全文获取类型
收费全文 | 714篇 |
免费 | 43篇 |
国内免费 | 8篇 |
专业分类
测绘学 | 21篇 |
大气科学 | 57篇 |
地球物理 | 248篇 |
地质学 | 242篇 |
海洋学 | 62篇 |
天文学 | 82篇 |
综合类 | 1篇 |
自然地理 | 52篇 |
出版年
2023年 | 6篇 |
2021年 | 18篇 |
2020年 | 10篇 |
2019年 | 14篇 |
2018年 | 31篇 |
2017年 | 23篇 |
2016年 | 22篇 |
2015年 | 24篇 |
2014年 | 29篇 |
2013年 | 50篇 |
2012年 | 32篇 |
2011年 | 36篇 |
2010年 | 48篇 |
2009年 | 41篇 |
2008年 | 41篇 |
2007年 | 40篇 |
2006年 | 28篇 |
2005年 | 30篇 |
2004年 | 30篇 |
2003年 | 32篇 |
2002年 | 23篇 |
2001年 | 14篇 |
2000年 | 12篇 |
1999年 | 13篇 |
1998年 | 12篇 |
1997年 | 7篇 |
1996年 | 8篇 |
1995年 | 4篇 |
1994年 | 3篇 |
1992年 | 4篇 |
1990年 | 3篇 |
1989年 | 5篇 |
1988年 | 2篇 |
1987年 | 10篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 5篇 |
1982年 | 2篇 |
1981年 | 5篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1974年 | 4篇 |
1973年 | 2篇 |
1971年 | 2篇 |
1907年 | 1篇 |
排序方式: 共有765条查询结果,搜索用时 15 毫秒
401.
402.
Simultaneous mass spectral detection and stable carbon isotope analysis was performed on individual indigenous n-alkanes isolated from single C4 and C3 plant species and on a series of aliphatic and polycyclic aromatic hydrocarbons (PAH) produced from the combustion of these same biomass materials. The analysis technique used a combined gas chromatograph-mass spectrometer/combustion/isotope ratio mass spectrometer (GC-MS/C/IRMS). Precision (2σ) for replicate measurements of individual compounds in standard solutions using this novel configuration ranged between 0.2 and 0.5‰ for n-alkanes and 0.3 and 0.8‰ for PAH. Accuracy of the n-alkane measurements ranged between 0.1 and 0.4‰ and that of the PAH measurements ranged between 0.2 and 0.9‰. Replicate GC-MS/C/IRMS measurements on the combustion-derived n-alkene/alkane pairs were performed to within a precision of between 0.1 and 1.1‰ and the precision for the combustion PAH was similar to the standard PAH solution. No notable isotopic effects were observed when altering the temperature of the combustion process from 900 to 700°C, or as a result of the individual n-alkenes/alkanes partitioning between the gaseous and condensate fractions. Combustion-derived n-alkenes/alkanes ranged from C11 to C31, and the C4-derived n-alkenes/alkanes were approx. 8‰ more enriched in 13C than the C3-derived compounds. Both the C4 and C3-derived n-alkenes/alkanes (C20-C30) were isotopically similar to the indigenous n-alkanes and were 2-3‰ more depleted in 13C than the lower mol. wt (C1111-C19) n-alkenes/alkanes, suggesting an independent origin for the lower mol. wt compounds. Combustion-generated C4 and C3-derived 2-, 3-, and 4-ring PAH were also isotopically distinct (Δδ = 10‰). Unlike the n-alkenes/alkanes, no compound-to-compound variations were observed between the low and high mol. wt PAH. This study demonstrates that the isotopic composition of original plant biomass material is mainly preserved in the aliphatic hydrocarbons and PAH generated by its combustion. Consequently, analyses of these compounds in sediments impacted by fire occurrences may provide useful information about paleo-fire activity that may help elucidate the impact biomass burning may have had and could have on climate-biosphere interactions. 相似文献
403.
The MICADO instrument, consisting of a Michelson interferometer, has observed winds and temperatures during three winter campaigns in the auroral zone, and during 2 years at the Observatoire de Haute-Provence. The instrument observed the O(1S) oxygen emission line. Emission from this line originates from both the E- and F-regions. A method to separate the contribution from these two regions is presented for cases when the thermospheric component is comparable to that for the mesosphere. For the auroral latitudes, a mean model of the meridional and zonal neutral wind components as a function of magnetic activity and time is presented and compared to predictions from recent empirical models. For the mid-latitudes, several properties of the semi-diurnal tides are shown and compared to radar observations and predictions from recent theoretical models. 相似文献
404.
Two- and three-dimensional gravel fabric analyses have been performed on a number of pediments located in central, western Saudi Arabia. Standard methods of vector analysis were used to measure orientation. It is concluded that long axes of pediment gravels tend to point downslope and that fabrics are remarkably uniform over pediment surfaces. The results indicate that agents such as sheetfloods, rather than channel flow, may be most effective in bringing about such fabrics. 相似文献
405.
Magnetic anomalies in the Shikoku Basin: a new interpretation 总被引:1,自引:0,他引:1
Nicolas Chamot-Rooke Vincent Renard Xavier Le Pichon 《Earth and Planetary Science Letters》1987,83(1-4)
Kaiko surveys over the Nankai Trough made available new magnetic and structural data for the northern Shikoku Basin. A survey of the oceanic lithosphere subducting below Southwest Japan along the central Nankai Trough revealed the existence of several north-south basement troughs. They are probably transform faults related to a north-south spreading system. We examine the possibility of a late phase of north-south spreading limited to the axial northernmost Shikoku Basin, active between 14 and 12 Ma. If this system was already active before that time, i.e. during the N55° opening of the southeastern basin, then a triple junction should be found between both areas.Based on these data and previous studies we present a new interpretation of magnetic anomalies over the whole basin. From early east-west rifting to late north-south spreading, opening of the Shikoku Basin proceeded through multiple episodes of spreading. The analysis of magnetic anomalies constrains the kinematic evolution of the basin through time and space. Two successive counter-clockwise rotations of the spreading direction are postulated, at anomaly 6 (19 Ma) and at anomaly 5B (14 Ma), involving segmentation and rotation of the spreading ridge. 相似文献
406.
407.
Deccan flood basalts at the Cretaceous/Tertiary boundary? 总被引:9,自引:0,他引:9
Vincent Courtillot Jean Besse Didier Vandamme Raymond Montigny Jean-Jacques Jaeger Henri Cappetta 《Earth and Planetary Science Letters》1986,80(3-4):361-374
Joint consideration of new paleomagnetic, paleontological and geochronological data from the Deccan continental flood basalts in India and critical discussion of earlier results lead us to suggest that volcanic activity may have lasted less than 1 Ma, thus possibly ranking as one of the largest volcanic catastrophes in the last 200 Ma. Available data are best satisfied if volcanism spanned the Cretaceous/Tertiary boundary, followed shortly afterwards by rifting of the Arabian Sea. These results point out the need for further work which may help in choosing between “external” and “internal” models of the Cretaceous/Tertiary boundary events. 相似文献
408.
The study highlights the dangers of limiting recharge calculations in humid climates to meteorological and soil considerations.
It highlights the importance of developing a conceptual understanding of the influence of geology on recharge mechanisms and
recharge rates. Uncertainties in recharge estimates through tills are examined by a study of the available literature, combined
with sensitivity analyses of soil moisture budget parameters and of hypothetical scenarios of till properties and hydraulic
gradients that are realistic for Irish conditions. The sensitivity analyses demonstrate that till properties have a greater
influence over recharge than the soil moisture budgeting parameters. Results are considered in terms of recharge coefficients,
representing the proportion of effective precipitation that becomes actual recharge to the aquifer. The literature review
identifies a range in recharge coefficients from 4% to 90% in selected field and catchment scale studies from bedrock aquifers
in Ireland and the U.K. The sensitivity analyses are able to simulate a similar range in recharge coefficient of 2% to 80%
by varying only till permeability, thickness and vertical hydraulic gradients. In the scenarios examined, the greatest sensitivity
to vertical till permeability lies between 0.001 m/day and 0.01 m/day. The sensitivity to soil moisture budgeting parameters
is much less apparent. It is concluded that this is due to the dominance of grassland in Ireland and the relatively wet summers,
resulting in the limited development of soil moisture deficits by comparison with some parts of the U.K.
Résumé Cette étude met en lumière les dangers résultants de la pratique consultant à limiter les calculs de recharge aux données météorologiques ainsi qu’aux considérations liées au sol. Elle met en lumière l’importance de développer des modèles conceptuels comprenant l’influence de la géologie sur les mécanismes de la recharge et les taux de recharge. Les incertitudes sur les estimations de la recharge à travers les argiles sont examinées à travers les données disponibles dans la littérature, combinées avec une analyse de sensibilité des paramètres du bilan hydrique et des scénarios hypothétiques reprenant les propriétés des argiles et les gradients hydrauliques qui sont réalistes dans le contexte irlandais de l’étude. Les analyses de sensibilité démontrent que les propriétés des argiles ont une plus grande influence sur la recharge que les paramètres du bilan hydrique des sols. Les résultats sont considérés en terme de coefficients de recharge, représentant la proportion de précipitation effective qui devient la recharge réelle de l’aquifère. La littérature identifie des coefficients de recharge allant de 4 à 90% sur des terrains sélectionnés à l’échelle de bassins-versants, sur les aquifères de socle en Irlande et au Royaume-Uni. Les analyses de sensibilité sont capables de simuler une variation aussi large de coefficients de recharge, en ne changeant rien que la perméabilité des argiles, leur épaisseur et les gradients hydrauliques verticaux. Dans les scénarios étudiés, la meilleure sensibilité à la perméabilité verticale se situe entre 0.001 et 0.01 m/jour. La sensibilité au bilan hydrique est beaucoup moins apparente. On en conclut que ceci est du à la dominance des prairies en Ireland et aux étés relativement humides, résultant des déficits en eau des sols relativement modérés en comparaison avec certaines zones du Royaume-uni.
Resumen El estudio resalta los peligros de limitar los cálculos de recarga en climas húmedos a las consideraciones meteorológicas y del suelo. Se destaca la importancia de desarrollar un entendimiento conceptual de la influencia de la geología en los mecanismos de recarga y los ritmos de recarga. Se examinan las incertidumbres en los estimados de recarga a través de tills mediante un estudio de la literatura disponible combinado con análisis de sensibilidad de parámetros de balance de humedad del suelo y de escenarios hipotéticos de las propiedades del till y gradientes hidráulicos que son realistas para condiciones típicas de Irlanda. Los análisis de sensibilidad demuestran que las propiedades del till tienen una influencia más grande en la recarga que los parámetros del balance de humedad del suelo. Se consideran los resultados en términos de coeficientes de recarga, representando la proporción de precipitación efectiva que se convierte en recarga real al acuífero. La revisión de literatura identifica un rango de coeficientes de recarga que varía de 4% a 90% en estudios seleccionados de cuencas y escala de campo realizados en acuíferos de macizo rocoso en Irlanda y el Reino Unido. Los análisis de sensibilidad pueden simular un rango similar en los coeficientes de recarga de 2% a 80% al variar únicamente la permeabilidad del till, el espesor y los gradientes verticales hidráulicos. En los escenarios examinados, la relación de sensibilidad más grande con la permeabilidad vertical del till se encuentra entre 0.001 m/día y 0.01 m/día. La sensibilidad en relación a los parámetros del balance de humedad del suelo es mucho menos aparente. Se concluye que esto se debe a la predominancia de tierras con hierba en Irlanda y los veranos relativamente húmedos, lo que resulta en el desarrollo limitado de déficit de humedad de suelo en comparación con algunas partes del Reino Unido.相似文献
409.
The excitation of structural components and liquid contents of surface impoundments by seismic waves can generate turbulence
that is large enough to overtop the bounding berms. In cases in which the liquids are wastes from industrial/municipal operations,
their release from impoundments can pose significant risks to the environment. In this analysis, the freeboard magnitudes
that can accommodate liquid head levels in impoundments are determined through linkage of configuration of waves in the liquid
surface to incident seismic wave characteristics, liquid characteristics and impoundment design. For an impoundment site in
a region of ground acceleration levels ranging from 0.2 to 1.0 g and impacted by seismic shear wave velocity of 180 m/s, freeboard
requirements are in the range of 0.004–2.0 m on soft soil; 0.008–0.7 m on medium-dense soil; and 0.002–0.1 m for dense soil.
For the same impoundment design, ground acceleration and incident wave characteristics, freeboard requirements are directly
proportional to the depth of the soil mantle over bedrock. The impoundment slope, which is a key parameter with regards to
liquid holding volumetric capacity of the impoundment, is a less significant parameter than depth to bedrock with regard to
the size of the required freeboard. This implies that siting of an impoundment should be considered to be critical to impoundment
performance in seismic zones. 相似文献
410.
The interaction of the lanthanides (Ln) with humic substances (HS) was investigated with a novel chemical speciation tool, Capillary Electrophoresis-Inductively Coupled Plasma Mass Spectrometry (CE-ICP-MS). By using an EDTA-ligand competition method, a bi-modal species distribution of LnEDTA and LnHS is attained, separated by CE, and detected online by sector field ICP-MS. We quantified the binding of all 14 rare earth elements (REEs), Sc and Y with Suwannee river fulvic acid, Leonardite coal humic acid, and Elliot soil humic acid under environmental conditions (pH 6-9, 0.001-0.1 mol L−1 NaNO3, 1-1000 nmol L−1 Ln, 10-20 mg L−1 HS). Conditional binding constants for REE-HS interaction (Kc) ranged from 8.9 < log Kc < 16.5 under all experimental conditions, and display a lanthanide contraction effect, ΔLKc: a gradual increase in Kc from La to Lu by 2-3 orders of magnitude as a function of decreasing ionic radius. HS polyelectrolyte effects cause Kc to increase with increasing pH and decreasing ionic strength. ΔLKc increases significantly with increasing pH, and likely with decreasing ionic strength. Based on a strong correlation between ΔLKc values and denticity for organic acids, we suggest that HS form a range of tri- to tetra-dentate complexes under environmental conditions. These results confirm HS to be a strong complexing agent for Ln, and show rigorous experimental evidence for potential REE fractionation by HS complexation. 相似文献