全文获取类型
收费全文 | 1376篇 |
免费 | 307篇 |
国内免费 | 311篇 |
专业分类
测绘学 | 186篇 |
大气科学 | 219篇 |
地球物理 | 505篇 |
地质学 | 652篇 |
海洋学 | 157篇 |
天文学 | 51篇 |
综合类 | 169篇 |
自然地理 | 55篇 |
出版年
2024年 | 10篇 |
2023年 | 30篇 |
2022年 | 55篇 |
2021年 | 57篇 |
2020年 | 58篇 |
2019年 | 78篇 |
2018年 | 51篇 |
2017年 | 84篇 |
2016年 | 61篇 |
2015年 | 79篇 |
2014年 | 88篇 |
2013年 | 95篇 |
2012年 | 93篇 |
2011年 | 114篇 |
2010年 | 86篇 |
2009年 | 82篇 |
2008年 | 92篇 |
2007年 | 111篇 |
2006年 | 106篇 |
2005年 | 85篇 |
2004年 | 77篇 |
2003年 | 64篇 |
2002年 | 53篇 |
2001年 | 29篇 |
2000年 | 37篇 |
1999年 | 36篇 |
1998年 | 38篇 |
1997年 | 20篇 |
1996年 | 22篇 |
1995年 | 15篇 |
1994年 | 18篇 |
1993年 | 13篇 |
1992年 | 10篇 |
1991年 | 7篇 |
1990年 | 5篇 |
1989年 | 13篇 |
1988年 | 8篇 |
1987年 | 8篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1978年 | 2篇 |
排序方式: 共有1994条查询结果,搜索用时 31 毫秒
11.
Radiocarbon and total carbonate data were obtained near the 1973 GEOSECS stations in the North Pacific along 30°N and along 175°E between 1993 and 1994. In these stations, we estimated radiocarbon originating from atomic bomb tests using tritium, trichlorofluoromethane and silicate contents. The average penetration depth of bomb radiocarbon during the two decades has deepened from 900 m to 1300 m. Bomb radiocarbon inventories above the average value for the whole North Pacific were found widely in the western subtropical region around 30°N both in the 1970s and 1990s, and its area in the 1990s was broader than that in the 1970s. In most of the North Pacific, while the bomb radiocarbon has decreased above 25.4, the bomb radiocarbon flux below 25.4 was over 1 × 1012 atom m-2yr-1 in the subtropical region around 30°N. In the tropical area south of 20°N, the bomb radiocarbon inventory below 25.4 increased from zero to over 10 × 1012 atom m-2 during the last three decades. These distributions suggest that the bomb radiocarbon removed from the surface is currently accumulated with bomb 14C flux of over 1 × 1012 atom m-2yr-1 below 25.4 in the subtropical region, mainly by advection from the higher latitude, and that part of the accumulated bomb 14C gradually spread southward with about 30 years. 相似文献
12.
基于矢量信号处理的水声定位系统 总被引:2,自引:1,他引:2
将传统的水声定位系统与矢量水听器相结合,设计了一种全新的轻便型长基线被动水声定位系统。介绍了系统的组成和工作原理,并结合近年来出现的矢量信号处理技术,设计了新的实时信号处理软件。经湖试和海试,系统的可行性得到了初步的验证。 相似文献
13.
Practical implementation of Hilbert-Huang Transform algorithm 总被引:12,自引:0,他引:12
Hilbert-Huang Transform (HHT) is a newly developed powerful method for nonlinear and non-stationary time series analysis. The empirical mode decomposition is the key part of HHT, while its algorithm was protected by NASA as a US patent, which limits the wide application among the scientific community. Two approaches, mirror periodic and extrema extending methods, have been developed for handling the end effects of empirical mode decomposition. The implementation of the HHT is realized in detail to widen the application. The detailed comparison of the results from two methods with that from Huang et al. (1998, 1999), and the comparison between two methods are presented. Generally, both methods reproduce faithful results as those of Huang et al. For mirror periodic method (MPM), the data are extended once forever. Ideally, it is a way for handling the end effects of the HHT, especially for the signal that has symmetric waveform. The extrema extending method (EEM) behaves as good as MPM, and it is better t 相似文献
14.
概述了应用子波分析对悬浮物测量中的声散射信号进行特征提取的方法 ,并对声散射信号识别的研究进行了探讨。对实测数据的实验取得了较好的结果 相似文献
15.
针对以往噪声信号发射换能器在实际工作中,存在着诸多影响正常科研试验的问题,并根据噪声信号的技术要求,以及探测方式的技术特征,我们对噪声发射换能器进行了重新设计,从选用超磁致伸缩材料作为换能器的材料入手,结合国内外一些新的设计理念,从材料性能分析,到理论设计,最后到可靠性实施方法,都进行了周密细致的研究,并完成了设计全过程。测试结果和使用性能表明,达到了预期的目的,这也是该材料在国内首次应用于单只宽频水声换能器中,开创了稀土超磁致伸缩材料应用的又一个先列。 相似文献
16.
High-resolution measurements of velocity and physio-chemistry were conducted before, during and after the passage of a transient front in a small subtropical system about 2.1 km upstream of the river mouth. Detailed acoustic Doppler velocimetry measurements, conducted continuously at 25 Hz, showed the existence of transverse turbulent shear between 300 s prior to the front passage and 1300 s after. This was associated with an increased level of suspended sediment concentration fluctuations, some transverse shear next to the bed and some surface temperature anomaly. 相似文献
17.
In this study, the impact of oceanic processes on the sensitivity of transient climate change is investigated using two sets of coupled experiments with and without tidal forcing, which are termed Exp_Tide and Exp_Control,respectively. After introducing tidal forcing, the transient climate response(TCR) decreases from 2.32 K to 1.90 K,and the surface air temperature warming at high latitudes decreases by 29%. Large ocean heat uptake efficiency and heat storage can explain the low TCR in Exp_Tide. Approximately 21% more heat is stored in the ocean in Exp_Tide(1.10×10~(24) J) than in Exp_Control(0.91×10~(24) J). Most of the large ocean warming occurs in the upper 1 000 m between 60°S and 60°N, primarily in the Atlantic and Southern Oceans. This ocean warming is closely related to the Atlantic Meridional Overturning Circulation(AMOC). The initial transport at mid-and high latitudes and the decline in the AMOC observed in Exp_Tide are both larger than those observed in Exp_Control. The spatial structures of AMOC are also different with and without tidal forcing in present experiments. The AMOC in Exp_Tide has a large northward extension. We also investigated the relationship between AMOC and TCR suggested by previous studies using the present experiments. 相似文献
18.
C. Debi Prasad T. Chandrasekhar J. N. Desai N. M. Ashok Vinod Krishan 《Journal of Astrophysics and Astronomy》1989,10(1):1-20
During the recent apparition of Comet Halley in 1985-86 a transient ionic event in the form of a blob of H2 O+ emission was recorded in thecoma at ~ 0h UT on 1986 March 13. Observations were carried out using a special IHW filter for H2 O+ emission at 7000 å/175 å, a 35 cm telescope, a Fabry-Perot interferometer and an image intensifier camera from Gurushikhar, Mt Abu. (24?39’N,72? 47’E). A Fabry-Perot inter-ferogram in Ha taken a few minutes later at the same location reveals strong hydrogen emission (Hα) associated with the blob. The velocity field in the blob is structured with relative velocities upto ~ 35km s?1. The event is interpreted as arising due to the sector boundary crossing of the interplanetary magnetic field by the comet 相似文献
19.
20.
This paper introduces a new method for GPS signal acquisition, which is based on the repeatability of successive code phase
measurements and the M-of-N search algorithm. The performance of the proposed method in terms of probability of signal detection
is similar to that of traditional methods, except that the calculation of the probability of detection does not rely on the
noise distribution or the Carrier-to-Noise ratio (C/N0). The code phase repeatability-based method is presented along with equations for probability of detection and probability
of false detection. If the distribution of the noise is known, it also provides an estimate of the C/N0. The proposed method is illustrated for coherent and non-coherent acquisition and C/N0 estimation. 相似文献