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   76526篇
  免费   1036篇
  国内免费   834篇
测绘学   2259篇
大气科学   5540篇
地球物理   14564篇
地质学   27285篇
海洋学   6669篇
天文学   18131篇
综合类   293篇
自然地理   3655篇
  2022年   472篇
  2021年   816篇
  2020年   839篇
  2019年   936篇
  2018年   2109篇
  2017年   2019篇
  2016年   2520篇
  2015年   1408篇
  2014年   2419篇
  2013年   4063篇
  2012年   2506篇
  2011年   3252篇
  2010年   2794篇
  2009年   3617篇
  2008年   3117篇
  2007年   3141篇
  2006年   3001篇
  2005年   2265篇
  2004年   2231篇
  2003年   2161篇
  2002年   2073篇
  2001年   1827篇
  2000年   1735篇
  1999年   1462篇
  1998年   1480篇
  1997年   1389篇
  1996年   1234篇
  1995年   1198篇
  1994年   1071篇
  1993年   924篇
  1992年   876篇
  1991年   887篇
  1990年   933篇
  1989年   788篇
  1988年   774篇
  1987年   888篇
  1986年   761篇
  1985年   953篇
  1984年   1043篇
  1983年   957篇
  1982年   915篇
  1981年   789篇
  1980年   759篇
  1979年   712篇
  1978年   704篇
  1977年   617篇
  1976年   556篇
  1975年   552篇
  1974年   548篇
  1973年   601篇
排序方式: 共有10000条查询结果,搜索用时 87 毫秒
611.
A method is proposed for the determination of the dispersion equation of Love waves propagating in a homogeneous layer lying over a laterally inhomogeneous half-space. The proposed method can be made to work only when the lateral inhomogeneities in the lower half-space are finite in nature so that their Fourier transforms are available. As an illustration the dispersion equation of Love waves is obtained for one of such media in which the shear-wave velocity and the rigidity in the lower half-space either increases or decreases along the direction of propagation of waves according as the parameter of heterogeneity is positive or negative.  相似文献   
612.
This paper deals with the construction procedure for regional travel-time curves and the new average global curve for P times. This paper also presents new average global values for PcP times constructed from observed travel times.  相似文献   
613.
614.
Recently the method of flotation is used as a simple step within a number of procedures of chemical analysis for enrichment of waters or industrial lyes to be investigated by ions. By the example of tin analysis all the possible variants are compared with regard to their efficiency. The advantages result from the using conventional methods of chemical analysis in connection with coprecipitation of traces and their flotation at trace levels of ng/ml. The flotation is a fast method of enrichment, it is easily to handle and sufficiently exact. As a proof example the chemical analysis of Sn, Au and Ag is described.  相似文献   
615.
We found extensive evidence that the vertical ground accelerations produced during the largest shock (M = 6.0) of the 1997 Umbria-Marche earthquake sequence exceeded 1g in two areas close to the heavily-damaged villages of Annifo and Colle Croce. This evidence comes from the striking observation of thousands of freshly fractured and broken rocks and stones in these areas. Some of the broken stones lie isolated on soft detritic soil while others had been previously piled up, probably a long time agoto clear the fields for farming. The freshness of the cuts and fractures and the consistency of the observations for thousands of rocks and stones in these areas indicate that these rocks were thrown upwards during the earthquake, with breakage occurring at the time of impact. Ground motion calculations consistent with the static deformation inferred from GPS and interferometry data, show that the broken stones and rocks are found in the zone where the strongest shaking took place during the earthquake and that most of the shaking there was vertical.  相似文献   
616.
Summary An entirely new procedure is proposed in this paper for interpreting anomalies of dykes. This reduces the ambiguity in magnetic interpretation. Measurements of the first vertical derivative simplifies the problem of interpretation and also supplies additional information concerning the parameters of a dyke. The first vertical derivative profile cuts the distance axis at two points whose separation is related to the depth and direction of magnetisation of a dyke.  相似文献   
617.
Summary A new significant correlation has been sought between high magnitude global seismicity and lateral surface wave velocity gradients. Rayleigh wave velocity divisioning of Eurasia, Africa, Pacific Ocean, Atlantic Ocean and Indian Ocean into regions of similar group velocity dispersion character of 30 sec period bySanto andSato [1]3) has been mainly used for calculating the gradients. It is quite striking to note that all earthquakes of magnitude 8.6 and above during 1897–1956 have occurred in regions having gradients of the order of 1.5·10–3 sec–1.Finally, some potential areas for high magnitude earthquake occurrences are predicted and the possible velocity gradients in regions, where division pattern is not yet investigated like South America and Australia, are also estimated.N.G.R.I. Contribution No. 70-170.  相似文献   
618.
Investigating adsorption of methyl parathion on the activated carbons Filtrasorb 400 and F 44 and on the adsorption polymer Wofatit Y 77 we found a stronger adsorption on the carbons at smaller concentration whereas the resin has the larger capacity at higher concentration. Adsorbents were regenerated through hydrolysis of the pesticide at pH = 11.7. The velocity of hydrolysis in the adsorbed state is reduced to a tenth of that in solution. However, a transport resistance in the pores of adsorbents seems to be negligible as may be concluded from the very small measured particle diameter dependence of the velocity constants. In adsorption-regeneration measurements the capacity of the adsorbents stabilized at about 70% of that of the fresh adsorbents.  相似文献   
619.
A simple and general technique to obtain a six-component earthquake input motion of a rigid foundation using three-linear-components earthquake record is presented. Using the multicomponent input, the maximum structural response to an arbitrarily oriented earthquake is derived. The method is applied to determine the response spectrum for a two-d.o.f. laterally and torsionally coupled linear system. The effects of foundation geometry, structural properties and apparent wave velocity on the structural response are investigated. It is found that increasing the foundation size reduces the peak values of the input motion; this reduction is more pronounced for a square foundation than for a circular one.  相似文献   
620.
By the example of some phenols and qninones, in aqueous solutions possibilities for the quantitative determination are tested separately and in the mixture. Methods for thin-layer chromatography and spectrophotometry were developed. There can be chromatographically deteeted between 5 and 80 μg of substance. Phenol and quinone concentrations of 1.5 · 10?3 … 0.5 · 10?4 M were spectrophotometrically investigated.  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