全文获取类型
收费全文 | 708篇 |
免费 | 140篇 |
国内免费 | 307篇 |
专业分类
测绘学 | 82篇 |
大气科学 | 5篇 |
地球物理 | 303篇 |
地质学 | 637篇 |
海洋学 | 33篇 |
天文学 | 1篇 |
综合类 | 44篇 |
自然地理 | 50篇 |
出版年
2024年 | 3篇 |
2023年 | 4篇 |
2022年 | 13篇 |
2021年 | 34篇 |
2020年 | 31篇 |
2019年 | 41篇 |
2018年 | 28篇 |
2017年 | 29篇 |
2016年 | 28篇 |
2015年 | 50篇 |
2014年 | 59篇 |
2013年 | 58篇 |
2012年 | 49篇 |
2011年 | 62篇 |
2010年 | 48篇 |
2009年 | 74篇 |
2008年 | 76篇 |
2007年 | 78篇 |
2006年 | 79篇 |
2005年 | 53篇 |
2004年 | 50篇 |
2003年 | 53篇 |
2002年 | 21篇 |
2001年 | 19篇 |
2000年 | 19篇 |
1999年 | 18篇 |
1998年 | 21篇 |
1997年 | 14篇 |
1996年 | 13篇 |
1995年 | 5篇 |
1994年 | 6篇 |
1993年 | 5篇 |
1992年 | 1篇 |
1991年 | 3篇 |
1990年 | 4篇 |
1989年 | 4篇 |
1988年 | 1篇 |
1987年 | 1篇 |
排序方式: 共有1155条查询结果,搜索用时 453 毫秒
991.
《国际泥沙研究》2023,38(5):673-697
The B1 dam of Córrego do Feijão Mine, owned by Vale, S.A. mining company and located on the Ferro-Carvão stream, collapsed and injected 2.8 Mm3 of clayey, silty, and sandy iron- and manganese-rich tailings into the Paraopeba River (Minas Gerais state, Brazil). The accident occurred on 25 January 2019 and the tailings have been co-transported with coarser natural sediment since then, being partly trapped in the Igarapé Weir reservoir located on the Paraopeba River nearly 45 km downstream the injection point. The general purpose of the current study was to model suspended sediment transport in the vicinity of the Igarapé Weir aiming to assess the concomitant barrier effect imposed by this structure. Specifically, the spatial distributions of suspended clay, silt, and very fine-grained sand fractions (CSS) of sediment were mapped around the Igarapé Weir under low-flow (16 m3/s) and high-flow (5 to 10,000 years return period stream discharge; 699–2,699 m3/s) regimes, using RiverFlow2D as the modelling tool. The concentrations of the various grain materials in the upstream and downstream sectors were quantified linking the barrier effect to concentration reductions in the direction of stream flow. It was also a study goal to calculate differences of iron and manganese concentrations in the sediment + tailings mixtures along the Paraopeba River. The study results showed reductions in the CSS between 6.6% and 18%, from upstream to downstream of the Igarapé Weir, related with backwater effects, free and submerged hydraulic jumps, bank sedimentation in periods of high flow, and streambed sedimentation controlled by channel sinuosity and tailings density. These reductions were accompanied by drops in the concentrations of iron and manganese present in the clay and silt fractions, which varied between 6% and 42% under low flows and between 16% and 44% under high flows. Bank sedimentation was viewed as a potential threat to the riparian vegetation in the long-term. Dredging is the potentially most effective mitigation measure to help lead the Paraopeba River to a pre-rupture condition. The retention of sediment + tailings transported in suspension is less effective than the trapping of bedload sediment + tailings behind the Igarapé weir. The efficacy of sediment trapping is expected to be larger for natural sediment because it is much coarser than the tailings. In that context, the simulations revealed for the low-flow period that 33.6% of the sediment deposition comprised suspended transport of natural sediment (thus, was comprised 66.4% of bedload transport), this proportion rose to 86.9% for mixtures of natural sediment + tailings, a result that did not differ much for the high-flow periods. 相似文献
992.
Emine Batürk 《洁净——土壤、空气、水》2019,47(12)
The water quality of Mamas?n Dam, the drinking water source for Aksaray in Turkey, is investigated for assessment of ecological and health risks according to data between December 2015 and November 2016. Water quality index, ecological risk, and health risk assessments are determined to depend on the intensity of occurrence of parameters and seasonal and spatial distributions at different depth ranges. As a result, some ions such as Al, As, B, Ba, Co, F, Mn, Ni, Pb, Se, and Zn were found to be exceeding the limit values of the national regulatory standards. Besides, Pb is listed in Priority Substances and Certain Other Pollutants according to Annex II. Because the WQI values of the water quality are low (below 60), the source is suitable for industrial and agricultural usage; however, it cannot be used for drinking water supply without treatment. The trace element levels, especially Ag, Ba, Cu, Cr, and Zn, are potential pollutants for organisms. The non‐carcinogenic health risks of As threaten both adults and children at “medium” and “high” levels through oral ingestion. The B, F, and Pb elements have Hazard Index values above 0.1 and may cause a “low” level risk chronic disease for children by oral ingestion. 相似文献
993.
以复杂地质条件的西藏高原地区旁多水电站实际工程为背景,探讨了有限元法与无限元法在深覆盖层悬挂式防渗墙土石坝结构非线性动力分析中的差异,分析和比较了无限元法和有限元法在西藏高原旁多水电站土石坝结构三维模型地震动响应的计算结果,验证了引入无限元法模拟西藏高原地区地基中复杂地质条件下的无限域或半无限域问题的可靠性和精确性。结果表明:在西藏高原地区复杂地质条件下,无限元法相比有限元法能量弥散现象较为明显。通过分析得到了西藏高原具有深覆盖层坝体结构地震动响应的规律性分析结果,为无限元法在西藏高原地区此类工程中的应用提供参考。 相似文献
994.
比例边界有限元方法是一种半解析的数值计算方法,具有降维、网格灵活、严格模拟无限域和无需基本解等特点。比例边界有限元方法的基本理论是在整体坐标与局部坐标的比例边界转换基础之上建立的,相似中心的选取是否合理对分析计算具有重要的影响,导致在模拟拱坝这种不规则的空间壳体结构时,具有一定的局限性。采用子结构方法,将坝体分为若干满足相似性要求的区域可解决上述问题,以某拱坝为例给出了合理的坝体子结构分区形式,验证了子结构方法的精确性,为建立基于比例边界有限元方法的坝体-库水-地基系统的计算模型奠定了基础。 相似文献
995.
996.
汉江上游是我国“十五”水环境规划的重点区域之一, 丹江口水库是具有战略意义的重点保护水源.汉江干流和丹江库区现状水质已达Ⅱ类标准, 但因为支流污染严重而形成巨大威胁, 主要污染物COD在“十五”有超标趋势.COD来源以生活源为主, 占77.2%;工业源中制浆造纸业为主要污染行业, COD排量占38.29%.列出了以城镇污水处理厂建设为主体的含A, B, C三种工程项目类型的污染控制方案.实施A类项目需投资27 500万元, 可使汉江上游COD在2005年降至32 682.25 t, 低于基准年1998年的排放水平, 落在省下达的目标总量控制范围内, 保证干流和丹江库区保持Ⅱ类水质; 接踵实施B, C类项目, 可保证其水质稳定达标, 并使支流水质得到明显改善. 相似文献
997.
为了解决上海缺乏深水港址和宁波、舟山港腹地范围狭小的问题,提出了两个相互关联的宏大构想:一是在杭州湾建造锁海大坝并展开大规模的围垦造陆;二是兴建宁波至舟山的跨海交通与输水工程。这两大构想一旦付诸实施,其直接效益主要有二:1.为长江三角洲增加数千平方公里的土地;2.今后可望在宁波-舟山建成世界超级大港和千万人口级特大城市。更进一步的影响则是在锁海大坝沿线和钱塘江下游沿岸将出现两个新的产业密集带与城镇 相似文献
998.
In this paper, the seismic response analysis of concrete gravity dams is presented using the concept of Continuum Damage Mechanics. The analysis is performed using the finite element technique and a proper material degradation/damage model. The damage criterion used here is a second order tensor model based on elastic-brittle characterization and on a power function of the principal tensile stress. The methodology employed is shown to be computationally efficient and consistent in its treatment of both damage growth and propagation. Other important features considered in the analysis are: (1) dam–foundation interaction (2) appropriate modelling of joined rock mass using continuum damage mechanics, and (3) proper modelling of unbounded domain of foundation rock. The infinite media representation of the foundation material has been achieved by using doubly asymptotic approximation. The results of the analysis indicate that the seismic response of a damaged concrete dam could be significantly different from that of an undamaged one. In particular, the analysis shows that during a seismic event, the microstructure of a damaged zone can significantly change due to growth and propagation of microcracks. 相似文献
999.
1000.
大柳树高坝F201断层避让距离研究 总被引:3,自引:0,他引:3
利用国内外类似构造位置上的走滑型、逆走滑型发震断层极震区IX度等震线短轴在断层上、下盘分布宽度的统计分析,探讨了大柳树高坝的F201断层避让距离问题。经统计分析,给出了活断层IX度等震线短轴在断层上盘的优势宽度为6~6.4km,在这个距离之内,活断层的粘滑或蠕滑运动都可能会在地表产生永久性形变,同震或震后灾害严重,对于工程而言存在抗断问题。因此,这个数据可作为F201发震断层避让距离的参考值。野外调查表明大柳树高坝距F201活断层仅1.5km,小于本研究得出的活断层避让距离,而且坝址位于F201活断层上盘,因此不适宜在大柳树坝址修建水利水电工程。 相似文献