全文获取类型
收费全文 | 1537篇 |
免费 | 69篇 |
国内免费 | 63篇 |
专业分类
测绘学 | 55篇 |
大气科学 | 52篇 |
地球物理 | 338篇 |
地质学 | 956篇 |
海洋学 | 102篇 |
天文学 | 82篇 |
综合类 | 12篇 |
自然地理 | 72篇 |
出版年
2023年 | 4篇 |
2022年 | 34篇 |
2021年 | 41篇 |
2020年 | 60篇 |
2019年 | 36篇 |
2018年 | 137篇 |
2017年 | 121篇 |
2016年 | 134篇 |
2015年 | 72篇 |
2014年 | 115篇 |
2013年 | 186篇 |
2012年 | 65篇 |
2011年 | 94篇 |
2010年 | 71篇 |
2009年 | 73篇 |
2008年 | 53篇 |
2007年 | 33篇 |
2006年 | 36篇 |
2005年 | 27篇 |
2004年 | 27篇 |
2003年 | 23篇 |
2002年 | 35篇 |
2001年 | 16篇 |
2000年 | 16篇 |
1999年 | 9篇 |
1998年 | 11篇 |
1997年 | 16篇 |
1996年 | 4篇 |
1995年 | 7篇 |
1994年 | 6篇 |
1993年 | 5篇 |
1992年 | 7篇 |
1991年 | 6篇 |
1990年 | 6篇 |
1988年 | 3篇 |
1987年 | 7篇 |
1986年 | 7篇 |
1985年 | 7篇 |
1984年 | 6篇 |
1983年 | 4篇 |
1982年 | 9篇 |
1981年 | 6篇 |
1980年 | 2篇 |
1979年 | 10篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1971年 | 2篇 |
排序方式: 共有1669条查询结果,搜索用时 15 毫秒
971.
Prioritization of sub-watersheds based on morphometric and land use analysis using remote sensing and GIS techniques 总被引:2,自引:0,他引:2
Akram Javed Mohd Yousuf Khanday Rizwan Ahmed 《Journal of the Indian Society of Remote Sensing》2009,37(2):261-274
Watershed prioritization has gained importance in natural resources management, especially in the context of watershed management.
Morphometric analysis has been commonly applied to prioritization of watersheds. The present study makes an attempt to prioritize
sub-watersheds based on morphometric and land use characteristics using remote sensing and GIS techniques in Kanera watershed
of Guna district, Madhya Pradesh. Various morphometric parameters, namely linear and shape have been determined for each sub-watershed
and assigned ranks on the basis of value/relationship so as to arrive at a computed value for a final ranking of the sub-watersheds.
Land use/land cover change analysis of the sub-watersheds has been carried out using multi-temporal data of IRS LISS II of
1989 and IRS LISS III of 2001. The study demonstrates the significant land use changes especially in cultivated lands, open
scrub, open forest, water bodies and wastelands from 1989 to 2001. Based on morphometric and land use/land cover analysis,
the sub-watersheds have been classified into three categories as high, medium and low in terms of priority for conservation
and management of natural resources. Out of the seven sub-watersheds, two sub-watersheds viz., SW1 and SW6 qualify for high
priority, whereas SW7 has been categorised as medium priority based on the integration of morphometric and land use change
analysis. 相似文献
972.
We have studied solar activity by analyzing naked-eye sunspot observations and aurorae borealis observed at latitudes below \(45^{\circ}\). We focused on the medieval epoch by considering the non-telescopic observations of sunspots from AD 974 to 1278 and aurorae borealis from AD 965 to 1273 that are reported in several Far East historical sources, primarily in China and Korea. After setting selection rules, we analyzed the distribution of these individual events following the months of the Gregorian calendar. In December, an unusual peak is observed with data recorded in both China and Japan, but not within Korean data.In extreme conditions, where the collection of events is reduced and discontinuous in some temporal intervals, we used the non-parametric kernel method. We opted for the plug-in approach of Sheather and Jones instead of cross-validation techniques to estimate the probability density functions (pdf) of the events. We obtained optimized bandwidths of 13.29 years for sunspots and 9.06 years for auroras, and 95% confidence intervals. The pdf curves exhibit multiple peaks occurring at quasi-periodic times with a very high positive correlation, \(r_{\mathrm{tt}} = 0.9958\), between the dates of occurrence of the nine extrema of sunspots and auroras. Furthermore, these extrema enabled us to evaluate mean periods at two standard deviations, \(66.77 \pm 7.25~\mbox{years}\) for sunspots and \(65.06 \pm 9.36~\mbox{years}\) for auroras. The accuracy of the average periods, 62.00 years for sunspots and 61.80 years for auroras, was improved by the use of the power spectrum method. The percentage of the total number of non-observed sunspots, using redundant data, from AD 1151 to 1275 was estimated to be greater than or equal to 78%. 相似文献
973.
Jebali J Banni M Gerbej H Boussetta H López-Barea J Alhama J 《Marine environmental research》2008,65(4):358-363
Metallothionein was quantified in sea bass Dicentrarchus labrax intraperitoneally (i.p.) injected with different Cu, Cd and Hg doses (50-250 microg kg(-1) wet wt) after 48 h exposure. A distinct peak with 16.8 min retention time was obtained by reversed-phase high performance liquid chromatography coupled to fluorescence detection (RP-HPLC-FD) with the three metals. Total metallothionein levels assayed in unheated liver extracts by RP-HPLC-FD were significantly higher (1.3-1.95-fold) than those obtained by the well-established spectrophotometric method. In the RP-HPLC-FD method, metallothionein increased linearly with Cu and Hg doses, being saturated beyond 100 mug kg(-1) Cd. Maximum induction was obtained at 100 microg kg(-1) Cd (5.3-fold), and 250 microg kg(-1) Cu or Hg (8- and 5.1-fold, respectively). At low doses no metallothionein induction was shown by the less sensitive spectrophotometric assay. 相似文献
974.
China Ocean Engineering - Hydrokinetic energy is a promising technology to harness predictable renewable energy from free-flowing water, tides and ocean currents. Many studies have been conducted... 相似文献
975.
Readily dispersible clay is the part of the clay fraction in soils that potentially disperses in water when a small amount of mechanical energy is applied to soil. Column and batch experiments were conducted to identify the effect of readily dispersible clay on the mobility of some metal ions in a disturbed soil sample. The clay fraction (<0.002 mm) was separated from an alkaline Vertisol from the Nile River Delta. X-ray diffraction technique was used to identify minerals present in the clay fraction. Clay suspensions and deionized H2O solutions of Cd2+, Cu2+, and Zn2+ were prepared and used as influents in soil columns. Adsorption capability of the studied soil among the three metal ions was investigated. The results showed high adsorption capacity of Cd2+, Cu2+, but not Zn2+ for the studied soil. Cu2+ was the highest adsorbed metal by soil and its sorption increased at small equilibrium concentrations compared with Cd2+ and Zn2+. For the three studied metal ions, Langmuir model represented the best fit to the adsorption data. The concentration of Zn2+ and Cd2+ in leachates increased as the leaching solution volume increased, while Cu2+ showed a homogeneous distribution throughout the soil column. According to DTPA extractable metals, Zn2+ was appeared at greater depths than Cd2+, while Cu2+ had homogeneous distribution through the soil column. 相似文献
976.
Existing uses for sulfur are relatively advanced and offer limited opportunities to consume significant new supplies. Currently,
sulfur is in net surplus on a global basis, and with environmental regulations, greater sulfur recovery from petroleum and
gas processing is mandated. The outlook is clear: there will be substantial and growing surpluses in global sulfur supply
in the foreseeable future. Sulfur prices are likely to be under pressure, and producers could face substantial and growing
disposal fees. Therefore, new markets must be found for sulfur to avoid disposal crises. One potential new market is the production
of sulfur-solidified concrete. This is a thermoplastic composite of mineral aggregates bound together with chemically modified
sulfur. The product is more durable than Portland cement. There may be monetizeable benefits in reducing greenhouse gas emissions
that would enhance the attraction of sulfur solidified cement applications. The main objective of this study is to evaluate
experimentally the potential use of sulfur as a solidifying agent for hazardous waste. To achieve this objective, the study
reviewed the sulfur concrete literature, analyzed locally produced sulfur, evaluated a number of sulfur polymer modifiers
and physical stabilizing agents, designed a set of experiments and evaluated the produced product for its hydro-mechanical-chemical
properties. The results indicated that the manufactured sulfur polymer cement is an excellent candidate for: waste management
practices such as solidification/stabilization of hazardous waste; barrier systems for landfilling of hazardous waste; and
waste water treatment plants. 相似文献
977.
Wissem Gallala Mohamed Essghaier Gaied Mabrouk Montacer 《Journal of African Earth Sciences》2009,53(4-5):159-170
The chemical and mineralogical composition of the Sidi Aïch Formation sandstones in central and southwestern Tunisia has been investigated in order to infer the provenance and tectonic setting, as well as to appraise the influence of weathering. The sixteen studied samples are mainly composed of quartz, feldspar, kaolinite and/or illite. Sidi Aïch sandstones are mainly arkosic, potassic feldspar-rich and immature. Much of the feldspar was transformed to kaolinite. Concerning the relation between sandstone detrital composition and their depositional setting, the Sidi Aïch Formation sandstone in the major studied localities, probably accumulated in relatively proximal small basins within the continental interior. However, for the Khanguet El Ouara study site, sandstones may have been deposited in a foreland basin which received recycled sediments from an adjacent orogenic belt.The source area may have included quartzose sedimentary rocks. The dominance of quartz and enrichment in immobile elements suggest that the depositional basins were associated with a passive margin. The petrography and geochemistry reflect a stable continental margin and sediments were derived from granitic and pegmatitic sources located in the southern parts of the Gafsa basin. High values for the chemical index of alteration (CIA) indicate that recycling processes might have been important. Particularly high CIA values in the Garet Hadid locality indicate more intense chemical alteration, either due to weathering processes or tectonic control. 相似文献
978.
Fatma Aloulou Monem Kallel Mohamed Dammak Boubaker Elleuch Alain Saliot 《Environmental Earth Sciences》2010,61(1):1-10
We investigated the mineral composition and trace element contents of dustfall with different deposited phases at the Minqin
Oasis, in China’s Hexi Corridor. The contents of elements such as Pb, Cd, Cu, As, Sb, Se, Rb, Cs, and Ba (believed to be of
anthropogenic origin) indicated that the significance of human impacts on dust emissions increased from 1970s to the early
2000s. However, variations in the content of elements including Sc, Mn, Na, Cr, V, Eu, Ce, Nb, Zr, La, and Ce (believed to
originate from natural processes such as physical and chemical weathering) show no obvious changes in the major potential
dust source areas (PSAs) in this region. In addition, analysis of the assemblage of trace elements in dustfall indicated that
the elements assumed to originate from anthropogenic and natural processes appear to be intertwined, leading to difficulties
and uncertainties in identifying the PSAs and evaluating the significance of human impacts on dust emissions. Comparisons
of the element contents of dustfall collected in Beijing and Minqin with similar data for Beijing show that under different
conditions some differences in ranges are permitted; the contents of most trace elements were similar even though the dustfall
in the two areas came from different sources. These results suggest that before attempting to identify major PSAs, researchers
should perform an integrated analysis that includes the mineral, elemental, geomorphological, vegetation cover, climatic conditions,
and other characteristics of the regions being studied. 相似文献
979.
Mohamed ABD EL MONSEF 《《地质学报》英文版》2020,94(5):1451-1470
Gold-silver deposits in the Atalla area occur as hydrothermal quartz veins in NE–SW pre-existing fractures within the Atalla granitic pluton. The orientation of such quartz veins has been attributed to extensional behavior related to the Atalla Shear Zone (ASZ). The Atalla area is covered by a variety of lithologies that are (from oldest to youngest): metasedimentary rocks, metavolcanic rocks, ophiolite assemblage (serpentinites/talc-carbonates), Atalla granite and Dokhan volcanic rocks. Microscopically, Atalla granite ranges in composition from granodiorite to monzogranite. Whole-rock geochemistry constrains the calc-alkaine affinity of the Atalla granite that was intruded within an orogenic (syn-collision) tectonic regime. The ore minerals are represented by gold/silver (electrum), pyrite (Py1 & Py2), arsenopyrite, pyrrhotite, sphalerite, chalcopyrite, galena, covellite and goethite. The temperature of ore formation ranges from 240 to 285°C and the estimated fluid pressure is in the range of 20–100 MPa. Based on the geological setting, ore textures and fluid characteristics; the Atalla Au-Ag deposits are considered to be orogenic in nature, formed from a continental collision (~653-590 Ma), synchronous with the emplacement of calc-alkaline magmatism during the evolutionary history of the Arabian Nubian Shield (ANS). The initial ore-forming fluid was primarily derived from a metamorphic source related to ophiolitic-serpentinite rocks under deep regional conditions of greenschist-amphibolite facies, where the Atalla granitic eruption provided the required temperature conditions for the metamorphic process to take place. Under such conditions, the transportation of ore metals as bisulfide complexes is favoured. The deposition of ore minerals was triggered by fluid-wallrock interaction through fracture pathways in conjunction with a temperature-pressure drop that is likely to have been related to uplift into the crustal levels. 相似文献
980.
华南花岗岩残积土红土化程度的地带性与香港该类土不发育的原因 总被引:8,自引:0,他引:8
从华南花岗岩残积土红土化程度的地域变化规律出发 ,通过香港与华南沿海各地花岗岩残积土的化学成分及红土化指标对比 ,作者发现香港花岗岩残积土无论厚度、类型 ,还是红土化程度都具有与所处气候环境不协调的特点。文章进一步讨论了香港花岗岩残积土不发育的原因和对工程性质的影响. 相似文献