全文获取类型
收费全文 | 1591篇 |
免费 | 137篇 |
国内免费 | 10篇 |
专业分类
测绘学 | 46篇 |
大气科学 | 175篇 |
地球物理 | 538篇 |
地质学 | 583篇 |
海洋学 | 108篇 |
天文学 | 205篇 |
综合类 | 7篇 |
自然地理 | 76篇 |
出版年
2023年 | 6篇 |
2022年 | 15篇 |
2021年 | 38篇 |
2020年 | 45篇 |
2019年 | 31篇 |
2018年 | 39篇 |
2017年 | 60篇 |
2016年 | 104篇 |
2015年 | 94篇 |
2014年 | 113篇 |
2013年 | 177篇 |
2012年 | 137篇 |
2011年 | 91篇 |
2010年 | 85篇 |
2009年 | 75篇 |
2008年 | 53篇 |
2007年 | 31篇 |
2006年 | 63篇 |
2005年 | 36篇 |
2004年 | 32篇 |
2003年 | 31篇 |
2002年 | 45篇 |
2001年 | 26篇 |
2000年 | 12篇 |
1999年 | 12篇 |
1998年 | 16篇 |
1997年 | 17篇 |
1996年 | 9篇 |
1995年 | 18篇 |
1994年 | 11篇 |
1993年 | 9篇 |
1992年 | 7篇 |
1991年 | 19篇 |
1990年 | 9篇 |
1989年 | 9篇 |
1987年 | 9篇 |
1986年 | 5篇 |
1985年 | 10篇 |
1984年 | 13篇 |
1983年 | 12篇 |
1982年 | 9篇 |
1981年 | 8篇 |
1980年 | 5篇 |
1979年 | 13篇 |
1977年 | 4篇 |
1976年 | 6篇 |
1975年 | 8篇 |
1974年 | 7篇 |
1973年 | 10篇 |
1969年 | 7篇 |
排序方式: 共有1738条查询结果,搜索用时 140 毫秒
91.
92.
Andesite and dacite genesis via contrasting processes: the geology and geochemistry of El Valle Volcano,Panama 总被引:3,自引:0,他引:3
Mare J. Defant Lee F. Clark Robert H. Stewart Mark S. Drummond Jelle Z. de Boer René C. Maury Hervé Bellon Thomas E. Jackson Juan F. Restrepo 《Contributions to Mineralogy and Petrology》1991,106(3):309-324
The easternmost stratovolcano along the Central American arc is El Valle volcano, Panama. Several andesitic and dacitic lava
flows, which range in age 5–10 Ma, are termed the old group. After a long period of quiescence (approximately 3.4 Ma), volcanic
activity resumed approximately 1.55 Ma with the emplacement of dacitic domes and the deposition of dacitic pyroclastic flows
0.9–0.2 Ma. These are referred to as the young group. All of the samples analyzed are calc-alkaline andesites and dacites.
The mineralogy of the two groups is distinct; two pyroxenes occur in the old-group rocks but are commonly absent in the young
group. In contrast, amphibole has been found only in the young-group samples. Several disequilibrium features have been observed
in the minerals (e.g., oscillatory zoning within clinopyroxenes). These disequilibrium textures appear to be more prevalent
among the old- as compared with the young-group samples and are most likely the result of magma-mixing, assimilation, and/or
polybaric crystallization. Mass-balance fractionation models for major and trace elements were successful in relating samples
from the old group but failed to show a relationship among the young-group rocks or between the old- and young-group volcanics.
We believe that the old-group volcanics were derived through differentiation processes from basaltic magmas generated within
the mantlewedge. The young group, however, does not appear to be related to more primitive magmas by differentiation. The
young-group samples cannot be related by fractionation including realistic amounts of amphibole. Distinctive geochemical features
of the young group, including La/Yb ratios〉15, Yb〈1, Sr/Y〉150, and Y〈6, suggest that these rocks were derived from the partial
melting of the subducted lithosphere. These characteristics can be explained by the partial melting of a source with residual
garnet and amphibole. Dacitic material with the geochemical characteristics of subducted-lithosphere melting is generated
apparently only where relatively hot crust is subducted, based on recent work. The young dacite-genesis at El Valle volcano
is related to the subduction of relatively hot lithosphere. 相似文献
93.
A modified parameterization of flux-profile relationships in the surface layer using different roughness length values for heat and momentum 总被引:1,自引:0,他引:1
The surface-layer flux-profile formulae of Louis (1979), used in many atmospheric models, are modified in a simple way to allow for different values of the roughness lengths for heat and momentum. The modified set of formulae simplifies the calculation of surface-layer fluxes over most natural land surfaces, where the roughness length for momentum can be almost an order of magnitude greater than that for heat. 相似文献
94.
This study presents a comparison of the water vapor and clear-sky greenhouse effect dependence on sea surface temperature for climate variations of different types. Firstly, coincident satellite observations and meteorological analyses are used to examine seasonal and interannual variations and to evaluate the performance of a general circulation model. Then, this model is used to compare the results inferred from the analysis of observed climate variability with those derived from global climate warming experiments. One part of the coupling between the surface temperature, the water vapor and the clear-sky greenhouse effect is explained by the dependence of the saturation water vapor pressure on the atmospheric temperature. However, the analysis of observed and simulated fields shows that the coupling is very different according to the type of region under consideration and the type of climate forcing that is applied to the Earth-atmosphere system. This difference, due to the variability of the vertical structure of the atmosphere, is analyzed in detail by considering the temperature lapse rate and the vertical profile of relative humidity. Our results suggest that extrapolating the feedbacks inferred from seasonal and short-term interannual climate variability to longer-term climate changes requires great caution. It is argued that our confidence in climate models' predictions would be increased significantly if the basic physical processes that govern the variability of the vertical structure of the atmosphere, and its relation to the large-scale circulation, were better understood and simulated. For this purpose, combined observational and numerical studies focusing on physical processes are needed. 相似文献
95.
Some turbiditic measures carried out by a limnological diffusiometer along with the physico-chemical determinations of the
Lake of Neuchatel have permitted to subdivide the water column in 4 distinct layers: the Epilimnium, the Clear Water Layer,
the Pelagic Nepheloid Layer and the Benthic Nepheloid Layer.
相似文献
96.
A least-squares prediction method is described to estimate horizontal coordinate distortions at lower order points of a network
using known coordinate differences (NAD27 coordinate distortions Δϕ′s and Δλ′s) at higher order points between NAD27 coordinates
and coordinates derived from a recent (MAY 76), relatively distortion free, adjustment of these points. Empirical autocovariance
functions of Δϕ and Δλ and crosscovariance function between Δϕ and Δλ are derived from some 5,250 data points and modelled
using series of exponential functions. Empirical mean square values of Δϕ and Δλ, which are a measure of the distortions in
NAD27 ϕ and λ, are 0.051 and 0.645 arcsecs2 respectively. The corresponding mean value of the product ΔϕΔλ, which is a measure of the correlation between Δϕ and Δλ,
is 0.056 arcsecs2. The accuracy obtainable for predicted Δϕ and Δλ at an arbitrary point (e.g., lower order station) is a function of the accuracy
and configuration of known Δϕ′s and Δλ′s in the surrounding area. Accuracies obtainable for various types of data configuration
are given. Under favorable conditions taking place in about 60% of cases, accuracies in terms of ms agreement with known values
of 0″.02 (0.6 m) and 0″.01 (0.2 m along parallel at latitude 50°) for the predicted latitude and longitude distortions are
obtainable. Finally, a comparison with a method based on the use of complex polynomials is made.
Presented at International Symposium on Geodetic Networks and Computations, Munich, August–September 1981. 相似文献
97.
André Lacam 《Physics and Chemistry of Minerals》1983,9(3-4):127-132
The objectives of this study of olivines are, to calibrate the variation of electrical conductivity with pressure, up to 200 kbar in a diamond-anvil cell, and to determine how this is influenced by chemical composition. Experimentally, we have found that the variation of the electrical conductivity of three synthetic olivines containing 50, 75 and 100 mole percent of fayalite, is an exponential function of pressure P, closely represented at room temperature by:σxP=σx·exp ·(B x·P) where x is the iron content of the olivine, σ x the extrapolated value of conductivity at normal pressure and B x the slope of the regression line in semi-logarithmic coordinates. It is thus possible to express the temperature dependence of conductivity through the Boltzmann relationship:σxPT= σαT· exp ·(-H*/RT)=σxT·exp ·[-(E*+PV * x )/RT] where H* is the activation enthalpy, E* the activation energy and V x * the activation volume. At constant temperature V * x =B x·RT and is approximately equal to 0,6 cm3/mole at 295 K. On the other hand, we have found that σxT is an exponential function of x and thus, B x and of course V x * are linear functions. The experimental procedure is described and the results discussed. 相似文献
98.
André Deprit Jesúus Palacián Etienne Deprit 《Celestial Mechanics and Dynamical Astronomy》2001,79(3):157-182
The relegation algorithm extends the method of normalization by Lie transformations. Given a Hamiltonian that is a power series = 0+ 1+ ... of a small parameter , normalization constructs a map which converts the principal part 0into an integral of the transformed system — relegation does the same for an arbitrary function [G]. If the Lie derivative induced by [G] is semi-simple, a double recursion produces the generator of the relegating transformation. The relegation algorithm is illustrated with an elementary example borrowed from galactic dynamics; the exercise serves as a standard against which to test software implementations. Relegation is also applied to the more substantial example of a Keplerian system perturbed by radiation pressure emanating from a rotating source.This revised version was published online in October 2005 with corrections to the Cover Date. 相似文献
99.
Wagner José Barreto Ieda Scarminio Maria Cristina Solci Dílson Norio Ishikawa Melissa Tiemi Ogasawara Sônia Naomi Nomi Sônia Regina Giancoli Barreto 《洁净——土壤、空气、水》2007,35(3):239-245
Seventeen physical and chemical parameters were obtained from a hydroelectric reservoir located in a tropical region in the south of Brazil. Multivariate Principal Component Analysis (PCA) and Hierarchical Group Analysis (HGA) were used to identify the parameters discriminating the origin of water from the Tibagi and the Primeiro de Maio River, after it has passed the mixing region. The study was conducted during the dry and rainy seasons in July 2002 and February 2003 at three depths and three sampling sites located 0, 5, and 10 km away from the mixing region. The statistical methods showed to be appropriate for identifying the contribution of each tributary in the water mixing site of a complex water system. The most important discriminating parameter was the absorbance relation A(253 nm)/A(203 nm), followed by the concentrations of Fe(III), Mn(III), and Ni(II). An anthropogenic interference was found in the reservoir due to high Ni(II) and orthophosphate concentrations caused by a nearby town sewage discharge. The interference was more important during the dry periods due to the lower dispersion of the pollutants. Urgent initiatives should be taken from the state government to build treatment stations for the wastewater of the small cities around the Capivara hydroelectric reservoir to prevent the drinking water quality from deteriorating. 相似文献
100.
André Salgado César Varajão Fabrice Colin Régis Braucher Angélica Varajão Herminio Nalini Jr 《地球表面变化过程与地形》2007,32(6):905-911
The present work quantifies the erosive processes in the two main substrates (schists–phyllites and granites–gneisses) of the upper Maracujá Basin in the Quadrilátero Ferrífero/MG, Brazil, a region of semi‐humid tropical climate. Two measuring methods of concentration were used: (i) in situ produced 10Be in quartz veins (surface erosion rates) and (ii) 10Be in fluvial sediments (basin erosion rates). The results confirm that (i) erosion tends to be more aggressive close to the headwaters than in the lower parts of the basin and (ii) the region is now affected by dissection. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献