全文获取类型
收费全文 | 12566篇 |
免费 | 476篇 |
国内免费 | 145篇 |
专业分类
测绘学 | 302篇 |
大气科学 | 955篇 |
地球物理 | 2823篇 |
地质学 | 4437篇 |
海洋学 | 1142篇 |
天文学 | 2017篇 |
综合类 | 30篇 |
自然地理 | 1481篇 |
出版年
2021年 | 133篇 |
2020年 | 164篇 |
2019年 | 182篇 |
2018年 | 240篇 |
2017年 | 234篇 |
2016年 | 320篇 |
2015年 | 300篇 |
2014年 | 296篇 |
2013年 | 708篇 |
2012年 | 386篇 |
2011年 | 524篇 |
2010年 | 457篇 |
2009年 | 597篇 |
2008年 | 501篇 |
2007年 | 448篇 |
2006年 | 472篇 |
2005年 | 385篇 |
2004年 | 434篇 |
2003年 | 412篇 |
2002年 | 407篇 |
2001年 | 251篇 |
2000年 | 261篇 |
1999年 | 233篇 |
1998年 | 229篇 |
1997年 | 159篇 |
1996年 | 182篇 |
1995年 | 175篇 |
1994年 | 182篇 |
1993年 | 167篇 |
1992年 | 155篇 |
1991年 | 175篇 |
1990年 | 151篇 |
1989年 | 133篇 |
1988年 | 129篇 |
1987年 | 184篇 |
1986年 | 144篇 |
1985年 | 221篇 |
1984年 | 263篇 |
1983年 | 202篇 |
1982年 | 179篇 |
1981年 | 195篇 |
1980年 | 163篇 |
1979年 | 172篇 |
1978年 | 175篇 |
1977年 | 147篇 |
1976年 | 145篇 |
1975年 | 125篇 |
1974年 | 103篇 |
1973年 | 118篇 |
1972年 | 76篇 |
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
361.
362.
New field, geochronological, geochemical and biostratigraphical data indicate that the central and northern parts of the Cordillera Occidental of the Andes of Ecuador comprise two terranes. The older (Pallatanga) terrane consists of an early to late (?) Cretaceous oceanic plateau suite, late Cretaceous marine turbidites derived from an unknown basaltic to andesitic volcanic source, and a tectonic mélange of probable late Cretaceous age. The younger (Macuchi) terrane consists of a volcanosedimentary island arc sequence, derived from a basaltic to andesitic source. A previously unidentified, regionally important dextral shear zone named the Chimbo-Toachi shear zone separates the two terranes. Regional evidence suggests that the Pallatanga terrane was accreted to the continental margin (the already accreted Cordillera Real) in Campanian times, producing a tectonic mélange in the suture zone. The Macuchi terrane was accreted to the Pallatanga terrane along the Chimbo-Toachi shear zone during the late Eocene, probably in a dextral shear regime. The correlation of Cretaceous rocks and accretionary events in the Cordillera Occidental of Ecuador and Colombia remains problematical, but the late Eocene event is recognised along the northern Andean margin. 相似文献
363.
Zhangxin Chen Richard E. Ewing Hao Lu Stephen L. Lyons Serguei Maliassov Michael B. Ray Tong Sun 《Computational Geosciences》2002,6(3-4):545-564
In this paper we employ mixed finite elements and numerically study an integrated two-dimensional model of fluid flow and compaction in a sedimentary basin. This model describes a single phase incompressible flow in a two-dimensional section of a sedimentary basin with vertical compaction. At each time step, an iterative algorithm is used to solve this model. The determination of the grid movement is based on the mass conservation and movement of sediments in the basin, while the mixed method is utilized to solve the fluid flow over the moving grid. Numerical experiments are presented to verify this iterative algorithm and show representative solutions for the model under consideration. 相似文献
364.
High-frequency (≥2 Hz) Rayleigh wave phase velocities can be inverted to shear (S)-wave velocities for a layered earth model up to 30 m below the ground surface in many settings. Given S-wave velocity (VS), compressional (P)-wave velocity (VP), and Rayleigh wave phase velocities, it is feasible to solve for P-wave quality factor QP and S-wave quality factor QS in a layered earth model by inverting Rayleigh wave attenuation coefficients. Model results demonstrate the plausibility of inverting QS from Rayleigh wave attenuation coefficients. Contributions to the Rayleigh wave attenuation coefficients from QP cannot be ignored when Vs/VP reaches 0.45, which is not uncommon in near-surface settings. It is possible to invert QP from Rayleigh wave attenuation coefficients in some geological setting, a concept that differs from the common perception that Rayleigh wave attenuation coefficients are always far less sensitive to QP than to QS. Sixty-channel surface wave data were acquired in an Arizona desert. For a 10-layer model with a thickness of over 20 m, the data were first inverted to obtain S-wave velocities by the multichannel analysis of surface waves (MASW) method and then quality factors were determined by inverting attenuation coefficients. 相似文献
365.
366.
367.
Thomas K. Nielsen Hans Christian Larsen John R. Hopper 《Earth and Planetary Science Letters》2002,200(3-4):271-286
We present new and reprocessed seismic reflection data from the area where the southeast and southwest Greenland margins intersected to form a triple junction south of Greenland in the early Tertiary. During breakup at 56 Ma, thick igneous crust was accreted along the entire 1300-km-long southeast Greenland margin from the Greenland Iceland Ridge to, and possibly 100 km beyond, the triple junction into the Labrador Sea. However, highly extended and thin crust 250 km to the west of the triple junction suggests that magmatically starved crustal formation occurred on the southwest Greenland margin at the same time. Thus, a transition from a volcanic to a non-volcanic margin over only 100–200 km is observed. Magmatism related to the impact of the Iceland plume below the North Atlantic around 61 Ma is known from central-west and southeast Greenland. The new seismic data also suggest the presence of a small volcanic plateau of similar age close to the triple junction. The extent of initial plume-related volcanism inferred from these observations is explained by a model of lateral flow of plume material that is guided by relief at the base of the lithosphere. Plume mantle is channelled to great distances provided that significant melting does not take place. Melting causes cooling and dehydration of the plume mantle. The associated viscosity increase acts against lateral flow and restricts plume material to its point of entry into an actively spreading rift. We further suggest that thick Archaean lithosphere blocked direct flow of plume material into the magma-starved southwest Greenland margin while the plume was free to flow into the central west and east Greenland margins. The model is consistent with a plume layer that is only moderately hotter, 100–200°C, than ambient mantle temperature, and has a thickness comparable to lithospheric thickness variations, 50–100 km. Lithospheric architecture, the timing of continental rifting and viscosity changes due to melting of the plume material are therefore critical parameters for understanding the distribution of magmatism. 相似文献
368.
Xiaoxian Zhang A. Glyn Bengough John W. Crawford Iain M. Young 《Journal of Hydrology》2002,260(1-4):75-87
The non-linear solvers in numerical solutions of water flow in variably saturated soils are prone to convergence difficulties. Many aspects can give rise to such difficulties and in this paper we address the gravity term and the prescribed-flux boundary in the Picard iteration. The problem of the gravity term in the Picard iteration is iteration-to-iteration oscillation as the gravity term is treated, by analogy with the time-step advance technique, ‘explicitly’ in the iteration. The proposed method for the gravity term is an improvement of the ‘implicit’ approach of Zhang and Ewen [Water Resour. Res. 36 (2000) 2777] by extending it to heterogeneous soil and approximating the inter-nodal hydraulic conductivity in the diffusive term and the gravity term with the same scheme. The prescribed-flux boundary in traditional methods also gives rise to iteration-to-iteration oscillation because there is no feedback to the flux in the solution at the new iteration. To reduce such oscillation, a new method is proposed to provide such a feedback to the flux. Comparison with traditional Picard and Newton iteration methods for a wide range of problems show that a combination of these two proposed methods greatly improves the stability and consequently the computational efficiency, making the use of small time step and/or under-relaxation solely for convergence unnecessary. 相似文献
369.
Climate Suitability: For Stable Malaria Transmission in Zimbabwe Under Different Climate Change Scenarios 总被引:1,自引:0,他引:1
Jessica Hartman Kristie Ebi K. John McConnell Nathan Chan John Weyant 《Global Change & Human Health》2002,3(1):42-54
As climate is one factor determining the potential range of malaria, climate change may work with or against efforts to bring
malaria under control. We developed a model of future climate suitability for stable Plasmodium falciparum malaria transmission in Zimbabwe. Current climate suitability for stable malaria transmission is based on the MARA/ARMA model
of climatic constraints on the survival and development of the Anopheles vector and the Plasmodium falciparum malaria parasite. We explored potential future geographic distributions of malaria using sixteen projections of climate in
2100. The results suggest that, assuming no future human-imposed constraints on malaria transmission, changes in temperature
and precipitation could alter the geographic distribution of malaria in Zimbabwe, with previously unsuitable areas of dense
human population becoming suitable for transmission. Among all scenarios, the highlands become more suitable for transmission,
while the lowveld and areas currently limited by precipitation show varying degrees of change, depending on climate sensitivity
and greenhouse gas emission stabilization scenarios, and depending on the general circulation model used. The methods employed
can be used within or across other African countries.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
370.
Richard Eckersley 《Global Change & Human Health》2002,3(1):55-57
Bjorn Lomborg's critique of environmentalism has been praised by The Economist as ‘right’ on its main points and ‘just’ in
its criticism of much green activism, and damned by Scientific American as a ‘failure’ in its purpose of describing the state
of the world. Most of the debate has centred on Lomborg's treatment of environmental issues. But the book is about more than
the environment, and its flaws extend beyond his use and interpretation of the data.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献