1,720,962 research outputs found
Estimation Of Formation Parameters Using Full Waveform Acoustic and Shear Wave Logs
Full text linkA combination of borehole Stoneley waves from full waveform acoustic logs and direct
shear wave logs was used to estimate formation permeability and shear wave velocity.
Data sets used here were collected by Area's array full waveform acoustic logging tool
and shear wave logging tool. The P- and S-wave velocities of the formation are determined
by threshold detection with cross-correlation correction from the full waveform
and the shear wave log, respectively. The full waveform acoustic logging data are also
processed using the Extended Prony's method to estimate the borehole Stoneley wave
phase velocity and attenuation as a function of frequency. Two different borehole models
are considered for the inversion of Stoneley wave velocity and attenuation data. They
are the isotropic elastic and the porous isotropic borehole models. Inversion parameters
include shear wave velocity and formation permeability. Inverted shear wave velocities
and permeabilities are compared with the shear wave log and the core permeability
measurements, respectively, for an integrated interpretation and possible identification
of shear wave anisotropy.Massachusetts Institute of Technology. Borehole Acoustics and Logging ConsortiumUnited States. Dept. of Energy (Grant DE-FG02-86ER13636
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
Finite Difference Modelling Of Acoustic Logs In Vertically Heterogeneous Biot Solids
Full text linkThis paper discusses the results of tests carried out on a finite difference formulation
of Biot's equations for wave propagation in saturated porous media which vary in range and depth (Stephen, 1987). A technique for modeling acoustic logs in two dimensionally
varying Biot solids will give insight into the behavior of tube waves at permeable fractures and fissures which intersect the borehole. The code agrees well with other finite difference codes and the discrete wavenumber code for small porosity in the elastic limit of Biot's equations. For large porosity (greater than one per cent)
in the elastic limit or for the acoustic limit, good agreement is not obtained with
the discrete wavenumber method for vertically homogeneous media. The agreement is
worst for amplitudes of the pseudo-Rayleigh wave. The amplitude of the Stoneley wave
and the phase velocities of both waves could be acceptable for some applications. An
example is shown of propagation across a horizontal high porosity stringer in a Berea
sandstone. Reflections from the stringer are observed but given the inaccuracies of
the pseudo-Rayleigh waves for vertically heterogeneous media the amplitudes for the
stringer model are questionable. We propose a three stage approach for further work:
1) Use the Virieux scheme instead of the Bhasavanija scheme for the finite difference
template. The Virieux scheme has been shown in other studies to be more accurate
for liquid-solid interfaces. 2) Run the present code for lower frequency sources to
emphasize Stoneley waves and diminish pseudo-Rayleigh waves. Stoneley waves are
most sensitive to permeability variations which are the primary objective of Biot wave
studies. 3) Develop a finite difference code for Biot media with the fluid-solid boundary
conditions specifically coded. This code would be suitable for studying constant radius
boreholes in vertically varying Biot media.Massachusetts Institute of Technology. Full Waveform Acoustic Logging Consortiu
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
Inversion For Permeability From Stoneley Wave Velocity And Attenuation
Full text linkThe in situ permeability of a formation is obtained by the inversion of Stoneley wave
phase velocity and attenuation, which are evaluated by applying the Extended Prony's
method to the array sonic logging data. The Maximum Likelihood inversion is used
together with logarithmic parameterization of the permeabilities. Formation shear
wave velocity is also inverted for. This process is tested on both synthetic and field
data. Logarithmic parameterization contributes to rapid convergence of the algorithm.
Permeabilities estimated from field data are in good agreement with core measurements.Massachusetts Institute of Technology. Full Waveform Acoustic Logging Consortiu
Fourth-Order Finite Difference Acoustic Logs In A Transversely Isotropic Formation
Full text linkIn this paper we present a finite difference scheme for seismic wave propagation in
a fluid-filled borehole in a transversely isotropic formation. The first-order hyperbolic
differential equations are approximated explicitly on a staggered grid using an algorithm
that is fourth-order accurate in space and second-order accurate in time. The grid
dispersion and grid anisotropy are analyzed. Grid dispersion and anisotropy are well
suppressed by a grid size of 10 points per wavelength. The stability condition is also
obtained from the dispersion analysis. This finite difference scheme is implemented
on the nCUBE2 parallel computer with a grid decomposition algorithm. The finite
difference synthetic waveforms are compared with those generated using the discrete
wavenumber method. They are in good agreement. The damping layers effectively
absorbed the boundary reflections. Four vertically heterogeneous borehole models: a
horizontal layered formation, a borehole with a radius change, a semi-infinite borehole,
and a semi-infinite borehole with a layer, are studied using the finite difference method. Snapshots from the finite difference results provide pictures of the radiating wavefields.Massachusetts Institute of Technology. Borehole Acoustics and Logging Consortiu
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