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   359篇
  免费   70篇
  国内免费   35篇
测绘学   76篇
大气科学   72篇
地球物理   104篇
地质学   72篇
海洋学   63篇
天文学   32篇
综合类   20篇
自然地理   25篇
  2024年   1篇
  2023年   6篇
  2022年   12篇
  2021年   14篇
  2020年   13篇
  2019年   22篇
  2018年   10篇
  2017年   26篇
  2016年   31篇
  2015年   22篇
  2014年   31篇
  2013年   18篇
  2012年   24篇
  2011年   45篇
  2010年   23篇
  2009年   31篇
  2008年   27篇
  2007年   25篇
  2006年   17篇
  2005年   14篇
  2004年   5篇
  2003年   4篇
  2002年   4篇
  2001年   4篇
  2000年   6篇
  1999年   3篇
  1998年   6篇
  1997年   3篇
  1996年   6篇
  1995年   1篇
  1994年   2篇
  1992年   1篇
  1991年   2篇
  1990年   1篇
  1989年   2篇
  1988年   1篇
  1976年   1篇
排序方式: 共有464条查询结果,搜索用时 250 毫秒
241.
Robust environmental management of deep-sea mining projects must be integrated into the planning and execution of mining operations, and developed concurrently. It should follow a framework indicating the environmental management-related activities necessary at each project phase, and their interrelationships. An environmental management framework with this purpose is presented in this paper; it facilitates the development of environmental information and decision-making throughout the phases of a mining project. It is based environmental management frameworks used in allied industries, but adjusted for unique characteristics of deep-sea mining. It defines the gathering and synthesis of information and its use in decision-making, and employs a conceptual model as a growing repository of claim-specific information. The environmental management activities at each phase have been designed to enable the implementation of the precautionary approach in decision making, while facilitating review of adaptive management measures to improve environmental management as the quantity and quality of data increases and technologies are honed. This framework will ensure fairness and uniformity in the application of environmental standards, assist the regulator in its requirements to protect the environment, and benefit contractors and financiers by reducing uncertainty in the process.  相似文献   
242.
Vesta, the second largest Main-Belt Asteroid, will be the first to be explored in 2011 by NASA’s Dawn mission. It is a dry, likely differentiated body with spectrum suggesting that is has been resurfaced by basaltic lava flows, not too different from the lunar maria.Here we present the first disk-resolved spectroscopic observations of an asteroid from the ground. We observed (4) Vesta with the ESO-VLT adaptive optics equipped integral-field near-infrared spectrograph SINFONI, as part of its science verification campaign. The highest spatial resolution of ∼90 km on Vesta’s surface was obtained during excellent seeing conditions (0.5) in October 2004.We observe spectral variations across Vesta’ surface that can be interpreted as variations of either the pyroxene composition, or the effect of surface aging. We compare Vesta’s 2 μm absorption band to that of howardite-eucrite-diogenite (HED) meteorites that are thought to originate from Vesta, and establish particular links between specific regions and HED subclasses. The overall composition is found to be mostly compatible with howardite meteorites, although a small area around 180°E longitude could be attributed to a diogenite-rich spot. We finally focus our spectral analysis on the characteristics of Vesta’s bright and dark regions as seen from Hubble Space Telescope’s visible and Keck-II’s near-infrared images.  相似文献   
243.
We relate a single-crystal FTIR (Fourier transform infrared) and neutron diffraction study of two natural cancrinites. The structural refinements show that the oxygen site of the H2O molecule lies off the triad axis. The water molecule is almost symmetric and slightly tilted from the (0001) plane. It is involved in bifurcated hydrogen bridges, with Ow···O donor–acceptor distances >2.7 Å. The FTIR spectra show two main absorptions. The first at 3,602 cm?1 is polarized for E ⊥ c and is assigned to the ν3 mode. The second, at 3,531 cm?1, is also polarized for ⊥ c and is assigned to ν1 mode. A weak component at 4,108 cm?1 could possibly indicate the presence of additional OH groups in the structure of cancrinite. Several overlapping bands in the 1,300–1,500 cm?1 range are strongly polarized for ⊥ c, and are assigned to the vibrations of the CO3 group.  相似文献   
244.
Globally, it is smaller urban settlements that are growing most rapidly, are most constrained in terms of adaptive capacity but increasingly looked to for delivering local urban resilience. Data from three smaller coastal cities and their wider regional governance systems in Florida, US; West Sussex, UK and São Paulo, Brazil are used to compare the influence of scale and sector on city adaptive capacity. These tensions are described through the lens of the Adaptive Capacity Index (ACI) approach. The ACI is built from structuration theory and presents an alternative to social-ecological systems framing of analysis on adaptation. Structuration articulates the interaction of agency and structure and the intervening role played by institutions on information flow, in shaping adaptive capacity and outcomes. The ACI approach reveals inequalities in adaptive capacity to be greater across scale than across government, private and civil society sector capacity in each study area. This has implications for adaptation research both by reinforcing the importance of scale and demonstrating the utility of structuration theory as a framework for understanding the social dynamics underpinning adaptive capacity; and policy relevance, in particular considering the redistribution of decision-making power across scale and/or compensatory mechanisms, especially for lower scale actors, who increasingly carry the costs for enacting resilience planning in cities.  相似文献   
245.
In 2003, we initiated a long-term Adaptive Optics campaign to study the orbit of various main-belt asteroidal systems. Here we present a consistent solution for the mutual orbits of four binary systems: 22 Kalliope, 45 Eugenia, 107 Camilla and 762 Pulcova. With the exception of 45 Eugenia, we did not detect any additional satellites around these systems although we have the capability of detecting a loosely-bound fragment (located at 1/4×RHill) that is ∼40 times smaller in diameter than the primary. The common characteristic of these mutual orbits is that they are roughly circular. Three of these binary systems belong to a C-“group” taxonomic class. Our estimates of their bulk densities are consistently lower (∼1 g/cm3) than their associated meteorite analogs, suggesting an interior porosity of 30-50% (taking CI-CO meteorites as analogs). 22 Kalliope, a W-type asteroid, has a significantly higher bulk density of ∼3 g/cm3, derived based on IRAS radiometric size measurement. We compare the characteristics of these orbits in the light of tidal-effect evolution.  相似文献   
246.
Seven main belt asteroids, 2 Pallas, 3 Juno, 4 Vesta, 16 Psyche, 87 Sylvia, 324 Bamberga, and 707 Interamnia, were imaged with the adaptive optics system on the 3 m Shane telescope at Lick Observatory in the near infrared, and their triaxial ellipsoid dimensions and rotational poles have been determined with parametric blind deconvolution. In addition, the dimensions and pole for 1 Ceres are derived from resolved images at multiple epochs, even though it is an oblate spheroid.  相似文献   
247.
基于灰度的数字图像立体匹配算法的中心问题是选择一个适当尺寸的窗口。窗口尺寸既要尽可能地大,以便为可靠匹配包容足够的灰度变化;又要尽可能地小,以避开投影畸变的影响。本文介绍一种通过测算局部灰度和视差变化来选取适当窗口的方法。该方法引入一个窗口范围内的视差分布统计模型(随机模型)。利用该模型可确定,在窗口内的视差变化及灰度变化引起的窗口中心点视差估值的统计误差。以此为基础,文中提出一种窗口搜索方法,以对整幅图像上每一像素求出一个统计误差为最小的视差估值。这种算法不仅能自适应地调节窗口的尺寸,而且亦能同时调整窗口的形状(矩形)。  相似文献   
248.
自适应抗差最小二乘估计   总被引:9,自引:3,他引:9  
杨元喜 《测绘学报》1996,25(3):206-211
抗差估计与最小二乘估计在相应条件下各具优点。特定的抗差估计极值函数只在特定的污染分布条件下显示其抗差性和优效性。本文研究最小二乘估计,抗差估计以及抗差估计类之间的自适应估计问题。重点讨论了适于测量平差计算的误差分布检验量以及自适应抗差极值函数和权函数的构造与选择,最后人出了自适应抗差最小二科解算积式。  相似文献   
249.
应用Rietveld -DBW 941 1和Pirum程序对湘潭锰矿次生氧化带中天然锰钾矿的X射线粉晶衍射数据进行了晶胞参数的修正 ,首次给出其晶胞参数 :a0 =0 .9974nm ,b0 =0 .2 86 3nm ,c0 =0 .9693nm ,β =91 .467° ,V =2 76 .66× 1 0 - 3nm3,同时指出天然锰钾矿晶体结构精细化的难点在于其呈隐晶质集合体的特征导致衍射峰宽化、背底高和不对称  相似文献   
250.
甘肃河西走廊实际水资源及其对林木适生程度的影响   总被引:5,自引:0,他引:5  
陈昌毓 《地理学报》1991,46(1):35-46
根据河西走廊近年来水资源研究中比较一致的结果,计算了各县市绿洲年、日平均气温≥10℃期间的实际水资源。采用张宝堃和H.L.彭曼的气候学方法,对各县市林木生长期的蒸散耗水量进行了估算,与实测和调查资料对照表明,两种估算值大致可分别反映林木基本正常生长和良好生长对水分的需求。提出了“实际干燥度”和“实际湿润度”的概念,并根据后者确定了各县市绿洲不同气候植被区林木的适生程度,以及应当选择的乔、灌木树种。  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