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   346篇
  免费   20篇
  国内免费   9篇
测绘学   1篇
大气科学   6篇
地球物理   100篇
地质学   96篇
海洋学   95篇
综合类   2篇
自然地理   75篇
  2023年   2篇
  2022年   1篇
  2021年   2篇
  2020年   3篇
  2019年   3篇
  2018年   5篇
  2017年   9篇
  2016年   9篇
  2015年   5篇
  2014年   10篇
  2013年   28篇
  2012年   18篇
  2011年   15篇
  2010年   16篇
  2009年   23篇
  2008年   27篇
  2007年   15篇
  2006年   31篇
  2005年   13篇
  2004年   22篇
  2003年   13篇
  2002年   11篇
  2001年   3篇
  2000年   12篇
  1999年   8篇
  1998年   8篇
  1997年   6篇
  1996年   5篇
  1995年   3篇
  1994年   2篇
  1993年   5篇
  1992年   7篇
  1991年   2篇
  1990年   2篇
  1989年   3篇
  1988年   4篇
  1987年   3篇
  1986年   4篇
  1985年   6篇
  1984年   2篇
  1983年   1篇
  1982年   2篇
  1981年   3篇
  1980年   1篇
  1979年   1篇
  1977年   1篇
排序方式: 共有375条查询结果,搜索用时 15 毫秒
41.
Research into learning, innovation, and clustering suggests that local socioeconomic practices, or conventions, are critical to economic growth and regional development. Yet when it comes to the wine industry, explanations of economic success often focus on natural advantages. This article seeks to address this issue, presenting findings on the local practices of wine producers in northern California. The argument is that there are important local social and industrial linkages that bolster the competitiveness of California's wine industry and serve to upgrade production practices. This article highlights the career histories and perceptions of several key informants who are offered as exemplars of more commonly shared industrial experience and practice.  相似文献   
42.
Late Quaternary slip across the Cañada David detachment has produced an extensive array of Quaternary scarps cutting alluvial-fans along nearly the entire length (~ 60 km) of the range-bounding detachment. Eight regional alluvial-fan surfaces (Q1 [youngest] to Q8 [oldest]) are defined and mapped along the entire Sierra el Mayor range-front. Terrestrial cosmogenic nuclide 10Be concentrations from individual boulders on alluvial-fan surfaces Q4 and Q7 yield surface exposure ages of 15.5 ± 2.2 ka and 204 ± 11 ka, respectively. Formation of the fans is probably tectonic, but their evolution is strongly moderated by climate, with surfaces developing as the hydrological conditions have changed in response to climate change on Milankovitch timescales. Systematic mapping reveals that the fault scarp array along active range-bounding faults in Sierras Cucapa and El Mayor can be divided into individual rupture zones, based on cross-cutting relationships with alluvial-fans. Quantitative morphological ages of the Laguna Salada fault-scarps, derived from linear diffusive degradation modeling, are consistent with the age of the scarps based on cross-cutting relationships. The weighted means of the maximum mass diffusivity constant for all scarps with offsets < 4 m is 0.051 and 0.066 m2/ka for the infinite and finite-slope solutions of the diffusion equation, respectively. This estimate is approximately an order of magnitude smaller than the lowest diffusivity constants documented in other regions and it probably reflects the extreme aridity and other microclimatic conditions that characterize the eastern margin of Laguna Salada.  相似文献   
43.
尹松  孔令伟  杨爱武  穆坤 《岩土力学》2016,37(Z2):287-293
为了研究花岗岩残积土的路用工程特性,通过击实试验、承载比CBR试验、固结试验以及室内基床系数试验分析了该类材料压实性能及基本力学特性,对压实度为92%的最优含水率和饱和压实土样进行了循环加载试验,研究了动力荷载作用下土体的变形特性。结果表明,花岗岩残积土在K为91%~97%时压实功效率较高,提高压实度对于增强土体局部抗变形能力较为有效;采用室内三轴法得到的基床系数K30值为188.25 MPa/m;最优含水率下花岗岩残积土动力变形稳定性较好,但含水率增加会大幅度增加土体塑性变形,降低土的动弹性模量,不利于变形稳定。所以作为路堤填料,应考虑作为受气候与动荷载影响较小的下路堤备用填料,作为铁路路堤本体及公路上、下路路床填料,应在进行土性改良且满足要求的论证基础上取舍。研究成果可为花岗岩残积土填料的工程应用及土体改良提供技术参考。  相似文献   
44.
《Urban geography》2013,34(4):301-322
Homelessness remains highly stigmatized in U.S. society, with homeless persons seen by the public as mentally disabled, criminal, and socially deviant. This paper focuses on homeless women with children and argues that, in addition to the material deprivation of poverty, the sociospatial stigmatization of homeless women constitutes a primary obstacle to their reentry into mainstream, domiciled society. How does sociospatial stigma constrain homeless women with children as they navigate the transition between homeless and homed? How do these women deflect and circumvent the social and spatial boundaries defined by stigmatization? An analysis of semistructured interviews with eight women with children in Orange County, California, highlights the ways that the social identity of women as homeless is reinforced by social service agencies, potential employers, family, and friends, and illustrates how homeless women attempt to circumvent the negative attributes associated with homelessness.  相似文献   
45.
《International Geology Review》2012,54(11):1315-1331
The Gulf of California is an excellent example of how new ocean basins form. Tectonically, the northern Gulf of California is an incipient ocean basin and studies on it have defined acoustic basement and reveal the presence of new oceanic crust and intrusive bodies. Some recent studies report fundamental differences between the basins of the northern and southern Gulf of California: that the latter have well-developed oceanic crust beneath a thin cover of sediments, whereas the northern basins show proto-ocean basins, which may reflect thermal insulation of the thick sedimentary cover, the presence of low-angle faults, and more diffuse and distributed deformation. During the 1970s, Petróleos Mexicanos (PEMEX) undertook a 2D seismic reflection survey in the northern Gulf of California, over many active rift basins, including the Consag Basin. Through the processing and interpretation of these data, we describe the structural characteristics of the Consag Basin beyond 2 km depths. Using seismic reflection data, we identified an intrusion in the central part of this basin that may represent new oceanic crust buried by more than 4 km of sediments.  相似文献   
46.
利用AIRS产品数据提取了2010年4月4日墨西哥下加利福尼亚MW7.2地震前后震中及其附近CO总量和CO体积混合比随时间变化的信息,讨论了CO气体地球化学变化与地震活动的关系。数据表明,下加利福尼亚地震前约一个月CO总量升高,高出前两年同期平均值2~4×1017 mole/cm2,在地震发生前后CO总量和近地表CO体积混合比有波动现象,且8日平均数据的标准偏差增大,约为非地震时段的2倍。该现象应该归因于地震孕育发生过程中地应力作用使岩石圈脱气增强、大气电磁和化学反应引起的CO含量的增加。研究结果表明利用卫星高光谱遥感数据提取与地震有关的CO气体地球化学信息,对地震监测预报有重要意义。  相似文献   
47.
Analysis of four historical bathymetric surveys over a 132-year period has revealed significant changes to the morphology of the San Francisco Bar, an ebb-tidal delta at the mouth of San Francisco Bay estuary. From 1873 to 2005 the San Francisco Bar vertically-eroded an average of 80 cm over a 125 km2 area, which equates to a total volume loss of 100 ± 52 million m3 of fine- to coarse-grained sand. Comparison of the surveys indicates the entire ebb-tidal delta contracted radially, with the crest moving landward an average of 1 km. Long-term erosion of the ebb-tidal delta is hypothesized to be due to a reduction in the tidal prism of San Francisco Bay and a decrease in coastal sediment supply, both as a result of anthropogenic activities. Prior research indicates that the tidal prism of the estuary was reduced by 9% from filling, diking, and sedimentation. Compilation of historical records dating back to 1900 reveals that a minimum of 200 million m3 of sediment has been permanently removed from the San Francisco Bay coastal system through dredging, aggregate mining, and borrow pit mining. Of this total, ∼54 million m3 of sand-sized or coarser sediment was removed from central San Francisco Bay. With grain sizes comparable to the ebb-tidal delta, and its direct connection to the bay mouth, removal of sediments from central San Francisco Bay may limit the sand supply to the delta and open coast beaches.  相似文献   
48.
锂镁皂土——一种稀缺矿种的形成机理和用途   总被引:7,自引:0,他引:7  
介绍了稀缺且具有极大经济价值的特殊粘土类矿产资源-锂镁皂土(汉克托石,hectorite)的基本概念、形成机理,特性及开发利用概况。  相似文献   
49.
ABSTRACT. Most of us have not known—or cared—where our electricity comes from. Our attitude is changing, however, as we turn toward wind energy, now the fastest‐growing renewable energy resource in the world. Because we cannot extract and transport the raw energy of the wind, reaping its many environmental benefits requires that we cope with the landscape presence of its development wherever it occurs. Sometimes this interferes with the value of open space, and sometimes it may be close to subdivisions. It is the immobility and very visibility of wind power that makes its presence unavoidable. In that regard it cannot be hidden underground, stored in tanks, or moved by trains. It is an energy resource that reminds us that our electricity comes from somewhere. The more we wish to tap the power of the wind, the less we will be able to avoid the responsibilities that our demand for energy brings. This necessary bargain, first evident near Palm Springs, California, is now being experienced wherever wind power is being developed.  相似文献   
50.
利用地震前后的Landsat-8和Sentinel-2光学遥感影像数据,基于频率域互相关算法提取加利福尼亚州MW7.1及MW6.4地震的同震形变场,同时针对形变数据易受轨道误差、条带误差及时间失相干的影响,分别采用最小二乘多项式曲线拟合、改进均值相减等方法去除系统误差项。研究表明,MW7.1主震以右旋走滑为主,地表形变的断层迹线呈NNW走向,长度达55 km,最大滑移量约为2.82 m;MW6.4地震的发震断层迹线呈NE走向,长度达15 km,最大滑移量约为1.05 m,推测2次地震的发震断层分别为NNW向右旋走滑断裂及NE向左旋走滑断裂,二者形成典型的共轭关系。  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