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   9231篇
  免费   2003篇
  国内免费   1204篇
测绘学   433篇
大气科学   688篇
地球物理   5652篇
地质学   3333篇
海洋学   1002篇
天文学   16篇
综合类   320篇
自然地理   994篇
  2024年   23篇
  2023年   115篇
  2022年   258篇
  2021年   304篇
  2020年   366篇
  2019年   420篇
  2018年   360篇
  2017年   413篇
  2016年   444篇
  2015年   418篇
  2014年   688篇
  2013年   600篇
  2012年   498篇
  2011年   566篇
  2010年   545篇
  2009年   623篇
  2008年   633篇
  2007年   603篇
  2006年   606篇
  2005年   505篇
  2004年   405篇
  2003年   400篇
  2002年   401篇
  2001年   269篇
  2000年   266篇
  1999年   223篇
  1998年   233篇
  1997年   199篇
  1996年   227篇
  1995年   202篇
  1994年   149篇
  1993年   140篇
  1992年   95篇
  1991年   57篇
  1990年   44篇
  1989年   34篇
  1988年   24篇
  1987年   13篇
  1986年   12篇
  1985年   7篇
  1984年   15篇
  1983年   3篇
  1981年   1篇
  1980年   4篇
  1979年   4篇
  1978年   8篇
  1977年   9篇
  1954年   6篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
62.
63.
The seismic expression of a salt-filled channel which cuts across the Mid North Sea High in Quadrant 37 is described, with features interpreted as being produced by salt-edge dissolution forming both eastern and western margins of the channel. The apparent half-graben nature of the channel is shown to be only superficial, and due to complex faulting associated with, but not defining, its western margin. The shallower faulting here is a Mesozoic to early Tertiary growth fault related to local dissolution of Zechstein salt. The dissolution effect appears in turn to have been localized by the presence of a deeper fault that was already downthrown to the east in Zechstein times, when it seems to have limited the eastward progradation of Zechstein shelf carbonates and anhydrites, and had probably ceased to move significantly before the onset of the Late Cimmerian erosional phase. The origin of this arcuate fault is tentatively ascribed to subsidence around a granite batholith. Zechstein salt originally spread some distance to the east and west of the channel; it was dissolved from the edges inwards, mainly before the Late Cretaceous, giving rise to a thicker Mesozoic sequence on parts of the flanks of the channel than in the middle. Besides providing an interesting structural case history, the features described have implications regarding Zechstein sedimentation, reservoir potential, the tectonic history of the North Sea, and the nature of the Mid North Sea High itself.  相似文献   
64.
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   
65.
基于不完备模态信息的海洋平台损伤诊断研究   总被引:2,自引:0,他引:2  
首先利用测量自由度正则化损伤指标向量得到损伤的大致区域;其次,在选定的区域内分别采用序列二次规划算法和最小二乘法来诊断损伤构件及其损伤程度,并对其结果进行了比较。数值模拟表明,仅需少量低阶模态和部分测量自由度,上述损伤诊断算法便可完成海洋平台的损伤诊断。  相似文献   
66.
鲫鱼(Carassius auratus)在微冻保鲜过程中的质量变化   总被引:2,自引:0,他引:2  
该文研究鲫鱼 ( Carassius auratus)在微冻 ( - 3℃ )保鲜过程中的质量变化规律。以感官评价、细菌总数、T- VBN值、p H值、ATPase活性、K值等作为质量指标。实验结果表明 ,微冻可以明显抑制细菌总数的增长 ,维持较低的 T- VBN和 K值。但是 ,微冻条件下鲫鱼的 ATPase活性下降速度较快  相似文献   
67.
提出将基于动态特性的检测方法与局部物理检测手段相结合的探伤思路。利用动力学方法进行实时监测及损伤区域的粗略定位,再由物理探测方法实现损伤程度和位置的精确判断,从而降低对动态检测方法的精度要求。此外,文中尝试直接由结构的动力响应信号构建能量指标识别结构损伤的方法,不需要进行模态参数识别,算法简单,有望应用于海洋平台、高层建筑等可简化为串联多自由度体系的结构的实时监测和损伤初步定位。  相似文献   
68.
69.
Detailed structural interpretation of the recently acquired deep seismic multichannel profiles along the Iberian Atlantic Margins (IAM Project) provides new results on the geodynamic evolution of the eastern part of the Azores-Gibraltar plate boundary. Thrusting and folding of the oceanic basement and of Mesozoic and Cenozoic sedimentary cover of the Gorringe Bank region are consistent with the N–S convergence of Iberia and Africa. Compressive structures in the Gorringe Bank region are spread over a wide area. Deformation under compression took place mainly in Tertiary times, as is evidenced by a basal unconformity and several discontinuities in Tertiary sediments, although some deformation has also been recorded in Quaternary sediments. The compressive structures in the Gulf of Cadiz are E–W oriented thrusts, folds and related diapiric structures. N–S oriented transpressive deformation is likely to occur in the western Portuguese platform. There is no continuity of structures from the oceanic to the continental domain, suggesting that deformation transfers from one side to the other through a transcurrent fault zone. The fault contact between the two domains is located in the ocean-continent transition zone.  相似文献   
70.
We describe industry funded contributions to the assessment of the SNA1 snapper (Pagrus auratus) fishery during the 1990s and discuss the advantages and disadvantages of industry funded research. We also provide background on the history of fisheries management in New Zealand, on the current assessment and management processes, and on the SNA1 snapper fishery. In the SNA1 fishery, the contributions of industry and the cooperation with Government scientists has resulted in high quality assessments. In our opinion, the advantages of industry funded research out weigh the disadvantages and suggest that industry funded assessments are highly desirable and should be an integral part of any management system.  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