全文获取类型
收费全文 | 4778篇 |
免费 | 122篇 |
国内免费 | 55篇 |
专业分类
测绘学 | 152篇 |
大气科学 | 411篇 |
地球物理 | 1068篇 |
地质学 | 1770篇 |
海洋学 | 439篇 |
天文学 | 828篇 |
综合类 | 16篇 |
自然地理 | 271篇 |
出版年
2023年 | 13篇 |
2022年 | 45篇 |
2021年 | 68篇 |
2020年 | 70篇 |
2019年 | 58篇 |
2018年 | 149篇 |
2017年 | 153篇 |
2016年 | 187篇 |
2015年 | 123篇 |
2014年 | 155篇 |
2013年 | 235篇 |
2012年 | 210篇 |
2011年 | 282篇 |
2010年 | 215篇 |
2009年 | 293篇 |
2008年 | 247篇 |
2007年 | 206篇 |
2006年 | 217篇 |
2005年 | 222篇 |
2004年 | 301篇 |
2003年 | 228篇 |
2002年 | 154篇 |
2001年 | 115篇 |
2000年 | 106篇 |
1999年 | 83篇 |
1998年 | 98篇 |
1997年 | 63篇 |
1996年 | 36篇 |
1995年 | 38篇 |
1994年 | 42篇 |
1993年 | 34篇 |
1992年 | 41篇 |
1991年 | 22篇 |
1990年 | 35篇 |
1989年 | 13篇 |
1988年 | 16篇 |
1987年 | 21篇 |
1986年 | 19篇 |
1985年 | 15篇 |
1984年 | 20篇 |
1983年 | 19篇 |
1982年 | 15篇 |
1981年 | 22篇 |
1978年 | 22篇 |
1977年 | 20篇 |
1976年 | 15篇 |
1975年 | 18篇 |
1974年 | 16篇 |
1973年 | 15篇 |
1971年 | 12篇 |
排序方式: 共有4955条查询结果,搜索用时 0 毫秒
991.
Suzan Pantaroto de Vasconcellos Elaine Crespim Georgiana Feitosa da Cruz Diego Barbosa Senatore Karen Christina Marques Simioni Eugênio Vaz dos Santos Neto Anita Jocelyne Marsaioli Valéria Maia de Oliveira 《Organic Geochemistry》2009,40(5):574-588
The detection of microorganisms with potential for biodeterioration and biodegradation in petroleum fields is of great relevance, since these organisms may be related to a decrease in petroleum quality in the reservoirs or damage in the production facilities. In this sense, petroleum formation water and oil samples were collected from the Campos Basin, Brazil, with the aim of isolating microorganisms and evaluating their ability to degrade distinct classes of hydrocarbon biomarkers (9,10-dihydrophenanthrene, phytane, nonadecanoic acid and 5α-cholestane). Twenty eight bacterial isolates were recovered and identified by sequencing their 16S rRNA genes. Biodegradation assays revealed that bacterial metabolism of hydrocarbons occurred through reactions based on oxidation, carbon–carbon bond cleavage and generation of new bonds or by the physical incorporation of hydrocarbons into microbial cell walls. Based on the biodegradation results, selective PCR-based systems were developed for direct detection in petroleum samples of bacterial groups of interest, namely Bacillus spp., Micrococcus spp., Achromobacter xylosoxidans, Dietzia spp. and Bacillus pumilus. Primer sets targeting 16S rRNA genes were designed and their specificity was confirmed in silico (i.e. computational analysis) and in PCR reactions using DNA from reference strains as positive and negative controls. Total DNA from oil was purified and the amplification tests revealed the presence of the target bacteria in the samples, unraveling a significant potential for petroleum deterioration in the reservoirs sampled, once proper conditions are present for hydrocarbon degradation. The application of molecular methods for rapid detection of specific microorganisms in environmental samples would be valuable as a supporting tool for the evaluation of oil quality in production reservoirs. 相似文献
992.
This paper presents the findings of a collaborative research project of the Geological Survey of Lower Saxony (NLfB) and the Programme Group Systems Analysis and Technology Evaluation (STE) of Research Centre Jülich on the GIS-based determination of the mean long-term groundwater recharge in Lower Saxony using high-resolution digital data (Dörhöfer and others 2001). The model calculations were performed on the basis of the water-balance model GROWA (Kunkel and Wendland 2002) with a spatial resolution of 100x100 m2. The accuracy of the calculated groundwater recharge values for the period 1961–1990 was verified on the basis of data from gauging stations and displayed a good agreement between observed runoff values and model results. 相似文献
993.
P.P. Povinec J. de Oliveira E.S. Braga J.-F. Comanducci J. Gastaud M. Groening I. Levy-Palomo U. Morgenstern Z. Top 《Estuarine, Coastal and Shelf Science》2008
Stable isotopes, tritium, radium isotopes, radon, trace elements and nutrients data were collected during two sampling campaigns in the Ubatuba coastal area (south-eastern Brazil) with the aim of investigating submarine groundwater discharge (SGD) in the region. The isotopic composition (δD, δ18O, 3H) of submarine waters was characterised by significant variability and heavy isotope enrichment. The stable isotopes and tritium data showed good separation of groundwater and seawater groups. The contribution of groundwater in submarine waters varied from a few % to 17%. Spatial distribution of 222Rn activity concentration in surface seawater revealed changes between 50 and 200 Bq m−3 which were in opposite relationship with observed salinities. Time series measurements of 222Rn activity concentration in Flamengo Bay (from 1 to 5 kBq m−3), obtained by in situ underwater gamma-spectrometry showed a negative correlation between the 222Rn activity concentration and tide/salinity. This may be caused by sea level changes as tide effects induce variations of hydraulic gradients, which increase 222Rn concentration during lower sea level, and opposite, during high tides where the 222Rn activity concentration is smaller. The estimated SGD fluxes varied during 22–26 November between 8 and 40 cm d−1, with an average value of 21 cm d−1 (the unit is cm3/cm2 per day). The radium isotopes and nutrient data showed scattered distributions with offshore distance and salinity, which implies that in a complex coast with many small bays and islands, the area has been influenced by local currents and groundwater–seawater mixing. SGD in the Ubatuba area is fed by coastal contaminated groundwater and re-circulated seawater (with small admixtures of groundwater), which claims for potential environmental concern with implications on the management of freshwater resources in the region. 相似文献
994.
A procedure to calculate the three-dimensional harmonic response of a infinitely long cylindrical shell of circular cross-section embedded in a layered viscoelastic half-space and subjected to harmonic plane waves impinging at an oblique angle with respect to the axis of the shell is validated by extensive comparisons with previous two- and three-dimensional results for the particular case of a shell embedded in a uniform half-space. New numerical results describing the motion and stresses within a shell embedded in a multilayered half-space and subjected to obliquely incident P-, SV- and SH-waves with different horizontal angles of incidence are presented and discussed. 相似文献
995.
996.
R. González Felipe H. J. Mosquera Cuesta A. Pérez Martínez H. Pérez Rojas 《中国天文和天体物理学报》2005,5(4):399-411
The equations of state for degenerate electron and neutron gases are studied in the presence of magnetic fields. After including quantum effects in the investigation of the structural properties of these systems, it is found that some hypermagnetized stars can be unstable according to the criterion of stability of pressures. Highly magnetized white dwarfs should collapse producing a supernova type Ia, while superstrong magnetized neutron stars cannot stand their own magnetic field and must implode, too. A comparison of our results with a set of the available observational data of some compact stars is also presented, and the agreement between this theory and observations is verified. 相似文献
997.
Prof. Juan L. de Vivero Suárez 《GeoJournal》1990,22(2):220-220
998.
999.
1000.