全文获取类型
收费全文 | 410篇 |
免费 | 15篇 |
国内免费 | 13篇 |
专业分类
测绘学 | 17篇 |
大气科学 | 17篇 |
地球物理 | 99篇 |
地质学 | 159篇 |
海洋学 | 61篇 |
天文学 | 43篇 |
综合类 | 2篇 |
自然地理 | 40篇 |
出版年
2021年 | 3篇 |
2020年 | 8篇 |
2019年 | 10篇 |
2018年 | 6篇 |
2017年 | 8篇 |
2016年 | 11篇 |
2015年 | 10篇 |
2014年 | 9篇 |
2013年 | 22篇 |
2012年 | 10篇 |
2011年 | 16篇 |
2010年 | 19篇 |
2009年 | 26篇 |
2008年 | 25篇 |
2007年 | 27篇 |
2006年 | 16篇 |
2005年 | 12篇 |
2004年 | 8篇 |
2003年 | 10篇 |
2002年 | 21篇 |
2001年 | 16篇 |
2000年 | 4篇 |
1999年 | 12篇 |
1998年 | 4篇 |
1997年 | 6篇 |
1996年 | 6篇 |
1995年 | 3篇 |
1993年 | 6篇 |
1992年 | 5篇 |
1991年 | 4篇 |
1990年 | 4篇 |
1989年 | 5篇 |
1988年 | 3篇 |
1985年 | 2篇 |
1984年 | 7篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1981年 | 6篇 |
1980年 | 8篇 |
1979年 | 10篇 |
1978年 | 5篇 |
1977年 | 9篇 |
1976年 | 5篇 |
1975年 | 4篇 |
1973年 | 2篇 |
1972年 | 3篇 |
1969年 | 2篇 |
1947年 | 3篇 |
1927年 | 2篇 |
1923年 | 1篇 |
排序方式: 共有438条查询结果,搜索用时 15 毫秒
41.
K.O. Mason M. Chester C. Gronwall S. Hunsberger S. Koch P.T. O'Brien P. Roming A. Wells R. Willingale N. Gehrels 《Icarus》2007,187(1):123-131
We report time-resolved imaging UV photometry of Comet 9P/Tempel 1 during the interval 2005 June 29-2005 July 21, including intensive coverage of the collision with the Deep Impact probe and its immediate aftermath. The nuclear flux of the comet begins to rise within minutes of the collision, and peaks about 3 h after impact. There is no evidence for a prompt flash at the time of impact. The comet exhibits a significant re-brightening about 40 h after the initial outburst, consistent with the rotation period of the comet, with evidence for further periodic re-brightenings on subsequent rotations. Modelling of the brightness profile of the coma as a function of time suggests two distinct velocity systems in the ejecta, at de-projected expansion speeds of 190 and 550 m/s, which we suggest are due to dust and gas, respectively. There is a distinct asymmetry in the slower-moving (dust) component as a function of position angle on the sky. This is confirmed by direct imaging analysis, which reveals an expanding plume of material concentrated in the impact hemisphere. The projected expansion velocity of the leading edge of this plume, measured directly from the imaging data, is 190 m/s, consistent with the velocity of the dust component determined from the photometric analysis. From our data we determine that a total of (1.4±0.2)×1032 water molecules were ejected in the impact, together with a total scattering area of dust at 300 nm of 190±20 km2. 相似文献
42.
Michael E. Zolensky Gordon L. Wells Helen M. Rendell 《Meteoritics & planetary science》1990,25(1):11-17
Abstract— The discovery of 154 meteorite fragments within an 11 km2 area of wind-excavated basins in Roosevelt County, New Mexico, permits a new calculation of the accumulation rate of meteorite falls at the Earth's surface. Thermoluminescence dating of the coversand unit comprising the prime recovery surface suggests the maximum terrestrial age of the meteorites to be about 16.0 ka. The 68 meteorite fragments subjected to petrological analyses represent a minimum of 49 individual falls. Collection bias has largely excluded carbonaceous chondrites and achondrites, requiring the accumulation rate derived from the recovered samples to be increased by a factor of 1.25. Terrestrial weathering destroying ordinary chondrites can be modelled as a first-order decay process with an estimated half-life of 3.5 ± 1.9 ka on the semiarid American High Plains. Having accounted for the age of the recovery surface, area of field searches, pairing of finds, collection bias and weathering half-life, we calculate an accumulation rate of 9.4 × 102 falls/a per 106 km2 for falls > 10 g total mass. This figure exceeds the best-constrained previous estimate by more than an order of magnitude. One possible reason for this disparity may be the extraordinary length of the fall record preserved in the surficial geology of Roosevelt County. The high accumulation rate determined for the past 16 ka may point to the existence of periods when the meteorite fall rate was significantly greater than at present. 相似文献
43.
Kevin R. Chamberlain Axel K. Schmitt Susan M. Swapp T. Mark Harrison Norbert Swoboda-Colberg Wouter Bleeker Tony D. Peterson Charles W. Jefferson Andrei K. Khudoley 《Precambrian Research》2010
An in situ U–Pb SIMS (IN-SIMS) method to date micro-baddeleyite crystals as small as 3 μm is presented with results from three samples that span a variety of ages and geologic settings. The method complements ID-TIMS geochronology by extending the range of dateable crystals to sizes smaller than can be recovered by physical separation. X-ray mapping and BSE imaging are used to locate target grains in thin section, followed by SIMS analysis on a CAMECA ims 1270, using the field aperture in the transfer column to screen out ions from host phases. Internal age precisions for the method are anticipated to range from 0.1% for Precambrian rocks to 3–7% for Phanerozoic rocks. Results establish a 2689 ± 5 Ma age for mafic dikes in the Wyoming craton, USA, a 1540 ± 30 Ma age for a subaerial lava flow from the Thelon Basin of northern Canada, and a 457 ± 34 Ma age for mafic dikes in the platform sequence of southeastern Siberia. The method is ideal for relatively non-destructive dating of small samples such as extraterrestrial rocks and precious terrestrial samples. 相似文献
44.
Management of freshwater lenses on small Pacific islands 总被引:3,自引:0,他引:3
The nature of shallow aquifers and the impacts of seawater intrusion in small islands within the Pacific Ocean are reviewed. Many Pacific islands rely on shallow fresh groundwater lenses in highly permeable aquifers, underlain and surrounded by seawater, as their principal freshwater source. It is argued here that, in small islands, the nature of fresh groundwater lenses and their host aquifers coupled with frequent natural and ever-present anthropogenic threats make them some of the most vulnerable aquifer systems in the world. A simple steady-state approximation is used to provide insight into the key climatic, hydrogeological, physiographic, and management factors that influence the quantity of, and saline intrusion into freshwater lenses. Examples of the dynamic nature of freshwater lenses as they respond to these drivers are given. Natural and human-related threats to freshwater lenses are discussed. Long dry periods strongly coupled to sea surface temperatures impact on the quantity and salinity of fresh groundwater. The vulnerability of small island freshwater lenses dictates careful assessment, vigilant monitoring, appropriate development, and astute management. Strategies to aid future groundwater sustainability in small islands are presented and suggested improvements to donor and aid programs in water are also advanced. 相似文献
45.
Atmospheric tracer monitoring and surface plume development at the ZERT pilot test in Bozeman,Montana, USA 总被引:1,自引:1,他引:0
Arthur Wells Brian Strazisar J. Rodney Diehl Garret Veloski 《Environmental Earth Sciences》2010,60(2):299-305
A controlled release of CO2 was conducted at a field site in Bozeman, Montana, USA in July of 2008 in a multi-laboratory study of near surface transport
and detection technologies. The development of a subsurface CO2 plume near the middle packer section of the horizontal release was studied using soil-gas and surface flux measurements of
CO2. A perfluorocarbon tracer was added to the CO2 released from this section of the horizontal well, and the development of atmospheric plumes of the tracer was studied under
various meteorological conditions using horizontal and vertical grids of monitors containing sorbent material to collect the
tracer. This study demonstrated the feasibility of using remote sensing for the ultra low level detection of atmospheric plumes
of tracers as means to monitor the near surface leakage of sequestered CO2. 相似文献
46.
Soil moisture is highly variable both spatially and temporally. It is widely recognized that improving the knowledge and understanding of soil moisture and the processes underpinning its spatial and temporal distribution is critical. This paper addresses the relationship between near‐surface and root zone soil moisture, the way in which they vary spatially and temporally, and the effect of sampling design for determining catchment scale soil moisture dynamics. In this study, catchment scale near‐surface (0–50 mm) and root zone (0–300 mm) soil moisture were monitored over a four‐week period. Measurements of near‐surface soil moisture were recorded at various resolutions, and near‐surface and root zone soil moisture data were also monitored continuously within a network of recording sensors. Catchment average near‐surface soil moisture derived from detailed spatial measurements and continuous observations at fixed points were found to be significantly correlated (r2 = 0·96; P = 0·0063; n = 4). Root zone soil moisture was also found to be highly correlated with catchment average near‐surface, continuously monitored (r2 = 0·81; P < 0·0001; n = 26) and with detailed spatial measurements of near‐surface soil moisture (r2 = 0·84). The weaker relationship observed between near‐surface and root zone soil moisture is considered to be caused by the different responses to rainfall and the different factors controlling soil moisture for the soil depths of 0–50 mm and 0–300 mm. Aspect is considered to be the main factor influencing the spatial and temporal distribution of near‐surface soil moisture, while topography and soil type are considered important for root zone soil moisture. The ability of a limited number of monitoring stations to provide accurate estimates of catchment scale average soil moisture for both near‐surface and root zone is thus demonstrated, as opposed to high resolution spatial measurements. Similarly, the use of near‐surface soil moisture measurements to obtain a reliable estimate of deeper soil moisture levels at the small catchment scale was demonstrated. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
47.
48.
49.
50.
Tony Waltham 《Geology Today》2001,17(1):22-24
A cold geyser appears to be a contradiction in terms. But a combination of carbon dioxide, effervescing groundwater and a fortuitous oil exploration well can create a very spectacular water fountain. 相似文献