全文获取类型
收费全文 | 9298篇 |
免费 | 1513篇 |
国内免费 | 2225篇 |
专业分类
测绘学 | 468篇 |
大气科学 | 2486篇 |
地球物理 | 1866篇 |
地质学 | 3519篇 |
海洋学 | 2140篇 |
天文学 | 127篇 |
综合类 | 574篇 |
自然地理 | 1856篇 |
出版年
2024年 | 78篇 |
2023年 | 218篇 |
2022年 | 337篇 |
2021年 | 424篇 |
2020年 | 440篇 |
2019年 | 496篇 |
2018年 | 379篇 |
2017年 | 445篇 |
2016年 | 446篇 |
2015年 | 492篇 |
2014年 | 595篇 |
2013年 | 671篇 |
2012年 | 625篇 |
2011年 | 599篇 |
2010年 | 505篇 |
2009年 | 597篇 |
2008年 | 541篇 |
2007年 | 616篇 |
2006年 | 545篇 |
2005年 | 468篇 |
2004年 | 418篇 |
2003年 | 379篇 |
2002年 | 327篇 |
2001年 | 294篇 |
2000年 | 304篇 |
1999年 | 255篇 |
1998年 | 215篇 |
1997年 | 232篇 |
1996年 | 186篇 |
1995年 | 161篇 |
1994年 | 145篇 |
1993年 | 119篇 |
1992年 | 121篇 |
1991年 | 78篇 |
1990年 | 65篇 |
1989年 | 46篇 |
1988年 | 34篇 |
1987年 | 19篇 |
1986年 | 27篇 |
1985年 | 22篇 |
1984年 | 13篇 |
1983年 | 20篇 |
1982年 | 12篇 |
1981年 | 9篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 8篇 |
1977年 | 3篇 |
1976年 | 1篇 |
1972年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
11.
Abstract. The major problem in coastal areas of developing countries is disturbance caused by anthropogenic influence. This disturbance can be quantified by analysing the distribution and composition of marine communities using uni- and multivariate techniques and the biotic index. A study of benthic macrofauna was carried out along the São Sebastião Channel, northern coast of São Paulo State, Brazil, in an area with a submarine outfall, a petroleum terminal and a commercial harbour. Sampling was undertaken seasonally, by means of a van Veen grab (0.1 m2 ) at 15 oceanographic stations, from November 1993 to August 1994. A total of 392 species were identified (129 Polychaeta, 127 Mollusca, 98 Crustacea, 28 Echinodermata and 10 other phyla). The sedimentary pattern recorded for the São Sebastião Channel is very heterogeneous, influenced mainly by strong wind-driven currents, and differs from nearby shelf areas. The study area could be divided into three regions based on sediment texture and fauna: one region dominated by medium and coarse sand with low values of organic carbon; a second area where pelitic fractions were dominant with high values of organic carbon and nitrogen; and a third region characterised by fine and very fine sand, presenting intermediate organic carbon values. No temporal significant variation in abundance and species composition was found, except in autumn. The use of the 'marine Biotic Coefficient' showed the same ecological trend as the faunal abundance and demonstrated that the central continental region of the channel is affected by human activities. 相似文献
12.
13.
14.
15.
An important task in modern geostatistics is the assessment and quantification of resource and reserve uncertainty. This uncertainty
is valuable support information for many management decisions. Uncertainty at specific locations and uncertainty in the global
resource is of interest. There are many different methods to build models of uncertainty, including Kriging, Cokriging, and
Inverse Distance. Each method leads to different results. A method is proposed to combine local uncertainties predicted by
different models to obtain a combined measure of uncertainty that combines good features of each alternative. The new estimator
is the overlap of alternate conditional distributions. 相似文献
16.
17.
在山西霍山七里峪林场,采用间接的光学测量方法对沿着海拔分布的植物群落主要层叶面积指数(LAI)进行了测量,从不同的角度对LAI的垂直分布规律进行分析。结果表明:1.海拔不同,LAI表现出很大的差异,海拔1750m左右的LAI值大于较高与较低海拔的;对LAI和海拔之间的关系进行回归,得到LAI分布的拟合方程:y=b0+b1x+b2x^2+b3x^3(R=0.712,P〈0.01)。2.同一海拔样带,阴坡LAI均大于阳坡,但对同种种群来说,阴坡树种平均胸径并不一定均大于阳坡。3.植被类型LAI比较表现为:低山针叶林〈低山灌丛〈落叶阔叶林〈落叶针叶林〈常绿针叶林;而主要群系LAI比较表现为:油松林(0.38)〈落叶松林(0.78)〈山杨林及其混交林(1.09)〈白桦林(2.00)〈辽东栎林及其混交林(3.73)。4.典型群落LAI值变化分析和人工林与天然林以,比较表明,LAI值在一定海拔达到最大值,这对于山西霍山人工林栽培和丰产林培育等具有重要意义。 相似文献
18.
Mass size distribution of the crustal elements (Al, Ca, Fe, Mg, Si, Ti), anthropogenic elements (Zn, Mn, Cr, Cu, K, P, Pb) and sea elements (Na, Cl) were obtained from measurements carried out with an inertial cascade impactor in Sfax. A fitting procedure by data inversion was applied to those data. This procedure yields accurate size distributions of aerosols in the diameter range 0.1–25 μm in two different sites. In a coastal industrial site, the mass distribution of the aerosol showed a bimodal structure; and in urban area, the lower particle mode cannot be observed. The elemental dry deposition flux was calculated as a function of particle size. The element flux size distribution increased rapidly with particle size. The modelling results indicate that the majority of the crustal and anthropogenic elements flux (>90%) was due to particles larger than 3 μm in diameter. 相似文献
19.
This paper presents an improved generalisation of cation distribution determination based on an accurate fit of all crystal-chemical
parameters. Cations are assigned to the tetrahedral and octahedral sites of the structure according to their scattering power
and a set of bond distances optimised for spinel structure. A database of 295 spinels was prepared from the literature and
unpublished data. Selected compositions include the following cations: Mg2+, Al3+, Si4+, Ti4+, V3+, Cr3+, Mn2+, Mn3+, Fe2+, Fe3+, Co2+, Ni2+, Zn2+ and vacancies. Bond distance optimisation reveals a definite lengthening in tetrahedral distance when large amounts of Fe3+ or Ni2+ are present in the octahedral site. This means that these cations modify the octahedral angle and hence the shared octahedral
edge, causing an increase in the tetrahedral distance with respect to the size of the cations entering it. Some applications
to published data are discussed, showing the capacity and limitations of the method for calculating cation distribution, and
for identifying inconsistencies and inaccuracies in experimental data.
Received: 19 February 2001 / Accepted: 1 June 2001 相似文献
20.