全文获取类型
收费全文 | 8267篇 |
免费 | 291篇 |
国内免费 | 96篇 |
专业分类
测绘学 | 201篇 |
大气科学 | 566篇 |
地球物理 | 1924篇 |
地质学 | 2980篇 |
海洋学 | 752篇 |
天文学 | 1221篇 |
综合类 | 19篇 |
自然地理 | 991篇 |
出版年
2021年 | 91篇 |
2020年 | 113篇 |
2019年 | 114篇 |
2018年 | 167篇 |
2017年 | 164篇 |
2016年 | 227篇 |
2015年 | 194篇 |
2014年 | 196篇 |
2013年 | 469篇 |
2012年 | 248篇 |
2011年 | 308篇 |
2010年 | 282篇 |
2009年 | 338篇 |
2008年 | 321篇 |
2007年 | 280篇 |
2006年 | 315篇 |
2005年 | 239篇 |
2004年 | 291篇 |
2003年 | 262篇 |
2002年 | 264篇 |
2001年 | 173篇 |
2000年 | 170篇 |
1999年 | 141篇 |
1998年 | 141篇 |
1997年 | 103篇 |
1996年 | 114篇 |
1995年 | 114篇 |
1994年 | 128篇 |
1993年 | 113篇 |
1992年 | 113篇 |
1991年 | 109篇 |
1990年 | 101篇 |
1989年 | 85篇 |
1988年 | 87篇 |
1987年 | 122篇 |
1986年 | 100篇 |
1985年 | 165篇 |
1984年 | 187篇 |
1983年 | 144篇 |
1982年 | 128篇 |
1981年 | 131篇 |
1980年 | 108篇 |
1979年 | 124篇 |
1978年 | 120篇 |
1977年 | 103篇 |
1976年 | 97篇 |
1975年 | 91篇 |
1974年 | 68篇 |
1973年 | 83篇 |
1972年 | 53篇 |
排序方式: 共有8654条查询结果,搜索用时 15 毫秒
41.
John Flynn 《GeoJournal》1992,28(3):386-386
The International Geographical Union Reports
IGU commission on the world political map 相似文献42.
Surface-layer flow in complex terrain: Comparison of models and full-scale observations 总被引:2,自引:0,他引:2
John L. Walmsley Ib Troen Demetrius P. Lalas Paul J. Mason 《Boundary-Layer Meteorology》1990,52(3):259-281
Four models of surface boundary-layer flow in complex terrain are compared with observations made at Blashaval Hill, North Uist, Scotland. The field experiment is described by Mason and King (1985). Three of the models are derived from the two-dimensional theory of Jackson and Hunt (1975) and are described in Mason and King (1985), Walmsley et al. (1986) and Troen and Petersen (1989). The fourth is a mass-consistent code based on Traci et al. (1979). The model results are in good agreement with each other and are generally within the observed range of variation ( ~ ± 16%) in normalized wind speed. For most wind direcions (7 of 11), model results of normalized wind speed at the summit were within 7% of the observed mean values. For some wind directions, calculations using the Guidelines of Walmsley et al. (1989) suggested that variations in surface roughness were important. This led us to apply one of our models incorporating nonuniform surface roughness. The lack of significant improvement for cases when water lay upstream of Blashaval Hill is attributed to compensating changes at summit and reference sites and to very local effects on the wind data. Sensitivity to topography lying to the west and northwest of Blashaval was also investigated. Results suggested an influence from those distant topographic features for some wind directions. When those features were incorporated, maximum errors in normalized wind speed at the summit were reduced from 18 to 13%. 相似文献
43.
The Siljan Ring is a 362-Ma-old impact structure formed in 1700-Ma-old I-type granites. A 6.8-km-deep borehole provides a vertical profile through granites and isolated horizontal diabase sills. Fluid-inclusion thermometry, and oxygen-isotope compositions of vein quartz, granite, diabase, impact melt, and pseudotachylite, reveal a complex history of fluid activity in the Siljan Ring, much of which can be related to the meteorite impact. In granites from the deep borehole, 18O values of matrix quartz increase with depth from near 8.0 at the surface to 9.5 at 5760 m depth. In contrast, feldspar 18O values decrease with depth from near 10 at the surface to 7.1 at 5760 m, forming a pattern opposite to the one defined by quartz isotopic compositions. Values of 18O for surface granites outside the impact structure are distinct from those in near-surface samples from the deep borehole. In the deep borehole, feldspar coloration varies from brick-red at the surface to white at 5760 m, and the abundances of crack-healing calcite and other secondary minerals decrease over the same interval. Superimposed on the overall decrease in alteration intensity with depth are localized fracture zones at 4662, 5415, and 6044 m depth that contain altered granites, and which provided pathways for deep penetration of surface water. The antithetic variation of quartz and feldspar 18O values, which can be correlated with mineralogical evidence of alteration, provides evidence for interaction between rocks and impact-heated fluids (100–300° C) in the upper 2 km of the pluton. Penetration of water to depths below 2 km was restricted by a general decrease in impact-fracturing with depth, and by a 60-m-thick diabase sill at 1500 m depth that may have been an aquitard. At depths below 4 km in the pluton, where water/rock ratios were low, oxygen isotopic compositions preserve evidence for limited high-temperature (>500° C) exchange between alkali feldspar and fluids. The high-temperature exchange may have been a post-impact event involving impact-heated fluids, or a post-magmatic event. 相似文献
44.
John D. Saxby 《Geochimica et cosmochimica acta》1978,42(2):215-217
A simple method is proposed for using gas Chromatographic data on the high molecular weight saturated fractions of crude oils to make comparisons of source materials, migration mechanisms and, in particular, alteration processes within reservoirs. The preferential loss of normal alkanes by bacterial attack on a light crude is one of several processes readily followed using this scheme. 相似文献
45.
John I. Hedges 《Geochimica et cosmochimica acta》1978,42(1):69-76
Sugars and amino acids condense readily to form polymers known as melanoidins that closely resemble natural humic substances. In this study melanoidins are formed by reacting glucose with each of three amino acids: glutamic acid, valine and lysine. The basic amino acid, lysine, reacts with glucose at a much greater rate than either of the other two compounds. The chemical properties of the melanoidins are strongly influenced by the amino acid from which they are formed. The nitrogen contents, acidities and electrophoretic behavior of the polymers all reflect functional group distributions inherited from the amino acids. These chemical differences strongly influence the extent of association of the melanoidins with kaolinite and montmorillonite clay minerals. Laboratory simulations suggest that melanoidin formation may be favored in marine environments where basic amino acids should preferentially condense with sugars to form nitrogen-rich polymers that have a great affinity for clay mineral surfaces. 相似文献
46.
Image-tube filter photographs calibrated against photoelectric filter photometry have been used to give maps of M42 in absolute flux units over the central 15 arc min of the nebula in Hα, [Nii] (λ 6584 Å), Hβ and continuum at λ 4700 Å. Maps of the ratios Hα/[Nii] and (for the first time) of continuum/Hβ have been produced with unprecedented spatial resolution. These show that the gas to dust ratio is high near the exciting stars and falls strongly in the vicinity of large scale ionization fronts marked by minima in the Hα/[Nii] ratio. These results are interpreted in terms of detailed shell models containing either ice or graphite or silicate scattering particles. In all models there must be a central hole in the distribution of scattering particles. The effect of neutral globules and intrusions is investigated. It is found that all types of grain are trapped inside neutral intrusions near the centre of the nebula by the pressure of the Lα light surrounding the globule, but in the early evolution of the nebula particles can escape into the ionized medium when fronts are R-type. Ice grains escaping at this time will be destroyed for distances to the exciting stars less than 1 pc. These results can explain both the central hole in dust and the underabundance of oxygen in the ionized gas observed earlier. Arguments depending on colour index of the scattered light indicate that mixtures of scattered light from ice in the globules and from ice in the ionized medium can explain the observations, but that the graphite and silicate particles fail. A schematic model of the Orion Nebula is presented to attempt to explain the large scale phenomena observed here. It demonstrates that simple shell models for this nebula are dubious. 相似文献
47.
48.
The U.S. and U.K. literatures have discussed “food deserts,” reflecting populated, typically urban, low‐income areas with limited access to full‐service supermarkets. Less is known about supermarket accessibility within Canadian cities. This article uses the minimum distance and coverage methods to determine supermarket accessibility within the city of Edmonton, Canada, with a focus on high‐need and inner‐city neighborhoods. The results show that for 1999 both of these areas generally had higher accessibility than the remainder of the city, but six high‐need neighborhoods had poor supermarket accessibility. We conclude by examining potential reasons for differences in supermarket accessibility between Canadian, U.S., and U.K. cities. 相似文献
49.
Robert Gilbert Sarah Crookshanks Kyle R. Hodder John Spagnol Roland B. Stull 《Journal of Paleolimnology》2006,35(4):737-745
Severe rainfall in mid October, 2003 produced the largest floods in almost a century of record on rivers in the Cordillera
of southwestern British Columbia. Sediment deposited in Lillooet Lake as a result of this event is clearly distinguished by
stratigraphy, colour, texture, magnetic properties, and organic content. Each of these physical properties is related to the
lacustrine processes, especially turbid underflow, that distributed the sediment through the lake. The flood, which lasted
less than a week, delivered 8–12 times the amount of sediment that accumulates in most entire years in the deepest, central
parts of the lake. Recognition of events of this type in the stratigraphic record offers a means of assessing the changing
nature of extreme hydroclimatic events, and their relation to more ubiquitous, lower-energy processes. 相似文献
50.
On the Social Lives of Maps and the Politics of Diagrams: A story of power, seduction, and disappearance 总被引:1,自引:0,他引:1
John Pickles 《Area》2006,38(3):347-350