1,720,960 research outputs found
Whole Earth tomography from delay times of P, PcP, and PKP phases: Lateral heterogeneities in the outer core or radial anisotropy in the mantle?
No full textWe perform tomographic inversions of compressional wave travel time measurements, in order to image the laterally varying radial anisotropy of P wave velocity in the entire mantle. In order to achieve an adequate sensitivity to both horizontal and vertical P velocity throughout the mantle, we include in our data set travel time measurements corresponding to ray paths of very different geometries. Specifically, we use data associated with the phases P, PcP(core-reflected) and PKPbc, PKPdf (core-refracted), all extracted from the bulletins of the International Seismological Centre, 1964-1993, PcP, PKPbc, and PKPdf data are also sensitive to the topography of the core-mantle boundary (CMB) and (PKP only) to the velocity structure of the Earth's core. These circumstances require that we also allow for CMB topographic anomalies as free parameters in our inversions and investigate the possibility of the existence of lateral heterogeneities in the outer core [e.g., Ritzwoller et al., 1986; Stevenson, 1987; Wahr and de Vries, 1989; Vasco and Johnson, 1998]. The solution models that we propose include alternatively a laterally heterogeneous outer core, a radially anisotropic mantle, or both, so that trade-offs between those features can also be evaluated. We conclude that only minor, and very localized, radially anisotropic anomalies are present in the middle and lower mantle, the most prominent being located in the deepest 400 km, under the western Pacific. We find a discrepancy, particularly evident by a comparison between different models of the CMB topography, between the travel time measurements that are sensitive td the Earth's core and those that are not; we show that it is not possible to explain this discrepancy only in terms of radial anisotropy of the mantle and/or lateral heterogeneity of the outer core
High- and low-resolution images of the Earth's mantle: Implications of different approaches to tomographic modeling
No full textRecently published images of the Earth's mantle are characterized by a nominal resolution much higher than that used in previous studies, where substantially different techniques were employed. The agreement between such "high-resolution" and "low-resolution" images often seems very poor. In an attempt to determine the reason for this discrepancy, we analyze how the choice of inversion algorithm (exact or iterative), regularization (norm or roughness minimization), and parameterization (spherical harmonics up to a variable degree, blocks) affects a tomographic model. We also investigate the effects of the varying density of the data coverage on the final solution. In our experiments we employ two seismic data sets: Rayleigh wave phase velocity at 75 s period and P wave travel times. We construct a new model of P velocity in the mantle (BDP98) based on the International Seismological Center bulletins 1964-1992. We use our findings in an evaluation of recent mantle models, including our own, focusing on similarities and discrepancies between models of different nominal resolution. In all the models the long-wavelength component is the most stable. However, consistent high-resolution details, probably corresponding to features of the real Earth, are also seen. In general, we conclude that most of the differences between existing tomographic models derive from the arbitrary choices made in the process of defining and solving the inverse problem, rather than from actual errors or approximations
Estimating lateral structure in the Earth's outer core
No full textLateral density structure could exist in the Earth's fluid outer core, owing to the gravitational perturbations induced by the asphericity of the mantle. We take a realistic tomographic image of lateral heterogeneities in the topography of the core-mantle boundary (CMB), as an upper bound for the amplitude of lateral structure in the surface of constant gravitational potential, at the same average depth. With this boundary condition, we apply the theory of Wahr and de Vries [1989], to estimate quantitatively the corresponding lateral density anomalies in the outer core structure; we find that their value, relative to PREM, must be everywhere smaller than similar to 0.1%. The resulting, theoretical maps of the outer core are dominated by long-wavelength structure, with a pattern closely resembling that of the assumed CMB topography; they do not correlate with any of the existing tomographic images of the core
On the relevance of Born theory in global seismic tomography
No full textDoes the application of seismic Born theory, as opposed to simpler ray theory, lead to an improvement in tomographic images of the Earth? In recent publications, Montelli et al. (2004a, 2004b) and van der Hilst and de Hoop ( 2005) among others have expressed opposite opinions. We propose a quantitative approach to the comparison of tomographic images, which we apply to the case of surface-wave phase velocity maps derived with Born vs. ray theory. RI Becker, Thorsten/A-6665-201
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
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