首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   294篇
  免费   6篇
  国内免费   5篇
测绘学   29篇
大气科学   28篇
地球物理   45篇
地质学   126篇
海洋学   5篇
天文学   63篇
综合类   1篇
自然地理   8篇
  2023年   1篇
  2022年   8篇
  2021年   11篇
  2020年   4篇
  2019年   6篇
  2018年   19篇
  2017年   21篇
  2016年   22篇
  2015年   9篇
  2014年   19篇
  2013年   23篇
  2012年   17篇
  2011年   23篇
  2010年   16篇
  2009年   17篇
  2008年   21篇
  2007年   5篇
  2006年   11篇
  2005年   6篇
  2004年   3篇
  2003年   1篇
  2002年   1篇
  2001年   1篇
  2000年   1篇
  1999年   6篇
  1998年   1篇
  1997年   3篇
  1995年   1篇
  1994年   1篇
  1993年   2篇
  1991年   5篇
  1990年   2篇
  1989年   2篇
  1987年   1篇
  1986年   2篇
  1985年   2篇
  1984年   2篇
  1983年   2篇
  1982年   1篇
  1980年   1篇
  1979年   1篇
  1974年   1篇
  1973年   1篇
  1971年   2篇
排序方式: 共有305条查询结果,搜索用时 15 毫秒
41.
Traditionally seismic design of structures supported on piled raft foundation is performed by considering fixed base conditions, while the pile head is also considered to be fixed for the design of the pile foundation. Major drawback of this assumption is that it cannot capture soil-foundation-structure interaction due to flexibility of soil or the inertial interaction involving heavy foundation masses. Previous studies on this subject addressed mainly the intricacy in modelling of dynamic soil structure interaction(DSSI) but not the implication of such interaction on the distribution of forces at various elements of the pile foundation and supported structure. A recent numerical study by the authors showed significant change in response at different elements of the piled raft supported structure when DSSI effects are considered. The present study is a limited attempt in this direction, and it examines such observations through shake table tests. The effect of DSSI is examined by comparing dynamic responses from fixed base scaled down model structures and the overall systems. This study indicates the possibility of significant underestimation in design forces for both the column and pile if designed under fixed base assumption. Such underestimation in the design forces may have serious implication in the design of a foundation or structural element.  相似文献   
42.
Composts with five different ratios of agricultural wastes, viz. rice straw (RS), wheat straw (WS), potato plant (PP), and mustard stover (MS) were prepared with or without fish pond bottom sediment to investigate the compost maturity and their suitability for field application. The composting process was monitored through the changes in physico‐chemical parameters and germination index (GI) at every 7 days interval of the composting process. All the composts were dark brown and smelled like forest soil within 56 days of composting, which reflected its matured status. On the basis of the physico‐chemical parameters (bulk density: 0.84 g/cm3; pH 7.05; electrical conductivity: 3.52 mS/cm; cation exchange capacity:82.4 cmol/kg; total carbon:321.4 g/kg; total nitrogen: 16.9 g/kg; As: 6.8 mg/kg; Cd: 2.96 mg/kg; Cr: 29.6 mg/kg, Cu: 243.6 mg/kg; Hg: 0.019 mg/kg; Ni: 24.3 mg/kg; Pb: 62.1 mg/kg and Zn: 812 mg/kg) and GI (89–96%), it could be concluded that RS/WS/PP/MS, 1:1:2:1 v/v/v/v with fish pond sediment produced better compost in accordance with the Indian compost standard. Application of a combined randomized block design analysis revealed that there is a significant difference in the responses of the five composts, in relation to the time of composting. Hierarchical clustering algorithm was applied with a view to form homogeneous groups of five different composts on the basis of different physico‐chemical parameters. Therefore, the ratio of waste incorporation is an important decision for composting and addition of pond sediment can improve the quality of compost.  相似文献   
43.
We investigate the effects of Hurricane Wilma??s storm surge (23?C24 October 2005) on the dominant tree Pinus elliottii var densa (South Florida slash pine) and rare plant species in subtropical pine rocklands of the Lower Florida Keys. We examine the role of elevation on species abundance in 1995 (Hurricane Betsy in 1965), 2005 (Hurricane Georges in 1998), and 2008 (Hurricane Wilma in 2005) to investigate if hurricanes influence abundance by eliminating plants at lower elevation on Big Pine Key, the largest island in the Lower Florida Keys. We compare densities before and after Hurricane Wilma over the 2005?C2008 sampling period and examine the role of elevation on changes in pine and rare species densities three years after Hurricane Wilma. We use elevation to assess the impact of hurricanes because elevation determined whether a location was influenced by storm surge (maximum surge of 2 m) in the Lower Florida Keys, where pine rocklands occur at a maximum elevation of 3 m. In 1995 (30 years after a major storm), elevation did not explain the abundance of South Florida slash pine or Chamaecrista lineata, but explained significant variation in abundance of Chamaesyce deltoidea. The latter two species are rare herbaceous plants restricted to pine rocklands. In 2008, 3 years after Hurricane Wilma, the positive relationship between elevation and abundance was strongest for South Florida slash pine, C. deltoidea, and C. lineata. Effects of Hurricane Wilma were not significant for rare species with wider distribution, occurring in plant communities adjacent to pine rocklands and in disturbed rocklands. Our results suggest that hurricanes drive population dynamics of South Florida slash pine and rare species that occur exclusively in pine rocklands at higher elevations. Rare species restricted to pine rocklands showed dramatic declines after Hurricane Wilma and were eliminated at elevations <0.5 m. Widely distributed rare species did not show significant changes in density after Hurricane Wilma. Abundance increased with elevation for South Florida slash pine and C. lineata after the hurricane. In an environment influenced by sea level rise, concrete plans to conserve pine ecosystems are warranted. Results from this study will help define conservation strategies by strengthening predictive understanding of plant responses to disturbance in the backdrop of sea level rise.  相似文献   
44.
The paper presents the nature of the variation of refractive index of atmospheric medium with time and altitude before, during and after the onset of thunderstorms over Gangetic West Bengal during the pre-monsoon period. A critical analysis shows that sharp depletion of the refractive index takes place before the onset of Nor??westers and possible explanations are also offered for the said occurrence.  相似文献   
45.
To understand the spatial variation in concentrations and compositions of organic micropollutants in marine plastic debris and their sources, we analyzed plastic fragments (∼10 mm) from the open ocean and from remote and urban beaches. Polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), dichloro-diphenyl-trichloroethane and its metabolites (DDTs), polybrominated diphenyl ethers (PBDEs), alkylphenols and bisphenol A were detected in the fragments at concentrations from 1 to 10,000 ng/g. Concentrations showed large piece-to-piece variability. Hydrophobic organic compounds such as PCBs and PAHs were sorbed from seawater to the plastic fragments. PCBs are most probably derived from legacy pollution. PAHs showed a petrogenic signature, suggesting the sorption of PAHs from oil slicks. Nonylphenol, bisphenol A, and PBDEs came mainly from additives and were detected at high concentrations in some fragments both from remote and urban beaches and the open ocean.  相似文献   
46.
In quartz-free Fe, Na-poor and high-Mg, Al schists from the Bundelkhand craton in north central India, corundum porphyroblasts in finely interleaved phlogopite–chlorite aggregates with rare clinozoisite are mantled by fine-grained (<3 μm) intergrowths of outer collars (>200 μm wide) of phengitic-muscovite and chlorite (phlogopite + corundum + H2O → phengitic-muscovite + chlorite), and <100 μm wide inner collars of margarite–muscovite (corundum + clinozoisite + phengitic muscovite → margarite + muscovite + H2O). Wide-beam electron probe microanalyses indicate Mg in the bi-layered corona increases from corundum outwards, with a complementary decrease in Al and K. Si and Ca increase and then decrease to matrix values. The sharp chemical gradients across the highly structured bi-layered corona are inferred to suggest that the corona-forming reactions were promoted by local grain-boundary-controlled thermodynamic instability as opposed to element transport by advective diffusion. The P–T convergence of KMASH reactions and NCKMASH pseudosection phase relations computed using micro-domain compositions indicate the chlorite–phengitic muscovite outer collar formed at 18–20 kbar and ca. 630°C. The NCKMASH margarite–muscovite inner collar yielded lower metamorphic P–T conditions of 11 ± 3 kbar, ca. 630°C. U–Th–Pb chemical dating of metamorphic monazite and LA-ICPMS U–Pb isotope dating of re-equilibrated zircon yield ca. 2.78 Ga ages, which are interpreted to date corona formation and Neoarchean high-P metamorphism in the Bundelkhand craton, hitherto unknown in the Indian Precambrian. (220)  相似文献   
47.
A total number of 328 groundwater samples are analysed to evaluate the groundwater flow systems in Bengal Delta aquifers, Bangladesh using environmental isotope (2H, 18O, 13C, 3H, and 14C) techniques. A well-defined Local Meteoric Water Line (LMWL) δ2H = 7.7 δ18O + 10.7 ‰ is constructed applying linear correlation analyses to the monthly weighted rainfall isotopic compositions (δ18O and δ2H). The δ18O and δ2H concentrations of all groundwater samples in the study area are plotted more or less over the LMWL, which provides compelling evidence that all groundwaters are derived from rainfall and floodwater with a minor localized evaporation effects for the shallow groundwaters. Tritium concentration is observed in 40 samples out of 41 with values varying between 0.3 and 5.0 TU, which represents an evidence of young water recharge to the shallow and intermediate aquifers. A decreasing trend of 14C activity is associated with the heavier δ13C values, which indicates the presence of geochemical reactions affecting the 14C concentration along the groundwater flow system. Both vertical and lateral decrease of 14C activity toward down gradient show the presence of regional groundwater flow commencing from the unconfined aquifers, which discharges along the coastal regions. Finally, shallow, intermediate, and deep groundwater flow dynamics has revealed in the Bengal Delta aquifers, Bangladesh.  相似文献   
48.
Felsic tuff beds with some presumed sedimentary components were reported from the Owk Shale (Kurnool Group; bearing Neoproterozoic fossils) in the upper part of the sedimentary succession in the Cuddapah basin in India by Saha and Tripathy (2012a). Our optical and SEM petrographic study of three thin sections, however, indicates that the parent samples are sandy mudstones with variable amounts of a felsic volcaniclastic component. New highquality U-Pb (SHRIMP and LA-MC-ICPMS) ages of 133 detrital zircon grains from a sample show that one grain is ca. 1880 Ma, one grain is ca. 3300 Ma, and the ages of the remaining 131 grains fall between 2690 Ma and 2429 Ma, the population averaging 2522 ± 36 Ma. The data indicate that the zircons are detrital grains derived from the ca. 2.5 Ga granitic/gneissic/greenstone basement of the Dharwar cratons that also host minor older Archean enclaves. The single 1880 Ma grain could have come from a ca. 1.9 Ga LIP. In the absence of any younger magmatic zircon, the absolute age of the Owk Shale remains elusive.  相似文献   
49.
The Ganges River water and riverbank shallow groundwater were studied during a single wet season using the hydrochemical and isotopic composition of its dissolved load. The dissolved concentrations of major ions (Cl?, SO4 2?, NO3 ?, HCO3 ?, Ca2+, Na+, Mg2+, and K+), trace elements (barium (Ba) and strontium (Sr)) and stable isotopes (O and D) were determined on samples collected from the Ganges River and its riverbank shallow aquifers. In the present study, the shallow groundwater differs significantly from the Ganges River water; it shows distinct high concentrations of Ca2+, Mg2+, HCO3 ?, Ba, and Sr due to water–rock interaction and this in particular suggests that the Ganges River may not contribute significantly to the riverbank shallow aquifers during wet season. Besides, the sum of the total cationic charge (∑+, in milliequivalents per liter) in the groundwater shows high values (2.48 to 13.91 meq/L, average 9.12 meq/L), which is much higher than the sum of the cations observed in the Ganges water (1.36 to 3.10 meq/L, average 1.94 meq/L). Finally, the more depleted stable isotopic (δ 18O and δ 2H) compositions of the Ganges River water are in contrast to those of the riverbank aquifer having enriched stable isotopic values during the wet season and the riverbank groundwater thus has a purely local origin from precipitation.  相似文献   
50.
Numerical simulation of fault reactivation phenomenon   总被引:1,自引:0,他引:1  
Two-dimensional finite element method was used for evaluating the effect of orthogonal compression on precursor faults. The tendency of reactivation of precursor faults as thrust or normal was analyzed involving the positions and angles of precursor faults with the stresses, strains and displacements. Twelve cases were taken up with different combinations of precursor fault angles (high, >45° and low, <45°) and fault positions for analysis. Different positions and angles of precursor faults are correlated with stresses, strains, and displacements and are discussed in detail. It is hoped that this would help in understanding the past and the present geodynamics of the earth’s crust.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号