A teleseismic shear-wave splitting study to investigate mantle flow around South America and implications for plate-driving forces   总被引：2，自引：0，他引：2  

George Helffrich  Douglas A. Wiens  Emilio Vera  Sergio Barrientos  Patrick Shore  Stacey Robertson  Rodrigo Adaros 《Geophysical Journal International》2002,149(1):F1-F7

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SENSITIVITY OF THE RUSSIAN AGRICULTURE TO CHANGES IN CLIMATE, CO2 AND TROPOSPHERIC OZONE CONCENTRATIONS AND SOIL FERTILITY     

Oleg D. Sirotenko  Helena V. Abashina  Vera N. Pavlova 《Climatic change》1997,36(1-2):217-232

Russian agriculture sensitivity to changes in climate, soil and atmosphere chemistry were analyzed. Calculated data are presented on crop productivity of grain crops and grasses (C₃) under arid and humid scenarios of climate taking account of one-, two-, three and four-factor natural environment impacts. All four factors under studies (climatic parameters, CO₂ and tropospheric ozone concentrations, soil degradation extent) greatly impact agriculture productivity. The effect of interaction between all considered factors on agroecosystem productivity is studied. It is established that a simple additive scheme for explaining the complex effect of some factors can be much violated. In this case, not only variations in the mean crop yield levels but also variations in the degree of crop stability have been assessed in some regions, that may be more important for determining the social-economic consequences. It turned out that the recurrence of critically very low yields in steppe regions may increase two fold as a result of global warming.  相似文献   

Boundary assessment under uncertainty: A case study     

Vera Pawlowsky  Ricardo A. Olea  John C. Davis 《Mathematical Geology》1993,25(8):1075-1076

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Steppe expansion in Patagonia?     

Thomas T. Veblen  Vera Markgraf 《Quaternary Research》1988,30(3)

Westward expansion of the Patagonian steppe and retrocession of Andean forests due to increasing aridity over the past one or two millennia has been a persistent theme in the ecological and paleoecological literature for at least half a century. New evidence from pollen profiles, tree-ring analysis, vegetation structure, and photographic and documentary historical sources does not show the expansion of the steppe. Instead, over the past century trees have invaded the steppe as a consequence mainly of human-induced changes in the fire regime, and trees have regenerated in forest areas that were heavily burnt at the onset of European colonization.  相似文献   

Chemical variability in francolites from Israeli phosphorite macrograins     

Shimshon Axelrod  Vera Rohrlich 《Mineralium Deposita》1982,17(1):1-16

The chemical composition (up to 7 elements) of francolite in 64 samples of Israeli phosphorites from 7 fields was determined. Samples comprising mainly skeletal macrograins contain, on the average, 1.5% less P₂O₅, than those with other types of macrograins (mostly pelletal). Average CO₂ in the skeletal variety is 4%, against 3% elsewhere. There is a strong positive correlation between Na and CO₂, and a strong negative one between Mg and P₂O₅. Of the principal components explaining more than 95% of the chemical variation, the first, which opposes phosphate to carbonate and to sodium, differentiates between types of macrograins; the second, based on calcium and sodium not coupled with carbonate, differentiates between phosphorite fields. This is clearly seen from the graph (Fig. 6) obtained by correspondence-factor analysis. The difference between skeletal and other magrograins could be due to origin or to diagenetic changes, which include francolitisation of carbonate hydroxyapatite.  相似文献   

San Agustin Plains, New Mexico: Age and paleoenvironmental potential reassessed     

Vera Markgraf  J. Platt Bradbury  R. M. Forester  G. Singh  R. S. Sternberg 《Quaternary Research》1984,22(3):336-343

Paleomagnetic analysis of samples from the original 600-m-long core from the San Agustin Plains, New Mexico, showed an unquestionable reversal stratigraphy dating the record back to at least 1.6 my. Analysis of pollen, ostracodes, and algae of a duplicate sample section in the vicinity of the original coring site and spanning the last 18,000 yr B.P. shows a much higher resolution than the earlier results.  相似文献   

Jumblies put out to sea again     

Vera Rich 《Marine pollution bulletin》1974,5(10):147-148

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Compensating for estimation smoothing in kriging   总被引：2，自引：0，他引：2  

Ricardo A. Olea  Vera Pawlowsky 《Mathematical Geology》1996,28(4):407-417

Smoothing is a characteristic inherent to all minimum mean-square-error spatial estimators such as kriging. Cross-validation can be used to detect and model such smoothing. Inversion of the model produces a new estimator—compensated kriging. A numerical comparison based on an exhaustive permeability sampling of a 4-ft² slab of Berea Sandstone shows that the estimation surface generated by compensated kriging has properties intermediate between those generated by ordinary kriging and stochastic realizations resulting from simulated annealing and sequential Gaussian simulation. The frequency distribution is well reproduced by the compensated kriging surface, which also approximates the experimental semivariogram well—better than ordinary kriging, but not as well as stochastic realizations. Compensated kriging produces surfaces that are more accurate than stochastic realizations, but not as accurate as ordinary kriging.  相似文献   

Uranium removal from groundwater using hydroxyapatite     

Franz Georg Simon  Vera Biermann  Burkhard Peplinski 《Applied Geochemistry》2008,23(8):2137-2145

The present work shows that U can be effectively removed from groundwater using permeable reactive barriers with hydroxyapatite (HAP) as reactive material. The main factor influencing the removal processes is the composition of the groundwater, namely the concentration of Ca and carbonate. Sorption of U onto the HAP surface seems to be the dominant removal process with the possibility of remobilisation. Newly formed U-phosphate minerals were detected by ESEM/EDX and XRD in samples with high U content indicating either a dissolution-precipitation mechanism or sorption onto the apatite surface followed by alteration of the structure. The formed U-phosphate minerals are stable under common groundwater conditions and can be remobilised only at high pH-values and high carbonate concentrations.  相似文献   

Subsidence Modeling Validation Through Back Analysis for an Italian Gas Storage Field     

Giulia Codegone  Vera Rocca  Francesca Verga  Christian Coti 《Geotechnical and Geological Engineering》2016,34(6):1749-1763

The multi-disciplinary work described in the paper was aimed at analyzing and predicting the cyclical ground surface movements induced by underground gas storage (UGS) activities in a depleted gas field located in the Po Plain (Italy). The field has been operated as a storage facility for nearly three decades. Currently, the possibility of delta-pressuring the reservoir (i.e. to increase the maximum operating pressure above the initial reservoir pressure) to enhance the storage performance is being considered. Significant information was collected over time: 2/3D seismic surveys, geological and sedimentological studies, 60+ logged wells, geotechnical lab tests and 50+ years of production history and monitoring were available for the development of a fully integrated static–dynamic–geomechanical analysis. The mechanical aspects of the study are the focus of this paper. The data coming from different sources at different scales were analysed and integrated to set up and characterize a 3D finite element method mechanical model to calculate the surface movements induced by UGS activity by adopting an elasto-plastic constitutive law. The model was then calibrated via a back analysis approach, i.e. the model parameters were fine-tuned so that the simulated subsidence/uplift would compare satisfactorily with the ground movements collected over nearly 10 years of monitoring via interferometric synthetic aperture radar analysis in the region under investigation. Eventually, the calibrated model was used as a forecasting tool for subsidence evaluation under different future storage strategies, including delta-pressuring conditions. Results proved that no significant subsidence is expected even if the maximum operating pressure reached 120 % of the initial formation pressure.  相似文献