全文获取类型
收费全文 | 33149篇 |
免费 | 666篇 |
国内免费 | 399篇 |
专业分类
测绘学 | 884篇 |
大气科学 | 3089篇 |
地球物理 | 6841篇 |
地质学 | 11505篇 |
海洋学 | 2649篇 |
天文学 | 7131篇 |
综合类 | 74篇 |
自然地理 | 2041篇 |
出版年
2020年 | 217篇 |
2019年 | 237篇 |
2018年 | 563篇 |
2017年 | 532篇 |
2016年 | 784篇 |
2015年 | 499篇 |
2014年 | 748篇 |
2013年 | 1521篇 |
2012年 | 849篇 |
2011年 | 1155篇 |
2010年 | 988篇 |
2009年 | 1398篇 |
2008年 | 1206篇 |
2007年 | 1054篇 |
2006年 | 1155篇 |
2005年 | 953篇 |
2004年 | 943篇 |
2003年 | 955篇 |
2002年 | 951篇 |
2001年 | 808篇 |
2000年 | 852篇 |
1999年 | 711篇 |
1998年 | 700篇 |
1997年 | 701篇 |
1996年 | 605篇 |
1995年 | 565篇 |
1994年 | 499篇 |
1993年 | 456篇 |
1992年 | 440篇 |
1991年 | 439篇 |
1990年 | 433篇 |
1989年 | 408篇 |
1988年 | 383篇 |
1987年 | 477篇 |
1986年 | 443篇 |
1985年 | 480篇 |
1984年 | 580篇 |
1983年 | 574篇 |
1982年 | 511篇 |
1981年 | 494篇 |
1980年 | 454篇 |
1979年 | 439篇 |
1978年 | 453篇 |
1977年 | 398篇 |
1976年 | 364篇 |
1975年 | 367篇 |
1974年 | 408篇 |
1973年 | 394篇 |
1972年 | 248篇 |
1971年 | 226篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
Establishing relative and absolute time frameworks for the sedimentary, magmatic, tectonic and gold mineralisation events in the Norseman-Wiluna Belt of the Archean Yilgarn Craton of Western Australia, has long been the main aim of research efforts. Recently published constraints on the timing of sedimentation and absolute granite ages have emphasized the shortcomings of the established rationale used for interpreting the timing of deformation events. In this paper the assumptions underlying this rationale are scrutinized, and it is shown that they are the source of significant misinterpretations. A revised time chart for the deformation events of the belt is established. The first shortening phase to affect the belt, D1, was preceded by an extensional event D1e and accompanied by a change from volcanic-dominated to plutonic-dominated magmatism at approximately 2685–2675 Ma. Later extension (D2e) controlled deposition of the ca 2655 Ma Kurrawang Sequence and was followed by D2, a major shortening event, which folded this sequence. D2 must therefore have started after 2655 Ma—at least 20 Ma later than previously thought and after the voluminous 2670–2655 Ma high-Ca granite intrusion. Younger transcurrent deformation, D3–D4, waned at around 2630 Ma, suggesting that the crustal shortening deformation cycle D2–D4 lasted approximately 20–30 Ma, contemporaneous with low-volume 2650–2630 Ma low-Ca granites and alkaline intrusions. Time constraints on gold deposits suggest a late mineralisation event between 2640–2630 Ma. Thus, D2–D4 deformation cycle and late felsic magmatism define a 20–30 Ma long tectonothermal event, which culminated with gold mineralisation. The finding that D2 folding took place after voluminous high-Ca granite intrusion led to research into the role of competent bodies during folding by means of numerical models. Results suggest that buoyancy-driven doming of pre-tectonic competent bodies trigger growth of antiforms, whereas non-buoyant, competent granite bodies trigger growth of synforms. The conspicuous presence of pre-folding granites in the cores of anticlines may be a result from active buoyancy doming during folding. 相似文献
972.
S. K. Deb M. H. Manghnani K. Ross R. A. Livingston P. J. M. Monteiro 《Physics and Chemistry of Minerals》2003,30(1):31-38
A Raman scattering and X-ray diffraction study of the thermal decomposition of a naturally occurring, ettringite-group crystal
is presented. Raman spectra, recorded with increasing temperature, indicate that the thermal decomposition begins at ≈55 °C,
accompanied by dehydration of water molecules from the mineral. This is in contrast to previous studies that reported higher
temperature breakdown of ettringite. The dehydration is completed by 175 °C and this results in total collapse of the crystalline
structure and the material becomes amorphous. The Raman scattering results are supported by X-ray diffraction results obtained
at increasing temperatures.
Received: 9 July 2001 / Accepted: 14 August 2002 相似文献
973.
Relative sea level (RSL) data derived from isolation basins at Innaarsuit, a site on the south shores of the large marine embayment of Disko Bugt, West Greenland, record rapid RSL fall from the marine limit (ca. 108 m) at 10,300-9900 cal yr B.P. to reach the present sea level at 3500 cal yr B.P. Since 2000 cal yr B.P., RSL rose ca. 3 m to the present. When compared with data from elsewhere in Disko Bugt, our results suggest that the embayment was deglaciated later and more quickly than previously thought, at or slightly before 10,300 cal yr B.P. The northern part of Disko Bugt experienced less rebound (ca. 10 m at 6000 cal yr B.P.) compared with areas to the south. Submergence during the late Holocene supports a model of crustal down-warping as a result of renewed ice-sheet growth during the neoglacial. There is little evidence for west to east differences in crustal rebound across the southern shores of Disko Bugt. 相似文献
974.
A new method is presented for the computation of the gravitational attraction of topographic masses when their height information is given on a regular grid. It is shown that the representation of the terrain relief by means of a bilinear surface not only offers a serious alternative to the polyhedra modeling, but also approaches even more smoothly the continuous reality. Inserting a bilinear approximation into the known scheme of deriving closed analytical expressions for the potential and its first-order derivatives for an arbitrarily shaped polyhedron leads to a one-dimensional integration with – apparently – no analytical solution. However, due to the high degree of smoothness of the integrand function, the numerical computation of this integral is very efficient. Numerical tests using synthetic data and a densely sampled digital terrain model in the Bavarian Alps prove that the new method is comparable to or even faster than a terrain modeling using polyhedra. 相似文献
975.
Geoid determination using adapted reference field, seismic Moho depths and variable density contrast 总被引:4,自引:0,他引:4
The traditional remove-restore technique for geoid computation suffers from two main drawbacks. The first is the assumption
of an isostatic hypothesis to compute the compensation masses. The second is the double consideration of the effect of the
topographic–isostatic masses within the data window through removing the reference field and the terrain reduction process.
To overcome the first disadvantage, the seismic Moho depths, representing, more or less, the actual compensating masses, have
been used with variable density anomalies computed by employing the topographic–isostatic mass balance principle. In order
to avoid the double consideration of the effect of the topographic–isostatic masses within the data window, the effect of
these masses for the used fixed data window, in terms of potential coefficients, has been subtracted from the reference field,
yielding an adapted reference field. This adapted reference field has been used for the remove–restore technique. The necessary
harmonic analysis of the topographic–isostatic potential using seismic Moho depths with variable density anomalies is given.
A wide comparison among geoids computed by the adapted reference field with both the Airy–Heiskanen isostatic model and seismic
Moho depths with variable density anomaly and a geoid computed by the traditional remove–restore technique is made. The results
show that using seismic Moho depths with variable density anomaly along with the adapted reference field gives the best relative
geoid accuracy compared to the GPS/levelling geoid.
Received: 3 October 2001 / Accepted: 20 September 2002
Correspondence to: H.A. Abd-Elmotaal 相似文献
976.
977.
A. Chakraborty H. C. Upadhyaya O. P. Sharma S. K. Deb 《Meteorology and Atmospheric Physics》2003,83(3-4):221-236
Summary ?A newly developed ocean general circulation model has been tested and verified with some idealized experiments. Generally
two types of idealized experiments have been done here. First types are called as “symmetric experiments” and second types
are called as “transport experiments”. The first types of experiment help to correct the model core and any deficiency from
boundary conditions. The second types of experiment are the type of validation experiment. In both the experiments there are
no continents, so in the first type of experiments where symmetric forcings are provided one can expect that model should
maintain the symmetric nature. In the second type of experiments one can expect that model should respond correctly to the
wind forcings, if no wind curl is present in the wind forcing there will be no circulation in the extratropics and if there
is no wind the equator there will be no circulation. The model reproduces the possible envisaged results of these experiments
and gives the confidence for performing the realistic integration.
Received February 20, 2002; accepted July 7, 2002
Published online: February 20, 2003 相似文献
978.
Numerical simulation of a South China Sea typhoon Leo (1999) 总被引:6,自引:0,他引:6
K.-H. Lau Z.-F. Zhang H.-Y. Lam S.-J. Chen 《Meteorology and Atmospheric Physics》2003,83(3-4):147-161
Summary ?A South China Sea typhoon, Leo (1999), was simulated using the Penn State/NCAR mesoscale model MM5 with the Betts-Miller
convective parameterization scheme (BMEX). The simulation had two nested domains with resolutions at 54 and 18 km, and the
forecast duration was 36 hours. The model was quite successful in predicting the track, the rapid deepening, the central pressure,
and the maximum wind speed of typhoon Leo as verified with reports from the Hong Kong Observatory (HKO). The structure of
the eye, the eye wall, and the spiral convective cloud band simulated in the model are found to be comparable to corresponding
features identified in satellite images for the storm, and also with those reported by other authors.
A trajectory analysis was performed. Three kinds of trajectory were found: (1) spirally rising trajectories near the eye wall;
(2) spirally rising/descending trajectories in the convective/cloud free belt; (3) straight and fast rising trajectories in
a heavy convection zone along one of the cloud bands on the periphery of the tropical cyclone.
Both the HKO and the U.S. Joint Typhoon Warning Center (JTWC) reported the rapid deepening of Leo started around 00 UTC 29
April. In the model, the eye was first formed in the lower troposphere, and it extended to the upper troposphere within a
few hours. We speculate that the spin-up of cyclonic rotation in the low-level eye enhanced the positive vorticity along the
low-level eye wall. The positive vorticity was then transported to the upper troposphere by convection, leading to an extension
and growth of the eye into the upper troposphere.
To examine the impact of convective parameterization scheme (CPS) on the simulation, the Grell scheme (GLEX) was also tested.
The GLEX predicted a weaker typhoon with a wilder eye that extended not as high up in the upper troposphere as BMEX. The different
structures of the eye between the BMEX and GLEX suggest that the mesoscale features of the eye are dependent on the convection.
In other words, the vertical and horizontal distribution of convective heating is essential to the development and structure
of the eye.
Received December 18, 2001; accepted May 7, 2002
Published online: March 20, 2003 相似文献
979.
980.
A. Heusel-Waltrop K. Diehl S. K. Mitra H. R. Pruppacher 《Journal of Atmospheric Chemistry》2003,44(2):211-223
Laboratory experiments have been carried out to investigate the uptake of sulfur dioxide by water drops containing heavy metal ions where the metal ions serve as catalysts to oxidise the taken up S(IV) into S(VI). During the gas uptake the drops were freely suspended at their terminal velocity in the airstream of the Mainz vertical wind tunnel. Two series of experiments were carried out, one with large millimeter size water drops containing manganese or iron ions, and the other with small water drops containing manganese ions and having radii in hundreds of micron size range. The experimental results were compared against model computations using the Kronig–Brink model and the fully mixed model, modified for the case that heavy metal ions present in the liquid phase act as catalysts for the oxidising process. The results of the model calculations show that there are only small differences between the predicted gas uptake according to the two models. In addition it was found that the experimental obtained results from the uptake of SO2 by water drops containing heavy metal ions for both, large and small water drops did agree with the model results. 相似文献