-quality metrics used to evaluate the suitability of such sites to host the seismic network. The background seismic noise can be generated by different sources such as, ocean waves (microseisms), atmospheric turbolences (strong wind and storms), and anthropogenic activities. Such disturbances are characterized by specific frequency bands, time-occurrence (diurnal and seasonal variation), and site location (close to populated area or to the coasts). Reducing the effect of these noise sources is one of the main challenges to face for designing seismic monitoring networks and, more specifically, when selecting the hosting site of a seismic stations. A solution to attenuate the seismic noise effect is obtained by deploying seismic stations in boreholes. The noise level reduction with depth has been observed and studied by different authors, however a general law estimating the sufficient depth to gain is still missing. In this study, we analyse the continuous seismic noise level at S. Potito-Cotignola gas storage in the Po Valley (Northern Italy) recorded from January 2019 to December 2021 by a broadband (BB) seismic station at surface and a vertical array composed by 6-short period 3-components seismometers installed at depth ranging between 35 to 285 m in borehole. We aim to characterize the seismic noise by computing the amplitude noise reduction in terms of dB as a function of depth for different frequencies and the SNR by selecting three seismic events, with different epicentral distance and magnitude. Our results show that the noise level decreases with depth following a logarithmic empirical trend and the lowest magnitude event records the maximum SNR difference between the deepest sensor and the one at the surface. The estimated empirical relationships can be used to help the design microseismic monitoring networks in similar geological settings.

How to cite: Rossi, C., Grigoli, F., Gasperini, P., Gandolfi, S., Cocorullo, C., Gukov, T., and Macini, P.: Estimation of amplitude noise reduction as a function of depth recorded by a deep vertical array (Northern Italy), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1171, https://doi.org/10.5194/egusphere-egu23-1171, 2023.

The Western Himalaya is one of the most complex and heterogeneous seismotectonic units of the Alpide-Himalaya seismic belt. The region has distinctive physiographic characteristics because of the way that they have changed and evolved over the course of time. The purpose of the present investigation is to understand the seismotectonic architecture beneath the study area which is seismically very active. Twenty broad-band seismic stations have been employed to record the surface wave data to study the crustal structure beneath the western Himalayas. We find phase and group velocities of Rayleigh waves for the region with periods between 4 and 30s. To obtain layered S wave velocity models, the dispersion curves are inverted. The crustal velocity structure beneath the region is found to vary significantly. The average estimated S-wave velocity is ~ 3.8 km/s down to 30 km depth. We also observed a low-velocity layer in the middle crust of the higher Himalayas section and the interpretation from the present analysis is consistent with available geological data.

How to cite: Biswal, S. and Kumar, S.: Crustal structure beneath the Western Himalayas from surface wave dispersion analysis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8432, https://doi.org/10.5194/egusphere-egu23-8432, 2023.

Pocket beaches, otherwise known as embayed beaches are beaches whose existence are traceable to human activity or natural occurrence. These beaches are found to be confined within the base of geological structures or artificially designed set up such as groin, hence, pocket beaches are always subject to various attacks via anthropogenic or natural forces. These attacks in many cases reflects in the thickness of the beach sediments and therefore determine the extent to which it can survive when hit by unfriendly environmental impact. Estimation of the sedimentary thickness of the beach is a vital tool in a successful geomorphological investigation geared towards the effective execution of coastal area management. This study aims to estimate the sedimentary thickness in several pocket beaches located in Malta using the passive seismic survey method of Horizontal-to-Vertical spectral ratio (H/V). This geophysical survey method employs seismic noise to estimate the depth or thickness of a sedimentary layer over a bedrock with higher shear wave velocity.  Although in recent years, this methodology has increased in popularity in geological investigations, however, there are few examples of the application of this approach to marine sandy beaches in the scientific literature, making this study a novel contribution to the field. The results of the study revealed clear H/V peaks on the beaches studied, with frequency variations corresponding to the expected sediment thickness variations. The latter was also computed by modelling of the H/V curves as well as the use of known data. The application of the H/V technique to the Maltese pocket beach system has demonstrated its effectiveness in providing valuable information for the effective use in the management of the coastal environments. This study has been supported by the SIPOBED project financed by the Malta Council of Science and Technology (MCST) - Space Reearch Funds

How to cite: Iregbeyen, P., D'Amico, S., Galone, L., and Colica, E.: Pocket beach management: the use of ambient noise to estimate beach sedimentary thickness, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14933, https://doi.org/10.5194/egusphere-egu23-14933, 2023.