1,721,038 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
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
Migrationsmuster von Injektions- und Postinjektionsinduzierter Seismizität: Anwendung auf Lokale und Regionale Maßstäbe
No full textIn many cases, fluid injections into the subsurface have been recognised to cause seismic activity. Understanding the driving physical processes of these man-made earthquakes and their occurrence in time and space is of significant scientific and public interest. Not only does a deeper insight into the physics of fluid-induced seismicity yield fundamental knowledge on seismogenic processes in the brittle Earth's crust but it is also a crucial point for reservoir characterisation and injection performance. In the end, these are inevitable steps towards seismic hazard assessment and risk mitigation.
The focus of this thesis is on spatio-temporal migration patterns of fluid-induced seismicity at different scales. In a first study, I work with seismicity induced by the rather local effect of single-borehole, high-pressure fluid injections. This part is followed by two studies on earthquakes caused by the regional impact of large-volume injections through numerous wells under gravity. Whereas high-pressure, single-borehole injections are typically used for hydraulic fracturing operations to enhance shale gas recovery from unconventional reservoirs and for hydraulic stimulations of geothermal heat production sites, gravity-driven, large-volume injections are part of wastewater disposal which has been performed throughout the last decade in the central U.S. In the context of this work, wastewater is a saline fluid, co-produced with natural oil and gas, which is re-injected into the subsurface at a later stage. Apart from the actual injection phase, I also consider seismic events that occur in the postinjection interval of high-pressure fluid injections and in times of decreasing wastewater disposal volumes. I assess the governing question which parameters and physical processes control features of fluid-induced seismicity in time and space by joining different methods.
By means of analytical solutions, I derive a novel scaling law for postinjection-induced seismicity in the first study. My findings suggest that the spatio-temporal evolution of the seismically active zone depends on the index of non-linearity and the Euclidean dimension of pore-fluid pressure diffusion.
I combine these results with numerical modelling of non-linear pore-fluid pressure diffusion in 3D. Based on the numerical pressure solutions, I generate catalogues of synthetic seismicity to validate the novel scaling relation. The subsequent successful application to different borehole injection case studies demonstrates that the relation can be used to estimate the two driving parameters of spatio-temporal features of seismicity induced by high-pressure fluid injections.
An analytical approach, combined with numerical modelling, also forms the basis of the second study. Using known relations for reservoir-induced seismicity, I present a new first-principle model for wastewater disposal-induced earthquakes observed in the crystalline basement in Central Oklahoma, U.S., called underground reservoir-induced seismicity (URIS). The model consists of the following physical mechanisms; a normal stress acting on the seismogenic basement induced by the mass of the disposal fluid added to the pore-space of the target injection formation, the diffusion of pore-fluid pressure in the basement, and poroelastic coupling which contributes to pore-fluid pressure- and stress changes in the basement.
I implement the novel conceptual model in a numerical finite element model, solving for poroelastic pressure- and stress changes. The obtained values are then used to calculate failure criterion stress changes and to generate synthetic clouds of seismicity. My findings demonstrate that the URIS model captures the observed time- and depth-distribution of earthquakes located in the study area both during constant and in times of declining injection volumes.
In the third study, I combine results from a time-dependent 2D cross-correlation with numerical modelling solutions to explain spatio-temporal patterns of wastewater disposal-induced seismicity in southern Kansas which seems to migrate away from the high-volume disposal area with time.
The cross-correlation reveals that the majority of earthquakes preferably occurs towards the east-northeast of the disposal wells. This feature may be explained by the directional migration of poroelastic stresses and pore-fluid pressure diffusion, probably caused by a large-scale, fault-induced anisotropic character of the basement permeability. Two-dimensional numerical modelling of poroelastic pressure- and stress changes for the study area suggests that the observed shift of the seismically active zone is guided by the high permeability of the injection formation and that the depth-migration of pore-fluid pressure and poroelastic stresses is driven by the basement permeability.
In general, the presented study verifies that poroelastic fluid-rock interaction may be crucial to explain seismic activity far away from large-volume disposal wells. Furthermore, it proposes that a volume reduction lowers seismicity rates only locally and that a state-wide reduction of the seismicity may require many more years.
Overall, the findings of the three works contribute to a general understanding of driving processes and parameters of fluid-induced seismicity across different scales. Thus, the presented novel model approaches may ultimately be used in future studies to assess and mitigate the risk posed by anthropogenic earthquakes
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
Earthquake Nucleation Processes Across Different Scales and Settings
Full text linkExtended nucleation phases of earthquakes have been regularly observed, yet the underlying mechanisms governing the initiation phase of rupture are yet to be understood in detail. Currently two end member models exist to explain earthquake nucleation: one model claiming that the nucleation phase of a small earthquake is indistinguishable from that of a large one, while the other proposes fundamental differences in the underlying process. Previous studies have been using the same seismological observations to argue for either model, leaving the need of further investigations into the nucleation behavior of earthquakes across scales and different settings. The thesis at hand contributes to the current discussion on earthquake nucleation by providing additional observational evidence for extended nucleation phases, complex rupture interaction and growth across a number of different scales and settings. Here, earthquake nucleation is investigated for three different scenarios, each with varying degrees of complexity: 1) the controlled case of induced seismicity in hydraulic stimulations of geothermal reservoirs, where rupture growth is assumed to be primarily governed by anthropogenic activity, 2) the partly-controlled setting of a geothermal field with a long history of fluid injection and production, and 3) the uncontrolled case of natural seismicity in the central Sea of Marmara, where earthquake nucleation is purely governed by the regional tectonics.
First, the temporal evolution of seismicity and the growth of observed moment magnitudes for a range of past and present hydraulic stimulation projects associated with the creation of enhanced geothermal systems are analyzed. They reveal a clear linear relation between injected fluid volume/hydraulic energy and cumulative seismic moments. For most projects studied, the observations are in good agreement with existing physical models that predict a relation between injected fluid volume and maximum seismic moment of induced events. This suggests that seismicity results from a stable, pressure controlled rupture process at least for an extended injection period. Overall evolution of seismicity is independent of tectonic stress regime and is most likely governed by reservoir specific parameters, such as the preexisting structural inventory. In contrast, a few stimulations reveal unbound increase in seismic moment suggesting that for these cases evolution of seismicity is mainly controlled by stress field, the size of tectonic faults and fault connectivity. The uncertainty over whether or not a transition between behavior is likely to occur at any point during the injection is what motivates the need for a next generation monitoring and traffic-light system accounting for the possibility of unstable rupture propagation from the very beginning of injection by observing the entire seismicity evolution at high resolution for an immediate reaction in injection strategy. Furthermore, the majority of pressure-controlled stimulations shows the potential of actively controlling the size of induced earthquakes, if an injection protocol is chosen based on continuous feedback from a near-real-time seismic monitoring system.
Second, moderate sized earthquakes at The Geysers geothermal field (California), where years of injection and production across hundreds of wells have led to a unique physical environment, are studied. While overall seismicity at The Geysers is generally governed by anthropogenic activities, contributions of individual wells or injection activities are hard to distinguish, thus making detailed managing of occurring magnitudes challenging. New high-resolution seismicity catalogs framing the occurrence of 20 ML > 2.5 earthquakes were created. The seismicity catalogs were developed using a matched filter algorithm, including automatic determination of P and S phase onsets and their inversion for absolute hypocenter locations with corresponding uncertainties. The selected 20 sequences sample different hypocentral depths and hydraulic conditions within the field. Seismic activity and magnitude frequency distributions displayed by the different earthquake sequences are correlated with their location within the reservoir. Sequences located in the northwestern part of the reservoir show overall increased seismic activity and low b values, while the southeastern part is dominated by decreased seismic activity and higher b values. Periods of high injection coincide with high b values and vice versa. These observations potentially reflect varying differential and mean stresses and damage of the reservoir rocks across the field. Additionally, a systematic search for seismicity localization using a multi-step cross-correlation analysis was performed. No evidence for increased correlation between the occurring seismicity and the mainshock for any of the 20 sequences could be seen, indicating that each main nucleation spot was seismically silent prior to the main rupture. However, a number of highly inter-correlated earthquakes for sequences below the reservoir and during high injection activity is observed. Under these conditions, the seismicity surrounding the future mainshock source region is more concentrated and might be evidence for a cascading nucleation process. About 50% of analyzed sequences exhibit no change in seismicity rate in response to the large main event. However, we find complex waveforms at the onset of the main earthquake, suggesting that small ruptures spontaneously grow into or trigger larger events, consistent with a cascading type nucleation.
Third, the spatiotemporal evolution of seismicity during a sequence of moderate (MW4.7 and MW5.8) earthquakes occurring in September 2019 at the transition between a creeping and a locked segment of the North Anatolian Fault in the central Sea of Marmara (Turkey) was analyzed. A matched filter technique was applied to continuous waveforms from the regional network, substantially reducing the magnitude threshold for detection. Sequences of foreshocks preceding the two mainshocks are clearly seen, exhibiting different behaviors: a migration of the seismicity along the entire fault segment on the long-term and a concentration around the epicenters of the large events on the short-term. Suggesting that both seismic and aseismic slip during the foreshock sequences change the stress state on the fault, bringing it closer to failure. Furthermore, the observations also suggest that the MW4.7 event contributed to weaken the fault as part of the preparation process of the MW5.8 earthquake.
Combining the results obtained from different settings, it becomes apparent that, regardless of the tectonic setting and degree of anthropogenic control over the seismicity, there is a wide range of complex nucleation behaviours not yet explained by any of the current models of earthquake nucleation. A simplistic view of earthquake nucleation as either a deterministic or a stochastic process seems inconsistent with the obtained results and fails to account for a more complex nucleation behaviour. Observations from The Geysers and the western Sea of Marmara earthquake sequence, suggest that both cascade triggering and aseismic slip can play major roles in the nucleation of moderate sized earthquakes. Both mechanisms seem to jointly contribute to fault initiation, even within the same rock volume. A separation of the two mechanisms can potentially be thought of at The Geysers, where cascade triggering seems to dominate in highly damaged parts of the reservoir, suggesting that the anthropogenic activity can at least partially influence the nucleation behavior of the occurring seismicity. This would be in agreement with the results obtained from analysis of hydraulic stimulations, where during the pressure-controlled phase of injection rupture growth is controlled by the injected fluid
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