1,721,665 research outputs found
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
Coupled seismic and electromagnetic wave propagation
No full textCoupled seismic and electromagnetic wave propagation is studied theoretically and experimentally. This coupling arises because of the electrochemical double layer, which exists along the solid-grain/fluid-electrolyte boundaries of porous media. Within the double layer, charge is redistributed, creating an excess electrical charge in the fluid along the boundary. Electrokinetic theory describes coupled seismic and electromagnetic wave propagation. It predicts that seismic waves disturb the fluid excess charge, thereby creating an electric streaming current (seismoelectric effect). Inversely, the theory predicts that electromagnetic waves generate mechanical/seismic signals (electroseismic effect). Electrokinetic conversions can potentially be used as an effective means of detecting hydrocarbon reservoirs: it inherently combines seismic resolution with electromagnetic hydrocarbon sensitivity. Validating electrokinetic wave theory is therefore of paramount importance. Electrokinetic theory predicts the existence of two seismoelectric effects: (1) a coseismic (electric) field that is coupled to seismic waves, and therefore propagates with seismic wave velocity, and (2) a seismic wave that traverses an interface with a contrast in electrical or mechanical properties and produces electromagnetic (EM) signals that propagate outside the support of the seismic waves with much higher EM-wave speeds. These are called the coseismic and interface response fields, respectively. Electroseismic counterparts of these fields exist as well. In this thesis, electrokinetic theory is reformulated along the lines sketched by Biot (1956a,b). The reformulation employs effective frequency-dependent densities, in which both viscous and electrokinetic coupling are comprised. The reformulated theory predicts the existence of four wave modes within a fluid-saturated porous medium: fast and slow P-waves, a shear wave and an EM-wave. The electrokinetic dispersion relations, which give wave speed and intrinsic attenuation of each wave, are expressed in terms of generalized elastic coefficients and the effective densities. Each of the wave modes has a specific fluid-solid amplitude displacement ratio. These are derived and expressed also in terms of generalized elastic coefficients and effective densities. Each wave mode also has a specific ratio of electric potential and solid displacement potential. These are expressed in terms of the fluid-solid ratios and the so-called (frequency-dependent) electrokinetic coupling coefficient. This coefficient describes the coupling between electric and mechanical fields. When the coupling coefficient is zero, the electrokinetic equations decouple into the familiar Biot’s poroelastic equations and Maxwell’s EM relations. Subsequently, the wave coupling at a fluid/porous-medium interface is theoretically solved, where the modified theoretical formulation is applied. First a straightforward scattering problem of an incident fluid P-wave into fluid electromagnetic and pressure waves, and porous medium waves is considered. Second, the electrokinetic scattering matrix for a fluid/porous-medium interface is derived. This matrix summarizes all electrokinetic reflection and transmission coefficients applicable to this boundary. These coefficients describe how incident P-waves are converted at electrical/mechanical interfaces to electromagnetic signals, and, vice-versa, how electromagnetic signals are converted into acoustic signals. They enter the full-waveform seismoelectric and electroseismic interface response field models. The seismoelectric model employs the Sommerfeld approach, while the electroseismic model uses wavefield (de)composition techniques. These models provide electrokinetic theoretical predictions. Laboratory seismoelectric fluid/porous-medium interface response field measurements are performed as a function of time, space and fluid salinity. These measurements are compared against the seismoelectric model predictions. It is found that the seismoelectric model predictions excellently describe the measured interface response fields in terms of waveform, spatial amplitude pattern, and travel times. One scalar amplitude scaling factor is needed to reconcile the amplitudes of the theoretical predictions and the measurements. The factor is shown to depend on electric conductivity, i.e., predicted amplitudes significantly deviate at low pore fluid conductivity (~10E?3 S/m), while they are close to the actual measurements for higher conductivities (~10E?2 S/m). The poroelastic, electromagnetic and specific electrokinetic parameters that enter the seismoelectric model are independently measured or known from literature. The seismoelectric Sommerfeld integral model incorporates a so-called directivity function that closely resembles the independently measured spatial distribution pattern of the incident acoustic field. As a check, seismoelectric origin of the measured electric potentials is confirmed. Next, an expanded seismoelectric model that predicts interface response fields at different boundaries, as well as coseismic fields, is presented and compared against measurements. The seismoelectric model predictions excellently describe the measured electric potentials in terms of travel times, waveform, polarity, amplitude and spatial amplitude decay, demonstrating that seismoelectric effects are comprehensively described by theory. Finally, an electroseismic laboratory setup is described and electroseismic measurements are presented.GeotechnologyCivil Engineering and Geoscience
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
Shock-induced borehole waves in fractured formations
No full textNatural or hydraulic fractures are of major importance for the productivity of hydrocarbon reservoirs. Besides fracture detection, also the aperture and extension of the fractures are essential for a correct reservoir productivity estimate. There are many ways to detect and measure fractures, such as borehole televiewers and electrical borehole scans. A practical approach to investigate fracture properties is by means of acoustic logging. In this thesis, borehole waves along fractured media are investigated theoretically and experimentally. Theoretically, the effect of a fracture intersecting a vertical borehole can be described by the introduction of a frequency-dependent (dynamic) borehole fluid compressibility which is measured in the laboratory. The dynamic fluid bulk modulus comprises the intrinsic fluid stiffness, the borehole wall distensibility, and the radial fluid seepage into the adjacent (horizontal) permeable zones. The latter two effects tend to diminish the intrinsic fluid’s stiffness, giving rise to a lower effective bulk modulus amplitude and thus to a lower wave speed in the borehole. The radial oscillatory fluid seepage causes viscous friction in the adjacent zones and results in a phase lag between the pressure increase and the compression of the borehole fluid, leading to attenuation of the borehole waves. This seepage effect is expressed in terms of a so-called borehole dynamic wall impedance specifying the radial fluid velocity at the borehole wall as a function of the borehole pressure variations. If a borehole wave travels down from an undamaged zone into a fracture zone, it will encounter an impedance contrast causing the wave to partially reflect and partially transmit, thus revealing the presence of permeable fracture zone adjacent to the borehole. Stoneley wave propagation in porous and fractured formations is studied experimentally by means of a vertical shock tube facility. In this set-up, shock waves in air are generated that travel downwards into a water-saturated cylindrical rock sample that has a borehole drilled along the center axis. In this way, high-energy borehole waves can be generated with excellent repeatability. A logging probe is installed in the borehole to measure the pressure profiles. Reflection from the water-sample interface and from the free water interface can be recorded by means of a fixed pressure transducer mounted in the wall of the shock tube above the sample in the water layer. The fractures in the formation are represented by small horizontal slits in composite cylinders whose upper and lower parts are separated by small spacer poles. In this way, a variable horizontal fracture (slit) aperture can be obtained. Obviously these fractures form an open connection between the borehole fluid and the fluid outside the cylinder. Also mandrel samples are used for horizontal slits that are not open to the fluid outside the cylinder, thus representing fractures with finite radial extension. Wave experiments show that varying fracture widths significantly alter the recorded Stoneley wave pressure signal at fixed depth. The reflection and transmission of borehole tube waves over 1 and 5 mm fractures are correctly predicted by theory. Other wave experiments show that attenuation in boreholes adjacent to porous zones without fractures can be predicted by theory. This technique even allows a direct measurement of the permeability, although the acoustically measured permeability and the permeability measured by falling-head technique still show a significant discrepancy. This technique is directly applicable to fractured porous reservoir core samples.Geoscience & EngineeringCivil Engineering and Geoscience
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
Electrokinetic conversion
No full textIn their search for improved and new exploration tools, geophysicists have improved seismic and electromagnetic techniques. Since the 1930s it is known that there is a coupling between seismic and electromagnetic waves in the shallow subsurface of the earth. Electroseismic surveying and its reciprocal process, seismic-to-electromagnetic conversion, are methods for remotely identifying the presence of hydrocarbons in the subsurface of the earth. In this study we investigate electrokinetic coupling theoretically and experimentally. The origin of this effect lies in a very thin nano-layer which is conventionally present at all solid-fluid interfaces where an excess charge density with respect to the bulk charge density in the pore fluid exists. Any hydraulic disturbances of this nano-layer cause electric currents that are opposed by ionic counterflows generating electric fields. These effects become manifest when acoustic waves impinge upon the interface between two adjacent porous layers having different electro-mechanical properties. The theoretical basis for the coupling phenomena under investigation is conceptually imbedded in the framework of the combined Biot-Maxwell equations. An important aspect of the theory is the so-called dynamic (i.e., frequency-dependent) coupling factor. This coupling factor is studied in a dedicated experimental set-up where an oscillating flow through a porous material generates electric fields. We address the frequency-dependency of the coupling coefficient, the mathematical description and experimental detection results. We validate that the governing model is capable of modeling the coupling effect.Geoscience & EngineeringCivil Engineering and Geoscience
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