全文获取类型
收费全文 | 523篇 |
免费 | 115篇 |
国内免费 | 99篇 |
专业分类
测绘学 | 32篇 |
大气科学 | 11篇 |
地球物理 | 266篇 |
地质学 | 243篇 |
海洋学 | 53篇 |
天文学 | 1篇 |
综合类 | 29篇 |
自然地理 | 102篇 |
出版年
2024年 | 2篇 |
2023年 | 2篇 |
2022年 | 6篇 |
2021年 | 16篇 |
2020年 | 19篇 |
2019年 | 25篇 |
2018年 | 19篇 |
2017年 | 19篇 |
2016年 | 17篇 |
2015年 | 16篇 |
2014年 | 32篇 |
2013年 | 33篇 |
2012年 | 29篇 |
2011年 | 24篇 |
2010年 | 30篇 |
2009年 | 42篇 |
2008年 | 29篇 |
2007年 | 30篇 |
2006年 | 41篇 |
2005年 | 35篇 |
2004年 | 27篇 |
2003年 | 14篇 |
2002年 | 21篇 |
2001年 | 18篇 |
2000年 | 19篇 |
1999年 | 18篇 |
1998年 | 28篇 |
1997年 | 16篇 |
1996年 | 24篇 |
1995年 | 8篇 |
1994年 | 15篇 |
1993年 | 14篇 |
1992年 | 8篇 |
1991年 | 7篇 |
1990年 | 7篇 |
1989年 | 9篇 |
1988年 | 5篇 |
1987年 | 7篇 |
1986年 | 5篇 |
1976年 | 1篇 |
排序方式: 共有737条查询结果,搜索用时 781 毫秒
51.
52.
依据线图的结构特点,首先提取节点信息快速抓住图像的总体拓扑结构,然后以节点信息指导进行矢量跟踪,能够有效解决复杂的交叉点问题,提高矢量化的效率和精度。基于节点信息指导和对不同连通体分类跟踪的方法避免了跟踪起始点选取的随意性和跟踪方向选取的盲目性,并具有抗噪性好和适应性强的特点。 相似文献
53.
Liu WenHui Chen MengJin Guan Ping Zheng JianJing Jin Qiang Li Jian Wang WanChun Hu GuoYi Xia YanQing Zhang DianWei 《中国科学D辑(英文版)》2007,50(10):1494-1503
The establishment of geochemical-tracing system of gas generation and accumulation is helpful to re-elucidating the gas migration and accumulation in time and space. To deduce the complex process of gas accumulation, a ternary geochemical-tracing system is set up, according to stable isotope inheritance of source rocks, kinetic fractionation of stable isotopes, time-accumulating effect of noble gas isotopes, mantle-derived volatile inheritance, and organic molecule inheritance of light hydrocarbons and thermally kinetic fractionation in their generation, in combination with the previous achievements of gas geochemistry and geochemical parameters of gas-source correlation. There are tight interactions for the geochemical parameters with much information about parent inheritance and special biomarkers, in which they are confirmed each other, reciprocally associated and preferentially used for the requirement so that we can use these geochemical parameters to effectively demonstrate the sources of natural gas, sedimentary environments and thermal evolution of source rocks, migration and accumulation of natural gas, and rearrangement of natural gas reservoirs. It is necessary for the ternary geochemical-tracing system to predict the formation of high efficient gas reservoir and their distribution in time and space. 相似文献
54.
How-Wei Chen 《Geophysical Journal International》1998,133(2):363-378
This paper presents a geometrically based algorithm for computing synthetic seismograms for energy transmitted through a 3-D velocity distribution. 3-D ray tracing is performed to compute the traveltimes and geometrical spreading (amplitude). The formulations of both kinematic and dynamic ray-tracing systems are presented. The two-point ray-tracing problem is solved by systematically updating the initial conditions and adjusting the ray direction until the ray intersects the specified endpoint. The amount of adjustment required depends on the derivatives of the position with respect to the given starting angles between consecutive rays. The algorithm uses derivatives to define the steepest-descent direction and to update the initial directions. The convergence rate depends on the complexity of the model.
Test seismograms compare favourably with those from a 2-D asymptotic ray theory algorithm and a 3-D Gaussian-beam algorithm. The algorithm is flexible in modelling arbitrary source and recorder geometries for various smoothly varying 3-D velocity distributions. The algorithm is further tested by simulating surface-to-tunnel vibroseis field data. Shear waves as well as compressional waves may be approximately included. Application of the algorithm to a data set from the Rainier Mesa of the Nevada Test Site produced a good fit to the transmitted (first arrival) traveltimes and amplitudes, with approximately 15 per cent variation in the local 3-D velocity. 相似文献
Test seismograms compare favourably with those from a 2-D asymptotic ray theory algorithm and a 3-D Gaussian-beam algorithm. The algorithm is flexible in modelling arbitrary source and recorder geometries for various smoothly varying 3-D velocity distributions. The algorithm is further tested by simulating surface-to-tunnel vibroseis field data. Shear waves as well as compressional waves may be approximately included. Application of the algorithm to a data set from the Rainier Mesa of the Nevada Test Site produced a good fit to the transmitted (first arrival) traveltimes and amplitudes, with approximately 15 per cent variation in the local 3-D velocity. 相似文献
55.
Quartz was studied with respect to its silicon isotopic composition and cathodoluminescence in micro-fine disseminated gold deposits in SW Guizhou and NW Guangxi.The results showed that quartz in wall rocks.ores and that in association with hydrothermal silicification are distinctive in silicon isotopes and cathodoluminescence characters.Quartz in association with primary silicification is non-luminescent while that in wall rocks and associated with secondary silicification exhibits striking luminescence.Based on the dynamic fractionation of silicon isotopes,it is suggested that the mineralization was accompanied by rapid transport of aprimary siliceous fluid along the major deep fault system into subordinate faults before ore components deposited in favorable strata via penetration and metasomatism.Therefore,a deep origin is implicit for gold deposits of this type. 相似文献
56.
铅同位素地球化学示踪进展 总被引:1,自引:0,他引:1
本文论述了铅同位素示在地学中的最新应用和发展。由于它在示踪物质来源,圈定找矿靶区,判别构造环境等方面的优势,是当前最重要的地球化学示踪手段之一和同位素地球化学研究的热点。 相似文献
57.
Olivia H. Devereux Karen L. Prestegaard Brian A. Needelman Allen C. Gellis 《水文研究》2010,24(11):1391-1403
Fine sediment sources were characterized by chemical composition in an urban watershed, the Northeast Branch Anacostia River, which drains to the Chesapeake Bay. Concentrations of 63 elements and two radionuclides were measured in possible land‐based sediment sources and suspended sediment collected from the water column at the watershed outlet during storm events. These tracer concentrations were used to determine the relative quantity of suspended sediment contributed by each source. Although this is an urbanized watershed, there was not a distinct urban signature that can be evaluated except for the contributions from road surfaces. We identified the sources of fine sediment by both physiographic province (Piedmont and Coastal Plain) and source locale (streambanks, upland and street residue) by using different sets of elemental tracers. The Piedmont contributed the majority of the fine sediment for seven of the eight measured storms. The streambanks contributed the greatest quantity of fine sediment when evaluated by source locale. Street residue contributed 13% of the total suspended sediment on average and was the source most concentrated in anthropogenically enriched elements. Combining results from the source locale and physiographic province analyses, most fine sediment in the Northeast Branch watershed is derived from streambanks that contain sediment eroded from the Piedmont physiographic province of the watershed. Sediment fingerprinting analyses are most useful when longer term evaluations of sediment erosion and storage are also available from streambank‐erosion measurements, sediment budget and other methods. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
58.
A global positioning system and ground penetrating radar surveys is used to produce digital elevation models of the surface and bed of Brewster Glacier. These are used to derive maps of subglacial hydraulic potential and drainage system structure using three different assumptions about the subglacial water pressure (Pw): (i) Pw = ice overburden; (ii) Pw = half ice overburden; (iii) Pw = atmospheric. Additionally, 16 dye‐tracing experiments at 12 locations were performed through a summer melt season. Dye return curve shape, together with calculations of transit velocity, dispersivity and storage, are used to infer the likely morphology of the subglacial drainage system. Taken together, the data indicate that the glacier is underlain by a channelised but hydraulically inefficient drainage system in the early summer in which water pressures are close to ice overburden. By mid‐summer, water pressures are closer to half‐ice overburden and the channelised drainage system is more hydraulically efficient. Surface streams that enter the glacier close to the location of major subglacial drainage pathways are routed quickly to the channels and then to the glacier snout. Streams that enter the glacier further away from the drainage pathways are routed slowly to the channels and then to the snout because they first flow through a distributed drainage system. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
59.
60.
自剥蚀地貌中形成的剥蚀产物搬运到沉积区或汇水盆地沉积下来的过程,称为“源-汇”系统。从这些“汇”中识别“源”的信息,结合地层的沉积时代,约束黄河的形成时代、水系格局演化、流域内构造运动以及环境变迁成为黄河水系演化研究的重要内容。然而,时至今日学者们对黄河何时出现的认识并不一致,存在“中新世内流黄河和中新世外流黄河”之争。为了解决这些争议,我们运用大河物源示踪研究中新兴的碎屑钾长石Pb同位素方法,对黄河中下游沉积物开展碎屑钾长石Pb同位素分析(n=378),将其与长江中下游和华夏板块已报道的碎屑和基岩钾长石Pb同位素结果进行对比,结果表明三者之间的钾长石Pb同位素组成明显不同。台湾岛是中国东部陆架海中新世地层出露最齐全的地区。因而,我们将黄河中下游与台湾岛中新统的钾长石Pb同位素比值进行对比,结果表明二者之间不存在物源联系。结合中国东部渤海湾盆地、南黄海盆地和东海盆地中新统的物源示踪研究结果,说明黄河物质在中新世未流入上述盆地。总体而言,中国北方中新世的构造活动控制了黄河的演化过程,导致其此时处于分段演化阶段。 相似文献