1,720,961 research outputs found
3D fourier reconstruction of irregularly sampled seismic Gathers : analysis of the extended model resolution matrix
No full textFourier reconstruction is basically a linear inverse problem that attempts to recover the Fourier spectrum of the seismic wave-field from irregularly sampled data along the spatial coordinates. The estimated Fourier coefficients are then used to reconstruct the data in a regular grid via a standard inverse Fourier transform (IDFT or IFFT). Unfortunately, this kind of inverse problem is usually under-determined and illconditioned. For this reason the Fourier reconstruction with minimum norm (FRMN) adopts a weighted damped least-squares inversion to retrieve a unique and stable solution. In this work we show how damping can introduce deleterious artifacts on the reconstructed 3D data. To quantitatively describe this issue, we introduce the concept of extended resolution matrix (ERM) and we formulate the reconstruction problem as an appraisal problem. Through the simultaneous analysis of the ERM and of the noise term, we can assess the validity of the reconstructed data and verify the possible bias introduced by the inversion process. Also, we can guide the parametrization of the forward problem to minimize the occurrence of unwanted artifacts. Real data from a 3D marine common shot gather are used to discuss our approach and to show the results of FMNR reconstruction
Studio delle riverberazioni su dati sismici Ocean Bottom Cable acquisiti su fondali a bassa profondità e applicazione di tecniche di Pz summation e Depegleg
No full textBayesian Estimation of Reservoir Properties by Means of Wide-angle AVA Inversion and a Petrophysical Zoeppritz Equation
No full textWe apply a target-oriented amplitude versus angle (AVA) inversion to estimate the petrophysical properties of a reservoir in offshore Nile delta. A linear empirical rock-physics model derived from well log data provides the link between the petrophysical properties (porosity, shaliness and saturation) and the P-wave, S-wave velocities and density, which we use to re-parameterize the exact Zoeppritz equations. The so derived equations are the forward model engine of a linearized Bayesian AVA inversion that, for each data gather, inverts the AVA of the target reflections to estimate the petrophysical properties of the reservoir layer, keeping fixed the cap-rock properties. We make use of the iterative Gauss-Newton method to solve the inversion problem. For each estimated petrophysical property, we discuss the benefits introduced by wide-angle reflections in constraining the inversion and we compare the posterior probability distribution (PPD) analytically obtained after local linearization of the inversion with the PPD numerically computed with a Markov Chain Monte Carlo (MCMC) method. It results that porosity is the best resolved parameter and that wide-angle reflections effectively constrain the shaliness estimates but do not guarantee reliable saturation estimates. It also results that the local linearization returns accurate PPDs in good agreement with the MCMC estimates
Probabilistic Seismic-petrophysical Inversion Applied for Reservoir Characterization in Offshore Nile Delta
No full textWe apply a two-step probabilistic seismic-petrophysical inversion for the characterization of a clastic, gas-saturated, reservoir in offshore Nile delta. The first step is a Bayesian linearized amplitude versus angle (AVA) inversion in which the reflection seismic data are used to derive the elastic properties in the reservoir zone and the associated uncertainties. The estimated elastic properties constitute the input to the second step, which is a probabilistic petrophysical inversion in which we take into account the uncertainties associated with the rock-physics model, with the estimated elastic properties and with the recorded seismic data. In particular, we formulate the petrophysical inversion considering a Gaussian mixture distribution for the petrophysical properties. This allows us to properly describe the multimodality and the correlation that characterize the distribution of these properties in the reservoir zone where sand channels are surrounded by thick shale sequences. In addition, in the petrophysical inversion we test two different rock-physics models that link the elastic properties to the petrophysical properties. The first is an empirical linear model, whereas the second is a theoretical non-linear model. The synthetic and the field data inversions confirm the reliability of the two different rock physics models and the applicability of the inversion method
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
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