全文获取类型
收费全文 | 12401篇 |
免费 | 612篇 |
国内免费 | 98篇 |
专业分类
测绘学 | 321篇 |
大气科学 | 1075篇 |
地球物理 | 3055篇 |
地质学 | 4797篇 |
海洋学 | 843篇 |
天文学 | 2224篇 |
综合类 | 48篇 |
自然地理 | 748篇 |
出版年
2023年 | 54篇 |
2022年 | 72篇 |
2021年 | 203篇 |
2020年 | 243篇 |
2019年 | 240篇 |
2018年 | 468篇 |
2017年 | 500篇 |
2016年 | 642篇 |
2015年 | 484篇 |
2014年 | 590篇 |
2013年 | 820篇 |
2012年 | 629篇 |
2011年 | 711篇 |
2010年 | 602篇 |
2009年 | 754篇 |
2008年 | 596篇 |
2007年 | 476篇 |
2006年 | 478篇 |
2005年 | 410篇 |
2004年 | 397篇 |
2003年 | 321篇 |
2002年 | 301篇 |
2001年 | 231篇 |
2000年 | 208篇 |
1999年 | 189篇 |
1998年 | 181篇 |
1997年 | 200篇 |
1996年 | 138篇 |
1995年 | 128篇 |
1994年 | 117篇 |
1993年 | 86篇 |
1992年 | 82篇 |
1991年 | 88篇 |
1990年 | 96篇 |
1989年 | 63篇 |
1988年 | 70篇 |
1987年 | 91篇 |
1986年 | 60篇 |
1985年 | 80篇 |
1984年 | 75篇 |
1983年 | 68篇 |
1982年 | 69篇 |
1981年 | 75篇 |
1980年 | 57篇 |
1979年 | 52篇 |
1978年 | 42篇 |
1977年 | 48篇 |
1975年 | 40篇 |
1974年 | 48篇 |
1973年 | 47篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Accurate measurements in highly turbulent flows, as they occur in nature, require reliable velocity measuring techniques that permit instantaneous velocity components to be locally recorded. Hot-element techniques and optical methods are available for local measurements of instantaneous velocity and the present paper summarizes the advantages and disadvantages of different techniques when applied to flow fields in the environment of vegetation. The paper points out the advantages of laser-Doppler anemometry for velocity measurements in highly turbulent flows under laboratory conditions and stresses the reliability of the technique for measurements in polluted air and water flows. The basic principles of the method are explained and developments are described that have yielded optical anemometer systems for measurements of the magnitudes and signs of the instantaneous velocity components. Both quantities have to be known if accurate measurements of the mean flow properties and turbulence characteristics in flow fields with unknown flow directions are required. Electronic data-processing systems for laser-Doppler anemometer measurements are surveyed, embracing frequency analysers, automatic filter banks, frequency trackers, photon correlators and frequency counters. Photon-correlation and counting techniques are introduced as the methods most likely to be employed for the laser-Doppler anemometer. Laser-Doppler anemometer investigations are described in boundary-layer flows along bean leaves and a metal model of plant leaves. These measurements formed the basis of heat and mass transfer predictions near leaves for a specific leaf position relative to the free stream and with different turbulence properties imposed onto the oncoming flow. These data are presented and discussed in some detail. Measurements are also presented that were carried out to investigate the velocity fields in different flow regimes around a square obstacle in a water flow. These flow properties were needed to understand the different growth rates of sea-weed observed in differing flow regimes. 相似文献
992.
993.
Anne Martine Tréhu 《Earth and Planetary Science Letters》1975,27(2):287-304
The straight-line relationship between depth and the square root of age predicted by recent variations of the thermal contraction model for ocean rise elevation is confirmed to an age of 80 m.y.We then examine this relationship in the immediate vicinity of the rise crest in an attempt to determine the sensitivity of the slopes thus obtained. Depth versus t1/2 profiles from a variety of rise types ranging from the topographically smooth, fast-spreading Pacific-Antarctic rise to the rough, slow-spreading Mid-Atlantic rise are discussed, ages having been assigned using a finite rotation pole. Because of the variety of superimposed anomalous features concentrated within a limited and well-surveyed region, the Galapagos Spreading Center has provided a suitable arena for determining the precision with which the method can decompose such an agglomeration into distinctly recognizable components. Although topographic “noise” precludes precise quantification of the slopes, it is concluded that, by removing the first-order effect of thermal contraction, the method can be quite revealing when topography is examined in relation to other data. Slopes for several profiles across the Pacific-Antarctic and Pacific-Nazca rises reveal the pattern expected in the case of asymmetric spreading, a conclusion which has independently been derived from the magnetic anomalies. In the Galapagos region “jumps” of the spreading center, a basic compositional difference, and uplift from below are exposed by their predictable effect on the slopes obtained from the depth versus t1/2 plots. 相似文献
994.
E. Juvigné 《Quaternary Research》1975,5(1):121-123
The following note on a new method for extracting pollen from fossil pollen grains summarizes two articles in French, published in Annales de la Société Géologique de Belgique (Juvigné, 1973a; 1973b).The new method proposed has three important parts: (1) Elutriation of the sediment in water; (2) the specific gravity of the heavy liquid must be between 2.25 and 2.5; and (3) the dispersion of the sediment in the heavy liquid is made with a high-speed mixer. 相似文献
995.
P. J. Patchett O. van Breemen R. F. Martin 《Contributions to Mineralogy and Petrology》1979,69(1):65-73
Late-crystallised interstitial alkali feldspars and a single epidote from selected Proterozoic dolerites in Sweden have higher initital 87Sr/86Sr ratios, (e.g., 0.709) than the early-crystallised minerals of the same rocks ( 0.704); anomalies in Rb and Sr concentrations are also noted. This radiogenically-enriched Sr must originate in the older host rocks of the intrusions. As the contaminated phases often occupy <1 % of the dolerite, only an aqueous fluid would have been capable of transporting the contamination through the 99% solid intrusions. Textural association of late feldspars with hydrous alteration products supports this interpretation. Feldspar structural data suggest that most dolerites have been affected by subsolidus aqueous fluids, causing extensive structural re-equilibration in interstitial K-feldspars, as well as occasional metasomatic effects. Anomalies in 87Sr developed only where the fluids interacted with host rocks. 相似文献
996.
Martin I. Hoffert Yeong-Cherng Wey Andrew J. Callegari Wallace S. Broecker 《Climatic change》1979,2(1):53-68
The possibility of controlling atmospheric carbon dioxide accumulation and attendant climatic effects from fossil-fuel burning by diverting a fraction of the combustion product and injecting it into the deep-ocean, as proposed by Marchetti, is analyzed using an atmosphere/mixed layer/diffusive deep-ocean model for the carbon cycle. The model includes the nonlinear buffering of CO2 at the air/sea interface, and considers the long term trends associated with consuming an assumed fossil-fuel reserve equivalent to 7.09 × 1015 kg carbon as a logistic function of time as in the projections of Siegenthaler and Oeschger, except that atmospheric carbon dioxide levels are computed for five alternate strategies: (a) 100% injected into atmosphere, (b) 50% injected at oceanic depth of 1500 m and 50% into atmosphere, (c) 50% injected at sea floor (4000 m) and 50% into atmosphere, (d) 100% at 1500 m depth and (e) 100% at sea floor. Since no carbon leaves the system, all runs approached the same post-fossil fuel equilibrium after several thousand years, C
a
- 1150 ppm, almost four times the pre-fossil fuel value (- 300 ppm). But the transient response of these cases showed a marked variation ranging from a peak overshoot value of 2800 ppm in the year 2130 for 100% atmospheric injection to a slight decrease to the pre-fossil fuel 300 ppm lasting till 2300 with a subsequent slow approach to equilibrium for the 100% deep-ocean injection. The implications of these results for an oceanic injection strategy to mitigate the climatic impact of fossil-fuel CO2 is discussed, as are the ingredients of a second generation carbon cycle model for carrying out such forecasts on an engineering design basis. 相似文献
997.
998.
P.Y. Gillot J. Labeyrie C. Laj G. Valladas G. Guérin G. Poupeau G. Delibrias 《Earth and Planetary Science Letters》1979,42(3):444-450
The reverse paleomagnetism of the lava flows of Laschamp and Olby, already discovered by Bonhommet and Babkine, is confirmed. The ages of these flows, measured by14C, thermoluminescence and K-Ar dating are respectively36,000 ± 4000 and42,000 ± 5000years. VGP's comparison of the “Laschamp event” with those of the 30,000-year Lake Mungo excursion does not show evidence of coincidence between these two events. 相似文献
999.
1000.
Algal humic-like substances were oxidized with KMnO4. The oxidation products were analyzed by GC-MS. Normal and branched fatty acids, dicarboxylic and benzene carboxylic acids were identified. Aliphatic acids were by far the most prominent compounds. Similarities between humic and fulvic acids of both Fucus and Laminaria algae were apparent. Fucus humic acids, however, showed a more aromatic structure than their Laminaria counterpart. The contribution of algae to marine sedimentary humus is discussed. 相似文献