全文获取类型
收费全文 | 44289篇 |
免费 | 542篇 |
国内免费 | 307篇 |
专业分类
测绘学 | 1063篇 |
大气科学 | 2897篇 |
地球物理 | 8889篇 |
地质学 | 15338篇 |
海洋学 | 3785篇 |
天文学 | 10860篇 |
综合类 | 97篇 |
自然地理 | 2209篇 |
出版年
2021年 | 412篇 |
2020年 | 454篇 |
2019年 | 458篇 |
2018年 | 1050篇 |
2017年 | 960篇 |
2016年 | 1170篇 |
2015年 | 659篇 |
2014年 | 1136篇 |
2013年 | 2194篇 |
2012年 | 1286篇 |
2011年 | 1741篇 |
2010年 | 1545篇 |
2009年 | 2180篇 |
2008年 | 1810篇 |
2007年 | 1834篇 |
2006年 | 1690篇 |
2005年 | 1273篇 |
2004年 | 1274篇 |
2003年 | 1187篇 |
2002年 | 1138篇 |
2001年 | 1020篇 |
2000年 | 962篇 |
1999年 | 841篇 |
1998年 | 857篇 |
1997年 | 821篇 |
1996年 | 693篇 |
1995年 | 714篇 |
1994年 | 637篇 |
1993年 | 543篇 |
1992年 | 503篇 |
1991年 | 515篇 |
1990年 | 585篇 |
1989年 | 500篇 |
1988年 | 462篇 |
1987年 | 587篇 |
1986年 | 489篇 |
1985年 | 620篇 |
1984年 | 697篇 |
1983年 | 668篇 |
1982年 | 583篇 |
1981年 | 610篇 |
1980年 | 508篇 |
1979年 | 476篇 |
1978年 | 481篇 |
1977年 | 440篇 |
1976年 | 427篇 |
1975年 | 422篇 |
1974年 | 401篇 |
1973年 | 432篇 |
1972年 | 261篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Vagle S. Foote K.G. Trevorrow M.V. Farmer D.M. 《Oceanic Engineering, IEEE Journal of》1996,21(3):298-305
A calibration technique has been adapted to render complete system calibrations of high-frequency acoustical instrumentation. This is based on standard targets; specifically, precisely manufactured spheres composed of tungsten carbide with 6% cobalt binder. The use of multiple sphere sizes was found to be advantageous, both as an independent check of the calibrations, and so that resonances in the sphere responses at certain frequencies could be avoided. Complete system gains and beam patterns, which include effects of bandpass filters and finite-pulse lengths, were determined by moving the spheres individually in the transducer far-fields. Use of this procedure ensures control over the acoustical characteristics of transducers, which may change from the time of manufacture and first testing due, for example, to platform mounting. It also provides a direct means of measuring the sampling volume at relatively high and constant signal-to-noise ratios. Implementation of this technique is discussed using a multifrequency sonar system as an example 相似文献
62.
The cumulative probability distributions for stream order, stream length, contributing area, and energy dissipation per unit length of channel are derived, for an ordered drainage system, from Horton's laws of network composition. It is shown how these distributions can be related to the fractal nature of single rivers and river networks. Finally, it is shown that the structure proposed here for these probability distributions is able to fit the observed frequency distributions, and their deviations from straight lines in a log-log plot. 相似文献
63.
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
64.
D.G. King-Hele 《Planetary and Space Science》1985,33(10):1145-1153
The satellite 1967-11G, which had an orbital inclination of 40°, passed through the 14th-order resonance with the Earth's gravitational field in 1974. The changes in its orbital inclination at resonance have been analysed to obtain values for four lumped 14th-order harmonics in the geopotential, with accuracies equivalent to about 5 cm in geoid height. Analysis of the eccentricity was also attempted, but did not yield useful results.As no previous satellite analysed at 14th-order resonance has had an inclination near 40°, the results have proved to be valuable in determining individual 14th-order harmonics in the geopotential. 相似文献
65.
66.
67.
During a campaign of optical observations at high latitude, a bi-dimensional study of the wave structure of the OH layer has been performed in December 1981 from Sodankyla (Finland). This site is one of the three stations of the EISCAT ionosphere sounding system. It has been found that a wave field covering an area of 1 million km2 may extend to latitudes as high as 70°N. The OH wave structure shows many similarities with noctilucent clouds. The fairly large horizontal wavelength, of the order of 40 km cannot easily be explained by a wave motion at an interface. The observed wave structure seems to be a result of the propagation of an internal gravity wave in the 80–100 km region. This wave structure was often recorded during the same time as an active aurora was present. As a result, it appears that the perturbation might be correlated with particle precipitations at auroral latitudes. 相似文献
68.
U–Th–Pb analyses of zircons from six granites and one metasediment collected in the accretionary Central belt of Taimyr, Arctic Siberia, demonstrate that Neoproterozoic (c. 900 Ma) granites intrude late Mesoproterozoic/early Neoproterozoic amphibolite facies metamorphic rocks. This is the first time in the Mamont–Shrenk region that Neoproterozoic ages have been recognized for these lithologies, previously thought to be Archaean/Palaeoproterozoic in age. The Mamont–Shrenk Terrane (MST) represents a Grenvillian age (micro?) continent intercalated with younger Neoproterozoic ophiolites during thrusting and accreted to the northern margin of the Siberian craton sometime before the late Vendian. Basement to the MST may have been derived from the Grenvillian belt of east Greenland. Viable tectonic reconstructions must allow for an active margin along northern Siberia (modern day coordinates) in the middle Neoproterozoic. 相似文献
69.
P. MAISONGRANDE A. RUIMY G. DEDIEU B. SAUGIER 《Tellus. Series B, Chemical and physical meteorology》1995,47(1-2):178-190
70.
The discovery of magnetic field strengths in the range of 500 million Gauss in the objects Grw+70°8247 and PG1031+224 has given enormous impetus to investigations of magnetic white dwarf stars. We have determined intensities of Balmer transitions for B>108 Gauss as a function of field and find strong oscillations of the transitions strengths, which are of importance to the quantitative analysis of the observed spectra and the interpretation of polarimetric measurements in these objects.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986. 相似文献