全文获取类型
收费全文 | 898篇 |
免费 | 33篇 |
国内免费 | 21篇 |
专业分类
测绘学 | 8篇 |
大气科学 | 121篇 |
地球物理 | 197篇 |
地质学 | 374篇 |
海洋学 | 30篇 |
天文学 | 194篇 |
综合类 | 4篇 |
自然地理 | 24篇 |
出版年
2021年 | 13篇 |
2020年 | 13篇 |
2019年 | 12篇 |
2018年 | 14篇 |
2017年 | 13篇 |
2016年 | 24篇 |
2015年 | 27篇 |
2014年 | 28篇 |
2013年 | 43篇 |
2012年 | 41篇 |
2011年 | 42篇 |
2010年 | 35篇 |
2009年 | 40篇 |
2008年 | 51篇 |
2007年 | 31篇 |
2006年 | 34篇 |
2005年 | 37篇 |
2004年 | 31篇 |
2003年 | 24篇 |
2002年 | 30篇 |
2001年 | 19篇 |
2000年 | 22篇 |
1999年 | 19篇 |
1998年 | 16篇 |
1997年 | 7篇 |
1996年 | 14篇 |
1995年 | 11篇 |
1994年 | 14篇 |
1993年 | 10篇 |
1992年 | 8篇 |
1991年 | 17篇 |
1990年 | 9篇 |
1989年 | 6篇 |
1988年 | 11篇 |
1987年 | 8篇 |
1986年 | 7篇 |
1985年 | 8篇 |
1984年 | 12篇 |
1983年 | 13篇 |
1982年 | 5篇 |
1981年 | 16篇 |
1980年 | 12篇 |
1979年 | 7篇 |
1978年 | 13篇 |
1977年 | 8篇 |
1976年 | 13篇 |
1975年 | 8篇 |
1974年 | 6篇 |
1973年 | 11篇 |
1972年 | 6篇 |
排序方式: 共有952条查询结果,搜索用时 0 毫秒
871.
Malte Thoma Christoph Mayer Klaus Grosfeld 《Earth and Planetary Science Letters》2008,269(1-2):242-247
Subglacial lakes provide unique habitats, but the exact nature of physical and geochemical conditions are still a matter of debate and await direct sampling of water. Due to its isolation from external atmospheric forcing other environmental parameters influence the flow characteristics within the lake. In this study we use an improved treatment of the physical processes at the ice–water boundary interface to identify and quantify the impact of (1) the geothermal heat flux, (2) the heat flux from the lake into the ice, (3) the influence of the salinity of the lake water, and (4) the ice thickness on the size of the freezing area and the freeze/melt rates. We show that the modelled basal mass imbalance (that is the produced melt water minus the re-frozen water) depends on the geothermal heating as well as the heat flux into the ice. The circulation and the temperature distribution within subglacial Lake Vostok are rather stable against variations of geothermal heat flux, heat flux into the ice sheet, salinity of the lake, and small changes of the ice thickness above the lake. However, the flow regime for any subglacial lake with less than 2000 m ice thickness above, will be substantially different from those that experience higher pressures. This is because the buoyancy–temperature relationship reverses at this depth. 相似文献
872.
Irene M. Farnham Klaus J. Stetzenbach Ashok K. Singh Kevin H. Johannesson 《Mathematical Geology》2000,32(8):943-968
The origin of groundwater discharging via evapotranspiration and from springs within Oasis Valley, Nevada, is of concern owing to the close proximity of the Nevada Test Site (NTS) and the possible contamination of groundwater as a result of underground nuclear testing. Principal components analysis, cluster analysis, and population partitioning, along with a Geographical Information System, were used to decipher groundwater flow patterns in Oasis Valley, Nevada. These multivariate statistical techniques were applied to the trace element chemistry of groundwater samples collected from 26 springs and wells within Oasis Valley, the NTS, and the Nellis Air Force Range. The results of all statistical analyses showed similar geographical trends in the trace element chemistry of the groundwaters included in this study. Differences are observed between the groundwaters from the NTS and those of Oasis Valley based on the concentrations of the elements Li, Ge, Mo, Rb, Ba, U, and Ru. A concentration gradient is observed from lower concentrations in the NTS to increasing concentrations toward Oasis Valley suggesting groundwater flow in an overall southwestward direction from the NTS. Also, a different trace element signature is observed for the waters collected in the northern and western region of Oasis Valley, suggesting another source of groundwater to this area. 相似文献
873.
Klaus Hornung Yuri G. Malama Khaim S. Kestenboim 《Astrophysics and Space Science》2000,274(1-2):355-363
We report on theoretical efforts to understand the process of vaporization and ion formation upon hypervelocity impact of
small cosmic dust particles on a solid surface. Such collisions occur at the surface of solid bodies within the planetary
system, which do not have an atmosphere as well as in various actual and upcoming space missions for in-situ measurements
of interplanetary, interstellar and cometary dust. The investigation uses Godunov's method to simulate the impact. For the
very high velocitites investigated, the impacting dust particle as well as parts of the target vaporize and some of the vapor
cloud may change to partially ionized. Numerical results of the impact process are communicated for an 80 kms-1impact of a slightly porous SiO2 particle on a compact SiO2surface. Values of the amount of vapor and liquid excavated from the target are given. Ionization rates are calculated for
the example investigated and an estimate is given how this extrapolates to the highest conceivable velocities in the planetary
system (above 100 km s-1).
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
874.
875.
The Preparation and Preliminary Characterisation of Eight Geological MPI-DING Reference Glasses for In-Situ Microanalysis 总被引:13,自引:4,他引:9
Klaus Peter Jochum Donald B. Dingwell Alexander Rocholl Brigitte Stoll Albrecht W. Hofmann S. Becker A. Besmehn D. Bessette H.-J. Dietze P. Dulski J. Erzinger E. Hellebrand P. Hoppe I. Horn K. Janssens G.A. Jenner M. Klein W.F. McDonough M. Maetz K. Mezger C. Müker I.K. Nikogosian C. Pickhardt I. Raczek D. Rhede H.M. Seufert S.G. Simakin A.V. Sobolev B. Spettel S. Straub L. Vincze A. Wallianos G. Weckwerth S. Weyer D. Wolf M. Zimmer 《Geostandards and Geoanalytical Research》2000,24(1):87-133
Eight silicate glasses were prepared by directly fusing and stirring 50-100 g each of basalt, andesite, komatiite, peridotite, rhyolite, and quartz-diorite. These are referred to as MPI-DING glasses and were made for the purpose of providing reference materials for geochemical, in-situ microanalytical work. Results from various analytical techniques indicate that individual glass fragments are well homogenised with respect to major and trace elements at the μm to mm scale. Heterogeneities due to quench crystallisation of olivine have been observed in small and limited areas of the two komatiitic glasses. In order to obtain concentration values for as many elements as possible, the glasses were analysed by a variety of bulk and microanalytical methods in a number of laboratories. The analytical uncertainties of most elements are estimated to be between 1% and 10%. From the analytical data, preliminary reference values for more than sixty elements were calculated. The analytical uncertainties of most elements are estimated to be between 1% and 10%. 相似文献
876.
Cristina?PulidoEmail author Kaj?Sand-Jensen Esther?C.?H.?E.?T.?Lucassen Jan?G.?M.?Roelofs Klaus?P.?Brodersen Ole?Pedersen 《Aquatic Sciences - Research Across Boundaries》2012,74(2):351-360
Isoetids, as indicators of near-pristine softwater lakes, have a high priority in national and international (European Water
Directive Framework) assessments of ecological lake quality. Our main goal was to identify the most important environmental
factors that influence the composition of plant communities and specifically determine the presence and abundance of the isoetid
Lobelia dortmanna in NW European softwater lakes. Geographical position and composition of surface water, porewater, sediment and plant communities
were examined in 39 lakes in four regions (The Netherlands, Denmark, West Norway and East Norway) distributed over a 1,200-km
long distance. We confirmed that lake location was accompanied by significant changes in environmental variables between NW
European lakes. Lake location was the single most important determinant of vegetation composition and it had significant individual
contributions independent of the coupling to environmental variables. This influence of location was supported by a significant
decline of community similarity with geographical distance between pairs of lakes at regional, inter-regional and international
scales. Combining the geographical position with environmental variables for surface water, porewater and sediment significantly
improved prediction of vegetation composition. Specifically, the combination of latitude, surface water alkalinity, porewater
phosphate and redox potential offered the highest correlation (BIO ENV correlation 0.66) to vegetation composition. This complex
analysis can also account for high sediment variability in the littoral zone of individual lakes, by using site-specific physico-chemical
sediment factors, and offer better predictions of vegetation composition when lake water chemistry is relatively homogeneous
among lakes within regions. 相似文献
877.
Much attention has recently been devoted to the fate and effects of pharmaceuticals in the water cycle. Removal of antibiotics in effluents by photo‐treatment or biodegradation is a topic currently under discussion. Degradation and removal efficiencies of sulfisomidine (SUI) by photodegradation and aerobic biodegradability were studied. SUI behavior was monitored during photolysis and photocatalysis (catalyst titanium dioxide) using 150‐W medium‐pressure Hg lamp. Also an aerobic bacterial degradation test from the OECD series (closed bottle test (CBT, OECD 301 D)) was performed. The primary elimination of SUI was monitored. Structures of photo‐degradation products were assessed by chromatographic separation on a C18 column with ultraviolet detection at 270 nm and ion trap MS. The results demonstrate that SUI is not readily biodegradable in CBT. Photo catalysis was more is effective than photolysis. SUI underwent photodegradation and several SUI photoproducts were identified. Accordingly, the photodegradation pathway of SUI was postulated. When reaching the aquatic environment, SUI and its photo products can constitute a risk to the environment. 相似文献
878.
Abstract— We present a numerical simulation technique for the retrieval of the electric properties relative permittivity and conductivity of planetary, asteroid, and cometary regolith. Our simulation techniques aim at accompanying hardware development and conducting virtual experiments, e.g., to assess the response of arbitrary heterogeneous conductivity and permittivity distributions or to scrutinize possibilities for spatial reconstruction methods using inverse schemes. In a first step, we have developed a finite element simulation code on the basis of unstructured, adaptive triangular grids for arbitrary two‐dimensional axisymmetric distributions of conductivity and permittivity. The code is able to take into account the spatial geometry of the probe and allows for possible inductive effects. In previous studies, the non‐inductive approach has been used to convert potential and phase data into apparent material properties. By our simulations, we have shown that this approach is valid for the frequency range from 102 Hz to 107 Hz and electric conductivities of 10?8 S/m that are typical for the near‐surface region of asteroids and comets composed of chondritic materials and/or frozen volatiles such as H2O and CO2 ice. We prove the accuracy of our code to be better than 10%, using mixed types of boundary conditions and present a simulated vertical log through a horizontally stratified subsurface layer as a representative example of a heterogeneous distribution of the electrical properties. Resolution studies for the given electrode separation reveal that the material parameters of layers having thicknesses of less than about half the electrode spread are not reconstructible if only apparent quantities are considered. Therefore, spatial distributions of the complex sensitivity are presented having in mind a future data inversion concept that will permit the multi‐dimensional reconstruction of material parameters in heterogeneous environments. 相似文献
879.
880.