全文获取类型
收费全文 | 49432篇 |
免费 | 644篇 |
国内免费 | 324篇 |
专业分类
测绘学 | 1335篇 |
大气科学 | 3833篇 |
地球物理 | 9298篇 |
地质学 | 16277篇 |
海洋学 | 4179篇 |
天文学 | 12165篇 |
综合类 | 108篇 |
自然地理 | 3205篇 |
出版年
2020年 | 340篇 |
2019年 | 338篇 |
2018年 | 831篇 |
2017年 | 814篇 |
2016年 | 1057篇 |
2015年 | 697篇 |
2014年 | 1170篇 |
2013年 | 2442篇 |
2012年 | 1111篇 |
2011年 | 1546篇 |
2010年 | 1437篇 |
2009年 | 1932篇 |
2008年 | 1778篇 |
2007年 | 1777篇 |
2006年 | 1680篇 |
2005年 | 1545篇 |
2004年 | 1508篇 |
2003年 | 1413篇 |
2002年 | 1358篇 |
2001年 | 1230篇 |
2000年 | 1156篇 |
1999年 | 1121篇 |
1998年 | 1057篇 |
1997年 | 1062篇 |
1996年 | 844篇 |
1995年 | 836篇 |
1994年 | 794篇 |
1993年 | 738篇 |
1992年 | 709篇 |
1991年 | 681篇 |
1990年 | 777篇 |
1989年 | 675篇 |
1988年 | 648篇 |
1987年 | 741篇 |
1986年 | 620篇 |
1985年 | 824篇 |
1984年 | 948篇 |
1983年 | 913篇 |
1982年 | 861篇 |
1981年 | 819篇 |
1980年 | 722篇 |
1979年 | 698篇 |
1978年 | 692篇 |
1977年 | 628篇 |
1976年 | 597篇 |
1975年 | 517篇 |
1974年 | 597篇 |
1973年 | 586篇 |
1972年 | 362篇 |
1971年 | 334篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
L. De Marchi J. Orm D. T. King D. R. Adrian J. J. Hagerty T. A. Gaither 《Meteoritics & planetary science》2019,54(11):2864-2878
Sedimentological (line‐logging) analysis of two drill cores, FC77‐3 and FC67‐3, situated, respectively, in the northwestern and southeastern quadrants of the Flynn Creek impact structure's crater‐moat area reveals that the ~27 m thick crater moat‐filling breccia consists of three subequal parts. These parts, which were deposited during early modification stage of this marine‐target impact structure, are distinguished on the basis of vertical trends in sorting, grain size, and counts of clasts per meter in comparison with other well‐known marine‐target impact structures, namely Lockne, Tvären, and Chesapeake Bay. The lower part is interpreted to represent mainly slump deposits, and the middle part is interpreted to represent a stage intermediate between slump and marine resurge, that is, a traction flow driven by overriding suspension flow. The upper part (size graded, and relatively well sorted and fine grained) is interpreted to represent marine resurge flow only. The upper part is capped by a relatively thin and relatively fine‐grained calcarenite to calcisiltite deposit. 相似文献
102.
We present new counts of stars in M15, using plates inB, V andU. We are able to explore relatively close to the central parts of the cluster (0.1 pc) and we derive the best fitting parameters for the star distribution. 相似文献
103.
T. R. P. Singh 《Mathematical Geology》1976,8(6):627-634
The application of the theory of random functions to problems of ore evaluation may involve computations of the covariance between the mean value of a given block and the functional value at a given point. However, an analytical solution for such a covariance does not exist for nonspherical blocks and for commonly applied models of covariance functions. Further, because this covariance is a function of the spatial arrangements of the block and the point, it has to be evaluated numerically each time for given point—block arrangements. This paper presents a readily available general solution to this problem in the form of a series of graduated curves which, together with some geometric manipulations, may be used to compute the covariance between a pointand a two-dimensional block for all possible point—block arrangements. The availability of the graph thus eliminates the necessity of using the time-absorbing programs on computers for such computations. Finally, many of the approximations that are made in order to avoid cumbersome covariance evaluations are no longer necessary due to the ease of such computations with the help of the graph provided. 相似文献
104.
M. Yu. Khlopov R. V. Konoplich R. Mignani S. G. Rubin A. S. Sakharov 《Astroparticle Physics》2000,12(4):367-372
The existence of macroscopic regions with antibaryon excess in the baryon asymmetric Universe with general baryon excess is the possible consequence of practically all models of baryosynthesis. Diffusion of matter and antimatter to the border of antimatter domains defines the minimal scale of the antimatter domains surviving to the present time. A model of diffused antiworld is considered, in which the density within the surviving antimatter domains is too low to form gravitationally bound objects. The possibility to test this model by measurements of cosmic gamma ray fluxes is discussed. The expected gamma ray flux is found to be acceptable for modern cosmic gamma ray detectors and for those planned for the near future. 相似文献
105.
A. K. Jain U. B. Jayanthi K. Kasturirangan U. R. Rao 《Astrophysics and Space Science》1976,45(2):433-438
The paper presents experimental evidence for the existence of fast intensity fluctuations with time scales of the order of a minute in the X-ray emission from Cyg X-1 at energies greater than 29 keV. Spectral variations over time intervals of 20–25 min are also observed in the same energy range. Whereas, similar intensity and spectral fluctuations have been reported earlier at lower energies the observations presented here is the first evidence for the existence of similar fluctuations at high energies. 相似文献
106.
The mechanism of concentration on the shaking table is discussed and a fresh attempt is made to obtain the theoretical model describing the velocity of a fluid in depth and time when in contact with a symmetrically reciprocating surface.The significance of the many design and operational variables and their interrelationships are examined. The various areas where the shaking table plays its role are then considered.The relevant features of micropanner, superpanner and the subsequent macropanner and pulsepanner are also treated since they all, like the shaking table, have the differential motion in common. 相似文献
107.
R. F. Griffin 《Journal of Astrophysics and Astronomy》1982,3(4):383-392
Photoelectric radial-velocity measurements have confirmed O. C. Wilson’s finding that BD 33° 2206, the secondary star in the
wide visual binary ADS 8470, is a spectroscopic binary. It has an eccentric orbit with a period of 100 days. Its γ-velocity
is close to the constant radial velocity of the visual primary, confirming the physical association of the stars. 相似文献
108.
109.
In this paper we report on the MEM power spectrum analysis of brightness temperature fluctuations observed at 2.8 GHz during the total solar eclipse of 16 February 1980. The observed periodicities range from 3.5 min to 64 min. These periodicities may arise due to spatial and/or temporal variations in the solar radio emission. The observed periodicities imply presence of scale sizes ranging from 70,000 to 600,000 km assuming that the brightness fluctuations arise because of spatial variation only. On the other hand, if these fluctuations are due to temporal variation, the observed periodicities correspond well to predicted modes of solar global oscillations. 相似文献
110.
R. L. Karale K. K. Narula V. Dayal K. M. Saini 《Journal of the Indian Society of Remote Sensing》1989,17(1):23-31
In order to check the premature siltation of the reservoirs and guard against the drop in the irrigation potential, the Government of India has launched the schemes of soil conservation and integrated watershed management in the catchments of RVPs and Flood Prone rivers. Owing to the large financial and manpower commitments needed to implement and execute soil conservation measures over vast catchment areas, a priority approach for treatment was identified. The methodology developed for prioritization of watersheds of a catchment area conceptualizes sedimentation of the reservoirs as a multiplicative function of erosivity value and the delivery ratio. This paper deals with the development of a computerized data base software module ‘WEIGHT’ for determination of erosivity values for the mapping units comprising assemblages of the varying combinations of climate, physiography and slope, land use and cover conditions, soil characteristics (texture, solumn thickness, permeability and pH) and the existing erosion and soil conservation measures. The WEIGHT software package is coded in FORTRON-4 for PDP 11/83 operating system. the data base comprises storage of the attributes of the different erosivity determinants of the mapping units with predetermined erosivity values sequentially on a disk and comparing the attributes of a new mapping unit to get the most probabilities erosivity value. The objective has been to eliminate the personal bias and bring about the objectivity in the process of assigning erosivity values to the different mapping units. The data base design, design logic and operational sequence of the data base are discussed in the paper. 相似文献