全文获取类型
收费全文 | 6092篇 |
免费 | 580篇 |
国内免费 | 168篇 |
专业分类
测绘学 | 257篇 |
大气科学 | 643篇 |
地球物理 | 2171篇 |
地质学 | 2406篇 |
海洋学 | 356篇 |
天文学 | 472篇 |
综合类 | 195篇 |
自然地理 | 340篇 |
出版年
2022年 | 7篇 |
2021年 | 27篇 |
2020年 | 15篇 |
2019年 | 26篇 |
2018年 | 449篇 |
2017年 | 382篇 |
2016年 | 277篇 |
2015年 | 165篇 |
2014年 | 135篇 |
2013年 | 161篇 |
2012年 | 677篇 |
2011年 | 464篇 |
2010年 | 153篇 |
2009年 | 177篇 |
2008年 | 145篇 |
2007年 | 157篇 |
2006年 | 161篇 |
2005年 | 864篇 |
2004年 | 889篇 |
2003年 | 675篇 |
2002年 | 195篇 |
2001年 | 88篇 |
2000年 | 54篇 |
1999年 | 30篇 |
1998年 | 25篇 |
1997年 | 29篇 |
1996年 | 21篇 |
1995年 | 15篇 |
1994年 | 13篇 |
1993年 | 14篇 |
1992年 | 11篇 |
1991年 | 18篇 |
1990年 | 16篇 |
1989年 | 11篇 |
1988年 | 13篇 |
1987年 | 19篇 |
1986年 | 9篇 |
1985年 | 14篇 |
1984年 | 24篇 |
1983年 | 22篇 |
1982年 | 13篇 |
1981年 | 20篇 |
1980年 | 14篇 |
1979年 | 8篇 |
1978年 | 17篇 |
1977年 | 12篇 |
1976年 | 16篇 |
1975年 | 18篇 |
1973年 | 15篇 |
1969年 | 6篇 |
排序方式: 共有6840条查询结果,搜索用时 15 毫秒
71.
Simulation of multigaussian stochastic fields can be made after a Karhunen-Loéve expansion of a given covariance function.
This method is also called simulation by Empirical Orthogonal Functions. The simulations are made by drawing stochastic coefficients
from a random generator. These numbers are multiplied with eigenfunctions and eigenvalues derived from the predefined covariance
model. The number of eigenfunctions necessary to reproduce the stochastic process within a predefined variance error, turns
out to be a cardinal question. Some ordinary analytical covariance functions are used to evaluate how quickly the series of
eigenfunctions can be truncated. This analysis demonstrates extremely quick convergence to 99.5% of total variance for the
2nd order exponential (‘gaussian’) covariance function, while the opposite is true for the 1st order exponential covariance
function. Due to these convergence characteristics, the Karhunen-Loéve method is most suitable for simulating smooth fields
with ‘gaussian’ shaped covariance functions. Practical applications of Karhunen-Loéve simulations can be improved by spatial
interpolation of the eigenfunctions. In this paper, we suggest interpolation by kriging and limits for reproduction of the
predefined covariance functions are evaluated. 相似文献
72.
73.
We study the importance of the zones of weakness and the pattern of downgoing flow in steady-state models of subducting lithosphere, which interacts mechanically and thermally with the ambient mantle. The non-linear system of governing equations consists of (i) the momentum equation in stream function formulation and (ii) the steady-state heat transfer equation including conduction and advection of heat and dissipation. A finite element method has been applied to this system. We consider the viscosity to be a non-linear function of both the temperature and the stream function. In steady-state two-dimensional (2D) flow, the stream function isolines follow material trajectories. They are used to follow the top of the subducting slab, which because of its possible increase in water content, is assumed to have a lower viscosity. The zone of weakness has been thus obtained in the self-consistent fashion since the stream function as well as the temperature are the output from our modeling and no a priori assumptions about the shape of the bending lithosphere are taken into account. It was shown that several orders decrease of viscosity in the zone of weakness is required to obtain the dip angle of about 45°. If the decrease of viscosity is not sufficient enough, the subducted slab either sinks almost vertically or does not exhibit a plate-like behavior. We have also demonstrated that shear heating can unrealistically increase at the zone of weakness for fast subductions if decrease of viscosity is underestimated. 相似文献
74.
A Modeling Study of the Effects of Anomalous Snow Cover over the Tibetan Plateau upon the South Asian Summer Monsoon 总被引:1,自引:0,他引:1
The effect of anomalous snow cover over the Tibetan Plateau upon the South Asian summer monsoon is investigated by numerical simulations using the NCAR regional climate model (RegCM2) into which gravity wave drag has been introduced. The simulations adopt relatively realistic snow mass forcings based on Scanning Multi-channel Microwave Radiometer (SNINIR) pentad snow depth data. The physical mechanism and spatial structure of the sensitivity of the South Asian early summer monsoon to snow cover anomaly over the Tibetan Plateau are revealed. The main results are summarized as follows. The heavier than normal snow cover over the Plateau can obviously reduce the shortwave radiation absorbed by surface through the albedo effect, which is compensated by weaker upward sensible heat flux associated with colder surface temperature, whereas the effects of snow melting and evaporation are relatively smaller.The anomalies of surface heat fluxes can last until June and become unobvious in July. The decrease of the Plateau surface temperature caused by heavier snow cover reaches its maximum value from late April to early May. The atmospheric cooling in the mid-upper troposphere over the Plateau and its surrounding areas is most obvious in May and can keep a fairly strong intensity in June. In contrast, there is warming to the south of the Plateau in the mid-lower troposphere from April to June with a maximum value in May.The heavier snow cover over the Plateau can reduce the intensity of the South Asian summer monsoon and rainfall to some extent, but this influence is only obvious in early summer and almost disappears in later stages. 相似文献
75.
Francis Leo Collins 《New Zealand geographer》2004,60(1):52-59
This paper discusses some of the ways that international students, particularly those from East Asia, are involved in the transformation of urban space in Auckland City. Using existing economic reports, local media accounts and the author's initial observations, the paper identifies how the embodied and linguistic practices of international students are involved in the changing landscapes of Auckland's inner city. This cursory reading serves to identify a research agenda and to reflect upon some key theoretical textxs. The insights of Arjun Appadurai, Michael Peter Smith and Ulrich Beck are used to suggest that the changes in Auckland, whilst certainly local, are also influenced fry factors that emerge from beyond the borders of international students' origin communities, cities and nations. 相似文献
76.
77.
The phase relations of quaternary systems are generally represented by projections onto ternary compositional planes. Such
projections often obscure relationships that would only be evident in a three-dimensional tetrahedral plot. The tetrahedral
plot requires that compositions of the minerals and melts be transformed into Cartesian coordinates. It is shown here how
this transformation is carried out. The application is demonstrated by tetrahedral plots of experimental melt compositions
of partially molten lherzolite. Furthermore, the plot can be used to evaluate whether or not a particular basaltic composition
represents a primary melt. The methods are applicable to any four-component system. 相似文献
78.
High-resolution historical records from Pettaquamscutt River basin sediments: 2. Pb isotopes reveal a potential new stratigraphic marker 总被引:2,自引:0,他引:2
Ana Lúcia Lima Bridget A. Bergquist Matthew K. Reuer Francis O. Dudas Timothy I. Eglinton 《Geochimica et cosmochimica acta》2005,69(7):1813-1824
A high-resolution record of Pb deposition in Rhode Island over the past 250 yr was constructed using a sediment core from the anoxic Pettaquamscutt River basin. The sedimentary Pb concentration record shows the well-described maximum associated with leaded gasoline usage in the United States. Diminished Pb variability during recorded periods of local industrial activity (1735 to 1847) supports the greater importance of regional atmospheric lead transport vs. local inputs. The Pb isotopic composition at this site shows a clear maximum in anthropogenic 206Pb/207Pb in the mid-1800s. Similar peaks have also been observed in sediments from Chesapeake Bay and the Great Lakes, suggesting a common source. Possible causes for this event include mining and smelting of Pb ores in the Upper Mississippi Valley district, which accounted for almost all Pb production in the United States in that period. The timing of this event can provide an important stratigraphic marker for sediments deposited in the past 200 yr in the Northeastern United States. The downcore profile of anthropogenic 206Pb/207Pb provides a classic example of how changes in the mixture of ores for production of tetraethyl lead caused a regional-scale shift in the sedimentary record, and suggests that coal could have played a secondary role in Pb emissions after 1920. 相似文献
79.
13C/12C and 18O/16O ratios of aragonite shells of modern land snails from the southern Great Plains of North America were measured for samples from twelve localities in a narrow east-west corridor that extended from the Flint Hills in North Central Oklahoma to the foothills of the Sangre de Cristo Mountains in Northern New Mexico, USA. Across the study area, shell δ18O values (PDB scale) ranged from −4.1‰ to 1.2‰, while δ13C values ranged from −13.2‰ to 0.0‰. δ18O values of the shell aragonite were predicted with a published, steady state, evaporative flux balance model. The predicted values differed (with one exception) by less than 1‰ from locality averages of measured δ18O values. This similarity suggests that relative humidity at the time of snail activity is an important control on the δ18O values of the aragonite and emphasizes the seasonal nature of the climatic information preserved in the shells. Correlated δ13C values of coexisting Vallonia and Gastrocopta suggest similar feeding habits and imply that these genera can provide information on variations in southern Great Plains plant ecology. Although there is considerable scatter, multispecies, transect average δ13C values of the modern aragonite shells are related to variations in the type of photosynthesis (i.e., C3, C4) in the local plant communities. The results of this study emphasize the desirability of obtaining isotope ratios representing averages of many shells in a locale to reduce possible biases associated with local variations among individuals, species, etc., and thus better represent the “neighborhood” scale temporal and/or spatial environmental variations of interest in studies of modern and ancient systems. 相似文献
80.
Segmentation and kinematics of the North America‐Caribbean plate boundary offshore Hispaniola 下载免费PDF全文
Sylvie Leroy Nadine Ellouz‐Zimmermann Jordane Corbeau Frédérique Rolandone Bernard Mercier de Lépinay Bertrand Meyer Roberte Momplaisir José‐Luis Granja Bruña Anne Battani Céline Baurion Evgueni Burov Valérie Clouard Rémy Deschamps Christian Gorini Youri Hamon Manfred Lafosse Jottin Leonel Laetitia Le Pourhiet Pilar Llanes Estrada Nicolas Loget Francis Lucazeau Daniel Pillot Jeffrey Poort Kevin R. Tankoo José‐Luis Cuevas José‐Fernando Alcaide Claude Jean Poix Alfonso Muñoz‐Martin Serge Mitton Yamil Rodriguez Julien Schmitz Leonardo Seeber Andres Carbo‐Gorosabel Santiago Muñoz 《地学学报》2015,27(6):467-478
We explored the submarine portions of the Enriquillo–Plantain Garden Fault zone (EPGFZ) and the Septentrional–Oriente Fault zone (SOFZ) along the Northern Caribbean plate boundary using high‐resolution multibeam echo‐sounding and shallow seismic reflection. The bathymetric data shed light on poorly documented or previously unknown submarine fault zones running over 200 km between Haiti and Jamaica (EPGFZ) and 300 km between the Dominican Republic and Cuba (SOFZ). The primary plate‐boundary structures are a series of strike‐slip fault segments associated with pressure ridges, restraining bends, step overs and dogleg offsets indicating very active tectonics. Several distinct segments 50–100 km long cut across pre‐existing structures inherited from former tectonic regimes or bypass recent morphologies formed under the current strike‐slip regime. Along the most recent trace of the SOFZ, we measured a strike‐slip offset of 16.5 km, which indicates steady activity for the past ~1.8 Ma if its current GPS‐derived motion of 9.8 ± 2 mm a?1 has remained stable during the entire Quaternary. 相似文献