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   933篇
  免费   53篇
  国内免费   8篇
测绘学   20篇
大气科学   74篇
地球物理   199篇
地质学   373篇
海洋学   88篇
天文学   171篇
综合类   4篇
自然地理   65篇
  2023年   5篇
  2022年   6篇
  2021年   18篇
  2020年   20篇
  2019年   14篇
  2018年   33篇
  2017年   30篇
  2016年   41篇
  2015年   30篇
  2014年   42篇
  2013年   56篇
  2012年   53篇
  2011年   60篇
  2010年   41篇
  2009年   64篇
  2008年   54篇
  2007年   52篇
  2006年   51篇
  2005年   42篇
  2004年   24篇
  2003年   34篇
  2002年   36篇
  2001年   27篇
  2000年   21篇
  1999年   15篇
  1998年   14篇
  1997年   10篇
  1996年   9篇
  1995年   10篇
  1994年   4篇
  1993年   9篇
  1991年   5篇
  1990年   5篇
  1989年   5篇
  1988年   6篇
  1987年   5篇
  1986年   5篇
  1985年   4篇
  1984年   4篇
  1983年   4篇
  1982年   3篇
  1980年   4篇
  1979年   3篇
  1978年   3篇
  1977年   3篇
  1976年   1篇
  1975年   2篇
  1974年   2篇
  1973年   3篇
  1967年   1篇
排序方式: 共有994条查询结果,搜索用时 15 毫秒
991.
Recent acoustic Doppler current profiler (ADCP)-measurements in the Scheldt estuary near Antwerp, Belgium, revealed anomalous, i.e. anti-clockwise circulations in a left bend during the major part of the flood period; these circulations were established shortly after the turn of the tide. During ebb, anti-clockwise circulations persisted, as predicted by classical theory. These data were analysed with a 3D and a 1DV-model. The 3D simulations reveal that the anomalous circulations are found when salinity is included in the computations—without salinity “normal” circulations were found. From analytical and 1DV simulations, it is concluded that a longitudinal salinity gradient ${\partial S} \mathord{\left/ {\vphantom {{\partial S} {\partial x}}} \right. \kern-0em} {\partial x}$ may induce a near-bed maximum in flow velocity reversing the direction of the secondary currents. The 1DV-model was then used to assess the contribution of various processes one by one. It was found that because of a reduction in vertical mixing, the vertical velocity profile is not at equilibrium during the first phase of accelerating tide, further enhancing the effects of ${\partial S} \mathord{\left/ {\vphantom {{\partial S} {\partial x}}} \right. \kern-0em} {\partial x}$ . A small vertical salinity gradient ${\partial S} \mathord{\left/ {\vphantom {{\partial S} {\partial z}}} \right. \kern-0em} {\partial z}$ appeared to have a very large effect as the crosscurrents of the secondary circulations induced by ${\partial S} \mathord{\left/ {\vphantom {{\partial S} {\partial x}}} \right. \kern-0em} {\partial x}$ became an order of larger magnitude. However, at the site under consideration, the effects of transverse salinity gradients, generated by differential advection in the river bend, were dominant: adverse directions of the secondary circulations were found even when the vertical velocity profile became more regular with a more or less logarithmic shape, i.e. when the effects of ${\partial S} \mathord{\left/ {\vphantom {{\partial S} {\partial x}}} \right. \kern-0em} {\partial x}$ and ${\partial S} \mathord{\left/ {\vphantom {{\partial S} {\partial z}}} \right. \kern-0em} {\partial z}$ did not play a dominant role anymore. It is argued that data on the secondary velocity structure, which can be measured easily owing to today’s developments in ADCP equipment, may serve as an indicator for the accuracy at which the salinity field is computed with 3D numerical models. Moreover, the large effect of the salinity structure on the velocity field must have a large impact on the morphological development of estuaries, which should therefore be accounted for in morphological modelling studies.  相似文献   
992.
Mitochondrial DNA (mtDNA) is a single, usually non‐recombining locus, and often uniparentally inherited. Therefore, its ability to reveal recent gene flow among populations is usually questioned. In this study, the genetic population structure of 16 populations of Tridacna crocea (n = 366) from the Indo‐Malay Archipelago (IMA) was examined with 10 microsatellite markers and compared to previous studies using mtDNA, in order to test if the revealed population structure was congruent between the two marker systems. The results showed that the genetic population structure revealed by the two marker systems was mostly congruent, with a high correlation between cytochrome c oxidase subunit I (COI) and microsatellites. The studied populations could be divided by both marker systems as follows: (i) Eastern Indian Ocean, (ii) Central IMA, and (iii) Western Pacific. Populations in the Central IMA showed high gene flow. However, populations in the Java Sea (Karimunjava, Pulau Seribu) were grouped into a separate cluster by mtDNA analysis, while this grouping was not detected by microsatellites. It was also noteworthy that there is obvious heterozygosity deficiency in most of the populations, which may be caused by null alleles, inbreeding or population expansion. Overall, these results indicate that the mitochondrial COI gene is applicable for population genetic analysis and precise recovery of connectivity patterns of giant clams. Therefore, the combination of mtDNA and nuclear DNA markers can lead to a more complete understanding of population genetics. Moreover, this study is expected to facilitate fully displaying the population genetic structure of giant clams combining with other researchers' results.  相似文献   
993.
Interactions between two identical monochromatic wave trains with a relative separation angle of 24? were experimentally investigated in a well-designed ‘X' configuration.Wave trains with different amplitudes and frequencies were generated.The results demonstrated that the interaction was strongly dependent on both wave amplitude and frequency.For nonbreaking and lower-frequency cases,the wave trains can approximately reestablish their initial state following the interaction.However,for larger waves,the interaction was enhanced,distorting the surfaces significantly-the wave trains were no longer two-dimensional after the encounter.During the interaction process,there was an obvious increase in wave height,reaching a maximum amplification in the middle of the interaction region that was approximately 1.55 times the initial height.Furthermore,the images captured by high-speed cameras illustrated that two wave trains entered the interaction region at the same time and then merged during the interaction process,resulting in an increase in wave amplitude.The combined wave crest was initially composed of two straight segments with a relative angle of 24? and gradually morphed into a single segment as is evident in the plan view.The wave then broke in the downstream,still within the interaction region,exhibiting a crescent pattern along the crest.  相似文献   
994.
In the Western Gneiss Region in Norway, mafic eclogites form lenses within granitoid orthogneiss and contain the best record of the pressure and temperature evolution of this ultrahigh-pressure (UHP) terrane. Their exhumation from the UHP conditions has been extensively studied, but their prograde evolution has been rarely quantified although it represents a key constraint for the tectonic history of this area. This study focused on a well-preserved phengite-bearing eclogite sample from the Nordfjord region. The sample was investigated using phase-equilibrium modelling, trace-element analyses of garnet, trace- and major-element thermobarometry and quartz-in-garnet barometry by Raman spectroscopy. Inclusions in garnet core point to crystallization conditions in the amphibolite facies at 510–600°C and 11–16 kbar, whereas chemical zoning in garnet suggests growth during isothermal compression up to the peak pressure of 28 kbar at 600°C, followed by near-isobaric heating to 660–680°C. Near-isothermal decompression to 10–14 kbar is recorded in fine-grained clinopyroxene–amphibole–plagioclase symplectites. The absence of a temperature increase during compression seems incompatible with the classic view of crystallization along a geothermal gradient in a subduction zone and may question the tectonic significance of eclogite facies metamorphism. Two end-member tectonic scenarios are proposed to explain such an isothermal compression: Either (1) the mafic rocks were originally at depth within the lower crust and were consecutively buried along the isothermal portion of the subducting slab or (2) the mafic rocks recorded up to 14 kbar of tectonic overpressure at constant depth and temperature during the collisional stage of the orogeny.  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