170,127 research outputs found
New insights into the basement structure of the West Siberian basin from forward and inverse modelling of GRACE satellite gravity data
No full textThe oil- and gas-rich West Siberian Basin is underlain by a layer of flood basalts of late Permian-Triassic age that are coeval with the Siberian traps. The extent and thickness of the basalts are unknown, but knowing their thickness is important for discussions on the end-Permian mass extinction because basalt volume constrains estimates of emitted volatiles. We have used GRACE satellite and terrestrial gravity data to study the structure of the crust and basalt distribution. Published seismic sections are used to constrain the sediment isopachs and to estimate a depth-density function. We use published models of crustal thickness and basement depth to reduce the observed gravity field to the basement level. The resulting three-dimensional density model gives information on density anomalies in the lower crust and upper mantle and on the basalt thickness. We identify several rift-graben structures that are presumably filled with basalt. The lower crust below the West Siberian Basin shows considerable density variations, and these variations allow the region to be divided into four major blocks. The eastern part of the basin, toward the Siberian platform, shows an arch-shaped density increase in the lower crust that is accompanied by a linear high-density anomaly at shallower depths. Our work demonstrates the way in which the GRACE-gravity field can be applied to map geological structures like buried rifts and large basins. The same techniques can be used for other large, remote basins such as those in cratonic South America
The GRACE-satellite gravity and geoid fields in analysing large-scale, cratonic or intracratonic basins
No full textThe recently released gravity potential field development derived from the Gravity
Recovery and Climate Experiment satellite allows an unprecedented opportunity to
use the gravity field to make global comparisons of structures of geological interest.
The spatial resolution of the gravity field is sufficiently good to map large-scale
or intracratonic and cratonic basins, as the areal extent of these basins is 0.5 ×
106 km2 and greater. We present the gravity anomaly, Bouguer, geoid and terrain
corrected geoid fields for a selection of nine large-scale basins and show that the
satellite-derived field can be used to successfully identify distinctive structures of these
basins, e.g., extinct rifts underlying the basins and generally the isostatic state. The
studied basins are the Eastern Barents Sea, West Siberian, Tarim, Congo, Michigan,
Amazon, Solim ˜ oes, Parnaiba and Paran`a basins. We complete the mapping of the
gravity field with a description of the basins in terms of areal extension and depth,
sedimentary age and presence and age of volcanism. Interpretation of the satellite
gravity anomalies and considerations regarding the crustal thickness as known from
seismic investigations, allows us to conclude that for the greater part of the basins
there is evidence for high-density material in the lower crust and/or upper mantle.
This density anomaly is, at least partly, compensating for the low-density sedimentary
infill instead of the crustal thinning mechanism. For our selection of basins, crustal
thickness variations and Moho topography cannot be considered as mechanisms of
compensation of the sedimentary loading, which is a clear difference to well-defined
rift basins.In press13JCR Journalreserve
The gravity potential derivatives as a means to classify the Barents Sea basin in the context of cratonic basins
No full textDetailed study of the gravity field and the isostatic state of the Barents Sea Region shows that the Eastern Barents Sea basins are not typical rift basins. They exhibit distinctive features such as large wavelengths, high lithospheric mantle density, thick sequences of sediments, a flat Moho and high elastic thickness. These attributes are normally associated with cratonic or intracratonic basins. To understand the geological history of the Eastern Barents Sea basins, we make a comparison with other
well studied cratonic basins: the West Siberian basin, the Michigan basin in North America, the Solimões, Amazon, Parnaìba and Paranà basins in South America, the Tarim basin in Central Asia and the Congo basin in Africa. For these basins, the structure, subsidence history and temperature evolution is relatively well known. Our analysis includes the characterization in terms of gravity, geoid undulations, isostatic state, age and igneous activity.
An important constraint in sedimentary basin evolution is the presence of the volcanism and the relative age of the volcanic strata with respect to the sedimentary package. In all the considered basins, except the Congo basin, volcanic masses are present at some time-stage and at some depth at the basin. Each of the basins exhibits some deviation from the classic isostatic equilibrium model that predicts the crustal thickness (thinning in this case) from the topographic and sedimentary load. Instead of crustal thinning, high density masses in the crust and mantle appear to be a typical feature. The basins may be divided into two groups, one in which the given basin correlates with the geoid, the second in which the geoid is independent. This iscrimination points towards different density characteristics in the integrated crustal column
The basalt geometry in the West Siberian basin from satellite gravity, magnetics and seismics
No full textThe West Siberian basin is underlain by an extensive basalt layer of end-Permian age, coeval to the large igneous province of the Siberian traps. Very little is known about its lateral extent and thickness, which is an important element to the thermal evolution of the basin. The top basalt has been imaged in several seismic sections, but the bottom of the layer has not been resolved by seismic studies. Interpretation of gravity and magnetic data helps to investigate the basalt, as basaltic rocks generate in general, due to the high density and magnetic susceptibility contrast with respect to the sediments, a strong signal. In our study we interpret the newest satellite gravity fields, the magnetic field, and the published seismic lines to define the geometry of the basalt layer. We first characterize the expected gravity and magnetic signal for the basalt. We then consider what the contribution of the ongoing satellite gravity GOCE mission will be for a better understanding of the West Siberian basin. Afterwards we build 2D models along the Ob rift in the western part of the basin. We find considerable depth and thickness variations of the basalt layer. It is interesting that the depth and thickness variations correlate with the transition zone from oil to gas fields
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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
Mitomycin C in highly myopic eyes - Author reply
No full textOphthalmology. 2005 Feb;112(2):208-18; discussion 219.
Mitomycin C modulation of corneal wound healing after photorefractive keratectomy in highly myopic eyes.
Gambato C, Ghirlando A, Moretto E, Busato F, Midena E.
SourceRefractive Surgery Service and Antimetabolite Therapy Research Unit, Department of Ophthalmology, University of Padova, Padova, Italy.
Abstract
PURPOSE: To evaluate the role of topical mitomycin C in corneal wound healing (CWH) after photorefractive keratectomy (PRK) in highly myopic eyes.
DESIGN: Prospective, double-masked, randomized clinical trial.
PARTICIPANTS: Seventy-two eyes of 36 patients affected by high (>7 diopters) myopia.
METHODS: In each patient, one eye was randomly assigned to PRK with intraoperative topical 0.02% mitomycin C application, and the fellow eye was treated with a placebo. Postoperatively, mitomycin C-treated eyes received artificial tears (3 times daily, tapered in 3 months), whereas the fellow eye was treated with fluorometholone sodium 2% and artificial tears (3 times daily, tapered in 3 months).
MAIN OUTCOME MEASURES: Uncorrected visual acuity (UCVA) and best-corrected visual acuity (BCVA), contrast sensitivity, manifest refraction, and biomicroscopy. Contrast sensitivity was determined using the Pelli-Robson chart. Corneal confocal microscopy documented CWH.
RESULTS: Mean follow-up was 18 months (range, 12-36). No side effects or toxic effects were documented. At 12-month follow-up examination, UCVAs (logarithm of the minimum angle of resolution) were 0.4+/-0.48 and 0.5+/-0.53 (P = .03) in mitomycin C-treated eyes and corticosteroid-treated eyes, respectively. At 1 year, corneal haze developed in 20% of corticosteroid-treated eyes, versus 0% of mitomycin C-treated eyes. At 12, 24, and 36 months, corneal confocal microscopy showed activated keratocytes and extracellular matrix significantly more evident in untreated eyes (Ps = 0.004, 0.024, and 0.046, respectively).
CONCLUSION: Topical intraoperative application of 0.02% mitomycin C can reduce haze formation in highly myopic eyes undergoing PRK.
Comment in
Ophthalmology. 2006 Feb;113(2):357; author reply 357-8
Dispelling the Myths Behind First-author Citation Counts
No full textWe 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
Hepatic artery hemodynamics suggest operation of a buffer response in the human fetus
No full textAfter birth, the hepatic artery buffer response helps to maintain liver perfusion. Here, the authors establish a Doppler technique to measure fetal hepatic artery flow velocity and test the hypothesis that the buffer response also operates prenatally. Women with low-risk pregnancies were recruited to a longitudinal study (N = 161). Measurement techniques and reference ranges for hepatic artery velocities and pulsatility index (PI) were established. Ductus venosus peak velocity (VDVps) represented the portocaval pressure gradient, and umbilical venous flow (QUV) represented portal flow. Reference ranges were established for the more accessible left hepatic artery branch. Hepatic artery PI was lower in fetuses with VDVps <10th centile (P < .05) and in those with QUV <10th centile ( P < .0001). Conversely, hepatic artery PI was higher in those with QUV >90th centile (P < .0001). The authors establish a method for measuring fetal hepatic arterial blood velocity, provide reference ranges, and show that the hepatic artery buffer response operates prenatally
Fetal celiac and splenic artery flow velocity and pulsatility index: longitudinal reference ranges and evidence for vasodilation at a low portocaval pressure gradient
No full textOBJECTIVES: To establish longitudinal reference ranges for the fetal celiac and splenic arteries flow velocity and pulsatility index (PI), and to determine their hemodynamic relationship to venous liver perfusion and distribution and to other essential arteries. METHODS: This was a prospective longitudinal study of 161 low-risk pregnancies. Doppler recordings of the celiac and splenic arteries were made on three to five occasions at 3-5-week intervals to establish reference ranges for blood velocity and PI measurements. Peak systolic velocity in the ductus venosus, a shunt between the umbilical and inferior caval veins, was used to represent the umbilicocaval (i.e. portocaval) pressure gradient, and the left portal vein blood velocity represented the umbilical distribution to the right liver lobe. The correlations between the celiac, splenic and hepatic arteries were determined, and their association with the middle cerebral and umbilical artery PIs (MCA-PI and UA-PI) was assessed. RESULTS: Longitudinal reference ranges for the fetal celiac and splenic arteries were established based on 510 and 521 observations, respectively, during gestational weeks 21-39. Terms for calculating conditional reference ranges to be used for repeat observations are provided. Celiac and splenic artery PIs were low when portocaval pressure and umbilical supply to the right lobe were low (P < 0.0001). Their peak systolic velocity and PI were correlated (r = 0.7 (95% CI, 0.6-0.8) and r = 0.5 (95% CI, 0.3-0.6), respectively), while the PI of the hepatic artery correlated weakly with those of the celiac and splenic arteries. They were positively associated with the MCA-PI and UA-PI (P < 0.0001). CONCLUSION: We provide longitudinal reference ranges for the fetal celiac and splenic arteries Doppler measurements and show that they are involved in maintaining portal liver perfusion independently from the hepatic arter
A Multi-Language Comparison of Influences on Author Verification using Character N-Grams
No full textWe create a new multi-language corpus for author verification based on Wikipedia talkpages, and evaluate the influence that differences in topic and time have on character n-gram author profiles. Topic alignment between two texts is found to increase author verification precision, and an authors writing style is found to change over time, but not more significantly after 3 years than after 1 year.Information ArchitectureWISElectrical Engineering, Mathematics and Computer Scienc
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