全文获取类型
收费全文 | 5787篇 |
免费 | 220篇 |
国内免费 | 48篇 |
专业分类
测绘学 | 112篇 |
大气科学 | 477篇 |
地球物理 | 1473篇 |
地质学 | 1920篇 |
海洋学 | 547篇 |
天文学 | 875篇 |
综合类 | 21篇 |
自然地理 | 630篇 |
出版年
2022年 | 28篇 |
2021年 | 66篇 |
2020年 | 68篇 |
2019年 | 79篇 |
2018年 | 119篇 |
2017年 | 94篇 |
2016年 | 148篇 |
2015年 | 147篇 |
2014年 | 152篇 |
2013年 | 272篇 |
2012年 | 190篇 |
2011年 | 279篇 |
2010年 | 205篇 |
2009年 | 281篇 |
2008年 | 245篇 |
2007年 | 229篇 |
2006年 | 239篇 |
2005年 | 206篇 |
2004年 | 186篇 |
2003年 | 169篇 |
2002年 | 184篇 |
2001年 | 95篇 |
2000年 | 116篇 |
1999年 | 96篇 |
1998年 | 103篇 |
1997年 | 78篇 |
1996年 | 72篇 |
1995年 | 101篇 |
1994年 | 92篇 |
1993年 | 79篇 |
1992年 | 75篇 |
1991年 | 55篇 |
1990年 | 83篇 |
1989年 | 70篇 |
1988年 | 70篇 |
1987年 | 74篇 |
1986年 | 77篇 |
1985年 | 88篇 |
1984年 | 108篇 |
1983年 | 92篇 |
1982年 | 88篇 |
1981年 | 65篇 |
1980年 | 85篇 |
1979年 | 69篇 |
1978年 | 76篇 |
1977年 | 52篇 |
1976年 | 66篇 |
1975年 | 57篇 |
1974年 | 49篇 |
1973年 | 63篇 |
排序方式: 共有6055条查询结果,搜索用时 312 毫秒
781.
Malcolm G. Anderson 《Applied geography (Sevenoaks, England)》1983,3(2):105-114
Slope topography is shown to be a significant factor in controlling the porewater pressures in road-cut slopes in the residual volcanic soils of St Lucia, West Indies. It is shown that this control diminishes as the permeability of the material decreases. This finding is confirmed by stability envelopes in which ‘stable’ and failed slopes are examined as a function of slope topography and material properties. 相似文献
782.
Gary L Mills Alfred K Hanson James G Quinn William R Lammela N.Dennis Chasteen 《Marine Chemistry》1982,11(4):355-377
Dissolved organic matter (DOM) and dissolved copper-organic complexes were isolated from the estuarine waters of Narragansett Bay, RI, using reverse-phase liquid chromatography employing C18 Sep-Pak cartridges (Waters Associates). The cartridges were found to have a constant retention efficiency for processing ? 1-l volumes of seawater. Fractionation of the isolated material, by sequential elution of the Sep-Pak with water: methanol mixtures of increasing organic solvent concentration, yielded a fraction of the organic matter with a specific copper activity six times greater than the overall activity for the isolated DOM. Analysis of this fraction by high performance liquid chromatography suggested that the organic components are of intermediate polarity and have appreciable aromatic character.An investigation of the protonation characteristics of the isolated complexes indicated that most of the copper is associated with a broad range of acidic sites on the DOM. Analysis by electron paramagnetic resonance spectroscopy confirmed the organic association of the isolated copper and also suggested the presence of several types of binding sites which probably involve oxygen donor ligands.Studies of the exchange of 64Cu with these binding sites on the isolated DOM indicated that 70% of the sites undergo rapid exchange with copper in seawater while 20% of these sites did not exchange in a 24-h time period. 相似文献
783.
Reverse-phase liquid chromatography was used for the isolation of dissolved organic matter and dissolved copper-organic complexes from the estuarine waters of Narragansett Bay, Rhode Island. The procedure isolates 10–30% of the organic matter and up to 50% of the total dissolved copper from various bay samples. Chromatograms obtained by high-performance liquid chromatography of the isolated organic matter showed qualitative differences between sampling stations progressing from the Providence River in the upper bay to the lower bay. 相似文献
784.
Analyses of Martian surface soil by Viking and Earth-based telescopes have been interpreted as indicating a regolith dominated by the weathering products of mafic or ultramafic rocks. Basaltic glass has previously been proposed as a more likely precursor than crystalline rock, given the low efficiency of surface weathering under present Martian conditions. On Earth large volumes of basaltic glass formed by quenching of magma by water. A similar interaction, between magma and ground ice, may have been a common occurrence on Mars. On the basis of this scenario palagonite, the alteration product of basaltic sideromelane glass, was studied as a possible analog to Martian soil. Samples from Iceland, Alaska, Antarctica, Hawaii, and the desert of New Mexico and Mexico were examined by optical and scanning electron microscopy, electron microprobe analysis, X-ray diffraction, spectrophotometry, and magnetic and thermogravimetric analysis. We suggest that palagonite is a good analog to the surface soil of Mars in chemical composition, particle size, spectral signature, and magnetic properties. Our model for the formation of fine-grained Martian surface soil begins with eruptions of basaltic magma through ground ice, forming deposits of glassy tuff. Individual glass shards are then altered by low-temperature hydrothermal systems to palagonitic material. Dehydration and aeolian abrasion strip the alteration rinds from the glass, and wind storms distribute the silt-sized palagonitic fragments in a planet-wide deposit. 相似文献
785.
Observed oscillatory current patterns in the southern basin of Lake Michigan, with a distinctive peak in the energy spectrum at a period of about 90 h, are simulated using a linear potential vorticity conservation model. Solutions of the forced vorticity equation in a paraboloidal basin show rotational, oscillatory motions tuned to the low-frequency topographic modes that are very similar to the observed flow patterns. Topography-controlled vorticity waves are excited most effectively by wind episodes with frequency nearly in resonance with the topographic modes. Bottom resistance has no significant effect on the frequency equation; it simply decays the waves slowly in the open lake and more quickly near the coast. Flow patterns of both the gravest free vorticity wave and the corresponding forced wave consist of two opposite circulation cells separated by a null streamline through the center of the basin and rotating cyclonically near the free wave and atmospheric forcing frequencies, respectively. Interactions between the forced and free waves result in an apparent rotational pattern with a frequency the median of the two. A combination of elliptic—paraboloidal basin and shorter period forced modes can approximate the observed Lake Michigan response. Doppler shift, due to the persistence of cyclonic vorticity in the flow field, is also determined to be a factor in shifting the elliptical basin mode to a higher frequency. 相似文献
786.
787.
A paleomagnetic study of the Mull lava succession 总被引:1,自引:0,他引:1
Summary. A paleomagnetic study has been made of a succession of 139 non-overlapping basaltic lavas, representing 91 per cent of the longest remaining succession of flows associated with the Paleogene Mull volcano. All the lavas have experienced considerable hydrothermal alteration, probably at up to several million years after initial magnetization and frequently with alteration to the opaque minerals and the production of new potentially magnetic phases. The question of whether directional remagnetization has taken place while preserving within-unit directional consistency and discreteness of unit mean direction is discussed. Extensive directional remagnetization is excluded as an explanation for the data. If stable directions obtained by alternating field remanence cleaning coincide with original TRM directions then a mean pole position for all temporally independent lava directions from the British Tertiary igneous province is at 71.9° N, 167.2° E, with k:22 and α95 :3.0°. This pole is significantly different from the geographic pole. If the difference in palaeomagnetic and geographic poles is interpreted in terms of absolute plate motion, then 2010 km of northwards motion of the western part of the Eurasian Plate, at 3.7 cm/yr, has taken place over the last 55 Myr. This motion has implications for the geological history of the Arctic and for the complexity of mantle motions. 相似文献
788.
789.
Philip B. James 《Icarus》1982,52(3):565-569
The 1979–1980 regression curve for the north polar cap of Mars, determined from Viking orbiter images, is compared to Viking observations of the same season 1 (Martin) year earlier and to telescopic observations. Differences between the two years cannot unambigously be attributed to dynamical effects because of uncertainties introduced by limited longitudinal coverage. 相似文献
790.