全文获取类型
收费全文 | 57343篇 |
免费 | 8607篇 |
国内免费 | 10727篇 |
专业分类
测绘学 | 5949篇 |
大气科学 | 6445篇 |
地球物理 | 13266篇 |
地质学 | 23336篇 |
海洋学 | 7210篇 |
天文学 | 8408篇 |
综合类 | 3681篇 |
自然地理 | 8382篇 |
出版年
2024年 | 182篇 |
2023年 | 611篇 |
2022年 | 1622篇 |
2021年 | 1936篇 |
2020年 | 2147篇 |
2019年 | 2732篇 |
2018年 | 1940篇 |
2017年 | 2339篇 |
2016年 | 2352篇 |
2015年 | 2523篇 |
2014年 | 3238篇 |
2013年 | 3639篇 |
2012年 | 3365篇 |
2011年 | 3740篇 |
2010年 | 3111篇 |
2009年 | 4126篇 |
2008年 | 3977篇 |
2007年 | 4099篇 |
2006年 | 3959篇 |
2005年 | 3288篇 |
2004年 | 3085篇 |
2003年 | 2691篇 |
2002年 | 2132篇 |
2001年 | 1912篇 |
2000年 | 1770篇 |
1999年 | 1595篇 |
1998年 | 1581篇 |
1997年 | 1092篇 |
1996年 | 940篇 |
1995年 | 895篇 |
1994年 | 803篇 |
1993年 | 711篇 |
1992年 | 527篇 |
1991年 | 379篇 |
1990年 | 295篇 |
1989年 | 301篇 |
1988年 | 225篇 |
1987年 | 153篇 |
1986年 | 126篇 |
1985年 | 111篇 |
1984年 | 68篇 |
1983年 | 35篇 |
1982年 | 46篇 |
1981年 | 34篇 |
1980年 | 43篇 |
1979年 | 26篇 |
1978年 | 26篇 |
1977年 | 36篇 |
1976年 | 28篇 |
1954年 | 28篇 |
排序方式: 共有10000条查询结果,搜索用时 671 毫秒
991.
992.
We have started a program of high-resolution (0.4/pixel) 10m imaging of bipolar outflow sources using the 10m camera CAMIRAS. We present recent results obtained at the Canada France Hawaii Telescope which reveal extended emission or IR companions in several luminous objects. The extended emission we detected probably arises from transiently heated very small grains, while the newly discovered companions could contribute significantly to the outflow activity and extended far-IR emission usually attributed to the main optical source. 相似文献
993.
Andrea Lami Frank Niessen Piero Guilizzoni Julieta Masaferro Claudio A. Belis 《Journal of Paleolimnology》1994,10(3):181-197
We use palaeolimnological techniques to reconstruct the eutrophication history of a volcanic lake (Lake Albano, central Italy) over the past three centuries. The presence of annual varves down to the bottom of the core (c. 1700 A.D.) indicated the lack of bioturbation and likely long-term meromixis. Sedimentation rates were estimated by varve counts (calcite/diatom couplets), indicating a mean rate of 0.15 cm yr–1. The reconstruction of eutrophication was traced using past populations of algal and photosynthetic bacteria (through their fossil pigment), and geochemistry, as well as fossil remains of chironomids. Phaeophorbidea and the red carotenoid astaxanthin were used to detect past zooplankton development.The first sign of trophic change related to human activities is datedc. 1870 A.D. From that period onward a sharp increase of authigenic CaCO3, nitrogen, N:P ratio, and dinoxanthin, a characteristic carotenoid of Chrysophyceae and Dinophyceae, is observed.Chironomid analyses showed the near absence of a deep water fauna throughout the core length. The populations of chironomid larvae are restricted to oxygenated littoral zones. In fact, the few fossil remains found are primarily of littoral origin, representing shallow water midges that were transported to profundal waters. The reduction of total chironomid in the uppermost layers of the core is to be related to human land uses. 相似文献
994.
We have used the ESO 10 m camera, TIMMI, to image with a very high angular resolution (PFoV: 0.3; FWHM:0.9) several main-sequence star disk candidates. Information on the -Pictoris dust disk has been obtained in a region largely inaccessible to previous observations: 0–80 AU, with a resolution of about 5 AU after deconvolution. Another promising target for 10 m imaging, 51 Ophiuchi, appears point-like.based on data collected at the European Southern Observatory (ESO), La Silla, Chile 相似文献
995.
996.
Chang-Jo F. Chung 《Mathematical Geology》1993,25(7):851-865
Multivariate statistical analyses have been extensively applied to geochemical measurements to analyze and aid interpretation of the data. Estimation of the covariance matrix of multivariate observations is the first task in multivariate analysis. However, geochemical data for the rare elements, especially Ag, Au, and platinum-group elements, usually contain observations the below detection limits. In particular, Instrumental Neutron Activation Analysis (INAA) for the rare elements produces multilevel and possibly extremely high detection limits depending on the sample weight. Traditionally, in applying multivariate analysis to such incomplete data, the observations below detection limits are first substituted, for example, each observation below the detection limit is replaced by a certain percentage of that limit, and then the standard statistical computer packages or techniques are used to obtain the analysis of the data. If a number of samples with observations below detection limits is small, or the detection limits are relatively near zero, the results may be reasonable and most geological interpretations or conclusions are probably valid. In this paper, a new method is proposed to estimate the covariance matrix from a dataset containing observations below multilevel detection limits by using the marginal maximum likelihood estimation (MMLE) method. For each pair of variables, sayY andZ whose observations containing below detection limits, the proposed method consists of three steps: (i) for each variable separately obtaining the marginal MLE for the means and the variances,
,
,
, and
forY andZ: (ii) defining new variables by
and
and lettingA=C+D andB=C–D, and obtaining MLE for variances,
and
forA andB; (iii) estimating the correlation coefficient YZ by
and the covariance
YZ
by
. The procedure is illustrated by using a precious metal geochemical data set from the Fox River Sill, Manitoba, Canada. 相似文献
997.
首都减灾圈,系首都减轻自然灾害预测防治圈。1991年12月20—21日在北京召开了《首都圈自然灾害及其减灾对策研讨会》。本文根据这次会议所提供的材料,在从整体上实现减灾的思想指导下,就首都减灾圈的“成灾背景”、“首都减灾圈的组成”、“首都减灾圈的灾害预测与防治状况”、“首都减灾圈的灾害关联性分析”、“首都减灾圈的减灾实效预估”和“首都减灾圈的减灾对策与实施”等六个带有共同性的问题,进行了讨论。供制定首都圈减灾方案时参考。 相似文献
998.
本分析了杠杆点的几何含义和统计特征,从杠杆点的保差性,矩阵的摄动和参数估值可靠性的观点出发讨论了杠杆点的不利和有利的两个方面,提出了一个全面的设计空间抗差方案。 相似文献
999.
Ernst Neßeler Michael Fröhler Sebastían Lebede 《International Journal of Earth Sciences》1993,82(4):765-768
The Wattkopftunnel, near Ettlingen (Nordschwarzwald), drives through the eastern margin of the Rheingraben. The tunnel passes cenozoic and mesozoic sediments. Early quarternary and tertiary beds are situated west of the main thrust of the Rheingraben. Fossil record indicates upper Oligocene age (Chatt) for parts of the tertiary sediments. At the eastern border of the Rheingraben, wedges of jurassic and middle triassic series are squeezed. East of the Rheingraben the tunnel drives in the lower triassic Bausandstein. The eastern margin of the Rheingraben was investigated in detail during tunneling. Faults of the Rheingraben margin are distributed in an 130 meter wide fault zone in the tunnel. Total stratigraphic separation by the normal faults reaches more than 2 000 meters. The cenozoic sequence suffered synsedimentary to early diagenetic deformation, while the mesozoic series are characterized by ruptural deformation. The fault- and joint system is directed in the rheinische Richtung (SSW-NNE). East of the Rheingraben a second direction occur, running parallel to the lower Albtal (W-E). 相似文献
1000.
R. W. Botz V. Georgiev P. Stoffers Kh. Khrischev V. Kostadinov 《International Journal of Earth Sciences》1993,82(4):663-666
The Precipitation of carbonate cements in the Pobitite Kamani area (Lower Eocene) began during early diagenesis of sediments. There is evidence, however, that calcite is still forming today.The negative 13C values to –29.2 suggest that the carbonate formed during degradation of 12C-enriched organic matter (perhaps partly from oxidation of methane). The 18O values of –0.9 to –1.6 reflect the marine origin of the early diagenetic carbonate cements. Most of the carbonates, however, formed during late diagenesis (at approximately 1300 m burial depth) and/or recently (after uplift) from percolating groundwaters. These carbonates have an isotopic composition characteristic of carbonates which precipitated from meteoric waters under normal sedimentary temperatures in isotopic equilibrium with 12C-enriched soil carbon dioxide. 相似文献