全文获取类型
收费全文 | 1464篇 |
免费 | 92篇 |
国内免费 | 20篇 |
专业分类
测绘学 | 66篇 |
大气科学 | 106篇 |
地球物理 | 355篇 |
地质学 | 466篇 |
海洋学 | 109篇 |
天文学 | 322篇 |
综合类 | 2篇 |
自然地理 | 150篇 |
出版年
2022年 | 2篇 |
2021年 | 32篇 |
2020年 | 34篇 |
2019年 | 40篇 |
2018年 | 56篇 |
2017年 | 39篇 |
2016年 | 51篇 |
2015年 | 56篇 |
2014年 | 55篇 |
2013年 | 83篇 |
2012年 | 67篇 |
2011年 | 82篇 |
2010年 | 62篇 |
2009年 | 98篇 |
2008年 | 72篇 |
2007年 | 78篇 |
2006年 | 86篇 |
2005年 | 78篇 |
2004年 | 76篇 |
2003年 | 63篇 |
2002年 | 56篇 |
2001年 | 46篇 |
2000年 | 31篇 |
1999年 | 39篇 |
1998年 | 35篇 |
1997年 | 13篇 |
1996年 | 13篇 |
1995年 | 14篇 |
1994年 | 12篇 |
1993年 | 14篇 |
1992年 | 4篇 |
1991年 | 8篇 |
1990年 | 1篇 |
1989年 | 8篇 |
1988年 | 7篇 |
1987年 | 7篇 |
1986年 | 8篇 |
1985年 | 7篇 |
1984年 | 6篇 |
1983年 | 5篇 |
1982年 | 2篇 |
1981年 | 9篇 |
1980年 | 5篇 |
1979年 | 6篇 |
1978年 | 3篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1967年 | 1篇 |
1890年 | 1篇 |
排序方式: 共有1576条查询结果,搜索用时 15 毫秒
101.
Catastrophic events reveal the dynamic nature of salt-marsh vegetation in Southern California 总被引:1,自引:0,他引:1
Joy B. Zedler Jordan Covin Chris Nordby Phil Williams John Boland 《Estuaries and Coasts》1986,9(1):75-80
Recent hydrological disturbances, including flooding, dry-season streamflow, and drought, greatly altered coastal wetland habitats in sourthern California. At Tijuana Estuary, a six-year study of salt-marsh vegetation patterns during these rare conditions documented substantial temporal variability in plant growth and distribution. Important to cordgrass (Spartina foliosa Trin.) dynamics were the amount and timing of streamflows, which reduced soil salinity and alleviated stresses on plant growth. Poorest growing conditions occurred in 1984 when both river and tidal flows were lacking; soils had low moisture and extreme salinities (avg.=104‰ in September). Plant stress was documented in 1984 as high mortality (62% fewer stems than in 1983) and reduced height (19% less than in 1983). Cordgrass height was greatest in 1980 following winter flooding (20% increase over 1979); densities were greatest in 1983 with summer freshwater influxes (60% increase over 1982). A carbon allocation model is proposed to explain the varied responses. 相似文献
102.
Chris C.Y. CHAN 《Geostandards and Geoanalytical Research》1986,10(2):131-134
The carbonate-carbon (CO2 ) content of forty-one geochemical reference samples has been determined by coulometric method following acid treatment of the sample for releasing CO2 . The method is superior to the conventional methods in speed, accuracy, sensitivity, specificity, and the coverage of CO2 range. The results on NBS limestone samples agree well with the certified values. The precision of the method is 0.5 % r.s.d., and the practical detection limit is 10 ppm C. 相似文献
103.
The ongoing human-induced emission of carbon dioxide (CO2) threatens to change the earth's climate. One possible way of decreasing CO2 emissions is to apply CO2 removal, which involves recovering of carbon dioxide from energy conversion processes and storing it outside the atmosphere. Since the 1980's, the possibilities for recovering CO2 from thermal power plants received increasing attention.In this study possible techniques of recovering CO2 from large-scale industrial processes are assessed.In some industrial processes, e.g. ammonia production, CO2 is recovered from the process streams to prevent it from interfering with the production process. The CO2 thus recovered can easily be dehydrated and compressed, at low cost. In the iron and steel industry, carbon dioxide can be recovered from blast furnace gas. In the petrochemical industry CO2 can be recovered from flue gases, using low-temperature heat for the separation process.Carbon dioxide can be recovered from large-scale industrial processes and in some cases the cost of recovery is significantly less than CO2 recovery from thermal power plants. Therefore this option should be studied further and should be considered if carbon dioxide removal is introduced on a wide scale. 相似文献
104.
105.
The experimental programme of Southard and Mackintosh (1981) provides a definitive refutation of the autosuspension criterion as it was originally defined. A simple energy budget for dilute two-phase flow is derived from continuum theory of mixtures; it shows that Bagnold's criterion is based on an arbitrary comparison of two elements of the complete two-phase system. It is recommended that facile, ab hoc criteria for the behaviour of such complex flows be avoided in favour of analyses based on fundamental equations of motion. 相似文献
106.
107.
Sonja Aulbach William L. Griffin Norman J. Pearson Suzanne Y. O’Reilly Buddy J. Doyle 《Contributions to Mineralogy and Petrology》2007,154(4):409-427
Major-element compositions of minerals in peridotite xenoliths from the Lac de Gras kimberlites provide constraints on the
mode of lithosphere formation beneath the central Slave Craton, Canada. Magnesia contents of reconstructed whole rocks correlate
positively with NiO and negatively with CaO contents, consistent with variable partial melt extraction. Alumina and Cr2O3 contents are broadly positively correlated, suggestive of melt depletion in the absence of a Cr–Al phase. Garnet modes are
high at a given Al2O3 content (a proxy for melt depletion), falling about a 7 GPa melt depletion model. These observations, combined with high
olivine Mg# and major-element relationships of FeO-poor peridotites (<7.5 wt%) indicative of melt loss at pressures >3 GPa
(residual FeO content being a sensitive indicator of melt extraction pressure), and similar high pressures of last equilibration
(∼4.2 to 5.8 GPa), provide multiple lines of evidence that the mantle beneath the central Slave Craton has originated as a
residue from high-pressure melting, possibly during plume subcretion. Apparent low melt depletion pressures for high-FeO peridotites
(>7.5 wt%) could suggest formation in an oceanic setting, followed by subduction to their depth of entrainment. However, these
rocks, which are characterised by low SiO2 contents (<43 wt%), are more likely to be the result of post-melting FeO-addition, leading to spuriously low estimates of
melt extraction pressures. They may have reacted with a silica-undersaturated melt that dissolved orthopyroxene, or experienced
olivine injection by crystallising melts. A secular FeO-enrichment of parts of the deep mantle lithosphere is supported by
lower average Mg# in xenolithic olivine (91.7) compared to olivine inclusions in diamond (92.6). 相似文献
108.
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
109.
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
110.
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