全文获取类型
收费全文 | 645篇 |
免费 | 37篇 |
国内免费 | 6篇 |
专业分类
测绘学 | 42篇 |
大气科学 | 54篇 |
地球物理 | 218篇 |
地质学 | 202篇 |
海洋学 | 45篇 |
天文学 | 71篇 |
综合类 | 4篇 |
自然地理 | 52篇 |
出版年
2024年 | 1篇 |
2023年 | 3篇 |
2022年 | 11篇 |
2021年 | 8篇 |
2020年 | 16篇 |
2019年 | 11篇 |
2018年 | 18篇 |
2017年 | 21篇 |
2016年 | 28篇 |
2015年 | 30篇 |
2014年 | 22篇 |
2013年 | 40篇 |
2012年 | 34篇 |
2011年 | 28篇 |
2010年 | 26篇 |
2009年 | 45篇 |
2008年 | 29篇 |
2007年 | 35篇 |
2006年 | 30篇 |
2005年 | 24篇 |
2004年 | 21篇 |
2003年 | 26篇 |
2002年 | 21篇 |
2001年 | 20篇 |
2000年 | 10篇 |
1999年 | 10篇 |
1998年 | 13篇 |
1997年 | 15篇 |
1996年 | 7篇 |
1995年 | 7篇 |
1994年 | 8篇 |
1993年 | 7篇 |
1992年 | 5篇 |
1991年 | 4篇 |
1990年 | 7篇 |
1989年 | 4篇 |
1988年 | 5篇 |
1987年 | 9篇 |
1986年 | 5篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1981年 | 2篇 |
1979年 | 1篇 |
1977年 | 3篇 |
1976年 | 1篇 |
1975年 | 5篇 |
1973年 | 2篇 |
1971年 | 1篇 |
排序方式: 共有688条查询结果,搜索用时 15 毫秒
631.
Rats were raised on a variety of isotopically controlled diets comprising 20% C3, C4 or marine protein and C3 and/or C4 non-protein or energy (i.e. sucrose, starch and oil) macronutrients. Compound specific stable carbon isotope (δ13C) analysis was performed on the cholesterol isolated from the diet (n=7 ) and bone (n=15 ) of these animals and the values compared with bulk δ13C measurements of bone collagen and apatite. The dietary signals reflected by these three bone biochemical components were investigated using linear regression analysis. δ13C values of bone cholesterol were shown to reflect whole diet δ13C values, collagen to reflect mainly dietary protein values and apatite to reflect whole diet values. Further correlations between dietary protein-to-energy spacings (Δ13Cprot-engy = δ13Cprotein - δ13Cenergy) and whole diet-to-bone component fractionations (Δ13Cbcomp-wdiet = δ13Cbone component - δ13Cwhole diet) indicates that for hypothetical diets where protein δ13C values are equal to energy values, fractionations between whole diet and bone biochemical fractions are -3.3‰ for cholesterol, +5.4‰ for collagen and +9.5‰ for apatite. Moreover, the narrow range of variation observed in apatite-to-cholesterol spacings (Δ13Capat-bchol) suggests that cholesterol δ13C values can potentially also be used as an independent test for the isotopic integrity of apatite δ13C values. These insights into bone cholesterol, collagen and apatite dietary signals, diet-to-bone fractionations and bone component-to-bone component spacings provide the basis for more accurate interpretations of the dietary behaviour of archaeological populations and food webs when the δ13C analysis of bone is employed. 相似文献
632.
Jiménez J. C. Raga G. B. Baumgardner D. Castro T. Rosas I. Báez A. Morton O. 《Natural Hazards》2004,31(1):21-37
A 17-day field campaign was carried out in April–May 1999 to determine thecontribution that gaseous volcanic emissions make to the compositionof solid particles,particularly to the presence and quantity of sulfates. Theexperimental site was located inTonantzintla (in the State of Puebla), only 30 km E from the volcanoPopocatépetl, whichhas been in an active phase since the end of 1993. An analysis ofthe carbon monoxide(CO) and sulfur dioxide (SO2) in the ambient air identifiedvolcanic influence in 6 out ofthe 17 days sampled. Particles collected in an 8-stage cascadeimpactor were analyzed for inorganic ions (by liquid chromatography).A non-parametric test indicates asignificant difference on the total particle mass and thesulfate fraction between days withand without volcanic influence. This difference was predominantlyobserved in the stages that collect the smaller particles. Windsat 500 mb (roughly corresponding to thesummit of the volcano) indicate a westerly transport from thevolcano to the experimentalsite, even though surface winds do not show a clear signal ofsuch a transport. Back trajectories from the experimental site werecalculated and clearly indicated that air parcels on the daysindependently identified as with volcanic influence had indeed passed over the volcano. 相似文献
633.
634.
J. Patrón I. González Hernández A. Jiménez S.J. Jiménez Reyes I. Martín P.L. Pallé F. Pérez Hernández C. Régulo T. Roca-Cortés 《Astrophysics and Space Science》1998,263(1-4):327-330
We investigate the conditions under which general scalar-tensor gravity theories relax towards General Relativity. We extend
the work of Damour and Nordtvedt [2] by studying the effects of the inclusion of a cosmological potential term. When the universe
is either radiation dominated or vacuum, we find that Einstein's gravity is indeed a cosmological attractor and, also, that
the universe exhibits inflationary expansion. This latter feature provides another striking argument in favour of the inflationary
paradigm, which in the present setting arises without the intervention of the usual inflaton field.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
635.
Luis Jiménez-Benito Angeles Díaz Roberto Terlevich Elena Terlevich 《Astrophysics and Space Science》1998,263(1-4):111-114
We have analyzed high signal-to-noise spectra of 10 active galaxies in order to investigate the stellar populations of these
objects and the kinematics of the gas and stars in their nuclei.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
636.
Recent protected-area designation in China: an evaluation of administrative and statutory procedures 总被引:3,自引:0,他引:3
Past protected-area (PA) designation in China followed a centralized administrative approach with minimum participation of lower-echelon governments. A period of deregulation and decentralization in 1979–91 without the benefit of relevant legislation resulted in few designations and poor management. Responding to national and international encouragement to augment the conservation of natural resources, the central government enacted statutory procedures in 1991 to encourage and regulate PA establishments at different administrative levels of government. The study evaluates the effectiveness of the new procedures in fulfilling conservation objectives. The new legislative and administrative regime reinforces deregulation and decentralization by devolving designation power and shifting financial and management responsibilities to lower levels of government, and involving more scientists and objective site assessment in PA design and assessment. Problems have arisen in the designation process due to the omission of key biota and ecosystems, bypassing the scientific assessment stage, too much emphasis on non-conservation gains, ignoring the needs of local communities, escalating people–park conflicts, intensifying paper-park syndrome, and the conflicting role of experts. The recently designated Shimentai Provincial Rank Nature Reserve in a mountain area in south China served as a case study to illustrate the procedures and problems. 相似文献
637.
An empirical test of the competing destinations model 总被引:2,自引:0,他引:2
It has long been believed that properties of spatial structure have a strong effect on trip distribution, which thus leads
to a bias in the estimated distance decay parameters of spatial interaction models. This paper is an attempt to identify to
what extent the spatial structure effect affects the trip distribution and determine whether the incorporation of a term to
account for the relative location of destinations into the conventional gravity models, results in a model that can more correctly
represent the actual trip distribution. The main focus is on the comparison of the origin–specific estimates of the distance
decay parameter, calibrated from the traditional production-constrained model and the production-constrained competing destinations
model. The results show that the competing destinations model is superior to the conventional model in both reproducing the
interaction flows and giving behavioral explanation to the distance decay parameters, but the essential aim of the competing
destinations model to remove the map pattern from the distance decay parameters of the conventional model has not been identified.
Received: 5 September 2001 / Accepted: 17 June 2002
We are grateful to Gloria. A. Swieczkowski for kindly providing the migration data. The authors also gratefully acknowledge
the comments of the referees. 相似文献
638.
Justin Butler Stuart Lane Jim Chandler & Ekaterini Porfiri 《The Photogrammetric Record》2002,17(99):419-439
Measurement of the structure of gravel-bed river surfaces is crucial for understanding both bed roughness and the sediment entrainment process. This paper describes the use of close range digital photogrammetry to measure and monitor change occurring in submerged river gravel-beds in both flume and field environments. High-resolution digital elevation models (DEMs) were obtained and two-media (through air and water) techniques were used to correct for the e.ects of refraction at the air/water interface. Although suitable refractive models have been developed, the use of proprietary software to generate DEMs automatically introduces the problem of how to re-establish collinearity. A simple refraction correction algorithm based upon analytical geometry was developed and is described. This algorithm was designed for use after initial DEM acquisition and allows any photogrammetric software package to be used for data acquisition. Application of this algorithm led to improvements in DEM accuracy by reducing the systematic, depth-dependent bias caused by refraction
Research carried out in a flume environment allowed the algorithm to be tested by measuring a flooded and drained bed surface. Non-systematic differences between the "dry" and "wet" DEMs arose from reductions in stereomatching success in the two-media case. This effect was thought to be due to light attenuation and the introduction of residual parallax. Results suggest that close range digital photogrammetry can be used to extract high quality DEMs of submerged topography in both flume and field fluvial environments, which represents a particularly exciting development for fluvial geomorphologists 相似文献
Research carried out in a flume environment allowed the algorithm to be tested by measuring a flooded and drained bed surface. Non-systematic differences between the "dry" and "wet" DEMs arose from reductions in stereomatching success in the two-media case. This effect was thought to be due to light attenuation and the introduction of residual parallax. Results suggest that close range digital photogrammetry can be used to extract high quality DEMs of submerged topography in both flume and field fluvial environments, which represents a particularly exciting development for fluvial geomorphologists 相似文献
639.
Unequivocal identification of irrigation return water through routine chemical analyses does not appear possible in tropical
volcanic islands. Several constituents, nevertheless, will suggest an agricultural source if they are present in large concentrations.
Nitrate is the most useful indicator; background concentrations are generally less than 2 ppm but return irrigation water
will commonly have concentrations of between 5 and 50 ppm. Sulfate appears to be the next most useful indicator. Calcium,
silica, and bicarbonate have also been reported to be significant. Where detailed studies have been made of agricultural water
balances, regional ground-water flow directions, and deep-well hydraulics, hydrochemistry can provide critical supporting
data for the identification of return irrigation water which could be a potential ground-water pollutant. 相似文献
640.