全文获取类型
收费全文 | 1531篇 |
免费 | 48篇 |
国内免费 | 26篇 |
专业分类
测绘学 | 31篇 |
大气科学 | 127篇 |
地球物理 | 350篇 |
地质学 | 490篇 |
海洋学 | 164篇 |
天文学 | 337篇 |
综合类 | 5篇 |
自然地理 | 101篇 |
出版年
2021年 | 11篇 |
2020年 | 23篇 |
2019年 | 26篇 |
2018年 | 20篇 |
2017年 | 28篇 |
2016年 | 39篇 |
2015年 | 30篇 |
2014年 | 33篇 |
2013年 | 84篇 |
2012年 | 42篇 |
2011年 | 67篇 |
2010年 | 58篇 |
2009年 | 62篇 |
2008年 | 69篇 |
2007年 | 64篇 |
2006年 | 70篇 |
2005年 | 48篇 |
2004年 | 39篇 |
2003年 | 38篇 |
2002年 | 48篇 |
2001年 | 53篇 |
2000年 | 50篇 |
1999年 | 31篇 |
1998年 | 39篇 |
1997年 | 35篇 |
1996年 | 27篇 |
1995年 | 33篇 |
1994年 | 19篇 |
1993年 | 18篇 |
1992年 | 18篇 |
1991年 | 11篇 |
1990年 | 23篇 |
1989年 | 20篇 |
1988年 | 22篇 |
1987年 | 22篇 |
1986年 | 19篇 |
1985年 | 11篇 |
1984年 | 24篇 |
1983年 | 21篇 |
1982年 | 14篇 |
1981年 | 23篇 |
1980年 | 15篇 |
1979年 | 31篇 |
1978年 | 21篇 |
1977年 | 21篇 |
1976年 | 11篇 |
1975年 | 11篇 |
1974年 | 8篇 |
1973年 | 9篇 |
1972年 | 7篇 |
排序方式: 共有1605条查询结果,搜索用时 15 毫秒
101.
102.
Infrared absorption spectra of the high-pressure polymorphs β-Mg2SiO4 and β-Co2SiO4 have been measured between 0 and 27 GPa at room temperature. Grüneisen parameters determined for 11 modes of β-Mg2SiO4 (frequencies of 300 to 1,050 cm?1) and 5 modes of β-Co2SiO4 (490 to 1,050 cm?1) range between 0.8 and 1.9. Averaging the mid-infrared spectroscopic data for β-Mg2SiO4 yields an average Grüneisen parameter of 1.3 (±0.1), in good agreement with the high-temperature thermodynamic value of 1.35. Similarly, we find a value of 1.05 (±0.2) for the average spectroscopic Grüneisen parameter of β-Co2SiO4. 相似文献
103.
Catastrophic events reveal the dynamic nature of salt-marsh vegetation in Southern California 总被引:1,自引:0,他引:1
Joy B. Zedler Jordan Covin Chris Nordby Phil Williams John Boland 《Estuaries and Coasts》1986,9(1):75-80
Recent hydrological disturbances, including flooding, dry-season streamflow, and drought, greatly altered coastal wetland habitats in sourthern California. At Tijuana Estuary, a six-year study of salt-marsh vegetation patterns during these rare conditions documented substantial temporal variability in plant growth and distribution. Important to cordgrass (Spartina foliosa Trin.) dynamics were the amount and timing of streamflows, which reduced soil salinity and alleviated stresses on plant growth. Poorest growing conditions occurred in 1984 when both river and tidal flows were lacking; soils had low moisture and extreme salinities (avg.=104‰ in September). Plant stress was documented in 1984 as high mortality (62% fewer stems than in 1983) and reduced height (19% less than in 1983). Cordgrass height was greatest in 1980 following winter flooding (20% increase over 1979); densities were greatest in 1983 with summer freshwater influxes (60% increase over 1982). A carbon allocation model is proposed to explain the varied responses. 相似文献
104.
105.
Jiahao Lin Guizhen Song Yong Sun F. W. Williams 《Soil Dynamics and Earthquake Engineering》1995,14(4)
In this paper, a direct Ritz method is presented for analysing the responses of non-uniform shear beams subjected to the action of an evolutionary random seismic excitation. Because of the use of a pseudo ground acceleration, time-dependent random responses can conveniently be calculated by solving a set of deterministic equations of motion. These responses include power spectral densities, variances and higher spectral moments of any required responses of the beam. A non-uniform beam example was solved and the time-dependent variances and second spectral moments of both the beam-top displacement and the beam-base shear force are given to show the simplicity and effectiveness of the method, which can also easily be applied to other continuous elastic structures. 相似文献
106.
C. C. Van Valin R. F. Pueschel E. W. Barrett G. M. Williams 《Boundary-Layer Meteorology》1982,24(3):331-343
In a study using the plume from the Four Corners power plant, near Farmington, N.M., lee waves were observed during times when the plume flowed across the Hogback. Wavelengths were typically about 1.2 km; wave amplitudes were more variable, ranging from 20 to 100 m. The observed amplitudes imply an obstacle that is broader and shallower than is actually the case. This is in agreement with laboratory studies that show the existence of regions of complex flow both upstream and downstream from an obstacle, which have the effect of broadening the region over which laminar flow occurs. Visual observation, measurement of the plume cross-sectional area both upstream and downstream from the Hogback, and measurement of plume aerosol concentrations show that turbulent and eddy flow over and downwind from the Hogback increase the rate of mixing of the plume with the surrounding atmosphere. This in turn increases the rate at which plume components come into contact with the ground. 相似文献
107.
David W. Graham Michael L. Bender Douglas F. Williams Lloyd D. Keigwin 《Geochimica et cosmochimica acta》1982,46(7):1281-1292
The ratio and other parameters have been measured in fossil planktonic foraminifera from the Atlantic and Pacific Ocean basins in order to evaluate the ratio of seawater during the last 75 million years. Results on well-preserved samples indicate that the ratio has increased to its present value by 10–15% during the Cenozoic, and that minima occurred between 55-45 Ma and 10-5 Ma, when the ratio was 15–25% less than at present. The long-term increase may reflect either decreasing deposition of aragonite with a high ratio in shallow seas, or decreasing seafloor spreading rates and consequently decreasing hydrothermal supply of Ca during the Cenozoic. Other geologic evidence suggests that the Eocene minimum (near 50 Ma) may have resulted from increased aragonite sedimentation, while the Late Miocene minimum (between 10-5 Ma) may have been caused by an increased rate of seawater-basalt exchange when seafloor spreading rates increased on the East Pacific Rise near 10 Ma. 相似文献
108.
109.
Overwash is a major component of a barrier island's response to high energy conditions. Examination of the 19-20 March, 1975, storm deposits at Assateague Island, showed that an overwash sequence could be divided into sedimentation units. Interpretation depends upon defining textural and mineralogical combinations that can be logically ordered as a vertical sequence. If the sequence is depositional and formed from a single material, normal grading usually results; if polymineralogical and erosionally truncated, inverse grading with a top layer of heavy minerals can be formed via an in situ sorting process dependent on disturbance depth and overwash influence. 相似文献
110.
A numerical model has been developed for the solution of the kinematic wave equation for a one-dimensional catchment, using the Lax-Wendroff technique. The model has been verified by comparing its solutions with some analytical solutions already available. Certain catchment parameters have been identified and the model applied with promising results to two actual catchments to ascertain their values. 相似文献