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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
New Applications In The Inversion Of Acoustic Full Waveform Logs - Relating Mode Excitation To Lithology
No full textExisting techniques for the quantitative interpretation of waveform data have been
based on one of two fundamental approaches: 1) simultaneous identification of compressional and shear wave velocities; and 2) least-squares minimization of the difference between experimental waveforms and synthetic seismograms. Techniques based on the first approach do not always work, and those based on the second seem too numerically
cumbersome for routine application during data processing. An alternative approach is
tested here, in which synthetic waveforms are used to predict relative mode excitation
in the composite waveform. Synthetic waveforms are generated for a series of lithologies
ranging from hard, crystalline rocks (V[subscript p] = 6.0 km/s and Poisson's ratio = 0.20) to soft, argillaceous sediments (V[subscript p] = 1.8 km/s and Poisson's ratio = 0.40). The series of waveforms illustrates a continuous change within this range of rock properties. Mode energy within a characteristic velocity window is computed for each of the modes in the set of synthetic waveforms. The results indicate that there is a consistent variation in mode excitation in lithology space that can be used to construct a unique relationship between relative mode excitation and lithology.Massachusetts Institute of Technology. Full Waveform Acoustic Logging ConsortiumChevronTexaco Firm (Fellowship
Modelling Of Downhole Seismic Sources II: An Analysis Of The Heelan/Brekhovskikh Results And Comparison Of Point Source Radiation To Radiation From Boreholes
Full text linkThe work of Heelan (1952, 1953a,b) was one of the first studies of wave propagation
from a cylindrical boundary. Heelan attempted to model the radiation emanating from
a cylindrical shot hole filled with dynamite. To do so he applied a constant stress to
a finite length of an empty infinite cylindrical cavity embedded in an infinite elastic,
homogeneous medium. The stresses he considered were axial, torsional, and radial
stresses. The radial and axial stresses were required to be proportional to each other
and of the same duration.
To date Heelan's work has been referenced in over 100 articles and 15 different
journals including recent works (Paulsson, 1988) . His results have also been compared
with results from the reciprocity theorem (White, 1953, 1960) and played an integral
part of important books including those by Brekhovskikh (1960, 1980) and White
(1965, 1983). His fundamental contributions were the description of shear wave lobes,
the famous four-leaved rose, generated from a radial source in a borehole and that the
radiation patterns for an axial source and a torsional source in a borehole have the
same geometries as the point axial and torsional sources in infinite media.
Despite the importance of this work, Heelan's results have been criticized by Jordan
(1962) who dismissed the work as mathematically unsound and Abo-Zena (1977) who
devoted an appendix of his 1977 paper to criticizing Heelan's results. The main point
of contention has been the use of contour analysis in his first paper (Heelan, 1953a).
Although Heelan's work did not include a fluid-filled borehole which is a crucial
omission for our purposes, his work may nonetheless be seen as a starting point for the
modelling of downhole seismic sources. For instance, Lee and Balch (1982) developed
radiation patterns for fluid boreholes which were simple extensions of Heelan's results.
Additionally, one particular application of Heelan's theory is in the preliminary development of downhole seismic Sources that often require dry holes until the electronics can be properly shielded. For that reason, an exhaustive examination of the mathematics and physics that went into Heelan's first paper was undertaken to determine if his formulation was correct.
The fundamental basis of Heelan's work was a variant of the Sommerfeld integral,
an integral of cylindrical waves, in which he unfortunately did not specify the contour.
To overcome this obstacle of an unknown contour a parallel method suggested by
Brekhovskikh (1960, 1980) was implemented. Brekhovskikh used the Weyl integral,
an integral over plane waves, to duplicate Heelan's results for the radial and torsional
stresses. However he does no justification of the extensive algebra or analysis involved
and does not include the effects of axial stress. Thus in this paper, we have completed
and elucidated the work that Brekhovskikh initiated and moreover indirectly verified
that Heelan's results were correct.
Additionally, we found that Abo-Zena's and Heelan's initial formulations were
equivalent. The only difference was in a reversal of the separation of variables procedure necessary to replicate this work and also in Abo-Zena's USe of the Laplace
transform where Heelan used the Fourier transform. However, Abo-Zena's results do
extend Heelan's by allowing the source function to vary over the distance in which it
is applied. The far field results of Abo-Zena and Heelan are equivalent (White, 1983)
only if a 1/μ correction is applied to Abo-Zena's results.
The first half of this paper is very involved mathematically but much of the algebra
is relegated to Appendix A. Having verified that Heelan's results were correct we then
proceed to compare Heelan's results with well established point source representations
known in the literature (White, 1983) and also with radiation patterns from point
sources and stress sources in a fluid-filled borehole (Lee and Balch, 1982). These comparisons will help us isolate the propagation effects of the fluid and the geometrical
effect of the borehole. One unique aspect to our approach will be the consideration of
radiation from boreholes surrounded by varying lithologies instead of just the Poisson
solid as is commonly done. The lithologies to be considered include a soft sediment
(Pierre shale) and two more indurated sediments, Berea sandstone and Solenhofen
limestone. By following this approach we show that the effect on the radiation magnitude
can be substantial due to changes in lithology in addition to isolating the relative
effects of the borehole and the fluid.Massachusetts Institute of Technology. Full Waveform Acoustic Logging Consortiu
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
Evaluation And Prediction Of Shear Wave Velocities In Soft Marine Sediments
Full text linkShear wave velocities from full waveform acoustic logs were determined at DSDP Site
613 using the spectral ratio inversion method. Discrete shear wave velocities for a 350
meter interval at 0.5-2 meter depth increments were calculated. Shear wave velocities
were not evaluated for the upper 130m of the log because of data recording problems.
The sediments of Site 613 represent a progression from carbonaceous-siliceous oozes
through partial lithification and cementation. A method for predicting shear wave
velocities using Wood's equation, the bulk moduli of water and carbonate grains, the
P-wave velocity and porosity from well logs will be described. The predictions of this
method provided a theoretical maximum value for the shear wave velocity to compare
with the inversion results. In general, the method works well for shear wave velocities
greater than 800 m/s. The inverted data fall just below the predicted theoretical
maximum value from Wood's equation and agree quite well with the trends. Below
this velocity threshold, trends with depth and Poisson's ratio and the divergence of
the inversion itself seem to indicate incorrect behavior.Massachusetts Institute of Technology. Full Waveform Acoustic Logging ConsortiumNational Science Foundation (U.S.) (Grant OCE84-08761)National Science Foundation (U.S.) (Grant OCE87-10226)National Science Foundation (U.S.) (Grant OCE87-20032
Modelling Of Downhole Seismic Sources I: Literature Review, Review Of Fundamentals, Impulsive Point Source In A Borehole
Full text linkThis paper represents the first of a two paper sequence comprising a multi-faceted introduction to the numerical and analytical modelling of seismic sources in a borehole.
Part one will present a literature review and a review of the fundamental mathematical
descriptions of wave propagation outside a borehole. By listing the mathematical
descriptions here we can show the equivalence and interrelationships of many treatments
presented in the literature. Part one will conclude with an outline of the familiar
discrete wavenumber technique as applied to modelling radiation outside a borehole
from a point source inside a borehole. Part two will attempt to isolate the effects of
the fluid-filled borehole on the radiation of a borehole source by comparing radiation
patterns for three cases: a point source in an infinite medium, a stress applied to the
wall of an empty borehole (Heelan's (1953) result) and a point source in a fluid-filled
borehole (Lee and Balch, 1982). Heelan's results will also be analyzed and defended
against criticism of them brought by Jordan (1962) and Abo-Zena (1978).
The literature review will be thorough and will include the work done directly on
modelling downhole seismic sources and the comparatively larger amount of work done
on modelling sources for acoustic logging purposes which is directly applicable.
Different authors publishing work on seismic sources have made widely different
symmetry assumptions ranging from no symmetry to axisymmetry to symmetry along
the z axis. These differences hamper the ability to directly compare results between
the different treatments. Compounding the differences in symmetry are the use of
different time dependencies ([superscript -iwl], [superscript iwl]) and the use of different Hankel function or modified Bessel function strategies. Therefore, the mathematical fundamentals of wave propagation in a borehole from the different symmetry systems are presented here in a consistent notation and are related to each other and treatments in the literature to help eliminate duplication of effort.
Finally, wave propagation outside a borehole is examined using synthetic seismograms.
For the synthetic seismograms, a point source inside the borehole is used as a
source and the synthetics are calculated using the discrete wavenumber method. The
synthetic seismograms are measured along vertical arrays of point receivers placed outside the borehole and for lithologies of Pierre shale, Solenhofen limestone, and Berea
sandstone. The method and the resulting seismograms are outlined in this paper along
with a brief description of the discrete wavenumber technique.Massachusetts Institute of Technology. Full Waveform Acoustic Logging Consortiu
Modelling Of Full Waveform Acoustic Logs In Soft Marine Sediments
No full textFull waveform acoustic logs obtained from the Deep Sea Drilling Project (DSDP) were
modelled using synthetic full waveform acoustic logs. These synthetic logs were calculated using the discrete wavenumber method. The model is that of a fluid-filled
borehole with a rigid logging tool in the center. Results from the modelling indicate
that V[subscript p],V[subscript 3] and V[subscript f] along with P wave attenuation (l/Q[subscript 3]) are the primary controls on the full waveform acoustic logs of these soft sediments. S-wave attenuation (l/Q[subscript 3]) does not play a major role because the S-wave velocities (V[subscript 3]) of these fluid-saturated marine
oozes are lower than the borehole fluid velocity (V[subscript f]), thus there is no refracted S-wave or pseudo-Rayleigh wave. However, the formation S-wave velocity does affect the amplitude of the observed P-wave train. Density variations by themselves have almost no discernible effect on the synthetics although in practice a change in density often is concurrent with a change in lithology and formation velocities. Matching the synthetic full waveform acoustic logs to those obtained during Leg 95 of the DSDP was formally done by a least squares linearized iteration inversion procedure. Only the P wavetrain and its associated leaky modes were taken into account. The forward model used in the inversion was a P-wave train generated by the branch cut integral method. Stable
results in V[subscript 3] and Q[subscript p] were obtained. Variations in the velocity and attenuation from the inversion correlates with sedimentary units delineated from conventional logs and lithologic units identified by shipboard stratigraphers for the Baltimore Canyon Trough area. Full waveform logs, in combination with conventional logs, help to identify changes in the physical properties of these sediments as a result of the diagenesis of biogenic silica and calcium carbonate.Massachusetts Institute of Technology. Full Waveform Acoustic Logging ConsortiumChevronTexaco FirmNational Science Foundation (U.S.) (Grant OCE-8408761)Deep Sea Drilling Projec
Secondary Shear Waves From Source Boreholes
Full text linkThe purpose of this paper is to synthesize the most important results of the thesis work
of Meredith (1990) concerning radiation from seismic sources in boreholes.
Previous studies of radiation from sources in boreholes have been far-field studies
and have neglected the explicit contribution of the borehole. In general, this is fine for P-wave radiation and for S-wave radiation into high velocity rocks. However, tube waves "leak" shear conical waves (Mach waves) which propagate when the tube wave velocity
is greater than the shear wave velocity of the surrounding medium. These Mach waves
are of high amplitude because of the dominance of the tube wave and radiate away from
the borehole in a fixed conical shape. The shape of the cone is dependent on the shear
wave velocity of the medium and the tube wave velocity.
This paper defines the conditions under which these Mach waves exist and thoroughly
describes them in a physical sense and less so in a mathematical sense. Finally,
the relationship of Mach waves to data sets is examined and how Mach waves may be
confused with receiver borehole tube waves. To keep the presentation simple, radiation
from axial or torsional sources or radiation from empty boreholes is omitted in this
paper but fully addressed in Meredith (1990).Massachusetts Institute of Technology. Full Waveform Acoustic Logging Consortiu
Determining Shear Wave Velocities in Soft Marine Sediments
Full text linkThe inversion technique presented in this volume (Cheng, 1987) that simultaneously
inverts full waveform acoustic logs for shear wave velocity (V[subscript 3]) and compressional wave attenuation (Q[subscript p]) was applied to selected full waveform acoustic logs taken in soft sediments from Deep Sea Drilling Project Site 613.
Besides V[subscript 3] and Q[subscript p], the sensitivity of the inversion to perturbations in the fixed parameters, P-wave velocity (V[subscript p]), fluid velocity (V[[subscript f]), borehole diameter, bulk density (P[subscript b]), and borehole fluid attenuation (Q[subscript f]), were tested. Our study shows that the inversion technique is most sensitive to the estimate of V[subscript p] because the inversion is based
on the P leaky mode energy portion of the spectrum. The Poisson's ratio, however,
which primarily controls the amplitude of the waveforms, is rather stable with different
estimates in V[subscript p]. The inversion technique is less sensitive to small perturbations in borehole diameter, P[subscript b], V[subscript f], and Q[subscript f]
The shear wave velocities inferred from these inversions correlate well with the attendant velocity logs run at Site 613 and the diagenetic changes identified by shipboard stratigraphers. For example, there is an increase in both V[subscript p] and V[subscript 3] at the diagenetic boundary between siliceous nannofossil oozes and porcellanite. This boundary is responsible for a sharp seismic reflector in a USGS. seismic line run nearby. Over the depth interval that we analyzed, from 390.0 to 582.0 meters below sea floor, we determined shear wave velocities ranging from 0.74 to 1.06 km/sec corresponding to compressional wave velocities from 1.70 to 2.20 km/sec.Massachusetts Institute of Technology. Full Waveform Acoustic Logging ConsortiumNational Science Foundation (U.S.) (Grant OCE84-08761)Chevron (Fellowship
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