全文获取类型
收费全文 | 1914篇 |
免费 | 334篇 |
国内免费 | 944篇 |
专业分类
测绘学 | 33篇 |
大气科学 | 9篇 |
地球物理 | 506篇 |
地质学 | 2032篇 |
海洋学 | 61篇 |
天文学 | 8篇 |
综合类 | 116篇 |
自然地理 | 427篇 |
出版年
2024年 | 8篇 |
2023年 | 15篇 |
2022年 | 59篇 |
2021年 | 96篇 |
2020年 | 103篇 |
2019年 | 140篇 |
2018年 | 136篇 |
2017年 | 116篇 |
2016年 | 124篇 |
2015年 | 127篇 |
2014年 | 155篇 |
2013年 | 183篇 |
2012年 | 132篇 |
2011年 | 159篇 |
2010年 | 159篇 |
2009年 | 148篇 |
2008年 | 126篇 |
2007年 | 149篇 |
2006年 | 131篇 |
2005年 | 105篇 |
2004年 | 105篇 |
2003年 | 100篇 |
2002年 | 79篇 |
2001年 | 89篇 |
2000年 | 77篇 |
1999年 | 57篇 |
1998年 | 56篇 |
1997年 | 68篇 |
1996年 | 32篇 |
1995年 | 32篇 |
1994年 | 36篇 |
1993年 | 24篇 |
1992年 | 22篇 |
1991年 | 11篇 |
1990年 | 9篇 |
1989年 | 7篇 |
1988年 | 4篇 |
1987年 | 2篇 |
1986年 | 6篇 |
1985年 | 4篇 |
1982年 | 1篇 |
排序方式: 共有3192条查询结果,搜索用时 609 毫秒
1.
The FAST/SKA site selection in Guizhou province 总被引:1,自引:0,他引:1
Many karst depressions with diameters of 300 m to 500 m, suitable for constructing Arecibo-style radio telescopes, were identified
in the south of Guizhou Province by Remote Sensing (RS) and Geographic Information System (GIS) technologies together with
field investigations. Fundamental topography and landform databases were established for 391candidate depressions, and using
GIS the3-dimensional images of depressions, at a scale of 1:10000, were then simulated to fit a spherical antenna.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
2.
对广西岩溶区三个铅锌硫化物矿床风化带观测结果表明,中低山岩溶区硫化物矿床具有更强烈的风化淋滤作用,但次生晕面积较小;岩溶平原区硫化物矿床风化与元素表生迁移特征较相似于非岩溶区的情况,表现在可产生较宽的pH偏低带和较大面积的次生晕 相似文献
3.
A. P. M. Vaughan L. A. Dowling F. J. G. Mitchell S.‐E. Lauritzen A. M. McCabe P. Coxon 《第四纪科学杂志》2004,19(6):577-590
Organic‐rich deposits, uncovered during overburden removal from mantled gypsum karst at Knocknacran opencast gypsum mine, Co. Monaghan, are the best candidate to date for a last interglacial record in Ireland. The two till and organic‐rich deposits (preserved at different quarry elevations) were emplaced on to a Tertiary dolerite surface during high‐energy flood events and subsequently folded and faulted by movement towards sinkholes in underlying gypsum. Uranium–thorium disequilibrium dating suggests that the organic‐rich deposits in the upper section were hydrologically isolated at ca. 41 ka and those in the lower section at ca. 86 ka. Interpretation of the pollen content, although tentative because of the depositional and post‐depositional history of the material, suggests that the organic material originated in a warm stage possibly warmer than the post‐Eemian interstadials. The unusual setting of preservation may indicate that in situ, last interglacial deposits have generally been removed by erosion in Ireland. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
4.
Claude R. Duguay Greg M. Flato Martin O. Jeffries Patrick Mnard Kim Morris Wayne R. Rouse 《水文研究》2003,17(17):3465-3483
A one‐dimensional thermodynamic model for simulating lake‐ice phenology is presented and evaluated. The model can be driven with observed daily or hourly atmospheric forcing of air temperature, relative humidity, wind speed, cloud amount and snowfall. In addition to computing the energy balance components, key model output includes the temperature profile at an arbitrary number of levels within the ice/snow (or the water temperature if there is no ice) and ice thickness (clear ice and snow‐ice) on a daily basis, as well as freeze‐up and break‐up dates. The lake‐ice model is used to simulate ice‐growth processes on shallow lakes in arctic, sub‐arctic, and high‐boreal forest environments. Model output is compared with field and remote sensing observations gathered over several ice seasons. Simulated ice thickness, including snow‐ice formation, compares favourably with field measurements. Ice‐on and ice‐off dates are also well simulated when compared with field and satellite observations, with a mean absolute difference of 2 days. Model simulations and observations illustrate the key role that snow cover plays on the seasonal evolution of ice thickness and the timing of spring break‐up. It is also shown that lake morphometry, depth in particular, is a determinant of ice‐off dates for shallow lakes at high latitudes. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
5.
在云南省西北部德钦县古水一带深切的澜沧江河谷中,发现了一套第四纪湖相沉积物,以纹层状粘土、粉砂质粘土和粉砂的互层组合为特征,构成了第三和第四级阶地的基座.笔者对该湖相剖面进行了U系、孢粉和若干地球化学指标的分析测试,并以这些结果讨论了古湖的形成时代和成因,以及古湖堆积时期的古植被和古气候.剖面顶部、中部和中下部粘土的U系法年龄测定结果分别为52.3±3.4 ka、64.2±5.6 ka和81.9±6.5 ka,说明古水古湖形成于晚更新世早中期的末次间冰期晚期和末次冰期的早冰阶与间冰阶早期,很可能是大型冰川(如下游的明永冰川等)或巨量的冰水沉积物堰塞了澜沧江河谷而造成的结果. 相似文献
6.
Using ALSM to map sinkholes in the urbanized covered karst of Pinellas County,Florida—1, methodological considerations 总被引:1,自引:0,他引:1
Larry D. Seale Lee J. Florea H. L. Vacher Robert Brinkmann 《Environmental Geology》2008,54(5):995-1005
Comparison of a database of interpreted sinkholes made using airborne laser swath mapping (ALSM) with databases of sinkholes
made from interpretation of aerial photographs reveals substantial disagreement. The discrepancies involve the location, number,
areas, and circularity of interpreted sinkholes. Methodological factors that contribute to the discrepancies include blockage
of ALSM laser returns in thick vegetation, multi-path reflections, and misrepresentation of the true extent of sinkholes.
Comparison of two ALSM-derived databases made (1) independently from versus (2) in combination with earlier air photo analysis
in undeveloped regions had the following result: one-fourth of the sinkholes interpreted by using the composite method were
missed by the independent analysis, and one-third of the sinkholes that were interpreted from the independent analysis were
not interpreted as sinkholes using the composite method. Subjective interpretation leads to a high level of uncertainty such
that the results of the remote sensing studies are suspect, if not invalid. 相似文献
7.
J. L. Simón M. A. Soriano L. E. Arlegui J. Gracia C. L. Liesa A. Pocoví 《Environmental Geology》2008,53(5):1057-1065
Space and time variations of alluvial karst subsidence in the central Ebro Basin are analysed in trenches (paleodolines),
aerial photographs (historical dolines) and field surveys (present-day active dolines). The measured subsidence rates, as
well as a comparison between densities of paleodolines computed in randomly selected scan-lines and density of active dolines
computed in maps, suggest that present-day subsidence in favourable areas is more intense than that of Pleistocene times.
According to diachronic maps drawn from (a) different aerial photographs taken after 1946 and (b) field surveys on deformations
in urbanized areas, subsidence behaved as a nearly steady process while the whole area was used for agriculture. In contrast,
rapid changes (through periods of several tens of years) have occurred in urbanized areas, in which dolines expand their boundaries
through small marginal collapses, shifting sharply their sinking centres to neighbouring sites. These rapid changes do not
have an equivalent in observed paleodolines. 相似文献
8.
Quantitative sinkhole hazard assessments in karst areas allow calculation of the potential sinkhole risk and the performance
of cost-benefit analyses. These estimations are of practical interest for planning, engineering, and insurance purposes. The
sinkhole hazard assessments should include two components: the probability of occurrence of sinkholes (sinkholes/km2 year) and the severity of the sinkholes, which mainly refers to the subsidence mechanisms (progressive passive bending or
catastrophic collapse) and the size of the sinkholes at the time of formation; a critical engineering design parameter. This
requires the compilation of an exhaustive database on recent sinkholes, including information on the: (1) location, (2) chronology
(precise date or age range), (3) size, and (4) subsidence mechanisms and rate. This work presents a hazard assessment from
an alluvial evaporite karst area (0.81 km2) located in the periphery of the city of Zaragoza (Ebro River valley, NE Spain). Five sinkholes and four locations with features
attributable to karstic subsidence where identified in an initial investigation phase providing a preliminary probability
of occurrence of 0.14 sinkholes/km2 year (11.34% in annual probability). A trenching program conducted in a subsequent investigation phase allowed us to rule
out the four probable sinkholes, reducing the probability of occurrence to 0.079 sinkholes/km2 year (6.4% in annual probability). The information on the severity indicates that collapse sinkholes 10–15 m in diameter
may occur in the area. A detailed study of the deposits and deformational structures exposed by trenching in one of the sinkholes
allowed us to infer a modern collapse sinkhole approximately 12 m in diameter and with a vertical throw of 8 m. This collapse
structure is superimposed on a subsidence sinkhole around 80 m across that records at least 1.7 m of synsedimentary subsidence.
Trenching, in combination with dating techniques, is proposed as a useful methodology to elucidate the origin of depressions
with uncertain diagnosis and to gather practical information with predictive utility about particular sinkholes in alluvial
karst settings: precise location, subsidence mechanisms and magnitude, and timing and rate of the subsidence episodes. 相似文献
9.
This article describes a unique flood hazard, produced by the dramatic expansion of wetlands in Nelson County, located within
the North American Prairie Pothole Region of North Dakota, USA. There has been an unprecedented increase in the number, average
size, and permanence of prairie wetlands, and a significant increase in the size of a closed lake (Stump Lake) due to a decade-long
wet spell that began in 1993 following a prolonged drying trend. Base-line land cover information from the 1992 USGS National
Land Cover Characterization dataset, and a Landsat TM scene acquired 9 July 2001 are used to assess the growth of the closed
lake and wetland pond surface areas, and to analyze the type and area of various land cover classes inundated between 1992
and 2001. The open water profile in Nelson County changed from one marked by relatively comparable coverage of closed lake
and wetland pond areas in 1992, to one in which wetland open water accounted for the vast majority of total open water in
2001. The bulk of the wetland pond area expansion occurred by displacing existing wetland vegetation and agricultural cropland.
Producers responded to the flood hazard by filing Federal Crop Insurance Corporation (FCIC) claims and enrolling cropland
in the Conservation Reserve Program (CRP), a federal land retirement program. Land taken out of agricultural production has
had an enormous impact upon the agricultural sector that forms the economic base of the rural economy. In 2001 the land taken
out of production due to CRP enrollment and preventive planting claims represented nearly 42% of Nelson County’s 205.2 K ha
base agricultural land. The patterns obtained from this detailed study of Nelson County are likely to be the representative
of the more publicized flood disaster occurring within the Devils Lake Basin of North Dakota. 相似文献
10.
喀斯特荒漠化是一种地质—生态灾难 总被引:23,自引:0,他引:23
杨汉奎 《海洋地质与第四纪地质》1995,15(3):137-147
KDN是一种GEOL-ECOL灾难,是发生潮湿的碳酸盐岩区的土壤侵蚀、土地退化与生态系统凋萎,致使人类很难生活在其中。KDN是我国四大GEOL-ECOL灾害中最难整治、最难摆脱贫困的地区。本文按国际公认的原则为准,认定“石山化”≠KDN,从地质结构性气候、物候和生态系统多因子厘定了KDN术语,认定人为扰动与人口超载是其产生的序参量。其过程最迟也是300多年前明清朝代屯垦开始,近半个世纪加速衰竭,森 相似文献