全文获取类型
收费全文 | 317篇 |
免费 | 32篇 |
国内免费 | 45篇 |
专业分类
测绘学 | 5篇 |
大气科学 | 21篇 |
地球物理 | 103篇 |
地质学 | 168篇 |
海洋学 | 53篇 |
天文学 | 6篇 |
综合类 | 6篇 |
自然地理 | 32篇 |
出版年
2023年 | 3篇 |
2022年 | 9篇 |
2021年 | 9篇 |
2020年 | 9篇 |
2019年 | 10篇 |
2018年 | 4篇 |
2017年 | 16篇 |
2016年 | 10篇 |
2015年 | 4篇 |
2014年 | 17篇 |
2013年 | 17篇 |
2012年 | 8篇 |
2011年 | 18篇 |
2010年 | 14篇 |
2009年 | 19篇 |
2008年 | 26篇 |
2007年 | 26篇 |
2006年 | 13篇 |
2005年 | 24篇 |
2004年 | 13篇 |
2003年 | 25篇 |
2002年 | 15篇 |
2001年 | 11篇 |
2000年 | 15篇 |
1999年 | 6篇 |
1998年 | 11篇 |
1997年 | 11篇 |
1996年 | 8篇 |
1995年 | 4篇 |
1993年 | 3篇 |
1992年 | 3篇 |
1991年 | 2篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1985年 | 3篇 |
1984年 | 1篇 |
1983年 | 1篇 |
排序方式: 共有394条查询结果,搜索用时 15 毫秒
51.
Analyses (n = 525) of chloride (Cl−), bromide (Br−), nitrate as nitrogen (NO3-N), sodium (Na+), calcium (Ca2+) and potassium (K+) in stream water, tile-drain water and groundwater were conducted in an urban-agricultural watershed (10% urban/impervious, 87% agriculture) to explore potential differences in the signature of Cl− originating from an urban source as compared with an agricultural source. Only during winter recharge events did measured Cl− concentrations exceed the 230 mg/L chronic threshold. At base flow, nearly all surface water and tile water samples had Cl− concentrations above the calculated background threshold of 18 mg/L. Mann–Whitney U tests revealed ratios of Cl− to Br− (p = .045), to NO3-N (p < .0001), to Ca2+ (p < .0001), and to Na+ (p < .0001) to be significantly different between urban and agricultural waters. While Cl− ratios indicate that road salt was the dominant source of Cl− in the watershed, potassium chloride fertilizer contributed as an important secondary source. Deicing in watersheds where urban land use is minimal had a profound impact on Cl− dynamics; however, agricultural practices contributed Cl− year-round, elevating stream base flow Cl− concentrations above the background level. 相似文献
52.
目前的土地估价不仅很少考虑地下空间价值,而且缺乏相对成熟的地下空间估价体系和方法。首先结合成本法、收益还原法及楼层效用比法,建立地下空间使用权价格评估模型。然后,分析了地下空间使用权价值的主要影响因素,发现区位、交通、商业经营等因素显著影响地下商业空间的效用;指出为了提高估价的准确性,地下商业空间效用应该分级、分类设置,而不能笼统地确定为同一水平,并据此测算出上海市地下空间使用权地价分配率。基于所构建的评估模型,以上海市地下商业空间为例,通过克里金内插法,廓清了上海市地下空间使用权价格的空间分布状况;最后,从典型购物中心、市级商业中心和全市商业用地成交地块三个维度,探讨上海市地下商业空间使用权价格的空间分异特征。结果表明:① 上海市地下空间使用权价格空间分布与地上土地使用权价格分布规律基本一致,并由市中心向郊区递减;② 地下商业空间使用权价格对地下轨道交通具有较强的依赖性;③ 与地上商业空间使用权价格分布相比,地下商业空间使用权价格分布的集聚性特征更明显。 相似文献
53.
The article presents a theoretical investigation of the propagation of normally and obliquely incident plane shear waves past a rectangular trench filled up with water saturated soil sandwiched between anisotropic elastic media. The motivation for this work is due to the effectiveness of the isolation of waves by the in-filled rectangular trench. Unlike the most of the previous researchers, this model considers the soil–structure interaction effects and directly determines the influence of barrier in the form of in-filled trench on the mode of wave propagation. It is of interest to determine the reflection and transmission coefficients, and the energy partition distribution of shear waves in the in-filled rectangular trench showing the influence of barrier on the propagation of waves. An extensive parametric study through numerical computation is carried out to investigate the influence of the material properties of the in-filled trench and the amplitude ratios on shear waves. The in-filled trench barrier directly declines the intensity of waves significantly in such a way that the waves do not create any hazards to the nearby structures, if exists at all. 相似文献
54.
A chert-phosphorite unit from the Sugetbrak section in the Tarim Basin was analyzed for rare earth elements (Ce, Eu), redox sensitive proxies, and carbon isotopic compositions (δ13Ccarb and δ 13Corg) in the lower Yurtus Formation of the Early Cambrian period. Redox sensitive element ratios (Th/U, V/Cr, Ni/Co, and V/Sc) were employed to determine the palaeoenvironmental conditions during this period. The ratios indicated that the depositional environment of the chert-phosphorite-black shale unit ranged from suboxic to anoxic. Negative Ce and positive Eu anomalies in the chert-phosphorite assemblages of the studied Yurtus Formation indicated the existence of a redox-stratified ocean, similar to that of South China. Overturn or upward expansion of the deep water-mass probably reached the shallow marine zone after the formation of the Yurtus phosphorites. The characteristics of the negative Ce anomaly may be due to phosphoritic inheritance from the Ce-depleted signature of the overlying water column. Subsequent hydrothermal inputs and reduced detrital supplies of the deep water caused by the upwelling affected certain redox sensitive elements in the sedimentary basin. δ 13Ccarb and δ13Corg negative excursions in the Yurtus chert-phosphorite unit may be related to a transgression phase when episodic basinal upwelling moved 12C- and P-rich waters from the pelagic basin floor to the continental shelf. Although carbon isotopic compositions in the Yurtus chert-phosphorite assemblages may have suffered from diagenetic alteration, they can be used to probe diagenetic conditions. Multi-proxy geochemical studies indicated that the δ 13Ccarb values of the Yurtus chert-phosphorite assemblages might be considered reflections of a predominantly suboxic environment that was subsequently affected by hydrothermal inputs due to the upwelling. 相似文献
55.
Organic Geochemistry of the Early Jurassic Oil Shale from the Shuanghu Area in Northern Tibet and the Early Toarcian Oceanic Anoxic Event 总被引:3,自引:0,他引:3
CHEN Lan YI Haisheng HU Ruizhong ZHONG Hong ZOU Yanrong Open Laboratory of Ore Deposit Geochemistry Institute of Geochemistry Chinese Academy of Sciences Guiyang Guizhou Graduate School of Chinese Academy of Sciences Beijing Institute of Sedimentary Geology Chengdu University of Technology Chengdu Sichuan State Key Laboratory of Organic Geochemistry Guangzhou Institute of Geochemistry Chinese Academy of Sciences Guangzhou Guangdong 《《地质学报》英文版》2005,79(3):392-397
This paper presents new geological and geochemical data from the Shuanghu area in northern Tibet, which recorded the Early Toarcian Oceanic Anoxic Event. The stratigraphic succession in the Shuanghu area consists mostly of grey to dark-colored alternating oil shales, marls and mudstones. Ammonite beds are found at the top of the Shuanghu oil shale section, which are principally of early Toarcian age, roughly within the Harplocearasfalciferrum Zone. Therefore,the oil shale strata at Shuanghu can be correlated with early Toarcian black shales distributing extensively in the European epicontinental seas that contain the records of an Oceanic Anoxic Event. Sedimentary organic matter of laminated shale anomalously rich in organic carbon across the Shuanghu area is characterized by high organic carbon contents, ranging from 1.8% to 26.1%. The carbon isotope curve displays the δ^13C values of the kerogen (δ^13Ckerogen) fluctuating from -26.22 to -23.53‰ PDB with a positive excursion close to 2.17‰, which, albeit significantly smaller, may also have been associated with other Early Toarcian Oceanic Anoxic Events (OAEs) in Europe. The organic atomic C/N ratios range between 6 and 43, and the curve of C/N ratios is consistent with that of the δ^13Ckerogen values. The biological assemblage,characterized by scarcity of benthic organisms and bloom of calcareous nannofossils (coccoliths), reveals high biological productivity in the surface water and an unfavorable environment for the benthic fauna in the bottom water during the Oceanic Anoxic Event. On the basis of organic geochemistry and characteristics of the biological assemblage, this study suggests that the carbon-isotope excursion is caused by the changes of sea level and productivity, and that the black shale deposition, especially oil shales, is related to the bloom and high productivity of coccoliths. 相似文献
56.
泊松比做为了解地球内部介质一个非常重要的参数,在地震学上可以通过P波和S波速度的比值来确定.利用远震P波波形记录中包含的壳-幔速度间断面的Ps转换波及其多次反射/转换波信息,采用在地壳厚度-波速比平面上对接收函数转换波能量的叠加搜索方法,可以测定台站下方区域内地壳的平均厚度和泊松比.我们对渤海湾盆地北西盆山边界地区68个宽频带台站近两年的远震记录资料进行了处理研究,得到了该区域的地壳厚度以及泊松比分布.结果表明,渤海湾盆地北西盆山边界的泊松比值分布横向变化复杂, 山区侧与盆地侧存在明显差异;山区侧泊松比值分布相对均匀,盆地侧变化剧烈,但存在小区域范围内(100~200km)的一致性,与地表出露岩石的对比表明:形成渤海湾盆地的新生代伸展作用在下地壳深度有明显的响应,从地震学上获得的泊松比分布对于了解该地区地壳物质组成可起到很好的约束作用. 相似文献
57.
Manish Kumar Bhawna Sharma AL. Ramanathan M. Someshwar Rao Bhishm Kumar 《Environmental Geology》2009,56(6):1171-1181
Present study is an effort to distinguish between the contributions of natural weathering and anthropogenic inputs towards
high salinity and nutrient concentrations in the groundwater of National Capital Territory (NCT) Delhi, India. Apart from
the source identification, the aquifer of entire territory has been characterized and mapped on the basis of salinity in space
and water suitability with its depth. Major element chemistry, conventional graphical plots and specific ionic ratio of Na+/Cl−, SO4
2−/Cl−, Mg2+/Ca2+ and Ca2+/(HCO3
− + SO4
2−) are conjointly used to distinguish different salinization sources. Results suggest that leaching from the various unlined
landfill sites and drains is the prime cause of NO3
− contamination while study area is highly affected with inland salinity which is geogenic in origin. The seasonal water level
fluctuation and rising water level increases nutrients concentration in groundwater. Mixing with old saline sub-surface groundwater
and dissolution of surface salts in the salt affected soil areas were identified as the principle processes controlling groundwater
salinity through comparison of ionic ratio. Only minor increase of salinity is the result of evaporation effect and pollution
inflows. The entire territory has characterized into four groups as fresh, freshening, near freshening and saline with respect
to salinity in groundwater. The salinity mapping suggests that in general, for drinking needs, groundwater in the fresh, freshening
and near freshening zone is suitable up to a depth of 45, 20 and 12 m, respectively, while the saline zones are unsuitable
for any domestic use. In the consideration of increasing demand of drinking water in the area; present study is vital and
recommends further isotopic investigations and highlights the need of immediate management action for landfill sites and unlined
drains. 相似文献
58.
采用荧光染色法对黄、东海春、秋季中华哲水蚤(Calanus sinicus)个体核酸含量进行了研究,初步探讨了RNA∶DNA比对中华哲水蚤生长状况的指示作用。结果表明,中华哲水蚤在饥饿培养24h后RNA∶DNA比值显著降低;雄性个体RNA含量和RNA∶DNA比值显著低于雌体和C5期幼体;春、秋季(2009年5月和2010年11月)个体RNA含量和RNA∶DNA比值存在显著的空间变化,两者最高值均出现在近岸;两个月份DNA含量相对稳定,在2.5—3.5μg/ind之间;个体RNA含量和RNA:DNA比值与叶绿素之间显著正相关,说明饵料是中华哲水蚤的生长过程中重要的影响因素。 相似文献
59.
This paper deals with the estimation of peak inelastic displacements of SDOF systems, representative of typical steel structures, under constant relative strength scenarios. Mean inelastic deformation demands on bilinear systems (simulating moment resisting frames) are considered as the basis for comparative purposes. Additional SDOF models representing partially‐restrained and concentrically‐braced (CB) frames are introduced and employed to assess the influence of different force‐displacement relationships on peak inelastic displacement ratios. The studies presented in this paper illustrate that the ratio between the overall yield strength and the strength during pinching intervals is the main factor governing the inelastic deformations of partially‐restrained models and leading to significant differences when compared with predictions based on bilinear structures, especially in the short‐period range. It is also shown that the response of CB systems can differ significantly from other pinching models when subjected to low or moderate levels of seismic demand, highlighting the necessity of employing dedicated models for studying the response of CB structures. Particular attention is also given to the influence of a number of scalar parameters that characterise the frequency content of the ground motion on the estimated peak displacement ratios. The relative merits of using the average spectral period Taver, mean period Tm, predominant period Tg, characteristic period Tc and smoothed spectral predominant period To of the earthquake ground motion, are assessed. This paper demonstrates that the predominant period, defined as the period at which the input energy is maximum throughout the period range, is the most suitable frequency content scalar parameter for reducing the variability in displacement estimations. Finally, noniterative equivalent linearisation expressions based on the secant period and equivalent damping ratios are presented and verified for the prediction of peak deformation demands in steel structures. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
60.
Spatial and temporal variations of sedimented organic matter in Xiaohai Lagoon, Hainan Island 总被引:1,自引:1,他引:0
The characteristics of grain-size,total organic carbon(TOC) and total nitrogen(TN) contents,TOC/TN ratios,stable carbon isotope(δ13C) and 210 Pb dating were measured in six sediment cores from the Xiaohai Lagoon.The results show distinct spatial and temporal variations in sedimentation patterns.The sediments are dominated by clayey silt,sandy silt and by silty sand in the southern,middle lagoon and the northern lagoon,respectively.TOC and TN contents decline from south to north.Sedimentation rates,determined by 210 Pb dating,tend to decrease from south to middle.However,the determination of sedimentation rate in the north is difficult.These spatial variations are related to the variations in sediment sources and hydrodynamic conditions in the Xiaohai Lagoon.The variations of organic matter signatures can be divided into two stages in the cores from the southern and middle lagoon.Before 1988,the organic matter signatures are relatively stable.The contribution of terrestrial organic carbon sources varies between 60% and 85%.After 1988,the organic matter signatures demonstrate significant variations.TOC and TN contents increase rapidly,TOC/TN ratios decrease,δ13C values shift to higher and the contribution of terrestrial organic carbon sources decreases to 40%-50%.The contributions of phytoplankton organic matter have increased in the sediment since 1988.Increasing aquaculture activities have had a significant impact on organic matter signatures since 1988.The sedimentation rates have increased rapidly in the southern and middle lagoon since 1988 due to the anthropogenic activities which include aquaculture,mining and deforestation.These activities have caused eutrophication and increased siltation in the southern and middle lagoon. 相似文献