1,721,056 research outputs found
Global multiple-frequency S-wave tomography of the Earth’s mantle
No full textAfin de mieux contraindre la structure interne de la Terre, de nouveaux développements théoriques, sur la propagation des ondes, ont émergé ces dernières années. Une de ces nouvelles méthodes est la tomographie multi-fréquences, qui vise à exploiter la dépendance en fréquence des temps de parcours des ondes de volume, liée aux effets de diffraction. En utilisant cette méthode, cette thèse a pour objectif d'obtenir un modèle tomographique 3-D du manteau en ondes de cisaillement à ``haute-résolution'', qui puisse contribuer à améliorer nos connaissances sur la dynamique de la Terre.For better constraining the structure of the Earth's interior, new theoretical developments on seismic wave propagation have emerged in recent years, and received increasing attention in tomography. One of these new methods is the multiple-frequency tomography, which aims at exploiting the frequency-dependency of body wave travel times related to diffraction effects. In this thesis, we have applied this method in order to obtain a ``high-resolution'' 3-D shear-wave tomographic model of the mantle, that could contribute to a better understanding of the Earth's dynamics
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Tomographie globale en ondes S multi-fréquences du manteau terrestre
No full textAfin de mieux contraindre la structure interne de la Terre, de nouveaux développements théoriques, sur la propagation des ondes, ont émergé ces dernières années. Une de ces nouvelles méthodes est la tomographie multi-fréquences, qui vise à exploiter la dépendance en fréquence des temps de parcours des ondes de volume, liée aux effets de diffraction. En utilisant cette méthode, cette thèse a pour objectif d'obtenir un modèle tomographique 3-D du manteau en ondes de cisaillement à ``haute-résolution'', qui puisse contribuer à améliorer nos connaissances sur la dynamique de la Terre.For better constraining the structure of the Earth's interior, new theoretical developments on seismic wave propagation have emerged in recent years, and received increasing attention in tomography. One of these new methods is the multiple-frquency tomography, which aims at exploiting the frquency-dependency of body wave travel times related to diffraction effects. In this thesis, we have applied this method in order to obtain a ``high-resolution'' 3-D shear-wave tomographic model of the mantle, that could contribute to a better understanding of the Earth's dynamics
Geophysical inversion with a neighbourhood algorithm - II. Appraising the ensemble
No full textMonte Carlo direct search methods, such as genetic algorithms, simulated annealing, etc., are often used to explore a finite-dimensional parameter space. They require the solving of the forward problem many times, that is, making predictions of observables from an earth model. The resulting ensemble of earth models represents all 'information' collected in the search process. Search techniques have been the subject of much study in geophysics; less attention is given to the appraisal of the ensemble. Often inferences are based on only a small subset of the ensemble, and sometimes a single member. This paper presents a new approach to the appraisal problem. To our knowledge this is the first time the general case has been addressed, that is, how to infer information from a complete ensemble, previously generated by any search method. The essence of the new approach is to use the information in the available ensemble to guide a resampling of the parameter space. This requires no further solving of the forward problem, but from the new 'resampled' ensemble we are able to obtain measures of resolution and trade-off in the model parameters, or any combinations of them. The new ensemble inference algorithm is illustrated on a highly non-linear waveform inversion problem. It is shown how the computation time and memory requirements scale with the dimension of the parameter space and size of the ensemble. The method is highly parallel, and may easily be distributed across several computers. Since little is assumed about the initial ensemble of earth models, the technique is applicable to a wide variety of situations. For example, it may be applied to perform 'error analysis' using the ensemble generated by a genetic algorithm, or any other direct search method
Variations on the Author
Get PDF“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
Geophysical inversion with a neighbourhood algorithm - I. Searching a parameter space
No full textThis paper presents a new derivative-free search method for finding models of acceptable data fit in a multidimensional parameter space. It falls into the same class of method as simulated annealing and genetic algorithms, which are commonly used for global optimization problems. The objective here is to find an ensemble of models that preferentially sample the good data-fitting regions of parameter space, rather than seeking a single optimal model. (A related paper deals with the quantitative appraisal of the ensemble.) The new search algorithm makes use of the geometrical constructs known as Voronoi cells to derive the search in parameter space. These are nearest neighbour regions defined under a suitable distance norm. The algorithm is conceptually simple, requires just two 'tuning parameters', and makes use of only the rank of a data fit criterion rather than the numerical value. In this way all difficulties associated with the scaling of a data misfit function are avoided, and any combination of data fit criteria can be used. It is also shown how Voronoi cells can be used to enhance any existing direct search algorithm, by intermittently replacing the forward modelling calculations with nearest neighbour calculations. The new direct search algorithm is illustrated with an application to a synthetic problem involving the inversion of receiver functions for crustal seismic structure. This is known to be a non-linear problem, where linearized inversion techniques suffer from a strong dependence on the starting solution. It is shown that the new algorithm produces a sophisticated type of 'self-adaptive' search behaviour, which to our knowledge has not been demonstrated in any previous technique of this kind
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe 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
Get PDFWe 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
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