全文获取类型
收费全文 | 4112篇 |
免费 | 951篇 |
国内免费 | 1235篇 |
专业分类
测绘学 | 198篇 |
大气科学 | 282篇 |
地球物理 | 1823篇 |
地质学 | 2551篇 |
海洋学 | 801篇 |
天文学 | 48篇 |
综合类 | 230篇 |
自然地理 | 365篇 |
出版年
2024年 | 8篇 |
2023年 | 51篇 |
2022年 | 110篇 |
2021年 | 199篇 |
2020年 | 214篇 |
2019年 | 249篇 |
2018年 | 214篇 |
2017年 | 206篇 |
2016年 | 220篇 |
2015年 | 219篇 |
2014年 | 273篇 |
2013年 | 336篇 |
2012年 | 258篇 |
2011年 | 323篇 |
2010年 | 268篇 |
2009年 | 289篇 |
2008年 | 290篇 |
2007年 | 323篇 |
2006年 | 316篇 |
2005年 | 259篇 |
2004年 | 230篇 |
2003年 | 199篇 |
2002年 | 158篇 |
2001年 | 132篇 |
2000年 | 132篇 |
1999年 | 144篇 |
1998年 | 85篇 |
1997年 | 100篇 |
1996年 | 90篇 |
1995年 | 78篇 |
1994年 | 73篇 |
1993年 | 47篇 |
1992年 | 46篇 |
1991年 | 32篇 |
1990年 | 23篇 |
1989年 | 20篇 |
1988年 | 18篇 |
1987年 | 12篇 |
1986年 | 11篇 |
1985年 | 4篇 |
1984年 | 7篇 |
1983年 | 7篇 |
1982年 | 7篇 |
1981年 | 11篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1973年 | 1篇 |
1954年 | 4篇 |
排序方式: 共有6298条查询结果,搜索用时 62 毫秒
81.
Assessing the Vibrational Frequencies of the Cathedral of Cologne (Germany) by Means of Ambient Seismic Noise Analysis 总被引:1,自引:0,他引:1
Andreas Fäcke Stefano Parolai Sandra M. Richwalski Lothar Stempniewski 《Natural Hazards》2006,38(1-2):229-236
Ambient seismic noise measurements were conducted inside the Cathedral of Cologne (Germany) for assessing its frequencies
of vibration and for checking whether these occur in the range where soil amplification is expected. If this is the case,
damages may increase in case of an earthquake due to an increased structural response of the building. Analysis of the ratio
between the horizontal and vertical components of the spectra recorded at stations located inside the building as well as
the ratio between the corresponding components of the spectra recorded simultaneously inside the building and at a reference
station placed in the basement of the cathedral indicated several modes of vibration. Facilitated by these results an assessment
of the seismic vulnerability was attempted for a 2D ground motion scenario using the finite element method. 相似文献
82.
A. B. Kuznetsov M. A. Semikhatov A. V. Maslov I. M. Gorokhov E. M. Prasolov M. T. Krupenin I. V. Kislova 《Stratigraphy and Geological Correlation》2006,14(6):602-628
New data on Sr-and C-isotopic systematics of carbonate rocks from the Upper Riphean stratotype (Karatau Group of the southern Urals) are obtained for several southwestern sections of the Bashkirian meganticlinorium, which have not been studied before. The results obtained supplement the Sr-and C-isotopic information for the group upper horizons thus detailing chemostratigraphic characterization of the entire succession. Limestone and dolostone samples used to analyze the Sr isotope composition satisfy strict geochemical criteria of the isotopic system retentivity and have been subjected to preliminary treatment in ammonium acetate to remove secondary carbonate phases. Data on 255 samples of carbonate rocks (171 studied for the first time) show that δ13C value varies in the Karatau Group succession from ?2.8 to +5.9 ‰ V-PDB with several in-phase excursions from the general trend in all the sections studied in the area 90 × 130 km. The δ13C variation trend demarcates several levels in the carbonate succession of the Karatau Group suitable for objectives of regional stratigraphy and for C-isotope chemostratigraphic subdivision of the Upper Riphean. The results of Sr isotopic analysis of 121 samples (51 unstudied before) from the Karatau Group imply that rocks in its lower part (the Katav Formation and basal horizon of the Inzer Formation) experienced considerable secondary alterations, while limestones and dolostones of the overlying interval of the group are frequently unaltered. In the “best” samples satisfying geochemical criteria of the isotopic system retentivity, the 87Sr/86Sr initial ratio increases from 0.70521–0.70532 in the lower Inzer deposits to 0.70611 in the upper Min’yar carbonates, decreasing to <0.70600 near the top of the latter. Above the regional hiatus separating the Min’yar and Uk formation, this ratio grows from 0.70533 to 0.70605–0.70609 in the limestone succession of the last formation. 相似文献
83.
M. V. M. S. Rao K. J. Prasanna Lakshmi L. P. Sarma K. B. Chary 《Journal of Earth System Science》2006,115(6):673-683
Compressional and shear wave velocities and attenuation measurements have been carried out in some of the borehole samples
of acidic, basic and intermediate granulites of Mahabalipuram, Tamil Nadu, India. The results have been obtained at ambient
conditions using ‘time-of-flight’ pulse transmission technique at 1.0 MHz frequency. The results show linear relationships
between velocity and density, and velocity and attenuation properties of the rocks. The acidic granulites show lower velocities
and higher attenuation than the intermediate and basic granulites. The average values of the Poisson’s ratio of acidic, intermediate
and basic granulites have been found to be 0.210, 0.241 and 0.279 respectively. The variations in velocities and attenuation
in these low porosity crystalline rocks are found to be strongly influenced by their mineral composition. The laboratory velocity
data (extrapolated to high pressure) of the present study and the published field velocity data from deep seismic sounding
studies indicate that these granulite facies rocks may belong to mid-crustal depths only. 相似文献
84.
Reconstructing recent limnological history often relies on lead-210 dating to accurately ascribe a chronology to a sediment
profile. In Lake Okeechobee, Florida, a large, shallow subtropical lake that may experience severe mixing, multiple dating
methods are required to confirm that conformable sedimentation has been preserved and that the assumptions of the 210Pb method are satisfied. This study uses stratigraphic profiles of heavy metals, 137Cs, PCBs and pollen as independent dating markers to validate the sediment chronology as determined by 210Pb for three cores from the central mud zone of the lake. Unsupported 210Pb and most dating markers show distinct concentration/depth profiles, suggesting that the sediments have not been severely
mixed for at least the last 75 years. Onset and maximum activity of the radioisotope 137Cs in the cores coincides with the 210Pb-dated interval of 1945–1970. This agrees well with the known timing of atmospheric deposition of 137Cs that resulted from above-ground nuclear testing during late 1940s until 1963. Sediment core profiles of atmospherically
deposited metals such as Zn and Pb, which reflect regional increases during industrialization and decreases after regulation
in the 1970s, exhibit expected concentration increases and peaks coinciding within 5–15 years of the predicted 210Pb dates. Uranium, a contaminant in some phosphate fertilizers, shows large concentration increases at core depths dated to
be about 1950 by 210Pb, matching the intensification of agriculture after WWII. PCBs, which are expected to peak in the early 1970s, were measured
in one core, and the observed peak corresponds to a 210Pb date of about 1960. Pollen makers were unable to verify specific events, but increases in disturbance taxa and declines
in native types correspond generally with the expected dates assigned by 210Pb dating. Conformity between the 210Pb defined dates and independent markers of < ±15 years confirm that Lake Okeechobee sediments do preserve a sequential and
reliable stratigraphic history of the lake, useful for reconstructing past limnological conditions. 相似文献
85.
Tackling social exclusion in European neighbourhoods: experiences and lessons from the NEHOM project
Anneli Kährik 《GeoJournal》2006,67(1):9-25
Disadvantaged urban neighbourhoods with their complex social, economic and physical situations have increasingly become the focus of governmental policies in the EU. The emphasis has shifted from improving physical qualities to building opportunities for social groups in these areas. While physical upgrade is easier to achieve, at least in the short run, initiatives often fail in outcomes on communities and local participation. This paper aims to improve understanding on how to tackle area-based social exclusion and create socially coherent communities. Building on the results of the NEHOM project, the focus is on examining how various interventions have impacted local communities and their opportunities for better inclusion. A comparison between neighbourhoods shows that mixing social groups to a certain degree is likely to be a necessary step towards opening long-term opportunities for a disadvantaged neighbourhood. The planned initiatives should at the same time directly benefit local communities, and the involvement of local groups in the planning and implementation process helps to guarantee that the overall changes are accepted by the local community. However, the proposed guidelines do not provide a universal recipe for a successful renewal suiting each neighbourhood—the contextual matters should always be a starting point when planning an initiative. 相似文献
86.
从分析大湾铀矿田各矿床(点)的铀镭平衡系数变化特征出发,结合放射性同位素比值特征分析铀镭迁移富集行为,指出了各矿床(点)氧化带的深度,以了解各铀矿床(点)潜在的工业价值. 相似文献
87.
库车坳陷侏罗系煤成气动力学模拟研究 总被引:1,自引:0,他引:1
应用黄金管—高压釜封闭体系热模拟实验与GC、GC-IRMS分析技术,结合KINETICS专用软件,对库车坳陷侏罗系煤成气进行了动力学模拟研究。库车坳陷侏罗系煤具有高的产气性,在高演化阶段主要产甲烷气;侏罗系煤热解气甲烷碳同位素为-36‰~-25‰,乙烷碳同位素为-28‰~-16‰;甲烷、C2-C5气态烃的生成活化能分别为(47~64k)calm/ol、(55~72k)calm/ol,频率因子各为5.265×1013s-1、5.388×1018s-1。在此基础上,进一步探讨了克拉2气田天然气的成因。研究认为,克拉2气田天然气属阶段捕获的煤成气,主要聚集了5~1Ma时期的天然气,其成熟度Ro分布范围为1.3%~2.5%。 相似文献
88.
Evangelos I. Stavridakis 《Geotechnical and Geological Engineering》2006,24(2):379-398
Major geotechnical problems in construction involving silty–clayey soils are due to their low strength, durability and high
compressibility of soft soils, and the swell–shrink nature of the overconsolidated swelling soils. Confronted with these problems,
a suitable ground improvement technique is needed, for deep excavations in soft clays, for stability, durability and deformation
control. Cement-stabilization is one of the alternatives. An increase in strength and durability, reduction in deformability
are the main aims of this method. Conventional cement-stabilization methods are used mainly for surface treatment. However,
the use of cement has recently been extended to a greater depth in which cement columns were installed to act as a type of
soil reinforcement (deep cement–soil mixing and cement jet grouting). In situ engineering properties of these silty–clayey soils are often variable and difficult to predict. For this reason cement-stabilization
methods have a basic target to control the aforementioned engineering properties of these clays so that the properties of
a silty–clayey soil become more like the properties of a soft rock such as clayey shale or lightly cemented sandstone. So
cement-stabilization of these soils is essential to control their engineering properties and to predict their engineering
behaviour for construction. In an effort to predict, classify and study the suitability of silty–clayey soils for cement-stabilization
both slaking and unconfined compressive strength tests were carried out on clayey–sand mixtures consisted of two types of
clays, kaolin and bentonite. Finally diagrams were prepared to study the variation of slaking and strength due to compaction,
curing time and cement percentage and also to predict areas of efficient cement-stabilization. 相似文献
89.
提出了一种用于处理多高光谱卫星数据的UPDM分析方法。研究结果证明.该方法应用干Landsat/TM(ETM^-)、Terra/MODIS和ADEOS—II/GLI等高光谱卫星传感器时.光谱重构均方根误差小于0.029适用于研究高光谱卫星遥感数据。 相似文献
90.