1,720,952 research outputs found
Resistivity imaging with controlled-source electromagnetic data: Depth and data weighting
No full textWe discuss some computational aspects of resistivity imaging by inversion of offshore controlled-source electromagnetic data. We adopt the classic approach to imaging by formulating it as an inverse problem. A weighted least-squares functional measures the misfit between synthetic and observed data. Its minimization by a quasi-Newton algorithm requires the gradient of the functional with respect to the model parameters. We compute the gradient with the adjoint-state technique. Preconditioners can improve the convergence of the inversion. Diagonal preconditioner based on a Born approximation are commonly used. In the context of CSEM inversion, the Born approximation is not really accurate, this limits the possibility of estimating a correct approximation of the Hessian in a smooth medium or, in fact, in any reference background that does not roughly account for the resistors. We hence rely on the limited memory BFGS approximation of the inverse of the Hessian and we improve the inversion convergence with the help of a heuristic data and depth weighting. Based on a numerical example, we show that a simple exponential depth weighting combined with an offset or frequency data weighting significantly improves the convergence rate of a deep-water controlled-source electromagnetic data inversion.Geoscience & EngineeringCivil Engineering and Geoscience
An Helmholtz iterative solver for the three-dimensional seismic imaging problem?
No full textDuring the last three decades high-frequency approximations or paraxial (one-way) approximations of the wave equation have been successfully used to process seismic data in two-dimensional or three-dimensional spaces. With the increase of computer power and the need to take into account complex geological structures, solutions of the exact wave equation either in time domain or in frequency domain are required. The time-domain wave equation can be solved with a marching approach in time and an explicit scheme. Currently three-dimensional imaging algorithms, so-called reverse time migrations, are developed based on time-domain solutions. The frequency-domain wave equation, namely the Helmholtz equation in the acoustic case with constant density, leads to a large sparse linear solver. In a two-dimensional space a direct solver, based on a LU decomposition, can be used. This leads to attractive and efficient imaging methods faster than the time-domain imaging methods. In a three-dimensional space, due to the size of the matrix, an iterative solver is required. In practice three-dimensional frequency-domain solutions are not used because most of the iterative solvers are not robust at seismic frequency or are too expensive. The difficulty arises because the matrix of the linear system is not definite and because the model size generally corresponds to several hundreds of wavelength in the three space directions. A new preconditioner based on a heavily damped wave equation and a multi-grid solver has been proposed by Erlangga et al.. The iterative solver is a preconditioned bicgstab. The preconditioner corresponds to the approximated inverse of the heavily damped wave-equation after one multigrid cycle. Numerical results have shown that this new iterative solver is robust. However does a frequency-domain imaging algorithm based on this solver compete with a time-domain imaging algorithm? To handle realistic three-dimensional geophysical problem the multigrid solver is implemented with a standard full-weighting coarsening, a tri-linear interpolation, a matrix-free approach, and an 8th order scheme in space. Three-dimensional numerical results indicate that the number of bicgstab iterations linearly increase with frequency. Based on this result, the number of the floating point operations needed for the time-domain and the frequency-domain imaging algorithms is roughly estimated. It is then shown that the migration algorithm is about one order of magnitude faster in the time domain and that the velocity modeling building algorithm based on a least-squares formulation and a scale separation is about three time faster in the frequency domain
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
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
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
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
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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