全文获取类型
收费全文 | 68383篇 |
免费 | 1204篇 |
国内免费 | 567篇 |
专业分类
测绘学 | 1704篇 |
大气科学 | 5467篇 |
地球物理 | 14224篇 |
地质学 | 22170篇 |
海洋学 | 5925篇 |
天文学 | 15665篇 |
综合类 | 160篇 |
自然地理 | 4839篇 |
出版年
2020年 | 513篇 |
2019年 | 527篇 |
2018年 | 992篇 |
2017年 | 981篇 |
2016年 | 1441篇 |
2015年 | 1071篇 |
2014年 | 1470篇 |
2013年 | 3353篇 |
2012年 | 1566篇 |
2011年 | 2364篇 |
2010年 | 2042篇 |
2009年 | 3017篇 |
2008年 | 2753篇 |
2007年 | 2469篇 |
2006年 | 2532篇 |
2005年 | 2200篇 |
2004年 | 2324篇 |
2003年 | 2162篇 |
2002年 | 2093篇 |
2001年 | 1902篇 |
2000年 | 1832篇 |
1999年 | 1552篇 |
1998年 | 1527篇 |
1997年 | 1514篇 |
1996年 | 1304篇 |
1995年 | 1239篇 |
1994年 | 1125篇 |
1993年 | 1019篇 |
1992年 | 957篇 |
1991年 | 822篇 |
1990年 | 1026篇 |
1989年 | 862篇 |
1988年 | 762篇 |
1987年 | 933篇 |
1986年 | 825篇 |
1985年 | 1039篇 |
1984年 | 1203篇 |
1983年 | 1140篇 |
1982年 | 1028篇 |
1981年 | 993篇 |
1980年 | 841篇 |
1979年 | 821篇 |
1978年 | 881篇 |
1977年 | 793篇 |
1976年 | 758篇 |
1975年 | 707篇 |
1974年 | 720篇 |
1973年 | 717篇 |
1972年 | 457篇 |
1971年 | 397篇 |
排序方式: 共有10000条查询结果,搜索用时 17 毫秒
251.
A scored lithological map including 10 radiometric units is established through applying factor analysis approach to aerial spectrometric data of Area-1, Syrian desert, which includes Ur, eU, eTh, K%, eU/eTh, eU/K%, and eTh/K%. A model of four rotated factors F1, F2, F3, and F4 is adapted for representing 234,829 data measured points in Area-1, where 86% of total data variance is interpreted. A geological scored pseudo-section derived from the lithological scored map is established and analyzed in order to show the possible stratigraphic and structural traps for uranium occurrences associated with phosphate deposits in the studied Area-1. These identified traps presented in this paper need detailed investigation and must be necessarily followed and checked by ground validations and subsurface well logging, in order to locate the anomalous uranium occurrences and explore with more confidence and certitude their characteristics as a function of depth. 相似文献
252.
A large submarine slide on the southern flank of the Bear Island Trough Mouth Fan, southwestern Barents Sea continental slope, has a run-out distance of about 400 km, a total volume of about 1100 km3, and is younger than 330 ka. Three seismic units, comprising mainly hemipelagic sediments has partly filled the slide scar. An increased sedimentation rate on the Bear Island Trough Mouth Fan from Late Pliocene time, probably in combination with abundant earthquakes, is the most likely cause of the slide. Based on these and previous studies, we suggest that large-scale slides were important sediment transport processes during Plio-Pleistocene. 相似文献
253.
We have used the spectra obtained by the Composite Infrared Spectrometer (CIRS) onboard the Cassini spacecraft to search for latitudinal variation in the 15N/14N ratio on Jupiter. We found no variations statistically significant given the observational and model uncertainties. The absence of latitudinal variations demonstrates that 15NH3 is not fractionated in Jupiter's atmosphere, and that the measured 15N/14N represents Jupiter's global value. Our mean value for the global jovian 15N/14N ratio of (2.22±0.52)×10−3 agrees with previous measurements made by Fouchet et al. (2000, Icarus 143, 223-243) and Owen et al. (2001, Astrophys. J. 553, L77-L79). We argue that the jovian isotopic 15N/14N ratio must represent the solar nitrogen isotopic composition. The solar 15N/14N ratio hence significantly differs from the terrestrial value: (15N/14N)⊕=3.68×10−3. This supports the proposition that terrestrial nitrogen originates from a nitrogen reservoir isolated from the main nitrogen reservoir in the proto-solar nebula. The origin and carrier of this isolated reservoir are still unknown. 相似文献
254.
L. V. Didkovsky D. L. Judge A. R. Jones E. J. Rhodes J. B. Gurman 《Astronomische Nachrichten》2006,327(4):314-320
An indirect proton flux measuring tool based on discrimination of the energy deposited by protons in 128 × 128 pixel EIT CCD areas outside the solar disk images is presented. Single pixel intensity events are converted into proton incident energy flux using modeled energy deposition curves for angles of incidence ±60° in four EIT spatial areas with different proton stopping power. The extracted proton flux is corrected for both the loss of one‐pixel events in the range of angles of incidence as well as for the contribution to the single pixel events resulting from scattered middle‐energy protons (low‐energy or high‐energy particles are stopped by the EIT components or pass through them, accordingly). A simple geometrical approach was found and applied to correct for a non‐unique relation between the proton‐associated CCD output signal and the incident proton energy. With this geometrical approximation four unique proton incident energy ranges were determined as 45–49, 145–154, 297–335, and 390–440 MeV. The indirect proton flux measuring tool has been tested by comparing Solar Energetic Particles (SEP) flux temporal profiles extracted from the EIT CCD frames and downloaded from the GOES database for the Bastille Day (BD) of 2000 July 14 and the more recent 2005 January 20 events. The SEP flux temporal profiles and proton spectra extracted from the EIT in the relatively narrow energy ranges between 45 and 440 MeV reported here are consistent with the related GOES profiles. The four additional EIT extracted ranges provide higher energy resolution of the SEP data. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
255.
256.
We report on results from the Goddard germanium Gamma-Ray Burst Spectrometer flown on the ISEE-3 spacecraft. Spectral and temporal studies of two events on 4 and 19 November, 1978 are presented. The power law spectral indices were found to be respectively –1.82 and –1.31, the latter being the hardest event thus far detected. Evidence for two lines in the 4 November event is presented — a broad feature at 420 keV and a narrower one at 740 keV. These are consistent with a common redshift if the parent lines are assumed to be the positron annihilation line (511 keV) and the first excited state of iron (847 keV). If this interpretation is correct, it is shown that additional lines from higher levels of iron and possibly other nuclei are expected and that they may represent a significant contribution to the total spectrum above 1 MeV.Paper presented at the Symposium on Cosmic-Ray Bursts, held at Toulouse, France, 26–29 November, 1979. 相似文献
257.
258.
C. J. Cohen E. C. Hubbard Claus Oesterwinter 《Celestial Mechanics and Dynamical Astronomy》1973,7(4):438-448
In 1950 Brouwer and van Woerkom published a secular theory of the variations of the planetary elements in analytical form. In the present paper we provide a graphical representation of this theory in the form of element plots for a time span of ten million years. 相似文献
259.
G.A. Jungclaus G.U. Yuen C.B. Moore J.G. Lawless 《Meteoritics & planetary science》1976,11(3):231-237
Alcohols, aldehydes and ketones identified in the Murchison C2 chondrite include: methanol, ethanol, 2-propanol, butyl alcohols, formaldehyde, acetaldehyde, propionaldehyde, acetone, 2-butanone, 3-pentanone and 2-pentanone. 相似文献
260.
J. Lewin 《Geological Journal》1978,13(1):25-36
Spatial and temporal patterns of meander development and sedimentation are assessed for a meander loop on the River Rheidol. Several scales and rates of change are distinguished for which different observational approaches are helpful. The meander loop itself developed over approximately 100 years while primary high-flow bedforms grew and migrated within the loop modifying its form. These were themselves variously dissected and embellished at intermediate discharges, but their residues are preserved as a major component of floodplain sedimentation. The characteristic and irregular pattern of point bar micro-relief common to rivers in mid-Wales suggests some adjustment of ideas on meander activity is required. 相似文献