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排序方式: 共有451条查询结果,搜索用时 15 毫秒
371.
372.
By monitoring and analysing three‐dimensional absolute displacement during tunnel excavation, it is possible to predict discontinuity zones ahead of tunnel face. This paper presents results of the three‐dimensional displacement obtained from three‐dimensional simulations using hybrid method for assessing the effect of discontinuity zones on monitoring data. The influence of fault zones and in situ initial stresses on the three‐dimensional tunnel movement is evaluated. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
373.
374.
The evolution of Alfvén turbulence due to three-wave interactions is discussed using kinetic theory for a collisionless, thermal plasma. In particular, we consider decay of Alfvén waves through three-wave coupling with an ion sound mode in the random-phase approximation. Two decay processes are of particular interest: an Alfvén wave decays into a backward propagating Alfvén wave and a forward propagating ion sound wave, and an Alfvén wave decays into a backward propagating fast magnetoacoustic wave and a forward ion sound wave. The former was widely discussed in the literature, particularly under the coherent wave assumption. The latter was not well explored and is discussed here. 相似文献
375.
The fidelity of melt inclusions as records of melt composition 总被引:5,自引:5,他引:0
Don R. Baker 《Contributions to Mineralogy and Petrology》2008,156(3):377-395
A series of experiments created melt inclusions in plagioclase and pyroxene crystals grown from a basaltic melt at 1,150°C,
1.0 GPa to investigate diffusive fractionation during melt inclusion formation; additionally, P diffusion in a basaltic melt
was measured at 1.0 GPa. Melt inclusions and melts within a few 100 microns of plagioclase–melt interfaces were analyzed for
comparison with melt compositions far from the crystals. Melt inclusions and melt compositions in the boundary layer close
to the crystal–melt interface were similar, but both differ significantly in incompatible element concentrations from melt
found greater than approximately 200 microns away from the crystals. The compositional profiles of S, Cl, P, Fe, and Al in
the boundary layers were successfully reproduced by a two-step model of rapid crystal growth followed by diffusive relaxation
toward equilibrium after termination of crystal growth. Applying this model to investigate possible incompatible element enrichment
in natural melt inclusions demonstrated that at growth rates high enough to create the conditions for melt inclusion formation,
∼10−9–10−8 m s−1, the concentration of water in the boundary layer near the crystal was similar to that of the bulk melt because of its high
diffusion coefficient, but sulfur, with a diffusivity similar to major elements and CO2, was somewhat enriched in the boundary layer melt, and phosphorus, with its low diffusion coefficient similar to other high-field
strength elements and rare earth elements, was significantly enriched. Thus, the concentrations of sulfur and phosphorus in
melt inclusions may over-estimate their values in the bulk melt, and other elements with similar diffusion coefficients may
also be enriched in melt inclusions relative to the bulk melt.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
376.
Per S. Andersson Don Porcelli Göran Björk Örjan Gustafsson 《Geochimica et cosmochimica acta》2008,72(12):2854-2867
The neodymium concentration, CNd, and isotopic composition, εNd, in seawater have been determined in the water column at five sites in the Barents Sea-Fram Strait area where most of the water exchange between the Arctic Ocean and the North Atlantic takes place. In the main Arctic Ocean inflow branch across the Barents Sea the concentration and isotopic composition (CNd = 15.5 pmol/kg and εNd = −10.8) are similar to those reported for the northeastern Nordic Seas, which is consistent with this region being a source area for the Arctic inflow. Due to the addition of Nd from Svalbard shelf sediments, the CNd in the surface waters above 150 m, in the Fram Strait inflow branch is higher by a factor of 2 and the εNd is shifted to lower values (−11.8).In the stratified Nansen Basin, where cold low salinity water overlies warmer Atlantic water the CNd and εNd do not vary with the vertical temperature-salinity structure but are essentially constant and similar to those of the Atlantic inflow throughout the entire water column, down to 3700 m depth, which indicates that the Nd is to a large extent of Atlantic origin.Compared to the Atlantic inflow water, the Nd in the major Arctic Ocean outflow, the Fram Strait, show higher CNd in the surface waters above 150 m, and a higher εNd (−9.8) throughout the entire water column down to 1300 m depth. Sources for the more radiogenic Nd isotopic composition in deep water of the Fram Strait outflow most likely involve boundary exchange with sediments on the shelf and slope as the water passes along the Canadian archipelago. River water is a possible source in the surface water but it also seems likely that Pacific water Nd, modified by interactions on the shelf, is an important component in the Fram Strait surface outflow. Changes in the relative proportions of inflow of river water and flow of Pacific water through the Arctic Ocean could thus influence the isotopic composition of Nd in the North Atlantic. 相似文献
377.
P. Humez B. Mayer M. Nightingale J. Ing V. Becker Don Jones Vien Lam 《Hydrogeology Journal》2016,24(1):109-122
Variability in baseline groundwater methane concentrations and isotopic compositions was assessed while comparing free and dissolved gas sampling approaches for a groundwater monitoring well in Alberta (Canada) over an 8-year period. Methane concentrations in dissolved gas samples (n?=?12) were on average 4,380?±?2,452 μg/L, yielding a coefficient of variation (CV) >50 %. Methane concentrations in free gas samples (n?=?12) were on average 228,756?±?62,498 ppm by volume, yielding a CV of 27 %. Quantification of combined sampling, sample handling and analytical uncertainties was assessed via triplicate sampling (CV of 19 % and 12 % for free gas and dissolved gas methane concentrations, respectively). Free and dissolved gas samples yielded comparable methane concentration patterns and there was evidence that sampling operations and pumping rates had a marked influence on the obtained methane concentrations in free gas. δ13CCH4 and δ2HCH4 values of methane were essentially constant (?78.6?±?1.3 and ?300?±?3?‰, respectively) throughout the observation period, suggesting that methane was derived from the same biogenic source irrespective of methane concentration variations. The isotopic composition of methane constitutes a robust and highly valuable baseline parameter and increasing δ13CCH4 and δ2HCH4 values during repeat sampling may indicate influx of thermogenic methane. Careful sampling and analytical procedures with identical and repeatable approaches are required in baseline-monitoring programs to generate methane concentration and isotope data for groundwater that can be reliably compared to repeat measurements once potential impact from oil and gas development, for example, may occur. 相似文献
378.
Bremer Leah L. Elshall Ahmed S. Wada Christopher A. Brewington Laura Delevaux Jade M.S. El-Kadi Aly I. Voss Clifford I. Burnett Kimberly M. 《Hydrogeology Journal》2021,29(5):1749-1765
Hydrogeology Journal - Groundwater sustainability initiatives, including sustainable yield and watershed policy protection policies, are growing globally in response to increasing demand for... 相似文献
379.
Isaac R. Santos Natasha Dimova Richard N. Peterson Benjamin Mwashote Jeffrey Chanton William C. Burnett 《Marine Chemistry》2009,113(1-2):137-147
We report the results of an experiment in which we measured 222Rn (15,000 observations), CH4 (40,000 observations), and associated variables in seawater nearly continuously at a coastal site in the Gulf of Mexico for almost two years. Significant correlations between 222Rn and CH4 imply that they are derived from a common source, most likely groundwater. However, we were unable to explain the overall tracer variability as a single function of groundwater table height, temperature, tidal range, and wind speed, indicating multiple, overlapping controls on SGD dynamics at this site. Methane and radon concentrations may vary 2-fold in a given well in the subterranean estuary over tidal time scales, demonstrating the complexity of determining SGD endmember concentrations and suggesting that unaccounted for temporal changes in groundwater may explain some of the patterns observed in seawater. Surprisingly, the variability of 222Rn and CH4 in seawater over short (e.g., hourly) time scales was generally comparable to or even more pronounced than fluctuations over much longer (e.g., monthly) scales. While high tracer concentrations usually occurred during low tide and low tracer concentrations during high tide, this pattern was occasionally inverted or absent indicating that no single model can be used to describe the entire data set. We also describe a sequence of events in which SGD tracers were depleted in coastal waters during storms and regenerated afterwards. We found no increase in radon activities immediately after the largest storm (75 mm rainfall) perhaps because of the short residence times of groundwater in contrast to the ingrowth time of radon. Marine controls appeared to be the most important SGD drivers with only minor influence relating to the shallow and deep aquifers. This implies that seasonal investigations of SGD tracers in the coastal ocean may be masked by short-term variability. 相似文献
380.
Using the All-Sky Monitor (ASM, 1.5∼12 keV) data of Rossi X-ray Timing Explorer (RXTE) from January 1996 to October 2011, we have analyzed in detail the power spectrum of the hardness ratio (HR) (5∼12 keV/3∼5 keV) of the X-ray binary Cyg X-1 as a black hole candidate. The results show that the HR exhibits the following periodical variations: (1) During MJD = 50087∼55841, the HR presented the the periods T ≈ 5.6 d, T ≈ 40.0 d, T ≈ 78.4 d, T ≈ 173.8 d, and T ≈ 400/800 d; (2) When Cyg X-1 was in the hard state, the HR exhibited the periods T ≈ 5.6 d, T ≈ 33.7/67.6 d, T ≈ 45.3 d, and T ≈ 165.3 d; (3) When Cyg X-1 was in the soft state, the HR exhibited the periods T ≈ 38.5 d, T ≈ d, and T ≈ 128.3 d. Moreover, using the viscosity theory and Zdziarski accretion disk model, we have made a discussion on the physical mechanism of this kind of periodicity. 相似文献