全文获取类型
收费全文 | 968篇 |
免费 | 24篇 |
国内免费 | 2篇 |
专业分类
测绘学 | 71篇 |
大气科学 | 66篇 |
地球物理 | 212篇 |
地质学 | 362篇 |
海洋学 | 52篇 |
天文学 | 152篇 |
综合类 | 2篇 |
自然地理 | 77篇 |
出版年
2022年 | 11篇 |
2021年 | 17篇 |
2020年 | 14篇 |
2019年 | 9篇 |
2018年 | 34篇 |
2017年 | 31篇 |
2016年 | 47篇 |
2015年 | 16篇 |
2014年 | 38篇 |
2013年 | 63篇 |
2012年 | 39篇 |
2011年 | 30篇 |
2010年 | 39篇 |
2009年 | 33篇 |
2008年 | 36篇 |
2007年 | 38篇 |
2006年 | 27篇 |
2005年 | 22篇 |
2004年 | 25篇 |
2003年 | 21篇 |
2002年 | 14篇 |
2001年 | 17篇 |
2000年 | 17篇 |
1999年 | 13篇 |
1998年 | 16篇 |
1997年 | 10篇 |
1996年 | 14篇 |
1995年 | 16篇 |
1994年 | 9篇 |
1993年 | 16篇 |
1992年 | 8篇 |
1991年 | 8篇 |
1990年 | 13篇 |
1989年 | 8篇 |
1988年 | 13篇 |
1987年 | 8篇 |
1986年 | 8篇 |
1985年 | 16篇 |
1984年 | 14篇 |
1983年 | 16篇 |
1982年 | 18篇 |
1981年 | 14篇 |
1980年 | 12篇 |
1979年 | 16篇 |
1978年 | 8篇 |
1977年 | 7篇 |
1975年 | 7篇 |
1973年 | 14篇 |
1972年 | 13篇 |
1968年 | 6篇 |
排序方式: 共有994条查询结果,搜索用时 15 毫秒
991.
Size‐selective harvesting can elicit a genetic response in target species through changes in population genetic subdivision, genetic diversity and selective regimes. While harvest‐induced genetic change has been documented in some commercially important species through the use of historic samples, many commonly harvested species, such as coastal molluscs, lack historic samples and information on potential harvest induced genetic change. In this study, we have genotyped six microsatellite markers from populations across much of the California mainland range of the size‐selectively harvested owl limpet (Lottia gigantea) to explore the genetic structure and diversity of this species. We found no significant genetic structure or differences in genetic diversity among populations of L. gigantea. Our results suggest high gene flow among populations and that differences in life history, demography, and body size previously observed between protected and exploited populations is largely due to phenotypic plasticity. From a conservation perspective, if proper actions are taken to curb harvesting, then exploited populations should be able to return to their pre‐impact state given sufficient time. 相似文献
992.
Byong-Jae Ryu Michael Riedel Ji-Hoon Kim Roy D. Hyndman Young-Joo Lee Bu-Heung Chung Il-Soo Kim 《Marine and Petroleum Geology》2009,26(8):1483-1498
Geophysical surveys and geological studies of gas hydrates in the western deep-water Ulleung Basin of the East Sea off the east coast of Korea have been carried out by the Korea Institute of Geoscience and Mineral Resources (KIGAM) since 2000. The work included a grid of 4782 km of 2D multi-channel seismic reflection lines and 11 piston cores 5–8 m long. In the piston cores, cracks generally parallel to bedding suggest significant in-situ gas. The cores showed high amounts of total organic carbon (TOC), and from the southern study area showed high residual hydrocarbon gas concentrations. The lack of higher hydrocarbons and the carbon isotope ratios indicate that the methane is primarily biogenic. The seismic data show areas of bottom-simulating reflectors (BSRs) that are associated with gas hydrates and underlying free gas. An important observation is the numerous seismic blanking zones up to 2 km across that probably reflect widespread fluid and gas venting and that are inferred to contain substantial gas hydrate. Some of the important results are: (1) BSRs are widespread, although most have low amplitudes; (2) increased P-wave velocities above some BSRs suggest distributed low to moderate concentration gas hydrate whereas a velocity decrease below the BSR suggests free gas; (3) the blanking zones are often associated with upbowing of sedimentary bedding reflectors in time sections that has been interpreted at least in part due to velocity pull-up produced by high-velocity gas hydrate. High gas hydrate concentrations are also inferred in several examples where high interval velocities are resolved within the blanking zones. Recently, gas hydrate recoveries by the piston coring and deep-drilling in 2007 support the interpretation of substantial gas hydrate in many of these structures. 相似文献
993.
K. H. V. Durga Rao Ambesh Kumar Singh P. S. Roy 《Journal of the Indian Society of Remote Sensing》2009,37(3):433-441
The scope of this research was to study lake morphology using spatial simulation technique, to develop revised elevation-capacity
curve, to develop elevation-water spread area curve, to study the relation between suspended sediment and remote sensing satellite
data, and to estimate suspended sediment load in the lake using a Geographic Information System coupled with ground truth.
The study area was the Bhopal Upper Lake, which has been classified as one of the major wetlands in India by the Ministry
of Water Resources, India. A precise digital elevation model was created using 0.5 meter interval contour information collected
from bathometric surveys. Water-spread areas at different water levels were simulated spatially in a Geographic Information
System (GIS) through the neighbourhood connectivity operator. Revised elevation-capacity curve and elevation-area curve of
the lake were prepared using the simulated results. Simulated water spread area at full tank level (FTL) was compared with
the actual water spread area delineated using remote sensing data. Water samples at different locations of the lake were collected
and located using the Global Positioning System (GPS) instrument. These samples were analysed in the laboratory for suspended
sediment concentration. Different image processing techniques were applied to LANDSAT 5 TM satellite digital data (except
thermal band). Correlation between radiance values of band 2 and suspended sediments was established and a positive linear
equation was found to fit the data best. Spatial distribution of suspended sediment load was estimated using the developed
regression equation and band 2 radiance image of the complete lake. Total suspended sediment load and loss of capacity at
full tank level were computed. 相似文献
994.
A range of mineral magnetic measurements have been carried out on archaeological sediments from Orkney and Cyprus. In a soil profile from Orkney, a magnetic enhancement factor of over 200 is observed in susceptibility data between the bedrock and a Norse sediment. The magnetic enhancement is associated with an increase in superparamagnetic grains probably caused by burning. Until now it has proved difficult to confirm the presence of superparamagnetic grains in natural samples using room temperature magnetic measurements. However, clear differences are to be found between the hysteresis loops of various magnetic domain states, including superparamagnetism. An algorithm has been developed to unmix hysteresis loops in terms of constituent domain states of ferrimagnetic iron oxides. Unmixing 128 hysteresis loops of archaeological sediments has shown that the dominant domain state in all sediments is superparamagnetic. Remarkably uniform superparamagnetic grain sizes of between 80 and 95 Å were found for all 128 sediments. © 1999 John Wiley & Sons, Inc. 相似文献