全文获取类型
收费全文 | 3406篇 |
免费 | 154篇 |
国内免费 | 46篇 |
专业分类
测绘学 | 87篇 |
大气科学 | 289篇 |
地球物理 | 723篇 |
地质学 | 1316篇 |
海洋学 | 242篇 |
天文学 | 615篇 |
综合类 | 12篇 |
自然地理 | 322篇 |
出版年
2023年 | 15篇 |
2021年 | 40篇 |
2020年 | 61篇 |
2019年 | 82篇 |
2018年 | 87篇 |
2017年 | 100篇 |
2016年 | 118篇 |
2015年 | 101篇 |
2014年 | 124篇 |
2013年 | 180篇 |
2012年 | 117篇 |
2011年 | 169篇 |
2010年 | 150篇 |
2009年 | 204篇 |
2008年 | 175篇 |
2007年 | 171篇 |
2006年 | 154篇 |
2005年 | 139篇 |
2004年 | 136篇 |
2003年 | 98篇 |
2002年 | 93篇 |
2001年 | 67篇 |
2000年 | 66篇 |
1999年 | 59篇 |
1998年 | 53篇 |
1997年 | 55篇 |
1996年 | 55篇 |
1995年 | 43篇 |
1994年 | 28篇 |
1993年 | 31篇 |
1992年 | 27篇 |
1991年 | 40篇 |
1990年 | 22篇 |
1989年 | 26篇 |
1988年 | 31篇 |
1987年 | 37篇 |
1986年 | 23篇 |
1985年 | 30篇 |
1984年 | 27篇 |
1983年 | 18篇 |
1982年 | 27篇 |
1981年 | 31篇 |
1980年 | 28篇 |
1979年 | 18篇 |
1978年 | 15篇 |
1977年 | 20篇 |
1976年 | 17篇 |
1974年 | 26篇 |
1973年 | 22篇 |
1971年 | 14篇 |
排序方式: 共有3606条查询结果,搜索用时 15 毫秒
91.
92.
Cécile Fabre Marie-Christine BoironJean Dubessy Aliouka ChabironBernard Charoy Tomas Martin Crespo 《Geochimica et cosmochimica acta》2002,66(8):1401-1407
Lithium is an important geochemical tracer for fluids or solids. However, because the electron microprobe cannot detect Li, variations of Li abundance at the micrometric scale are most often estimated from bulk analyses. In this study, the Li intense emission line at 670.706 nm in optical emission spectroscopy was used to perfect the analysis of Li at the micrometric scale by means of laser-induced breakdown spectroscopy (LIBS). To estimate lithium content for different geological materials, LIBS calibration of the emission line at 670.706 nm was achieved by use of synthetic glasses and natural minerals. The detection limit for this method is ∼5 ppm Li. Three applications to geological materials show the potential of LIBS for lithium determination, namely for Li-bearing minerals, melt inclusions, quartz, and associated fluid inclusions.For spodumene and petalite from granite pegmatite dikes (Portugal), the Li2O concentrations are 7.6 ± 1.6 wt% and 6.3 ± 1.3 wt%, respectively, by use of LIBS. These values agree with ion microprobe analyses, bulk analyses, or both. For eucryptite crystals, the Li concentrations are scattered because grain size is smaller than the LIBS spatial resolution (6 to 8 μm). Lithium concentrations of melt inclusions from the Streltsovka U deposit (Siberia) are in the range of 2 to 6.2 wt% (Li2O) for Li-rich daughter minerals. Lithium estimations on silicate glasses display values between 90 and 400 ppm.Lithium was also analyzed as a trace element in quartz. Transverse profiles were performed in hydrothermal barren quartz veins from the Spanish Central System (Sierra de Guadarrama). The highest Li concentrations (250 to 370 ppm) were found in specific growth bands in conjunction with the observed variation in optical cathodoluminescence intensity. Considering the fluid inclusion analysis, the source of fluid responsible to the Li enrichment in quartz is probably high-salinity fluids derived from sedimentary basins. 相似文献
93.
Uncertainties in quantitative time-lapse seismic analysis 总被引:1,自引:0,他引:1
Martin Landrø 《Geophysical Prospecting》2002,50(5):527-538
Most seismic time-lapse studies so far have been of a qualitative nature. Identification of areas with minor or no seismic changes has been used to plan new infill drilling targets. Increased accuracy in seismic acquisition methods, in both conventional streamer surveys and newer methods such as multicomponent sea-bed seismic and permanent sensors, opens possibilities for the next step: quantitative time-lapse analysis. Quantitative methods here mean the estimation of, for instance, a change in fluid saturation from 20% water to 90% water or the estimation of a pore pressure change of 5 MPa. Explicit expressions for the uncertainties associated with estimated changes in, for instance, reservoir pressure and fluid saturation are derived. These formulae can be used to compare relative uncertainties between estimated parameters as well as to identify the critical factors in various estimation techniques. The importance of accurate rock physics input, as well as that of highly repeatable time-lapse seismic data, is emphasized. Furthermore, uncertainty analysis can be used to find optimal weight factors when the same parameter (e.g. saturation change) is estimated by two or three different techniques. 相似文献
94.
电离层水平不均匀结构是引起掩星资料反演的主要误差源.本文模拟不均匀结构对无线电掩星资料反演的影响.首先利用3-D全球射线追踪技术进行了模拟.结果指出:电离层电子密度20%的变化可能引起GPS的时延增加约12m.其次,将90%的掩星时延对应的路径长度,定义为电离层掩星测量的水平分辨率,其值为1200-2000 km.最后,计算了不同纬度电离层当地正午、子夜和日出时的电离层水平不对称指数.它表明:低纬夏季日出时,不对称指数达-78%,反演结果可能有较大误差. 相似文献
95.
Helen J Fraser , Martin R S McCoustra and David A Williams present a simple guide to astrochemistry.
Molecules play a fundamental role in many regions of our universe. The science where chemistry and astronomy overlap is known as astrochemistry, a branch of astronomy that has risen in importance over recent years. In this article we review the significance of chemistry in several astronomical years. IN this article we review the significance of chemistry in several astronomical environments including the early universe, interstellar clouds, starforming regions and protoplanetary disks. We discuss theoretical models, laboratory experiments and observational data, and present several recent and exciting results that challenge our perception of the "molecular universe". 相似文献
Molecules play a fundamental role in many regions of our universe. The science where chemistry and astronomy overlap is known as astrochemistry, a branch of astronomy that has risen in importance over recent years. In this article we review the significance of chemistry in several astronomical years. IN this article we review the significance of chemistry in several astronomical environments including the early universe, interstellar clouds, starforming regions and protoplanetary disks. We discuss theoretical models, laboratory experiments and observational data, and present several recent and exciting results that challenge our perception of the "molecular universe". 相似文献
96.
In conventional seismic processing, the classical algorithm of Hubral and Krey is routinely applied to extract an initial macrovelocity model that consists of a stack of homogeneous layers bounded by curved interfaces. Input for the algorithm are identified primary reflections together with normal moveout (NMO) velocities, as derived from a previous velocity analysis conducted on common midpoint (CMP) data. This work presents a modified version of the Hubral and Krey algorithm that is designed to extend the original version in two ways, namely (a) it makes an advantageous use of previously obtained common-reflection-surface (CRS) attributes as its input and (b) it also allows for gradient layer velocities in depth. A new strategy to recover interfaces as optimized cubic splines is also proposed. Some synthetic examples are provided to illustrate and explain the implementation of the method. 相似文献
97.
98.
Annett Bartsch Martin Gude Christer Jonasson & Dieter Scherer 《Geografiska Annaler: Series A, Physical Geography》2002,84(3-4):171-178
Sediment transport processes in the Kärkevagge are investigated concerning their spatial and temporal characteristics due to long–term monitoring. Within this study remote sensing techniques and GIS modelling in connection with geomorphic mapping are applied for identification and characterization of geomorphic process units. Relationships between geomorphometric parameters and slope processes like solifluction, talus creep and rockfall have been analysed. Multitemporal Landsat–TM5 scenes are used as source for landcover characteristics (Normalized Difference Vegetation Index) after preprocessing involving orthorectification and topographic normalization in order to remove possible terrain–induced effects. Additionally, a digital elevation model with a resolution of 20 m for the Kärkevagge catchment is developed and parameters like slope gradient, slope aspect and profile curvature are extracted as input for the analysis of the sediment transport system. The combination of landcover information, geomorphometrical and topological features allows the definition of areas for single process activities. They show specific sediment displacement characteristics depending on material conditions, topological and geometrical features. Geomorphic process units, which show a homogenous composition, are extracted from these available layers. 相似文献
99.
Martin Roberge 《The Professional geographer》2002,54(2):175-189
In‐stream gravel mining, massive bridge piers, and channelization have all contributed to the geomorphic instability of the Lower Salt River channel in Arizona. Dam closure, changing dam operating rules, and the frequent modification of the channel bed have decreased our ability to predict the Salt River hydrology. Engineering practice has adapted to this situation and to a public that is increasingly intolerant of service disruptions by constructing larger bridges and extending levees. Building these larger structures may be counterproductive; future construction should not constrict the channel and should re‐establish a braided river to decrease the energy available to the system. 相似文献
100.