全文获取类型
收费全文 | 2715篇 |
免费 | 149篇 |
国内免费 | 33篇 |
专业分类
测绘学 | 113篇 |
大气科学 | 229篇 |
地球物理 | 614篇 |
地质学 | 996篇 |
海洋学 | 229篇 |
天文学 | 441篇 |
综合类 | 14篇 |
自然地理 | 261篇 |
出版年
2023年 | 16篇 |
2022年 | 23篇 |
2021年 | 66篇 |
2020年 | 89篇 |
2019年 | 79篇 |
2018年 | 90篇 |
2017年 | 108篇 |
2016年 | 131篇 |
2015年 | 95篇 |
2014年 | 118篇 |
2013年 | 167篇 |
2012年 | 128篇 |
2011年 | 175篇 |
2010年 | 150篇 |
2009年 | 157篇 |
2008年 | 140篇 |
2007年 | 104篇 |
2006年 | 99篇 |
2005年 | 102篇 |
2004年 | 84篇 |
2003年 | 84篇 |
2002年 | 67篇 |
2001年 | 52篇 |
2000年 | 47篇 |
1999年 | 33篇 |
1998年 | 28篇 |
1997年 | 32篇 |
1996年 | 31篇 |
1995年 | 29篇 |
1994年 | 14篇 |
1993年 | 16篇 |
1992年 | 22篇 |
1991年 | 18篇 |
1990年 | 22篇 |
1989年 | 12篇 |
1988年 | 15篇 |
1987年 | 19篇 |
1986年 | 8篇 |
1985年 | 21篇 |
1984年 | 26篇 |
1983年 | 18篇 |
1982年 | 19篇 |
1981年 | 22篇 |
1980年 | 14篇 |
1979年 | 8篇 |
1978年 | 12篇 |
1977年 | 13篇 |
1976年 | 10篇 |
1975年 | 14篇 |
1974年 | 16篇 |
排序方式: 共有2897条查询结果,搜索用时 9 毫秒
351.
352.
Daniel R. Lynch 《Advances in water resources》1984,7(2):67-75
Numerical mass balance relations are derived for common formulations of the hydraulic and species transport equations, by summing the Galerkin equations. Precise mass balance is demonstrated, provided the Galerkin equation is retained at all boundaries. The effects of quadrature, variable coefficients, transients and irregular geometry are addressed, and numerical experiments verify the algebra. 相似文献
353.
Adrian Gilli Flavio S. Anselmetti Daniel Ariztegui† Milan Beres† Judith A. McKenzie Vera Markgraf‡ 《Sedimentology》2005,52(1):1-23
The results of a seismic stratigraphic analysis of a closed lake basin, Lago Cardiel, in southernmost South America are reported. Very few high-resolution, continental records spanning the Late Quaternary have been obtained from this region. Seismic sequence stratigraphic analysis allows a reconstruction of lake level variations. Two major hiatuses of unknown age occurred during the early evolution of the basin with the deposition of an alluvial fan in a restricted area in the intervening time period. Following the development of a relatively shallow lake during the late Pleistocene and a short desiccation pulse around 11 220 14C yr BP, a transgression of over 135 m occurred at the beginning of the Holocene. The transgression was associated with the formation of beach ridges preserved in the lake stratigraphy on the floor of the modern Lago Cardiel at four different elevations. The preservation of largely unreworked beach ridges indicates a stepwise rise in the lake level. There is no seismic evidence of a major lowering of the lake below modern level during the entire Holocene. Deposition since the mid-Holocene is marked by strong lateral differences in sediment accumulation with a depocentre slightly to the north of the basin midpoint and a pronounced mounded distribution. Seismic reflection geometries, as well as sedimentological characteristics indicate a lacustrine contourite drift covering an area of 80–100 km2. As Lago Cardiel is under the influence of westerly winds, these most likely drove lake circulation. The identification of drowned beach ridges and of contourite drifts illustrates that high-resolution seismic stratigraphy is not only a powerful tool in reconstructing past lake level elevations for closed lake basins, but it can also provide information about the rate of lake level changes and the presence and strength of lake currents. 相似文献
354.
Using density functional simulations, within the generalized gradient approximation and projector-augmented wave method, we study structures and energetics of CaSiO3 perovskite in the pressure range of the Earths lower mantle (0–150 GPa). At zero Kelvin temperature the cubic
CaSiO3 perovskite structure is unstable in the whole pressure range, at low pressures the orthorhombic (Pnam) structure is preferred. At 14.2 GPa there is a phase transition to the tetragonal (I4/mcm) phase. The CaIrO3-type structure is not stable for CaSiO3. Our results also rule out the possibility of decomposition into oxides.
相似文献
Daniel Y. JungEmail: Phone: +41-44-6323744Fax: +41-44-6321133 |
355.
Summary results from a pilot study conducted around an oil production platform on the Northwest Shelf of Australia 总被引:3,自引:0,他引:3
King SC Johnson JE Haasch ML Ryan DA Ahokas JT Burns KA 《Marine pollution bulletin》2005,50(11):1163-1172
The Australian Institute of Marine Science (AIMS) conducted a pilot study around the Harriet A oil production platform on the Northwest Shelf of Australia. We evaluated hepatic ethoxyresorufin-O-deethylase (EROD) activity, fluorescent aromatic compounds (FACs) in bile and immunodetection of CYP1A-like proteins in two Australian tropical fish species, Gold-Spotted Trevally (Carangoides fulvoguttatus) and Bar-Cheeked Coral Trout (Plectropomus maculatus) to assess exposure to petroleum hydrocarbons associated with produced formation water (PFW). Additionally, the incidence of hydrocarbon-degrading bacteria isolated from the liver and bile of all fish captured was examined. Low EROD activity was found in both species, with EROD activity in C. fulvoguttatus showing significant site differences. FACs and CYP1A protein levels in C. fulvoguttatus showed a clear trend in hydrocarbon exposure consistent with hydrocarbon chemistry data: Harriet A>Harriet C>reference site. P. maculatus showed elevated levels of FACs at Harriet A as compared to the reference site and demonstrated detectable levels of CYP1A-like proteins at these two sites. Hydrocarbon-degrading bacteria were found in the liver and bile of both species, yet there was no correlation by sites. Our results demonstrate that C. fulvoguttatus and P. maculatus have potential as indicator species for assessing the effects from exposure to petroleum hydrocarbons. Both FACs and CYP1A are providing warning signs that there is potential for biological effects on fish populations exposed to PFW around the Harriet A production platform. 相似文献
356.
Brian Warner Anthony P. Fairall M. Daniel Overbeek 《Astrophysics and Space Science》1988,143(1):211-213
High-speed photometry, spectroscopy, and visual observations of TU Oph show it to be a Mira variable, not a dwarf nova as originally classified or a UV Cet star as tentatively classified. 相似文献
357.
Emma?S.?Msaky Daniel?Livingstone Owen?K.?DavisEmail author 《Journal of Paleolimnology》2005,34(1):73-83
Pollen spectra from seven short cores taken from deltaic sites in the central and northern parts of Lake Tanganyika provide information about vegetation changes around the lake during the last 5000 years. Pollen analysis was undertaken to understand the history and timing of catchment deforestation and its causal linkage to excess sedimentation and ecosystem change in Lake Tanganyika. The spectra are dominated by grass pollen at all levels in every core. Grass pollen percentage values range between 40 and 80%. Low values of arboreal pollen taxa (1–20%) were documented from most cores except core LT-98-2M. Core LT-98-2M represents the longest duration vegetation record of this study (close to 5000 years BP), and records the onset of increasingly arid conditions in the Late Holocene, especially after 500 A.D., with the probable replacement of forest by open grassland in the Mahale Mountains region. The pollen/spore content for other cores showed a consistent trend of a decrease in grass pollen and an increase in pteridophyte and forest indicator pollen taxa during the last few centuries, contemporaneous with other indications of increased watershed disturbance from forest clearing (especially isotopes and lake faunal change). The timing or strength of this trend is not tied to specific levels of watershed disturbance. However, increasing fern spore abundance does occur progressively later towards the south, where modern human population densities are lower. Although increasing fern spore abundance is consistent with a land-clearing hypothesis, the rising arboreal pollen percentages are seemingly counterintuitive. One possible explanation is that increasing arboreal pollen proportions reflects the recycling of abundant pollen of this type from rapidly eroding soils. Another likely explanation for this finding is that land clearing involved the replacement of miombo woodland, with its mixture of trees producing little pollen and understory grasses producing large amounts of pollen, by the present day cassava, banana, and legume agricultural systems, all of which are poor pollen producers. This shift in catchment vegetation would increase the relative contribution of Afromontane pollen transported long distances from the surrounding highland regions. This hypothesis is consistent with both the lack of correlation of palynological history with specific watershed deforestation attributes, as well as the broader historical record of human habitation in the Lake Tanganyika region. This study also highlights the need for both modern pollen transect data from the region and comparative cores from low elevation swamps or ponds (wetlands) in the region with smaller catchment areas. 相似文献
358.
Geographical data sets sometimes contain missing observations that need to be estimated. A statistical approach to the problem is discussed for multivariate normal spatial data sets satisfying the first-order spatial Markov property with constant mean, where the information at neighboring or contiguous observed sites is used to estimate the missing values. The completed data are used to estimate the parameters of the distribution. The procedure is iterative. The approach is a special case of the Orchard and Woodbury missing information principle. The paper concludes with an illustrative empirical example using rainfall data from an area of Kansas and Nebraska. The quality of the estimates for different sites are compared. 相似文献
359.
In zones washed by rain (façades, pinnacles, pilasters), fine siliceous sandstones are covered by a black varnish, which gives a dirty appearance to monuments. In composition, structure and areas covered by it, it differs from the usual black gypsum crust. Experiments carried out on blocks covered by black varnish show the modifications due to their growth. Results show that despite their low thickness and the coherence of the stone below, the accumulation of hydrophobic matters and the wetting-drying cycles modify the transfer properties of sandstones several centimeters below the surface. Thus, although sandstones seem to be protected by the black varnish, in the long run, a flake detachment process, in the areas covered by it, affects them. Therefore, damage to sandstones by black varnish can be explained by the fact that a wetting and drying cycle modifies the pore network. 相似文献
360.
Salvito D 《Marine pollution bulletin》2006,52(10):1316-7; author reply 1318-9