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   269篇
  免费   10篇
  国内免费   2篇
测绘学   5篇
大气科学   20篇
地球物理   77篇
地质学   122篇
海洋学   12篇
天文学   18篇
自然地理   27篇
  2020年   4篇
  2019年   8篇
  2018年   3篇
  2017年   6篇
  2016年   5篇
  2015年   7篇
  2014年   7篇
  2013年   13篇
  2012年   9篇
  2011年   6篇
  2010年   6篇
  2009年   11篇
  2008年   14篇
  2007年   8篇
  2006年   8篇
  2005年   7篇
  2004年   5篇
  2003年   10篇
  2002年   7篇
  2001年   2篇
  1999年   5篇
  1998年   4篇
  1997年   3篇
  1996年   6篇
  1995年   6篇
  1994年   3篇
  1993年   7篇
  1992年   1篇
  1991年   3篇
  1990年   3篇
  1989年   1篇
  1988年   1篇
  1987年   3篇
  1986年   5篇
  1985年   9篇
  1984年   5篇
  1983年   10篇
  1982年   2篇
  1981年   8篇
  1980年   4篇
  1979年   6篇
  1978年   13篇
  1977年   3篇
  1976年   2篇
  1975年   1篇
  1974年   6篇
  1973年   9篇
  1970年   1篇
  1966年   1篇
  1949年   1篇
排序方式: 共有281条查询结果,搜索用时 390 毫秒
91.
92.
Houses in inner-city areas, because of their age, suffer the depredations of functional obsolescence and decay. Some, however, have sound locations and are able to assemble economic forces that precipitate rehabilitation by both renters and owners. Accompanying restoration is a concomitant increase in market value and in population density, the corollary of which is a reduction in lot size. This process is modeled by graphically synthesizing elements of location theory, urban growth theory, and principles of economics.  相似文献   
93.
94.
This study demonstrates that the Bunkers Hill borehole, which is located within the Hampshire Basin, contains a carbon isotope excursion (CIE) in the Upnor Formation followed by a positive recovery from another CIE within the overlying Reading Formation. This study has also shown that the CIE in the Upnor Formation predates the Paleocene/Eocene Thermal Maximum (PETM), while the carbon isotope recovery at the base of the Reading Formation is most likely related to it. The positive carbon isotope trend, before a return to more negative values, seen throughout the majority of the Reading Formation can also be seen in coeval deposits in the Jubilee 404T borehole in the London Basin and in the Tienen Formation in the Doel and Kallo boreholes from the Belgium Basin. This indicates that this post-PETM carbon isotope signal may, therefore, be regional if not global in nature.  相似文献   
95.
Unequivocal planktonic foraminifera have been discovered in Oxfordian strata from Dorset and Scotland. These assemblages are, in part, coeval with previously reported occurrences of planktonic taxa in the Oxfordian of Normandy and Seine Maritime (France). Three species are now reported from the United Kingdom for the first time: Globuligerina oxfordiana ( Grigelis, 1958 ), Haeuslerina helvetojurassica ( Haeusler, 1881 ) and Compactogerina sp. cf. C. stellapolaris ( Grigelis, 1977 ). There appears to be a close relationship between the distribution of these planktonic taxa in the UK and a marked sea-level highstand.  相似文献   
96.
The status of dark matter searches with inorganic scintillator detectors at Boulby mine is reviewed and the results of tests with a CsI(Tl) crystal are presented. The objectives of the latter experiment were to study anomalous fast events previously observed and to identify ways to remove this background. Clear indications were found that these events were due to surface contamination of crystals by alphas, probably from radon decay. A new array of unencapsulated NaI(Tl) crystals immersed either in liquid paraffin or pure nitrogen atmosphere is under construction at Boulby. Such an approach allows complete control of the surface of the crystals and the ability to remove any surface contamination. First data from the unencapsulated NaI(Tl) do not show the presence of anomalous fast events.  相似文献   
97.
Marine sedimentary section across the Paleocene/Eocene (P/E) boundary interval is preserved in the Dungan Formation (Lower Indus Basin), Pakistan. Four dinoflagellate zones in the P/E interval of the Rakhi Nala section (Lower Indus Basin) are identified and correlated. The quantitative analysis of the dinoflagellate cyst assemblages together with geochemical data are used to reconstruct the palaeoenvironment across the P/E interval. The dinocyst assemblages allow the local correlation of the Dungan Formation (part) of the Sulaiman Range with the Patala Formation (part) of the Upper Indus Basin and global correlation of the Zone Pak-DV with the Apectodinium acme Zone of the Northern and Southern hemispheres. The onset of the carbon isotopic excursion (CIE) associated with Paleocene Eocene Thermal Maximum (PETM) is used globally to identify the P/E boundary. The CIE for the total organic carbon (fine fraction) δ13CFF is of a magnitude of ?1.7‰ is recorded for the first time in the Indus Basin. The Apectodinium acme precedes and straddles the onset of the CIE in the Indus Basin. This Apectodinium acme is also accompanied by a planktonic and benthonic foraminifera “barren zone.” The CIE in the Indus Basin, coupled with the changes in the dinocyst distribution and the benthonic and planktonic foraminifera assemblages, provides evidence of the changes associated with the PETM in this little-known part of the world. The benthonic foraminiferal assemblage indicates bathyal environment of deposition at the time of P/E boundary interval; the presence of dominantly open marine dinoflagellates and high planktonic foraminiferal ratio suggest that the water column at this site was well connected with the rest of the Tethys.  相似文献   
98.
Membrane behavior in naturally occurring and engineering systems refers to the restriction of solute migration through a membrane relative to the migration of the solvent. Hyperfiltration is the net flux that results when two solutions of different concentration are separated by a membrane and an external force is applied in excess of the osmotic pressure. Clay membranes containing layered fabric have higher efficiencies than membranes with random fabrics. Therefore, low-permeability rocks with a foliated fabric might exhibit membrane properties. Four hyperfiltration experiments were conducted on samples of Darrington Phyllite from the Easton Metamorphic Suite of the Northwest Cascades, Washington (USA). Chloride solutions were passed through thin, intact discs at relatively low heads. At the end of the experiments, dissolved chloride concentrations had increased 110–140 % and calculated reflection coefficients ranged from 0.87 to 0.88; this was attributed to partial solute rejection by the phyllite. Natural scenarios in which phyllite might exhibit membrane properties include (1) shallow perched aquifers bounded by phyllite, (2) overpressured aquifers bounded by phyllite, (3) phyllite-bounded aquifers with significant vertical groundwater flows, and (4) ultrafiltration during metamorphic devolatilization at depth. Membrane processes exhibited by phyllite may also contribute to the formation of some low-temperature ore bodies.  相似文献   
99.
Abstract

Erosion rates in residual limestone soils in a humid climate were measured for 10 years at one site, and for 4 years at another site, using erosion pins. Erosion pins were placed in gully floors and on convex divides between adjacent gullies, on abandoned land where vegetation had been removed. We measured an average erosion rate of 20 mm yr?1 over 10 years at one site and only 5 mm yr?1 over 4 years at another site where chert gravel was common on the surface. The 10-year average erosion rate of divides (26 mm yr?1) was significantly greater than the average erosion rate of gullies (14 mm yr?1), suggesting control by different processes, some of which may be seasonal. In winter, it was observed that frost action produced a thin layer of loose soil on the surface of divides. In summer, a hardpan developed on divides, as the soil loosened by winter frosts was transported to gullies, likely by rainsplash or dry ravel. The diffusive processes of frost action, rainsplash, and dry ravel appear to shape the convex divides in this study. Down-cutting of gullies requires channelized flow produced by intense rainfall, which is more common in summer for this location.  相似文献   
100.
This paper provides an explanation of an automated solution for correctly interpolating phase-lags across abrupt boundaries. Although an automated solution to this problem has existed for several years, this is not commonly known and so many researchers continue to perform corrections manually. Interpolation errors commonly occur when tidal propagation surfaces are generated for regimes with amphidromic points. General correction methods are manual, clunky and prone to operator error. The problem can be solved by applying a simple method to scalarize the phase-lag vectors pre-interpolation. This approach successfully and automatically generates correct tidal phase-lag interpolation values and may be applied to any surface mapping software used to interpolate phase-lags.  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