全文获取类型
收费全文 | 1440篇 |
免费 | 24篇 |
国内免费 | 11篇 |
专业分类
测绘学 | 12篇 |
大气科学 | 92篇 |
地球物理 | 300篇 |
地质学 | 524篇 |
海洋学 | 114篇 |
天文学 | 282篇 |
综合类 | 4篇 |
自然地理 | 147篇 |
出版年
2020年 | 12篇 |
2018年 | 14篇 |
2017年 | 20篇 |
2016年 | 19篇 |
2015年 | 16篇 |
2014年 | 27篇 |
2013年 | 51篇 |
2012年 | 27篇 |
2011年 | 33篇 |
2010年 | 55篇 |
2009年 | 47篇 |
2008年 | 45篇 |
2007年 | 47篇 |
2006年 | 57篇 |
2005年 | 39篇 |
2004年 | 42篇 |
2003年 | 56篇 |
2002年 | 45篇 |
2001年 | 19篇 |
2000年 | 25篇 |
1999年 | 26篇 |
1998年 | 19篇 |
1997年 | 22篇 |
1996年 | 31篇 |
1995年 | 19篇 |
1994年 | 28篇 |
1993年 | 24篇 |
1992年 | 22篇 |
1991年 | 17篇 |
1990年 | 25篇 |
1989年 | 23篇 |
1988年 | 19篇 |
1987年 | 20篇 |
1986年 | 26篇 |
1985年 | 36篇 |
1984年 | 44篇 |
1983年 | 34篇 |
1982年 | 33篇 |
1981年 | 31篇 |
1980年 | 32篇 |
1979年 | 31篇 |
1978年 | 27篇 |
1977年 | 21篇 |
1976年 | 19篇 |
1975年 | 21篇 |
1974年 | 20篇 |
1973年 | 25篇 |
1972年 | 11篇 |
1971年 | 15篇 |
1970年 | 13篇 |
排序方式: 共有1475条查询结果,搜索用时 31 毫秒
131.
Generalized Ray Theory for a Layered Sphere 总被引:1,自引:0,他引:1
132.
Donald J. Jezewski 《Celestial Mechanics and Dynamical Astronomy》1976,14(1):105-111
A method for developing the missing general K/S (Kustaanheimo/Stiefel) boundary conditions is presented, with use of the formalism of optimal control theory. As an illustrative example, the method is applied to the K/S Lambert problem to derive the missing terminal condition. The necessary equations are developed for a solution to this problem with the generalized eccentric anomaly,E, as the independent variable. This formulation, requiring the solution of only one nonlinear, well-behaved equation in one unknown,E, results in considerable simplification of the problem.Presented also at the AAS/AIAA Astrodynamics Specialist Conference, Nassau, Bahamas, July 1975 (AAS Paper No. 75-032). 相似文献
133.
134.
Theoretical values for the relative intensities of the Fe XIII lines 110747, 110798, and 3388 are presented as functions of electron density using recent values of the cross-section for collisionl excitation by protons. Our results are compared with those of Chevalier and Lambert. 相似文献
135.
136.
Lance A.M. BennerSteven J. Ostro R.Scott HudsonKeith D. Rosema Raymond F. JurgensDonald K. Yeomans Donald B. CampbellJohn F. Chandler Irwin I. Shapiro 《Icarus》2002,158(2):379-388
We report Doppler-only (cw) radar observations of basaltic near-Earth asteroid 3908 Nyx obtained at Arecibo and Goldstone in September and October of 1988. The circular polarization ratio of 0.75±0.03 exceeds ∼90% of those reported among radar-detected near-Earth asteroids and it implies an extremely rough near-surface at centimeter-to-decimeter spatial scales. Echo power spectra over narrow longitudinal intervals show a central dip indicative of at least one significant concavity. Inversion of cw spectra yields two statistically indistinguishable shape models that have similar shapes and dimensions but pole directions that differ by ∼100°. We adopt one as our working model and explore its implications. It has an effective diameter of 1.0±0.15 km and radar and visual geometric albedos of 0.15±0.075 and 0.16+0.08−0.05. The visual albedo supports the interpretation by D. P. Cruikshank et al. (1991, Icarus89, 1-13) that Nyx has a thermal inertia consistent with that of bare rock. The model is irregular, modestly asymmetric, and topographically rugged. 相似文献
137.
Time delays associated with processes leading to a failure or stress relaxation in materials and earthquakes are studied in
terms of continuum damage mechanics. Damage mechanics is a quasi-empirical approach that describes inelastic irreversible
phenomena in the deformation of solids. When a rock sample is loaded, there is generally a time delay before the rock fails.
This period is characterized by the occurrence and coalescence of microcracks which radiate acoustic signals of broad amplitudes.
These acoustic emission events have been shown to exhibit power-law scaling as they increase in intensity prior to a rupture.
In case of seismogenic processes in the Earth's brittle crust, all earthquakes are followed by an aftershock sequence. A universal
feature of aftershocks is that their rate decays in time according to the modified Omori's law, a power-law decay. In this
paper a model of continuum damage mechanics in which damage (microcracking) starts to develop when the applied stress exceeds
a prescribed yield stress (a material parameter) is introduced to explain both laboratory experiments and systematic temporal
variations in seismicity. 相似文献
138.
Charles R. Cornish Christopher S. Bretherton Donald B. Percival 《Boundary-Layer Meteorology》2006,119(2):339-374
Statistical tools based on the maximal overlap discrete wavelet transform (MODWT) are reviewed, and then applied to a dataset of aircraft observations of the atmospheric boundary layer from the tropical eastern Pacific, which includes quasi-stationary and non-stationary segments. The wavelet methods provide decompositions of variances and covariances, e.g. fluxes, between time scales that effectively describe a broadband process like atmospheric turbulence. Easily understood statistical confidence bounds are discussed and applied to these scale decompositions, and results are compared to Fourier methods for quasi-stationary turbulence. The least asymmetric LA(8) wavelet filter yields coefficients that exhibit better uncorrelatedness across scales than the Haar filter and is better suited for decomposition of broadband turbulent signals. An application to a non-stationary segment of our dataset, namely vertical profiles of the turbulent dissipation rate, highlights the flexibility of wavelet methods. 相似文献
139.
Comparison of 15 evaporation methods applied to a small mountain lake in the northeastern USA 总被引:7,自引:0,他引:7
Donald O. Rosenberry Thomas C. Winter Donald C. Buso Gene E. Likens 《Journal of Hydrology》2007,340(3-4):149-166
Few detailed evaporation studies exist for small lakes or reservoirs in mountainous settings. A detailed evaporation study was conducted at Mirror Lake, a 0.15 km2 lake in New Hampshire, northeastern USA, as part of a long-term investigation of lake hydrology. Evaporation was determined using 14 alternate evaporation methods during six open-water seasons and compared with values from the Bowen-ratio energy-budget (BREB) method, considered the standard. Values from the Priestley–Taylor, deBruin–Keijman, and Penman methods compared most favorably with BREB-determined values. Differences from BREB values averaged 0.19, 0.27, and 0.20 mm d−1, respectively, and results were within 20% of BREB values during more than 90% of the 37 monthly comparison periods. All three methods require measurement of net radiation, air temperature, change in heat stored in the lake, and vapor pressure, making them relatively data intensive. Several of the methods had substantial bias when compared with BREB values and were subsequently modified to eliminate bias. Methods that rely only on measurement of air temperature, or air temperature and solar radiation, were relatively cost-effective options for measuring evaporation at this small New England lake, outperforming some methods that require measurement of a greater number of variables. It is likely that the atmosphere above Mirror Lake was affected by occasional formation of separation eddies on the lee side of nearby high terrain, although those influences do not appear to be significant to measured evaporation from the lake when averaged over monthly periods. 相似文献
140.
Donald W. Northfelt Michael J. DeNiro Samuel Epstein 《Geochimica et cosmochimica acta》1981,45(10):1895-1898
The and C ratios of the cellulose nitrate and saponifiable lipid fractions prepared from eleven annual growth rings of a California redwood were determined. The ratios of the two fractions are related to one another for the annual rings in the sapwood portion of the tree, but not for those in the heartwood or in the wood undergoing the transition from sapwood to heartwood. No relationship was observed between the ratios of the two fractions. These results suggest that analysis of the hydrogen isotopic composition of the saponifiable lipid fractions in plants will provide information useful for climatic reconstruction provided the initial isotopic record has not been changed by subsequent physiological or diagenetic processes. 相似文献