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   1284篇
  免费   226篇
  国内免费   297篇
测绘学   83篇
大气科学   254篇
地球物理   458篇
地质学   511篇
海洋学   165篇
天文学   22篇
综合类   67篇
自然地理   247篇
  2024年   8篇
  2023年   28篇
  2022年   44篇
  2021年   54篇
  2020年   70篇
  2019年   75篇
  2018年   71篇
  2017年   74篇
  2016年   80篇
  2015年   74篇
  2014年   105篇
  2013年   115篇
  2012年   77篇
  2011年   77篇
  2010年   60篇
  2009年   93篇
  2008年   73篇
  2007年   78篇
  2006年   79篇
  2005年   60篇
  2004年   63篇
  2003年   45篇
  2002年   33篇
  2001年   32篇
  2000年   32篇
  1999年   23篇
  1998年   30篇
  1997年   35篇
  1996年   24篇
  1995年   21篇
  1994年   17篇
  1993年   21篇
  1992年   7篇
  1991年   12篇
  1990年   2篇
  1989年   5篇
  1988年   1篇
  1987年   2篇
  1986年   3篇
  1985年   2篇
  1984年   1篇
  1979年   1篇
排序方式: 共有1807条查询结果,搜索用时 15 毫秒
991.
The building losses in Adapazari following the 17 August 1999 Kocaeli earthquake are estimated for both ground-shaking and ground-failure induced damage and the predictions compared with field observations. The ground-shaking damage is estimated using the capacity spectrum approach and accommodating the results of a previously published calibration of this model. These results, which do not incorporate ground-failure damage, are compared to the observed damage patterns in Adapazari to illustrate the significance of not including the more complex ground failure component in a loss model for a region of high liquefaction susceptibility. A preliminary estimation of ground-failure-induced damage is then presented based upon the HAZUS (FEMA, 2003) default methodology. The benefits of these additional calculations to the overall loss model are assessed to provide some quantitative decision-making guidance for those producing loss estimation models. The findings suggest that the benefits of specifically incorporating ground failure into a loss model only start to be obtained if a detailed approach using in situ geotechnical data as well as adequate representation of building foundations is adopted. Otherwise, the additional input data required is not commensurate with the small potential refinement in the estimated losses, particularly considering the compounded uncertainties associated with the simplified approach.  相似文献   
992.
The porosity and hence volume of argillaceous sediments is determined by: (1) the magnitude of the effective stress acting within the sediment; (2) the previous stress history of the sediment; and (3) at shallow depths of burial, by features such as the mineralogy and the nature of the depositional environment. Stress paths and the critical state diagrams for a number of clays are used to investigate the range of porosities possible in argillaceous sediments as the effective stresses increase. It is found that all porosity/effective stress curves converge at large stresses. The change in porosity is strongly dependent on the mean effective stress but largely independent of the deviatoric stress, and thus is largely independent of the nature of the stress field acting on the basin (compressional, extensional etc.). Because of the dependence of porosity on the mean effective stress, no simple relationship exists between porosity and depth of burial but in the absence of overpressured pore fluids and assuming the sediment is not overconsolidated, it is possible to contour the porosity/effective stress diagram in terms of burial depths. These data should assist in recalculating stratigraphic thicknesses for basin reconstruction and stratigraphic correlation studies.  相似文献   
993.
以某海洋钻井公司购置自升式深水钻井平台为例,建立了海洋平台购置项目经济风险评估的综合概率模型。考虑未来的平台购置价格、作业租金、年营运天数、年维修费、日耗油量、还贷资金利率等多种因素的影响,对项目投资的经济评价指标进行了风险计算。通过主要参数的灵敏度分析,使得决策指标更趋于客观实际。  相似文献   
994.
995.
从患病的杜氏(鱼师)肾脏分离出Y-1菌.人工感染试验,当注射9.0×107CFU/尾时,Y-1菌可致试验鱼100%死亡;注射9.0×106CFU/尾时,死亡率为50%,症状与自然发病相似,结果表明该分离菌为致病菌.Y-1菌为革兰氏阴性,有动力,杆状,极生单鞭毛;发酵葡萄糖,还原硝酸盐,氧化酶、触酶阳性,赖氨酸脱羧酶、鸟氨酸脱羧酶、精氨酸双水解酶阴性.根据形态及生理生化特性,Y-1菌应为弧菌Vibriosp..药敏试验结果表明,该菌对氨基糖甙类、呋喃类、喹诺酮类及大环内脂类药物敏感.  相似文献   
996.
视网膜视蛋白组成是动物识别光强与颜色视觉的基础,其会随着不同生长环境和发育阶段而发生变化。大菱鲆(Scophthalmus maximus)为重要的海水养殖鱼种,在发育过程中经历了从浮游生活到底栖生活的转变;生活习性的转变伴随着生存环境的变化,尤其是光照环境的变化。我们推测大菱鲆光照感知功能包括视蛋白种类及表达模式等,在不同阶段应有所不同,目前未见相关研究报道。本研究以1、4和9月龄大菱鲆为研究对象,通过qRT-PCR探查了不同阶段大菱鲆视蛋白基因表达的特征规律,通过分析三个发育阶段的视蛋白基因表达比例,推测鱼体光谱敏感性变化规律。结果表明,红视蛋白(LWS)基因表达量随鱼体发育逐渐降低,视紫红质(RH1)、紫外视蛋白(SWS1)和蓝视蛋白(SWS2)随鱼体发育表达量逐渐升高,而绿视蛋白(RH2)中高表达的RH2b1无显著变化。LWS表达比例由1月龄57.2%降至九月龄11.4%,RH2b1表达比例由1月龄29.9%升至九月龄73.3%。随着大菱鲆由早期浮游生活转入底栖生活,表达占主导地位的视蛋白,逐渐由LWS转变为RH2b1,其光谱敏感性可能由红色敏感向绿色敏感转变。大菱鲆视蛋白在不同发育阶段表达的特征变化引发的光谱敏感性变化,可认为是其对不同光环境的适应策略。  相似文献   
997.
Upper Ocean Sensitivity to Wind Forcing in the South China Sea   总被引:2,自引:0,他引:2  
The Naval Research Laboratory (NRL) Layered Ocean Model (NLOM) has been used to investigate the sensitivity of the upper South China Sea (SCS) circulation to various atmospheric wind forcing products. A 1/16° 6-layer, thermodynamic Pacific Ocean north of 20°S version of NLOM has been integrated using observed climatological monthly mean winds (Hellerman and Rosenstein, 1983) and climatologies based on two atmospheric prediction models: the European Centre for Medium-Range Weather Forecasts (ECMWF) and the National Centers for Environmental Prediction (NCEP). ECMWF products include the 10 meter winds (at both 1.125° and 2.5° resolution) and surface stresses (1.125°). The NCEP forcing (1.875°) is a surface stress product. Significant differences exist in the wind stress curl patterns and this is reflected in the upper ocean model response, which is compared to observational data. The model experiments suggest the generation of the West Luzon Eddy is controlled by positive wind stress curl. The degree of Kuroshio intrusion into the SCS, however, is not affected by wind stress curl but is governed by the coastline geometry of the island chain within Luzon Strait. The summertime offshore flow from the Vietnamese coast is present in all simulations but the dipole structure on either side of the jet is variable, even among experiments with similar wind stress curl patterns. The ECMWF surface stresses exhibit spurious coastal wind stress curl patterns, especially in locations with significant orographic features. This manifests itself in unrealistic small scale coastal gyres in NLOM. High resolution basin-scale and coastal models might be adversely affected by these stresses. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
998.
999.
生态敏感性分析是城市规划编制研究的重要依据,运用GIS空间分析技术可以有效地实现生态敏感性定量分析。文中以环滆湖地区为研究区,选择海拔、水域、农田、水源保护区、生态保护区和建成区六项有区域代表性的生态因子,借助GIS空间缓冲区分析、叠加分析等功能,采用因子叠加取最值法得到规划研究区的总体生态敏感性区划,对研究区生态敏感性进行定量化分析与评价,为环滆湖地区的生态环境保护和空间管制规划策略制定与用地空间合理布局提供科学依据、方法支撑和决策支持。  相似文献   
1000.
石漠化敏感性指的是区域在自然状况下发生石漠化现象的可能性大小,开展石漠化敏感性评价对区域生态环境的建设和可持续发展具有重要意义。本文以OLI影像为数据源,选取植被覆盖率、裸岩率和坡度作为评价指标,以地理信息技术为支撑开展禄劝县石漠化敏感性评价。评价结果显示:禄劝县轻度敏感面积为2 091 km~2,占总面积的57.492%;中度敏感面积为1 470 km~2,占总面积的40.418%;重度敏感面积为75.46 km~2,占总面积的2.075%;极度敏感面积为0.533 km~2,占总面积的0.015%。从空间分布上看,轻度敏感区主要分布于中西部地区;中度敏感集中分布于北部及南部地区;重度敏感主要分布于北部金沙江流域、东南部普渡河流域和云龙水库内流河沿线区域;极度敏感区主要分布在普渡河下游地区。总体而言,禄劝县石漠化敏感性相对较高,在区域开发与保护过程中应引起高度重视。  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