全文获取类型
收费全文 | 41380篇 |
免费 | 1080篇 |
国内免费 | 1068篇 |
专业分类
测绘学 | 1099篇 |
大气科学 | 3435篇 |
地球物理 | 8599篇 |
地质学 | 15375篇 |
海洋学 | 3546篇 |
天文学 | 8657篇 |
综合类 | 255篇 |
自然地理 | 2562篇 |
出版年
2022年 | 309篇 |
2021年 | 529篇 |
2020年 | 518篇 |
2019年 | 537篇 |
2018年 | 1036篇 |
2017年 | 1010篇 |
2016年 | 1237篇 |
2015年 | 843篇 |
2014年 | 1232篇 |
2013年 | 2099篇 |
2012年 | 1441篇 |
2011年 | 1939篇 |
2010年 | 1722篇 |
2009年 | 2227篇 |
2008年 | 1829篇 |
2007年 | 1879篇 |
2006年 | 1816篇 |
2005年 | 1294篇 |
2004年 | 1196篇 |
2003年 | 1104篇 |
2002年 | 1093篇 |
2001年 | 920篇 |
2000年 | 896篇 |
1999年 | 739篇 |
1998年 | 777篇 |
1997年 | 753篇 |
1996年 | 624篇 |
1995年 | 627篇 |
1994年 | 551篇 |
1993年 | 456篇 |
1992年 | 460篇 |
1991年 | 417篇 |
1990年 | 497篇 |
1989年 | 403篇 |
1988年 | 388篇 |
1987年 | 468篇 |
1986年 | 366篇 |
1985年 | 471篇 |
1984年 | 576篇 |
1983年 | 502篇 |
1982年 | 490篇 |
1981年 | 444篇 |
1980年 | 455篇 |
1979年 | 384篇 |
1978年 | 398篇 |
1977年 | 371篇 |
1976年 | 337篇 |
1975年 | 334篇 |
1974年 | 362篇 |
1973年 | 381篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
82.
M.?B.?BellEmail author L.?W.?Avery J.?M.?MacLeod J.?P.?Vallée 《Astrophysics and Space Science》2011,333(2):377-388
Sensitive spectral observations made in two frequency bands near 6.0 and 17.6 GHz are described for Orion and W51. Using frequency
switching we were able to achieve a dynamic range in excess of 10,000 without fitting sinusoidal or polynomial baselines.
This enabled us to detect lines as weak as T
A
∼1 mK in these strong continuum sources. Hydrogen recombination lines with Δn as high as 25 have been detected in Orion. In the Orion data, where the lines are stronger, we have also detected a systematic
shift in the line center frequencies proportional to linewidth that cannot be explained by normal optical depth effects. 相似文献
83.
84.
The population and distribution of carbon-oxidizing and sulfate-reducing bacteria in the soils of the Mozhaisk Reservoir are studied. 相似文献
85.
We present here, for the O and B type stars in the Catalogue of Stellar Ultraviolet Fluxes, an approach which does not require
a precise knowledge of spectral type and luminosity class for derivingE(B-V) colour excesses. The method is based on the use of an UV-visual two-colour diagram; galactic variations in the interstellar
extinction law are analyzed and fully taken into account. Our results have been compared with those derived by using the differences
between observed and intrinsic colours for stars with known spectral classification. The very good agreement in a large number
of cases (94 per cent) demonstrates that our approach permits the derivation of reliable colour excess values for early type
stars even if only a rough spectral classification is available. 相似文献
86.
Robert O. Fournier Robert J. Rosenbauer James L. Bischoff 《Geochimica et cosmochimica acta》1982,46(10):1975-1978
The solubility of quartz in 2, 3, and 4 molal NaCl was measured at 350°C and pressures ranging from 180 to 500 bars. The molal solubility in each of the salt solutions is greater than that in pure water throughout the measured pressure range, with the ratio of solubility in NaCl solution to solubility in pure water decreasing as pressure is increased. The measured solubilities are significantly higher than solubilities calculated using a simple model in which the water activity in NaCl solutions decreases either in proportion to decreasing vapor pressure of the solution as salinity is increased or in proportion to decreasing mole fraction of water in the solvent. 相似文献
87.
88.
Summary. Lg -wave observations at NORSAR from a sequence of explosions straddling the Central Graben in the North Sea, show that the phase is almost extinguished by passing through this structure. For models of the graben structure, based on seismic refraction experiments, with crustal thinning beneath a zone of thickened sediments, numerical modelling of Lg propagation shows very poor transmission. The inclusion of attenuation simulating the effects of scattering from faults and other complex structures further diminishes the transmittivity. The theoretical effect of a crustal pinch is to redistribute S energy over a wide range of groups velocities, a pattern that fits well with the observations.
Three components recordings at NORSAR of explosive charges fired in water covering a wide range of azimuths, show significant transverse components to the seismograms. The transverse energy builds up with travel time and for Lg is of the same order as the vertical component. These results are consistent with the theoretical prediction of progressive net transfer of energy from Rayleigh to Love modes as the wavetrain propagates through a three-dimensionally heterogeneous medium. 相似文献
Three components recordings at NORSAR of explosive charges fired in water covering a wide range of azimuths, show significant transverse components to the seismograms. The transverse energy builds up with travel time and for Lg is of the same order as the vertical component. These results are consistent with the theoretical prediction of progressive net transfer of energy from Rayleigh to Love modes as the wavetrain propagates through a three-dimensionally heterogeneous medium. 相似文献
89.
In well velocity surveys made to calibrate Sonic (CV) Logs the calibration survey uses frequencies around 50 Hz whereas the Sonic Logging tool uses frequencies around 20 kHz. There thus exists the possibility of making a direct measure of velocity dispersion. In any one survey the disturbing factors, both instrumental and operational, will often mask any dispersive effect that might exist. Consequently this paper reports on a statistical analysis of the velocity differences resulting from calibration surveys and Sonic logs. Only Borehole Compensated Sonic Logs were used. Four areas were investigated: the North Sea, Abu Dhabi, Libya and Alaska. After rejecting logs and calibration records which were obviously in error there remained 424000 feet (about 130 km) of usable log distributed throughout 66 wells. The four areas were analysed separately and in no case was the estimated dispersion significantly different from zero. However, the mean values did correlate with lithology from (? 0.17 ± 0.18)% for the essentially carbonate section in Abu Dhabi to (+ 0.45 ± 0.25)% for the sand-shale section in Alaska, a positive sign meaning that the higher frequencies travelled faster. Except for Alaska the calibration surveys were made with a wall-clamp geophone, and for these areas amplitude measurements were made. After suitable corrections estimates of the absorption parameter Q were obtained. These varied from 20 to 200 with mean values of 63 for Libya, 70 for Abu Dhabi and 88 for the North Sea (excluding the Tertiary). If, as is usually assumed, the absorption mechanism is linear and is described by a Q which is independent of frequency, then these values would necessarily imply dispersion of several percent. As instanced above no such dispersion was observed. It is possible that the expected dispersion was compensated for by invasion of the mud filtrate into the borehole walls, but it is more likely that the absorption mechanism was substantially non-linear. 相似文献
90.
Volcanogenic massive sulfide deposits in ophiolite complexes are usually attributed to the Cyprus type. They associate with
basaltic volcanics that are formed in mid-ocean or back-arc spreading centers and much less frequently in intra-plate settings.
The deposits are characterized by copper or copper-zinc ores that are enriched in Ni, Co, and in places Mn and As, but are
very poor in Pb and demonstrate a low to moderate content of Ag and Au. Typically, the deposits are low to very low in ore
and metal reserves. Cyprus-type deposits were irregularly distributed during geological history. The most ancient of them
were formed in the Neoproterozoic, while the bulk of the deposits are Ordovician or Cretaceous in age. Their possible Paleoproterozoic
analogues can be found in the Svecofennian belt (Outokumpu ore district), while modern ones are confined to the Explorer and
Endeavour Ridges and southern segment of the Juan de Fuca Ridge. 相似文献