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   389篇
  免费   51篇
  国内免费   60篇
测绘学   18篇
大气科学   56篇
地球物理   111篇
地质学   203篇
海洋学   50篇
天文学   4篇
综合类   10篇
自然地理   48篇
  2023年   4篇
  2022年   6篇
  2021年   6篇
  2020年   10篇
  2019年   10篇
  2018年   8篇
  2017年   17篇
  2016年   14篇
  2015年   24篇
  2014年   20篇
  2013年   24篇
  2012年   14篇
  2011年   12篇
  2010年   16篇
  2009年   28篇
  2008年   20篇
  2007年   27篇
  2006年   40篇
  2005年   26篇
  2004年   22篇
  2003年   23篇
  2002年   27篇
  2001年   14篇
  2000年   9篇
  1999年   9篇
  1998年   10篇
  1997年   7篇
  1996年   9篇
  1995年   10篇
  1994年   6篇
  1993年   3篇
  1992年   3篇
  1991年   5篇
  1990年   2篇
  1989年   4篇
  1988年   3篇
  1987年   3篇
  1986年   3篇
  1983年   1篇
  1981年   1篇
排序方式: 共有500条查询结果,搜索用时 15 毫秒
51.
One of the reasons for the failure of some Marine Protected Areas (MPAs) is the lack of respect for their boundaries and regulations, which intensifies the need to assess the attitudes of stakeholders affected by MPAs. To this end, it is necessary to know the perception and behavior of resource users in these areas in relation to the management process. This study addressed the perception of different groups of fishermen in three MPAs that allow sustainable use of resources on the Brazilian northeastern coast. The perception analysis was based on four aspects: biodiversity conservation, flexibility and adaptability of fishermen, participation in management and opinions about the MPA. The interviewed fishermen (n=100) were classified into natives or immigrants,≥than 40 years old or <40, predominant use of selective or nonselective fishing gear and part or full time fishermen. The results showed that younger fishermen and the ones who use selective fishing gear presented a more conservation prone perception; nonselective fishermen and part-time fishermen were more flexible and adaptable to changes; and younger fishermen tended to agree more with the establishment of the MPAs. Taking these differences in perceptions among fishermen into account could serve as a basis for improvements in the management and conservation of fishing resources, besides helping predict possible future behavior due to changes in management policies.  相似文献   
52.
53.
The diet of Pomatoschistus microps has been studied using both gut content and stable isotope analyses. In the Roscoff Aber Bay (Brittany, France), this fish is commonly found on sandy muddy intertidal flats. Gut content analyses were also interpreted using trophic indices. Owing to the large diversity of prey consumed, these indices emphasised the opportunistic feeding behaviour of P. microps. Here, this species fed mainly on endofauna with meiofauna being of high relative importance. The main biotic components of its trophic habitat, characterized by δ13C and δ15N, provided evidence of a major trophic pathway based on drift Enteromorpha sp. Trophic positions estimated by both diet analyses and isotopic analyses led to similar results. In this bay, P. microps is a first‐order predator with a low degree of omnivory. Despite a preferential consumption of the amphipod Corophium arenarium, we assumed that this goby behaves as a generalist feeding on a uniform variety of endofauna taxa.  相似文献   
54.
The SEM-EDX technique was applied to investigate Au, and Cu+Sn alloyed grains in the mineralization of the Um Shashoba mine for achieving further understanding of occurrences, internal structures and microchemistry of Au and Cu alloys and associated minerals, and mineralization type. This study is aiming at the genetic history of ore-bearing fluid events, geochemical evaluation and exploration significance. The results showed that the mineralization could be considered as a single major episode generated by metamorphic mesothermal solution rich in sulfides and unsaturated respect to Au. It was differentiated into many stages; started with formation of auriferous pyrite that was pseudomorphed by secondary hematite, limonite and goethite. Three phases of Au alloy were precipitated, and Cu+Sn and Ag-rich alloys were produced respectively and followed by deposition of two generations of barren pyrite. Calcite and ankerite were crystalized, surrounded and partially replaced some of early formed minerals. Finally, barren muscovite recrystallized around and inside both later formed carbonate minerals that were free of any sign of Au in their structures. The processes of deformation, recrystallization, annealing, dissolution, remobilization and re-precipitation played the most important roles in the genetic history of the mineralization.  相似文献   
55.
主要介绍了在IMU/DGPS辅助空三的情况下,通过对不同的布点方案进行空三加密,对这些空三成果进行精度比较与分析,分析POS辅助空三在无地面控制点或少量的地面控制点的情况下能获得怎样的精度,能满足多大比例尺的成图要求。  相似文献   
56.
In the context of dust samples collections in space, the COMET experiment (Collecte en Orbite de Matière ExtraTerrestre) was proposed for the first time in 1982. The idea of such an experiment was to collect grains with identified cometary parent body, instead of mixing all extraterrestrial contributions present in low Earth orbit. It was thus proposed to install collectors inside hermetic boxes, to have these boxes mounted outside a space station, orbiting the Earth and to have the capability of choosing the time and duration of the collection. Since 1985, the COMET experiment has been exposed three times to space (COMET-1, in October 1985 during the encounter of the Earth with the Draconid meteor stream; the EUROMIR-95 instrument, exposing collectors, during the crossing by the Earth of the Orionid meteor stream associated to comet P/Halley and, in November 1998, during the crossing by the Earth of the Leonid meteor stream associated to comet Temple-Tuttle, COMET-99). Specific collection techniques, and corresponding analytical procedures have been developed. The collected particles are the only ones accessible in the laboratory with a known cometary origin, before the return to Earth (2006) of the Stardust mission, which will collect cometary grains in the tails of comet Wild 2. Such a challenge justifies the tremendous efforts brought into play, and that are summarized here.  相似文献   
57.
针对综导系统卡尔曼滤波器的特点,深入研究了灰关联分析法在滤波器精度效能评估中的应用。给出卡尔曼滤波器进行灰关联分析的方法和步骤,并对某一类型的滤波器进行精度效能评估。仿真结果表明了该方法的可行性和有效性。  相似文献   
58.
Observations of relative sea‐level change and local deglaciation in western Scotland provide critical constraints for modelling glacio‐isostatic rebound in northern Britain over the last 18 000 years. The longest records come from Skye, Arisaig and Knapdale with a shorter, Holocene, record from Kintail. Biostratigraphic (diatom, pollen, dinoflagellate, foraminifera and thecamoebian), lithological and radiocarbon analyses provide age and elevation parameters for each sea‐level index point. All four sites reveal relative sea‐level change that is highly non‐monotonic in time as the local vertical component of glacio‐isostatic rebound and eustasy (or global meltwater influx) dominate at different periods. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   
59.
本文对视超光速源样本的有关数据进行统计分析,检验活动星系核统一模式。给出各类视超光速源的R(核光度与扩展区光度之比)分布和喷流与视线的平均夹角 。分析源样本的R、洛伦兹因子r、喷流的投影尺度d和线性尺度l间的关系,发现R和d之间存在反相关、r和l之间存在相关的特性,这些结果有利于说明活动星系核统一模式的取向效应。  相似文献   
60.
In a sequence of glacigenic sediments at Aberdaron, Foraminifera were obtained from samples located specifically in order to differentiate between opposing models of depositional environment. All the diamict samples yielded remarkably uniform assemblages, with similar numbers of benthic specimens and benthic species per unit weight of sediment, similar planktic : benthic ratios, and similar ratios of clearly allochthonous to possibly autochthonous elements. This is precisely as predicted by the terrestrial model of sedimentation, where all of the sediments are interpreted as being derived from the melting of glacier ice rich in marine debris entrained during passage along the Irish Sea Basin. The results lend no support to a glacial marine model, since no faunal responses to increasingly distal sedimentary environments are observed. However, the fauna is dominated by the Foraminifera Elphidium excavatum (Terquem) forma clavata Cushman, which is commonly assumed to indicate glacial marine conditions. The modern distribution of similar assemblages suggests that it is just as likely to represent the cold, reduced salinity conditions that would have prevailed in the northern Irish Sea Basin for much of the Quaternary.  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