1,721,088 research outputs found
Improved Green’s functions from seismic interferometry
No full textUnder certain theoretical assumptions, the theory of seismic interferometry allows the construction of artificial (or virtual) sources and
receivers at the locations of receivers in a physical experiment. This
is done by redatuming the physical sources to be at the locations of
the physical receivers. Each redatumed trace is formed by stacking
the cross-correlations of appropriate recorded traces from each physical shot. For the resulting stacked traces to be a valid approximation
certain requirements, like an adequate number of surface sources with
a small enough spacing in the acquisition geometry, must be met. If
these requirements are not met, the resulting virtual shot gather will
contain artifacts. In this paper, we analyze both the sets of correlated
traces (correlograms) and their stack. We observe that it is possible
to reduce certain artifacts in the stacked traces by novel filtering operations. These filtering operations may have broad utility in all of
seismic interferometric applications.Massachusetts Institute of Technology. Earth Resources Laborator
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
Fractured Reservoir Characterization using Azimuthal AVO
No full textOrdinary least squares is used to investigate the ability to detect changes in physical properties
using Amplitude Versus Offset (AVO) information collected from seismic data. In order to characterize
vertically aligned fractures within a reservoir, this method is extended to Azimuthal AVO (AVOA)
analysis. Azimuthal AVO has the potential not only to detect fractured zones, but to spatially describe the
fracture strike orientation and changes in fracture or fluid properties. Depending on the data acquisition
geometry, signal-to-noise ratio, and extent of fracturing, AVOA analysis can be marginally successful. A
study of the robustness and limitations of AVOA analysis is therefore first classified with synthetic data.
These methods are then applied to seismic data collected during an Ocean Bottom Cable (OBC) survey
over a known fractured reservoir.Massachusetts Institute of Technology. Earth Resources LaboratoryUnited States. Dept. of Energy (Grant DE-FC26-02NT15346)Eni S.p.A. (Firm
Comparison Of Scattered Energy Using Point Scatterers Versus Full 3D Finite Difference Modeling
No full textWe present results of 3D numerical modeling using a series of simple point scatterers to create synthetic seismic shot records collected over regular, discrete, vertical fracture systems. The background medium is taken to be constant velocity. The model contains a series of point scatterers delineating the top tip and bottom tip of each vertical fracture. We use these results to gain an understanding of some of the features seen in full 3D elastic modeling of vertical fractures. We compare our results to those of Willis et al (2003) and Willis et al (2004) for their 5 layer model with 50m spacing between discrete, vertical fractures. Our modeling shows that a series of back scattered events with both positive and negative moveouts are observed when the shot record is oriented normal to the direction of fracturing. When the shot record is both located in the middle of the fractured zone and is oriented normal to the direction of fracturing, a complicated series of beating is observed in the back scattered energy. When the shot record is oriented parallel to the fracturing, ringing wavetrains are observed which moveouts similar to reflections from many horizontal layers. The point scattering models are, in general, very consistent with the full 3D elastic modeling results.Eni S.p.A. (Firm)United States. Dept. of Energy (Grant number DE-FC26-02NT15346)Massachusetts Institute of Technology. Earth Resources Laborator
Interferometric correlogram-space analysis
No full textSeismic interferometry is a method of obtaining a virtual shot
gather from a collection of actual shot gathers. The set of traces
corresponding to multiple actual shots recorded at two receivers is
used to synthesize a virtual shot located at one of the receivers and
a virtual receiver at the other. An estimate of a Green’s function between
these two receivers is obtained by first cross-correlating pairs
of traces from each of the common shots and then stacking the resulting
cross-correlograms. In this paper, we study the structure of
cross-correlograms obtained from a VSP acquisition geometry using a
surface source reflected by flat or dipping layers and/or diffracted by
point inclusions. The model is purely acoustic. The shape of events
in the cross-correlogram space can be used to infer the location and
geometry of a subsurface structure. A pilot wavelet created by a curvilinear
stacking process is used as a detector of predicted events in the
cross-correlogram. Results of a semblance-based velocity scan of the
cross-correlograms using curvilinear stacks can be used to improve the
quality of the virtual gather.Massachusetts Institute of Technology. Earth Resources Laborator
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