首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   27881篇
  免费   26965篇
  国内免费   26394篇
测绘学   3310篇
大气科学   17514篇
地球物理   10407篇
地质学   29798篇
海洋学   13432篇
天文学   704篇
综合类   3773篇
自然地理   2302篇
  2024年   451篇
  2023年   510篇
  2022年   732篇
  2021年   978篇
  2020年   1868篇
  2019年   4586篇
  2018年   4665篇
  2017年   4421篇
  2016年   4453篇
  2015年   4108篇
  2014年   3588篇
  2013年   4086篇
  2012年   3960篇
  2011年   3740篇
  2010年   3730篇
  2009年   3181篇
  2008年   2639篇
  2007年   2673篇
  2006年   2356篇
  2005年   2186篇
  2004年   2313篇
  2003年   2083篇
  2002年   1899篇
  2001年   1669篇
  2000年   1374篇
  1999年   1338篇
  1998年   1360篇
  1997年   1427篇
  1996年   1116篇
  1995年   1077篇
  1994年   938篇
  1993年   918篇
  1992年   785篇
  1991年   573篇
  1990年   568篇
  1989年   472篇
  1988年   394篇
  1987年   301篇
  1986年   240篇
  1985年   209篇
  1984年   224篇
  1983年   140篇
  1982年   159篇
  1981年   113篇
  1980年   82篇
  1979年   84篇
  1978年   39篇
  1977年   34篇
  1971年   40篇
  1970年   39篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
To meet the increasing demand for air transportation, Ministry of Land, Infrastructure and Transport had carried out the offshore extension project at Tokyo International Airport (Haneda Airport). The airport should be constructed to be perfectly level, but the ground conditions with very thick layers of super soft dredged soils and need for reclamation made the task very difficult. Because of large amount of consolidation settlement, ground improvement by combined vertical drain method was applied to decrease residual and differential settlement after opening of new airport. This paper is aimed to introduce the design concept and method of ground improvement for this project. In addition the result of ground improvement is keeping satisfactory performance for operating airport confirmed by long-term monitoring for airport facilities.  相似文献   
102.
吴涛  肖龙  马昌前  黄婉 《岩石学报》2013,29(10):3567-3580
同普杂岩体位于藏东江达地区。本文对该杂岩体岩相学、年代学、主微量元素以及Sr-Nd同位素进行了详细的研究。结果表明,该杂岩体主要由石英闪长岩、花岗闪长岩、含黑云母花岗岩和斑状花岗岩4种岩石类型组成,前三者形成时代分别为262.8±1.5Ma、263.9±1.9Ma、263.7±1.6Ma。其中,含黑云母花岗岩与斑状花岗岩呈渐变过渡关系,它们和花岗闪长岩一起均具有高SiO2(65.1%~76.6%)、高K2O+Na2O(4.84%~8.13%),低MgO(0.25%~2.28%),低FeOT(0.99%~4.44%),高A/CNK值(除一件样品外两者A/CNK值均大于1.1),以及Ba、Sr、Eu亏损等,均符合强过铝质S型花岗岩的特征。但前两者与后者CaO/Na2O、Rb/Ba、Rb/Sr值均存在明显差异,哈克图解上也不存在线性关系,表明它们源区存在差异。石英闪长岩则具有相对低的SiO2(54.2%~55.4%)、低K2O+Na2O(3.15%~4.72%),高MgO(3.78%~4.79%),高FeOT(6.13%~8.09%)等特征。其还具有明显的Nb负异常,不具有Sr的负异常,以及轻稀土相对富集等,均符合岛弧岩浆岩的特征。Sr-Nd同位素特征显示,斑状花岗岩与花岗闪长岩具有相似的初始Sr比值(分别为0.7099,0.7125)和εNd(t)值(分别为-8.6,-10.3),而石英闪长岩则具有较低的初始Sr比值(0.7062),较高的εNd(t)值(1.37)。前两者均落入研究区S型花岗岩与高硅流纹岩区域,后者落在金沙江MORB的下方。通过综合分析,本文认为同普杂岩体形成于火山弧环境,且各岩石类型具有不同的源区:含黑云母花岗岩、斑状花岗岩与花岗闪长岩的源区分别为古老的泥质变质岩和贫泥质的变质砂岩;石英闪长岩则来自富集地幔。表明在晚二叠世早期(~263Ma)金沙江洋盆仍处于俯冲阶段,江达地区主要受金沙江洋盆俯冲消减体系控制而非地幔柱体系。  相似文献   
103.
胶州--莱阳盆地白垩纪莱阳群中的震积岩及构造意义讨论   总被引:14,自引:2,他引:14  
殷秀兰  杨天南 《地质论评》2005,51(5):502-506
通过对胶州-莱阳盆地沉积层的野外考察和岩心观察,系统分析了莱阳群的沉积特征及其变化规律,鉴别出一些较典型的震积岩类型;较详细描述了其中的粉砂质液化脉、震裂液化角砾岩、震褶岩和砂枕构造等地震液化特征。最后从盆地的区域地质背景出发,初步认为白垩纪早期胶州-莱阳盆地的形成与其南部苏鲁造山带的后期隆升同时,造成研究区总体处于伸展-裂解的构造环境之中。强烈的同沉积构造活动伴随频繁的地震发生,致使盆地内早期沉积的莱阳群以普遍发育震积岩为特征。  相似文献   
104.
瑶琳仙境洞穴动物的研究   总被引:6,自引:2,他引:6  
1980年11月至1986年5月,在瑶琳洞内各种生境作了观察、采集,并对微形动物进行了实验室培养。已经鉴定的洞穴动物有35种,隶属于28科、32属。从蜘蛛种类同五个不同面积溶洞的关系看,相关系数(r)为0.9707,两者为正相关。并分析了洞穴动物的营养源和食物链,在此基础上,还阐述了洞穴动物的保护利用问题。   相似文献   
105.
载炭泡塑吸附法对金有良好的吸附性能,但只能用于抽滤吸附不能振荡吸附,分析手续繁杂。本文以载炭泡塑振荡吸附-电感耦合等离子体发射光谱法测定金矿石的金量。样品在650℃高温灼烧2 h,用50%王水和10%氯化铁加热溶解,溶液冷却后加入5%高锰酸钾氧化,用中密度规格的载炭泡塑两次振荡吸附溶液中的金,然后于580℃高温灼烧后以50%王水溶解灰分,直接用ICP-OES测定金量。方法检出限(3σ)为0.002μg/g,精密度(RSD,n=11)小于3.7%。本方法对金的吸附率大于99.9%,测定范围为0.01~90μg/g,对不同类型金矿石的适应性强,解决了以往泡塑吸附法吸附率较低、标准系列与样品需同时预处理的问题,对低含量和高含量样品均有较高的准确度。  相似文献   
106.
It is becoming easier to combine geographical data and dynamic models to provide information for problem solving and geographical cognition. However, the scale dependencies of the data, model, and process can confuse the results. This study extends traditional scale research in static geographical patterns to dynamic processes and focuses on the combined scale effect of multiscale geographical data and dynamic models. The capacity for topographical expression under the combined scale effect was investigated by taking multiscale topographical data and meteorological processes in Hong Kong as a case study. A meteorological simulation of the combined scale effect was evaluated against data from Hong Kong Observatory stations. The experiments showed that (1) a digital elevation model (DEM) using 3 arc sec data with a 1 km resolution Weather Research and Forecasting (WRF) model gives better topographical expression and meteorological reproduction in Hong Kong; (2) a fine-scale model is sensitive to the resolution of the DEM data, whereas a coarse-scale model is less sensitive to it; (3) better topographical expression alone does not improve weather process simulation; and (4) uncertainty arising from a scale mismatch between the DEM data and the dynamic model may account for 38 % of the variance in certain meteorological variables (e.g., temperature). This case study gives a clear explanation of the significance and implementation of scale matching for multiscale geographical data and dynamic models.  相似文献   
107.
Movement of seasonal eddies in the Bay of Bengal (BOB) and its relation with cyclonic heat potential (CHP) and cyclogenesis points have been investigated in this study using 6 years (2002–2007) of global ocean monthly analysis datasets based on the Simple Ocean Data Assimilation (SODA) package (SODA v2.0.4) of Carton et al. (2005) and Indian Meteorological Department cyclogenesis points. The region dominated by anticyclonic eddies with CHP greater than 70 × 107 J/m2 as well as good correlations (>0.9) with sea surface height (SSH) and 26°C isothermal depth (D 26) can be a potential region of cyclogenesis. The region dominated by cyclonic eddies with CHP greater than 50 × 107 J/m2 and good correlation (>0.9) with both SSH and D 26 can serve as a potential region of high-level depression. Potential cyclogenesis regions are the southern BOB (5°N–12°N) for the post-monsoon season and the head of BOB (north of 15°N) during southwest monsoon. Seven potential regions are identified for the eddy formation for different seasons, which are consistent with the cyclogenesis points. The CHP distributions alone are able to explain the cyclone tracks for the pre-monsoon and post-monsoon seasons but not for the monsoon season.  相似文献   
108.
Frozen ground is significantly stiffer than unfrozen ground. For bridges supported on deep foundations, bridge stiffness is also measurably higher in winter months. Significant changes due to seasonal freezing in bridge pier boundary conditions require additional detailing in order to ensure a ductile performance of the bridge during a design earthquake event. This paper reports the latest results obtained from a project that systematically investigated the effects of seasonally frozen soil on the seismic behavior of highway bridges in cold regions. A bridge was chosen and was monitored to study its seismic performance and assess the impact of seasonally frozen soil on its dynamic properties. A Finite Element (FE) model was created for this bridge to analyze the impact of seasonal frost. It was found that when frost depth reaches 1.2 m, the first transverse modal frequency increases about 200% when compared with the no-frost case. The results show that seasonal frost has a significant impact on the overall dynamic behavior of bridges supported by pile foundations in cold regions, and that these effects should be accounted for in seismic design.  相似文献   
109.
分析了TDRS卫星的轨道特性及传统的地基测距跟踪技术定轨精度不高的现状,研究了基于空基的用户星精密轨道的TDRS卫星定轨,解决了基于空基的一般GEO卫星定轨问题。  相似文献   
110.
Fractal modelling has been applied extensively as a means of characterizing the spatial distribution of geological phenomena that display self-similarity at differing scales of measurement. A fractal distribution exists where the number of objects exhibiting values larger than a specified magnitude displays a power-law dependence on that magnitude, and where this relationship is scale-invariant. This paper shows that a number of distributions, including power-function, Pareto, log-normal and Zipf, display fractal properties under certain conditions and that this may be used as the mathematical basis for developing fractal models for data exhibiting such distributions. Population limits, derived from fractal modelling using a summation method, are compared with those derived from more conventional probability plot modelling of stream sediment geochemical data from north-eastern New South Wales. Despite some degree of subjectivity in determining the number of populations to use in the models, both the fractal and probability plot modelling have assisted in isolating anomalous observations in the geochemical data related to the occurrence of mineralisation or lithological differences between sub-catchments. Thresholds for the main background populations determined by the fractal model are similar to those established using probability plot modelling, however the summation method displays less capacity to separate out anomalous populations, especially where such populations display extensive overlap. This suggests, in the geochemical data example provided, that subtle differences in the population parameters may not significantly alter the fractal dimension.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号