全文获取类型
收费全文 | 598篇 |
免费 | 103篇 |
国内免费 | 125篇 |
专业分类
测绘学 | 63篇 |
大气科学 | 311篇 |
地球物理 | 175篇 |
地质学 | 98篇 |
海洋学 | 64篇 |
天文学 | 67篇 |
综合类 | 18篇 |
自然地理 | 30篇 |
出版年
2024年 | 2篇 |
2023年 | 4篇 |
2022年 | 13篇 |
2021年 | 8篇 |
2020年 | 23篇 |
2019年 | 15篇 |
2018年 | 22篇 |
2017年 | 37篇 |
2016年 | 21篇 |
2015年 | 27篇 |
2014年 | 37篇 |
2013年 | 31篇 |
2012年 | 35篇 |
2011年 | 29篇 |
2010年 | 26篇 |
2009年 | 39篇 |
2008年 | 50篇 |
2007年 | 46篇 |
2006年 | 40篇 |
2005年 | 30篇 |
2004年 | 32篇 |
2003年 | 26篇 |
2002年 | 39篇 |
2001年 | 22篇 |
2000年 | 13篇 |
1999年 | 17篇 |
1998年 | 16篇 |
1997年 | 20篇 |
1996年 | 11篇 |
1995年 | 13篇 |
1994年 | 11篇 |
1993年 | 18篇 |
1992年 | 7篇 |
1991年 | 6篇 |
1989年 | 5篇 |
1988年 | 8篇 |
1987年 | 1篇 |
1986年 | 4篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 7篇 |
1977年 | 3篇 |
1954年 | 1篇 |
排序方式: 共有826条查询结果,搜索用时 31 毫秒
731.
福建沿海剪切波速与土层参数定量关系研究 总被引:2,自引:0,他引:2
通过对大量实测数据的分析,按照剪切波速与土层密度、标贯击数、土层埋深之间的关系特征,利用单因子及多因子回归分析方法,给出剪切波速与土层密度、标贯击数、土层埋深的定量关系。 相似文献
732.
This article introduces a technique for using a combination of remote sensing imagery and open-channel flow principles to estimate depths for each pixel in an imaged river. This technique, which we term hydraulically assisted bathymetry (HAB), uses a combination of local stream gage information on discharge, image brightness data, and Manning-based estimates of stream resistance to calculate water depth. The HAB technique does not require ground-truth depth information at the time of flight. HAB can be accomplished with multispectral or hyperspectral data, and therefore can be applied over entire watersheds using standard high spatial resolution satellite or aerial images. HAB also has the potential to be applied retroactively to historic imagery, allowing researchers to map temporal changes in depth.We present two versions of the technique, HAB-1 and HAB-2. HAB-1 is based primarily on the geometry, discharge and velocity relationships of river channels. Manning's equation (assuming average depth approximates the hydraulic radius), the discharge equation, and the assumption that the frequency distribution of depths within a cross-section approximates that of a triangle are combined with discharge data from a local station, width measurements from imagery, and slope measurements from maps to estimate minimum, average and maximum depths at a multiple cross-sections. These depths are assigned to pixels of maximum, average, and minimum brightness within the cross-sections to develop a brightness–depth relation to estimate depths throughout the remainder of the river.HAB-2 is similar to HAB-1 in operation, but the assumption that the distribution of depths approximates that of a triangle is replaced by an optical Beer–Lambert law of light absorbance. In this case, the flow equations and the optical equations are used to iteratively scale the river pixel values until their depths produce a discharge that matches that of a nearby gage.R2 values for measured depths versus depths estimated by HAB-1 and HAB-2 are 0.51 and 0.77, respectively, in the relatively simple Brazos River, Texas. R2 values for HAB-1 and HAB-2 are 0.46 and 0.26, respectively, in the Lamar River, a complex mountain river system in Yellowstone National Park. Although the R2 values are moderate, depth maps and cross-sections derived from the HAB techniques are consistent with typical stream geomorphology patterns and provide far greater spatial coverage and detail than could be achieved with ground-based survey techniques. Improved depth estimates can be achieved by stratifying the river into different habitat types that normalize for differences in turbulence and substrate. 相似文献
733.
A. Akhperjanian R. Kankanian V. Sahakian A. Heusler C.-A. Wiedner H. Wirth 《Experimental Astronomy》1998,8(2):135-152
The raytracing technique was used to derive a suitable design for the HEGRA system of Cherenkov telescopes, which is at present commissioned at La Palma. The reflectors with a diameter of 3.9 m consist of 30 spherical mirrors with focal lengths in the range of 4.88 – 4.94 m. It is shown that 93% of the photons from the Cherenkov light emitted by an extended air shower are contained in the camera pixels, 0.25° in diameter, for the full field of view of = ± 2.5°. The optical performance of the HEGRA design is compared to other layouts. 相似文献
734.
735.
The methodology applied in this research work allows the evaluation of the vertical vulnerability of the Esino River alluvial
aquifer (Marche). Main factors determining the ease with which possible pollutant may come from the soil surface to reach
the groundwater beneath, through a prevalently vertical path, have been evaluated. Following a "raster" logic, the study area
has been discretized into a set of elemental areas. For each of these areas, dimensionless indices able to characterize the
actual possibility for the groundwater to be reached by polluting substances have been estimated.
Received: 25 February 1997 · Accepted: 13 November 1997 相似文献
736.
Masaharu Watanabe Yasunobu Iwasaka Takashi Shibata Masahiko Hayashi Motowo Fujiwara Roland Neuber 《Atmospheric Research》2004,69(3-4):199-215
Balloon-borne aerosol measurements were performed with an optical particle counter between 1994 and 2000 at Ny-Ålesund (79°N), Svarbard. Throughout the observation period, continuous decay was found in the concentrations of particles with 0.4–0.6 μm in radius in the Arctic stratosphere, suggesting that Pinatubo aerosols remained even at the end of the 1990s. The decay rate was clearly higher for larger particle sizes, and higher at higher altitude (e-folding time of 970–526 days), suggesting a gravitational sedimentation effect. For smaller particles with R<0.4 μm, slight increases in concentration with time were found, which agreed with the measurements at mid-latitude. The sulfate mass mixing ratio in the Arctic stratosphere before 1998 showed values higher than those at middle latitude, while values were almost the same in both regions after 1998. A possible explanation of the latitudinal difference is a time lag (of 0.5–1 year) in the arrival of Pinatubo aerosols in the Arctic. 相似文献
737.
利用MODIS可见光通道气溶胶光学厚度的卫星遥感和523 nm波长微脉冲激光雷达 (MPL LIDAR) 对气溶胶消光系数垂直分布的观测,分析了珠江三角洲地区2003年6月一次气溶胶污染过程中气溶胶光学厚度的分布特征、气溶胶消光系数廓线的演变,认为这次污染过程是弱高压控制下的区域性污染,而香港地区污染物浓度的上升与区域性输送有直接关系,结果表明卫星和激光雷达的光学遥感方法提供了研究大气污染的可行手段。 相似文献
738.
The Sahelian Margin of West Africa is widely recognised as an area of recent environmental catastrophe and human suffering arising from food shortage and land degradation associated with prolonged drought. The propensity of this region to suffer drought has been related, using environmental data collected during the period of instrumental records, to a combination of low mean annual rainfall levels and a high degree of rainfall variability which relates to sea surface temperature anomalies in the adjacent tropical Atlantic Ocean. Despite the significant environmental and human consequences of such droughts, there is a paucity of long-term environmental data for the West African Sahel. Aeolian dune reactivations in this area are a potentially highly useful environmental archive of past periods of extended drought conditions, which may have resulted in localised or widespread dune reactivation. Here we describe the initial results from an ongoing programme of research, which seeks to develop a detailed record of past dune reactivations in Mali. We find evidence for repeated Holocene dune reactivation events and a significant number of reactivations, which commenced at the time of onset of the last major drought cycle in the early 1970s. We obtain ages as young as 20–30 years for some significant dune units (thickness up to 1 m) and describe the results of experiments which test the performance of our dating exercise. We specifically test for the significance of preheat temperature on single aliquot regeneration (SAR) equivalent dose determinations and recycling ratios; neither are found vary significantly as a function of preheating. Optical dating of sand sized quartz could provide a useful tool for palaeogeographical mapping of ancient and historical dune reactivations in this region and elsewhere. 相似文献
739.
Optical dating of Holocene sediments from a variety of geomorphic settings using single grains of quartz 总被引:1,自引:0,他引:1
This paper presents an improved method for the optical dating of Holocene sediments from a variety of geomorphic settings. We have measured the equivalent dose (De) in individual grains of quartz, using green laser light for optical stimulation, and have simulated the De distributions for multiple-grain ‘synthetic’ aliquots using the single-grain data. For 12 samples of known (independent) age, we show that application of a ‘minimum age model’ to the single-grain and ‘small’ (10-grain) aliquot De data provides the most accurate estimate of the burial dose for nine of the samples examined (3 aeolian, 5 fluvial, and 1 marine). The weighted mean De (as obtained using the ‘central age model’) gives rise to burial age overestimates of up to a factor of 10 for these nine samples, whether single grains, small aliquots, or ‘large’ (100-grain) aliquots are used. For the other three samples (two aeolian and one fluvial), application of either the minimum age model or the central age model to the single-grain, small aliquot, and large aliquot De data yields burial ages in accord with the independent age control. We infer that these three samples were well bleached at the time of deposition. These results show that heterogeneous bleaching of the optical dating signal is commonplace in nature, and that aeolian transport offers no guarantee that the sample will be well bleached at the time of deposition. We also show that grains sensitive to infrared (IR) stimulation can give rise to low De values, which will result in significant underestimation of the burial dose and, hence, of the age of deposition. We demonstrate that use of a modified single-aliquot regenerative-dose protocol incorporating IR stimulation prior to green light stimulation deals effectively with contamination by IR-sensitive grains. We conclude that application of the modified protocol to single grains or small aliquots of quartz, using the lowest De population to estimate the burial dose, is the best means of obtaining reliable ages for Holocene sediments from a wide range of depositional environments. 相似文献
740.
在研究水云的微物理性质与光学性质关系的基础上,提出了三种用于长波区域窄带和宽带水云光学性质计算的参数化方案,研究了应用不同的水云光学性质参数化方案和不同的宽带数目对云辐射性质的影响。分析了云滴的散射作用对有云大气中长波辐射通量和冷却率的影响。 相似文献