全文获取类型
收费全文 | 2004篇 |
免费 | 68篇 |
国内免费 | 8篇 |
专业分类
测绘学 | 31篇 |
大气科学 | 136篇 |
地球物理 | 457篇 |
地质学 | 612篇 |
海洋学 | 154篇 |
天文学 | 425篇 |
综合类 | 5篇 |
自然地理 | 260篇 |
出版年
2021年 | 12篇 |
2020年 | 15篇 |
2019年 | 20篇 |
2018年 | 48篇 |
2017年 | 32篇 |
2016年 | 30篇 |
2015年 | 53篇 |
2014年 | 52篇 |
2013年 | 111篇 |
2012年 | 46篇 |
2011年 | 83篇 |
2010年 | 82篇 |
2009年 | 100篇 |
2008年 | 79篇 |
2007年 | 93篇 |
2006年 | 77篇 |
2005年 | 67篇 |
2004年 | 76篇 |
2003年 | 67篇 |
2002年 | 65篇 |
2001年 | 45篇 |
2000年 | 62篇 |
1999年 | 25篇 |
1998年 | 29篇 |
1997年 | 45篇 |
1996年 | 37篇 |
1995年 | 42篇 |
1994年 | 21篇 |
1993年 | 30篇 |
1992年 | 34篇 |
1991年 | 32篇 |
1990年 | 28篇 |
1989年 | 24篇 |
1988年 | 13篇 |
1987年 | 19篇 |
1986年 | 23篇 |
1985年 | 28篇 |
1984年 | 35篇 |
1983年 | 29篇 |
1982年 | 24篇 |
1981年 | 27篇 |
1980年 | 24篇 |
1979年 | 17篇 |
1978年 | 23篇 |
1977年 | 31篇 |
1976年 | 16篇 |
1975年 | 18篇 |
1974年 | 18篇 |
1973年 | 16篇 |
1971年 | 9篇 |
排序方式: 共有2080条查询结果,搜索用时 437 毫秒
131.
Russell C. Bunker 《Quaternary Research》1982,18(1):17-31
Catastrophic floods from glacial Lake Missoula entered the Pasco Basin in south-central Washington and backflooded its marginal valleys. Badger Coulee, one such valley, contains beds of fine-grained slackwater sediment deposited by these floods. The slackwater sediment contains two ash layers of the Mount St. Helens set S tephra, about 13,000 yr old. The ash was deposited on a ground surface developed atop slackwater sediment deposited during preash flooding. Evidence of the former ground surface includes the reworked ash, inferred trace fossils, stream and debris-flow deposits, slopewash and/or eolian sediment, and colluvium at the ash horizon. These features and the ash were buried by slackwater sediment deposited during postash flooding. Nonflood, subaerial deposits are not present atop other beds. Instead, beds commonly are reversely graded across “contacts,” suggesting that multiple beds were continuously deposited. The exposed beds thus record at least two late-Wisconsin floods, one preash, the other postash. The pre- and postash floods may be correlative with earlier-reported floods thought to have occurred 17,500-14,000 and 14,000–13,000 yr B.P., respectively. 相似文献
132.
Douglas G. Brookins 《Environmental Geology》1982,4(3-4):201-208
The Oklo natural reactor, Gabon, permits study of fission-produced elemental behavior in a natural geologic environment. The
uranium ore that sustained fission reactions formed about 2 billion years before present (BYBP), and the reactor was operative
for about 5 × 105 yrs between about 1.95 to 2 BYBP. The many tons of fission products can, for the most part, be studied for their abundance
and distribution today. Since reactor shutdown, many fissiogenic elements have not migrated from host pitchblende, and several
others have migrated only a few tens of meters from the reactor ore. Only Xe and Kr have apparently been largely removed from
the reactor zones. An element by element assessment of the Oklo rocks' ability to retain the fission products, and actinides
and radiogenic Pb and Bi as well, leads to the conclusion that no widespread migration of the elements occurred. This suggests
that rocks with more favorable geologic characteristics are indeed well suited for consideration for the storage of radioactive
waste. 相似文献
133.
Seth Stein Joseph F. Engeln Douglas A. Wiens Robert C. Speed Kazuya Fujita 《Tectonophysics》1983,99(2-4)
The Lesser Antilles subduction zone is an extreme case of the subduction of old (~ 90 m.y.) lithosphere at a slow (~ 2 cm/y) convergence rate. Focal mechanisms of the largest earthquakes in the area have been obtained using body and surface wave data. During the time period (1950–1978) studied the subduction seismicity appears to represent primarily intraplate rather than interplate deformation. All three large (magnitude seven) earthquakes were from intraplate normal faults; no large thrust faulting earthquakes and few small ones occurred. These observations suggest that the plate boundary is largely decoupled, that subduction is at least partially aseismic, and that the downgoing slab is in a state of extension. 相似文献
134.
The judicious choice of dipolar dephasing times or carbon magnetization holding times has been shown to improve resolution in solid state nuclear magnetic resonance (NMR) spectra of complex materials. Signals from protonated and alkylated aromatic carbons are reduced to enhance resolution of aromatic oxygenated groups. Rapidly rotating methyl groups can be resolved from other aliphatic carbon types. These techniques were used to investigate the structure of a brown coal, xylite fractions of a brown coal, a bituminous coal, an oil shale and a solvent-refined coal. The results allow estimates of the fraction of aromatic carbon that is protonated in coal to be made, and demonstrate that methyl groups in coal rotate rapidly at room temperature. 相似文献
135.
Fluctuations in US Freezing Rain Days 总被引:1,自引:0,他引:1
Freezing rain occurrences during a 50-year period, 1949/1950–1998/1999, derived from carefully examined records of 161 first-order stations distributed across the United States, were assessed for temporal fluctuations and trends. Classification of station fluctuations based on five 10-year periods revealed five unique distribution types in areas east of the Rockies. One of these five distributions, for stations located in the western Great Plains, experienced its greatest 10-year value at the end of the 50-year period. The other four regional distributions experienced their highest 10-year value in either of the first two 10-year periods. Nationally, the 10-year period when the greatest number of stations experienced their maximum value was 1949/1950–1958/1959, while the period when the greatest number of stations experienced their minimum value fell near the end of the 50-year record (1979/1980–1988/1989). The 50-year linear trends defined one region, the western Great Plains, with increasing values, while three areas of decreasing trend were identified; the Great Lakes, the eastern Ohio River valley, and southern New England. These analyses also indicate the need to examine and consider such time–space changes in the frequency of climate variables at various spatial scales when assessing weather risks and developing climate change scenarios. 相似文献
136.
M.G. Shepherd D.L. Wu I.N. Fedulina S. Gurubaran J.M. Russell M.G. Mlynczak G.G. Shepherd 《Journal of Atmospheric and Solar》2007,69(17-18):2309
Temperature observations at 20–90 km height and 5–15°N during the winter of 1992–1993, 1993–1994 and 2003–2004, from the Wind Imaging Interferometer (WINDII) and Microwave Limb Sounder (MLS) experiments on the Upper Atmosphere Research Satellite (UARS) satellite and the Sounding the Atmosphere using Broadband Emission Radiometry (SABER) experiment on the Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED) satellite are analyzed together with MF radar winds and UK Meteorological Office (UKMO) assimilated fields. Mesospheric cooling is observed at the time of stratospheric warming at the tropics correlative with stratospheric warming events at middle and high latitudes. Planetary waves m=1 with periods of 4–5, 6–8, 10 and 12–18 days are found to dominate the period. Westward 7- and 16-day waves at the tropics appear enhanced by stationary planetary waves during sudden stratospheric warming events. 相似文献
137.
W. Douglas Thompson Daniel Wartenberg 《Stochastic Environmental Research and Risk Assessment (SERRA)》2007,21(5):635-646
Much research in environmental epidemiology relies on aggregate-level information on exposure to potentially toxic substances
and on relevant covariates. We compare the use of additive (linear) and multiplicative (log-linear) regression models for
the analysis of such data. We illustrate how both additive and multiplicative models can be fit to aggregate-level data sets
in which disease incidence is the dependent variable, and contrast these results with similar models fitted to individual-level
data. We find (1) that for aggregate-level data, multiplicative models are more likely than additive models to introduce bias
into the estimation of rates, an effect not found with individual-level data; and (2) that under many circumstances multiplicative
models reduce the precision of the estimates, an effect also not found in individual-level models. For both additive and multiplicative
models of aggregate-level data, we find that, in the presence of covariates, narrow confidence interval are obtained only
when two or more antecedent factors are strongly related to the measured covariate and/or the exposure of primary substantive
interest. We conclude that the equivalency of fitting additive versus multiplicative models in studies with individual-level
binary data does not carry over to studies that analyze aggregate-level information. For aggregate data, we strongly recommend
use of additive models.
Supported by Grant #1 U19 EH000102 from the National Center for Environmental Health, Centers for Disease Control and Prevention,
Atlanta, GA. 相似文献
138.
John Douglas 《Bulletin of Earthquake Engineering》2007,5(3):363-376
Accurate estimates of the ground motions that occurred during damaging earthquakes are a vital part of many aspects of earthquake
engineering, such as the study of the size and cause of the uncertainties within earthquake risk assessments. This article
compares a number of methods to estimate the ground shaking that occurred on Guadeloupe (French Antilles) during the 21st
November 2004 (M
w
6.3) Les Saintes earthquake, with the aim of providing more accurate shaking estimates for the investigation of the sources
of uncertainties within loss evaluations, based on damage data from this event. The various techniques make differing use
of the available ground-motion recordings of this earthquake and by consequence the estimates obtained by the different approaches
are associated with differing uncertainties. Ground motions on the French Antilles are affected by strong local site effects,
which have been extensively investigated in previous studies. In this article, use is made of these studies in order to improve
the shaking estimates. It is shown that the simple methods neglecting the spatial correlation of earthquake shaking lead to
uncertainties similar to those predicted by empirical ground-motion models and that these are uniform across the whole of
Guadeloupe. In contrast, methods (such as the ShakeMap approach) that take account of the spatial correlation in motions demonstrate
that shaking within roughly 10 km of a recording station (covering a significant portion of the investigated area) can be
defined with reasonable accuracy but that motions at more distant points are not well constrained. 相似文献
139.
140.
R.I. Waller A.J. Russell T.A.G.P. van Dijk & Ó. Knudsen 《Geografiska Annaler: Series A, Physical Geography》2001,83(1-2):29-38
During the initial stages of the November 1996 jökulhlaup at Skeiðarárjökull, Iceland, floodwaters burst onto the glacier surface via a series of fractures. This supraglacial drainage led to the formation of a number of distinct ice surface depressions, one of which is investigated in detail. The morphology and structural characteristics of this feature are described, as well as the sedimentology of an associated assemblage of debris-filled fractures. This work suggests that debris-charged subglacial floodwaters travelled up to the glacier surface, where supraglacial flow occurred initially via an extensive network of fractures, orientated parallel to the glacier margin. Supraglacial discharge became progressively more focused into a series of discrete outlets, leading to the mechanical erosion of a number of depressions on the glacier surface. The associated transfer of subglacially derived floodwaters to high levels within the glacier resulted in the rapid entrainment of large volumes of sediment which may influence the patterns, processes and products of ice-marginal sedimentation in the future. 相似文献