1,720,960 research outputs found
Converted-wave seismic imaging: Amplitude-balancing source-independent imaging conditions
Get PDFWe have developed crosscorrelational and deconvolutional forms of a source-independent converted-wave imaging condition (SICW-IC) and show the relationship between them using a concept of conversion ratio coefficient, a concept that we developed through reflection, transmission, and conversion coefficients. We applied the SICW-ICs to a two half-space model and the synthetic Marmousi I and II models and show the sensitivity of the SICW-ICs to incorrect wave speed models. We also compare the SICW-ICs and source-dependent elastic reverse time migration. The results of SICW-ICs highlight the improvements in spatial resolution and amplitude balancing with the deconvolutional forms. This is an attractive alternative to active and passive source elastic imaging
Data-driven estimation of the sensitivity of target-oriented time-lapse seismic imaging to source geometry
Get PDFThe goal of time-lapse imaging is to identify and characterize regions in which the earth’s material properties have changed between surveys. This requires an effective deployment of sources and receivers to monitor the region where changes are anticipated. Because each source adds to the acquisition cost, we should ensure that only those sources that best image the target are collected and used to form an image of the target region. This study presents a data-driven approach that estimates the sensitivity of target-oriented imaging to source geometry. The approach is based on the propagation of the recorded baseline seismic data backward in time through the entire medium and coupling it with the estimated perturbation in the subsurface. We test this approach using synthetic surface seismic and time-lapse VSP field-data from the SACROC field. These tests show that the use of the baseline seismic data enhances the robustness of the sensitivity estimate to errors, and can be used to select data that best image a target zone, thus increasing the signal-to-noise ratio of the image of the target region and reducing the cost of time-lapse acquisition, processing, and imaging.Massachusetts Institute of Technology. Earth Resources Laborator
Converted-wave seismic imaging: Amplitude-balancing source-independent imaging conditions
Get PDFWe have developed crosscorrelational and deconvolutional forms of a source-independent converted-wave imaging condition (SICW-IC) and show the relationship between them using a concept of conversion ratio coefficient, a concept that we developed through reflection, transmission, and conversion coefficients. We applied the SICW-ICs to a two half-space model and the synthetic Marmousi I and II models and show the sensitivity of the SICW-ICs to incorrect wave speed models. We also compare the SICW-ICs and source-dependent elastic reverse time migration. The results of SICW-ICs highlight the improvements in spatial resolution and amplitude balancing with the deconvolutional forms. This is an attractive alternative to active and passive source elastic imaging. Keywords: imaging; converted wave; elastic; illumination; multicomponen
Theory and application of source independent full wavefield elastic converted phase seismic imaging and velocity analysis
No full textThesis: Ph. D., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2015.Cataloged from PDF version of thesis.Includes bibliographical references (pages 169-177).The recorded seismic signal contains full information about the source that generated the seismic waves and the path along which the seismic waves travel and interfere with subsurface. However, source information is not an explicit part of the seismic record and thus is a large source of uncertainty in seismic imaging and velocity analysis applications. In this thesis, we develop source-independent methods for seismic imaging, seismic trace interpolation and velocity analysis using the interference between pure (PP and SS) and converted-phase (PS and SP) waves. For seismic imaging, we develop amplitude-balancing source-independent converted-phase seismic imaging conditions and introduce a concept of conversion ratio coefficients to provide a physical and mathematical foundation for source-independent converted-phase (SICP) imaging. For seismic trace interpolation, we develop a scheme for migration/de-migration to suppress migration-based artifacts due to sparse station deployments. For velocity analysis, we present first a source-independent space-lag domain Extended SICP imaging condition (ESICP-IC). Then, we mathematically derive an optimization scheme for source independent converted-phase wave equation migration velocity analysis (SICP-WEMVA). We investigate numerically the stability and convergence of SICP-ICs, SICP interpolation and SICP-WEMVA with synthetic data. Finally, using the developed methodologies, we investigate the subsurface structure of the Hengill geothermal area in Iceland using the abundant micro-seismic activity of the region. The constructed SICP seismic images show detailed subsurface structure of the Hengill area that is well correlated with previous seismic and resistivity studies. Also, we find that the amplitudes of the images are well correlated with a low resistivity region of the geothermal area. The reason for this correlation is not fully understood, but may provide an additional tool for investigation of the Hengill site.by Andrey Hanan Shabelansky.Ph. D
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
Monitoring Seismic Attenuation Changes Using a 4D Relative Spectrum Method in Athabsca Heavy Oil Reservoir, Canada
Get PDFHeating heavy oil reservoirs is a common method for reducing the high viscosity of heavy oil and thus increasing the recovery factor. Monitoring these changes in the reservoir is essential for delineating the heated region and controlling production. In this study, we measure the changes in the seismic wave attenuation of a heavy oil reservoir by constructing time-lapse Q[superscript -1] factor maps using a 4D-relative spectrum method. This method estimates seismic attenuation from surface reflection seismic surveys by calculating, for each trace in each survey, the attenuation (Q[superscript -1]) using the spectral ratio (Toksoz et al. (1979)) between a reference reflector above the reservoir and a second reflector below the reservoir. The results of our study on a real data set exhibit alignment along the injection wells, indicating that seismic attenuation can be used to monitor changes in a heavy oil reservoir
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
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