On the amplitude of surface waves obtained by noise correlation and the capability to recover the attenuation: a numerical approach

International audienceCross-correlation of ambient seismic noise recorded by a pair of stations is now commonly recognized to contain the Green's function between the stations. Although traveltimes extracted from such data have been extensively used to get images of the Earth interior, very few studies have attempted to exploit the amplitudes. In this work, we investigate the information contained in the amplitudes and we probe the capability of noise correlations to recover anelastic attenuation. To do so, we carry out numerical experiments in which we generate seismic noise at the surface of a 1-D Earth model. One of the advantages of our approach is that both uniform and non-uniform distributions of noise sources can be taken into account. In the case of a uniform distribution, we find that geometrical spreading as well as intrinsic attenuation are retrieved, even after strong non-linear operations such as one-bit normalization and spectral whitening applied to the noise recordings. In the case of a non-uniform distribution of sources, the geometrical spreading of the raw noise correlations depends on the distribution, but intrinsic attenuation is preserved. For the one-bit noise and whitened noise correlations, the interpretation of observed amplitude decays requires further study

Multiscale full waveform inversion

We develop and apply a full waveform inversion method that incorporates seismic data on a wide range of spatio-temporal scales, thereby constraining the details of both crustal and upper-mantle structure. This is intended to further our understanding of crust-mantle interactions that shape the nature of plate tectonics, and to be a step towards improved tomographic models of strongly scale-dependent earth properties, such as attenuation and anisotropy. The inversion for detailed regional earth structure consistently embedded within a large-scale model requires locally refined numerical meshes that allow us to (1) model regional wave propagation at high frequencies, and (2) capture the inferred fine-scale heterogeneities. The smallest local grid spacing sets the upper bound of the largest possible time step used to iteratively advance the seismic wave field. This limitation leads to extreme computational costs in the presence of fine-scale structure, and it inhibits the construction of full waveform tomographic models that describe earth structure on multiple scales. To reduce computational requirements to a feasible level, we design a multigrid approach based on the decomposition of a multiscale earth model with widely varying grid spacings into a family of single-scale models where the grid spacing is approximately uniform. Each of the single-scale models contains a tractable number of grid points, which ensures computational efficiency. The multi-to-single-scale decomposition is the foundation of iterative, gradient-based optimization schemes that simultaneously and consistently invert data on all scales for one multi-scale model. We demonstrate the applicability of our method in a full waveform inversion for Eurasia, with a special focus on Anatolia where coverage is particularly dense. Continental-scale structure is constrained by complete seismic waveforms in the 30-200s period range. In addition to the well-known structural elements of the Eurasian mantle, our model reveals a variety of subtle features, such as the Armorican Massif, the Rhine Graben and the Massif Central. Anatolia is covered by waveforms with 8-200s period, meaning that the details of both crustal and mantle structure are resolved consistently. The final model contains numerous previously undiscovered structures, including the extension-related updoming of lower-crustal material beneath the Menderes Massif in western Anatolia. Furthermore, the final model for the Anatolian region confirms estimates of crustal depth from receiver function analysis, and it accurately explains cross-correlations of ambient seismic noise at 10s period that have not been used in the tomographic inversion. This provides strong independent evidence that detailed 3-D structure is well resolve

Testing Scenarios on Geological Models: Local Interface Insertion in a 2D Mesh and its Impact on Seismic Wave Simulation

In this work, we consider a relatively simple case of fluid monitoring in a subsurface gas reservoir. Seismic wave velocities for porous rocks vary depending on fluid saturation, and our objective is to quantitatively evaluate the impact of the water/gas contact depth on elastic wave propagation. To efficiently test different contact depth scenarios and assess their impact on wave propagation, we propose to locally modify a 2D geological model and run time-dependent elastic simulations. The input model is a triangulated surface conforming to geological structures and representing physical properties. The 2D meshed model is locally updated, meaning that only a given region is modified and that the other parts of the mesh remain identical. To create several models by modifying only the reservoir layer, we insert a water/gas contact defined by a level-set at several depths with MMG. During the insertion, specific care is taken to maintain the conformity of the output mesh. As compared to global remeshing, the local modification reduces the cost of recomputing physical properties over the updated mesh. We run the numerical simulations by using Hou10ni2D code, which is based on a Discontinuous Galerkin method. Our results on a gas reservoir show a consistent behavior: we observe a correlation between the depth difference and L2-norm, the larger the distance from the reference depth contact, the higher the L2-norm. This approach could therefore be integrated into an inversion loop to determine the position of the fluid contact and reduce uncertainties in the reservoir model

Anatomy of the high-frequency ambient seismic wave field at the TCDP borehole.

International audienceThe Taiwan Chelungpu-fault Drilling Project (TCDP) installed a vertical seismic array between 950 and 1270 m depth in an active thrust fault environment. In this paper we analyze continuous noise records of the TCDP array between 1 and 16 Hz. We apply multiple array processing and noise correlation techniques to study the noise source process, properties of the propagation medium, and the ambient seismic wave field. Diurnal amplitude and slowness patterns suggest that noise is generated by cultural activity. The vicinity of the recording site to the excitation region, indicated by a narrow azimuthal distribution of propagation directions, leads to a predominant ballistic propagation regime. This is evident from the compatibility of the data with an incident plane wave model, polarized direct arrivals of noise correlation functions, and the asymmetric arrival shape. Evidence for contributions from scattering comes from equilibrated earthquake coda energy ratios, the frequency dependent randomization of propagation directions, and the existence of correlation coda waves. We conclude that the ballistic and scattered propagation regime coexist, where the first regime dominates the records, but the second is weaker yet not negligible. Consequently, the wave field is not equipartitioned. Correlation signal-to-noise ratios indicate a frequency dependent noise intensity. Iterations of the correlation procedure enhance the signature of the scattered regime. Discrepancies between phase velocities estimated from correlation functions and in-situ measurements are associated with the array geometry and its relative orientation to the predominant energy flux. The stability of correlation functions suggests their applicability in future monitoring efforts

Palaeogenomics of Upper Palaeolithic to Neolithic European hunter-gatherers

: Modern humans have populated Europe for more than 45,000 years1,2. Our knowledge of the genetic relatedness and structure of ancient hunter-gatherers is however limited, owing to the scarceness and poor molecular preservation of human remains from that period3. Here we analyse 356 ancient hunter-gatherer genomes, including new genomic data for 116 individuals from 14 countries in western and central Eurasia, spanning between 35,000 and 5,000 years ago. We identify a genetic ancestry profile in individuals associated with Upper Palaeolithic Gravettian assemblages from western Europe that is distinct from contemporaneous groups related to this archaeological culture in central and southern Europe4, but resembles that of preceding individuals associated with the Aurignacian culture. This ancestry profile survived during the Last Glacial Maximum (25,000 to 19,000 years ago) in human populations from southwestern Europe associated with the Solutrean culture, and with the following Magdalenian culture that re-expanded northeastward after the Last Glacial Maximum. Conversely, we reveal a genetic turnover in southern Europe suggesting a local replacement of human groups around the time of the Last Glacial Maximum, accompanied by a north-to-south dispersal of populations associated with the Epigravettian culture. From at least 14,000 years ago, an ancestry related to this culture spread from the south across the rest of Europe, largely replacing the Magdalenian-associated gene pool. After a period of limited admixture that spanned the beginning of the Mesolithic, we find genetic interactions between western and eastern European hunter-gatherers, who were also characterized by marked differences in phenotypically relevant variants

Palaeogenomics of Upper Palaeolithic to Neolithic European hunter-gatherers

Publisher Copyright: © 2023, The Author(s).Modern humans have populated Europe for more than 45,000 years1,2. Our knowledge of the genetic relatedness and structure of ancient hunter-gatherers is however limited, owing to the scarceness and poor molecular preservation of human remains from that period3. Here we analyse 356 ancient hunter-gatherer genomes, including new genomic data for 116 individuals from 14 countries in western and central Eurasia, spanning between 35,000 and 5,000 years ago. We identify a genetic ancestry profile in individuals associated with Upper Palaeolithic Gravettian assemblages from western Europe that is distinct from contemporaneous groups related to this archaeological culture in central and southern Europe4, but resembles that of preceding individuals associated with the Aurignacian culture. This ancestry profile survived during the Last Glacial Maximum (25,000 to 19,000 years ago) in human populations from southwestern Europe associated with the Solutrean culture, and with the following Magdalenian culture that re-expanded northeastward after the Last Glacial Maximum. Conversely, we reveal a genetic turnover in southern Europe suggesting a local replacement of human groups around the time of the Last Glacial Maximum, accompanied by a north-to-south dispersal of populations associated with the Epigravettian culture. From at least 14,000 years ago, an ancestry related to this culture spread from the south across the rest of Europe, largely replacing the Magdalenian-associated gene pool. After a period of limited admixture that spanned the beginning of the Mesolithic, we find genetic interactions between western and eastern European hunter-gatherers, who were also characterized by marked differences in phenotypically relevant variants.Peer reviewe

Heterogeneities and Anisotropy in the Earth

International audienc

Simulation par la méthode des éléments spectraux des formes d'onde obtenues par corrélation de bruit sismique

The waveform we can obtain by correlating ambient seismic noise recorded at two different stations gives an interesting signature of the media between these stations. This provides a new type of data that can be used to investigate the Earth's structure, as the earthquakes records do. Only the phase information has been considered from noise correlations for now and we would like to know in this work whether the amplitude can also be used or not. Ultimately, our aim is to process a spectral element simulation of the entire waveforms. In a first time, we study the features of correlations generated with synthetic seismic noise. Both uniform and non-uniform sources distributions are investigated and the effect of attenuation is carefully detailed for raw, 1-bit and whitened noise. In a second time, we compute a spectral element simulation of these synthetic correlations. The main difficulty is to take into account the amplitudes of the anisotropic noise flux that goes across the stations network. To do so, we use the time-reversal technique and we create a extended source that is positioned at a station and then propagated with the SEM ina certain Earth's model to retrieve waveforms corresponding to correlations between this station and the other receivers of the network. The result is demonstrated numerically as well as theoretically using the representation theorem. Moreover, the spectral element code we use is presented, with an accurated validation and two examples of wavefields generated by earthquakes in a 3D model of Europe. To end up, we apply our time-reversal process to correlations from real seismic noise. We discuss problems due to space-time variations of the noise sources and we show very encouraging resultsLa forme d'onde que l'on peut obtenir en corrélant le bruit de fond sismique enregistré à deux stations distinctes constitue une signature extrêmement intéressante du milieu qui sépare les deux stations. Récemment apparu, ce type de données vient compléter les enregistrements de séismes pour étudier la structure interne de la Terre. Malheureusement, seule l'information sur la phase a été considérée jusqu'ici, et nous nous demandons dans ce travail de thèse s'il est également possible d'utiliser l'amplitude, le but ultime étant d'élaborer une simulation des formes d'onde en question par la méthode des éléments spectraux (SEM). Dans un premier temps, nous nous intéressons aux caractéristiques de corrélations obtenues grâce à du bruit généré de manière synthétique. Deux distributions de sources sont alors envisagées, l'une uniforme et l'autre non-uniforme, et l'atténuation est soigneusement étudiée, que le bruit corrélé soit brut, binarisé ou blanchi. Nous procédons ensuite à la simulation éléments spectraux de ces corrélations synthétiques. Pour cela, il est nécessaire de prendre en compte les amplitudes spectrales du flux de bruit anisotrope qui traverse le réseau de stations considéré. Nous nous servons alors du renversement temporel et créons une source étendue qui, placée en un récepteur et se propageant grâce à la SEM dans un milieu donné, permet de retrouver les corrélations de bruit effectuées entre ce récepteur et les autres stations du réseau. Le résultat est démontré non seulement numériquement mais aussi d'un point de vue théorique en manipulant le théorème de représentation. Par ailleurs, le code éléments spectraux utilisé est présenté en détails avant d'être validé. Il permet de modéliser la propagation des ondes sismiques à l'échelle régionale et est employé, à titre d'exemple, pour simuler des séismes dans un modèle 3D de l'Europe. Enfin, une application à des corrélations issues du bruit sismique réel est exposée. Nous y discutons les problèmes liés à la variation spatio-temporelle des sources de bruit et montrons des résultats qui laissent présager un bel avenir à notre méthod