全文获取类型
收费全文 | 624篇 |
免费 | 57篇 |
国内免费 | 13篇 |
专业分类
测绘学 | 22篇 |
大气科学 | 101篇 |
地球物理 | 156篇 |
地质学 | 201篇 |
海洋学 | 47篇 |
天文学 | 100篇 |
综合类 | 1篇 |
自然地理 | 66篇 |
出版年
2022年 | 5篇 |
2021年 | 22篇 |
2020年 | 19篇 |
2019年 | 16篇 |
2018年 | 20篇 |
2017年 | 27篇 |
2016年 | 55篇 |
2015年 | 33篇 |
2014年 | 30篇 |
2013年 | 46篇 |
2012年 | 54篇 |
2011年 | 39篇 |
2010年 | 42篇 |
2009年 | 51篇 |
2008年 | 35篇 |
2007年 | 30篇 |
2006年 | 34篇 |
2005年 | 26篇 |
2004年 | 19篇 |
2003年 | 14篇 |
2002年 | 6篇 |
2001年 | 13篇 |
2000年 | 14篇 |
1999年 | 7篇 |
1998年 | 8篇 |
1997年 | 7篇 |
1996年 | 3篇 |
1994年 | 2篇 |
1993年 | 2篇 |
1989年 | 1篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1980年 | 1篇 |
1973年 | 2篇 |
1971年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有694条查询结果,搜索用时 15 毫秒
101.
Frank Dennis John N. Andrews Andrew Parker Jason Poole Manfred Wolf 《Applied Geochemistry》1997,12(6):763-773
The chemical and isotopic composition of groundwater from 52 sites in the London (U.K.) area was determined as part of a project aimed at assessing the spatial variation in the age of Chalk groundwater, and in determining the relationship between fracture and matrix groundwater in this dual porosity system.Systematic changes in groundwater chemistry take place in the downgradient direction in response to several chemical processes. These processes include early concentration by evaporation and congruent dissolution of calcite followed by widespread incongruent dissolution and ion exchange in addition to local oxidation-reduction reactions, gypsum dissolution and saline intrusion. As a result of the above processes, Chalk groundwater follows an evolutionary path from Ca bicarbonate type to Na bicarbonate type.The age of Chalk groundwater was modelled using14C, δ13C,3H, δ2H and δ180. There is a general increase in the groundwater age in a downgradient direction with the oldest water found in N central areas of the basin. Groundwater in the unconfined zones and in areas S of the Greenwich fault is almost entirely of unevolved, modem composition. Carbon-14 modelling suggests that Chalk groundwater in the S basin is generally less than 10000 a old while that in the north is generally between 10000 and 25000 a old. The presence of3H in concentrations of up to 7 TU in groundwater which yields ages of several 1000 a, however, indicates that mechanisms exist for the rapid introduction of recent groundwater to the confined aquifer. Results of palaeorecharge temperature determinations using δ2H, δ180 and noble gas analytical results suggest that significant Devensian recharge did indeed occur in the aquifer.A model of the development of the Chalk recognizes that it is a classic dual porosity aquifer in which groundwater flow occurs predominantly in the fracture system. The upper 50 m of the aquifer was flushed with fresh water during the 2–3 × 106 a of the Quaternary and therefore meteoric water largely replaced the Tertiary and Cretaceous marine water that previously saturated the system. Most processes which control the chemistry of the groundwater occur in the matrix where the surface area is exceptionally high. Although fracture flow dominates the flow regime, diffusion from the matrix into the fracture porosity controls the chemistry of Chalk groundwater. 相似文献
102.
103.
An investigation into the effects of damping and nonlinear geometry models in earthquake engineering analysis
下载免费PDF全文
![点击此处可从《地震工程与结构动力学》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The growing emphasis of considering the behavior of structures at extreme performance states, such as collapse, has necessitated the characterization of the effects of varying attributes of the structural model. One source of variability that has not previously been considered is variability in the mathematical model. This study investigated the effects of changing the geometric nonlinearity approach and damping model on a four‐story buckling restrained braced frame, a four‐story steel moment resisting frame, and an eight‐story steel moment resisting frame. The variations in behavior are quantified using the maximum interstory drift ratio as the performance metric and qualified by comparing the relative displaced shapes and component response histories at the collapse performance state. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
104.
Very slow erosion rates and landscape preservation across the southwestern slope of the Ladakh Range,India
下载免费PDF全文
![点击此处可从《地球表面变化过程与地形》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Craig Dietsch Jason M. Dortch Scott A. Reynhout Lewis A. Owen Marc W. Caffee 《地球表面变化过程与地形》2015,40(3):389-402
Erosion rates are key to quantifying the timescales over which different topographic and geomorphic domains develop in mountain landscapes. Geomorphic and terrestrial cosmogenic nuclide (TCN) methods were used to determine erosion rates of the arid, tectonically quiescent Ladakh Range, northern India. Five different geomorphic domains are identified and erosion rates are determined for three of the domains using TCN 10Be concentrations. Along the range divide between 5600 and 5700 m above sea level (asl), bedrock tors in the periglacial domain are eroding at 5.0 ± 0.5 to 13.1 ± 1.2 meters per million years (m/m.y.)., principally by frost shattering. At lower elevation in the unglaciated domain, erosion rates for tributary catchments vary between 0.8 ± 0.1 and 2.0 ± 0.3 m/m.y. Bedrock along interfluvial ridge crests between 3900 and 5100 m asl that separate these tributary catchments yield erosion rates <0.7 ± 0.1 m/m.y. and the dominant form of bedrock erosion is chemical weathering and grusification. Erosion rates are fastest where glaciers conditioned hillslopes above 5100 m asl by over‐steepening slopes and glacial debris is being evacuated by the fluvial network. For range divide tors, the long‐term duration of the erosion rate is considered to be 40–120 ky. By evaluating measured 10Be concentrations in tors along a model 10Be production curve, an average of ~24 cm is lost instantaneously every ~40 ky. Small (<4 km2) unglaciated tributary catchments and their interfluve bedrock have received very little precipitation since ~300 ka and the long‐term duration of their erosion rates is 300–750 ky and >850 ky, respectively. These results highlight the persistence of very slow erosion in different geomorphic domains across the southwestern slope of the Ladakh Range, which on the scale of the orogen records spatial changes in the locus of deformation and the development of an orogenic rain shadow north of the Greater Himalaya. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
105.
Heterogeneous distributions of CO2 may be more important for dissolution and karstification in coastal eogenetic limestone than mixing dissolution
下载免费PDF全文
![点击此处可从《地球表面变化过程与地形》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Jason D. Gulley Jonathon B. Martin Paul J. Moore Amy Brown Patricia D. Spellman John Ezell 《地球表面变化过程与地形》2015,40(8):1057-1071
Mixing dissolution, a process whereby mixtures of two waters with different chemical compositions drive undersaturation with respect to carbonate minerals, is commonly considered to form cavernous macroporosity (e.g. flank margin caves and banana holes) in eogenetic karst aquifers. On small islands, macroporosity commonly originates when focused dissolution forms globular chambers lacking entrances to the surface, suggesting that dissolution processes are decoupled from surface hydrology. Mixing dissolution has been thought to be the primary dissolution process because meteoric water would equilibrate rapidly with calcium carbonate as it infiltrates through matrix porosity and because pCO2 was assumed to be homogeneously distributed within the phreatic zone. Here, we report data from two abandoned well fields in an eogenetic karst aquifer on San Salvador Island, Bahamas, that demonstrate pCO2 in the phreatic zone is distributed heterogeneously. The pCO2 varied from less than log ?2.0 to more than log ?1.0 atm over distances of less than 30 m, generating dissolution in the subsurface where water flows from regions of low to high pCO2 and cementation where water flows from regions of high to low pCO2. Using simple geochemical models, we show dissolution caused by heterogeneously distributed pCO2 can dissolve 2.5 to 10 times more calcite than the maximum amount possible by mixing of freshwater and seawater. Dissolution resulting from spatial variability in pCO2 forms isolated, globular chambers lacking initial entrances to the surface, a morphology that is characteristic of flank margin caves and banana holes, both of which have entrances that form by erosion or collapse after cave formation. Our results indicate that heterogeneous pCO2, rather than mixing dissolution, may be the dominant mechanism for observed spatial distribution of dissolution, cementation and macroporosity generation in eogenetic karst aquifers and for landscape development in these settings. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
106.
107.
The concept of the hybrid passive control system is studied analytically by investigating the seismic response of steel frame structures. Hybrid control systems consist of two different passive elements combined into a single device or system. The hybrid systems investigated in this research consist of a rate‐dependent damping device paired with a rate‐independent energy dissipation element. The innovative configurations exploit individual element strengths and offset their weaknesses through multiphased behavior. A nine‐story, five‐bay steel moment‐frame was used for the analysis. Six different seismic resisting systems were analyzed and compared. The conventional systems included a special moment‐resisting frame (SMRF) and a dual SMRF–buckling‐restrained brace (BRB) system. The final four configurations are hybrid passive systems. The different hybrid configurations utilize a BRB and either a high‐damping rubber damper or viscous fluid damper. The analyses were run in the form of an incremental dynamic analysis. Several damage measures were calculated, including maximum roof drift, base shear, and total roof acceleration. The results demonstrate the capability of hybrid passive control systems to improve structural response compared with conventional lateral systems and to be effective for performance‐based seismic design. Each hybrid configuration improved some aspect of structural response with some providing benefits for multiple damage measures. The multiphased nature provides improved response for frequent and severe seismic events. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
108.
Dietmar Grimm Ivan Barkhorn David Festa Kate Bonzon Judd Boomhower Valerie Hovland Jason Blau 《Marine Policy》2012,36(3):644-657
What are the effects of transitioning traditionally managed fisheries to incentive-based catch shares fisheries? In a study of all major United States federal catch share fisheries and associated shared stock fisheries in British Columbia, catch shares result in environmental improvements, economic improvements, and a mixture of changes in social performance, relative to the race for fish under traditional management. Environmentally, compliance with total allowable catch increases and discards decrease. Economically, vessel yields rise, total revenues grow, and long-term stock increases are encouraged. Socially, safety increases, some port areas modestly consolidate, needed processing capacity often reduces, and labor markets shift from part time jobs to full time jobs with similar total employment. Newer catch shares address many social concerns through careful design. 相似文献
109.
Organic carbon inputs,common ions and degassing: rethinking mixing dissolution in coastal eogenetic carbonate aquifers
下载免费PDF全文
![点击此处可从《地球表面变化过程与地形》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Caves deliver freshwater from coastal carbonate landscapes to estuaries but how these caves form and grow remains poorly understood. Models suggest fresh and salt water mixing drives dissolution in eogenetic limestone, but have rarely been validated through sampling of mixing waters. Here we assess controls on carbonate mineral saturation states using new and legacy geochemical data that were collected in vertical profiles through three cenotes and one borehole in the Yucatan Peninsula. Results suggest saturation states are primarily controlled by carbon fluxes rather than mixing. Undersaturation predicted by mixing models that rely on idealized end members is diminished or eliminated when end members are collected from above and below actual mixing zones. Undersaturation due to mixing is limited by CO2 degassing from fresh water in karst windows, which results in calcite supersaturation. With respect to saline groundwater, controls on capacity for mixing dissolution were more varied. Oxidation of organic carbon increased pCO2 of saline groundwater in caves (pCO2 = 10–2.06 to 10–0.96 atm) relative to matrix porosity (10–2.39 atm) and local seawater (10–3.12 atm). The impact of increased pCO2 on saturation state, however, depended on the geochemical composition of the saline water and the magnitude of organic carbon oxidation. Carbonate undersaturation due to mixing was limited where gypsum dissolution (Cenote Angelita) or sulfate reduction (Cenote Calica) increased concentrations of common ions (Ca2+ or HCO3?, respectively). Maximum undersaturation was found to occur in mixtures including saline water that had ion concentrations and ratios similar to seawater, but with moderately elevated pCO2 (Cenote Eden). Undersaturation, however, was dominated by the initial undersaturation of the saline end member, mixing was irrelevant. Our results add to a growing body of literature that suggests oxidation of organic carbon, and not mixing dissolution, is the dominant control on cave formation and enlargement in coastal eogenetic karst aquifers. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
110.