全文获取类型
收费全文 | 485篇 |
免费 | 12篇 |
国内免费 | 3篇 |
专业分类
测绘学 | 9篇 |
大气科学 | 30篇 |
地球物理 | 72篇 |
地质学 | 153篇 |
海洋学 | 26篇 |
天文学 | 107篇 |
综合类 | 1篇 |
自然地理 | 102篇 |
出版年
2021年 | 7篇 |
2020年 | 4篇 |
2019年 | 4篇 |
2018年 | 10篇 |
2017年 | 5篇 |
2016年 | 9篇 |
2015年 | 12篇 |
2014年 | 11篇 |
2013年 | 17篇 |
2012年 | 12篇 |
2011年 | 19篇 |
2010年 | 13篇 |
2009年 | 19篇 |
2008年 | 25篇 |
2007年 | 26篇 |
2006年 | 20篇 |
2005年 | 14篇 |
2004年 | 18篇 |
2003年 | 20篇 |
2002年 | 13篇 |
2001年 | 13篇 |
2000年 | 8篇 |
1999年 | 20篇 |
1998年 | 15篇 |
1997年 | 14篇 |
1996年 | 10篇 |
1995年 | 6篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1992年 | 5篇 |
1991年 | 6篇 |
1990年 | 5篇 |
1989年 | 3篇 |
1988年 | 7篇 |
1987年 | 3篇 |
1986年 | 7篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 3篇 |
1981年 | 16篇 |
1980年 | 8篇 |
1979年 | 2篇 |
1977年 | 8篇 |
1976年 | 5篇 |
1975年 | 4篇 |
1974年 | 7篇 |
1973年 | 5篇 |
1972年 | 6篇 |
1970年 | 3篇 |
1967年 | 3篇 |
排序方式: 共有500条查询结果,搜索用时 818 毫秒
141.
John D. Monnier Stefan Kraus Michael J. Ireland Fabien Baron Amelia Bayo Jean-Philippe Berger Michelle Creech-Eakman Ruobing Dong Gaspard Duchêne Catherine Espaillat Chris Haniff Sebastian Hönig Andrea Isella Attila Juhasz Lucas Labadie Sylvestre Lacour Stephanie Leifer Antoine Merand Ernest Michael Stefano Minardi Christoph Mordasini David Mozurkewich Johan Olofsson Claudia Paladini Romain Petrov Jörg-Uwe Pott Stephen Ridgway Stephen Rinehart Keivan Stassun Jean Surdej Theo ten Brummelaar Neal Turner Peter Tuthill Kerry Vahala Gerard van Belle Gautam Vasisht Ed Wishnow John Young Zhaohuan Zhu 《Experimental Astronomy》2018,46(3):517-529
The Planet Formation Imager (PFI, www.planetformationimager.org) is a next-generation infrared interferometer array with the primary goal of imaging the active phases of planet formation in nearby star forming regions. PFI will be sensitive to warm dust emission using mid-infrared capabilities made possible by precise fringe tracking in the near-infrared. An L/M band combiner will be especially sensitive to thermal emission from young exoplanets (and their disks) with a high spectral resolution mode to probe the kinematics of CO and H2O gas. In this paper, we give an overview of the main science goals of PFI, define a baseline PFI architecture that can achieve those goals, point at remaining technical challenges, and suggest activities today that will help make the Planet Formation Imager facility a reality. 相似文献
142.
Alfredo Reyes-Salazar José M. Sauceda-Pimentel Sonia E. Ruiz Edén Bojórquez Juan Bojorquez 《Bulletin of Earthquake Engineering》2018,16(11):5557-5590
A numerical investigation regarding the seismic behavior of complex-3D steel buildings with perimeter moment resisting frames (PMRF) and interior gravity frames (GF) is conducted. The interior connections are assumed to be first perfectly pinned (PP) and then semi-rigid (SR); the two model responses are compared. Three steel building models representing low-, medium- and high-rise buildings, and several strong motions are used. The relative stiffness of SR connections is calculated according to the Richard Model and the Beam Line Theory. The Ruaumoko Computer Program is used to perform the required step-by-step nonlinear seismic analysis. Results indicate that interstory shears and interstory displacements at PMRF may be significantly reduced when interior connections are modeled as SR. Average reductions of up to 20, 46 and 11% are observed for interstory shears, for low-, medium-, and high-rise buildings, respectively. The corresponding reductions for interstory displacements are about 14, 44 and 15%. The contribution of GF to the lateral resistance is considerable, which significantly increases when the connections are modeled as SR; relative contributions larger than 80% are observed. The dissipated energy (DE) at PMRF is larger for the buildings with PP than for the buildings with SR connections indicating that damage at PMRF is reduced. Thus, the effect of the stiffness and the DE at interior connections should not be ignored. However, the design of some elements, particularly columns of the GF, has to be revised; these members may not be able to support the loads produced by the neglected lateral contribution if they are not properly designed. 相似文献
143.
Precise geodesy with the Very Long Baseline Array 总被引:2,自引:2,他引:0
Leonid Petrov David Gordon John Gipson Dan MacMillan Chopo Ma Ed Fomalont R. Craig Walker Claudia Carabajal 《Journal of Geodesy》2009,83(9):859-876
We report on a program of geodetic measurements between 1994 and 2007 which used the Very Long Baseline Array (VLBA) and up
to ten globally distributed antennas. One of the goals of this program was to monitor positions of the array at a 1 mm level
of accuracy and to tie the VLBA into the international terrestrial reference frame. We describe the analysis of these data
and report several interesting geophysical results including measured station displacements due to crustal motion, earthquakes,
and antenna tilt. In terms of both formal errors and observed scatter, these sessions are among the very best geodetic very
long baseline interferometry experiments. 相似文献
144.
John W. Johnston Todd A. Thompson Douglas A. Wilcox Steve J. Baedke 《Journal of Paleolimnology》2007,37(3):349-364
A common break was recognized in four Lake Superior strandplain sequences using geomorphic and sedimentologic characteristics.
Strandplains were divided into lakeward and landward sets of beach ridges using aerial photographs and topographic surveys
to identify similar surficial features and core data to identify similar subsurface features. Cross-strandplain, elevation-trend
changes from a lowering towards the lake in the landward set of beach ridges to a rise or reduction of slope towards the lake
in the lakeward set of beach ridges indicates that the break is associated with an outlet change for Lake Superior. Correlation
of this break between study sites and age model results for the strandplain sequences suggest that the outlet change occurred
sometime after about 2,400 calendar years ago (after the Algoma phase). Age model results from one site (Grand Traverse Bay)
suggest an alternate age closer to about 1,200 calendar years ago but age models need to be investigated further. The landward
part of the strandplain was deposited when water levels were common in all three upper Great Lakes basins (Superior, Huron,
and Michigan) and drained through the Port Huron/Sarnia outlet. The lakeward part was deposited after the Sault outlet started
to help regulate water levels in the Lake Superior basin. The landward beach ridges are commonly better defined and continuous
across the embayments, more numerous, larger in relief, wider, have greater vegetation density, and intervening swales contain
more standing water and peat than the lakeward set. Changes in drainage patterns, foreshore sediment thickness and grain size
help in identifying the break between sets in the strandplain sequences. Investigation of these breaks may help identify possible
gaps in the record or missing ridges in strandplain sequences that may not be apparent when viewing age distributions and
may justify the need for multiple age and glacial isostatic adjustment models.
This is the third in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were
presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004), held at the University
of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M. Lewis were guest editors of this special issue.
The U.S. Government's right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged. 相似文献
145.
146.
A. V. Solov’ev T. N. Palechek M. N. Shapiro S. A. Johnston J. I. Garver D. M. Ol’shanetskii 《Stratigraphy and Geological Correlation》2007,15(1):112-119
In the Sredinnyi Range of Kamchatka, the Baraba Formation of continental conglomerates is assumed to be of the late Campanian age based on found flora remains, but data of isotopic geochronology suggest the Eocene age of these deposits. New data on radiolarians from cherty pebbles are considered in this work along with results of fission-track dating of zircons from pebbles and matrix of the Baraba conglomerates. Fission-track dates obtained for zircons from matrix approve the Eocene age of the Baraba Formation, and new dates characterizing pebbles are not contradicting this conclusion. The Baraba Formation structural position can hardly be lower, therefore, than that of the Irunei Formation. 相似文献
147.
S. Johnston N. Wex L. Nicastro R.N. Manchester A.G. Lyne 《Monthly notices of the Royal Astronomical Society》2001,326(2):643-648
We report here on multifrequency radio observations of the pulsed emission from PSR B1259−63 around the time of the closest approach (periastron) to its B2e companion star. There was a general increase in the dispersion measure (DM) and scatter-broadening of the pulsar, and a decrease in the flux density towards periastron although fluctuation in these parameters were seen on time-scales as short as minutes. The pulsed emission disappeared 16 d prior to periastron and remained undetectable until 16 d after periastron.
The observations are used to determine the parameters of the wind from the Be star. We show that a simple model, in which the wind density varies with radius as r−2 , provides a good fit to the data. The wind is highly turbulent with an outer scale of ≤1010 cm and an inner scale perhaps as small as 104 cm, a mean density of ∼106 cm−3 and a velocity of ∼2000 km s−1 at a distance of ∼50 stellar radii. We find a correlation between DM variations and the pulse scattering times, suggesting that the same electrons are responsible for both effects. 相似文献
The observations are used to determine the parameters of the wind from the Be star. We show that a simple model, in which the wind density varies with radius as r
148.
A Conceptual Framework for Enhancing the Utility of Rainfall Hazard Forecasts for Agriculture in Marginal Environments 总被引:1,自引:0,他引:1
Muhammad?T.?UsmanEmail author Emma?Archer Peter?Johnston Mark?Tadross 《Natural Hazards》2005,34(1):111-129
Semi-arid and dry sub-humid areas (especially in the tropics) are characterized by high inter-annual and intra-seasonal rainfall variability. Agriculture, which employs the bulk of the rapidly increasing populations, is largely rain-fed, low-input based and highly resource dependent. Recent spates of drought have, therefore, exacerbated the now-too-familiar specter of famine and starvation in these areas with glaring examples being the recurring episodes in sub-Saharan Africa since the great Sahel drought of 1969–1973. A great need for accurate and timely hazard forecast products in aid of agriculture thus exists.Several schemes are currently employed by various agencies around the globe in this direction. There does remain, however, a gap between product provision and user expectations. This paper examines this gap suggesting a five-point framework within which it can be addressed as an action agenda for the climate science community. The paper posits that changes are possible to existing methodologies (related to three of these points), which, within the context of current science, can greatly enhance the utility of forecast products for agriculture in marginal areas. The remaining two points have, however, been identified as requiring additional applied research and necessary pointers for addressing these issues are provided. First is the need for appropriate impact-related indicators for intra-seasonal and interannual rainfall variability that are easy to compute, amenable to forecasting and follow closely the experiences of farmers in marginal areas. The second is a consideration of appropriate forecast information formatting and communication medium that guarantee effective feedback between forecast producers and users. Specific examples of the status quo and of work currently underway are cited from southern Africa – a region currently attracting international attention as a result of recent droughts and the threat of famine. 相似文献
149.
150.