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   4309篇
  免费   146篇
  国内免费   92篇
测绘学   158篇
大气科学   296篇
地球物理   1117篇
地质学   1518篇
海洋学   377篇
天文学   695篇
综合类   21篇
自然地理   365篇
  2022年   30篇
  2021年   48篇
  2020年   47篇
  2019年   64篇
  2018年   85篇
  2017年   105篇
  2016年   119篇
  2015年   110篇
  2014年   137篇
  2013年   258篇
  2012年   130篇
  2011年   231篇
  2010年   164篇
  2009年   226篇
  2008年   238篇
  2007年   208篇
  2006年   191篇
  2005年   171篇
  2004年   137篇
  2003年   162篇
  2002年   136篇
  2001年   84篇
  2000年   88篇
  1999年   61篇
  1998年   70篇
  1997年   57篇
  1996年   67篇
  1995年   52篇
  1994年   57篇
  1993年   42篇
  1992年   48篇
  1991年   42篇
  1990年   35篇
  1989年   37篇
  1988年   37篇
  1987年   40篇
  1986年   44篇
  1985年   57篇
  1984年   47篇
  1983年   50篇
  1982年   52篇
  1981年   57篇
  1980年   48篇
  1979年   39篇
  1978年   47篇
  1977年   28篇
  1976年   29篇
  1975年   24篇
  1974年   31篇
  1973年   29篇
排序方式: 共有4547条查询结果,搜索用时 15 毫秒
1.
This paper explores links between transport and housing security issues for the urban poor using the example of the Klang Valley in Malaysia. The interface between these issues is identified as a gap in the literature, including policy debates, on both housing and transport. A number of linkages are shown to be important and likely to be relevant in many cities of the South, especially those with rapid motorisation and large numbers of "squatters". A simple framework for understanding these linkages is presented. Key examples include displacement to make way for transport infrastructure and the impact on transport problems for the poor of policies affecting the location of urban poor housing, including relocation sites and transit accommodation. The case study of the Klang Valley is used to illustrate and test the relevance of a focus on this issue and the utility of the conceptual framework. Some policy implications of the investigation and case study are suggested.  相似文献   
2.
11 million years of Oligocene geomagnetic field behaviour   总被引:2,自引:0,他引:2  
An 11 million year long record of the Oligocene geomagnetic field has been obtained from pelagic sediments of DSDP Hole 522 An average sample spacing of 4 cm yielded approximately one specimen per 4 to 8 kyr. The rock magnetics are remarkabh consistent across the entire interval. Previous work demonstrated a magnetic mineralogy dominated by magnetically stable magnetite. The natural remanent magnetism (NRM) carries an Oligocene polarity timescale that is in excellent agreement with the Oligocene reversal record as determined from marine magnetic anomalies (MMAs), including many of the so-called 'crypto-chrons'. Normalized NRM intensities from the undisturbed portions of the record yield a time series of variations with features consistent with a number of other palaeointensity time series derived from both sedimentary and lava sequences. These features include consistent, major decreases in palaeointensity (DIPs) at reversal boundaries, and occasional DIPs between reversal boundaries that could correspond to lineated 'tiny wiggles' in the MMA records. The data set suggests that the overall field strength was 40 per cent higher in the first half of the Oligocene when the average reversal frequency was 1.6 Myr-1 than in the second half when the reversal frequency was 4 Myr-1. There is also a weak dependence of average field strength on length of polarity interval. Finally, in the three cores suited to spectral analysis (of coherent polarity and relative intensity independent of lithological contamination), there is a persistent ca. 30–50ka periodicity in the variations of the relative intensity, suggesting that the geomagnetic field 'pulses' at about this frequency, not only during the Brunhes (as demonstrated by Tauxe & Shackleton 1994), but in the Oligocene as well.  相似文献   
3.
4.
5.
News     
  相似文献   
6.
We present the first continuous paleolimnological reconstruction from the North Island of New Zealand (37°S) that spans the last 48.2?cal kyr. A tephra- and radiocarbon-based chronology was developed to infer the timing of marked paleolimnological changes in Lake Pupuke, Auckland, New Zealand, identified using sedimentology, magnetic susceptibility, grain size and geochemistry (carbon, nitrogen and sulphur concentrations and fluxes, carbon and nitrogen stable isotopes). Variable erosional influx, biomass and benthic REDOX conditions are linked to changing effective precipitation and seasonality within three inferred broad intervals of climatic change: (1) the Last Glacial Coldest Phase (LGCP) of reduced effective precipitation and cooler temperatures, from 28.8 to 18.0?cal kyr BP, (2) the Last Glacial Interglacial Transition (LGIT) of increasing effective precipitation and warmer conditions, from 18.0 to 10.2?cal?kyr?BP, and (3) a Holocene interval of high effective precipitation, beginning with a warm period of limited seasonality from 10.2?cal?kyr?BP and followed by increasing seasonality from 7.6?cal?kyr?BP. The LGCP and LGIT also contain millennial-scale climate events, including the coldest inferred glacial conditions during the LGCP from 27.8 to 26.0 and 22.0?C19.0?cal?kyr?BP, and a climate reversal in the LGIT associated with lower lake level, from 14.5 to 13.8?cal?kyr?BP, coeval with the Antarctic Cold Reversal. The onset of seasonal thermal stratification occurred at 5.7?cal?kyr?BP and was linked to natural eutrophication of Lake Pupuke, which produced enhanced organic sedimentation.  相似文献   
7.
In this paper, the application of 2D and 3D electrical resistivity methods in geotechnical investigations is explored through a case study in Northern Greece. These two methods were employed at a lignite surface mining operation where fracture zones and discontinuities have been recently observed close to the pit boundaries. The main aim of the geophysical survey was to estimate the inclination of the contact between the Neogene and Schist/Carbonate formations near the southern limits of the pit, as well as to estimate the thickness of the carbonate rocks on top of the Schist formations to evaluate the stability of the southern slopes. Synthetic data were initially generated to help plan an efficient electrical tomography survey, in a region with complex geology and irregular terrain. Three configurations (Wenner–Schlumberger and dipole–dipole or pole–dipole) proved essential in such conditions and helped improving the resolution of the resistivity section. The sections were then calibrated by boreholes. Finally, the geophysical survey provided invaluable data regarding the geometry of the bedrock and possible faults, which was essential for the slope stability calculations.  相似文献   
8.
Estimates of the number of undiscovered deposits on a statewide basis offer a different perspective on the nation's undiscovered resources of gold, silver, copper, lead, and zinc. Mean estimates of the number of undiscovered deposits statewide were extracted from the estimates of undiscovered deposits nationwide. More than 50 undiscovered deposits are estimated to occur in Alaska, Arizona, Nevada, and Wisconsin. Estimating the number of undiscovered deposits statewide serves as a measure of a state's total remaining mineral resources in known conventional deposit types.  相似文献   
9.
This article explores the use of nighttime satellite imagery for mapping urban and peri-urban areas of Australia. A population-weighted measure of urban sprawl is used to characterize relative levels of sprawl for Australia's urban areas. In addition, the expansive areas of low light surrounding most major metropolitan areas are used to map the urban–bush interface of exurban land use. Our findings suggest that 82 percent of the Australian population lives in urban areas, 15 percent live in peri-urban or exurban areas, and 3 percent live in rural areas. This represents a significantly more concentrated human settlement pattern than presently exists in the United States.  相似文献   
10.
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