全文获取类型
收费全文 | 114034篇 |
免费 | 1384篇 |
国内免费 | 1709篇 |
专业分类
测绘学 | 3654篇 |
大气科学 | 8559篇 |
地球物理 | 21550篇 |
地质学 | 42289篇 |
海洋学 | 9121篇 |
天文学 | 23224篇 |
综合类 | 2388篇 |
自然地理 | 6342篇 |
出版年
2021年 | 834篇 |
2020年 | 922篇 |
2019年 | 954篇 |
2018年 | 6500篇 |
2017年 | 5727篇 |
2016年 | 4991篇 |
2015年 | 1695篇 |
2014年 | 2684篇 |
2013年 | 4829篇 |
2012年 | 3521篇 |
2011年 | 5886篇 |
2010年 | 4948篇 |
2009年 | 6140篇 |
2008年 | 5292篇 |
2007年 | 5755篇 |
2006年 | 3458篇 |
2005年 | 2944篇 |
2004年 | 3134篇 |
2003年 | 2960篇 |
2002年 | 2710篇 |
2001年 | 2242篇 |
2000年 | 2119篇 |
1999年 | 1838篇 |
1998年 | 1878篇 |
1997年 | 1774篇 |
1996年 | 1502篇 |
1995年 | 1457篇 |
1994年 | 1345篇 |
1993年 | 1205篇 |
1992年 | 1193篇 |
1991年 | 1144篇 |
1990年 | 1254篇 |
1989年 | 1048篇 |
1988年 | 1046篇 |
1987年 | 1150篇 |
1986年 | 1004篇 |
1985年 | 1306篇 |
1984年 | 1468篇 |
1983年 | 1356篇 |
1982年 | 1301篇 |
1981年 | 1215篇 |
1980年 | 1105篇 |
1979年 | 1073篇 |
1978年 | 1045篇 |
1977年 | 904篇 |
1976年 | 877篇 |
1975年 | 811篇 |
1974年 | 843篇 |
1973年 | 866篇 |
1971年 | 537篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
31.
Kunde Yang Yuanliang Ma Chao Sun Miller J.H. Potty G.R. 《Oceanic Engineering, IEEE Journal of》2004,29(4):964-972
This paper discusses the results of geoacoustic inversion carried out using explosive charge data from the Asian Seas International Acoustic Experiment (ASIAEX) East China Sea (ECS) Experiment. A multifrequency incoherent matched-field inversion processor and a genetic algorithm (GA) are used for the inversion. A multistep matched field inversion approach is presented, which makes use of the varying sensitivities of wave fields at various frequencies to reduce the inversion problem into a sequence of smaller inversions with fewer unknowns to estimate at each stage. Different parameters are estimated using data at different frequencies according to their sensitivities. Inversion results for different areas in the ECS region are summarized and compared with core data. 相似文献
32.
We report multi-frequency and multi-epoch radio continuum observations with multi-spatial resolution for the low-luminosity active galactic nucleus (LLAGN) NGC 266. In the centimetre regime, we find diffuse components with Very Large Array (VLA) observations, and a variable compact core with a rising spectrum with Very Long Baseline Array (VLBA) observations. Although the spectral index of the rising spectrum is consistent with the prediction of the simple advection-dominated accretion flow (ADAF) model, the observed radio power is slightly high compared with that of the model prediction. A spectral break at centimetre-to-millimetre wavelengths is inferred from the upper limits of flux densities from Nobeyama Millimetre Array (NMA) and James Clerk Maxwell Telescope (JCMT) data at millimetre and submillimetre wavelengths, respectively. More complicated considerations are required for the theoretical model to interpret such observed radio properties. 相似文献
33.
Using photoclinometry, topographic profiles across europan ridges have been produced. These profiles allow the identification of bulges in the terrain adjacent to the ridges. The bulges are assumed to have been produced by flexure of the elastic lithosphere due to the load of the ridges, which lie along cracks in the crust. The distance from the crack to these “fore-bulges” depends on the thickness of the elastic plate being flexed. Based on a survey of ridges in Galileo images with resolution <300 m/pixel, the thickness of the elastic lithosphere has been determined by this method at a wide variety of sites along the leading and trailing hemispheres of Europa. The average thickness is about 200 m. The elastic lithosphere underneath smooth dilational bands tends to be thicker than plains morphology, an effect that is pronounced at Thynia Linea and Astypalaea Linea. Among the ridges investigated here, more recent loading correlates with a thicker elastic lithosphere, which may either reflect an intrinsically thicker layer, or less viscous relaxation over the shorter time period. 相似文献
34.
35.
36.
In the coastal and estuarine waters of Goa, particulate organic carbon (POC) varied from 0.52 to 2.51 mg l?1 and from 0.28 to 5.24 mg l?1 and particulate phosphorus (PP) varied from 0.71 to 5.18 μg l?1 and from 0.78 to 20.34 μg l?1, respectively. The mean values of chlorophyll and primary productivity were 1.94 mg m?3 and 938.1 mg C m?2 day?1 in the coastal waters and 4.3 mg m?3 and 636.5 mg C m?1 day?1 in the estuarine waters, respectively. ratios were low in June and October even when POC values were quite high. The POC in surface waters was linearly correlated with the chlorophyll content. Also PP increased when chlorophyll and primary productivity remained high. The results suggest that the phytoplankton was sharply increasing and contributed to POC and PP content. The percentage of detritus calculated from the intercept values of chlorophyll on POC varied from 46 to 76% depending on season. Results indicate that the major portion of POC and PP during postmonsoon (October–January) is derived from phytoplankton production while the allochthonous matter predominate during monsoon (June–September). 相似文献
37.
R. L. Macklin 《Astrophysics and Space Science》1985,115(1):71-83
Neutron capture measurements made on a sample of fission-product zirconium containing 20%93Zr(t
1/2=1.5×106a) at the Oak Ridge Electron Linear Accelerator time-of-flight facility resulted in the identification of 138 resonance peaks for the93Zr isotope at neutron energies up to 21.5 keV. Average capture cross sections from 20 to 300 keV were derived by subtracting neutron capture yields of the stable zirconium isotopes90, 91, 92, 94, 96Zr and additional backgrounds. The average cross sections found were significantly less than those of JENDL-1. While generally 30% higher than those of ENDF/B —V below 60 keV, the binned data overlapped the smooth ENDF/B —V curve. The average for a Maxwellian neutron spectrum withkT=30 keV is (95±10) mb and the resonance contribution to the capture resonance integral is (15.0±0.5)b.Research sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences, under contract DE-AC05-840R21400 with the Martin Marietta Energy Systems, Inc. 相似文献
38.
Bennett R. Burns J. Nastav F. Lipkin J. Percival C. 《Oceanic Engineering, IEEE Journal of》1985,10(1):17-22
Two single-sensor piezometer probes, 8 mm in diameter, were developed for deep-ocean geotechnical investigations. These probes were tested in a hyperbaric chamber pressurized to 55 MPa (8000 psi). Testing was performed for a period of five weeks under high hydrostatic pressure with the probes inserted in reconstituted illitic marine sediment. Small differential pore-water pressures were generated in response to both mechanically and thermally generated forcing functions. During deep-ocean simulated pressure tests, the sensors exhibited excellent sensitivity and stability. These developments in piezometer-probe technology provide a quantitative means of assessing important geotechnical parameters of fine-grained seabed deposits. 相似文献
39.
The great continental ice sheets of the Pleistocene represented a significant topographic obstacle to the westerly winds
in northern midlatitudes. This work explores how consequent changes in the atmospheric stationary wave pattern might have
affected the shape and growth of the ice sheets themselves. A one dimensional (1-D) model is developed which permits an examination
of the types and magnitudes of the feedbacks that might be expected. When plausible temperature perturbations are introduced
at the ice-sheet margin which are proportional to the stationary wave amplitude, the equilibrium shape of the ice sheet is
significantly altered, and depends on the sign of the perturbation. The proposed feedback also affects the response of the
ice sheet to time-varying climate forcing. The results suggest that the evolution of a continental-scale ice sheet with a
land-based margin may be significantly determined by the changes it induces in the atmospheric circulation.
Received: 1 October 1999 / Accepted: 17 July 2000 相似文献
40.
The light curve modeling of binary stars has continued to evolve since its founding by Henry Norris Russell (see Russell and Merrill 1952 and citations therein) nearly a century ago, accelerated in the 1950s by Kopal's introduction of Roche geometry into models and by the development of synthetic light curve computer code in the 1970's. Improved physics and the use of more kinds of observational input are providing another round of important advances that promise to enlarge our knowledge of both binary stars and ensembles containing them. Here we discuss the newer horizons of light curve modeling and the steps being taken toward them. 相似文献