全文获取类型
收费全文 | 99422篇 |
免费 | 1685篇 |
国内免费 | 902篇 |
专业分类
测绘学 | 2505篇 |
大气科学 | 7280篇 |
地球物理 | 20156篇 |
地质学 | 34897篇 |
海洋学 | 8526篇 |
天文学 | 22593篇 |
综合类 | 220篇 |
自然地理 | 5832篇 |
出版年
2021年 | 807篇 |
2020年 | 908篇 |
2019年 | 980篇 |
2018年 | 2175篇 |
2017年 | 1987篇 |
2016年 | 2560篇 |
2015年 | 1468篇 |
2014年 | 2433篇 |
2013年 | 4884篇 |
2012年 | 2676篇 |
2011年 | 3697篇 |
2010年 | 3327篇 |
2009年 | 4529篇 |
2008年 | 3778篇 |
2007年 | 3795篇 |
2006年 | 3613篇 |
2005年 | 2918篇 |
2004年 | 2877篇 |
2003年 | 2739篇 |
2002年 | 2695篇 |
2001年 | 2404篇 |
2000年 | 2292篇 |
1999年 | 1942篇 |
1998年 | 1914篇 |
1997年 | 1925篇 |
1996年 | 1658篇 |
1995年 | 1604篇 |
1994年 | 1449篇 |
1993年 | 1295篇 |
1992年 | 1235篇 |
1991年 | 1224篇 |
1990年 | 1309篇 |
1989年 | 1216篇 |
1988年 | 1135篇 |
1987年 | 1394篇 |
1986年 | 1226篇 |
1985年 | 1462篇 |
1984年 | 1660篇 |
1983年 | 1563篇 |
1982年 | 1429篇 |
1981年 | 1461篇 |
1980年 | 1244篇 |
1979年 | 1189篇 |
1978年 | 1186篇 |
1977年 | 1130篇 |
1976年 | 1022篇 |
1975年 | 996篇 |
1974年 | 1019篇 |
1973年 | 1049篇 |
1972年 | 645篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A simple sand trap is used to measure swash and backwash bedload transport rates on intertidal profiles. Data from sixty-eight beach experiments are used to calculate a mean value of 12.78 kg m?4s?2 for the calibration coefficient in the Bagnold beach equation. 相似文献
992.
Anaerobic salt marsh sediments were amended with a variety of organic pollutants and the effects on methanogenesis, sulfate reduction and carbon dioxide evolution were examined. Addition of 1000 μg g?1 (dry weight sediment) Arochlor 1221, lindane, endrin, benzene and phenanthrene resulted in no significant effects on the activities studied. Methanogenesis was inhibited by 1000 μg g?1toxaphene, PCP, chlordane, naphthalene, DDT, Kepone and heptachlor and by 100 μg g?1 PCP and toxaphene. At 1000 μg g?1 naphthalene and toxaphene and 100 μg g?1 PCP, a period of initial inhibition of methanogenesis was followed by stimulation relative to controls. Arochlor 1254 (1000 μg g?1) and Temik (500 and 10 μg g?1) stimulated methanogenesis from the outset. Temik at 500 μg g?1 gave the greatest stimulation of methanogenesis (900% of controls) of any of the compounds studied. Sulfate reduction was inhibited by 1000 μg g?1 PCP, toxaphene, naphthalene and chlordane and by 500 μg g?1 atrazine and 100 μg g?1 heptachlor. Sustained inhibition of sulfate reduction by naphthalene, toxaphene and PCP may have contributed to the stimulation of methanogenesis. Carbon dioxide evolution was not significantly affected by most of the compounds studied except for 100 μg g?1 PCP and 1000 μg g?1 aphthalene, each of which gave significant inhibition in only one of three experiments.Concentrations of individual organic pollutants required to cause observable effects were high. It is concluded that, except for highly polluted sediments, methanogenesis, sulfate reduction and CO2 evolution would not be affected by the compounds studied here at concentrations typically found in the environment. 相似文献
993.
The Upper Cretaceous and Paleocene Gottero Sandstone was deposited as a small deep-sea fan on ophiolitic crust in a trench-slope basin. It was thrust northeastward as an allochthonous sheet in Early and Middle Cenozoic time. The Gottero, as thick as 1500 m, was probably derived from erosion of Hercynian granites and associated metamorphic rocks in northern Corsica. Outcrops of inner-fan channel, middle-fan channel and interchannel, outer-fan lobe, fan-fringe, and basin-plain facies associations indicate that the depositional model of Mutti and Ricci Lucchi for mixed-sediment deep-sea fans can be used. The original fan had a radius of 30 to 50 km. 相似文献
994.
K. G. Robertson 《Marine Geophysical Researches》1990,12(1-2):3-8
Accurate navigation forms an essential part of all research at sea and the deep ocean imposes it's own unique problems. This chapter discusses several of the techniques in current use on the research vessels of the Natural Environment Research Council (NERC), concentrating on those systems which provide global navigation facilities, as opposed to the more localised, coastal aids. Whilst most of the systems rely on surface propagation of radio waves, the use of acoustics and sea-bed mapping instruments constitute accurate alternatives for some sub-sea applications. 相似文献
995.
Yurii D. Shuisky 《Marine Geology》1989,90(4):289-296
With the very extensive coastline in the north and northeast and the relatively shorter coastlines in the south, cheniers in the USSR can occur in a great variety of climatological conditions ranging from subarctic to subtropical. Cheniers in the USSR have been studied since approximately 1900, the investigations being mainly concerned with the palaeogeographical aspects. Since the 1960's, investigations have focussed on contemporary morphodynamics. Chenier formation appears to depend largely on the relative influence that marine and fluvial factors have on the formation of marginal deltaic plains. Research in this field is briefly reviewed. 相似文献
996.
The first map of the sea bed morphology and sedimentary features within the RMSTitanic search area is proposed from the interpretation of SAR side-scan sonar images. Downslope sedimentary features such as erosional
furrows and crown scarps constitute a 7 km wide instability corridor. A large field (15 km2) of asymmetrical sediment waves indicating a downslope transport is identified. Current-induced features corresponding to
associated sand ribbons and barchan dunes resulting from the Western Boundary Undercurrent action are mapped. The morphology
of theTitanic Canyon is also precised from the SAR images. Finally, the origin of the sea bed features is discussed in an attempt to link
each bed form to a sedimentary process. 相似文献
997.
Descriptive Analysis and Classification of Benthic Communities in Some Mediterranean Coastal Lagoons (Central Italy) 总被引:3,自引:0,他引:3
Abstract. Benthic samples were taken seasonally at 32 stations in five coastal lagoons of Central Italy (Fogliano, Monaci, Caprolace, Fondi, and Lungo) during a two-year period (1982–1984). Composition and distribution of benthic populations in each lagoon as well as seasonal trends of species richness, density, diversity, and redundancy are presented and discussed.
The faunal differentiation appeared to be mainly due to the different degree of water exchange, both with marine and continental systems. 相似文献
The faunal differentiation appeared to be mainly due to the different degree of water exchange, both with marine and continental systems. 相似文献
998.
Geological and geophysical surveys of active submarine volcanoes offshore and southeast of Epi Island, Vanuatu, New Hebrides
Arc, have delineated details of the structure and acoustic stratigraphy of three volcanic cones. These submarine cones, named
Epia, Epib, and Epic, are aligned east-west and spaced 3.5 km apart on the rim of a submerged caldera. At least three acoustic
sequences, of presumed Quaternary age, can be identified on single-channel seismic-reflection profiles. Rocks dredged from
these cones include basalt, dacite, and cognate gabbroic inclusions with magmatic affinities similar to those of the Karua
(an active submarine volcano off the southeastern tip of Epi) lavas. 相似文献
999.
Extensive surveys were made to establish the concentrations of several metals (Zn, Cd, Pb, Cu and Hg) in various marine organisms from different parts of the Adriatic Sea in the period from 1979 to 1985. The results obtained are based on analyses which were performed in conjunction with carefully tested procedures. Proper sampling, sample storage, treatment of samples and final analytical determination has excluded artefacts which would influence the accuracy, or cause contamination and/or trace metal losses.The metal concentrations in shellfish from the coastal areas of the Adriatic Sea are in the range of the lowest values obtained for similar organisms from other localities throughout the Mediterranean Sea. Special attention has been paid to the anthropogenic influence on trace metal concentrations in organisms living in the vicinity of domestic sewage and/or industrial waste-water outflows. 相似文献
1000.
The paper describes an analytical method to determine an operability index for a marine vehicle when its geometric characteristics, the geographical area in which the vessel is supposed to operate, and the limiting criteria for operations are known. The computer program based on strip theory provides reliable results not only for conventional hullforms, but also for catamarans, offshore vehicles, etc. The results are presented in the form of figures for different vessels, namely, crane barge, naval vessels and SWATH type catamarans. The authors are of the opinion that the proposed method would provide the designer with a valid tool to improve the seakeeping qualities of a vessel, while taking into account the limiting conditions imposed due to seaway operations. 相似文献