全文获取类型
收费全文 | 47361篇 |
免费 | 718篇 |
国内免费 | 411篇 |
专业分类
测绘学 | 1406篇 |
大气科学 | 3522篇 |
地球物理 | 9768篇 |
地质学 | 15655篇 |
海洋学 | 4163篇 |
天文学 | 11079篇 |
综合类 | 128篇 |
自然地理 | 2769篇 |
出版年
2021年 | 373篇 |
2020年 | 422篇 |
2019年 | 517篇 |
2018年 | 1074篇 |
2017年 | 1008篇 |
2016年 | 1298篇 |
2015年 | 757篇 |
2014年 | 1225篇 |
2013年 | 2304篇 |
2012年 | 1381篇 |
2011年 | 1849篇 |
2010年 | 1688篇 |
2009年 | 2240篇 |
2008年 | 2002篇 |
2007年 | 1989篇 |
2006年 | 1901篇 |
2005年 | 1440篇 |
2004年 | 1467篇 |
2003年 | 1341篇 |
2002年 | 1374篇 |
2001年 | 1192篇 |
2000年 | 1120篇 |
1999年 | 992篇 |
1998年 | 969篇 |
1997年 | 966篇 |
1996年 | 795篇 |
1995年 | 756篇 |
1994年 | 701篇 |
1993年 | 616篇 |
1992年 | 559篇 |
1991年 | 551篇 |
1990年 | 558篇 |
1989年 | 566篇 |
1988年 | 506篇 |
1987年 | 596篇 |
1986年 | 525篇 |
1985年 | 625篇 |
1984年 | 779篇 |
1983年 | 668篇 |
1982年 | 648篇 |
1981年 | 600篇 |
1980年 | 524篇 |
1979年 | 529篇 |
1978年 | 514篇 |
1977年 | 464篇 |
1976年 | 408篇 |
1975年 | 411篇 |
1974年 | 375篇 |
1973年 | 419篇 |
1972年 | 294篇 |
排序方式: 共有10000条查询结果,搜索用时 29 毫秒
931.
This paper discusses the possibility of measurement of the current velocity vector characteristics averaged over the time of continuous regular down-up probings from a moving carrier. The measurements can be carried out either from a moving vessel or from a helicopter, floating on or hovering above the sea surface. As an illustration for the suggested technique,in situ observations compiled by a free-component (±XYZ) VEGA-3 probe in the Guiana current region in March 1990 are provided. Comparison of the results with buoy data shows good agreement within the accuracy limits of the instruments employed.Translated by Vladimir A. Puchkin. 相似文献
932.
The growth rate and potential production of bacterioplankton in cold hydrocarbon seeps located along the Louisiana coast were determined using a pulse-labeling technique. Surprisingly, community doubling times are on the order of 1.1 h, which compares to laboratory-grown cultures. We also found that there are differences in growth rates on relatively small geographic scales, suggesting the influence of site-specific geological features (e.g., gas hydrate mounds). Proceeding downslope to deeper waters, methane-oxidizing bacteria appeared to play a more significant role in community productivity. These preliminary experiments indicated, quite unexpectedly, that water column microbes are growing at a more rapid rate than in any other marine system so far studied and that methane may serve as a primary nutrient (carbon) source in these seep-associated microbial assemblages. 相似文献
933.
From 1988 to 1993, 23 satellite-tracked drifting buoys entered the Kamchatka Current. The buoy trajectories showed a well-formed, high-speed current that originated near Shirshov Ridge, and flowed southward through Kamchatka Strait. During some years, the buoys turned eastward at 50°N, while in other years they were transported as far south as Japan (40°N). Only one buoy entered the Sea of Okhotsk. Eddies were evident in many of the buoy trajectories. Greatest maximum daily velocities (>100 cm s–1) were observed south of Kamchatka Strait, with 50–60 cm s–1 being more common. 相似文献
934.
Donald J. Walter Douglas N. Lambert David C. Young Kevin P. Stephens 《Geo-Marine Letters》1997,17(4):260-267
Real-time trackline plots of surficial sediment acoustic impedance delineate several sedimentary facies off Garden Key in
the Dry Tortugas. The sea floor within a 6×6 km surveyed area consists of carbonate muds (silts), sands and shell, rock, and
live corals. The 4-kHz acoustic data supports this finding by providing a pictographic representation of the distribution
and structure of several sediment facies types. Plotting the gridded acoustic data with commercial mapping software (Surfer)
provides a three-dimensional (3D) perspective of the bottom topography with a color contour map of surficial sediment impedance
(upper 0.4 m) draped over the 3D surface. 相似文献
935.
Ship hull drag reduction using bottom air injection 总被引:1,自引:0,他引:1
Robert Latorre Dr. 《Ocean Engineering》1997,24(2):161-175
The idea of bottom air injection to reduce ship hull resistance is not new. Early patents envisioned planing hull applications. Recent planing hull tests speed realized an increase of 7–12 knots. River barges and ship fitted with an air injection system results are presented to show a 10–15% reduction in the frictional resistance. Graphs for making initial estimates for displacement hulls with bottom air injection are presented. It is clear from these results that improvements in high speed planing catamarans and full form hull resistance can be realized by using bottom air injection. 相似文献
936.
Robert D. Pierce 《Ocean Engineering》1985,12(6):493-529
A procedure is presented for the estimation of extreme values of stationary Gaussian random processes with arbitrary bandwidths. This approach is based on the analytic envelope defined by the Hilbert Transform; this envelope is Rayleigh distributed regardless of bandwidth. For experimentally derived data that has been converted into digital form, the Hilbert Transform is approximated using algorithms implemented on a digital computer to produce samples of the envelope's time history. Next, the degree of correlation between these envelope samples is taken into account using a method developed from simulation studies of a series of synthetic Gaussian time histories with varying bandwidths. Once this correlation effect has been estimated, the standard methods of order statistics are applied to these samples using the Rayleigh probability density function. Examples of applying this procedure to experimentally derived data are presented. 相似文献
937.
Pristane (2,6,10,14 -tetramethylpentadecane) occurs ubiquitously in the marine environment. This hydrocarbon may be of biogenic or petrogenic origin.1 Recently it has been shown that residual amounts of this branched alkane increased in marine organisms after an oil spill.2,3 The lack of data on the fate of pristane in fish, added with the fact that this compound was considered by some authors as a non-metabolisable substance in vertebrates, including man,4 led us to investigate the capability of fish to metabolise pristane. In this study, urinary and fecal excretion, tissue distribution and metabolism of 3H-pristane were analysed in Salmo gairdneri R. after a single intragastric dose. In addition to unchanged hydrocarbon, various labelled compounds have been isolated and identified in liver, bile, faeces, urine and surrounding water, demonstrating that pristane was first oxidised to alcohols (pristanol and pristane-diol) and to acid (pristanic acid). The elimination of these compounds occurred in the form of conjugated products (primarily glucuronides) as well as free metabolites. 相似文献
938.
Coals are oil source rocks in many of the Tertiary basins of Southeast Asia. The precursors of these hydrogen rich and oxygen poor coals are coastal plain peats which have mainly developed in an everwet and tropical climate. In these environments water flow and reworking can concentrate liptinitic kerogen in preference to vitrinitic kerogen. The distribution, petrography and chemistry of the coaly Miocene source rocks present in the Kutai Basin are described. The recognition of environmental controls on the accumulation of potentially oil-prone coals and coaly shales in deltaic environments is an aid to predictive source bed recognition in petroleum exploration. Comments on the environment of deposition of coaly sediments in the basins of the Norwegian Sea are discussed with reference to their possible oil and/or gas sourcing potential. The Triassic - Jurassic coals of the Haltenbanken area may become more oil-prone towards the delta margins, and facies mapping could aid oil exploration in this area. 相似文献
939.
Eggs from 69 females of spring spawning herring from the German Baltic coast (Travemünde, April 1979) were incubated in clean sea water (20‰ S, temperature 8°C) under standard conditions. Sixty-one trials could be used for the evaluation of hatching success. Viable hatch was taken as a measure to evaluate the effects of chlorinated hydrocarbons accumulated in gonads, liver and muscle of parental fish.PCB levels in running ripe females ranged on a wet weight basis between 19 and 241 ng g?1 (gonad), 20 and 377 ng g?1 (liver) and 11 and 1820 ng g?1 (muscle). Concentrations of other chlorinated hydrocarbons (DDD, DDE, γ-HCH, etc.) were in the same range as reported by other authors for Baltic herring (Huschenbeth, 1973, 1977). Viable hatch was significantly affected at ovary DDE concentrations higher than 18 ng g?1 (wet wt) and PCB concentrations of more than 120 ng g?1 (wet wt).Results are compared with data obtained during earlier investigations with flounder eggs. 相似文献