全文获取类型
收费全文 | 56283篇 |
免费 | 21778篇 |
国内免费 | 45734篇 |
专业分类
测绘学 | 4436篇 |
大气科学 | 24599篇 |
地球物理 | 17998篇 |
地质学 | 44993篇 |
海洋学 | 18135篇 |
天文学 | 1992篇 |
综合类 | 6559篇 |
自然地理 | 5083篇 |
出版年
2024年 | 242篇 |
2023年 | 659篇 |
2022年 | 1554篇 |
2021年 | 1964篇 |
2020年 | 2879篇 |
2019年 | 6299篇 |
2018年 | 6630篇 |
2017年 | 6343篇 |
2016年 | 6536篇 |
2015年 | 5679篇 |
2014年 | 5426篇 |
2013年 | 6108篇 |
2012年 | 5719篇 |
2011年 | 5605篇 |
2010年 | 5509篇 |
2009年 | 4814篇 |
2008年 | 3916篇 |
2007年 | 4058篇 |
2006年 | 3445篇 |
2005年 | 3244篇 |
2004年 | 3306篇 |
2003年 | 3051篇 |
2002年 | 2792篇 |
2001年 | 2591篇 |
2000年 | 2354篇 |
1999年 | 2669篇 |
1998年 | 2488篇 |
1997年 | 2524篇 |
1996年 | 2103篇 |
1995年 | 1968篇 |
1994年 | 1793篇 |
1993年 | 1672篇 |
1992年 | 1367篇 |
1991年 | 1030篇 |
1990年 | 923篇 |
1989年 | 759篇 |
1988年 | 683篇 |
1987年 | 503篇 |
1986年 | 422篇 |
1985年 | 324篇 |
1984年 | 333篇 |
1983年 | 225篇 |
1982年 | 250篇 |
1981年 | 174篇 |
1980年 | 130篇 |
1979年 | 130篇 |
1978年 | 61篇 |
1977年 | 53篇 |
1972年 | 45篇 |
1971年 | 55篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
601.
Analysis of Dynamic Characteristics of Submarine Free Spanning Pipelines by Complex Damping Method 总被引:2,自引:0,他引:2
Considering the effect of the internal flowing fluid and the external marine environmental condition, the differential equation for the vortex-induced vibration (V1V) of the free spanning pipeline is derived and is discretized by the Hermit interpolation function. The free vibration equation with the damping term is solved by the complex damping method for the natural frequency, and then the effect of fluid damping on the natural frequency of the free spanning pipeline is analyzed.The results show that fluid damping has a significant influence on the damped natural frequency of the free spanning pipeline in the lock-in state, while it has little influence when the pipeline is out of the lock-in state. In the meantime,the change of the free span length has the same effect on the damped natural frequency and the undamped natural frequency. 相似文献
602.
An optimization mathematical model of the pile forces for piled breasting dolphins in the open sea under various loading conditions is presented. The optimum layout with the well distributed pile forces and the least number of piles is achieved by the multiplier penalty function method. Several engineering cases have been calculated and compared with the result of the conventional design method. It is shown that the number of piles can be reduced at least by 10%—20% and the piles‘ bearing state is improved greatly. 相似文献
603.
Spatial and Temporal Variations of Sound Speed at the PN Section 总被引:3,自引:0,他引:3
Gridded sound speed data were calculated using Del Grosso's formulation from the temperature and salinity data at the PN section
in the East China Sea covering 92 cruises between February 1978 and October 2000. The vertical gradients of sound speed are
mainly related to the seasonal variations, and the strong horizontal gradients are mainly related to the Kuroshio and the
upwelling. The standard deviations show that great variations of sound speed exist in the upper layer and in the slope zone.
Empirical orthogonal function analysis shows that contributions of surface heating and the Kuroshio to sound speed variance
are almost equivalent.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
604.
Roles of Continental Shelves and Marginal Seas in the Biogeochemical Cycles of the North Pacific Ocean 总被引:4,自引:0,他引:4
Chen-Tung Arthur Chen Andrey Andreev Kyung-Ryul Kim Michiyo Yamamoto 《Journal of Oceanography》2004,60(1):17-44
Most marginal seas in the North Pacific are fed by nutrients supported mainly by upwelling and many are undersaturated with
respect to atmospheric CO2 in the surface water mainly as a result of the biological pump and winter cooling. These seas absorb CO2 at an average rate of 1.1 ± 0.3 mol C m−2yr−1 but release N2/N2O at an average rate of 0.07 ± 0.03 mol N m−2yr−1. Most of primary production, however, is regenerated on the shelves, and only less than 15% is transported to the open oceans
as dissolved and particulate organic carbon (POC) with a small amount of POC deposited in the sediments. It is estimated that
seawater in the marginal seas in the North Pacific alone may have taken up 1.6 ± 0.3 Gt (1015 g) of excess carbon, including 0.21 ± 0.05 Gt for the Bering Sea, 0.18 ± 0.08 Gt for the Okhotsk Sea; 0.31 ± 0.05 Gt for
the Japan/East Sea; 0.07 ± 0.02 Gt for the East China and Yellow Seas; 0.80 ± 0.15 Gt for the South China Sea; and 0.015 ±
0.005 Gt for the Gulf of California. More importantly, high latitude marginal seas such as the Bering and Okhotsk Seas may
act as conveyer belts in exporting 0.1 ± 0.08 Gt C anthropogenic, excess CO2 into the North Pacific Intermediate Water per year. The upward migration of calcite and aragonite saturation horizons due
to the penetration of excess CO2 may also make the shelf deposits on the Bering and Okhotsk Seas more susceptible to dissolution, which would then neutralize
excess CO2 in the near future. Further, because most nutrients come from upwelling, increased water consumption on land and damming
of major rivers may reduce freshwater output and the buoyancy effect on the shelves. As a result, upwelling, nutrient input
and biological productivity may all be reduced in the future. As a final note, the Japan/East Sea has started to show responses
to global warming. Warmer surface layer has reduced upwelling of nutrient-rich subsurface water, resulting in a decline of
spring phytoplankton biomass. Less bottom water formation because of less winter cooling may lead to the disappearance of
the bottom water as early as 2040. Or else, an anoxic condition may form as early as 2200 AD.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
605.
606.
607.
一种基于BP算法学习的小波神经网络 总被引:2,自引:1,他引:2
为发展 Szu的基于信号表示的小波神经网络 ,提出一种多输入多输出的小波网络模型 ,网络隐层采用框架小波函数、输出层采用 Sigmoid激励函数 ,并选用“熵误差函数”以加速网络的学习速度。奇偶判别和混沌时间序列预测例子的实验结果表明了它具有良好的函数逼近能力和推广能力 ,收敛速度和均方误差均优于相同结构的多层感知器模型。 相似文献
608.
根据Taylor的湍流扩散理论和Hay、Pasquill提出的拉格朗日流和欧拉流的相关函数的相似性,用黄河三角洲埕北海域的实测资料,按四种不同方案计算出该海域的水平扩散系数和谱密度分布。指出:该海区潮流扩散系数主要受潮流的影响,其分布特征与潮流的分布基本一致,所求的水平扩散系数主要是由周期性的潮运动给予潮流扰动所致。方案4是通过富氏变换滤波消去了周期性潮流扰动,本方案计算的水平扩散系数是由周期为6h以下的小尺度涡提供能量。 相似文献
609.
Semidiurnal and diurnal tidal waves in the Taiwan Strait are computed simultaneously by using finite difference method with two-dimensional nonlinear hydrodynamic equations. The agreement between the computed and observed results is considered satisfactory. 相似文献
610.
Two new species (Nitzschia sinensis Liu, sp. nov. and Podosira granulata Liu, sp. nov.) and one new variety (Xanthiopyxis microspinosa var. ellipticus Liu, var. nov.) collected from the urface sediments off the southern Huanghai Sea and the East China Sea are described and a list of diatoms from the surface sediments in the survey area and some new records in China are attached. 相似文献