全文获取类型
收费全文 | 250990篇 |
免费 | 5130篇 |
国内免费 | 3273篇 |
专业分类
测绘学 | 6681篇 |
大气科学 | 18657篇 |
地球物理 | 52549篇 |
地质学 | 86663篇 |
海洋学 | 20826篇 |
天文学 | 55404篇 |
综合类 | 975篇 |
自然地理 | 17638篇 |
出版年
2021年 | 2197篇 |
2020年 | 2561篇 |
2019年 | 2822篇 |
2018年 | 2846篇 |
2017年 | 2569篇 |
2016年 | 5325篇 |
2015年 | 4139篇 |
2014年 | 6849篇 |
2013年 | 14137篇 |
2012年 | 6169篇 |
2011年 | 7205篇 |
2010年 | 6348篇 |
2009年 | 8961篇 |
2008年 | 7848篇 |
2007年 | 7186篇 |
2006年 | 9525篇 |
2005年 | 7517篇 |
2004年 | 7439篇 |
2003年 | 6925篇 |
2002年 | 6557篇 |
2001年 | 5883篇 |
2000年 | 5870篇 |
1999年 | 5177篇 |
1998年 | 5198篇 |
1997年 | 4991篇 |
1996年 | 4643篇 |
1995年 | 4389篇 |
1994年 | 4073篇 |
1993年 | 3815篇 |
1992年 | 3599篇 |
1991年 | 3572篇 |
1990年 | 3739篇 |
1989年 | 3495篇 |
1988年 | 3285篇 |
1987年 | 3831篇 |
1986年 | 3391篇 |
1985年 | 4205篇 |
1984年 | 4716篇 |
1983年 | 4387篇 |
1982年 | 4300篇 |
1981年 | 3906篇 |
1980年 | 3629篇 |
1979年 | 3501篇 |
1978年 | 3471篇 |
1977年 | 3272篇 |
1976年 | 3035篇 |
1975年 | 2953篇 |
1974年 | 2911篇 |
1973年 | 3070篇 |
1972年 | 2022篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
681.
An efficient and robust method has been developed to locate multiple impulsive sources in an ocean environment. Global position system (GPS) receivers were installed on sonobuoys to obtain their locations within a few meters of accuracy. A sonobuoy field was deployed in a ring-type pattern. Charges were then set off at arbitrary locations within the ring, High-resolution plots were used to obtain direct path and/or first bottom bounce arrivals on each buoy. A model grid of arrival times was constructed, corresponding to the dimensions of the buoy field. A ray model previously developed here at the Applied Research Laboratories at the University of Texas at Austin (ARL:UT) was used to obtain model travel times. The minimum value of the least-square-type error between the real arrival times and the modeled travel times resulted in an unambiguous location of the source, within the limits of the grid spacing chosen. This value was calculated by picking one receiver as the reference and then summing the timing errors of the remaining receivers relative to the reference. Successive iterations with finer grid spacings result in source localization within the accuracy of the buoy locations. The localization routine was extended by allowing permutations of the pulse arrivals on each buoy to account for multiple sources closely separated in time and/or space. An automated correlation technique is presented as an alternative to the leading edge-detection method used here for obtaining relative arrival times. Two proof-of-concept experiments were performed and some results of data obtained at Lake Travis and the Gulf of Mexico are presented 相似文献
682.
Studies of soil productivity must compensate for the effects of temporal trends in order to examine the pattern of crop yields along spatial gradients. An analysis of the published yield estimates for 30 soils in 233 counties, however, did not find consistent yield increases over the past three decades. On the contrary, the yield estimates for many soils were markedly uniform since 1972. The uniformity appears to have two causes: the acknowledged difficulty of making yield estimates in a time of increasing variability in soil and crop management, both within and between regions, and surveyors' awareness of data stored in a national soils data base. The effect is to cast doubt on soil productivity data reported in county soil surveys published between 1973 and 1988. 相似文献
683.
684.
685.
We study the temporal intensity variations of Mgib bright features and investigate the corresponding Hα velocity pattern.
The network bright features are well visible in the continuum, in images averaged over the duration of the observations (130 min).
We detected `flashing' bright features, which appear and disappear within two to five minutes, while the rest of the bright
features undergo small variations of either their shape and/or their intensity. A power spectrum analysis reveals a 10-min
oscillation for approximately half of the stable bright features. The 5-min oscillations are detected mainly at the network
boundaries, where stable bright features are located, while 3-min oscillations coincide with few bright features, but are
also quite intense inside the network cells. The majority of bright features are associated with Hα downflows. The downflow
is very intense at the location of `flashing' bright features. 相似文献
686.
James F. Kasting 《Astrophysics and Space Science》1996,241(1):3-24
The goal of this conference is to consider whether it is possible within the next few decades to detect Earth-like planets around other stars using telescopes or interferometers on the ground or in space. Implicit in the term “Earth-like” is the idea that such planets might be habitable by Earth-like organisms, or that they might actually be inhabited. Here, I shall address two questions from the standpoint of planetary atmosphere evolution. First, what are the chances that habitable planets exist around other stars? And, second, if inhabited planets exist, what would be the best way to detect them? 相似文献
687.
The photometric U orbital variations of the symbiotic binary AG Peg, caused by occultation of a bright gaseous region by the
M giant in this system, are used to determine the geometry of this region whose existence is explained in the framework of
the colliding winds model. The emission of the bright region varies and a particular value of its mean density of 2–3 1010 cm-3 has been derived. The causes for the irregular variations of the light in the orbital minimum and maximum are discussed.
It is also obtained, when the mass-loss rate of the hot companion of the system decreases to about 5 10-10
Ms
⊙ yr-1, this bright region will disappear.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
688.
689.
W. Goudswaard 《Geophysical Prospecting》2001,49(6):625-627
690.