全文获取类型
收费全文 | 251725篇 |
免费 | 5136篇 |
国内免费 | 3301篇 |
专业分类
测绘学 | 6703篇 |
大气科学 | 18703篇 |
地球物理 | 52729篇 |
地质学 | 87027篇 |
海洋学 | 20867篇 |
天文学 | 55498篇 |
综合类 | 977篇 |
自然地理 | 17658篇 |
出版年
2021年 | 2213篇 |
2020年 | 2565篇 |
2019年 | 2829篇 |
2018年 | 2928篇 |
2017年 | 2649篇 |
2016年 | 5397篇 |
2015年 | 4152篇 |
2014年 | 6859篇 |
2013年 | 14145篇 |
2012年 | 6201篇 |
2011年 | 7263篇 |
2010年 | 6417篇 |
2009年 | 9046篇 |
2008年 | 7890篇 |
2007年 | 7237篇 |
2006年 | 9531篇 |
2005年 | 7536篇 |
2004年 | 7463篇 |
2003年 | 6937篇 |
2002年 | 6569篇 |
2001年 | 5891篇 |
2000年 | 5872篇 |
1999年 | 5180篇 |
1998年 | 5203篇 |
1997年 | 4995篇 |
1996年 | 4643篇 |
1995年 | 4390篇 |
1994年 | 4076篇 |
1993年 | 3818篇 |
1992年 | 3601篇 |
1991年 | 3572篇 |
1990年 | 3739篇 |
1989年 | 3497篇 |
1988年 | 3288篇 |
1987年 | 3832篇 |
1986年 | 3391篇 |
1985年 | 4206篇 |
1984年 | 4716篇 |
1983年 | 4387篇 |
1982年 | 4300篇 |
1981年 | 3908篇 |
1980年 | 3630篇 |
1979年 | 3502篇 |
1978年 | 3472篇 |
1977年 | 3273篇 |
1976年 | 3035篇 |
1975年 | 2954篇 |
1974年 | 2911篇 |
1973年 | 3071篇 |
1972年 | 2022篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
42.
43.
JOKOB J. MØLLER TOR K. DANIELSEN ARNE FJALSTAD 《Boreas: An International Journal of Quaternary Research》1992,21(1):1-13
Morphological, seismic and lithostratigraphic investigations of a moraine deposit at Bleik (the Bleik moraine), northern Andøya, show short-distance transported till overlying long-distance transported predominantly glaciofluvial ice-marginal deposits. Consolidated glaciomarine sediments from a core at present sea-level, c . 400 m distally to the moraine complex, contain 31 species of foraminifera, among which Cassidulina reniforme, Islandiella helenae and Trifarina fluens dominate, and fragments of the molluscs Mya truncata and Astarte sp. and the echinoid Strongylocentrotus sp. Amino acid analyses of the foraminifera Cibicides lobatulus and the mollusc Mya truncata indicate ages between 22,000 and 16,000 BP. Radiocarbon dating of fragments of Mya truncata from the upper part of the core gave an age of 17,940 ± 245 BP, while a dating of unidentified shell fragments from the lower part gave an infinite age (>40,000 BP). The sediment was probably disturbed by icebergs beyond the end moraine zone, and the radiocarbon and amino acid dating of Mya truncata therefore represent a maximum age for this process. This new evidence indicates two phases with a higher relative sea-level than at present at Bleik, c . 18,000 and >40,000 BP. The Bleik moraine probably represents the early Late Weichselian glacial maximum ( c . 22,000 BP), while the underlying deglaciation deposit and associated beach-ridge (Bruvollen) is of pre-Late Weichselian age. Moraine ridges 3–4 km to the south of Bleik probably indicate advances of local glaciers between 22,000 and 18,000 BP. 相似文献
44.
Isolation of resonance in acoustic backscatter from elastic targetsusing adaptive estimation schemes
It has been shown that at certain frequencies the acoustic backscatter from elastic targets exhibits certain resonance behavior which closely relates to the physical properties of the target such as dimension, thickness, and composition. The purpose of this paper is to develop an automated approach for identifying the presence of resonance in the acoustic backscatter from an unknown underwater target by isolating the resonance part from the specular contribution. An adaptive transversal filter structure is used to estimate the specular part of the backscatter and consequently the error signal would provide an estimate of the resonance part. An important aspect of this scheme lies in the fact that it does not require an underlying model for the elastic return. The adaptation rule is based upon fast recursive least squares (RLS) learning. The approach taken in this paper is general in the sense that it can be applied to targets of unknown geometry and thickness and, further, does not require any a priori information about the target and/or the environment. Test results on acoustic data are presented which indicate the effectiveness of the proposed approach 相似文献
45.
J. C. Eichelberger 《Bulletin of Volcanology》1989,51(1):72-75
This work was sponsored at Sandia National Laboratories (contract DE-AC04-76DP00789) by the US Department of Energy, Office of Basic Energy Sciences, as part of the Continental Scientific Drilling Program 相似文献
46.
47.
48.
Mallory S. E. Roberts Crystal L. Brogan Bryan M. Gaensler Jason W. T. Hessels C.-Y. Ng Roger W. Romani 《Astrophysics and Space Science》2005,297(1-4):93-100
A remarkable number of pulsar wind nebulae (PWN) are coincident with EGRET γ-ray sources. X-ray and radio imaging studies of unidentified EGRET sources have resulted in the discovery of at least six new pulsar wind nebulae (PWN). Stationary PWN (SPWN) appear to be
associated with steady EGRET sources with hard spectra, typical for γ-ray pulsars. Their toroidal morphologies can help determine the geometry of the
pulsar which is useful for constraining models of pulsed γ-ray emission. Rapidly moving PWN (RPWN) with more cometary morphologies
seem to be associated with variable EGRET sources in regions where the ambient medium is dense compared to what is typical for the ISM. 相似文献
49.
50.
Abstract The Yaxcopoil‐1 (Yax‐1) drill hole comprises Cretaceous limestones and calcarenites, the K/P boundary cocktail unit (including impact breccia), and a Danian marly clay layer overlain by calcareous marls. The biostratigraphy, paleobathymetry, and environmental turnover across the K/P interval were inferred after analyzing the planktic and benthic foraminiferal assemblages. The Cretaceous samples only contain a few poorly preserved planktic foraminifera of a middle Campanian to Maastrichtian age, while low‐diversity benthic foraminiferal assemblages suggest a sufficient nutrient supply to the sea floor and a shallow neritic, occasionally stressed environment. The impact breccia and the redeposited suevite are overlain by a 46 cm‐thick dolomitic calcareous sandstone unit that contains scarce, reworked planktic foraminiferal specimens. This unit probably represents the uppermost part of the initial infill of the crater. The uppermost centimeters of this unit are bioturbated, and its top represents a hiatus that spans at least the G. cretacea, Pv. eugubina, and part of the P. pseudobulloides biozones. This unit is overlain by a 3–4 cm‐thick marly clay layer that represents a condensed layer. Benthic foraminiferal assemblages suggest a low food supply to the sea floor and environmental instability during the deposition of the marly clay layer. The increase in diversity of the assemblages indicates that the environmental conditions improved and stabilized from the G. compressa biozone toward the A. uncinata (P2) biozone. The Danian planktic and benthic foraminiferal assemblages indicate a deeper, probably bathyal environment. 相似文献