全文获取类型
收费全文 | 756篇 |
免费 | 109篇 |
国内免费 | 140篇 |
专业分类
测绘学 | 6篇 |
大气科学 | 25篇 |
地球物理 | 240篇 |
地质学 | 454篇 |
海洋学 | 154篇 |
天文学 | 39篇 |
综合类 | 28篇 |
自然地理 | 59篇 |
出版年
2024年 | 3篇 |
2023年 | 4篇 |
2022年 | 17篇 |
2021年 | 12篇 |
2020年 | 17篇 |
2019年 | 26篇 |
2018年 | 16篇 |
2017年 | 43篇 |
2016年 | 41篇 |
2015年 | 32篇 |
2014年 | 51篇 |
2013年 | 34篇 |
2012年 | 33篇 |
2011年 | 55篇 |
2010年 | 33篇 |
2009年 | 71篇 |
2008年 | 74篇 |
2007年 | 41篇 |
2006年 | 66篇 |
2005年 | 48篇 |
2004年 | 28篇 |
2003年 | 28篇 |
2002年 | 26篇 |
2001年 | 26篇 |
2000年 | 31篇 |
1999年 | 25篇 |
1998年 | 24篇 |
1997年 | 21篇 |
1996年 | 21篇 |
1995年 | 12篇 |
1994年 | 8篇 |
1993年 | 10篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1988年 | 4篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 6篇 |
1981年 | 1篇 |
1978年 | 4篇 |
排序方式: 共有1005条查询结果,搜索用时 312 毫秒
891.
The study area is the southern depocenter (depth > 4200 m) of the Mesohellenic Basin which extends between Kipourio and Grevena, central Greece. The Mesohellenic Basin is a Middle-Tertiary intramontane basin developed within the Hellenide orogen. Previous studies have focused on the depositional environments, configuration and hydrocarbon potential of the basin. In this paper we present additional geochemical and petrographic data from outcrop samples of the basin's southern depocenter, which is considered the most promising area, in terms of hydrocarbon prospectivity. A total number of thirty six samples were analysed: Rock-Eval pyrolysis, maceral analysis, vitrinite reflectance and thermal alteration index, bitumens extraction, liquid chromatography, and GC-MS. The samples were collected from deltaic deposits and submarine fan sediments of Late Eocene to Late Oligocene age. The TOC values of the analysed samples range between rich and very rich and the organic matter consists mainly of type III kerogen and the organic matter consider to be predominately gas prone. The thermal maturity assessed from Tmax and vitrinite reflectance shows an immature stage of the organic matter along with the presence of layers having reached the very early mature stage. Vitrinite reflectance measurements and maturity calculations (applying the Lopatin modeling), reveal that the lower part of the depocenter sediments falls within the ‘oil window’. The extractable organic matter (EOM) (mg bitumens/g TOC) indicate the existence of samples (from deltaic deposits) with high ratio of transformation (EOM) (> 100 mg bitumen/g TOC). The GC and GC-MS analyses of the biomarkers indicate mainly the occurrence of terrestrial organic matter reflecting oxidizing conditions and both immature and very early mature stages. The results of the Rock-Eval pyrolysis and the distribution of the isoprenoids support the assumption of the input of an organic matter mixture. 相似文献
892.
There are at least two sapropelic units associated with Late Palaeozoic black shales in Central Europe. The older unit, of Late Carboniferous age, is the lower part of the Anthracosia Shales in the Intrasudetic Basin, and the younger one is the well-known Zechstein Kupferschiefer in both the Foresudetic Monocline and the Northsudetic Basin. The first unit is of lacustrine origin, while the second one represents deposition in a shallow marine depositional environment. Both units contain high amounts of organic matter, thus being typical black shales.The organic matter dispersed in these shales was studied petrographically. In general, the vitrinite reflectance of the shales studied indicates variable, but moderate organic matter maturity (0.68–1.25%), equivalent to the oil window. Detailed microscopic studies of the organic material dispersed in the lower unit of the Anthracosia Shales showed that liptinite, especially alginite is the most abundant component. Secondary altered organic matter, i.e. solid hydrocarbons, rarely occurs. Organic components together with mineral matter constitute a lacustrine sapropelic association, a humic (terrestrial) association and an intermediary association. The character and predominance of alginite and lacustrine sapropelic association are indicative of an open-lacustrine depositional environment. In general, this organic composition is typical of type I kerogen.Microscopic analysis of the Kupferschiefer revealed a mixture of liptinite, vitrinite and inertinite macerals, and other organic components such as amorphous sapropelic mass (ASM) and solid bitumens. The most common organic components are liptinite macerals. Bituminite and alginite predominate, and are diagnostic macerals of this unit. The amount of bituminite locally exceeds 85 vol.%. Other liptinite macerals such as sporinite and liptodetrinite, are present in significantly lower amounts, one exception being ASM, which may be present in higher amounts. Humic constituents (vitrinite and inertinite) are rare, present in small amounts in the Kupferschiefer beds. The organic matter composition points to type II kerogen for this unit. 相似文献
893.
以福宁高速公路福鼎分水关至福安坞段A3标段八尺门互通区滑坡整治工程为实例,对现场采用的压力分散型锚索进行抗拔试验研究,详细介绍了该新型锚索的抗拔试验过程及其锚索受力测试方法,并就锚索试验中的若干问题进行了分析和探讨,试验研究成果为锚索设计和施工提供了直接依据,具有实际的应用和参考价值。 相似文献
894.
Martin Melles Julie Brigham-Grette Olga Yu. Glushkova Pavel S. Minyuk Norbert R. Nowaczyk Hans-W. Hubberten 《Journal of Paleolimnology》2007,37(1):89-104
The ca. 13 m long sediment core PG1351, recovered in 1998 from the central part of Lake El’gygytgyn, NE Siberia, was investigated
for lithostratigraphy, water content, dry bulk density (DBD), total organic carbon (TOC), total nitrogen (TN), total sulphur
(TS) and biogenic silica (opal) contents, and for TOC stable isotope ratios (δ13CTOC). The event stratigraphy recorded in major differences in sediment composition match variations in regional summer insolation,
thus confirming a new age model for this core, which suggests that it spans the last 250 ka BP. Four depositional units of
contrasting lithological and biogeochemical composition have been distinguished, reflecting past environmental conditions
associated with relatively warm, peak warm, cold and dry, and cold but more moist climate modes. A relatively warm climate,
resulting in complete summer melt of the lake ice cover and seasonal mixing of the water column, prevailed during the Holocene
and Marine Isotope Stages (MIS) 3, 5.1, 5.3, 6.1, 6.3, 6.5, 7.1–7.3, 7.5, 8.1 and 8.3. MIS 5.5 (Eemian) was characterized
by significantly enhanced aquatic primary production and organic matter supply from the catchment, indicating peak warm conditions.
During MIS 2, 5.2, 5.4, 6.2 and 6.4 the climate was cold and dry, leading to perennial lake ice cover, little regional snowfall,
and a stagnant water body. A cold but more moist climate during MIS 4, 6.6, 7.4, 8.2 and 8.4 is thought to have produced more
snow cover on␣the perennial ice, strongly reducing light penetration and biogenic primary production in␣the lake. While the
cold–warm pattern during␣the past three glacial–interglacial cycles is probably controlled by changes in regional summer insolation,
differences in the intensity of the warm phases and in the degree of aridity (changing snowfall) during cold phases likely
were due to changes in atmospheric circulation patterns.
This is the seventh in a series of eleven papers published in this special issue dedicated to initial studies of El'gygytgyn Crater Lake and
its catchment in NE Russia. JulieBrigham-Grette, Martin Melles, Pavel Minyuk were guest editors of this special issue. 相似文献
895.
896.
Vertical drains have been used as remediation against earthquake-induced soil liquefaction for many years. These are seen to begin fluid dissipation from deeper deposits first. Drains are not necessarily installed to the full depth of the liquefiable layer. To determine the effect of this on the efficiency of drain systems, centrifuge test results are presented. It is seen that not installing all drains through the full liquefiable depth significantly retards their performance, due to the dominance of vertical dissipation. It will be shown that a standard design chart may over-predict an improvement in drain performance. 相似文献
897.
Four highly contaminated sediment samples obtained from three sampling locations of the Teltow Canal, Berlin, were investigated by quantitation of extractable and nonextractable organic contaminants. The selection of the anthropogenic contaminants (including chlorinated and brominated naphthalenes, 2, 4, 6‐tribromoaniline, phthalates, tri‐n‐butylphosphate, 2, 2, 4‐trimethyl‐1, 3‐pentanediol diisobutyrate, bisphenol A, butylated nitrophenols, 4‐nitrobenzoic acid, galaxolide, and tonalide) based on the results of extended GC‐MS screening analyses applied to the extracts of the sediment samples as well as to the extracts derived from selective chemical degradation procedures. In detail, alkaline hydrolyses, BBr3‐treatment and RuO4‐oxidation were applied to the pre‐extracted sediment samples in both a separate and a sequential mode. 相似文献
898.
社会经济的发展和资源的利用产生了一系列环境问题,尤其是化肥的超量施用,已经危及包气带土体和地下水的质量安全。大量的氮素没有被农作物吸收,经微生物硝化作用后,以硝态氮的形式进入包气带土体,使包气带土体普遍受到氮素不同程度的污染。本文简述了包气带土体硝态氮污染的室内微生态修复、阻控技术和实验研究,探讨了今后应用的可行性。 相似文献
899.
Ryan Holifield 《Geoforum》2004,35(3):285-297
In response to the failings of the aggressive neoliberalism of the Reagan era, the Clinton administration sought new ways to promote and deepen the neoliberal project. One of its strategies was to develop “neocommunitarian” programs to support political empowerment and economic self-sufficiency in marginalized communities. Its environmental justice policy became an important vehicle for delivering these programs. This paper examines ways in which one regional office of the US Environmental Protection Agency translated the Clinton administration's environmental justice policy into practical guidelines for its managers. It investigates how these guidelines affected the work of personnel in the EPA's program to clean up hazardous waste sites. It asks how the Clinton administration's approach to environmental justice--emphasizing data analysis, managed public participation, and economic opportunity--helped both to deepen the neoliberal project and to transform the geography of risk and remediation. Although the EPA was unable to “normalize” the environmental justice community--to make it a standardized target for the Clinton administration's empowerment programs--its environmental justice policy both contributed to subtle changes in the distribution of hazardous waste risk and made the delivery of neocommunitarian programs an important part of the work of remedial personnel. 相似文献
900.
R. F. Sachsenhofer V. A. Privalov A. Izart M. Elie J. Kortensky E. A. Panova A. Sotirov M. V. Zhykalyak 《International Journal of Coal Geology》2003,55(2-4):225-259
The Carboniferous succession in the Donets Basin hosts about 130 seams, each with a thickness over 0.45 m. Nine economically important seams from the (south)western Donets Basin are studied using organic petrographical, inorganic geochemical, and organic geochemical techniques. The main aim of the study is the reconstruction of peat facies of Serpukhovian (Mississippian) and Moscovian (Middle Pennsylvanian) coals.Formation of major coal seams commenced during Serpukhovian times. Early Serpukhovian coal accumulated in a relatively narrow shore-zone and is rich in inertinite and liptinite. Very low ash yields, low to moderate sulphur contents, and upward increasing inertinite contents suggest coal deposition in raised mires.Moscovian coal has a significantly wider lateral extension and is generally rich in vitrinite. Coal properties vary widely in response to different peat facies. Low-sulphur, low-ash k7 coal was formed in a raised mire or in a low-lying mire without detrital input. l1 and l3 seams containing several fluvial partings were formed in low-lying mires. Both seams are more than 2 m thick. Seams m2 and m3 contain high-sulphur coal, a consequence of deposition in a peat with marine influence. In contrast, syngenetic sulphur content is low in the m51 upper seam, which was formed in a lacustrine setting. The late Moscovian n1 seam, up to 2.4 m thick, accumulated in a swamp with a vegetation rich in bryophytes and pteridophytes. The properties of the n1 seam are transitional between those of Serpukhovian and other Moscovian seams. Differences in maceral composition between Serpukhovian and Moscovian coals probably reflect changes in climate and vegetation type.Tuff layers are observed in the l1, l3, and m3 seams. The l3 and m3 seams contain abundant authigenic quartz. Trace element contents are high in many seams. As contents are especially high in seams c102, k7, l3 and m3. Ash in the l3 seam contains up to 8000 ppm As. Co is enriched near the base of several seams. Maxima up to 2400 ppm occur in the ash of the k7 and l3 seams. Cd contents in ash are frequently as high as 30 or 40 ppm. 相似文献