1,721,013 research outputs found
Impact of log-transmissivity variogram structure on groundwater flow and transport predictions
No full textWe analyze the impact of the choice of the variogram model adopted to characterize the spatial variability of natural log-transmissivity on the evaluation of leading (statistical) moments of hydraulic heads and contaminant travel times and trajectories within mildly (randomly) heterogeneous two-dimensional porous systems. The study is motivated by the fact that in several practical situations the differences between various variogram types and a typical noisy sample variogram are small enough to suggest that one would often have a hard time deciding which of the tested models provides the best fit. Like-wise, choosing amongst a set of seemingly likely variogram models estimated by means of geostatistical inverse models of flow equations can be difficult due to lack of sensi-tivity of available model discrimination criteria. We tackle the problem within the framework of numerical Monte Carlo simulations for mean uniform and radial flow scenarios. The effect of three commonly used isotropic variogram models, i.e., Gaus-sian, Exponential and Spherical, is analyzed. Our analysis clearly shows that (ensemble) mean values of the quantities of interest are not considerably influenced by the variogram shape for the range of parameters examined. Contrariwise, prediction vari-ances of the quantities examined are significantly affected by the choice of the variogram model of the log-transmissivity field. The spatial distribution of the larg-est/lowest values of the relative differences observed amongst the tested models de-pends on a combination of variogram shape and parameters and relative distance from internal sources and the outer domain boundary. Our findings suggest the need of de-veloping robust techniques to discriminate amongst a set of seemingly equally likely al-ternative variogram models in order to provide reliable uncertainty estimates of state variables
Impact of the choice of the variogram model on flow and travel time predictors in radial flows
No full textPrediction of hydraulic head, flux and contaminant travel time/trajectories in natural aquifers is uncertain due to the geologic media complexity and lack of information. Hence it is appropriate to cast the equations that govern groundwater flow and contaminant transport within a stochastic framework. The latter is oriented towards rendering ensemble moments of the analyzed quantities. In this view the spatial variable transmissivity is usually modeled as a Stochastic Continuum, characterized by a set of parameters (covariance shape, geometric mean, variance and correlation length). These are generally assumed to be known with certainty even though they are usually derived using a limited amount of experimental data, which are often not enough for a complete characterization.
Full-Bayesian approaches (e.g. Woodbury and Rubin 2000; Woodbury and Ulrych 2000) take into account the uncertainty in the knowledge of the variogram parameters (geometric mean, variance and correlation length). Feyen et al. (2002) illustrate an application of these methodologies to determine the uncertainty asso-ciated with the delineation of well capture zones. Hendricks Franssen et al. (2002) investigate the impact of the uncertainty of variogram parameters on the same topic using sequential Gaussian simulation (Gómez-Hernández and Journel 1993) to generate transmissivity fields and the sequential self-calibrated method for in-verse conditioning. In all these works the shape of the correlation structure of the natural logarithm of transmissivity is fixed and assumed known without uncertainty. Salandin and Rinaldo (1989) analyze the influence of the form of the log-conductivity covariance on dispersion coefficients in random permeability fields under mean uniform flow conditions.
Here, we focus on the impact of the choice of the functional form for the log-transmissivity variogram on (ensemble) moments of hydraulic head and contami-nant residence time under convergent flow conditions, such as those created by a single pumping well.
Although of high relevance in practical applications, problems associated to contaminant transport in the vicinity of extraction wells in heterogeneous media have been tackled only recently (e.g. Guadagnini and Franzetti 1999, Riva et al. 1999, Dagan and Indelman 1999, van Leeuwen et al. 2000, Feyen et al. 2002).
We perform a numerical Monte Carlo analysis of (a) the predictors of hydraulic head and residence time (rendered by their means) for conservative solute particles injected at various radial distances from the well, and (b) the associated prediction errors (rendered by the variance of the state variables investigated).
The natural logarithm of aquifer transmissivity, Y, is modeled as a statistically homogeneous Gaussian random field. Three functional forms of the variogram (namely Exponential, Gaussian and Spherical), chosen amongst the most common models used in the literature, are considered. The impact of the choice of the variogram model on flow and travel time predictors is analyzed for different domain sizes in terms of correlation scale of Y (i.e. extent of the aquifer within which the effects of pumping are not negligible) and degrees of heterogeneity (in terms of the variance of Y )
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
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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