全文获取类型
收费全文 | 3410篇 |
免费 | 404篇 |
国内免费 | 919篇 |
专业分类
测绘学 | 333篇 |
大气科学 | 149篇 |
地球物理 | 621篇 |
地质学 | 1766篇 |
海洋学 | 577篇 |
天文学 | 78篇 |
综合类 | 310篇 |
自然地理 | 899篇 |
出版年
2024年 | 19篇 |
2023年 | 51篇 |
2022年 | 136篇 |
2021年 | 185篇 |
2020年 | 142篇 |
2019年 | 159篇 |
2018年 | 134篇 |
2017年 | 139篇 |
2016年 | 173篇 |
2015年 | 154篇 |
2014年 | 206篇 |
2013年 | 232篇 |
2012年 | 177篇 |
2011年 | 202篇 |
2010年 | 167篇 |
2009年 | 240篇 |
2008年 | 200篇 |
2007年 | 226篇 |
2006年 | 221篇 |
2005年 | 216篇 |
2004年 | 170篇 |
2003年 | 183篇 |
2002年 | 130篇 |
2001年 | 142篇 |
2000年 | 138篇 |
1999年 | 103篇 |
1998年 | 66篇 |
1997年 | 77篇 |
1996年 | 58篇 |
1995年 | 50篇 |
1994年 | 57篇 |
1993年 | 47篇 |
1992年 | 31篇 |
1991年 | 23篇 |
1990年 | 22篇 |
1989年 | 18篇 |
1988年 | 10篇 |
1987年 | 14篇 |
1986年 | 6篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1982年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
排序方式: 共有4733条查询结果,搜索用时 17 毫秒
31.
雪宝顶碱性花岗岩岩石地球化学与成矿控制 总被引:7,自引:0,他引:7
对雪宝顶碱性花岗岩具有高丰度的W、Sn、Be含量和W、Sn矿化花岗岩的微量元素组合特征进行了分析,结果显示:稀土元素具有∑REE含量与LREE/HREE比值偏低和负Eu异常的特点;元素比值K/Rb,U/Th,Nb/Ta的规律性变化以及矿脉产状特征等表明W-(Sn)-Be脉状矿床形成于岩浆演化晚期,与岩浆期后的热液活动有关。 相似文献
32.
Waterfront retaining walls supporting dry backfill are subjected to hydrostatic pressure on upstream face and earth pressure on the downstream face. Under seismic conditions, if such a wall retains a submerged backfill, additional hydrodynamic pressures are generated. This paper pertains to a study in which the effect of earthquakes along with the hydrodynamic pressure including inertial forces on such a retaining wall is observed. The hydrodynamic pressure is calculated using Westergaard's approach, while the earth pressure is calculated using Mononobe-Okabe's pseudo-static analysis. It is observed that when the horizontal seismic acceleration coefficient is increased from 0 to 0.2, there is a 57% decrease in the factor of safety of the retaining wall in sliding mode. For investigating the effect of different parameters, a parametric study is also done. It is observed that if φ is increased from 30° to 35°, there is an increase in the factor of safety in the sliding mode by 20.4%. Similar observations were made for other parameters as well. Comparison of results obtained from the present approach with [Ebeling, R.M., Morrison Jr, E.E., 1992. The seismic design of waterfront retaining structures. US Army Technical Report ITL-92-11. Washington DC] reveal that the factor of safety for static condition (kh=0), calculated by both the approaches, is 1.60 while for an earthquake with kh=0.2, they differ by 22.5% due to the consideration of wall inertia in the present study. 相似文献
33.
NPZ Models of Plankton Dynamics: Their Construction, Coupling to Physics, and Application 总被引:9,自引:0,他引:9
Peter J. S. Franks 《Journal of Oceanography》2002,58(2):379-387
Nutrient-phytoplankton-zooplankton (NPZ) models have been in use in oceanography for at least three decades, and are still
a common research tool. Given the discoveries of the last two decades, particularly concerning the role of bacteria in the
plankton, there are questions as to whether NPZ models can still be supported as a useful tool in planktonic research. Here
I review the construction of NPZ models, and some of the physical platforms they have been coupled to. I then discuss the
applications of NPZ-physical models, and conclude that they still constitute an important and viable research tool, provided
that the questions being explored are clearly stated.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
34.
数字地球与海洋科学的发展 总被引:3,自引:0,他引:3
本文从数字地球的概念出发,根据海洋科学的发展要求,详细论述了数字地球与海洋科学的发展关键。并提出了海洋科学的新的发展战略和配套技术的开发研究理论,从而为我国海洋事业的发展和数字地球的建立有一个协调和统一的认识观点。 相似文献
35.
Fisheries in boreal ecosystems 总被引:2,自引:1,他引:2
36.
This study takes cognizance of the fact that the TIOMIN (TIOMIN Resources Inc. of Canada) project has resulted in controversy over its handling of environmental issues and especially the Environmental Impact Assessment (EIA). The authors address many of the protracted issues that have slowed the development of the mining project in Kwale. The main emphasis is on the impacts of the mining and mineral separation processes on the environment, including the governing legislation, the role of consultation and public participation, and socioeconomic issues. In their public documents TIOMIN has specified neither the type of minerals it wants to extract from the area nor their chemical composition. It is well known, however that the titanium minerals and zircon targeted have impurities of iron, thorium and uranium. In the absence of an Environmental Management Plan, the effects of stockpiling radioactive wastes and other impurities that could possibly lead to environmental degradation in both the terrestrial and marine environments have not been publically addressed. The measures proposed to mitigate ecological damage as a result of the establishment of a minerals processing plant in the area seem inadequate. Pollution resulting from accidental spillage or breakage could have significant impact on marine life and residents living near the mining site. Other issues that have not been addressed satisfactorily pertain to the use of surface and underground water. The area already faces a huge water deficit and the calculations presented on aquifer recharge and stream flow rates do not indicate the large quantities of water that would be required in the mineral processing plant. The project, if approved in its present state, risks violation of international conventions. Furthermore, it could cause a conflict between Kenya and Tanzania in the event of an oil spill at the proposed ship loading facility at Shimoni. The proposed mining area includes the district's most fertile land, is home to many fisherfolk and is a major tourist destination. An analysis of the effects of this project on other available opportunities must be thoroughly understood to ascertain the economic and environmental benefits and costs of the mining venture. The proposed compensation rate of $1,000 per acre, for resettlement for example, appears to be grossly inadequate. Compensation should take into account family size and structure family assets and the cost of relocation. 相似文献
37.
38.
39.
40.
Combining indicator trends to assess ongoing changes in exploited fish communities: diagnostic of communities off the coasts of France 总被引:1,自引:1,他引:1