全文获取类型
收费全文 | 82544篇 |
免费 | 1754篇 |
国内免费 | 1417篇 |
专业分类
测绘学 | 2219篇 |
大气科学 | 6108篇 |
地球物理 | 16037篇 |
地质学 | 29923篇 |
海洋学 | 7519篇 |
天文学 | 18890篇 |
综合类 | 405篇 |
自然地理 | 4614篇 |
出版年
2022年 | 577篇 |
2021年 | 939篇 |
2020年 | 976篇 |
2019年 | 1066篇 |
2018年 | 2226篇 |
2017年 | 2131篇 |
2016年 | 2623篇 |
2015年 | 1510篇 |
2014年 | 2533篇 |
2013年 | 4341篇 |
2012年 | 2858篇 |
2011年 | 3722篇 |
2010年 | 3261篇 |
2009年 | 4182篇 |
2008年 | 3584篇 |
2007年 | 3643篇 |
2006年 | 3497篇 |
2005年 | 2584篇 |
2004年 | 2502篇 |
2003年 | 2352篇 |
2002年 | 2260篇 |
2001年 | 1945篇 |
2000年 | 1861篇 |
1999年 | 1516篇 |
1998年 | 1597篇 |
1997年 | 1487篇 |
1996年 | 1287篇 |
1995年 | 1251篇 |
1994年 | 1072篇 |
1993年 | 956篇 |
1992年 | 941篇 |
1991年 | 920篇 |
1990年 | 1010篇 |
1989年 | 819篇 |
1988年 | 780篇 |
1987年 | 915篇 |
1986年 | 782篇 |
1985年 | 973篇 |
1984年 | 1135篇 |
1983年 | 976篇 |
1982年 | 954篇 |
1981年 | 853篇 |
1980年 | 820篇 |
1979年 | 788篇 |
1978年 | 765篇 |
1977年 | 671篇 |
1976年 | 621篇 |
1975年 | 617篇 |
1974年 | 594篇 |
1973年 | 645篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
951.
952.
953.
A marine magnetic total-field survey of Hudson Bay was conducted from 1975 to 1978. Approximately 62 000 line-km of data, or 35 000 five-minute averages, were acquired. The data were corrected for secular variation by means of a polynomial in latitude, longitude, and time, first fitted to annual mean differences from nine observatories and then integrated over time. The data were corrected for diurnal and transient variations by means of a linear interpolation, over geomagnetic latitude, of running hourly mean values from two neighbouring magnetic stations (numbering seven in all). Crossover differences are analysed in terms of a mixture of normal distributions resulting from the intermittent transient variations, and indicate the effectiveness of the corrections. The variance of the data is reduced, as a result of the corrections, by 87% during quiet periods and by 53% during disturbed periods, the weighted variance being reduced by 66%. Navigational accuracy is estimated, from a novel method utilizing crossover differences and field gradients, to be approximately 300 m. Anomalies are presented in profile form, and discussed briefly.Contribution from the Earth Physics Branch No. 973. 相似文献
954.
Ice effects have caused extensive damage. Enormous forces have been recorded. On the other hand, ice has been used as a structural material under various situations. To use ice beneficially and to assess the danger of ice forces, it is necessary to know the mechanical properties of ice.Results from the large number of ice tests carried out in laboratories show the difficulties of sampling and testing procedures. Most of the tests have not reproduced faithfully enough the conditions existing in nature. To obtain adequate results, laborious and slow samplings and preparations of test specimens have been necessary. Recently, extensive use has been made of in situ tests in order to reduce the number of factors whixh can induce errors in measurements. Large scale tests have simulated actual structures. General ice properties have become available from small-scale in situ tests. Since these tests give rapid results, a large number are already available with a good coverage of field conditions.A theoretical basis has been devised for these in situ tests which measure essentially the unconfined and confined compressive strengths of ice, and practical devices have been developed to execute these tests. A correlation of measurements from laboratory tests and in situ tests has established a link between natural and artificial environments. Brittle elastic and clastic theories have been applied, to find the best correlations for ice tests. Test results have been related to the mechanical properties as used in engineering practice.A very summary conclusion of results is that cold sea ice and cold fresh water ice are both very strong. However, depending on the load rate, ice can be quite brittle, which mitigates dangerous effects from ice action against massive structures. 相似文献
955.
Regoli F Gorbi S Frenzilli G Nigro M Corsi I Focardi S Winston GW 《Marine environmental research》2002,54(3-5):419-423
An integrate approach will be discussed for investigations on oxidative stress in xenobiotic toxicity. While the analysis of individual antioxidants is useful for their sensitivity and to understand the mode of action of a stressor, the integration with the analysis of the total antioxidant capacity provides a more holistic assessment of the overall biological significance of such variations. TOSC has a greater predictive value on the health condition of the organisms and allows to discriminate the different role of specific ROS in oxidative stress syndrome. 相似文献
956.
Gassy sediments are detectable acoustically in subbottom profiles of Canada's Fraser Delta slope and prodelta areas. Interstitial gas is typically represented by zones of acoustic turbidity on high-resolution seismic profiles and by gas brightening on air-gun profiles. The top of the acoustically turbid zone is generally highest within the sediment column closest to the river mouths and lies within 10 ms (two-way travel time) of the sea floor in a nearly continuous area that covers over 530 km2. Most of the gas is believed to be biogenic in origin, although thermogenic gas derived from underlying Tertiary sedimentary rocks may be present in places. 相似文献
957.
The northern parts of the Prathap and Laccadive Ridge system, eastern Arabian Sea, consist of three parallel basement ridge peaks at varied depths. The topographic highs are associated with either well-developed or subdued magnetic signatures. Model studies, constrained by seismic results, determine the varied nature and depth to the top of the causative basement bodies. Similarities of the geophysical signatures of the ridges and their structural resemblance perhaps point to their common origin. Hence we propose that the Prathap Ridge complex may be a part of the Chagos-Laccadive Ridge system and formed because of the Reunion hotspot activity. 相似文献
958.
L. D. Wright Z. -S. Yang B. D. Bornhold G. H. Keller D. B. Prior W. J. Wiseman Jr. Y. B. Fan Z. Su 《Geo-Marine Letters》1986,6(2):115-120
Internal waves with periods of about 5 minutes and trough to crest heights of up to 6.2 m were observed acoustically over the actively accreting delta front of the Huanghe (Yellow River) in the western portion of the Gulf of Bohai, Peoples Republic of China. The radian frequency of the internal waves was close to the locally-observed Brunt-Vaisala frequency. Through the relatively short (one hour) duration of an internal wave train, the amplitude was observed to decrease progressively with time. These internal waves may cause resuspension of delta-front sediments. 相似文献
959.
960.