全文获取类型
收费全文 | 972篇 |
免费 | 34篇 |
国内免费 | 7篇 |
专业分类
测绘学 | 22篇 |
大气科学 | 43篇 |
地球物理 | 234篇 |
地质学 | 305篇 |
海洋学 | 104篇 |
天文学 | 174篇 |
综合类 | 1篇 |
自然地理 | 130篇 |
出版年
2021年 | 14篇 |
2020年 | 13篇 |
2019年 | 18篇 |
2018年 | 30篇 |
2017年 | 22篇 |
2016年 | 26篇 |
2015年 | 26篇 |
2014年 | 30篇 |
2013年 | 75篇 |
2012年 | 31篇 |
2011年 | 52篇 |
2010年 | 42篇 |
2009年 | 57篇 |
2008年 | 47篇 |
2007年 | 46篇 |
2006年 | 39篇 |
2005年 | 27篇 |
2004年 | 31篇 |
2003年 | 33篇 |
2002年 | 21篇 |
2001年 | 28篇 |
2000年 | 19篇 |
1999年 | 23篇 |
1998年 | 13篇 |
1997年 | 20篇 |
1996年 | 6篇 |
1995年 | 7篇 |
1994年 | 15篇 |
1993年 | 15篇 |
1992年 | 8篇 |
1991年 | 7篇 |
1990年 | 13篇 |
1989年 | 5篇 |
1988年 | 7篇 |
1987年 | 13篇 |
1986年 | 7篇 |
1985年 | 9篇 |
1984年 | 10篇 |
1983年 | 12篇 |
1982年 | 9篇 |
1981年 | 8篇 |
1980年 | 10篇 |
1979年 | 8篇 |
1978年 | 4篇 |
1976年 | 8篇 |
1975年 | 6篇 |
1974年 | 7篇 |
1973年 | 5篇 |
1969年 | 4篇 |
1968年 | 5篇 |
排序方式: 共有1013条查询结果,搜索用时 15 毫秒
51.
52.
53.
The International Geographical Union Reports
IGU Commission on Population Geography 相似文献54.
A.B. Cadle B. Cairncross A.D.M. Christie D.L. Roberts 《International Journal of Coal Geology》1993,23(1-4)
The coal-bearing sediments and coal seams of the Karoo Basin, Southern Africa are described and discussed. The Karoo Basin is bounded on its southern margin by the Cape Fold Belt, onlaps onto the Kaapvaal Craton in the north and is classified as a foreland basin. Coal seams are present within the Early Permian Vryheid Formation and the Triassic Molteno Formation.The peats of the Vryheid Formation accumulated within swamps in a cool temperate climatic regime. Lower and upper delta plain, back-barrier and fluvial environments were associated with peat formation. Thick, laterally extensive coal seams have preferentially accumulated in fluvial environments. The coals are in general inertinite-rich and high in ash. However, increasing vitrinite and decreasing ash contents within seams occur from west to east across the coalfields. The Triassic Molteno coal seams accumulated with aerially restricted swamps in fluvial environments. These Molteno coals are thin, laterally impersistent, vitrinite-rich and shaly, and formed under a warm temperate climatic regime.Palaeoclimate, depositional systems, differential subsidence and basin tectonics influence to varying degrees, the maceral content, thickness and lateral extent of coal seams. However, the geographic position of peat-forming swamps within a foreland basin, coupled with basin tectonics and differential subsidence are envisaged as the primary controls on coal parameters. The Permian coals are situated in proximal positions on the passive margin of the foreland basin. Here, subsidence was limited which enhanced oxidation of organic matter and hence the formation of inertinitic coals. The coals in this tectonic setting are thick and laterally extensive. The Triassci coals are situated within the tectonically active foreland basin margin. Rapid subsidence and sedimentation rates occurred during peat formation which resulted in the preservation of thin, laterally impersistent, high ash, vitrinite-rich, shaly coals. 相似文献
55.
56.
Digital techniques for the correction of signal distortions that arise in planar laser-induced fluorescence (PLIF) measurements (by PC-based video digitizing systems) of jets and plumes and the production of flow statistics in both time and space are reviewed. The entire concentration field is repeatedly imaged in
s intervals over hundreds of thousands a points in a plane. By the recognition of signal distortion sources and the employment of corrections, a clearer picture of tracer concentrations may be realized.Fluorescence studies are made with a planar sheet of laser light 430 mm tall and 1.5 mm thick. The fluorescence excitation produced from trace concentrations of Rhodamine 6G is used to visualize and measure the propagation of a jet or plume in a density stratified laboratory tank. The emitted light is collected by a CCD camera in a 512 × 480 pixel format over a 940 × 715 mm field of view. The captured images are corrected for transverse laser sheet intensity distribution; laser beam attenuation; refraction; lens vignette; time varying and spatial noise; digitization aspect ratio; camera response. The measurement and methods of correction are discussed in detail. The resulting image data can then be used to collect tracer concentration statistics for jets and plumes. Instantaneous (i.e. over
of a second intervals), average, maximum, minimum, standard deviation, and coefficient of variation are given as introductory examples of image statistics realizable for a buoyant jet. 相似文献
57.
58.
59.
60.
David A. Noever Helen C. Matsos Raymond J. Cronise Loren L. Looger Rachna A. Relwani Vladimir I. Nikora 《Climatic change》1994,27(3):299-320
Phytoplankton is considered a key component mediating the ocean-atmospheric exchange of carbon dioxide and oxygen. Lab simulations which model biological responses to atmospheric change are difficult to translate into natural settings owing in part to the vertical migration of phytoplankton. In the sea this vertical migration acts to regulate actual carbon dioxide consumption. To capture some critical properties of this vertical material transfer, we monitored the effects of atmospheric CO2 on dense suspensions of bioconvecting microorganisms. Bioconvection refers to the spontaneous patterns of circulation which arise among such upwardly swimming cells as alga, protozoa, zoospore and large bacteria. Gravity, phototaxis and chemotaxis have all been implicated as affecting pattern-forming ability. The ability of a biologically active suspension to detect atmospheric changes offers a unique method to quantify organism adjustment and vertical migration. With increasing CO2, bioconvection patterns in alga (P. parva) and protozoa (T. pyriformis) lose their robustness, and surface cell populations retreat from the highest CO2 regions. Cell movement (both percent motile and mean velocity) generally diminishes. A general program of image analysis yields statistically significant variations in macroscopic migration patterns; both fractal dimension and various crystallographic parameters correlate strongly with carbon dioxide content. 相似文献