全文获取类型
收费全文 | 4669篇 |
免费 | 333篇 |
国内免费 | 306篇 |
专业分类
测绘学 | 170篇 |
大气科学 | 186篇 |
地球物理 | 468篇 |
地质学 | 834篇 |
海洋学 | 233篇 |
天文学 | 2776篇 |
综合类 | 67篇 |
自然地理 | 574篇 |
出版年
2024年 | 26篇 |
2023年 | 49篇 |
2022年 | 76篇 |
2021年 | 83篇 |
2020年 | 81篇 |
2019年 | 125篇 |
2018年 | 64篇 |
2017年 | 69篇 |
2016年 | 80篇 |
2015年 | 97篇 |
2014年 | 133篇 |
2013年 | 153篇 |
2012年 | 148篇 |
2011年 | 181篇 |
2010年 | 120篇 |
2009年 | 412篇 |
2008年 | 355篇 |
2007年 | 426篇 |
2006年 | 414篇 |
2005年 | 340篇 |
2004年 | 311篇 |
2003年 | 306篇 |
2002年 | 216篇 |
2001年 | 195篇 |
2000年 | 149篇 |
1999年 | 162篇 |
1998年 | 184篇 |
1997年 | 63篇 |
1996年 | 55篇 |
1995年 | 42篇 |
1994年 | 33篇 |
1993年 | 38篇 |
1992年 | 28篇 |
1991年 | 12篇 |
1990年 | 19篇 |
1989年 | 12篇 |
1988年 | 8篇 |
1987年 | 11篇 |
1986年 | 10篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1982年 | 5篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 1篇 |
1971年 | 2篇 |
1954年 | 1篇 |
排序方式: 共有5308条查询结果,搜索用时 15 毫秒
891.
J. M. Colberg S. D. M. White N. Yoshida T. J. MacFarland A. Jenkins † C. S. Frenk F. R. Pearce A. E. Evrard H. M. P. Couchman G. Efstathiou J. A. Peacock P. A. Thomas 《Monthly notices of the Royal Astronomical Society》2000,319(1):209-214
We use very large cosmological N -body simulations to obtain accurate predictions for the two-point correlations and power spectra of mass-limited samples of galaxy clusters. We consider two currently popular cold dark matter (CDM) cosmogonies, a critical density model ( τ CDM) and a flat low density model with a cosmological constant (ΛCDM). Our simulations each use 109 particles to follow the mass distribution within cubes of side 2 h −1 Gpc ( τ CDM) and 3 h −1 Gpc (ΛCDM) with a force resolution better than 10−4 of the cube side. We investigate how the predicted cluster correlations increase for samples of increasing mass and decreasing abundance. Very similar behaviour is found in the two cases. The correlation length increases from for samples with mean separation to for samples with The lower value here corresponds to τ CDM and the upper to ΛCDM. The power spectra of these cluster samples are accurately parallel to those of the mass over more than a decade in scale. Both correlation lengths and power spectrum biases can be predicted to better than 10 per cent using the simple model of Sheth, Mo & Tormen. This prediction requires only the linear mass power spectrum and has no adjustable parameters. We compare our predictions with published results for the automated plate measurement (APM) cluster sample. The observed variation of correlation length with richness agrees well with the models, particularly for ΛCDM. The observed power spectrum (for a cluster sample of mean separation ) lies significantly above the predictions of both models. 相似文献
892.
893.
A geophysical survey was conducted to determine the depth of the base of the water-table aquifer in the southern part of
Jackson Hole, Wyoming, USA. Audio-magnetotellurics (AMT) measurements at 77 sites in the study area yielded electrical-resistivity
logs of the subsurface, and these were used to infer lithologic changes with depth. A 100–600 ohm-m geoelectric layer, designated
the Jackson aquifer, was used to represent surficial saturated, unconsolidated deposits of Quaternary age. The median depth
of the base of the Jackson aquifer is estimated to be 200 ft (61 m), based on 62 sites that had sufficient resistivity data.
AMT-measured values were kriged to predict the depth to the base of the aquifer throughout the southern part of Jackson Hole.
Contour maps of the kriging predictions indicate that the depth of the base of the Jackson aquifer is shallow in the central
part of the study area near the East and West Gros Ventre Buttes, deeper in the west near the Teton fault system, and shallow
at the southern edge of Jackson Hole. Predicted, contoured depths range from 100 ft (30 m) in the south, near the confluences
of Spring Creek and Flat Creek with the Snake River, to 700 ft (210 m) in the west, near the town of Wilson, Wyoming.
Received, May 1997 · Revised, February 1998 · Accepted, April 1998 相似文献
894.
Hazard assessment of dangerous natural phenomena is critical because of their evident results concerning loss of human life and property, especially in dense populated areas. Earthquakes are probably the most devastating phenomenon since their immediate and long-term consequences are severe. This study is focused on the earthquake data analysis in different regions of Greece, characterised by different seismicity levels. In specific, a novel model is proposed based on evolutionary computation methods, such as symbolic regression by genetic programming and genetic algorithms in order to elucidate preliminary hidden mathematical relations and patterns found in the seismological signals under study. Furthermore, the model is calibrated using reverse engineering and closes the loop from the data collection to initial hypothesis. In this way, the model formation is achieved. The presented simulation results qualitatively and quantitatively reveal some of the fundamental characteristics of each studied geographical region located in Greece that stem from its geodynamic properties. 相似文献
895.
896.
B. M. Schäfer 《Monthly notices of the Royal Astronomical Society》2008,388(3):1403-1408
The subject of this paper is the derivation of the integrated Sachs–Wolfe (iSW) effect in cosmologies with coupled dark matter and dark energy fluids. These couplings influence the iSW effect in three ways: the Hubble function assumes a different scaling, the structure growth rate shows a different time evolution and, in addition, the Poisson equation, which relates the density perturbations to fluctuations in the gravitational potential, is changed, due to the violation of the scaling ρ∝ a −3 of the matter density ρ with scalefactor a . Exemplarily, I derive the iSW spectra for a model in which dark matter decays into dark energy, investigate the influence of the dark matter decay rate and the dark energy equation of state on the iSW signal, and discuss the analogies for gravitational lensing. Quite generally, iSW measurements should reach similar accuracy in determining the dark energy equation of state parameter and the coupling constant. 相似文献
897.
898.
河口海岸环境沉积速率研究方法 总被引:22,自引:0,他引:22
沉积速率是河口海岸沉积环境的重要参数,对于河口海岸地质历史时期沉积速率的研究,大多采用^14C年龄值计算;现代河口海岸沉积速率的研究方法较多,常见的有河流输沙法,海图对比法,GIS法,放射性同位素测年法等。^14C法结合考古,孢粉等方法,能比较真实地反映河口海岸地质历史时期的沉积速率,对于现代河口海岸沉积,由于GIS和DEM方法的发展和计算机功能的支持,使海图对比的方法克服了手工带来的误差,目前仍然是一种可以使用的方法。放射性同位素^210Pb,^137Cs,^239,240Pu法使沉积速率的计算趋于定量化,在一定时间区域范围内,多种同位素测年同时运用,相互印证,从而使河口现代沉积速率更加准确。 相似文献
899.
C. R. Kaiser 《Monthly notices of the Royal Astronomical Society》2006,367(3):1083-1094
Flat radio spectra with large brightness temperatures at the core of active galactic nuclei and X-ray binaries are usually interpreted as the partially self-absorbed bases of jet flows emitting synchrotron radiation. Here we extend previous models of jets propagating at large angles to our line of sight to self-consistently include the effects of energy losses of the relativistic electrons due to the synchrotron process itself and the adiabatic expansion of the jet flow. We also take into account energy gains through self-absorption. Two model classes are presented. The ballistic jet flows, with the jet material travelling along straight trajectories, and adiabatic jets. Despite the energy losses, both scenarios can result in flat emission spectra; however, the adiabatic jets require a specific geometry. No re-acceleration process along the jet is needed for the electrons. We apply the models to observational data of the X-ray binary Cygnus X-1. Both models can be made consistent with the observations. The resulting ballistic jet is extremely narrow with a jet opening angle of only 5 arcsec. Its energy transport rate is small compared to the time-averaged jet power and therefore suggests the presence of non-radiating protons in the jet flow. The adiabatic jets require a strong departure from energy equipartition between the magnetic field and the relativistic electrons. These models also imply a jet power of two orders of magnitude higher than the Eddington limiting luminosity of a 10-M⊙ black hole. The models put strong constraints on the physical conditions in the jet flows on scales well below achievable resolution limits. 相似文献
900.