1,720,960 research outputs found
Seismological forward and inverse modelling for upper mantle seismic anisotropy studies
Full text linkSeismic anisotropy is the dependence of seismic wave velocity on the propagation direction and it is mainly generated by strain-induced lattice preferred orientation (LPO) of intrinsically anisotropic minerals. Despite previous studies have demonstrated that neglecting anisotropy introduces notable imaging artifacts, most tomographic methods rely on the assumption of isotropy, interpreting fast and slow velocity anomalies as related to seismically isotropic sources (e.g., temperature anomalies, presence of a liquid phase, etc). In this Thesis we carried out numerical simulations aiming at improving strain-induced fabric estimates and predicting realistic elastic properties in 2-D and 3-D synthetic domains. We generated synthetic datasets with forward waveform modelling and explored different inverse methodologies (e.g., P- and S-wave travel time tomography, automatic partitioned waveform inversion of surface waves) both with real and synthetic data. Among the results, we present ani-NEWTON21, the first 3D anisotropic teleseismic P-wave tomography revealing upper mantle structures and dynamics beneath the Central Mediterranean. By performing synthetic seismic data inversions we tested how ray density, data quality and regularization (i.e., damping and smoothing factors) influence the tomographic image. Finally, from the comparison of purely isotropic and anisotropic tests, we observed that the first-order effect of including anisotropy in the inversion is to reduce the magnitude of isotropic anomalies, more significantly for low-velocity zones relative to high-velocity zones. The research activities described in this Thesis altogether provide important insights for predicting and isolating seismic anisotropy, and for obtaining more reliable and physically consistent imaging of the Earth’s internal structure.Seismic anisotropy is the dependence of seismic wave velocity on the propagation direction and it is mainly generated by strain-induced lattice preferred orientation (LPO) of intrinsically anisotropic minerals. Despite previous studies have demonstrated that neglecting anisotropy introduces notable imaging artifacts, most tomographic methods rely on the assumption of isotropy, interpreting fast and slow velocity anomalies as related to seismically isotropic sources (e.g., temperature anomalies, presence of a liquid phase, etc). In this Thesis we carried out numerical simulations aiming at improving strain-induced fabric estimates and predicting realistic elastic properties in 2-D and 3-D synthetic domains. We generated synthetic datasets with forward waveform modelling and explored different inverse methodologies (e.g., P- and S-wave travel time tomography, automatic partitioned waveform inversion of surface waves) both with real and synthetic data. Among the results, we present ani-NEWTON21, the first 3D anisotropic teleseismic P-wave tomography revealing upper mantle structures and dynamics beneath the Central Mediterranean. By performing synthetic seismic data inversions we tested how ray density, data quality and regularization (i.e., damping and smoothing factors) influence the tomographic image. Finally, from the comparison of purely isotropic and anisotropic tests, we observed that the first-order effect of including anisotropy in the inversion is to reduce the magnitude of isotropic anomalies, more significantly for low-velocity zones relative to high-velocity zones. The research activities described in this Thesis altogether provide important insights for predicting and isolating seismic anisotropy, and for obtaining more reliable and physically consistent imaging of the Earth’s internal structure
Imaging upper mantle anisotropy with traveltime and splitting intensity observations from teleseismic shear waves: insights from tomographic reconstructions of subduction simulations
Full text linkTeleseismic traveltime tomography remains one of the most popular methods for obtaining images of Earth's upper mantle. However, despite extensive evidence for an elastically anisotropic mantle, the isotropic assumption remains commonplace in such imaging studies. This can result in significant model artefacts which in turn may yield misguided inferences regarding mantle dynamics. The nature of anisotropy-induced apparent velocity anomalies has been well-documented in P-wave imaging and various strategies have been proposed to constrain both isotropic and anisotropic heterogeneity from these data. In contrast, few studies have explored the consequences for shear wave tomography and no practical framework for the anisotropic inversion of S-wave delays exists. Here, we propose a new method for constraining arbitrarily oriented hexagonal anisotropy using both traveltime and splitting intensity observations from direct S phases. Our approach accounts for polarization and finite-frequency effects and allows for isotropic starting models. The imaging method is validated through the tomographic analysis of a realistic synthetic dataset produced from waveform simulations through a geodynamic model of subduction. Results illustrate that neglecting anisotropy produces distortions in slab geometry and the appearance of sub- and supraslab low-velocity zones. Anisotropic inversions remove these artefacts while also constraining geodynamically relevant fabric properties including dip
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
INVERSIONE DI ONDE DI RAYLEIGH MEDIANTE CURVE DI DISPERSIONE E ALGORITMI DI FULL WAVEFORM INVERSION LOCALE
No full textL’inversione delle onde superficiali è un utile strumento per la stima dei parametri elastici della sottosuperficie e nello specifico il principale parametro che può esse-re stimato è la velocità delle onde di taglio.
Esistono diversi metodi per la ricostruzione dell’andamento delle Vs nel terreno che sfruttano proprio le onde di superficie, più precisamente le onde di Rayleigh. Nel mio lavoro di tesi mi sono occupato di analizzare il metodo classico di inversione delle curve di dispersione e il più recente metodo di full waveform inversion con ottimizzazione locale.
Il metodo classico è ormai in uso da diversi decenni, esso però si basa su approssimazioni fisiche come considerare il terreno una pila di strati paralleli con eterogeneità sulla sola verticale. I risultati di questi esperimenti, effettuati su modelli sintetici che puntano a ricreare situazioni geologiche complesse, hanno messo in luce i limiti del metodo che risiedono principalmente nell’introduzione di errori nel modello predetto in presenza di eterogeneità laterali e nella soggettività del picking delle curve di dispersione. È stata inoltre studiata l’influenza delle informazioni a priori sulla predizione del risultato dell’inversione, mostrando un miglioramento di quest’ultimo all’aumentare delle informazioni.
Gli stessi modelli sono stati sottoposti ad esperimenti di full waveform inversion elastica con metodo dei gradienti coniugati precondizionato come strumento di ottimizzazione locale. Uno degli aspetti più importanti su cui si è focalizzato il mio lavoro e di cui tener conto ogni qual volta ci si approcci all’uso di FWI è la scelta dei modelli iniziali e il loro effetto sulla convergenza dell’algoritmo verso una corretta soluzione. Per questo motivo ho eseguito per ogni modello due inversioni con differenti modelli di partenza dei parametri Vp (velocità delle onde di compressione), Vs (velocità delle onde di taglio) e densità. Una prima inversione è stata effettuata usando modelli iniziali costanti, ottenuti da una media aritmetica dei valori di ognuno dei tre parametri osservati sul modello reale, caratterizzati quindi da un basso contenuto di informazioni. Una seconda inversione è stata effettuata con modelli di partenza generati da precedenti esperimenti di full waveform inversion globale attraverso l’uso di algoritmi genetici. Da questo secondo set di inversioni è emerso come la FWI locale sia in grado di predire risultati con dettagli ad alta risoluzione, quando i modelli di partenza sono a loro volta predizioni ottenute da FWI con algoritmi genetici. Le strutture a grande lunghezza d’onda individuate dagli algoritmi genetici vengono ulteriormente perfezionate da FWI con ottimizzazione locale. Infine è stato eseguito un test preliminare su dati sismici reali con lo scopo di applicare quanto visto e sperimentato sui dati sintetici anche a dati di campo.
Surface waves inversion is a useful tool to estimate the elastic parameters of the subsurface and, in particular, of the shear wave velocity (Vs). There are several methods trying to reconstruct the Vs structure in subsurface that exploit surface waves, precisely Rayleigh waves. In this work, I analyzed the classic inversion method of the dispersion curves and the most recent full waveform inversion method with local optimization.
The classic method has been in use for several decades, but it is based on physical approximations such as considering the ground as a stack of horizontal layers with no lateral variations. The results of these experiments were carried out on synthetic models in order to recreate realistic geological situations, revealing the limitations of the method; the latter mainly consist in the introduction of errors when lateral heterogeneities are present and in the subjectivity of the picking of the dispersion curves. It was also studied the influence of adding a priori information on the prediction of the inversion result, showing an improvement as the information increases.
The same models were subjected to full waveform inversion elastic experiments using preconditioned conjugate gradients as a local optimization tool. One of the most important aspects on which my work focused is the choice of the initial models and evaluating its effects on the convergence of the algorithm to a correct solution. For this reason, I performed for each model two
inversions with different starting models of Vp, Vs and density parameters. The first inversion was performed using constant initial models obtained from the arithmetic mean of the values of each of the three parameters observed on the real model, thus introducing little a priori information. The second inversion was performed with starting models generated from previous full waveform inversion experiments with genetic algorithms as the optimization tool. This second set of inversions revealed that local FWI is able to predict results with high-resolution details, when the starting models are predictions of previous genetic algorithms full waveform inversion. The large wavelength structures detected by genetic algorithms are further enhanced by FWI with local optimization
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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