全文获取类型
收费全文 | 1182篇 |
免费 | 61篇 |
国内免费 | 10篇 |
专业分类
测绘学 | 14篇 |
大气科学 | 70篇 |
地球物理 | 271篇 |
地质学 | 389篇 |
海洋学 | 86篇 |
天文学 | 313篇 |
综合类 | 1篇 |
自然地理 | 109篇 |
出版年
2022年 | 6篇 |
2021年 | 17篇 |
2020年 | 11篇 |
2019年 | 14篇 |
2018年 | 34篇 |
2017年 | 28篇 |
2016年 | 51篇 |
2015年 | 24篇 |
2014年 | 41篇 |
2013年 | 61篇 |
2012年 | 25篇 |
2011年 | 45篇 |
2010年 | 47篇 |
2009年 | 68篇 |
2008年 | 60篇 |
2007年 | 66篇 |
2006年 | 58篇 |
2005年 | 50篇 |
2004年 | 46篇 |
2003年 | 41篇 |
2002年 | 36篇 |
2001年 | 28篇 |
2000年 | 27篇 |
1999年 | 24篇 |
1998年 | 37篇 |
1997年 | 18篇 |
1996年 | 13篇 |
1995年 | 14篇 |
1994年 | 19篇 |
1993年 | 11篇 |
1992年 | 10篇 |
1991年 | 6篇 |
1990年 | 12篇 |
1989年 | 14篇 |
1988年 | 11篇 |
1987年 | 11篇 |
1986年 | 12篇 |
1985年 | 17篇 |
1984年 | 8篇 |
1983年 | 8篇 |
1982年 | 10篇 |
1981年 | 7篇 |
1980年 | 11篇 |
1979年 | 10篇 |
1978年 | 6篇 |
1977年 | 8篇 |
1976年 | 8篇 |
1972年 | 12篇 |
1971年 | 5篇 |
1970年 | 5篇 |
排序方式: 共有1253条查询结果,搜索用时 14 毫秒
51.
Survey and Geographic Information System (GIS) data analysis describes the relative influence of biophysical and human variables on site choices made by marine farmers in New Zealand. Community conflicts have grown in importance in determining farm location and different government planning strategies leave distinct signature patterns. Recent legislation empowers local governments to choose among three strategies for future regional aquaculture development. This paper suggests each strategy could result in different spatial outcomes. Simulation modelling of the type described here can provide a better understanding of farmer responses to management approaches and the range of futures that could result from planning choices made today. 相似文献
52.
53.
A. J. R. White 《Australian Journal of Earth Sciences》2013,60(4):551-555
Dehydration (vapour absent) partial melting reactions in the Earth's crust produce a hydrous granitic melt phase, new anhydrous minerals that are mostly pyroxenes, and new plagioclase more calcic than the initial plagioclase. These solid phases of the melt reaction are restite. If the restite is carried to high levels in the crust as a component of the magma, cooling and crystallisation to granite will result in back reactions in which the H2O in the melt phase is consumed and is not then available to form a hydrothermal solution. Even in magmas in which some restite has been removed there will be some back reaction and again less H2O. Only fractional crystallisation will enrich the H2O in the magma in sufficient amounts to form a substantial quantity of hydrothermal solution and possible mineralisation. 相似文献
54.
A riparian ecosystem downstream of a small dam in central Texas was instrumented for sap flow, soil moisture content, and stream level from 2001. Stable isotopes in water (D and 18O) were analysed from rainfall, stream, lake, and cored sapwood cellulose from cedar elm (Ulmus crassifolia). The isotope signature of water source to cedar elm was identified by back calculation starting with the water isotopes in cellulose, and accounting for leaf‐water evaporation and biological fractionation during cellulose synthesis. The estimated mean isotope of the source water to cedar elm was enriched above rainfall in similarity to stream water during 2002. Flow paths that may have contributed to estimated variability from regional base flow and recharge water were identified using the variably saturated HYDRUS‐2D model. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
55.
Civil society is a critical arena both for exploring Sustainability itself and for sustaining trajectories towards it through innovation, experimentation and debate. Innovations can be mould breaking and can challenge local institutions. Concurrently, initiatives may be fragile due to the development of new working relationships, reliance on voluntary labour and goodwill, and dependence on grant funding. Here we examine different aspects of what it takes to sustain grassroots trajectories for ‘communal growing’, given the pressures that groups and intermediary organisations practicing and supporting this activity experience, and the consequential need to build qualities like ‘resilience’. Attending carefully to the definition of this otherwise slippery concept, a particular focus is given to how contrasting aspects of temporality and agency lead to divergent constructions of ‘resilience’ and strategies for sustaining growing. We draw on fieldwork that explores the practice and support of communal growing in East Sussex, England, and directly associated activities at a national level.We find important interdependencies between communal growing projects and the intermediary organisations supporting them. Additionally there is huge diversity within and between both projects and the organisations that support them, including with respect to the ends to which growing is seen as a means. These ends link growing initiatives – both antagonistically and synergistically – to food, education and health systems. This diversity can be seen positively as: a source of innovation; facilitating the open and bottom up nature of growing; and, enabling the securing of greater financial support for the endeavour. What is less clear is how this plays into framing and configuring communal growing specifically in relation to achieving a more Sustainable and localised food system. We discuss the conceptual and methodological implications of these empirically derived observations with regards future research on grassroots innovations. 相似文献
56.
57.
R. W. Parkinson T. C. Wang J. R. White J. R. David M. E. Hoffman 《Environmental Geology》1993,22(1):26-32
This project was designed to: (1) document the distribution and migration of organochlorine pesticide residues within marsh substrates of 18 St. Lucie County mosquito control impoundments located along the Indian River Lagoon estuary, and (2) evaluate the impact of water management techniques on residue mobility. Our results indicate that detectible concentrations of organochlorine compounds, applied between the late 1940s and early 1950s, are present in 16 of the 18 St. Lucie County mosquito control impoundments. These compounds are primarily restricted to the surficial, organic-rich wetland sediment, which, based upon geotechnical analysis, was exposed to the atmosphere at a time when the impoundments were subjected to pesticide treatment. Contaminated sediments are present below the surficial, organic-rich layer, suggesting that some vertical migration of pesticides has occurred. It is unlikely that leaching associated with the downward percolation of impounded water was responsible for this migration as pesticide residues were never detected within thein situ pore waters. An alternative explanation is that biological processes (e.g., rooting, burrowing) facilitated the downward flux of organochlorine compounds into sediment horizons not subjected to direct treatment. Eighty-eight surface water samples obtained from two impoundments subjected to contrasting water management techniques were analyzed for pesticide content. None of the surficial water samples collected in association with these impoundments contained detectible concentrations of organochlorine compounds. These samples were unfiltered and contained as much as 25 mg/1 of particulate organic matter. This suggests that the currently preferred management technique (RIM), which is designed to maintain water quality, limit mosquito production, and provide for ecological continuity, does not hydraulically mobilize pesticide residues into the Indian River Lagoon estuary. 相似文献
58.
59.
Epithermal environments and styles of mineralization: Variations and their causes, and guidelines for exploration 总被引:3,自引:0,他引:3
Epithermal precious- and base-metal deposits are diverse, reflecting the different tectonic, igneous and structural settings in which they occur, the complexities of their local setting, and the many processes involved in their formation. Most epithermal deposits form at shallow crustal levels where abrupt changes in physical and chemical conditions result in metal deposition and attendant hydrothermal alteration. The principal factors that influence the conditions prevailing in the epithermal environment, and which ultimately determine the sites and character of mineralization, include: geology (structure, stratigraphy, intrusions and rock type, which affect the style and degree of permeability and the reactivity of the host); pressure and temperature (which in the epithermal environment are related on the boiling point with depth curve); hydrology (the relationship between permeability and topography which governs fluid flow, and discharge/recharge characteristics, as well as access of steam-heated waters); chemistry of the mineralizing fluid (which determines the metal-carrying capacity, as well as the associated vein and alteration assemblage); and syn-hydrothermal development of permeability and/or changes in hydraulic gradients.Many attempts have been made to classify epithermal deposits based on mineralogy and alteration, the host rocks, deposit form, genetic models, and standard deposits. All have their strengths and weaknesses. We prefer a simple approach using the fundamental fluid chemistry (high or low sulfidation, reflecting relatively oxidized or reduced conditions, respectively) as readily inferred from vein and alteration mineralogy and zoning, together with the form of the deposit, and using comparative examples to clarify the character of the deposit.Guidelines for exploration vary according to the scale at which work is conducted, and are commonly constrained by a variety of local conditions. On a regional scale the tectonic, igneous and structural settings can be used, together with assessment of the depth of erosion, to select areas for project area scale exploration. At project area scale, direct (i.e. geochemical) or indirect guidelines may be used. Indirect methods involve locating and interpreting hydrothermal alteration as a guide to ore, with the topographic and hydrologic reconstruction of the system being of high priority. These pursuits may involve mineralogic, structural, geophysical or remote sensing methods. On a prospect scale, both direct and indirect methods may be used; however, they can only be effective in the framework of a sound conceptual understanding of the processes that occur in the epithermal environment, and the signatures they leave. 相似文献
60.
Antony White 《Physics of the Earth and Planetary Interiors》1990,60(1-4):147-154
Three seafloor magnetometers made recordings for up to 95 days between July and October 1986 at site across the continental slope of southeast Australia along the profile of the 1984 Tasman Project of Seafloor Magnetotelluric Exploration (TPSME). Land magnetometers at TPSME sites at the coast and Canberra were reactivated to give simultaneous recordings. The seafloor magnetometers were at depths of 140 m, 2240 m and 3380 m and spanned the continental shelf between the coastline and the closest inshore TPSME ocean floor site (depth 4850 m). This experiment complements the TPSME by giving a much sharper definition of the geomagnetic coast effect in this critical region.
Data are presented in the form of Parkinson arrows for comparison with previously derived TPSME results along the profile. They show a strong coast effect with the maximum mid-way down the continental slope. The situation is closely two dimensional, and using this approximation some simple models have been computed. One which gives a relatively good fit at a period of 1 h comprises simply a conductive ocean overlying a uniform conductor at depth. Further work will be needed to determine whether lateral conductivity structure at depth is required to fit the data more closely. 相似文献