全文获取类型
收费全文 | 70088篇 |
免费 | 1503篇 |
国内免费 | 1267篇 |
专业分类
测绘学 | 1603篇 |
大气科学 | 5053篇 |
地球物理 | 14104篇 |
地质学 | 25810篇 |
海洋学 | 6314篇 |
天文学 | 15187篇 |
综合类 | 318篇 |
自然地理 | 4469篇 |
出版年
2022年 | 505篇 |
2021年 | 832篇 |
2020年 | 886篇 |
2019年 | 980篇 |
2018年 | 1766篇 |
2017年 | 1717篇 |
2016年 | 1946篇 |
2015年 | 1183篇 |
2014年 | 1917篇 |
2013年 | 3436篇 |
2012年 | 2336篇 |
2011年 | 3119篇 |
2010年 | 2712篇 |
2009年 | 3473篇 |
2008年 | 3016篇 |
2007年 | 3123篇 |
2006年 | 2968篇 |
2005年 | 2072篇 |
2004年 | 2028篇 |
2003年 | 1928篇 |
2002年 | 1838篇 |
2001年 | 1565篇 |
2000年 | 1488篇 |
1999年 | 1279篇 |
1998年 | 1298篇 |
1997年 | 1283篇 |
1996年 | 1090篇 |
1995年 | 1061篇 |
1994年 | 913篇 |
1993年 | 813篇 |
1992年 | 780篇 |
1991年 | 759篇 |
1990年 | 833篇 |
1989年 | 713篇 |
1988年 | 672篇 |
1987年 | 777篇 |
1986年 | 717篇 |
1985年 | 892篇 |
1984年 | 1023篇 |
1983年 | 934篇 |
1982年 | 888篇 |
1981年 | 800篇 |
1980年 | 794篇 |
1979年 | 711篇 |
1978年 | 677篇 |
1977年 | 695篇 |
1976年 | 612篇 |
1975年 | 610篇 |
1974年 | 609篇 |
1973年 | 667篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
In recent years, interface waves such as the Scholte wave have become important tools in the study of the geoacoustic properties of near-bottom seafloor sediments. Traditionally, these waves have been generated by explosive or pneumatic sources deployed at or near the seafloor and monitored by ocean-bottom seismographs or geophone arrays. While these sources generate the requisite interface waves, they also produce higher frequency compressional waves in the water and sediment that tend to contaminate the surface wave and make inversion of the data difficult in the near field. In this paper, a new source consisting of a freely falling projectile instrumented with an accelerometer is described. When the projectile impacts the bottom, the exact time history of the vertical force applied to the sediment is known and therefore may be convolved with the transfer function of a sediment geoacoustic model to produce accurate synthetic seismograms. Moreover, the vertical force applied to the seafloor is very efficient in generating surface wave motion while producing very little compressional wave energy so that the near-field signals are much more easily analyzed. An example of the use of the new source is presented including inversion of the received signals to obtain shear-wave velocity and attenuation as a function of depth in the near bottom sediments at a shallow-water site 相似文献
72.
73.
R. K. Pant N. Basavaiah N. Juyal N. K. Saini M. G. Yadava E. Appel A. K. Singhvi 《第四纪科学杂志》2005,20(5):485-492
The southwest monsoon that dominated Central Himalaya has preserved loessic silt deposits preserved in patches that are proximal to periglacial areas. The occurrence of such silts suggests contemporary prevalence of cold and dry northwesterly winds. Field stratigraphy, geochemistry, mineral magnetism, infrared stimulated luminescence (IRSL) and radiocarbon dating has enabled reconstruction of an event chronology during the past 20 ka. Three events of loess accretion could be identified. The first two events of loess deposition occurred betweem 20 and 9 ka and were separated by a phase of moderate weathering. Pedogenesis at the end of this event gave rise to a well‐developed soil that was bracketed around 9 to > 4 ka. This was followed by the third phase of loess accretion that occurred around 4 to > 1 ka. Episodes of loess deposition and soil formation are interpreted in terms of changes in the strength of the Indian southwest monsoon. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
75.
76.
Quartz‐rich veins in metapelitic schists of the Sanandaj‐Sirjan belt, Hamadan region, Iran, commonly contain two Al2SiO5 polymorphs, and, more rarely, three coexisting Al2SiO5 polymorphs. In most andalusite and sillimanite schists, the types of polymorphs in veins correlate with Al2SiO5 polymorph(s) in the host rocks, although vein polymorphs are texturally and compositionally distinct from those in adjacent host rocks; e.g. vein andalusite is enriched in Fe2O3 relative to host rock andalusite. Low‐grade rocks contain andalusite + quartz veins, medium‐grade rocks contain andalusite + sillimanite + quartz ± plagioclase veins, and high‐grade rocks contain sillimanite + quartz + plagioclase veins/leucosomes. Although most andalusite and sillimanite‐bearing veins occur in host rocks that also contain Al2SiO5, kyanite‐quartz veins crosscut rocks that lack Al2SiO5 (e.g. staurolite schist, granite). A quartz vein containing andalusite + kyanite + sillimanite + staurolite + muscovite occurs in andalusite–sillimanite host rocks. Textural relationships in this vein indicate the crystallization sequence andalusite to kyanite to sillimanite. This crystallization sequence conflicts with the observation that kyanite‐quartz veins post‐date andalusite–sillimanite veins and at least one intrusive phase of a granite that produced a low‐pressure–high‐temperature contact aureole; these relationships imply a sequence of andalusite to sillimanite to kyanite. Varying crystallization sequences for rocks in a largely coherent metamorphic belt can be explained by P–T paths of different rocks passing near (slightly above, slightly below) the Al2SiO5 triple point, and by overprinting of multiple metamorphic events in a terrane that evolved from a continental arc to a collisional orogen. 相似文献
77.
Abstract— Five hundred stony cosmic spherules collected from deep-sea sediments, polar ice, and the stratosphere have been analyzed for major and some minor element composition. Typical spherules are products of atmospheric melting of millimeter sized and smaller meteoroids. The samples are small and modified by atmospheric entry, but they are an important source of information on the composition of asteroids. The spherules in this study were all analyzed in an identical manner, and they provide a sampling of the solar system's asteroids that is both different and less biased than provided by studies of conventional meteorites. Volatile elements such as Na and S are depleted due to atmospheric heating, while siderophiles are depleted by less understood causes. The refractory nonsiderophile elements appear not to have been significantly disturbed during atmospheric melting and provide important clues on the elemental composition of millimeter sized meteoroids colliding with the Earth. Typical spherules have CM-like composition that is distinctively different than ordinary chondrites and most other meteorite types. We assume that C-type asteroids are the primary origin of spherules with this composition. Type S asteroids should also be an important source of the spherules, and the analysis data provide constraints on their composition. A minor fraction of the spherules are melt products of precursor particles that did not have chondritic elemental compositions. The most common of these are particles that are dominated by olivine. The observed compositions of spherules are inconsistent with the possibility that an appreciable fraction of the spherules are simply chondrules remelted during atmospheric entry. 相似文献
78.
E. F. Arias C. Boucher M. Feissel J. -F. Lestrade 《Astrophysics and Space Science》1991,177(1-2):187-192
The relative orientations of various VLBI celestial reference frames are evaluated on the basis of coordinate differences of common sources. It is shown that an accuracy better than 0.001 can be achieved. Possible regional deformations in the different catalogues are investigated; they are found to reach a few 0.001 in some restricted zones. The application of these studies to the realisation of a combined celestial reference frame consistent with the BIH Terrestrial System is outlined. 相似文献
79.
P. L. Bornmann 《Solar physics》1985,102(1-2):111-130
The light curves of soft X-ray lines, observed by the Flat Crystal Spectrometer on Solar Maximum Mission during eight solar flares are modeled to determine the plasma temperature and emission measure as functions of time using the method first presented by Bornmann (1985, Paper I), but modified to include a 2 search routine. With this modification the technique becomes more general, more accurate, and applicable throughout the gradual phase of the flare. The model reproduces the light curves of the soft X-ray lines throughout these flares. Model fits were repeated for each flare using five different sets of published line emissivity calculations. The emissivities of Mewe and Gronenschild (1981) consistenly gave the best fits to the observed light curves for each flare. 相似文献
80.
Using plane wave theory and assuming a given input wavelet the shape of the reflected (or transmitted) wavelet from a layered boundary is derived. Several types of boundaries are considered, among them the weathered layer and a wedge shaped intermediate layer. Different angles of incidence and all internal multiples are taken into account. The examples shown in the figures can be used for a direct comparison between theoretical and observed shapes of reflected (or transmitted) wavelets from special boundaries. 相似文献