全文获取类型
收费全文 | 1130篇 |
免费 | 336篇 |
国内免费 | 385篇 |
专业分类
测绘学 | 173篇 |
大气科学 | 91篇 |
地球物理 | 332篇 |
地质学 | 849篇 |
海洋学 | 205篇 |
天文学 | 65篇 |
综合类 | 59篇 |
自然地理 | 77篇 |
出版年
2024年 | 7篇 |
2023年 | 20篇 |
2022年 | 31篇 |
2021年 | 59篇 |
2020年 | 85篇 |
2019年 | 66篇 |
2018年 | 48篇 |
2017年 | 74篇 |
2016年 | 79篇 |
2015年 | 68篇 |
2014年 | 112篇 |
2013年 | 95篇 |
2012年 | 98篇 |
2011年 | 77篇 |
2010年 | 97篇 |
2009年 | 86篇 |
2008年 | 67篇 |
2007年 | 77篇 |
2006年 | 77篇 |
2005年 | 58篇 |
2004年 | 65篇 |
2003年 | 66篇 |
2002年 | 50篇 |
2001年 | 47篇 |
2000年 | 40篇 |
1999年 | 41篇 |
1998年 | 24篇 |
1997年 | 32篇 |
1996年 | 20篇 |
1995年 | 23篇 |
1994年 | 28篇 |
1993年 | 14篇 |
1992年 | 4篇 |
1991年 | 6篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1988年 | 3篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1983年 | 1篇 |
排序方式: 共有1851条查询结果,搜索用时 31 毫秒
121.
The frequency and extent of debris flows have increased tremendously due to the extreme weather and the Wenchuan earthquake on May 12, 2008. Previous studies focused on the debris flow from gullies damming the mountain streams. In this paper, an equation for the run-out distance of debris flow in the main river is proposed based on the dynamic equation of debris flow at different slopes given by Takahashi. By undertaking field investigations and flume experiments, a new calculation method of the volume of debris flow damming large river is obtained. Using the percolation theory and the renormalization group theory it was deduced that the large particles should comprise more than 50% for forming a stable debris flow dam. Hence, the criteria of damming large river by debris flow is presented in terms of run-out distance and grain composition which was then validated through the event of damming river by debris flow at Gaojia gully, the upper reaches of the Minjiang River, Sichuan, China, on July 3, 2011. 相似文献
122.
In isotope 137 Cs tracing studies, it is a premise to determine suitable 137 Cs reference inventory(CRI) plots and the CRI values. Owing to the heterogeneous spatial distribution of 137 Cs deposition in the ground and diverse, or even irregular, operations in sampling and testing procedures, CRI determination is usually faced with many difficulties and uncertainties. In addition, more difficulties occur in an investigation of a large-scale region because of time constraints and measurement cost limitations. In this study, traditional CRI acquiring methods were summarized first, and then a new complex scheme was established, involving seven core steps and coupling the model estimate and sample measurement. The above CRI determination methodology was implemented in the central-eastern Inner Mongolia Plateau. The case study results showed that the CRI in the dark chestnut soil sub-region, located in the east and south of Xing'an City, exhibited 2447 Bq·m–2; the CRI in the aeolian sandy soil sub-region, positioned in the south central Tongliao City and central Chifeng City, showed 2430 Bq·m–2; the CRI in the sandy chernozem soil sub-region, situated in the northwestern Chifeng City, presented 2384 Bq·m–2; and the CRI in the chestnut soil sub-region, in the southern Xilin Gol City, was 2368 Bq·m–2. The newly proposed CRI determination scheme was proved effective, and the determined CRI plots and CRI values were convincing. The methodology offered a framework for 137 Cs tracing studies in large-scale regions or long-distance transects. 相似文献
123.
利用1951~2012年新疆哈密市的观测资料和NCEP再分析资料,对造成哈密市大降水(12.0 mm)的大气环流特征进行合成分析。结果发现造成哈密站大降水的大气环流分为4种类型:横槽型、低涡型、低槽(ω)型和不稳定小槽型。其中出现暴雨以上降水(24 h降水量24.0 mm)的个例集中在低槽(ω)型;横槽型个例的降水量在13.0~22.0 mm之间;低涡型个例的降水量分布相对均匀,在15.0~20.0 mm之间;不稳定小槽型个例的降水量偏少,全部在18.0 mm以下。4种类型的大气环流特征主要在对流层中高层差异明显。横槽型中低纬度环流平直,其横槽的北部是西北气流,南亚高压是青藏高压西部型;低涡型的中低纬度,在孟加拉湾有低槽,低槽的北部有低涡存在,低涡西面的脊偏强,其南亚高压是青藏高压东部型;低槽(ω)型东部的脊偏强,呈西北东南向,南亚高压是伊朗高压型;不稳定小槽型的南亚高压为伊朗高原到青藏高原东部的带状,500 hPa位势高度场上,85°E以西是平直的偏西气流,新疆东部地区有一小的短波槽。 相似文献
124.
Using ANSYS-CFX, a general purpose fluid dynamics program, the vortex-induced vibration (VIV) of a variable cross-section cylinder is simulated under uniform current with high Reynolds numbers. Large eddy simulation (LES) is conducted for studying the fluid-structure interaction. The vortex shedding in the wake, the motion trajectories of a cylinder, the variation of drag and lift forces on the cylinder are analyzed. The results show that the vortices of variable cross-section cylinder are chaotic and are varying along the cylinder. In places where cross-sections are changing significantly, the vortices are more irregular. The motion trail of the cylinder is almost the same but irregular. The drag and lift coefficients of the cylinder are varying with the changes of diameters. 相似文献
125.
Dynamics of large-truncated mooring systems coupled with a catenary moored semi-submersible 总被引:2,自引:2,他引:0
With the floating structures pushing their activities to the ultra-deep water,model tests have presented a challenge due to the limitation of the existing wave basins.Therefore,the concept of truncated mooring system is implemented to replace the full depth mooring system in the model tests,which aims to have the same dynamic responses as the full depth system.The truncated mooring system plays such a significant role that extra attention should be paid to the mooring systems with large truncation factor.Three different types of large truncation factor mooring system are being employed in the simulations,including the homogenously truncated mooring system,non-homogenously truncated mooring system and simplified truncated mooring system.A catenary moored semi-submersible operating at 1000 m water depth is presented.In addition,truncated mooring systems are proposed at the truncated water depth of 200 m.In order to explore the applicability of these truncated mooring systems,numerical simulations of the platform’s surge free decay interacting with three different styles of truncated mooring systems are studied in calm water.Furthermore,the mooring-induced damping of the truncated mooring systems is simulated in the regular wave.Finally,the platform motion responses and mooring line dynamics are simulated in irregular wave.All these simulations are implemented by employing full time domain coupled dynamic analysis,and the results are compared with those of the full depth simulations in the same cases.The results show that the mooring-induced damping plays a significant role in platform motion responses,and all truncated mooring systems are suitable for model tests with appropriate truncated mooring line diameters.However,a large diameter is needed for simplified truncated mooring lines.The suggestions are given to the selection of truncated mooring system for different situations as well as to the truncated mooring design criteria. 相似文献
126.
在低精度MEMS-IMU和GPS组合导航中,由于IMU的精度问题,无法通过传统的解析方法实现方位失准角的粗对准,造成了大方位失准角问题,从而导致系统的强非线性。通过变换状态量,用方位失准角的两个三角函数代替方位失准角作为状态量,建立了新的线性系统方程。用改进奇异值分解法对新对准系统进行可观测度分析,完成了车载导航试验,结果表明:本初始对准方案在低精度的组合导航中具有很好的对准精度和对准速度。 相似文献
127.
128.
在系统收集山东已完成的1∶5万、1∶20万和1∶25万区域地质资料基础上,通过编制山东省1∶25万实际材料图和建造构造图、1∶50万大地构造相五要素工作底图和1∶50万大地构造相图,全面介绍了大地构造相的定义、研究内容、指导思想、基本原则与编图程序。采用大地构造相编图作为大陆动力学研究的主要方法,是揭示不同陆块之间相互作用、形成过程和演化历史、研究山东大陆块体离散、会聚、碰撞、造山的大陆动力学过程的主要载体和具体表达形式。进而对山东大地构造相进行了划分,共划出华北陆块区、秦祁昆造山系和中国东部陆缘弧盆系相系3大相系,鲁西陆块、渤海陆块、大别苏鲁结合带、威海弧盆系、胶辽岩浆弧和华北裂谷盆地6个大相以及20个相和59个亚相,明确了其鉴别标志。对陆块区、造山系和陆缘弧盆系相系以及大相、相的基本特征进行了描述;简要总结了大地构造相与成矿尤其是金矿的关系,探讨了不同矿床与大地构造相的成因联系。 相似文献
129.
Arnold J. H. Reesink Philip J. Ashworth Gregory H. Sambrook Smith James L. Best Daniel R. Parsons Mario L. Amsler Richard J. Hardy Stuart N. Lane Andrew P. Nicholas Oscar Orfeo Steven D. Sandbach Christopher J. Simpson Ricardo N. Szupiany 《Sedimentology》2014,61(4):1055-1085
To date, published studies of alluvial bar architecture in large rivers have been restricted mostly to case studies of individual bars and single locations. Relatively little is known about how the depositional processes and sedimentary architecture of kilometre‐scale bars vary within a multi‐kilometre reach or over several hundreds of kilometres downstream. This study presents Ground Penetrating Radar and core data from 11, kilometre‐scale bars from the Río Paraná, Argentina. The investigated bars are located between 30 km upstream and 540 km downstream of the Río Paraná – Río Paraguay confluence, where a significant volume of fine‐grained suspended sediment is introduced into the network. Bar‐scale cross‐stratified sets, with lengths and widths up to 600 m and thicknesses up to 12 m, enable the distinction of large river deposits from stacked deposits of smaller rivers, but are only present in half the surface area of the bars. Up to 90% of bar‐scale sets are found on top of finer‐grained ripple‐laminated bar‐trough deposits. Bar‐scale sets make up as much as 58% of the volume of the deposits in small, incipient mid‐channel bars, but this proportion decreases significantly with increasing age and size of the bars. Contrary to what might be expected, a significant proportion of the sedimentary structures found in the Río Paraná is similar in scale to those found in much smaller rivers. In other words, large river deposits are not always characterized by big structures that allow a simple interpretation of river scale. However, the large scale of the depositional units in big rivers causes small‐scale structures, such as ripple sets, to be grouped into thicker cosets, which indicate river scale even when no obvious large‐scale sets are present. The results also show that the composition of bars differs between the studied reaches upstream and downstream of the confluence with the Río Paraguay. Relative to other controls on downstream fining, the tributary input of fine‐grained suspended material from the Río Paraguay causes a marked change in the composition of the bar deposits. Compared to the upstream reaches, the sedimentary architecture of the downstream reaches in the top ca 5 m of mid‐channel bars shows: (i) an increase in the abundance and thickness (up to metre‐scale) of laterally extensive (hundreds of metres) fine‐grained layers; (ii) an increase in the percentage of deposits comprised of ripple sets (to >40% in the upper bar deposits); and (iii) an increase in bar‐trough deposits and a corresponding decrease in bar‐scale cross‐strata (<10%). The thalweg deposits of the Río Paraná are composed of dune sets, even directly downstream from the Río Paraguay where the upper channel deposits are dominantly fine‐grained. Thus, the change in sedimentary facies due to a tributary point‐source of fine‐grained sediment is primarily expressed in the composition of the upper bar deposits. 相似文献
130.
《Atmósfera》2014,27(4):353-365
This study attempts to understand why the frequency of tropical cyclones (TC) over the western North Pacific (WNP) was a record low during the 2010 season, by analyzing the effect of several large-scale factors. The genesis potential index (GPI) can represent, to some extent, the spatial distribution of formation in 2010. However, the GPI does not explain the extremely low TC frequency. No robust relationship between the TC number and El Niño Southern Oscillation (ENSO) was found. A comparison of the extreme inactive TC year 2010 and extreme active year 1994 was performed, based on the box difference index that can measure the quantitative difference of large-scale environmental factors. Dynamic factors were found to be important in differentiating TC formation over the WNP basin between 2010 and 1994. The remarkable difference of monsoon flows in the WNP basin between these two years may be the cause of the difference in TC formation. The unfavorable conditions for TC genesis in 2010 may have also been due to other large scale factors such as: (1) weak activity of the Madden-Julian Oscillation during the peak season; (2) warming of the sea surface temperature in the tropical Indian Ocean during the peak season, causing the development of an anticyclone over the WNP basin and associated with the westward motion of the monsoon trough, and (3) the phase change of the Pacific Decadal Oscillation (more negative) and the two strong La Niña events that have evolved since 2006. 相似文献