全文获取类型
收费全文 | 2941篇 |
免费 | 150篇 |
国内免费 | 39篇 |
专业分类
测绘学 | 92篇 |
大气科学 | 358篇 |
地球物理 | 745篇 |
地质学 | 888篇 |
海洋学 | 253篇 |
天文学 | 428篇 |
综合类 | 9篇 |
自然地理 | 357篇 |
出版年
2022年 | 13篇 |
2021年 | 50篇 |
2020年 | 61篇 |
2019年 | 56篇 |
2018年 | 68篇 |
2017年 | 83篇 |
2016年 | 111篇 |
2015年 | 101篇 |
2014年 | 108篇 |
2013年 | 205篇 |
2012年 | 127篇 |
2011年 | 175篇 |
2010年 | 136篇 |
2009年 | 159篇 |
2008年 | 170篇 |
2007年 | 152篇 |
2006年 | 144篇 |
2005年 | 124篇 |
2004年 | 105篇 |
2003年 | 107篇 |
2002年 | 94篇 |
2001年 | 58篇 |
2000年 | 67篇 |
1999年 | 50篇 |
1998年 | 41篇 |
1997年 | 43篇 |
1996年 | 37篇 |
1995年 | 33篇 |
1994年 | 24篇 |
1993年 | 25篇 |
1992年 | 16篇 |
1991年 | 22篇 |
1990年 | 20篇 |
1989年 | 34篇 |
1988年 | 15篇 |
1987年 | 23篇 |
1986年 | 14篇 |
1985年 | 31篇 |
1984年 | 20篇 |
1983年 | 26篇 |
1982年 | 20篇 |
1981年 | 25篇 |
1980年 | 16篇 |
1979年 | 19篇 |
1978年 | 10篇 |
1977年 | 11篇 |
1976年 | 11篇 |
1975年 | 9篇 |
1974年 | 12篇 |
1973年 | 8篇 |
排序方式: 共有3130条查询结果,搜索用时 266 毫秒
91.
Myers developed a matrix form of the cokriging equations, but one that entails the solution of a large system of linear equations.
Large systems are troublesome because of memory requirements and a general increase in the matrix condition number. We transform
Myers’s system into a set of smaller systems, whose solution gives the classical kriging results, and provides simultaneously
a nested set of lower dimensional cokriging results. In the course of developing the new formulation we make an interesting
link to the Cauchy-Schwarz condition for the invertibility of a system, and another to a simple situation of coregionalization.
In addition, we proceed from these new equations to a linear approximation to the cokriging results in the event that the
crossvariograms are small, allowing one to take advantage of a recent results of Xie and others which proceeds by diagonalizing
the variogram matrix function over the lag classes. 相似文献
92.
Global monsoons in the mid-Holocene and oceanic feedback 总被引:10,自引:3,他引:10
The response of the six major summer monsoon systems (the North American monsoon, the northern Africa monsoon, the Asia monsoon, the northern Australasian monsoon, the South America monsoon and the southern Africa monsoon) to mid-Holocene orbital forcing has been investigated using a coupled ocean–atmosphere general circulation model (FOAM), with the focus on the distinct roles of the direct insolation forcing and oceanic feedback. The simulation result is also found to compare well with the NCAR CSM. The direct effects of the change in insolation produce an enhancement of the Northern Hemisphere monsoons and a reduction of the Southern Hemisphere monsoons. Ocean feedbacks produce a further enhancement of the northern Africa monsoon and the North American monsoon. However, ocean feedbacks appear to weaken the Asia monsoon, although the overall effect (direct insolation forcing plus ocean feedback) remains a strengthened monsoon. The impact of ocean feedbacks on the South American and southern African monsoons is relatively small, and therefore these regions, especially the South America, experienced a reduced monsoon regime compared to present. However, there is a strong ocean feedback on the northern Australian monsoon that negates the direct effects of orbital changes and results in a strengthening of austral summer monsoon precipitation in this region. A new synthesis is made for mid-Holocene paleoenvironmental records and is compared with the model simulations. Overall, model simulations produce changes in regional climates that are generally consistent with paleoenvironmental observations. 相似文献
93.
Mitigating the Effects of Climate Change on the Water Resources of the Columbia River Basin 总被引:5,自引:3,他引:5
Jeffrey T. Payne Andrew W. Wood Alan F. Hamlet Richard N. Palmer Dennis P. Lettenmaier 《Climatic change》2004,62(1-3):233-256
The potential effects of climate change on the hydrology and water resources of the Columbia River Basin (CRB) were evaluated using simulations from the U.S. Department of Energy and National Center for Atmospheric Research Parallel Climate Model (DOE/NCAR PCM). This study focuses on three climate projections for the 21st century based on a `business as usual' (BAU) global emissions scenario, evaluated with respect to a control climate scenario based on static 1995 emissions. Time-varying monthly PCM temperature and precipitation changes were statistically downscaled and temporally disaggregated to produce daily forcings that drove a macro-scale hydrologic simulation model of the Columbia River basin at 1/4-degree spatial resolution. For comparison with the direct statistical downscaling approach, a dynamical downscaling approach using a regional climate model (RCM) was also used to derive hydrologic model forcings for 20-year subsets from the PCM control climate (1995–2015) scenario and from the three BAU climate(2040–2060) projections. The statistically downscaled PCM scenario results were assessed for three analysis periods (denoted Periods 1–3: 2010–2039,2040–2069, 2070–2098) in which changes in annual average temperature were +0.5,+1.3 and +2.1 °C, respectively, while critical winter season precipitation changes were –3, +5 and +1 percent. For RCM, the predicted temperature change for the 2040–2060 period was +1.2 °C and the average winter precipitation change was –3 percent, relative to the RCM controlclimate. Due to the modest changes in winter precipitation, temperature changes dominated the simulated hydrologic effects by reducing winter snow accumulation, thus shifting summer streamflow to the winter. The hydrologic changes caused increased competition for reservoir storage between firm hydropower and instream flow targets developed pursuant to the Endangered Species Act listing of Columbia River salmonids. We examined several alternative reservoir operating policies designed to mitigate reservoir system performance losses. In general, the combination of earlier reservoir refill with greater storage allocations for instream flow targets mitigated some of the negative impacts to flow, but only with significant losses in firm hydropower production (ranging from –9 percent in Period1 to –35 percent for RCM). Simulated hydropower revenue changes were lessthan 5 percent for all scenarios, however, primarily due to small changes inannual runoff. 相似文献
94.
95.
Late Miocene movement within the Himalayan Main Central Thrust shear zone, Sikkim, north-east India 总被引:7,自引:0,他引:7
E. J. Catlos C. S. Dubey T. M. Harrison M. A. Edwards 《Journal of Metamorphic Geology》2004,22(3):207-226
In the Sikkim region of north‐east India, the Main Central Thrust (MCT) juxtaposes high‐grade gneisses of the Greater Himalayan Crystallines over lower‐grade slates, phyllites and schists of the Lesser Himalaya Formation. Inverted metamorphism characterizes rocks that immediately underlie the thrust, and the large‐scale South Tibet Detachment System (STDS) bounds the northern side of the Greater Himalayan Crystallines. In situ Th–Pb monazite ages indicate that the MCT shear zone in the Sikkim region was active at c. 22, 14–15 and 12–10 Ma, whereas zircon and monazite ages from a slightly deformed horizon of a High Himalayan leucogranite within the STDS suggest normal slip activity at c. 17 and 14–15 Ma. Although average monazite ages decrease towards structurally lower levels of the MCT shear zone, individual results do not follow a progressive younging pattern. Lesser Himalaya sample KBP1062A records monazite crystallization from 11.5 ± 0.2 to 12.2 ± 0.1 Ma and peak conditions of 610 ± 25 °C and 7.5 ± 0.5 kbar, whereas, in the MCT shear zone rock CHG14103, monazite crystallized from 13.8 ± 0.5 to 11.9 ± 0.3 Ma at lower grade conditions of 525 ± 25 °C and 6 ± 1 kbar. The P–T–t results indicate that the shear zone experienced a complicated slip history, and have implications for the understanding of mid‐crustal extrusion and the role of out‐of‐sequence thrusts in convergent plate tectonic settings. 相似文献
96.
Exploring the attractions of city centre living: evidence and policy implications in British cities 总被引:1,自引:0,他引:1
The British city centre has been the focus of dynamic housing development and repopulation strategies as a key dimension of the government's urban renaissance programme. Through large scale interview surveys in the provincial city centres of Bristol and Swansea, this paper explores the positive and negative features of city centre living, and how these vary amongst a range of social and physical characteristics. The findings suggest that the practical and mundane attractions of city centre living are more important than the more widely publicised lifestyle and cultural attractions of the city centre commonly emphasised in the gentrification literature. Moreover, the research points to differences in attitudes between age groups, with younger adults placing greater value on the range of social and cultural activities in the city centre, whilst older people express high levels of satisfaction with the attractiveness of the environment. A number of policy implications emerge to encourage inclusive marketing strategies within the continued urban renaissance, particularly to challenge the negative perceptions and promote the varied attractions of city centre living to a mix of social groups. 相似文献
97.
Ian J. Basson Samantha Perritt Michael K. Watkeys Andrew H. Menzies 《Gondwana Research》2004,7(1):57-73
The whole-rock geochemistry of metamorphosed greywackes, arenites and arkoses within the Mesoproterozoic Namaqua-Natal-Maudheim Province is interpreted with the aim of establishing geochemical correlations and defining common sediment source terrains. Metasediments of the Mfongosi Group of the Natal Sector of the Namaqua-Natal Metamorphic Province were sampled from their type area in the Mfongosi Valley. Metagreywackes from the northern limits of the Mfongosi Valley, directly adjacent to the Kaapvaal Craton, show ocean island arc signatures while metagreywackes from the southern limits of the Mfongosi Valley, near the contact with the Madidima Thrust of the Natal nappe zone, show mainly active continental margin signatures. Interleaved, geochemically distinct low-Ca+Na, high-K metamorphosed arkoses to lithic arkoses indicate a minor passive margin sediment component. Geochemical classification of low-grade Ahlmannryggen Group greywackes, arenites and arkoses of the Grunehogna Province, Antarctica, indicates both active and passive continental margin sediment sources. An oceanic island arc signature is not evident in Ahlmannryggen Group data. The active continental margin signature in both Natal Sector and Grunehogna Province metasediments potentially provides for a common link between these terranes. Discriminant Function Analysis, using three pre-defined provenance sub-sets within the Mfongosi Group and two pre-defined provenance sub-sets within the Ahlmannryggen Group, indicate that metasediments with active continental margin signatures from both groups are geochemically identical, implying that the active continental margin of the Grunehogna Province shed immature sediments westwards (African azimuths) into the developing, narrow or restricted Mesoproterozoic ‘Mfongosi Basin.’ This was accompanied by minor sediment influx from a stable continental platform, potentially the Kaapvaal Craton. Oblique and diachronous collision, initiated in the southwestern portions of the combined Natal Sector/Grunehogna Province system produced a laterally variable Mfongosi Group, which formed in the ‘Mfongosi Basin’. Coarse-grained sediments dominated in its eastern portions while basalts with thin sapropelite units dominated in its western portions. 相似文献
98.
Andrew McGregor 《The Australian geographer》2004,35(2):141-149
This paper presents group research as a potentially powerful research tool for cultural geographers interested in the ways knowledge is produced at the interpersonal level. I argue that groups can create unique research spaces that incorporate 'sociality', the everyday social norms, logics and languages that guide ordinary conversations, into the research agenda. By concentrating on these discursive norms, insights can be obtained into the ways that knowledge is negotiated, produced and constrained in different places. To demonstrate the value of group research, a case study involving Australian environmentalists is analysed, to show how forests are constructed, contested and naturalised within this particular subculture. The article concludes by emphasising the importance of group research to critical geographers interested in social empowerment and change. 相似文献
99.
The Elusive AD 1826 Tsunami, South Westland, New Zealand 总被引:1,自引:0,他引:1
James R. Gofr Andrew Wells Catherine Chagu‐Goff Scott L. Nichol Robert J.N. Devoy 《New Zealand geographer》2004,60(2):28-39
In AD 1826 sealers reported earthquake and tsunami activity in Fiordland, although, contemporary or near‐contemporary accounts of tsunami inundation at the time are elusive. A detailed analysis of recent sediments from Okarito Lagoon builds on contextual evidence provided by earlier research concerning past tsunami inundation. Sedimentological, geochemical, micropalaeontological and geochronological data are used to determine palaeoenvironments before, during and after what was most probably tsunami inundation in AD 1826. The most compelling chronological control is provided fry a young cohort of trees growing on a raised shoreline bench stranded fry a drop in the lagoon water level following tsunami inundation. 相似文献
100.
The MER rover Opportunity has carried out the first outcrop-scale investigation of ancient sedimentary rocks on Mars. The rocks, exposed in craters and along fissures in Meridiani Planum, are sandstones formed via the erosion and re-deposition of fine grained siliciclastics and evaporites derived from the chemical weathering of olivine basalts by acidic waters. A stratigraphic section more than seven meters thick measured in Endurance crater is dominated by eolian dune and sand sheet facies; the uppermost half meter, however, exhibits festoon cross lamination at a length scale that indicates subaqueous deposition, likely in a playa-like interdune setting. Silicates and sulfate minerals dominate outcrop geochemistry, but hematite and Fe3D3 (another ferric iron phase) make up as much as 11% of the rocks by weight. Jarosite in the outcrop matrix indicates precipitation at low pH. Cements, hematitic concretions, and crystal molds attest to a complex history of early diagenesis, mediated by ambient ground waters. The depositional and early diagenetic paleoenvironment at Meridiani was arid, acidic, and oxidizing, a characterization that places strong constraints on astrobiologial inference. 相似文献