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   1733篇
  免费   360篇
  国内免费   527篇
测绘学   62篇
大气科学   218篇
地球物理   392篇
地质学   1088篇
海洋学   468篇
天文学   23篇
综合类   112篇
自然地理   257篇
  2024年   11篇
  2023年   21篇
  2022年   51篇
  2021年   67篇
  2020年   76篇
  2019年   84篇
  2018年   54篇
  2017年   72篇
  2016年   76篇
  2015年   86篇
  2014年   114篇
  2013年   96篇
  2012年   90篇
  2011年   99篇
  2010年   117篇
  2009年   124篇
  2008年   117篇
  2007年   104篇
  2006年   146篇
  2005年   115篇
  2004年   99篇
  2003年   77篇
  2002年   85篇
  2001年   64篇
  2000年   81篇
  1999年   81篇
  1998年   67篇
  1997年   64篇
  1996年   50篇
  1995年   29篇
  1994年   37篇
  1993年   40篇
  1992年   18篇
  1991年   25篇
  1990年   20篇
  1989年   11篇
  1988年   11篇
  1987年   6篇
  1986年   6篇
  1985年   8篇
  1984年   9篇
  1983年   2篇
  1982年   3篇
  1981年   1篇
  1980年   1篇
  1978年   4篇
  1973年   1篇
排序方式: 共有2620条查询结果,搜索用时 15 毫秒
1.
应用显微光度术、显微傅立叶红外光谱(Micro-FT.IR)和飞行时间二次离子质谱(TOF-SIMS)等原位微分析技术并结合均一温度测量对胜利油气区下第三系沙河街组沙三段中有机包裹体进行了研究。区分出两类有机包裹体,即原生有机包裹体和次生有机包裹体。结果表明两类有机包裹体特征不同,二者物质组成、有机质成分及化学结构、热演化程度等差别也较大。结合地质分析表明原生有机包裹体是沙三段烃源岩生成烃类运移产物,具“自生自储”特点,而次生有机包裹体是沙四段烃源岩生成的烃类二次运移的产物。沙三段是沙河街组油气运移和聚集的主要层位,因而是寻找油气资源的主要目标层。研究表明,有机包裹体是研究油气生成、运移、聚集和演化等成藏系统最有效的手段之一,在油气勘探中有重要的应用意义。  相似文献   
2.
Surface waters of Alsea Bay, an unpolluted estuary on the Oregon coast, were analysed for nitrous oxide, nitrate and nitrite on a weekly or biweekly basis during the summer of 1979. The estuary was found to be a variable source of N2O to the atmosphere. Large and rapid increases in the concentrations of N2O, NO3?, and NO2? occurred at the beginning of the sampling period and are attributed to the influx of nutrient-rich upwelling water into the estuary with the tide. The subsequent decline in concentrations of nitrate, nitrite and nitrous oxide over the remainder of the summer is attributed to a decrease in upwelling intensity, a decline in nitrification rates and to assimilatory nitrate reduction. Measurements of nitrous oxide at six stations along the Alsea River were also made in September and October before and after the onset of the rainy season. Samples taken after flood conditions were established were systematically 50% higher than pre-flood samples. The data suggest that soil runoff results in elevated concentrations of N2O in rivers.  相似文献   
3.
An examination is made of the circulation in narrow estuaries subject to a predominant tidal forcing. Velocity structures are derived separately for residual flow components associated with (a) river flow, (b) wind stress, (c) a well-mixed longitudinal density gradient and (d) a fully stratified saline wedge. Dimensionless parameters are introduced to indicate the magnitude of each component and these parameters are evaluated for 9 major estuaries, thereby revealing their sensitivity to each component.For a channel of constant breadth and depth, formulae are deduced for the length of saline intrusion, L. Comparisons with observed data show that such formulae may be used with confidence to predict changes in L arising from variations in river flow, tidal range or channel depths.The level of stratification is shown to be related to a product of two parameters, one associated with velocity structure and a second involving the square of the ‘flow ratio’ uu? (i.e. residual velocity/amplitude of the tidal velocity). This relationship provides a simple classification system for estuarine stratification which can be used to indicate the sensitivity of any particular estuary to changing conditions.  相似文献   
4.
Parameters for ion exchange selectivity and aluminium hydroxide dissolution in the soil chemical submodels used in applications of the Birkenes model and of MAGIC are compared and several discrepancies identified for organic soils. A laboratory column simulation of the soil chemical submodels is proposed and applied to soils from the Loch Dee area in Galloway. Experimental results were well predicted by a simplified version of MAGIC, with ion exchange selectivity parameters similar to those used in a previous simulation of one subcatchment of Loch Dee. The aluminium hydroxide dissolution parameter used previously was found to be too low for the organic soil materials, where a value of 106 predicted the experimental results more closely. The model developed also included a simple silicate weathering reaction to release base cations into the system. It is concluded that such simple laboratory simulations are useful for independent calibration of the soil chemical submodel of catchment models.  相似文献   
5.
Observation of fluid parcel pathways in the Kuroshio Stream revealed a striking crossstream pattern associated with the Kuroshio‘‘ s meanders. In order to understand the observed pattern, a two-dimensional kinematic model of a meandering jet was developed which could be used to examine the relationship between streamfunction patterns and fluid parcel trajectories. The experiments designed to investigate the dependence of the water exchanges between the Kuroshio water and its surroundings on the characteristics of the Kuroshio‘‘s meanders involved the downstream speed,phase speed and the amplitude of other propagating waves. The results suggested that fluid parcels could cross the meandering jet to and fro; and that the water exchanges between the Kuroshio and its surroundings increased with a) increasing meander amplitude, b) decreasing downstream speed, and c) increasing wave phase speed. The results also showed that when the meandering jet was disturbed by other propagating waves, more effective water exchanges could be induced.  相似文献   
6.
抗旱保水剂膨润土-聚丙烯酸盐吸水树脂研究   总被引:2,自引:0,他引:2  
利用丙烯酸盐在蒙脱石层间进行原位聚合,研究开发一种适用于农林业的抗旱保水剂。实验结果表明:聚合产物的综合性能较为理想,吸水树脂的保水作用主要体现在能够吸收大量的水,并且在压力下不脱水。  相似文献   
7.
土壤中氟的形态分析   总被引:11,自引:3,他引:8  
以宁夏盐池地区高氟土壤为例,采用连续提取法对土壤样品中各形态氟进行提取,离子色谱法测定各形态氟的含量。根据研究目的及土壤特点将氟的形态划分为水溶态、离子交换态、可还原态、可氧化态及残渣态5种形态;对各种形态连续提取过程中使用的提取液进行了选择。采用建立的方法获得提取土壤中F-的检出限为0.76μg/g;方法精密度(RSD,n=7)各形态氟为水溶态氟11.3%,离子交换态氟13.5%,可还原态氟10.7%,可氧化态氟8.9%。  相似文献   
8.
Tourmaline is widespread in metapelites and pegmatites from the Neoproterozoic Damara Belt, which form the basement and potential source rocks of the Cretaceous Erongo granite. This study traces the B-isotope variations in tourmalines from the basement, from the Erongo granite and from its hydrothermal stage. Tourmalines from the basement are alkali-deficient schorl-dravites, with B-isotope ratios typical for continental crust (δ11B average −8.4‰ ± 1.4, n = 11; one sample at −13‰, n = 2). Virtually all tourmaline in the Erongo granite occurs in distinctive tourmaline-quartz orbicules. This “main-stage” tourmaline is alkali-deficient schorl (20–30% X-site vacancy, Fe/(Fe + Mg) 0.8–1), with uniform B-isotope compositions (δ11B −8.7‰ ± 1.5, n = 49) that are indistinguishable from the basement average, suggesting that boron was derived from anatexis of the local basement rocks with no significant shift in isotopic composition. Secondary, hydrothermal tourmaline in the granite has a bimodal B-isotope distribution with one peak at about −9‰, like the main-stage tourmaline, and a second at −2‰. We propose that the tourmaline-rich orbicules formed late in the crystallization history from an immiscible Na–B–Fe-rich hydrous melt. The massive precipitation of orbicular tourmaline nearly exhausted the melt in boron and the shift of δ11B to −2‰ in secondary tourmaline can be explained by Rayleigh fractionation after about 90% B-depletion in the residual fluid. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   
9.
对陕西省白河县月儿潭风化严重的绿松石样品进行了岩石薄片观察、化学成分测试、X射线粉晶衍射和红外吸收光谱分析等研究.结果表明,绿松石风化分解由天蓝色退色成黄绿色,最后变成白色,结构也由致密细腻变得松散多孔,主要原因是发生了硫酸根对磷酸根的交代作用,最终的交代产物主要是磷钙铝矾.  相似文献   
10.
利用1981 ̄1990年环渤海地区的地面气象资料,探空资料和土地利用资料,计算了该地区大气水份平衡方程中各分量,分析了其变化特征,并对它们之间的相互关系进行了研究。  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