全文获取类型
收费全文 | 171篇 |
免费 | 43篇 |
国内免费 | 26篇 |
专业分类
测绘学 | 5篇 |
大气科学 | 92篇 |
地球物理 | 39篇 |
地质学 | 59篇 |
海洋学 | 3篇 |
天文学 | 2篇 |
综合类 | 9篇 |
自然地理 | 31篇 |
出版年
2023年 | 5篇 |
2022年 | 3篇 |
2021年 | 7篇 |
2020年 | 6篇 |
2019年 | 11篇 |
2018年 | 4篇 |
2017年 | 6篇 |
2016年 | 4篇 |
2015年 | 16篇 |
2014年 | 12篇 |
2013年 | 12篇 |
2012年 | 7篇 |
2011年 | 16篇 |
2010年 | 4篇 |
2009年 | 15篇 |
2008年 | 15篇 |
2007年 | 10篇 |
2006年 | 12篇 |
2005年 | 9篇 |
2004年 | 8篇 |
2003年 | 10篇 |
2002年 | 4篇 |
2001年 | 3篇 |
2000年 | 4篇 |
1999年 | 3篇 |
1998年 | 1篇 |
1997年 | 5篇 |
1996年 | 7篇 |
1995年 | 2篇 |
1994年 | 3篇 |
1993年 | 6篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1978年 | 2篇 |
排序方式: 共有240条查询结果,搜索用时 443 毫秒
211.
B. Broder 《Pure and Applied Geophysics》1981,119(5):978-989
The diurnal cycle of ozone at the surface and within the PBL is examined. It is concluded that the intrusion of air masses from ozone-rich layers in the lower troposphere results in late evening ozone maxima observed at ground stations on mountain slopes as well as within distinct layers of the PBL above the valley bottom. The ozone-rich layers are assumed to exist due to meso- or large-scale horizontal transport of smog air masses and a local cross-valley wind system is shown to be responsible for the occurrence of the intrusion processes. 相似文献
212.
The physical and chemical characteristics of Lake Bonney, a permanently ice-covered closed basin lake in Taylor Valley, Antarctica are influenced significantly by local climate. The rising lake-levels of the past thirty years indicate a recent change in the local climate. We explored the significance of twentieth century changes in lake-level as a climate-change indicator by using a hydrologic model for the basin and a Monte Carlo simulation based on the variability in the available 30 yrs of hydrologic record. We compared the lake-level in the retrospective simulations with a measurement surveyed by Robert Scott's party in 1903. All the retrospective simulations based on the observed inflows yielded estimates that the lake was dry in 1903. It was necessary to remove 6 yrs from the observed 21-yr record for the retrospective simulation to match the measured 1903 lake-level for 50% of the simulations. From these analyses, we conclude that the period from 1969 to the present has had greater availability of solar radiation for meltwater generation, possibly brought about by changing cloud-cover patterns and coupled with a gradual warming trend. A third simulation indicated that an annual increase in inflow of about 3% between 1903 and 1973 would be required to match the 1903 measurement. 相似文献
213.
JAMES BOCKHEIM 《Geografiska Annaler: Series A, Physical Geography》2007,89(3):153-165
Soil‐forming processes and soil development rates are compared and contrasted on glacial deposits in two adjacent and coeval valleys of the Quartermain Mountains, which are important because they display Miocene glacial Stratigraphy and some of the oldest landforms in the McMurdo Dry Valleys. More than 100 soil profiles were examined on seven drift sheets ranging from 115 000 to greater than 11.3 million years in age in Beacon Valley and Arena Valley. Although the two valleys contain drifts of similar age, they differ markedly in ice content of the substrate. Whereas Arena Valley generally has ‘dry‐frozen’ permafrost in the upper 1 m and minimal patterned ground, Beacon Valley contains massive ice buried by glacial drift and ice cored rock glaciers and has ice‐cemented perma‐frost in the upper 1 m and considerable associated patterned ground. Arena Valley soils have twice the rate of profile salt accumulation than Beacon Valley soils, because of lower available soil water and minimal cryoturbation. The following soil properties increase with age in both valleys: weathering stage, morphogenetic salt stage, thickness of the salt pan, the quantity of profile salts, electrical conductivity of the horizon of maximum salt enrichment, and depth of staining. Whereas soils less than 200 000 years and older soils derived from sandstone‐rich ground moraine are Typic Anhyorthels and Anhyturbels, soils of early Quaternary and older age, particularly on dolerite‐rich drifts, are Petronitric Anhyorthels. Arena Valley has the highest pedodiversity recorded in the McMurdo Dry Valleys. The soils of the Quartermain Mountains are the only soils in the McMurdo Dry Valleys known to contain abundant nitrates. 相似文献
214.
The size ratio of adults to neonates varies greatly in cladocerans across taxa. In this study we tested the impact of this ratio (using length basis) on the selected life history parameters of nine species of freshwater cladocerans (Alona rectangula, Pleuroxus aduncus, Scapholeberis mucronata, Moina macrocopa, Moina wierzejskii, Ceriodaphnia dubia, Daphnia laevis, Diaphanosoma birgei and Daphnia pulex). Experiments were conducted on a mixed diet of Chlorella vulgaris (1.0·106 cells mL–1) and Scenedesmus acutus (0.5·106 cells mL–1) at (22 ± 1)°C. Using a cohort life table experimental design, we quantified age‐specific reproductive output. Body sizes of neonates and adults as well as dry weight were determined using standard procedures. We found that the ratio of neonate to adult body length decreased with increasing body size of the adult. In terms of clutch size, however, smaller taxa produced lower numbers of offspring than larger ones. The influence of ratio neonate/adult size on average lifespan, age at first reproduction and at the maximum clutch showed U‐shaped curves. In smaller taxa (< 700 μm) and larger taxa (> 1500 μm) the ratio between the neonate and adult body size was less than 0.4 or more than 0.6. In medium sized taxa (1000?1500 μm), the ratio of neonate to adult size was between 0.4 and 0.6. The results have been discussed in relation to the life history strategies of cladocerans. 相似文献
215.
J. Moeyersons 《地球表面变化过程与地形》1978,3(2):115-128
This study reveals that the Quaternary mantle of reworked Kalahari Sands undergoes a post-sedimentary composite process at the archaeological site of Gombe (Kinshasa, Zaïre). There is a biological activity which consists of an upworking of soil particles, mainly by termites and worms. This causes a diminishing dry volume weight of the sediment. In response, the mantle reconsolidates. When the soil surface is horizontal, the particles which are not brought up will sink down in the soil profile as a result of the reconsolidation. When the soil surface is inclined, it is only the resettlement of the sediment which can give rise to creep, caused by wetting and drying. Creep of the dead mineralogical material alone seems to be very improbable once the structure and settlement of the sediment are in equilibrium with a given slope. Only a new phase of bioturbation can induce further creep movements. The experiments have shown that stone implements in reconsolidating Kalahari Sands do not exactly accompany the compaction movement of the sediment. Depending on their form, their size, their orientation and the water content of the sediment around them, stone implements can undergo differential movements during the reconsolidation process. This can result in a dispersion of the implements over levels of different ages. As creep is a phenomenon, going hand in hand with the resettlement of the sediment after biogenic upworking, partially analogous movements of stone implements can be expected in a creeping Kalahari Sands mantle. Partial or total reconcentration of the stone implements at the base of affective biogenic activity can be expected. This can lead to typical stone-line profiles which must be interpreted very carefully. 相似文献
216.
P. Tommasi P. Campedel C. Consorti R. Ribacchi 《Rock Mechanics and Rock Engineering》2008,41(1):37-58
Summary. The runout of dry rock avalanches produced by planar rockslides affecting a limestone formation with clayey interbeds is analysed
by means of distinct element modelling. Potential and past rock avalanche events are described with reference to the geotechnical
and structural conditions of the slope, typical of several Alpine valleys. Runout prediction analyses of potential rock avalanches
performed with the PFC2D code are based on independent measurement of strength, energy dissipation and stiffness parameters of the rock mass and are
validated by means of the back analysis of a historical rockslide occurred in the investigated area. Physical aspects of the
avalanching process evidenced by modelling are also discussed.
Author’s address: Paolo Tommasi, CNR – Institute for Geo-Engineering and Environmental Geology, c/o Facoltá di Ingegneria,
Via Eudossiana 18, 00184 Rome, Italy 相似文献
217.
Tayel El-Hasan Kamal Momani Jafar Al-Nawayseh Khaled Al-Nawayseh 《Environmental Geology》2008,54(1):103-110
The Dead Sea as a unique geological and geographical phenomenon has an effect on its adjacent areas. Therefore, 17 sampling
sites at the eastern highlands facing the Dead Sea; beside three blank sites were collectedlocated during summer (2005). The
aim was to investigate such influence on the chemical and mineralogical composition of dry deposition, and to measure the
settling rate. The investigations showed that the depositional rate at the studied sites was much lower than other areas at
central and southern Jordan. The average heavy metal contents are almost similar in all sampling sites and the blanks, and
they exhibit similar enrichment series, whereas, the meaningful difference between sampling sites and blank was in cation
and anion content, which caused different enrichment series between the two sites. The index of pollution (IP) confirms that
mainly cations and anions have IP > 1.0 and they dominate the southern and the closest sampling sites to the Dead Sea. The
XRD results reveals that the studied samples have minor phases such as halite, gypsum, and dolomite. Meanwhile, these mineral
phases are not found in blank samples. All these results indicates the influence of the Dead Sea, as it is a highly saline
large water mass, which accompanied with by high evaporation rates causing causes the atmosphere over the Sea to be enriched
with these cations, anions, elements and minerals, which eventually are adsorbed in air particulate or carried out as dry
deposition and transported by the NW–SE prevailing winds, and fall over the eastern highlands. 相似文献
218.
We examined air trapped in ancient ice from three shallow cores (<35 m deep) recovered from stagnant portions of the Mullins glacier, an 8 km long debris-covered alpine glacier in the McMurdo Dry Valleys that is overlain by several in-situ volcanic ash-fall deposits. Previously reported 40Ar/39Ar dates on ash-fall in the vicinity of the core sites average 4.0 Ma, and underlying ice is presumably as old in some areas. We analyzed the elemental and isotopic composition of O2, N2, and Ar and total air content of the glacial ice. We also dated the trapped air directly to an uncertainty of ±220 kyr (1σ) by measuring its 40Ar/36Ar and 38Ar/36Ar ratios. Our results suggest that the air analyzed is likely a mixture of ancient atmosphere trapped at the time of ice formation and more recent air introduced via cracks in the ice that penetrate to at least 33 m. The isotopic signatures of gases have been complicated by gas loss, as well as a mixture of thermal and gravitational fractionation. The oldest age estimated for the trapped air dates to 1.6 Ma, indicating that the original air is at least as old as 1.6 ± 0.2 Ma. A convergence to older ice ages with increasing depth in the deepest core analyzed (33 m) hints at the possibility that pristine air might be recovered at greater depths. Minor interstitial debris present in the glacial ice (<1%), along with geochemical evidence for in-situ microbial respiration, prohibit direct analysis of CO2. We measured the triple isotopic composition of O2 as a proxy for CO2 and infer that, in the air represented in our ice samples, CO2 concentrations are within the range observed over the last 800 ka. 相似文献
219.
Existing relationships for lateral backfill pressures on pipelines assume that the trench is adequately wide to contain the failure surface. This condition is commonly violated in design and construction practice, putting at risk the pipeline safety. In this context, size and shape effects for trenches excavated in stiff soils and rocks, are numerically investigated, through experimentally-calibrated parametric analyses. It is shown that, for narrow trenches, ultimate pressures and yield displacements may increase up to an order of magnitude compared to “infinite-trench” values, while excavation of inclined walls reduces the above detrimental effects. Simplified relations are developed to aid pipeline design. 相似文献
220.
Semi-arid areas are often considered to be environmentally and economically marginal, a situation which has been exacerbated by economic change, shifts in agricultural production and land use, and changing state policy. These themes are explored with reference to a semi-arid landscape, namely the Karoo, which covers some 40% of the geographic space of South Africa and is used primarily for extensive livestock farming. Despite long-term decline in agricultural output, the traditional mainstay of the region, and weakening small town economies, the Karoo's population and the economies of its largest service centres are growing. There are, real socio-economic needs and development backlogs, and the situation has been exacerbated by recent political marginalisation. In this study, the small towns of the region are focal points of investigation and provide a lens to investigate the changing demographic and economic dynamics of the Karoo. Most of the region's population lives in these centres which are service, collection, and distribution points for what traditionally has been an agriculture-based regional economy. This paper explores the concept of marginalisation with specific reference to historical, economic, and demographic change. 相似文献