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   1235篇
  免费   311篇
  国内免费   960篇
测绘学   168篇
大气科学   49篇
地球物理   280篇
地质学   1774篇
海洋学   68篇
天文学   5篇
综合类   52篇
自然地理   110篇
  2024年   2篇
  2023年   16篇
  2022年   47篇
  2021年   75篇
  2020年   82篇
  2019年   90篇
  2018年   84篇
  2017年   51篇
  2016年   89篇
  2015年   92篇
  2014年   115篇
  2013年   90篇
  2012年   160篇
  2011年   173篇
  2010年   142篇
  2009年   173篇
  2008年   144篇
  2007年   137篇
  2006年   157篇
  2005年   105篇
  2004年   122篇
  2003年   66篇
  2002年   55篇
  2001年   36篇
  2000年   37篇
  1999年   32篇
  1998年   27篇
  1997年   17篇
  1996年   17篇
  1995年   12篇
  1994年   20篇
  1993年   10篇
  1992年   2篇
  1991年   2篇
  1990年   4篇
  1989年   2篇
  1988年   5篇
  1987年   7篇
  1986年   4篇
  1985年   3篇
  1978年   1篇
  1975年   1篇
排序方式: 共有2506条查询结果,搜索用时 31 毫秒
991.
万家寨引黄入晋工程是一项大型的国家重点调水工程 ,工程总设计造价 180亿元。目前 ,对于从万家寨到太原的一期工程而言 ,大多数工程已经阶段性竣工 ,其余工程也正在施工 ,预计 2 0 0 1年通水到太原。从它的超长距离、大流量、多隧洞、超长隧洞和具有大型地下泵站等特点来看 ,确系全国之最、举世罕见。在该工程的施工测量中 ,有许多施工测量工作是超规范作业 ,存在着大量的特殊问题需要进行研究。作者对这些问题进行了大量深入地研究 ,取得了一些有益的结论。这些结论已经被应用于该工程的施工测量中 ,并且它们的正确性大部分已被工程实践所验证  相似文献   
992.
针对某工程隧洞进口闸室段开挖跨度大,上覆岩体薄,围岩为V类的具体情况,本文以减小开挖断面跨度以及新奥法施工方法的基本原理为出发点,对该隧洞进口闸室段的两种开挖方案进行了分析研究。用有限元方法得到了洞室外围岩的位移场、应力场分布。比较这两种方案,最终得出了在围岩稳定以及支护结构稳定情况下较为理想的开挖与支护方案。   相似文献   
993.
为了诊断运营中隧道病害的程度,需要对隧道衬砌作出系统评价.本文结合工程实际,阐述了采用地质雷达对隧道衬砌进行检测的原理及方法,采取提高检测效果的方法和技术措施,并通过典型雷达图像分析了衬砌结构完整性和衬砌厚度识别判定特征,取得了较好的检测效果,为隧道整体健康诊断提供依据.  相似文献   
994.
针对龙溪隧道在施工过程中常出现软弱围岩变形现象,选取其左线LK22 083~LK22 153段为试验段:采用H20型钢施作初期支护.对锚杆轴力、围岩压力和钢拱架应变开展监控量测工作,验证H20型钢支护是否合理.在试验段范围内选取三个监测断面,对应里程桩号:LK22 084、LK22 100和LK22 125;每个断面在拱顶、两侧拱腰和边墙5个测点埋设监测仪器,每个测点处的锚杆计、土压力计和钢拱架应变计埋设在同一点,便于在数据分析时相互补充说明;通过实测数据分析,结合施工实践,综合表明H20型钢支护是合理的,确保了隧道施工顺利快速通过F8断层.  相似文献   
995.
???????????輼??????????????η????????????KNNS??ICP??????????η???????????????????????????????????????????????????????????????????????????????????Σ????ó????????????λ????????????????????KNNS??ICP???????????????????????????????????????????????ж???????????????????????????????????????????????????????????????????????仯????????????????η?????????÷??????????????????????????????÷???????Ч???  相似文献   
996.
????и?????????????η????????????????????????С?????????????????????????????????????????????“С??????????”???????????????????????????????????????????????????GM(1,1)??????????????г??????η????????????????????????????????????????????????Ч???????????????е???????????????????????????????????????????????????GM(1,1)???????????????????GM(1,1)??????????????????????η?????????????????????????  相似文献   
997.
Application of TSD tunnel seismic prediction system in karst area   总被引:1,自引:0,他引:1  
TSD is one of the classical methods of tunnel seismic prediction based on higher accuracy multi-wave multi-component seismology. The working principle of the TSD and an application example of the TSD on tunnel prediction in Chongqing are introduced in this paper. This system has two ports for speed signal and acceleration signal, and the equipment is more portable and easy to use. According to the application results we can conclude that the TSD prediction system is accurate and it has the wide application prospect in tunnel seismic detection.  相似文献   
998.
This paper presents an analytical‐numerical approach to obtain the distribution of stresses and deformations around a reinforced tunnel. The increase in the radial stress of the reinforced tunnel, based on the performance of a bolt, is modeled by a function, which its maximum value is in the vicinity of the bolt periphery and it exponentially decreases in the far distance from the bolt. On the basis of this approach, the shear stiffness between the bolt and the rock mass and the shear stress distribution around the bolt within the rock mass are also analytically obtained. The results are compared with those obtained by the assumption of ‘uniform increase of radial stress’ method, which is made by the previous studies. The analyses show when the bolts' spacing is large, the safety factor must be increased if the ‘uniform increase of radial stress’ method is used for the design. Finally, a procedure is introduced to calculate the non‐equal deformation of the rock mass between the bolts at any radius that can be useful to compute the bending moment in shotcrete layer in New Austrian Tunnelling Method (NATM) approach. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   
999.
The Palaeo‐Mesoproterozoic metapelite granulites from northern Garo Hills, western Shillong‐Meghalaya Gneissic Complex (SMGC), northeast India, consist of resorbed garnet, cordierite and K‐feldspar porphyroblasts in a matrix comprising shape‐preferred aggregates of biotite±sillimanite+quartz that define the penetrative gneissic fabric. An earlier assemblage including biotite and sillimanite occurs as inclusions within the garnet and cordierite porphyroblasts. Staurolite within cordierite in samples without matrix sillimanite is interpreted to have formed by a reaction between the sillimanite inclusion and the host cordierite during retrogression. Accessory monazite occurs as inclusions within garnet as well as in the matrix, whereas accessory xenotime occurs only in the matrix. The monazite inclusions in garnet contain higher Ca, and lower Y and Th/U than the matrix monazite outside resorbed garnet rims. On the other hand, matrix monazite away from garnet contains low Ca and Y, and shows very high Th/U ratios. The low Th/U ratios (<10) of the Y‐poor garnet‐hosted monazite indicate subsolidus formation during an early stage of prograde metamorphism. A calculated P–T pseudosection in the MnCKFMASH‐PYCe system indicates that the garnet‐hosted monazite formed at <3 kbar/600 °C (Stage A). These P–T estimates extend backward the previously inferred prograde P–T path from peak anatectic conditions of 7–8 kbar/850 °C based on major mineral equilibria. Furthermore, the calculated P–T pseudosections indicate that cordierite–staurolite equilibrated at ~5.5 kbar/630 °C during retrograde metamorphism. Thus, the P–T path was counterclockwise. The Y‐rich matrix monazite outside garnet rims formed between ~3.2 kbar/650 °C and ~5 kbar/775 °C (Stage B) during prograde metamorphism. If the effect of bulk composition change due to open system behaviour during anatexis is considered, the P–T conditions may be lower for Stage A (<2 kbar/525 °C) and Stage B (~3 kbar/600 °C to ~3.5 kbar/660 °C). Prograde garnet growth occurred over the entire temperature range (550–850 °C), and Stage‐B monazite was perhaps initially entrapped in garnet. During post‐peak cooling, the Stage‐B monazite grains were released in the matrix by garnet dissolution. Furthermore, new matrix monazite (low Y and very high Th/U ≤80, ~8 kbar/850–800 °C, Stage C), some monazite outside garnet rims (high Y and intermediate Th/U ≤30, ~8 kbar/800–785 °C, Stage D), and matrix xenotime (<785 °C) formed through post‐peak crystallization of melt. Regardless of textural setting, all monazite populations show identical chemical ages (1630–1578 Ma, ±43 Ma). The lithological association (metapelite and mafic granulites), and metamorphic age and P–T path of the northern Garo Hills metapelites and those from the southern domain of the Central Indian Tectonic Zone (CITZ) are similar. The SMGC was initially aligned with the southern parts of CITZ and Chotanagpur Gneissic Complex of central/eastern India in an ENE direction, but was displaced ~350 km northward by sinistral movement along the north‐trending Eastern Indian Tectonic Zone in Neoproterozoic. The southern CITZ metapelites supposedly originated in a back‐arc associated with subducting oceanic lithosphere below the Southern Indian Block at c. 1.6 Ga during the initial stage of Indian shield assembly. It is inferred that the SMGC metapelites may also have originated contemporaneously with the southern CITZ metapelites in a similar back‐arc setting.  相似文献   
1000.
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