全文获取类型
收费全文 | 395篇 |
免费 | 16篇 |
国内免费 | 2篇 |
专业分类
测绘学 | 6篇 |
大气科学 | 27篇 |
地球物理 | 128篇 |
地质学 | 113篇 |
海洋学 | 25篇 |
天文学 | 85篇 |
综合类 | 1篇 |
自然地理 | 28篇 |
出版年
2022年 | 5篇 |
2021年 | 4篇 |
2020年 | 10篇 |
2019年 | 7篇 |
2018年 | 11篇 |
2017年 | 7篇 |
2016年 | 10篇 |
2015年 | 11篇 |
2014年 | 7篇 |
2013年 | 14篇 |
2012年 | 14篇 |
2011年 | 6篇 |
2010年 | 17篇 |
2009年 | 14篇 |
2008年 | 16篇 |
2007年 | 18篇 |
2006年 | 9篇 |
2005年 | 12篇 |
2004年 | 4篇 |
2003年 | 12篇 |
2002年 | 6篇 |
2001年 | 6篇 |
2000年 | 8篇 |
1999年 | 5篇 |
1998年 | 4篇 |
1997年 | 6篇 |
1996年 | 3篇 |
1995年 | 8篇 |
1994年 | 12篇 |
1993年 | 9篇 |
1992年 | 4篇 |
1991年 | 7篇 |
1990年 | 6篇 |
1989年 | 4篇 |
1987年 | 6篇 |
1986年 | 7篇 |
1985年 | 13篇 |
1984年 | 7篇 |
1983年 | 5篇 |
1982年 | 9篇 |
1981年 | 9篇 |
1980年 | 3篇 |
1979年 | 8篇 |
1978年 | 6篇 |
1977年 | 8篇 |
1976年 | 6篇 |
1975年 | 4篇 |
1973年 | 6篇 |
1971年 | 5篇 |
1969年 | 4篇 |
排序方式: 共有413条查询结果,搜索用时 0 毫秒
101.
David K. Rea Jack Dymond G. Ross Heath Donald F. Heinrichs Stephen H. Johnson Donald M. Hussong 《Earth and Planetary Science Letters》1973,19(2):225-229
Detailed surveys of the crest of the East Pacific Rise at 6° and 11°S form the basis for estimates of sea-floor spreading rates of 8.2 and 8.3 cm/yr, respectively. These estimates are significantly higher than previous ones of 6.5 to 7.5 cm/yr. The high quality of the magnetic profile at 6°S allows identification of shorter magnetic events such as the two Olduvai events of Cox. DSDP drill-hole data indicate that the present spreading rates have persisted throughout the Cenozoic. 相似文献
102.
103.
Jack Hardisty 《地球表面变化过程与地形》1993,18(9):855-862
The use of time domain and frequency domain techniques in the analysis of geomorphological time series is discussed and the techniques of spectral analysis are introduced. An Excel worksheet is used to analyse wave-current data from Slapton Sands in Devon and the advantages and limitations of the technique are listed. 相似文献
104.
Robert K. Dixon Jack K. Winjum Kenneth J. Andrasko Jeffrey J. Lee Paul E. Schroeder 《Climatic change》1994,27(1):71-92
Degraded or sub-standard soils and marginal lands occupy a significant proportion of boreal, temperate and tropical biomes. Management of these lands with a wide range of existing, site-specific, integrated, agroforest systems represents a significant global opportunity to reduce the accumulation of greenhouse gases in the atmosphere. Establishment of extensive agricultural, agroforest, and alternative land-use systems on marginal or degraded lands could sequester 0.82–2.2 Pg carbon (C) per year, globally, over a 50-year time-frame. Moreover, slowing soil degradation by alternative grassland management and by impeding desertification could conserve up to 0.5–1.5 Pg C annually. A global analysis of biologic and economic data from 94 nations representing diverse climatic and edaphic conditions reveals a range of integrated land-use systems which could be used to establish and manage vegetation on marginal or degraded lands. Promising land-use systems and practices identified to conserve and temporarily store C include agroforestry systems, fuelwood and fiber plantations, bioreserves, intercropping systems, and shelterbelts/windbreaks. For example, successful establishment of low-intensity agroforestry systems can store up to 70 Mg C/ha in boreal, temperate and tropical ecoregions. The mean initial cost of soil rehabilitation and revegetation ranges from $500–3,000/ha for the 94 nations surveyed. Natural regeneration of woody vegetation or agro-afforestation establishment costs were less than $1000/ha in temperate and tropical regions. The costs of C sequestration in soil and vegetation systems range from $1-69/Mg C, which compares favorably with other options to reduce greenhouse gas emissions to the atmosphere. Although agroforestry system projects were recently established to conserve and sequester C in Guatemala and Malaysia, constraints to wide-spread implementation include social conditions (demographic factors, land tenure issues, market conditions, lack of infrastructure), economic obstacles (difficulty of demonstrating benefits of alternative systems, capital requirements, lack of financial incentives) and, ecologic considerations (limited knowledge of impacts and sustainability of some systems).The information in this document has been funded by the U.S. Environmental Protection Agency. It has been subject to the Agency's peer and administrative review, and it has been approved for publication as an EPA document. Mention of trade names or commercial products does not constitute endorsement for use. 相似文献
105.
Jack H. Shreffler 《Boundary-Layer Meteorology》1982,24(3):345-356
During the Regional Air Pollution Study (RAPS), winds in the St. Louis region were recorded by surface stations (hourly averages) and by multiple upper air releases (on-the-hour). This study analyzes differences (1) between upper air and surface resultant winds, (2) among upper air winds at 4 sites, and (3) among surface winds at 25 sites. The extensive data set provides a statistical basis for indicating the representativeness of individual observations, and the results show considerable variation between simultaneously-measured winds determined by alternate means. Based on Gaussian plume dispersion, the spatial variations in wind direction are translated into expected departures between predictions and observations of pollutant concentrations. Inaccurate specification of winds in air quality simulation models is likely to be a substantial contributor to differences between short-term predictions and observations on an urban scale.On assignment from the National Oceanic and Atmospheric Administration, U. S. Department of Commerce. 相似文献
106.
Willard S. Moore Teh-Lung Ku J. Douglas Macdougall Virginia Mee Burns Roger Burns Jack Dymond Mitchel W. Lyle David Z. Piper 《Earth and Planetary Science Letters》1981,52(1):151-171
Fluxes of metals to the top and bottom surfaces of a manganese nodule were determined by combining radiochemical (230Th,231Pa,232Th,238U,234U) and detailed chemical data. The top of the nodule had been growing in its collected orientation at 4.7 mm Myr?1 for at least 0.5 Myr and accreting Mn at 200 μg cm?2 kyr?1. The bottom of the nodule had been growing in its collected orientation at about 12 mm Myr?1 for at least 0.3 Myr and accreting Mn at about 700 μg cm?2 yr?1. Although the top of the nodule was enriched in iron relative to the bottom, the nodule had been accreting Fe 50% faster on the bottom.232Th was also accumulating more rapidly in the bottom despite a 20-fold enrichment of230Th on the top.The distribution of alpha-emitting nuclides calculated from detailed radiochemical measurements matched closely the pattern revealed by 109-day exposures of alpha-sensitive film to the nodule. However, the shape and slope of the total alpha profile with depth into the nodule was affected strongly by226Ra and222Rn migrations making the alpha-track technique alone an inadequate method of measuring nodule growth rates.Diffusion of radium in the nodule may have been affected by diagenetic reactions which produce barite, phillipsite and todorokite within 1 mm of the nodule surface; however, our sampling interval was too broad to document the effect. We have not been able to resolve the importance of nodule diagenesis on the gross chemistry of the nodule. 相似文献
107.
Jack Dymond Richard Cobler Louis Gordon Pierre Biscaye Guy Mathieu 《Earth and Planetary Science Letters》1983,64(3):417-429
Hydrothermal waters collected by “Alvin” from the Galapagos Spreading Center are enriched in222Rn by factors of 50–200 over bottom waters. The226Ra in the same samples, however, is enriched by less than a factor of four over bottom waters. Enrichments of222Rn result primarily from α-recoil from rock surfaces while226Ra enrichments are dominantly produced by high-temperature alteration of cooling ridge volcanics. The abundances of both nuclides exhibit positive correlations with temperature. The data extrapolate to bottom water temperatures and compositions, demonstrating the importance of seawater mixing. Different vents, however, have different mixing lines, and vents with high222Rn have low226Ra. We propose these patterns result from variations in the extent of low-temperature crustal interaction with the hydrothermal fluids. Low-temperature crustal waters can maintain high steady state222Rn contents due to the α-recoil additions to the fluids. The226Ra, however, is strongly adsorbed at low-temperatures resulting in low concentrations of this nuclide in low-temperature crustal waters. Thus, physical mixing of a crustal water component with hydrothermal waters or variable crustal path lengths of the hydrothermal fluids can account for the variable mixing lines and222Rn/226Ra values of the hot springs.The222Rn/226Ra value appears to be a sensitive indicator of low-temperature crustal interaction. Values > 100 have experienced extensive crustal interaction and are indicative of diffuse hydrothermal flow. Values between 1 and 10 are indicative of primary hydrothermal fluids which have not experienced significant interaction with the crust. Values of222Rn/226Ra between 103 and 104 are indicative of interaction of the hydrothermal fluids with sediments. Such values are observed in water samples from the Galapagos hydrothermal mounds. 相似文献
108.
Radiocaesium, introduced into coastal waters in the effluent from the B.N.F.L. nuclear fuel reprocessing plant at Windscale, is present at significant levels throughout the Clyde Sea Area. Concentrations reached maximum in April, 1977, and have recently declined. Around 30% of Windscale radiocaesium output passes through the Clyde Sea Area but less than 0.3% remains in sediments. There appears to be no major radiological safety hazard. The radiocaesium is, however, an excellent tracer species for both waters and sediments. The Irish Sea-derived component comprises over 90% of Clyde seawater and may well introduce major inventories of pollutants. The average water residence time in the Clyde system is 4.5 months, although significantly longer entrainment is evident in northern sea lochs. Study of radiocaesium variations has enabled improved assessment of the Clyde's response to marine pollution. 相似文献
109.
Jack H. Shreffler 《Boundary-Layer Meteorology》1975,9(2):191-204
Numerical solutions are obtained for a two-dimensional eddy diffusion equation describing aerosol particles in the atmospheric surface layer. Particles are assumed to be from either an elevated line source or a surface area source. Characteristics of the solutions are exhibited as a function of source type, lower boundary condition, and the ratio of terminal velocity () to friction velocity (u
*). This theory indicates that suspended particles tend to escape the near-surface layer, and are thus subject to long-range transport, when the ratio /u
* is less than von Karman's constant. This prediction is consistent with available data. 相似文献
110.
Spectral shape,epsilon and record selection 总被引:4,自引:0,他引:4
Selection of earthquake ground motions is considered with the goal of accurately estimating the response of a structure at a specified ground motion intensity, as measured by spectral acceleration at the first‐mode period of the structure, Sa(T1). Consideration is given to the magnitude, distance and epsilon (ε) values of ground motions. First, it is seen that selecting records based on their ε values is more effective than selecting records based on magnitude and distance. Second, a method is discussed for finding the conditional response spectrum of a ground motion, given a level of Sa(T1) and its associated mean (disaggregation‐based) causal magnitude, distance and ε value. Records can then be selected to match the mean of this target spectrum, and the same benefits are achieved as when records are selected based on ε. This mean target spectrum differs from a Uniform Hazard Spectrum, and it is argued that this new spectrum is a more appropriate target for record selection. When properly selecting records based on either spectral shape or ε, the reductions in bias and variance of resulting structural response estimates are comparable to the reductions achieved by using a vector‐valued measure of earthquake intensity. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献