全文获取类型
收费全文 | 3580篇 |
免费 | 470篇 |
国内免费 | 635篇 |
专业分类
测绘学 | 46篇 |
大气科学 | 591篇 |
地球物理 | 1426篇 |
地质学 | 957篇 |
海洋学 | 896篇 |
天文学 | 27篇 |
综合类 | 103篇 |
自然地理 | 639篇 |
出版年
2024年 | 23篇 |
2023年 | 60篇 |
2022年 | 108篇 |
2021年 | 126篇 |
2020年 | 167篇 |
2019年 | 169篇 |
2018年 | 154篇 |
2017年 | 148篇 |
2016年 | 145篇 |
2015年 | 139篇 |
2014年 | 185篇 |
2013年 | 266篇 |
2012年 | 161篇 |
2011年 | 192篇 |
2010年 | 169篇 |
2009年 | 255篇 |
2008年 | 257篇 |
2007年 | 249篇 |
2006年 | 196篇 |
2005年 | 158篇 |
2004年 | 193篇 |
2003年 | 147篇 |
2002年 | 125篇 |
2001年 | 117篇 |
2000年 | 113篇 |
1999年 | 119篇 |
1998年 | 68篇 |
1997年 | 88篇 |
1996年 | 62篇 |
1995年 | 39篇 |
1994年 | 53篇 |
1993年 | 35篇 |
1992年 | 35篇 |
1991年 | 21篇 |
1990年 | 28篇 |
1989年 | 20篇 |
1988年 | 14篇 |
1987年 | 9篇 |
1986年 | 13篇 |
1985年 | 10篇 |
1984年 | 12篇 |
1983年 | 13篇 |
1982年 | 6篇 |
1981年 | 6篇 |
1980年 | 5篇 |
1979年 | 1篇 |
1978年 | 3篇 |
1977年 | 1篇 |
1973年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有4685条查询结果,搜索用时 15 毫秒
61.
By taking the Yong River for example in this paper, based on the multiple measured data during 1957 to 2009, the change process of runoff, tide feature, tidal wave, tidal influx and sediment transport are analyzed. Then a mathematical model is used to reveal the influence mechanism on hydrodynamic characteristics and sediment transport of the wading engineering groups such as a tide gate, a breakwater, reservoirs, bridges and wharves, which were built in different periods. The results showed the hydrodynamic characteristics and sediment transport of the Yong River changed obviously due to the wading engineering groups. The tide gate induced deformation of the tidal wave, obvious reduction of the tidal influx and weakness of the tidal dynamic, decrease of the sediment yield of flood and ebb tide and channel deposition. The breakwater blocked estuarine entrances, resulting in the change of the tidal current and the reduction of the tidal influx in the estuarine area. The large-scale reservoirs gradually made the decrease of the Yong River runoff. The bridge and wharf groups took up cross-section areas, the cumulative affection of which caused the increase of tidal level in the tidal river. 相似文献
62.
海底管线是海洋油气输运系统的重要组成成分,其稳定性和安全性尤为重要。通过有限元软件ABAQUS,应用Mohr-Coulomb模型,模拟海底裸置与埋置管线自沉过程。通过平衡初始地应力,设置管土接触,考虑重力和浮力作用,进行土体固结分析,计算裸置与埋置管线的土体与管线竖直位移,并进行管线悬跨研究。计算结果显示,对于裸置管线,参数的改变使土体更容易发生塑性屈服,土体的竖直位移量更大;对于埋置管线,当管线埋深较浅、管线上方土体重量不大时,参数的改变使管线平均密度与周围土体密度相差越大土体越容易发生塑性屈服,土体的竖直位移量越大。当管线悬空时,悬跨长度过大易使管线两端支撑土体被压溃,管线易产生大变形而失稳,因此在管线安装与维护过程中,一定要采取措施降低悬跨长度,保证管线运营安全。 相似文献
63.
Simplified equations of fluid mud motion, which is described as Bingham-Plastic model under waves and currents, are presented by order analysis. The simplified equations are non-linear ordinary differential equations which are solved by hybrid numerical-analytical technique. As the computational cost is very low, the effects of wave current parameters and fluid mud properties on the transportation velocity of the fluid mud are studied systematically. It is found that the fluid mud can move toward one direction even if the shear stress acting on the fluid mud bed is much smaller than the fluid mud yield stress under the condition of wave and current coexistence. Experiments of the fluid mud motion under current with fluctuation water surface are carried out. The fluid mud transportation velocity predicted by the presented mathematical model can roughly match that measured in experiments. 相似文献
64.
根据历史资料、数据和相关研究,结合研究区域背景,分析苏北废黄河三角洲的演变。结果显示,岸线演变在发育阶段和侵蚀阶段分别为向海延伸约90 km和侵蚀后退约22 km,面积相差约800 km2,三角洲地貌演变表现为岸线平直-曲折-平滑-平直的过程。在废三角洲陆海相互作用的基础上,运用演化模式分析三角洲的演变过程。该三角洲演变可以分为7个演变阶段,发育期在径流和潮流作用下以沙洲并陆淤积延伸方式进行,侵蚀期在波浪和潮流作用下以沙洲合并侵蚀后退和淤积外长交替侵蚀的方式。泥沙输运、人类活动和气候变化对废三角洲的演变有重要影响,巨量的来沙是三角洲发育的原因,泥沙平衡被打破是侵蚀的主要原因。发育期中,泥沙输运影响淤积速度和位置,人类活动和气候变化影响黄河河道迁移、输沙量和产沙量;侵蚀期中,泥沙输运影响侵蚀状态,人类活动在一定程度上影响海岸带冲/淤,气候变化将影响三角洲的演变趋势。 相似文献
65.
ZHANG Weiyan JIN Haiyan YAO Xuying JI Zhongqiang ZHANG Xiaoyu YU Xiaoguo ZHANG Fuyuan GAO Aigen 《海洋学报(英文版)》2015,34(10):46-56
Surface sediments from the Changjiang River(Yangtze River) Estuary,Hangzhou Bay,and their adjacent waters were analyzed for their grain size distribution,organic carbon(OC) concentration,and stable carbon isotope composition(δ13C).Based on this analysis,about 36 surface sediment samples were selected from various environments and separated into sand(0.250 mm,0.125–0.250 mm,0.063–0.125 mm) and silt(0.025–0.063 mm)fractions by wet-sieving fractionation methods,and further into silt-(0.004–0.025 mm) and clay-sized(0.004mm) fractions by centrifugal fractionation.Sediments of six grain size categories were analyzed for their OC andδ13C contents to explore the grain size composition and transport paths of sedimentary OC in the study area.From fine to coarse fractions,the OC content was 1.18%,0.51%,0.46%,0.42%,0.99%,and 0.48%,respectively,while theδ13C was –21.64‰,–22.03‰,–22.52‰,–22.46‰,–22.36‰,and –22.28‰,respectively.In each size category,the OC contribution was 42.96%,26.06%,9.82%,5.75%,7.09%,and 8.33%,respectively.The OC content in clay and fine silt fractions(0.025 mm) was about 69.02%.High OC concentrations were mainly found in offshore modern sediments in the northeast of the Changjiang River Estuary,in modern sediments in the lower estuary of the Changjiang River and Hangzhou Bay,and in Cyclonic Eddy modern sediments to the southwest of the Cheju Island.Integrating the distribution of terrestrial OC content of each grain size category with the δ13C of the bulk sediment indicated that the terrestrial organic material in the Changjiang River Estuary was transported seaward and dispersed to the Cyclonic Eddy modern sediments to the southwest of the Cheju Island via two pathways:one was a result of the Changjiang River Diluted Water(CDW) northeastward extending branch driven by the North Jiangsu Coastal Current and the Yellow Sea Coastal Current,while the other one was the result of the CDW southward extending branch driven by the Taiwan Warm Current. 相似文献
66.
Integration of 2D and 3D seismic data from the Qiongdongnan Basin along the northwestern South China Sea margin has enabled the seismic stratigraphy, seismic geomorphology and emplacement mechanisms of eight separate, previously undocumented, mass–transport complexes (MTCs) to be characterized. These eight MTCs can be grouped into two types:(1) Localized detached MTCs, which are confined to submarine canyons and cover hundreds of km2, consist of a few tens of km3 remobilized sediments and show long striations at their base. They resulted from small-scale mass-wasting processes induced by regional tectonic events and gravitational instabilities on canyon margins.(2) Regional attached MTCs, which occur within semi-confined or unconfined settings and are distributed roughly perpendicular to the strike of the regional slope. Attached MTCs occupy hundreds to thousands of km2 and are composed of tens to hundreds of km3 of remobilized sediments. They contain headwall escarpments, translated blocks, remnant blocks, pressure ridges, and basal striations and cat-claw grooves. They were created by large-scale mass-wasting processes triggered by high sedimentation rates, slope oversteepening by shelf-edge deltas, and seismicity.Our results show that MTCs may act as both lateral and top seals for underlying hydrocarbon reservoirs and could create MTC-related stratigraphic traps that represent potential drilling targets on continental margins, helping to identify MTC-related hydrocarbon traps. 相似文献
67.
68.
Sediment source and transport trends are influenced by various hydrodynamic factors, and thus play important roles in sedimentary evolution and coastal stability. To examine sediment transport trends around the abandoned Yellow River delta promontory and its erosion mechanism, we employ empirical orthogonal function (EOF) analysis to study sedimentary characteristics and transport trends of the abandoned Yellow River delta in northern Jiangsu Province, China. The results showed that: (1) the main sediment source in the abandoned Yellow River delta is the submarine coastal slope and both sides of the abandoned Yellow River Delta; (2) the main hydrodynamics controlling sediment transport is the current that runs along the shore, coupled with waves, especially southward currents; (3) the sediment of the study area was redistributed under hydrodynamics; coarse sediments were eroded and broadly transported to the south. Therefore, it is concluded that the sediment sources and transport have important influence on coastal evolution: the sediment source area shows mass loss of deposits and erosion; deposits in the submarine coastal slope provide the source and were continuously eroded to provide materials to other places as a sediment source. 相似文献
69.
Wave shapes that induce velocity skewness and acceleration asymmetry are usually responsible for onshore sediment transport, whereas undertow and bottom slope effect normally contribute to offshore sediment transport. By incorporating these counteracting driving forces in a phase-averaged manner, the theoretically-based quasi-steady formula of Wang (2007) is modified to predict the magnitude and direction of net cross-shore total load transport under the coaction of wave and current. The predictions show an excellent agreement with the measurement data on medium and fine sand collected by Dohmen-Janssen and Hanes (2002) and Schretlen (2012) in a full-scale wave flume at the Coastal Research Centre in Hannover, Germany. The modified formula can predict the net onshore transport of fine sand in sheet flows. In particular, it can predict the net offshore transport of medium sand in rippled beds through enlarged bed roughness, as well as the net offshore transport of fine-to-coarse sand in sheet flows with the aid of a new criterion to judge the occurrence of net offshore transport. 相似文献
70.