全文获取类型
收费全文 | 13333篇 |
免费 | 2318篇 |
国内免费 | 3564篇 |
专业分类
测绘学 | 1427篇 |
大气科学 | 2693篇 |
地球物理 | 2851篇 |
地质学 | 6402篇 |
海洋学 | 2278篇 |
天文学 | 538篇 |
综合类 | 1198篇 |
自然地理 | 1828篇 |
出版年
2024年 | 58篇 |
2023年 | 183篇 |
2022年 | 519篇 |
2021年 | 627篇 |
2020年 | 540篇 |
2019年 | 639篇 |
2018年 | 668篇 |
2017年 | 583篇 |
2016年 | 730篇 |
2015年 | 703篇 |
2014年 | 789篇 |
2013年 | 885篇 |
2012年 | 939篇 |
2011年 | 954篇 |
2010年 | 859篇 |
2009年 | 846篇 |
2008年 | 772篇 |
2007年 | 813篇 |
2006年 | 670篇 |
2005年 | 662篇 |
2004年 | 565篇 |
2003年 | 441篇 |
2002年 | 596篇 |
2001年 | 546篇 |
2000年 | 485篇 |
1999年 | 487篇 |
1998年 | 345篇 |
1997年 | 326篇 |
1996年 | 290篇 |
1995年 | 271篇 |
1994年 | 253篇 |
1993年 | 194篇 |
1992年 | 186篇 |
1991年 | 161篇 |
1990年 | 119篇 |
1989年 | 100篇 |
1988年 | 88篇 |
1987年 | 71篇 |
1986年 | 42篇 |
1985年 | 31篇 |
1984年 | 34篇 |
1983年 | 20篇 |
1982年 | 35篇 |
1981年 | 21篇 |
1980年 | 13篇 |
1979年 | 9篇 |
1977年 | 6篇 |
1976年 | 9篇 |
1972年 | 5篇 |
1971年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
991.
992.
Precambrian Geochronology, Chronotectonic Framework and Model of Chronocrustal Structure of the Zhongtiao Mountains 总被引:8,自引:0,他引:8
The Zhongtiao Mountains, a typical exposure area of Precambrian rocks, are a concentration area of vari-ous types of copper deposit. The rocks were dated using several dating methods. Based on the age data ob-tained by means of three methods, several aspects are dealt with: (1) the dating results obtained by differentmethods and their geological implications have been compared; (2) a chronotectonic framework has been con-structed by means of these reliable ages, which indicates that the ages of the rocks of the Proterozoic mobilebelt fall in a time span of 2400 to 2000 Ma during which global magmatic records are lacking and reflects theages of important events in the mobile belt; (3) according to the ages of inherited zircons, neodymium modelage of the depleted mantle sources (T_(DM)) and geochemical and geological data, a model of Precambrianchronocrustal structure has been constructed, representing a typical for the North China craton. 相似文献
993.
A lot of researches onb value have been made in seismology. Since the 1960’s Mogi, Scholz and others have studied AE of rock specimens in laboratory
and discovered that it is related to natural earthquakes. All former researchers used integral specimens to studyb value in the laboratory. However a major earthquake is usually related to a existing seismic-fault in that area. For this
reason, a series of fracture experiments with rock and glass specimens having pre-existing crack or notch is performed in
order to examine the effect of preexisting crack tob value. The experimental results show that theb value begins to decrease as soon as the initiation of the crack and finally drop to a very low value when the specimen breaks
unstably. Based on these, a brief discussion on the possible mechanism ofb value change for natural earthquakes is given.
The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,9, 393–400, 1987.
Projects sponsored by the Chinese Joint Seismological Science Foundation. 相似文献
994.
995.
对湖南上堡、郴县及胶东地区5个黄铁矿样品和5个黄铜矿样品进行的穆斯堡尔谱及电子探针(波谱仪)的分析得出:含Fe~(2+)的黄铁矿中Fe的纯离子浓度I_(La)/I_(Ka)值为0.65±0.05,含Fe~(3+)的黄铜矿中Fe的纯离子浓度,I_(La)/I_(ka)值为0.40士0.05.可见,随Fe价态的升高,比值I_(La)/I_(ka)反而降低. 相似文献
996.
Soil amplification characteristics of earthquake ground motion were investigated in terms of peak ground acceleration and transfer function based on the Chiba array observation records. The amplification of peak ground acceleration occurred mostly at the top soft layer and was similar for the three components. The effects of non-linear response of soil deposits on the transfer function were examined. Transfer functions calculated by ensemble average were close for the two horizontal components while those obtained from a smoothing operation were generally different. Both the transfer functions from the ensemble average and the smoothing operation underestimated the gain factor around the natural frequencies. A two-step smoothing procedure was proposed and a rotary spectrum was used to improve the estimation of the transfer function. Microtremors were observed at the locations of the boreholes where seismometers are buried. The power spectrum and spatial coherency of the microtremors were compared with those of the earthquake ground motion. Emphasis was placed on the wavetypes which dominated the peaks in the power spectra. 相似文献
997.
In order to reduce the size of problems involving analysis of the dynamic response of structural systems, a transformatio based on appropriately selected Ritz shapes is commonly employed. The lower mode shapes may at times serve a effective Ritz shapes. However, the computation of mode shapes is a time consuming task; in addition, the mode shapes may not form the best basis for representing the spatial distribution of loads. The recently developed load dependent vectors, which are derived from a static solution for the applied loads, address some of the problems inherent in the use of mode shapes. However, both the natural mode shapes and the load dependent vectors fail to account for the frequency content of the loading, a parameter that may influence strongly the response, particularly for loading with a high frequency content. A procedure is presented here for the generation of frequency dependent vectors. A combination of load dependent and frequency dependent vectors will often form a very efficient basis for the representation of the response, as illustrated by several examples presented here. 相似文献
998.
Array observation is an efficient tool to investigate various characteristics of earthquake ground motion. However, seismographs used in arrays may involve unexpected errors in their orientations. Methods of orientation error estimation were developed in three-dimensional space by comparing recorded ground motions at a reference point with those at a checking point. A maximum cross-correlation method and a maximum coherence method were proposed and their accuracy was demonstrated. The earthquake ground motions recorded in the Chiba array and in two other arrays were used in numerical examples. Non-trivial orientation errors were detected for all these arrays. The cross-correlation coefficients and the coherence values between two points increased significantly by correcting the estimated orientation errors. 相似文献
999.
球粒陨石金属相的 Ir、Os、Co等亲铁元素的浓度随氧化程度的升高而显著增大 ,其浓度以及铁纹石 Co含量和橄榄石 Fa值均反映了球粒陨石的氧化 -还原程度具有连续变化的特征。根据铁纹石 Co含量、橄榄石 Fa值、低 Ca辉石 Fs值、金属相的 Ir、Os、Co浓度以及其他球粒陨石分类参数 ,可能存在介于 E与 H、H与 L、L与 L L以及 L L与 C之间的球粒陨石过渡型 ,即E/H、H/L、L/L L和 LL/C化学群 ,从而将原有的 9个球粒陨石化学群增加到 1 3个。球粒陨石氧化 -还原程度的变化特征以及球粒陨石过渡型的发现都表明早期太阳星云的化学分馏作用具有连续变化的特点。 相似文献
1000.
岩相古地理及古地理学的研究,在我国已有相当长的历史并有着十分丰硕的成果。各家以不同的理论为指导、采取不同的编图方法,从不同的研究角度编制了中国的岩相古地理图。刘鸿允(1955)以古生物地层学的理论为指导,编写了我国第一部古地理学的专著《中国古地理图》,从此,正式揭开了中国古地理学研究的序幕;卢衍豪等(1965)以期为单位,编制了中国寒武纪各期的岩相古地理图;关士聪等(1984),以大地构造学和沉积学的理论为指导,编制了《中国海陆变迁海域沉积相与油气》专著,建立中国各地质时代的 相似文献