全文获取类型
收费全文 | 47170篇 |
免费 | 4914篇 |
国内免费 | 7226篇 |
专业分类
测绘学 | 3816篇 |
大气科学 | 6156篇 |
地球物理 | 9952篇 |
地质学 | 23883篇 |
海洋学 | 4606篇 |
天文学 | 2522篇 |
综合类 | 4238篇 |
自然地理 | 4137篇 |
出版年
2024年 | 133篇 |
2023年 | 439篇 |
2022年 | 1278篇 |
2021年 | 1441篇 |
2020年 | 1209篇 |
2019年 | 1375篇 |
2018年 | 5959篇 |
2017年 | 5133篇 |
2016年 | 3819篇 |
2015年 | 1577篇 |
2014年 | 1607篇 |
2013年 | 1689篇 |
2012年 | 2638篇 |
2011年 | 4337篇 |
2010年 | 3630篇 |
2009年 | 3793篇 |
2008年 | 3340篇 |
2007年 | 3570篇 |
2006年 | 1200篇 |
2005年 | 1225篇 |
2004年 | 1163篇 |
2003年 | 1089篇 |
2002年 | 951篇 |
2001年 | 770篇 |
2000年 | 763篇 |
1999年 | 808篇 |
1998年 | 650篇 |
1997年 | 573篇 |
1996年 | 545篇 |
1995年 | 438篇 |
1994年 | 408篇 |
1993年 | 360篇 |
1992年 | 312篇 |
1991年 | 212篇 |
1990年 | 176篇 |
1989年 | 145篇 |
1988年 | 114篇 |
1987年 | 76篇 |
1986年 | 66篇 |
1985年 | 49篇 |
1984年 | 35篇 |
1983年 | 28篇 |
1982年 | 26篇 |
1981年 | 38篇 |
1980年 | 31篇 |
1979年 | 19篇 |
1978年 | 12篇 |
1976年 | 10篇 |
1958年 | 11篇 |
1954年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
721.
Emil V. Stanev Jörg-Olaf Wolff Hans Burchard Karsten Bolding Götz Flöser 《Ocean Dynamics》2003,53(1):27-51
In this paper we use a combination of numerical modeling and data analysis to gain a better understanding of the major characteristics
of the circulation in the East Frisian Wadden Sea. In particular, we concentrate on the asymmetry of the tidal wave and its
modulation in the coastal area, which results in a complex pattern of responses to the sea-level forcing from the North Sea.
The numerical simulations are based on the 3-D primitive equation General Estuarine Transport Model (GETM) with a horizontal
resolution of 200 m and terrain-following vertical coordinates. The model is forced at its open boundaries with sea-level
data from an operational model for the German Bight (German Hydrographic Office). The validation data for our model simulations
include time series of tidal gauge data and surface currents measured at a pile in the back-barrier basin of the Island Langeoog,
as well as several ADCP transects in the Accumer Ee tidal inlet.
Circulation and turbulence characteristics are investigated for typical situations driven by spring and neap tides, and the
analysis is focused on dominating temporal and spatial patterns. By investigating the response of five back-barrier basins
with rather different morphologies to external forcing, an attempt is made to elucidate the dominating physical balances controlling
the circulation in the individual sub-basins. It is demonstrated that the friction at the seabed tends to slow down the tidal
signal in the shallow water. This leads to the establishment of flood dominance in the shallow sea north of the barrier islands.
South of the islands, where the water volume of the channels at low tide is smaller than the tidal prism, the asymmetry of
the tidal signal is shifted towards ebb dominance, a feature which is particularly pronounced at spring tide.
At the northern open boundary, the tidal wave propagating from west to east generates a sea-level difference of ∼1 m along
the boundary, and thereby triggers vigorous alongshore currents. The frictional control in the model is located in the inlets,
as well as along the northern boundary. The correlation between velocity and turbulent kinetic energy tends to the establishment
of a net southward transport, giving theoretical support to the observed accumulation of sediments on the intertidal flats.
Weak turbulence along the northern shores of the barrier islands and the small magnitude of the residual currents there promote
accumulation of suspended matter in these areas, although wave action will generally counteract this effect.
Received: 29 May 2002 / Accepted: 26 September 2002
Responsible Editor: Jean-Marie Beckers
Acknowledgements We are indebted to S. Dick for providing the data from the operational model of BSH and to B. Flemming for the useful discussions.
The topography data and Fig. 1 have been prepared in cooperation with F. Meyer. Figure 2 has been prepared by G. Brink-Spalink.
We also thank for the comments from an anonymous reviewer which helped to improve our paper. 相似文献
722.
— Seismic responses of weathered and non-weathered ridge models were simulated to study the ridge effects on the ground motion characteristics. The range of ridge slope from 19.98° to 45° was considered to produce a possible set of generalized results. 2.5-D modeling based on parsimonious staggered grid approximation of elastodynamic wave equations was adopted in simulations. Computed results reveal an increase of amplitude of incoming waves with both elevation and the slope of the ridge. Further, the characteristics of surface waves are highly ridge slope dependent. The analysis of responses of weathered and non-weathered ridge models reveals that ridge has caused a strong generation of surface waves near its top. The surface waves are not dominating on the top of the ridge but at some lower elevation. The increase of weathering of ridge further intensified the ridge effect. Analysis in frequency domain, based on spectral ratio method, does not indicate any pattern in the spectral amplification factor and is very much sensitive to slope, source focal mechanism and location. However, on an average there is a continuous decrease of amplification with slope in the vertical component and increase in the transverse component, and it is increasing in the radial component up to slope =38.0° and thereafter decreasing. 相似文献
723.
Tidal mixing plays an important role in the modification of dense water masses around the Antarctic continent. In addition to the vertical (diapycnal) mixing in the near-bottom layers, lateral mixing can also be of relevance in some areas. A numerical tide simulation shows that lateral tidal mixing is not uniformly distributed along the shelf break. In particular, strong mixing occurs all along the Ross Sea and Southern Weddell Sea shelf breaks, while other regions (e.g., the western Weddell Sea) are relatively quiet. The latter regions correspond surprisingly well to areas where indications for cross-shelf exchange of dense water masses have been found. The results suggest that lateral tidal mixing may account for the relatively small contribution of Ross Sea dense water masses to Antarctic Bottom Water. 相似文献
724.
Petrogenesis of the Ulsan carbonate rocks from the south-eastern Kyongsang Basin, South Korea 总被引:1,自引:0,他引:1
Abstract The petrogenesis of the Ulsan carbonate rocks in the Mesozoic Kyongsang Basin of South Korea, which have previously been interpreted as limestone of Paleozoic age, is reconsidered in the present study. Within the Kyongsang Basin, a small volume of carbonate rocks, containing a magnetite deposit and spatially associated ultramafic rocks, is surrounded by sedimentary, volcanic and granitic rocks of the Mesozoic age. The simple cross‐cutting relationships and other outcrop features of the area indicate that the carbonate rocks are an intrusive phase and younger than the other surrounding Mesozoic rocks. The Ulsan carbonates have low concentrations of rare earth elements (REE) and trace elements with the carbon and oxygen isotope values in the range of δ13CPDB = 2.4 to 4.0‰ and δ18OSMOW = 17.0 to 19.5‰. Outcrop evidence and geochemical signatures indicate that the Ulsan carbonates were formed from crustal carbonate melts, which were generated by the melting/fluxing of crustal carbonate materials, caused by the emplacement‐related processes of alkaline A‐type granitic rocks. Compared to typical mantle‐derived carbonatites associated with silica‐undersaturated, strongly peralkaline systems, the relatively small size and geochemical characteristics of the Ulsan carbonates reflect carbonatite genesis in a silica‐saturated, weakly alkali intrusive system. Major deep‐seated tectonic fractures formed by the collapse of the cauldron or the rift system associated with the opening of the East Sea (Japan Sea) might have facilitated the ascent of the crustal carbonate melts. 相似文献
725.
0.7Ma以来的念青唐古拉山脉隆升过程——来自冰川剥蚀作用的证据 总被引:1,自引:0,他引:1
通过对念青唐古拉山冰碛地层划分及冰碛物同位素测年,发现最早一期冰碛物形成于0.7~0.6MaBP,指示自中更新世以来念青唐古拉山脉开始隆升,主峰地区发生了大规模的冰川剥蚀作用,形成了大面积分布的冰碛高平台;0.2~0.14MaBP念青唐古拉山又快速隆升,并堆积了刚刚伸出各大沟谷口的高侧碛;0.07~0.03MaBP念青唐古拉山再次小规模隆起,形成各大沟谷内的侧碛和终碛垄;0.01Ma BP还有小规模冰川活动。念青唐古拉山主峰地区的冰川剥蚀作用反映出的山脉隆升过程,可较好地与青藏高原的隆起过程相对比,它应是青藏高原隆升的响应。 相似文献
726.
SHRIMP U-Pb zircon age, geochemical and Nd isotopic data are reported for the Neo-proterozoic Guandaoshan pluton in the Yanbian region, SW Sichuan. This pluton is of typical I-type granite and emplaced at (857 ± 13) Ma. Geochemical and Nd isotopic characters suggest that the pluton was generated by partial melting of pre-existing, young (late Mesoproterozoic to early Neo-proterozoic) low-K tholeiitic protolith within an intraplate anorogenic setting. The Guandaoshan pluton probably records the earliest magmatism induced by the proposed ca. 860–750 Ma mantle superplume beneath the supercontinent Rodinia. 相似文献
727.
728.
阐述研究空间数据质量问题的必要性和重要性,进而依据GIS数据流程系统分析了产生数据质量问题的各种因素;并重点介绍了空间数据质量问题中基本的误差和不确定性分析方法以及常用的误差控制与校正方法。 相似文献
729.
730.