全文获取类型
收费全文 | 74616篇 |
免费 | 588篇 |
国内免费 | 1220篇 |
专业分类
测绘学 | 2345篇 |
大气科学 | 4505篇 |
地球物理 | 14193篇 |
地质学 | 30592篇 |
海洋学 | 5826篇 |
天文学 | 13742篇 |
综合类 | 2273篇 |
自然地理 | 2948篇 |
出版年
2022年 | 577篇 |
2021年 | 858篇 |
2020年 | 897篇 |
2019年 | 1005篇 |
2018年 | 6691篇 |
2017年 | 5792篇 |
2016年 | 4545篇 |
2015年 | 1061篇 |
2014年 | 1902篇 |
2013年 | 2812篇 |
2012年 | 3015篇 |
2011年 | 5072篇 |
2010年 | 4250篇 |
2009年 | 4912篇 |
2008年 | 4175篇 |
2007年 | 4892篇 |
2006年 | 2353篇 |
2005年 | 1488篇 |
2004年 | 1605篇 |
2003年 | 1573篇 |
2002年 | 1403篇 |
2001年 | 1175篇 |
2000年 | 987篇 |
1999年 | 706篇 |
1998年 | 727篇 |
1997年 | 735篇 |
1996年 | 549篇 |
1995年 | 573篇 |
1994年 | 535篇 |
1993年 | 433篇 |
1992年 | 449篇 |
1991年 | 428篇 |
1990年 | 477篇 |
1989年 | 421篇 |
1988年 | 388篇 |
1987年 | 409篇 |
1986年 | 336篇 |
1985年 | 463篇 |
1984年 | 468篇 |
1983年 | 453篇 |
1982年 | 432篇 |
1981年 | 407篇 |
1980年 | 412篇 |
1979年 | 347篇 |
1978年 | 378篇 |
1977年 | 320篇 |
1976年 | 293篇 |
1975年 | 293篇 |
1974年 | 274篇 |
1973年 | 344篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Terrorist attacks and natural disasters have potentially severe economic consequences in terms of property damage and business
interruption. However, experience from the September 11 World Trade Center attack and other disasters indicates that the economy
has a great deal of resilience. This refers to the ability to dampen the maximum potential economic output (business interruption)
loss. One of the most prominent sources of resilience is the ability of businesses to reschedule, or recapture, lost production
after the event. Although there have been applications of a fixed parameter recapture factor for each of several aggregated
sectors of the economy, there has been little formal analysis of this resilience action. This study offers a theoretic framework
for analyzing production rescheduling. It distinguishes the major conditions influencing two aspects that have previously
been neglected: (1) the maximum time span over which the rescheduling can take place and (2) the likely decline of the maximum
recapture as the business interruption increases. We divide the relevant time path into two periods after recovery. One is
a function of a recaptured output path after recovery to the status of normal production. The other is a function for the
maximum recaptured production, based on the recaptured output path. The recaptured output path function is assumed to follow
a normal distribution function, and hence, total recaptured output follows the cumulative normal distribution function over
time after productive capacity is restored. Also, we develop a new cumulative normal distribution function for interruption
time duration, which is symmetric with respect to the output axis. This recapture function has unknown parameters. Empirical
data on the recaptured amounts following an actual disaster can be used to estimate the parameters of this function using
simulation methods. 相似文献
992.
Redox problems in the “metallogenic specialization” of magmatic rocks and the genesis of hydrothermal ore mineralization 总被引:1,自引:0,他引:1
I. N. Kigai 《Petrology》2011,19(3):303-321
Considering the history and current state of the problem of the so-called metallogenic specialization of magmatic rocks, the
paper places emphasis onto various aspects of the genesis of ore mineralization depending on the redox state of magmas (as
a logical continuation of S. Ishihara’s works), fluids, and host rocks. These problems were inadequately poorly explored and
discussed by researchers dealing with ore deposits. Various possible variants of ore-forming redox processes for different
types of mineral deposits, with ore mineralization affiliated to granites (Ta, Sn, W, Mo, and Be) and mafic magmas (Au, Ag,
U, Cu, Zn, Pb, As, Sb, and Hg) and, accordingly, to crustal and mantle origin, are discussed. On the basis of analyzed geological
data, including those published over the past three decades, it is shown that the redox state of ore-producing magmas commonly
significantly impacted not only the ore potential of magmatic complexes but also the genetic type of the ore mineralization.
The redox state of the fluids predetermined the transport and precipitation speciation of metals. Influence mechanisms of
hydrocarbons from sedimentary country rocks and gaseous products of their pyrolysis on ore deposition of various metals are
considered. Understanding these mechanisms can be helpful for predicting the possible precipitation sites of ore mineralization
of noble, radioactive, and chalcophile metals. 相似文献
993.
In order to ascertain the forms in which uranium is present in ores of the Melovoe rare metal sedimentary deposit of uranium
and rare earth minerals (South Mangyshlak), we investigated a series of typical ore samples that were collected earlier; both
the uranium content and the total content of rare earth metals in them lay within 0.1–0.3%. The study was carried out by analytical
electron microscopy using transmission electron microscopy and scanning electron microscopy, electron microdiffraction, and
microprobing. It was ascertained that both uranium and rare earth elements are present in ore mostly associated with biogenic
phosphate in the form of natural minerals, such as uraninite, ningyoite, coffinite, autenite, and churchite. Iron hydroxides
and graphitized organic matter are present in some samples. It is assumed that the co-occurrence of uranium and rare earth
elements, which is nontypical for the sedimentary process, resulted from secondary epigenetic processes and alternation of
reducing and oxidizing environmental conditions. 相似文献
994.
NILANJAN DASGUPTA TARITWAN PAL JOYDEEP SEN TAMOGHNO GHOSH 《Journal of Earth System Science》2011,120(4):617-626
The study involves the characterization of pegmatoidal granite, southeast of Beawar, Ajmer district, Rajasthan. Earlier researchers
had described this granite as part of the BGC, basement to the Bhim Group of the Delhi Super Group rocks. However, the present
study indicates that it is younger than the rocks of Bhim Group of South Delhi Fold Belt, into which it is intrusive. The
intrusion is structurally controlled and the outcrop pattern is phacolithic. The granite had intruded post-D2 deformation of the Delhi orogeny along the axial planes of D2 folds. The intrusion has also resulted in the formation of a contact aureole about the calc gneisses. 相似文献
995.
A Leaky-Conduit Model of Transient Flow in Karstic Aquifers 总被引:1,自引:0,他引:1
Karst Flow Model (KFM) simulates transient flow in an unconfined karstic aquifer having a well-developed conduit system. KFM
treats the springshed as a two-dimensional porous matrix containing a triangulated irregular network of leaky conduits. The
number and location of conduits can be specified arbitrarily, perhaps using field information as a guide, or generated automatically.
Conduit networks can be tree-like or braided. Rainwater that has infiltrated down from the surface leaks into the conduits
from the adjacent porous matrix at a rate dictated by Darcy’s law, then flows turbulently to the spring via the conduits.
KFM is calibrated using the known steady state; geometry and recharge determine the steady fluxes in the conduits, and the
head distribution determines conduit gradients and sizes. Spring flow can vary with time due to spatially and temporally variable
recharge and due to prescribed variations in the elevation of the spring. KFM is illustrated by four examples run on a test
aquifer consisting of 27 nodes, 42 elements, and 26 conduits. Three examples (drought, uniform rainstorm, storm-water input
to one element) are simulations, while the fourth uses data from a spring-basin flooding event. The qualitative fit between
the predicted and observed spring discharge in the fourth example provides support of the hypothesis that the dynamic behavior
of a karst conduit system is an emergent property of a self-organized system, largely independent of the locations and properties
of individual conduits. 相似文献
996.
Catarina Vinagre João Salgado Henrique N. Cabral Maria J. Costa 《Estuaries and Coasts》2011,34(4):663-674
River flow variability is known to influence estuarine production, yet knowledge on its effect upon estuarine food webs dynamics
is still scarce. Stable carbon and nitrogen isotopes were used to assess the effect of river flow in the connectivity and
food web interactions between the two main fish nursery areas of the Tagus estuary. The aims of the present work were to investigate
the seasonal variation in food web structure and the exchange rate of individuals of marine juvenile fish among estuarine
nurseries, to compare the spring of a rainy year (2001) with that of an average year (2000), and to investigate the impact
of the winter floods of 2001. A low level of connectivity was observed for the fish species that use these areas as nurseries.
In low river flow conditions, two isotopically distinct food webs were established in each nursery area. These food webs were
very sensitive to small variations in the freshwater input. Winter floods seem to disrupt the localized food webs that are
established in low river flow periods, leading to the re-establishment of a wider food web. While in rainy years this wide
food web is maintained until spring, in average years the food web undergoes fragmentation into two localized and isotopically
distinctive food webs. The increase in frequency of droughts due to climate change should lower the connectivity of the estuarine
fish nurseries food webs, causing habitat fragmentation and consequent loss in complexity and resilience. 相似文献
997.
The coexisting pyroxenes and olivines from nine British andIcelandic Tertiary porphyritic acid glasses have been separatedand analysed chemically. Eight new analyses of augite and ferroaugites,one of orthopyroxene, and five of iron-rich olivines are presented,together with their optical properties. The trend of crystallizationof the ferroaugites is discussed. New optical determinativecurves for these ferroaugites and the iron-rich olivines arepresented. The relationship of the pyroxenes and olivines totheir analysed residual glasses is considered. 相似文献
998.
999.
The problem of calculating equivalent grid block permeability tensors for heterogeneous porous media is addressed. The homogenization method used involves solving Darcy's equation subject to linear boundary conditions with flux conservation in subregions of the reservoir and can be readily applied to unstructured grids. The resulting equivalent permeability tensor is stable as defined relative to G-convergence. It is proposed to use both conforming and mixed finite elements to solve the local problems and compute approximations from above and below of the equivalent permeability, respectively. Comparisons with results obtained using periodic, pressure and no-flux boundary conditions and the renormalization method are presented. A series of numerical examples demonstrates the effectiveness of the methodology for two-phase flow in heterogeneous reservoirs. 相似文献
1000.
Extensive and economically-important lithium pegmatites have been described from the eastern flank of the large S-type peraluminous Leinster Granite batholith in southeast Ireland. This flank area is also the locus of a major dip-slip ductile shear zone which was active during emplacement of the Tullow Lowlands pluton of the batholith. All the known bedrock occurrences of lithium pegmatite are contained within this marginal deformation zone. Most previous studies have suggested derivation of the lithium pegmatites by fractionation from the Leinster Granite itself, though none are conclusive. More recently, an anatectic model has been proposed whereby the pegmatites were derived by melting of Li-rich sediments adjacent to the batholithic contact. Our investigation has been concerned with a detailed study of the Rb-Sr isotope systematics and trace element geochemistry of the pegmatites and their host granitoids in an attempt to resolve the problem of pegmatite genesis. The results suggest a strong geochemical and temporal coherence between the Leinster Granite and the lithium pegmatites. A model is proposed in which Li enrichment in residual magmas may be linked to the development of muscovite instead of biotite in the Leinster Granite itself. The Li excluded from entry to biotite then accumulates in highly mobile, low-viscosity pegmatitic fluids which migrate into the marginal shear zone of the Tullow Lowlands pluton where they crystallize at considerably lower temperatures. 相似文献