全文获取类型
收费全文 | 824篇 |
免费 | 116篇 |
国内免费 | 249篇 |
专业分类
测绘学 | 14篇 |
大气科学 | 36篇 |
地球物理 | 353篇 |
地质学 | 565篇 |
海洋学 | 167篇 |
天文学 | 5篇 |
综合类 | 11篇 |
自然地理 | 38篇 |
出版年
2024年 | 7篇 |
2023年 | 6篇 |
2022年 | 10篇 |
2021年 | 24篇 |
2020年 | 28篇 |
2019年 | 41篇 |
2018年 | 49篇 |
2017年 | 59篇 |
2016年 | 42篇 |
2015年 | 47篇 |
2014年 | 58篇 |
2013年 | 78篇 |
2012年 | 44篇 |
2011年 | 50篇 |
2010年 | 53篇 |
2009年 | 67篇 |
2008年 | 49篇 |
2007年 | 71篇 |
2006年 | 62篇 |
2005年 | 45篇 |
2004年 | 45篇 |
2003年 | 30篇 |
2002年 | 30篇 |
2001年 | 26篇 |
2000年 | 22篇 |
1999年 | 27篇 |
1998年 | 22篇 |
1997年 | 17篇 |
1996年 | 10篇 |
1995年 | 9篇 |
1994年 | 11篇 |
1993年 | 8篇 |
1992年 | 7篇 |
1991年 | 5篇 |
1990年 | 5篇 |
1989年 | 4篇 |
1988年 | 1篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1973年 | 2篇 |
1972年 | 2篇 |
排序方式: 共有1189条查询结果,搜索用时 31 毫秒
71.
Late-glacial lake sediments containing the Laacher See Tephra (LST, 11 000 yr B.P.) have been analyzed for their pollen and diatom content at three sites at varying distances from the volcano and on different bedrock geologies. The aim was to test the null hypothesis that this major volcanic eruption had no effect on terrestrial pollen or aquatic diatom assemblages. The pollen spectra at all sites show a short-lived increase in grass pollen following the LST. Partial redundancy analysis and associated Monte Carlo permutation tests suggest, however, that the LST had no statistically significant effect at two sites but it had a statistically significant impact on the pollen assemblages at the site nearest (60 km) to the volcano.The diatom assemblages at the three sites changed individualistically after the LST deposition, with increases inAchnanthes minutissima at one site, an expansion ofAulacoseira species at another, and an increase ofAsterionella formosa andFragilaria brevistriata at the third site. Partial redundancy analysis and associated permutation tests suggest a statistically significant change in diatoms in relation to the LST and associated changes in sediment lithology at the one site situated on acidic bedrock. No significant impacts were found at the sites on volcanic or calcareous rocks. Due to the interaction between tephra and sediment lithology, it is not possible to conclude if the statistically significant diatom changes were a direct result of the LST deposition or an indirect result of lithological changes following LST deposition.This is the first paper in a series of papers published in this issue on high-resolution paleolimnology. These papers were presented at the Sixth International Paeolimnology Symposium held 19–21 April, 1993 at the Australian National University, Canberra, Australia. Dr A. F. Lotter and Dr. M. Sturm served as guest editors for these papers. 相似文献
72.
Determination of shear strength of conglomerates using a caterpillar D9 ripper and comparison with alternative methods 总被引:1,自引:0,他引:1
J. P. Savely 《Geotechnical and Geological Engineering》1990,8(3):203-225
Summary A method of determining shear strength in Gila Conglomerates at Inspiration Mine (now Cyprus Miami Mine), Arizona, using Caterpillar D9H and D9L rippers is described. The results are compared with data obtained using laboratory direct shear tests — both large and small scale, back analysis and boulder modelling. The significance of scale, particularly relating to boulder size is discussed. 相似文献
73.
We have theoretically considered the problem of interpretation of nutrient profiles in the upper ocean (100–2000m). We compare
the experimental depth profiles of nonconservative tracers, both stable and radioactive, with solutions of one-dimensional
steady state transport equations of increasing complexity including situations not encountered in the real oceans. Apart from
gaining insight into the nutrient transport processes, this analysis is useful in offering a way to obtain operational estimates
of depth dependent/independent eddy diffusivity and dissolution fluxes in the ocean. These parameters are essential for estimating
new production, total production and burial of carbon in the sediments. 相似文献
74.
Sitharam G. Thallak Sireesh Saride Sujit Kumar Dash 《Geotechnical and Geological Engineering》2007,25(5):509-524
This paper presents the results of laboratory model tests carried out to develop an understanding of the behaviour of geocell-reinforced
soft clay foundations under circular loading. Natural silty clay was used in this study. The geocells were prepared using
biaxial polymer grid. The performance of the reinforced bed is quantified using non-dimensional factors i.e., Bearing capacity
improvement factor (If) and Percentage reduction in footing settlement (PRS). The test results demonstrate that the geocell mattress redistributes
the footing load over a wider area thereby improving the performance of the footing. The load carrying capacity of the clay
bed is increased by a factor of up to about 4.5 times that of unreinforced bed. From the pressure-settlement responses, it
is observed that the geocell-reinforced foundation bed behaves as a much stiffer system compared to the unreinforced case
indicating that a substantial reduction in footing settlement can be achieved by providing geocell reinforcement in the soft
clay bed. The maximum reduction in footing settlement obtained with the provision of geocell mattress of optimum size placed
close to the footing is around 90%. Further improvement in performance is obtained with provision of an additional planar
geogrid layer at the base of the geocell mattress. 相似文献
75.
There is an exceptionally rich aquatic fauna in the epikarst, the skin of karst. High species richness in the epikarst, coupled
with its special vulnerability as the first point of entry of nearly all toxic spills, makes its protection especially important.
The epikarst fauna may also be an useful tool in tracing the potential route of pollutants. Copepods in epikarst have extremely
local distributions, and their body size is such that they are largely at the mercy of directional flows. In a series of caves
in southwest Slovenia and West Virginia, a significant fraction of the copepod species occur in less than 100 m of linear
extent. This suggests a pattern of highly restricted lateral flow under normal conditions and the distribution of copepods
could potentially be used to trace water movement. Under high flow conditions as would often be the case with toxic spill,
mounding of water may increase the lateral radius of flow. Nevertheless, copepods may be useful tracers. 相似文献
76.
Sokrat Amataj Todor Anovski Ralf Benischke Romeo Eftimi Laurence L. Gourcy Liliana Kola Ioannis Leontiadis Eftim Micevski Alqiviadis Stamos Jovan Zoto 《Environmental Geology》2007,51(5):749-753
Prespa Lake and Ohrid Lake constitute a hydraulic system shared between Albania, FYR of Macedonia and Greece. Karst rocks
separate both lakes. The elevation of Prespa Lake is about 150 m higher than that of Ohrid Lake. Considering these facts,
Cvijić formulated in 1906 the hypothesis that Prespa Lake recharges the St. Naum and Tushemisht springs at Ohrid lakeside.
Environmental isotopes demonstrated that Prespa Lake recharges about 37–42 and 52–54% of water emerging in St. Naum, and Tushemisht
springs, respectively. An artificial tracer experiment carried out in 2002 physically demonstrated the underground connection
between both lakes. This experiment confirmed the supposed underground connection and brought important information about
the groundwater velocity, transit time, and karst water conduits development. 相似文献
77.
Transport and deposition of suspended particles in saturated porous media: hydrodynamic effect 总被引:1,自引:1,他引:1
Nasre-Dine Ahfir Hua Qing Wang Ahmed Benamar Adbellah Alem Nicolas Massei Jean-Paul Dupont 《Hydrogeology Journal》2007,15(4):659-668
A laboratory study was undertaken to determine the transport and deposition rate of suspended particles in columns of saturated porous media (gravel and glass beads), where the porous media were subjected to steady-state flow. Silt particles with a mode of 14 μm diameter (used as the suspended particles) and fluorescein (as the conservative tracer) were injected into the columns in short pulses. The breakthrough curves were competently described with the analytical solution of a convection–dispersion equation with a first-order deposition rate. The experiments were performed using different flow rates. The suspended particle size distribution, the porous media, and the flow rates themselves were the main factors retained in this study to investigate the mechanisms governing the transport and deposition kinetics in detail. The results showed the existence of a flow rate, beyond which suspended particles travel faster than the conservative tracer. A decrease of the deposition rate of suspended particles beyond a critical flow velocity was also observed. Such behaviour led to consideration of the couple hydrodynamic-gravity forces at high flow rates. As the hydrodynamic force increases, particle deposition rates are reduced due to the effect of hydrodynamic forces inhibiting the deposition. 相似文献
78.
This paper presents a new method, called the equivalent force control method, for solving the nonlinear equations of motion in a real‐time substructure test using an implicit time integration algorithm. The method replaces the numerical iteration in implicit integration with a force‐feedback control loop, while displacement control is retained to control the motion of an actuator. The method is formulated in such a way that it represents a unified approach that also encompasses the effective force test method. The accuracy and effectiveness of the method have been demonstrated with numerical simulations of real‐time substructure tests with physical substructures represented by spring and damper elements, respectively. The method has also been validated with actual tests in which a Magnetorheological damper was used as the physical substructure. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
79.
80.
Conservative tracers are necessary to obtain groundwater transport velocities at the field scale. Deuterated water is an
effective tracer for this purpose due to its similarity to water, chemical stability, non-reactivity, ease of handling and
sampling, relatively neutral buoyancy, and reasonable price. Reliable detection limits of 0.1 mg deuterium/L may be obtained
in field tests. A field example is presented in which deuterated water, bromide, and pentafluorobenzoic acid are used as groundwater
tracers. Deuterated water appeared to be transported conservatively, producing almost identical breakthrough curves as that
of other soluble tracers.
Electronic Publication 相似文献