全文获取类型
收费全文 | 1556篇 |
免费 | 101篇 |
国内免费 | 20篇 |
专业分类
测绘学 | 68篇 |
大气科学 | 120篇 |
地球物理 | 375篇 |
地质学 | 457篇 |
海洋学 | 135篇 |
天文学 | 342篇 |
综合类 | 2篇 |
自然地理 | 178篇 |
出版年
2021年 | 34篇 |
2020年 | 32篇 |
2019年 | 43篇 |
2018年 | 53篇 |
2017年 | 43篇 |
2016年 | 55篇 |
2015年 | 60篇 |
2014年 | 54篇 |
2013年 | 95篇 |
2012年 | 76篇 |
2011年 | 77篇 |
2010年 | 68篇 |
2009年 | 97篇 |
2008年 | 75篇 |
2007年 | 76篇 |
2006年 | 85篇 |
2005年 | 75篇 |
2004年 | 77篇 |
2003年 | 62篇 |
2002年 | 62篇 |
2001年 | 47篇 |
2000年 | 31篇 |
1999年 | 36篇 |
1998年 | 33篇 |
1997年 | 19篇 |
1996年 | 17篇 |
1995年 | 12篇 |
1994年 | 11篇 |
1993年 | 12篇 |
1992年 | 7篇 |
1991年 | 10篇 |
1990年 | 5篇 |
1989年 | 10篇 |
1988年 | 9篇 |
1987年 | 7篇 |
1986年 | 9篇 |
1985年 | 8篇 |
1984年 | 8篇 |
1983年 | 12篇 |
1982年 | 10篇 |
1981年 | 8篇 |
1980年 | 4篇 |
1979年 | 10篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1975年 | 4篇 |
1974年 | 3篇 |
1972年 | 6篇 |
1971年 | 8篇 |
1967年 | 2篇 |
排序方式: 共有1677条查询结果,搜索用时 15 毫秒
101.
A study on two closed salt lake basins, Tal Chapar and Parihara in the eastern margin of the Thar Desert, Rajasthan, was carried
out to unravel late Quaternary geomorphic evolution of these saline lakes. Both lakes are elliptical in shape bordered by
stabilised dunes, and are oriented in a NE-SW direction, i.e., in the direction of the prevailing summer monsoon wind. Both
lakes have been formed in the wind-shadow zones of isolated hills of Precambrian quartzite. Our study indicates that the late
Quaternary sediments in the lakes began with the cyclic deposition of laminated fine silt layers (0.5 m thick), rich in organic
matter, alternating with ripple cross-bedded sand layers (each ∼1.5–2 m thick). Sand layers that are moderately sorted are
separated by laminated silt-clay layers with gypsum/calcite and this unit occurs in the upper most 4 m sequence in deeper
sections. The presence of gypsum crystals within the laminated sediments suggests a high concentration of Ca in the inflowing
water.
At Parihara Lake the organic carbon-rich sediments at 95 cm depth was dated to 7,375 + 155/−150 year BP. At Tal Chapar radiocarbon
dates of 7,190 + 155/−150 and 9,903 + 360/−350 was obtained from the sediments rich in organic carbon occurring at a depth
of 1.35 m and 1.80 m, respectively. The study reveals strong hydrologic oscillations during the past ∼14,000 year BP (13,090 + 310/−300 year
BP).
Quaternary geomorphic processes, especially the strong aeolian processes during dry climatic phases, played a major role in
the formation of the lake basins, as well as the fringing linear dunes. Geochemical and mineralogical analyses of the lacustrine
sediments, supported by radiocarbon dates indicate the existence of an ephemeral lake earlier than ∼13,000 year BP as sediments
began to be deposited in a lacustrine environment implying sustained runoff in the catchments. A freshwater lake formed between
9,000 year and 7,000 year BP. The lake dried periodically and this strong fluctuating regime continued until about ∼7,000 year
BP. Mid-Holocene was wet and this was possibly due to higher winter rains A saline lake existed between 6,000 year and 1,300 year
BP and finally present day semi arid conditions set in since 1,200 year BP. Remnants of a habitation site (hearth and charred
bones) on stabilised dune at Devani near Tal Chapar were dated to 240 ± 120 year, while that at Gopalpura was dated to 335 ± 90 year.
These historical sites on stabilised dunes were, according to the local accounts, settlements of people who used the lake
brine for manufacturing salt. 相似文献
102.
Location-based services: technical and business issues 总被引:13,自引:0,他引:13
Geographical Information System (GIS) and Global Positioning System (GPS) technologies are expanding their traditional applications
to embrace a stream of consumer-focused, location-based applications. Through an integration with handheld devices capable
of wireless communication and mobile computing, a wide range of what may be generically referred to as 'Location-Based Services'
(LBS) may be offered to mobile users. A location-based service is able to provide targeted spatial information to mobile workers
and consumers. These include utility location information, personal or asset tracking, concierge and route-guidance information,
to name just a few of the possible LBS. The technologies and applications of LBS will play an ever increasingly important
role in the modern, mobile, always-connected society. This paper endeavours to provide some background to the technology underlying
location-based services, and to discuss some issues related to developing and launching LBS. These include whether wireless
mobile technologies are ready to support LBS, which mobile positioning technologies can be used and what are their shortcomings,
and how GIS developers manipulate spatial information to generate appropriate map images on mobile devices (such as cell phones
and PDAs). In addition, the authors discuss such issues as interoperability, privacy protection and the market demand for
LBS.
Electronic Publication 相似文献
103.
DAI LiwenHAN ShaoweiChris Rizos 《地球空间信息科学学报》2001,4(4):9-18
1 IntroductionReal_timekinematicGPSprecisepositioninghasbeenplayinganincreasingroleinbothsurveyingandnavigation ,andhasbecomeanessentialtoolforpreciserelativepositioning .However,reliableandcorrectambiguityresolutiondependsonobserva tionsuponalargenumbe… 相似文献
104.
中国南京与美国德克萨斯稻田甲烷排放的比较 总被引:8,自引:0,他引:8
Field measurements of methane emission from rice paddies were made in Nanjing, China and in Texas, USA, respectively. Soil temperature at approximately 10 cm depth of the flooded soils was automatically recorded. Aboveground biomass of rice crop was measured approximately every 10 days in Nanjing and every other week in Texas. Seasonal variation of soil temperature in Nanjing was quite wide with a magnitude of 15.3℃ and that in Texas was narrow with a magnitude of 2.9℃. Analysis of methane emission fluxes against soil temperature and rice biomass production demonstrated that the seasonal course of methane emission in Nanjing was mostly attributed to soil temperature changes, while that in Texas was mainly related to rice biomass production. We concluded that under the permanent flooding condition, the seasonal trend of methane emission would be determined by the soil temperature where there was a wide variation of soil temperature, and the seasonal trend would be mainly determined by rice biomass production if there are no additional organic matter inputs and the variation of soil temperature over the rice growing season is small. 相似文献
105.
106.
Techniques are described in this paper for improving the Ambiguity Function Method (AFM) for differential GPS positioning using phase observations, (a) that take advantage of optimal dual-frequency observable combinations to improve thereliability of the AFM, and (b) that significantly shorten the computation time necessary for the AFM. The procedure can be used for kinematic positioning applications if a Kalman filter predicted position is accurate enough as an initial position for the suggested AFM searching procedure, or pseudokinematic mode using say a triple-difference solution as an initial position for static positioning if the baseline length is short (typically <5km). 相似文献
107.
Chris Garrett 《Dynamics of Atmospheres and Oceans》1996,23(1-4)
Models for the evolution of the surface mixed layer need to be improved to include dominant processes such as Langmuir circulation. It is shown that the wave forcing in Langmuir circulation models is much stronger than that due to a surface buoyancy loss, and studies of the erosion by the cells of a pre-existing stratification are described. Mixed layer models will also need to allow for horizontal inhomogeneity. It is shown, for example, that the horizontal buoyancy gradient that may be left behind after a storm produces restratification that can be significant. The nonlinearity of the equation of state is another real-world factor; it gives rise to an annual average surface buoyancy that is misleading as it is compensated by interior cabbeling. Current work linking the mixed layer to water mass formation is also introduced. 相似文献
108.
Thomas C. Malone Daniel J. Conley Thomas R. Fisher Patricia M. Glibert Lawrence W. Harding Kevin G. Sellner 《Estuaries and Coasts》1996,19(2):371-385
The scales on which phytoplankton biomass vary in response to variable nutrient inputs depend on the nutrient status of the plankton community and on the capacity of consumers to respond to increases in phytoplankton productivity. Overenrichment and associated declines in water quality occur when phytoplankton growth rate becomes nutrient-saturated, the production and consumption of phytoplankton biomass become uncoupled in time and space, and phytoplankton biomass becomes high and varies on scales longer than phytoplankton generation times. In Chesapeake Bay, phytoplankton growth rates appear to be limited by dissolved inorganic phosphorus (DIP) during spring when biomass reaches its annual maximum and by dissolved inorganic nitrogen (DIN) during summer when phytoplankton growth rates are highest. However, despite high inputs of DIN and dissolved silicate (DSi) relative to DIP (molar ratios of N∶P and Si∶P>100), seasonal accumulations of phytoplankton biomass within the salt-intruded-reach of the bay appear to be limited by riverine DIN supply while the magnitude of the spring diatom bloom is governed by DSi supply. Seasonal imbalances between biomass production and consumption lead to massive accumulations of phytoplankton biomass (often>1,000 mg Chl-a m?2) during spring, to spring-summer oxygen depletion (summer bottom water <20% saturation), and to exceptionally high levels of annual phytoplankton production (>400 g m?2 yr?1). Nitrogen-dependent seasonal accumulations of phytoplankton biomass and annual production occur as a consequence of differences in the rates and pathways of nitrogen and phosphorus cycling within the bay and underscore the importance of controlling nitrogen inputs to the mesohaline and lower reaches of the bay. 相似文献
109.
Dispersion of the salt-marsh periwinkle Littoraria irrorata: Effects of water level,size, and season
This paper documents horizontal and vertical dispersion patterns of a Texas population of the saltmarsh periwinkle, Littoraria irrorata, over a 15-month period. The study was conducted within a tidal marsh on the Anahuac National Wildlife Refuge in Galveston Bay. Two mark-recapture experiments demonstrated that L. irrorata rarely move more than 2 m from their release point over long periods of time and do not home to individual Spartina plants. Adult L. irrorata forage farther away from the base of Spartina stalks at low tide than do juvenile snails. Remaining near the plant base may decrease both temperature and desiccation stress on juveniles. During warm months, L. irrorata climb grass stalks with tidal inundation and forage on the substratum at low tide. Snails are inactive and aggregate in detrital debris at the base of Spartina clumps during the winter. *** DIRECT SUPPORT *** A01BY058 00016 相似文献
110.