全文获取类型
收费全文 | 8925篇 |
免费 | 2247篇 |
国内免费 | 2924篇 |
专业分类
测绘学 | 313篇 |
大气科学 | 1544篇 |
地球物理 | 2114篇 |
地质学 | 6531篇 |
海洋学 | 971篇 |
天文学 | 81篇 |
综合类 | 1149篇 |
自然地理 | 1393篇 |
出版年
2024年 | 70篇 |
2023年 | 183篇 |
2022年 | 322篇 |
2021年 | 351篇 |
2020年 | 333篇 |
2019年 | 404篇 |
2018年 | 356篇 |
2017年 | 436篇 |
2016年 | 418篇 |
2015年 | 458篇 |
2014年 | 580篇 |
2013年 | 541篇 |
2012年 | 696篇 |
2011年 | 645篇 |
2010年 | 612篇 |
2009年 | 650篇 |
2008年 | 619篇 |
2007年 | 707篇 |
2006年 | 685篇 |
2005年 | 581篇 |
2004年 | 507篇 |
2003年 | 449篇 |
2002年 | 431篇 |
2001年 | 434篇 |
2000年 | 351篇 |
1999年 | 359篇 |
1998年 | 317篇 |
1997年 | 268篇 |
1996年 | 229篇 |
1995年 | 211篇 |
1994年 | 178篇 |
1993年 | 178篇 |
1992年 | 121篇 |
1991年 | 96篇 |
1990年 | 84篇 |
1989年 | 56篇 |
1988年 | 59篇 |
1987年 | 35篇 |
1986年 | 15篇 |
1985年 | 15篇 |
1984年 | 8篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 9篇 |
1979年 | 4篇 |
1978年 | 12篇 |
1977年 | 13篇 |
1976年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 17 毫秒
291.
The exponential decline in saturated hydraulic conductivity with depth: a novel method for exploring its effect on water flow paths and transit time distribution
下载免费PDF全文
![点击此处可从《水文研究》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The strong vertical gradient in soil and subsoil saturated hydraulic conductivity is characteristic feature of the hydrology of catchments. Despite the potential importance of these strong gradients, they have proven difficult to model using robust physically based schemes. This has hampered the testing of hypotheses about the implications of such vertical gradients for subsurface flow paths, residence times and transit time distribution. Here we present a general semi‐analytical solution for the simulation of 2D steady‐state saturated‐unsaturated flow in hillslopes with saturated hydraulic conductivity that declines exponentially with depth. The grid‐free solution satisfies mass balance exactly over the entire saturated and unsaturated zones. The new method provides continuous solutions for head, flow and velocity in both saturated and unsaturated zones without any interpolation process as is common in discrete numerical schemes. This solution efficiently generates flow pathlines and transit time distributions in hillslopes with the assumption of depth‐varying saturated hydraulic conductivity. The model outputs reveal the pronounced effect that changing the strength of the exponential decline in saturated hydraulic conductivity has on the flow pathlines, residence time and transit time distribution. This new steady‐state model may be useful to others for posing hypotheses about how different depth functions for hydraulic conductivity influence catchment hydrological response. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
292.
Inverse modelling using PS‐InSAR data for improved land subsidence simulation in Las Vegas Valley,Nevada
下载免费PDF全文
![点击此处可从《水文研究》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Las Vegas Valley has had a long history of groundwater development and subsequent surface deformation. InSAR interferograms have revealed detailed and complex spatial patterns of subsidence in the Las Vegas Valley area that do not coincide with major pumping regions. This research represents the first effort to use high spatial and temporal resolution subsidence observations from InSAR and hydraulic head data to inversely calibrate transmissivities (T), elastic and inelastic skeletal storage coefficients (Ske and Skv) of the developed‐zone aquifer and conductance (CR) of the basin‐fill faults for the entire Las Vegas basin. The results indicate that the subsidence observations from InSAR are extremely beneficial for accurately quantifying hydraulic parameters, and the model calibration results are far more accurate than when using only groundwater levels as observations, and just a limited number of subsidence observations. The discrepancy between distributions of pumping and greatest levels of subsidence is found to be attributed to spatial variations in clay thickness. The Eglington fault separates thicker interbeds to the northwest from thinner interbeds to the southeast and the fault may act as a groundwater‐flow barrier and/or subsidence boundary, although the influence of the groundwater barrier to this area is found to be insignificant. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
293.
294.
This paper describes how measurements of the movement of identifiable features at the edge of a turbulent plume can be interpreted to determine the properties of the mean flow and consequently, using plume theory, can be used to make estimates of the fluxes of volume (mass), momentum, and buoyancy in a plume. This means that video recordings of smoke rising from a chimney or buoyant material from a source on the sea bed can be used to make accurate estimates of the source conditions for the plume. At best we can estimate the volume flux and buoyancy flux to within about 5% and 15% of the actual values, respectively. Although this is restricted to the case of a plume rising in a stationary and unstratified environment, we show that the results may be of practical use in other more complex situations. In addition, we demonstrate that large-scale (turbulent) coherent structures at the plume edge form on a scale approximately 40% of the local (mean) plume half-width and travel at almost 60% of the average local (mean) velocity in the plume. 相似文献
295.
深层水平位移监测(即测斜)能直接反映土体或支护结构的变形特征,是高风险建筑基坑工程安全的重要保证。本文对测斜技术原理进行了阐述,并根据工程实践经验对测斜监测的作业要点进行了探讨与总结。 相似文献
296.
Pinch and swell structures occur where a more competent layer in a weaker matrix is subjected to layer-parallel extension. In this contribution, we use numerical models to explore the use of pinch and swell structure shape symmetry and asymmetry as a determinant of relative viscosity between layers. Maximum asymmetry is attained when the matrix viscosity on one side is subtly weaker than the competent layer, while the other side is significantly weaker.Our numerical results are directly applied to asymmetrically developed pinch and swell structures in exposed lower continental crust. Here, shape geometries observed in a shear zone comprised of plagioclase-dominated, garnet-dominated and mixed amphibole-plagioclase-dominated bands, reveals that the plagioclase-dominated band is the most competent band and is marginally stronger (2×) and significantly stronger (10–40×) than the fine grained garnet-dominated and mixed amphibole-plagioclase-dominated band, respectively. Based on the experimentally determined viscosity of a plagioclase-dominated material and quantitative microstructural analysis, the viscosity range of the natural rock bands is 2.8 × 1015 to 1.1 × 1017 Pa s. Consequently, the assumption that the experimentally-derived plagioclase flow law is an appropriate proxy for the middle to lower continental crust may lead to a viscosity over-estimation by up to forty times. 相似文献
297.
Microstructural, electron backscatter diffraction (EBSD), and misorientation analyses of a migmatitic granulite-facies orthogneiss from the exhumed lower crust of a Cretaceous continental arc in Fiordland, New Zealand show how deformation was accommodated during and after episodes of melt infiltration and high-grade metamorphism. Microstructures in garnet, omphacite, plagioclase, and K-feldspar suggest that an early stage of deformation was achieved by dislocation creep of omphacite and plagioclase, with subsequent deformation becoming partitioned into plagioclase. Continued deformation after melt infiltration resulted in strain localization in the leucosome of the migmatite, where a change of plagioclase deformation mechanism promoted the onset of grain boundary sliding, most likely accommodated by diffusion creep, in fine recrystallized plagioclase grains. Our results suggest three distinctive transitions in the rheology of the lower crust of this continental arc, where initial weakening was primarily achieved by deformation of both omphacite and plagioclase. Subsequent strain localization in plagioclase of the leucosome indicates that the zones of former melt are weaker than the restite, and that changes in deformation mechanisms within plagioclase, and an evolution of its strength, primarily control the rheology of the lower crust during and after episodes of melting and magma addition. 相似文献
298.
内蒙古黄花滩铜镍矿区辉长岩LA-ICP-MS锆石U-Pb定年及地球化学特征 总被引:1,自引:0,他引:1
在乌拉特后旗—达茂旗—四子王旗一带的内蒙古中部地区分布着超基性-酸性连续岩浆系列,开展其岩体年代学、地球化学研究可为内蒙古中部地区与超基性-基性岩有关的铜镍矿成矿规律研究提供基础地质资料,还可以推断岩浆侵位时期的区域大地构造背景。本文以达茂旗黄花滩铜镍矿区出露的辉长岩为研究对象,采用LA-ICP-MS仪器对无裂隙、高透明度、阴极发光均匀、环带清晰的锆石进行U-Pb定年,获得岩体加权平均年龄为262.4±1.1 Ma,并利用XRF和ICP-MS进行岩石地球化学分析显示黄花滩辉长岩具有后碰撞拉伸环境下的岩浆特征。综合前人相关研究认为,内蒙古中部地区于中二叠世晚期(255~275 Ma)已经进入后碰撞构造阶段,而古亚洲洋在该区域的闭合应发生在晚二叠世前(~285 Ma),此结论丰富了古亚洲洋闭合时间上限及中晚二叠世期间内蒙古中部地区构造背景方面的证据。 相似文献
299.
广西油麻坡钨钼矿床成岩成矿年代学研究及其地质意义 总被引:2,自引:1,他引:1
广西博白_岑溪断裂带位于钦杭成矿带南段,既是一条长期活动的地体边界断裂带,也是一条岩浆活动频繁的W_Mo_Sn_Pb_Zn_Sb_Au_Ag多金属成矿带。文章以该带中的广西博白县油麻坡矽卡岩型钨钼矿床为研究对象,在详细的岩性鉴别和划分的基础上,对矿区内花岗岩类进行了单颗粒锆石LA_ICP_MS锆石U_Pb测年,获得深灰色花岗斑岩的等时线年龄为(479.7±3.9)Ma,灰色黑云母花岗闪长岩为(109.7±1.1)Ma,浅灰色细粒白云母花岗岩为(103.3±1.2)Ma。辉钼矿Re_Os测年获得加权平均年龄为(97.4±1.9)Ma。由此可见,油麻坡岩体为一个复式岩体,由早奥陶世(加里东期)的花岗斑岩和早白垩世(燕山晚期)的黑云母花岗闪长岩和细粒白云母花岗岩组成,其中,黑云母花岗闪长岩构成油麻坡岩体的主体。钨钼矿形成于早白垩世晚期(即燕山晚期),与黑云母花岗闪长岩和白云母花岗岩密切相关。这些高精度测年数据的获得,进一步表明博白断裂带是一条长期活动的断裂,其在加里东期已经活动,并伴有岩浆岩的侵位。燕山晚期,该断裂重新复活,并导致大量的岩浆活动和一定强度的W_Mo_Sn_Pb_Zn_Sb_Au_Ag成矿作用,形成由多个大_中型矿床组成的多金属成矿带。博白_岑溪成矿带属于华南100~80 Ma大规模成矿的一部分,成矿背景与华南地区白垩纪的地壳伸展、钦杭成矿带的再次裂陷有关。 相似文献
300.
中国叠生型铁矿床成矿特征探讨 总被引:3,自引:1,他引:2
叠生成矿作用主要是指早期成矿作用被晚期成矿作用叠加、复合和改造。晚期成矿作用的性质常与早期成矿作用不同,也就是说,在早先己有矿床(或矿体、矿源层)的基础上,叠加复合了晚期成矿作用,即成矿时间上有先后、空间产出上有重叠、并对早先形成的矿床进行复杂的复合、叠加和改造,使成矿作用具有多样性、复杂性,并可形成大矿、富矿。铁矿床中叠生成矿作用广泛发育。按矿床或矿体产出和形成的地质特征,中国叠生型铁矿床可分为风化淋滤型、热液叠加改造型和热液叠加复合型3个亚类。风化淋滤型铁矿床在中国分布有限,规模不大,工业利用价值不大,因而中国的叠生型铁矿床主要是指热液叠加改造型和热液叠加复合型两个亚类。热液叠加改造型主要是指早期的铁矿床(或矿体、矿源层)经后期热液叠加改造,使早期的较贫铁矿床(或矿体、矿源层)成为较富铁矿床(或矿体),这是中国BIF型铁矿床中最重要的富铁矿类型,以鞍本地区弓长岭二矿区为典型代表。弓长岭二矿区铁矿,早期在新太古代形成条带状磁铁石英岩(2528 Ma,贫矿石),后期在古元古代,含矿热液交代改造贫铁矿形成富铁矿(1840 Ma)。热液叠加复合型主要是指后期脉型铁(或稀土元素等)矿床叠加在早期(沉积或其他成因)铁等矿床上而形成的矿床,如白云鄂博铁-铌-稀土元素矿床和黔西菱铁矿矿床。白云鄂博铁-铌-稀土元素矿床的形成与火成碳酸岩有关,在中元古代(1.3 Ga)左右,区内火成碳酸岩的侵位,在早期主要形成以岩浆熔离作用为主的铁-铌-稀土元素矿,晚期叠加了加里东期稀土-铌矿化热液脉。古陆边缘构造带或陆内活化带是形成叠生型铁矿床的有利构造空间,较大的地球化学块体,为形成多期、多成因的矿床提供物质来源,叠生型铁矿床的形成明显受构造的控制。叠生成矿是复杂地质过程的一种具体表现。热液叠加改造型和热液叠加复合型的叠生型铁矿床的形成是因中国独特的大地构造环境决定的。叠生成矿作用的研究,尚处在初步阶段。加强对叠生成矿作用的研究,了解其形成的地质背景、成矿机制、作用过程、控矿因素等,对发展矿床学研究,认识区域成矿特征和指导地质找矿具有重要的理论和实际意义。 相似文献