全文获取类型
收费全文 | 116篇 |
免费 | 4篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 5篇 |
大气科学 | 4篇 |
地球物理 | 29篇 |
地质学 | 40篇 |
海洋学 | 8篇 |
天文学 | 20篇 |
综合类 | 5篇 |
自然地理 | 10篇 |
出版年
2022年 | 1篇 |
2021年 | 3篇 |
2020年 | 4篇 |
2019年 | 1篇 |
2018年 | 8篇 |
2017年 | 5篇 |
2016年 | 10篇 |
2015年 | 5篇 |
2014年 | 7篇 |
2013年 | 7篇 |
2012年 | 3篇 |
2011年 | 7篇 |
2010年 | 8篇 |
2009年 | 6篇 |
2008年 | 7篇 |
2007年 | 1篇 |
2006年 | 6篇 |
2005年 | 6篇 |
2004年 | 2篇 |
2003年 | 4篇 |
2002年 | 2篇 |
2001年 | 1篇 |
1998年 | 3篇 |
1997年 | 2篇 |
1995年 | 1篇 |
1993年 | 1篇 |
1992年 | 3篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1982年 | 1篇 |
1979年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有121条查询结果,搜索用时 15 毫秒
61.
P. Purushothaman M. Someshwar Rao Y. S. Rawat C. P. Kumar Gopal Krishan T. Parveen 《Environmental Earth Sciences》2014,72(3):693-706
Agricultural activities act as dominant polluter of groundwater due to increased fertilizers and pesticides usage. Bist-Doab region, Punjab, India, is one such region facing deterioration of groundwater quality due to usage of fertilizers. This study aims in delineating and evaluating the groundwater quality in the region. Water samples are collected from canals, reservoir, and shallow and deep groundwater. Water types in canal and reservoir in Kandi region are Mg2+HCO3 ? and Mg2+Ca2+Na+HCO3 ?, respectively. While water types of shallow and deep groundwaters are found to be of two types: Na+Mg2+Ca2+HCO3 ? and Ca2+Mg2+Na+HCO3 ?. Presence of Mg2+ in groundwater at locations adjoining canals indicates recharge due to canal. The major ion (Na+, Mg2+, Ca2+, HCO3 ?) chemistry of the region is due to weathering of rocks that are rich in sodic minerals and kankar. Deep groundwater quality in the region meets BIS and WHO standards for drinking purpose, unlike shallow groundwater which is of poor quality at many locations. Both shallow and deep groundwater with high sodium concentration (>1.5 meq/l) affect cropping yield and permeability of soil matrix. High concentration of SO4 2? and NO3 2? (>1 meq/l) in shallow groundwater at few locations indicates influence of anthropogenic (fertilizer) activity. Factor analysis indicates that the major cations, bicarbonate and chloride are derived from weathering/dissolution of source rocks. Higher concentration of nitrate and presence of sulphate in shallow groundwater at few locations is due to usage of fertilizers and pesticides. 相似文献
62.
Global nature of the Paleoproterozoic Lomagundi carbon isotope excursion: A review of occurrences in Brazil,India, and Uruguay 总被引:2,自引:0,他引:2
Anil Maheshwari Alcides N. Sial Claudio Gaucher Jorge Bossi Andrey Bekker Valderez P. Ferreira Antonio W. Romano 《Precambrian Research》2010
Positive carbon isotope excursion is reported from Paleoproterozoic carbonates of the Aravalli Supergroup (northwestern India), the Minas Supergroup (Brazil), and new sections of the Paso Severino Formation (Uruguay). The 2.42 Ga Gandarela Formation, Minas Gerais, Brazil, contains red carbonate-facies BIF grading into dolostones and limestones and yielding δ13C values ranging from −1.6 to +0.4‰ V-PDB. The positive C-isotope excursion (up to + 11‰ V-PDB) in marine shallow-water carbonates in India and Brazil (Jhamarkotra Formation in northwestern India, and Cercadinho and Fecho do Funil formations in Minas Gerais State, Brazil) is comparable to that observed in 2.22–2.1 Ga carbonate successions worldwide that were deposited during the Lomagundi excursion. In Uruguay, δ13C values up to +11.6‰ V-PDB in the deep-water Paso Severino Formation of the Piedra Alta Terrane are compatible with deposition at ca. 2.15 Ga, as indicated by the 2146 ± 7 Ma U–Pb age of dacites occurring at the top of the unit. Negative δ13C values are also present in carbonates of the Paso Severino Formation, but an origin related to organic-matter remineralization cannot be ruled out. Thin carbonate beds in the Rio Itapicuru greenstone belt, Bahia State, Brazil, are associated, as in the Paso Severino Formation, with deep-water black shales and have carbon isotope values up to +9‰ V-PDB. High metamorphic grade carbonates of the Jacurici terrane in the Medrado-Ipueira area, Bahia, Brazil, have carbon isotope values up to +6.9‰ V-PDB, consistent with their minimum age of 2085 ± 5 Ma inferred from the intrusive contact with and the age of the Medrado norite. No evidence was found in India, Brazil, or Uruguay for Paleoproterozoic glacial events recognized in the 2.45–2.22 Ga sedimentary successions worldwide. Unconformities between the Gandarela and Cercadinho formations in Brazil and the banded gneissic Complex and the Lower Aravalli Supergroup in India might explain the absence of glacial record. Compositional and isotopic data presented here for studied Paleoproterozoic carbonate successions allow their integration into the global record of the Paleoproterozoic evolution as well as correlation with other successions of similar age. The study highlights the global nature of the Lomagundi excursion. Furthermore, it indicates that the Lomagundi excursion is recorded in both shallow-water (Aravalli and Minas supergroups) and deep-water carbonates (Paso Severino Formation and Rio Itapicuru greenstone belt) negating a significant impact of stromatolite productivity and hypersaline conditions on carbon isotope values of carbonates deposited in shallow-water, open-marine and isolated basins. 相似文献
63.
Landsat imagery were used to interpret the geomorphic festures in the northern part of Tamilnadu between latitudes 11° and 12°30′ N and longitudes 77° and 80° E. Different geomorphic regions were identified with the aid of ground truths. These geomorphic regions, in general, are controlled by the underlying rock types. As the different geomorphic regions have different signatures in the Landsat images, the geomorphic interpretation, which helps to bring out the regional geology is possible with the aid of ground truths. 相似文献
64.
65.
Conventional methods of soil nitrogen extraction are time consuming, expensive and tedious. Remote sensing and Geographical Information System technologies can be used for the rapid and efficient prediction of the presence of soil nitrogen. However, studies are limited by and large to fields of larger and homogeneous units. This research concentrates on the prediction of topsoil nitrogen from harvested, scattered and small-sized agricultural fields of India using hyperspectral data. Spaceborne hyperspectral Hyperion data are used for the prediction of the presence of nitrogen. Multivariate partial least square regression method was used to predict the presence of nitrogen from reflectance. Reflectance data were pretreated using moving average and Savitzky–Golay filters which resulted in moderate prediction of R2 0.65 and 0.63 for calibration and validation, respectively. It can be inferred that Hyperion data can be effectively used for the prediction of the presence of soil nitrogen with a moderate level of accuracy even in case of scattered fields and fields of sizes approximately equal to the spatial resolution of the satellite. 相似文献
66.
Priti Maheshwari 《国际地质力学数值与分析法杂志》2008,32(12):1479-1494
In this paper, a beam subjected to end concentrated loads has been modeled and analyzed to estimate its flexural response. The beam has been assumed to rest on reinforced earth beds with reinforcing elements having some finite bending stiffness. The reinforcing elements have been idealized as beams with smooth surface characteristics. The foundation system has been assumed to react only in compression (tensionless foundation), i.e. the separation between the upper beam and the ground surface has been taken into consideration. Winkler springs of different stiffnesses have been used to idealize the upper dense and lower poor soils. As the analysis considers the separation between the upper beam and the soil, the weight of the upper beam has been taken into account. The governing differential equations have been derived and presented in a non‐dimensional form. These equations have been solved using finite difference method with the help of appropriate boundary and continuity conditions. The response of the foundation system has been compared with the case when the beam is in perfect contact with the ground surface. The parametric study shows that the response of the model is greatly affected by the length ratio of beams, ratio of stiffness of upper and lower soil layers, ratio of flexural rigidity of upper and lower beams and weight of the upper foundation beam. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
67.
In the present paper, soil-structure interaction analysis of an infinite beam resting on extensible geosynthetic reinforced
earth beds has been carried out for an applied load moving with constant velocity. The viscous damping of the soil-foundation
system has been given due consideration in the analytical procedure. The infinite beam has been treated as resting on a granular
fill layer overlying the naturally occurring weak soil layer. Geosynthetic layer has been provided in the granular fill layer
and has been considered as extensible. This extensible nature has been incorporated with the help of no slip and the compatibility
conditions at the interface between reinforcing layer and the neighboring soil. These conditions help in eliminating the two
interfacial shear stress parameters and in considering the tensile modulus of geosynthetic layer in the analysis. The influence
of various parameters, like magnitude and velocity of applied moving load, viscous damping, relative stiffness of granular
fill and tensile modulus of geosynthetic layer, on the response of soil-foundation system has been studied. It has been observed
that all these parameters affect the response significantly, however, the effect of velocity of moving load and viscous damping
has been found to be more significant especially at higher velocities. 相似文献
68.
Feasibility of photocatalytic dechlorination of PCB‐138 and Aroclor‐1254 using platinum loaded TiO2 (P25) in an alkaline 2‐propanol medium has been investigated. Experiments were conducted in batch mode using a Rayonet photo‐reactor under aerated and deaerated conditions with black lamps emitting around 350 nm as light source. Two forms of platinum deposition were investigated: Photochemically loaded P25 as well as addition of potassium hexachloroplatinate into the reaction medium. Partial dechlorination of PCBs was observed after 7 h of irradiation with pure TiO2 as catalyst when the reaction vessel was kept open to air. Complete dechlorination was observed after 5 h of irradiation in a deaerated environment. Enhanced photocatalytic efficiency was observed when platinum deposited P25 was used as catalyst, with the best result being obtained when aqueous potassium hexachloroplatinate was added into the reaction medium. Lower potassium hexachloroplatinate concentration (0.1 mM) yielded better performance. 相似文献
69.
70.
E × B-drifting jets have been generally ignored for the past 25 years even though they may well describe all the astrophysical
jet sources, both on galactic and stellar scales. Here we present closed-form solutions for their joint field-and-particle
distribution, argue that the observed jets are near equipartition, with extremely relativistic, monoenergetic e±-pairs of bulk Lorentz factor γ ≲ 104, and are first-order stable. We describe plausible mechanisms for the jets’ (i) formation, (ii) propagation, and (iii) termination. Wherever a beam meets with resistance, its frozen-in Poynting flux transforms the delta-shaped energy distribution
of the pairs into an almost white power law,E
2
N
E ∼E
−∫ with ∫ ≳ 0, via single-step falls through the huge convected potential. 相似文献