全文获取类型
收费全文 | 1990篇 |
免费 | 61篇 |
国内免费 | 23篇 |
专业分类
测绘学 | 61篇 |
大气科学 | 165篇 |
地球物理 | 482篇 |
地质学 | 719篇 |
海洋学 | 177篇 |
天文学 | 324篇 |
综合类 | 12篇 |
自然地理 | 134篇 |
出版年
2021年 | 24篇 |
2020年 | 21篇 |
2019年 | 25篇 |
2018年 | 55篇 |
2017年 | 36篇 |
2016年 | 58篇 |
2015年 | 33篇 |
2014年 | 50篇 |
2013年 | 102篇 |
2012年 | 59篇 |
2011年 | 111篇 |
2010年 | 75篇 |
2009年 | 86篇 |
2008年 | 88篇 |
2007年 | 70篇 |
2006年 | 59篇 |
2005年 | 66篇 |
2004年 | 51篇 |
2003年 | 43篇 |
2002年 | 61篇 |
2001年 | 37篇 |
2000年 | 53篇 |
1999年 | 54篇 |
1998年 | 35篇 |
1997年 | 48篇 |
1996年 | 46篇 |
1995年 | 34篇 |
1994年 | 30篇 |
1993年 | 28篇 |
1992年 | 20篇 |
1991年 | 22篇 |
1990年 | 21篇 |
1989年 | 14篇 |
1988年 | 13篇 |
1987年 | 34篇 |
1986年 | 18篇 |
1985年 | 24篇 |
1984年 | 29篇 |
1983年 | 28篇 |
1982年 | 35篇 |
1981年 | 22篇 |
1980年 | 29篇 |
1979年 | 20篇 |
1978年 | 19篇 |
1977年 | 21篇 |
1976年 | 21篇 |
1975年 | 25篇 |
1974年 | 18篇 |
1973年 | 18篇 |
1971年 | 12篇 |
排序方式: 共有2074条查询结果,搜索用时 15 毫秒
981.
982.
This study was designed to examine the environmental exposure of surface‐ and groundwater pollution in remote mountainous regions of northern Vietnam. In 2008, we monitored the loss of four commonly applied pesticides (imidacloprid, fenitrothion, fenobucarb, dichlorvos) from paddy rice farming systems to a receiving stream on the watershed scale and quantified groundwater pollution. For the entire monitoring period, runoff loss of pesticides from the watershed was estimated to range between 0.4% (dichlorvos) and 16% (fenitrothion) of the total applied mass. These losses were correlated well with the octanol–water partition coefficient and water solubility of pesticides (r2 = 0.78–0.99). In the groundwater collected from eight wells, all target pesticides were frequently detected. Maximum measured concentrations were 0.47, 0.22, 0.17, and 0.07 µg L?1 for fenitrothion, imidacloprid, fenobucarb, and dichlorvos, respectively. Our results strongly indicate that under the current management practice pesticide use in paddy fields poses a serious environmental problem in mountainous regions of northern Vietnam. 相似文献
983.
Over the last decade several computational models, and several types of model, have been developed to simulate the response of river systems to environmental change over time scales of decades to millennia: hydrological models, flood inundation models, channel morphology models, channel network models, models of river meandering and river braiding, alluvial stratigraphy models, and landscape evolution models. Each type of model simulates different aspects of a river's response to changes in environmental inputs such as climate and land-use — and to changes in these inputs. And each type of model has its abilities, advantages and limitations. We provide an overview of the different types of model that have been developed, and we evaluate their suitability for testing hypotheses about past environmental conditions, as well as for investigating the response of alluvial river systems to future environmental change. Additionally, we discuss the general issues and problems of computational modelling (e.g. scale and resolution, data availability, process representation, process parameterization, model calibration, non-linearity, and uncertainty), and the extent to which these hamper the usefulness of the models as a tool in environmental landscape studies. Finally, we identify trends in computational modelling research to outline possible future directions of the discipline. 相似文献
984.
For data acquired with conventional acquisition techniques, surface multiples are usually considered as noise events that obscure the primaries. However, in this paper we demonstrate that for the situation of blended acquisition, meaning that different sources are shooting in a time‐overlapping fashion, multiples can be used to ‘deblend’ the seismic measurements. We utilize the recently introduced estimation of primaries by sparse inversion (EPSI) methodology, in which the primary impulse responses are considered to be the unknowns in a large‐scale inversion process. With some modifications the estimation of primaries by sparse inversion method can be used for blended seismic data. As output this process gives unblended primary impulse responses with point sources and receivers at the surface, which can be used directly in traditional imaging schemes. It turns out that extra information is needed to improve on the deblending of events that do not have much associated multiple energy in the data, such as steep events at large offsets. We demonstrate that this information can be brought in during acquisition and during processing. The methodology is illustrated on 2D synthetic data. 相似文献
985.
A striking characteristic of glacial climate in the North Atlantic region is the recurrence of abrupt shifts between cold stadials and mild interstadials. These shifts have been associated with abrupt changes in Atlantic Meridional Overturning Circulation (AMOC) mode, possibly in response to glacial meltwater perturbations. However, it is poorly understood why they were more clearly expressed during Marine Isotope Stage 3 (MIS3, ~60?C27?ka BP) than during Termination 1 (T1, ~18?C10?ka BP) and especially around the Last Glacial Maximum (LGM, ~23?C19?ka BP). One clue may reside in varying climate forcings, making MIS3 and T1 generally milder than LGM. To investigate this idea, we evaluate in a climate model how ice sheet size, atmospheric greenhouse gas concentration and orbital insolation changes between 56?ka BP (=56k), 21k and 12.5k affect the glacial AMOC response to additional freshwater forcing. We have performed three ensemble simulations with the earth system model LOVECLIM using those forcings. We find that the AMOC mode in the mild glacial climate type (56k and 12.5k), with deep convection in the Labrador Sea and the Nordic Seas, is more sensitive to a constant 0.15?Sv freshwater forcing than in the cold type (21k), with deep convection mainly south of Greenland and Iceland. The initial AMOC weakening in response to freshwater forcing is larger in the mild type due to an early shutdown of Labrador Sea deep convection, which is completely absent in the 21k simulation. This causes a larger fraction of the freshwater anomaly to remain at surface in the mild type compared to the cold type. After 200?years, a weak AMOC is established in both climate types, as further freshening is compensated by an anomalous salt advection from the (sub-)tropical North Atlantic. However, the slightly fresher sea surface in the mild type facilitates further weakening of the AMOC, which occurs when a surface buoyancy threshold (?0.6?kg?m?3 surface density anomaly to the 56k reference state) is stochastically crossed in the Nordic Seas. While described details are model-specific, our results imply that a more northern location of deep convection sites during milder glacial times may have amplified frequency and amplitude of abrupt climate shifts. 相似文献
986.
Aleš Vaněk Vladislav Chrastný Leslaw Teper Jerzy Cabala Vít Penížek Michael Komárek 《Journal of Geochemical Exploration》2011
Distribution of Tl, Zn, Cd, Pb, K, Ca, Mg and Mn and Pb isotopic composition in tree rings of Scots pine (Pinus sylvestris L.) from an area affected by primary Zn smelting (Olkusz, southern Poland) were investigated. Elevated concentrations of Tl (up to 0.8 mg kg−1) in pine trees imply that conifers tend to accumulate this metal to some extent. A generally positive relationship between soil and tree-ring Tl levels was identified. The Tl patterns in stem wood did not correspond to changes in Tl deposition; the lateral translocation of Tl in the sapwood and its accumulation at the sapwood-heartwood boundary (i.e., in the inner sapwood) is suggested. It is probable that the specific behavior of Tl in trees results from its biogeochemical analogy with K. In contrast, tree-ring patterns of Zn and Cd significantly correlated with their deposition; nevertheless, a partial shift of these metals towards the stem center cannot be excluded. The isotopic composition of Pb (206Pb/207Pb ~ 1.172–1.184) in trees and underlying soils revealed the predominant influence of smelter emissions (206Pb/207Pb ~ 1.17) on Pb contamination. Analysis of main nutrients (Ca, Mg and Mn) in wood reflected environmental changes related to acid deposition. 相似文献
987.
988.
Stephanie Simpson Barbara L. Sherriff Jamie Van GulckElena Khozhina Kathleen LondryNikolay Sidenko 《Applied Geochemistry》2011,26(11):1843-1854
This study was to investigate the source, mobility and attenuation of As at the New Britannia Mine, Snow Lake, Manitoba. One major source of As contamination was determined to be an arsenopyrite residue stockpile (ARS) containing refractory Au in a waste rock impoundment. It appears that As is still moving through glacial clay at the base of the ARS into a confined aquifer even though the pile was capped in the year 2000. Arsenic is also being mobilized from a deposit of tailings, which formed following spills by previous owners, Nor Acme. Arsenic from the tailings is being mobilized by oxidation of arsenopyrite and reduction of arsenate to the more mobile arsenite by arsenate-reducing bacteria. This contamination is affecting a shallow unconfined aquifer and surface water flowing from the tailings through wetlands towards Snow Lake. Arsenic is being attenuated by adsorption to hydrated ferric oxides (HFO) in the tailings, wetland soils and aquatic plants. Although As in surface water, soils and plants along the flow path from the mine to Snow Lake are above Canadian drinking water guidelines, efficient natural attenuation by HFO in soils and plants of the wetlands have limited the concentration in Snow Lake to below drinking water standards. 相似文献
989.
Orbital timing of the Indian,East Asian and African boreal monsoons and the concept of a ‘global monsoon’ 总被引:1,自引:0,他引:1
Thibaut Caley Bruno Malaizé Marie Revel Emmanuelle Ducassou Karine Wainer Mohamed Ibrahim Dina Shoeaib Sébastien Migeon Vincent Marieu 《Quaternary Science Reviews》2011,30(25-26):3705-3715
Our understanding of monsoon circulation timing’s at the orbital scale is currently a matter of debate. Here, we compare previous and recently published results of Indian, East Asian, West African and East African monsoon variability. We note different timings between the East African, West African, Indian and East-Asian monsoon systems for the most recent 45 ka, where the age models are constrained by AMS dating. On this basis, we construct different orbital forcing “reference curves” and apply them to the 200 ka time period for the different monsoon systems. Our results indicate that the ‘global monsoon’ concept at the orbital scale is a misnomer. We find real regional differences in the timing of the monsoon response to orbital forcing and differences in the weight of precession and obliquity in the monsoon records. This work highlights the necessity of studies aimed at understanding the underlying physics of these regional response patterns. This is crucial to a better understanding of monsoon dynamics and improved climate model simulations and comparisons with proxy data. 相似文献
990.
By means of new photoelectric observations made in 1974 an attempt to determine the poles of asteroids 9 and 44 was made. Following a method based upon the magnitude-aspect and amplitude-aspect relations, the coordinates of the poles for 9 and 44 were found to be, respectively, λ0 = 191° ± 5°, β0 = 56° ± 6° and λ0 = 100° ± 10°, β0 = 50° ± 10°. The previously published pole for asteroid 22, λ0 = 215° ± 10°, β0 = 45° ± 15°, was confirmed. From its phase relation we determined the phase coefficient of 44 Nysa, a very high albedo object (pv = 0.377). The very low phase coefficient obtained (βv = 0.018 mag/deg) agrees very well with an inverse relation between geometrical albedo and phase coefficient. The results are summarized in a table. 相似文献