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   12955篇
  免费   3650篇
  国内免费   3404篇
测绘学   1045篇
大气科学   1926篇
地球物理   2105篇
地质学   8461篇
海洋学   1063篇
天文学   78篇
综合类   2329篇
自然地理   3002篇
  2024年   48篇
  2023年   186篇
  2022年   446篇
  2021年   510篇
  2020年   507篇
  2019年   619篇
  2018年   511篇
  2017年   655篇
  2016年   641篇
  2015年   710篇
  2014年   926篇
  2013年   843篇
  2012年   1050篇
  2011年   1106篇
  2010年   982篇
  2009年   851篇
  2008年   863篇
  2007年   940篇
  2006年   932篇
  2005年   891篇
  2004年   755篇
  2003年   682篇
  2002年   554篇
  2001年   593篇
  2000年   463篇
  1999年   460篇
  1998年   367篇
  1997年   353篇
  1996年   268篇
  1995年   230篇
  1994年   222篇
  1993年   192篇
  1992年   146篇
  1991年   103篇
  1990年   78篇
  1989年   70篇
  1988年   74篇
  1987年   55篇
  1986年   39篇
  1985年   30篇
  1984年   8篇
  1983年   4篇
  1982年   4篇
  1981年   4篇
  1980年   9篇
  1979年   4篇
  1978年   11篇
  1977年   13篇
  1976年   1篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
971.
据个旧锡矿开发勘探资料,在计算机上实现不同网度稀空试验的结果、讨论了误差估计问题,提出了矿石品位、矿体厚度、矿体投影面积、矿体体积和金属储量等误差估值公式。以实例建立了不同穿透样锡品位变化系数值的金属储量误差与工程数量的回归模型,并将其应用于金属储量误差估计和工程效果的预测。  相似文献   
972.
本文研究了金厂峪金矿太古代斜长角闪岩中构造岩的特征和分布特点;根据片岩带内存在纯片岩地段、片岩带内有岩墙和两种含金脉体地段所呈现的无对称分带性、岩墙和脉体的接触关系,论述了断裂构造性质、控矿作用、热液充填期次、成矿时代;指出产于斜长角闪岩中的金矿床,并非都形成于太古代,在冀东燕山准地槽内应注意燕山期成矿作用。  相似文献   
973.
本文在大量实验和实践工作的基础上,对钋法找矿探测深度、钋室内分析流程(简称流程)的一些影响因素及土样化学成分对流程的影响等问题进行讨论。同时,对钋法找矿中采用归一化流程的可行性、必要性进行了论述。因而本文对钋法找矿工作的开展有一定的实际参考价值。  相似文献   
974.
富钆镝易解石的发现   总被引:1,自引:0,他引:1  
该矿物产于铀钍混合矿化的中粗粒黑云母花岗岩中。本文研究了产状、共生矿物、结晶特性、物理及化学性质并与易解石、钍易解石、铀易解石、钇易解石等作了对比。该矿物铀、钍、铅等含量均较高,在稀土含量中,钆、镝含量高达稀土总量的27%,故定名富钆镝易解石。  相似文献   
975.
Low frequency electrostatic waves are studied in magnetized plasmas with an electron temperature which varies with position in a direction perpendicular to the magnetic field. For wave frequencies below the ion cyclotron frequency, the waves need not follow any definite dispersion relation. Instead a band of phase velocities is allowed, with a range of variation depending on the maximum and minimum values of the electron temperature. Simple model equations are obtained for the general case which can be solved to give the spatial variation of a harmonically time varying potential. A simple analytical model for the phenomenon is presented and the results are supported by numerical simulations carried out in a 2.5-dimensional particle-in-cell numerical simulation. We find that when the electron temperature is striated along B0 and low frequency waves (ci) are excited in this environment, then the intensity of these low frequency waves will be striated in a manner following the electron temperature striations. High frequency ion acoustic waves (ci) will on the other hand have a spatially more uniform intensity distribution.  相似文献   
976.
Hydrodynamical Modeling Of Oceanic Vortices   总被引:1,自引:0,他引:1  
Mesoscale coherent vortices are numerous in the ocean.Though they possess various structures in temperature and salinity,they are all long-lived, fairly intense and mostly circular. Thephysical variable which best describes the rotation and the density anomaly associated with coherent vortices is potential vorticity. It is diagnostically related to velocity and pressure, when the vortex is stationary. Stationary vortices can be monopolar (circular or elliptical) or multipolar; their stability analysis shows thattransitions between the various stationary shapes are possible when they become unstable. But stable vortices can also undergo unsteady evolutions when perturbed by environmental effects, likelarge-scale shear or strain fields, -effect or topography. Changes in vortex shapes can also result from vortex interactions. such as the pairing, merger or vertical alignment of two vortices, which depend on their relative polarities and depths. Such interactions transfer energy and enstrophy between scales, and are essential in two-dimensional and in geostrophic turbulence. Finally, in relation with the observations, we describe a few mechanisms of vortex generation.  相似文献   
977.
Dissolved (dialysis in situ) and total concentrations ofCu, Zn, Cd and Al in eight mining polluted rivers in the Røros area, central Norway, were determinedby atomic absorption spectrometry (flame and graphite furnace) and compared to pH, Caconcentration and alkalinity through seasonal variations in river discharge. Totalconcentrations of the metals were highest during early spring flood and during summer andautumn rain episodes. Dissolved concentrations also increased as the spring floodproceeded, but small discharge peaks within this 2 month period as well as a considerableautumn flood episode appeared to lower rather than to raise the dissolved metal concentrations.Consequently the dissolved fractions of Zn, Cd and Al showed a significant negative correlationwith river discharge, and were low at the discharge peaks. Possibly high sediment concentrationsoccurring at high flood conditions more than counteracted desorption induced by pHdecrease, and led to decreased dissolved fractions through adsorption. Cu speciationon the other hand seemed to be more closely linked to pH. Alkalinity and Ca concentration,both assumed to protect aquatic life from metal pollution, were significantly lowerduring episodes with high Cu and Al total concentrations.  相似文献   
978.
五莲七宝山金铜矿床赋存于隐爆角砾岩筒中 ,与次火山岩关系密切。火山机构受NE ,NW向断裂联合控制。根据隐爆角砾岩筒的地质标志 ,结合对物、化、遥等资料的综合分析认为七宝山地区还有较好的找矿前景  相似文献   
979.
将流的标准化变差度概念应用到各年南海夏季风建立研究中去,并用其作为大气环流调整的客观定量指标。用该指标定义的南海夏季风建立的预兆日期与用传统天气气候学方法确定的南海夏季风的来临日期,在绝大多数具体年份两者均很接近,故可作南海夏季风建立的先兆指标。但有一些年份,南海季风的建立不伴随着低空环流的突变过程,两种方法都可能不准确,可靠的方法也许是用场相似度作指标。此外,南海夏季风建立前,对流层顶和平流层下层就出现了环流调整,该调整为南海夏季风建立打下基础,而南海夏季风爆发则表现为低空环流的大调整。南海夏季风的爆发是高、低空全球大气环流发生显著调整的结果,并非限于南海范围局部,南海夏季风建立不能看作是发生在南海的局部现象。  相似文献   
980.
We statistically analyze the tropical typhoon forming in the South China Sea and use TC (Tropical Cyclone) for short in the following) by typhoon yearbook. The typhoon quantity is very different in different months and years. TC appears in all months except March, and the most TC quantity in a year is 11, the least is 1 and 6.2 on average. The most TC quantity in a month is 5 and the least is 0. TC lands most in August and no TC lands on Chinese continent from December to the following April. The primary landing area is between Shantou and Hainan Island. The sustaining period of TC is usually between 4 days to 7days, and the longest is 19 days. Only 15% of the TC forming in the South China Sea can intensify to typhoon, and they all form in the ocean area deeper than 150m. The South China Sea is the ocean area over which the TC occurs frequently.  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