全文获取类型
收费全文 | 6986篇 |
免费 | 387篇 |
国内免费 | 37篇 |
专业分类
测绘学 | 177篇 |
大气科学 | 729篇 |
地球物理 | 1794篇 |
地质学 | 2804篇 |
海洋学 | 374篇 |
天文学 | 1190篇 |
综合类 | 28篇 |
自然地理 | 314篇 |
出版年
2023年 | 32篇 |
2022年 | 47篇 |
2021年 | 126篇 |
2020年 | 145篇 |
2019年 | 111篇 |
2018年 | 318篇 |
2017年 | 323篇 |
2016年 | 424篇 |
2015年 | 310篇 |
2014年 | 367篇 |
2013年 | 527篇 |
2012年 | 414篇 |
2011年 | 401篇 |
2010年 | 369篇 |
2009年 | 405篇 |
2008年 | 280篇 |
2007年 | 222篇 |
2006年 | 213篇 |
2005年 | 170篇 |
2004年 | 185篇 |
2003年 | 140篇 |
2002年 | 132篇 |
2001年 | 125篇 |
2000年 | 104篇 |
1999年 | 79篇 |
1998年 | 91篇 |
1997年 | 114篇 |
1996年 | 69篇 |
1995年 | 84篇 |
1994年 | 78篇 |
1993年 | 52篇 |
1992年 | 33篇 |
1991年 | 45篇 |
1990年 | 68篇 |
1989年 | 32篇 |
1988年 | 26篇 |
1987年 | 53篇 |
1986年 | 33篇 |
1985年 | 40篇 |
1984年 | 41篇 |
1983年 | 39篇 |
1982年 | 41篇 |
1981年 | 45篇 |
1980年 | 26篇 |
1979年 | 32篇 |
1978年 | 26篇 |
1977年 | 29篇 |
1975年 | 21篇 |
1974年 | 22篇 |
1973年 | 24篇 |
排序方式: 共有7410条查询结果,搜索用时 31 毫秒
411.
Progress in Carrier Phase Time Transfer 总被引:1,自引:0,他引:1
Jim Ray Felicitas Arias Gérard Petit Tim Springer Thomas Schildknecht Jon Clarke Jan Johansson 《GPS Solutions》2001,4(4):47-54
The progress of the joint Pilot Project for time transfer, formed by the International GPS Service (IGS) and the Bureal International
des Poids et Mesures (BIPM), was recently reviewed. Three notable milestones were set. (1) The IGS will implement, at least
in a test mode, an internally realized time scale based on an integration of combined frequency standards within the IGS network.
This will eventually become the reference time scale for all IGS clock products (instead of the current GPS broadcast time).
(2) A new procedure for combined receiver and satellite clock products will be implemented officially in November 2000. Receiver
clocks are an entirely new product of the IGS. (3) The BIPM will coordinate an effort to calibrate all Ashtech Z12-T (and
possibly other) receivers suitable for time transfer applications, either differentially or absolutely. Progress reports will
be presented publicly in the spring 2001. ? 2001 John Wiley & Sons, Inc. 相似文献
412.
Effects of the spherical terrain on gravity and the geoid 总被引:3,自引:0,他引:3
413.
The surface energy balance of a snow cover: comparing measurements to two differentsimulation models 总被引:1,自引:0,他引:1
Summary We compared two one-dimensional simulation models for heat and water fluxes in the soil-snow-atmosphere system with respect
to their mathematical formulations of the surface heat exchange and the snow pack evolution. They were chosen as examples
of a simple one-layer snow model and a more detailed multiple-layer snow model (SNTHERM). The snow models were combined with
the same one-dimensional model for the heat and water balance of the underlying soil (CoupModel). Data from an arable field
in central Sweden (Marsta), covering two years (1997–1999) of soil temperature, snow depth and eddy-correlation measurements
were successfully compared with the models. Conditions with a snow pack deeper or shallower than 10 cm and bare soil resulted
in similar discrepancies. The simulated net radiation and sensible heat flux were in good agreement with that measured during
snow-covered periods, except for situations with snowmelt when the downward sensible heat flux was overestimated by 10–20 Wm−2. The results showed that the uncertainties in parameter values were more important than the model formulation and that both
models were useful in evaluating the limitations and uncertainties of the measurements.
Received November 1, 1999 Revised April 20, 2000 相似文献
414.
An approach to generate artificial earthquakeaccelerograms on hard soil sites is presented. Eachtime-history of accelerations is considered as arealization of a non-stationary gaussian stochasticprocess, with statistical parameters depending onmagnitude and source-to-site distance. In order tolink the values of these parameters for each groundmotion record with the corresponding magnitude andsource-to-site distance, semi-empirical functionalrelations called generalized attenuationfunctions are determined. The set of realground-motion time histories used to obtain thesefunctions correspond to shocks generated at differentsources and recorded at different sites in thevicinity of the southern coast of Mexico. The resultsshow significant dispersion in the parameters of themodel adopted, which reflect that associated with thereal earthquakes included in the sample employed.The problem of conditional simulation of artificialacceleration time histories for prescribed intensitiesis briefly presented, but its detailed study is leftfor a companion paper. The criteria and modelsproposed are applied to generate two families ofartificial acceleration records for recurrenceintervals of 100 and 200 years at a specific sitelocated in the region under study. The results shownin this article correspond to acceleration timehistories recorded on firm ground for earthquakesgenerated at the subduction zone that runs along thesouthern coast of Mexico, and cannot be generalized tocases of earthquakes generated at other sources orrecorded at other types of local conditions. Thismeans that the methods and functional forms presentedhere are applicable to these other cases, but thevalues of the parameters that characterize thosefunctions may differ from those presented here. 相似文献
415.
416.
E. L. Gégo G. S. Johnson M. Hankins 《Stochastic Environmental Research and Risk Assessment (SERRA)》2001,15(1):47-64
Stochastic techniques, such as Monte Carlo experiments, are more and more frequently used for the study of flow and transport
in heterogeneous aquifers. When the aquifer is composed of distinct hydrofacies, a common way to model heterogeneity is to
first generate equally-possible hydrofacies fields, and then convert these hydrofacies fields into hydraulic conductivity
(K) fields by assigning a single K value to each facies. This technique assumes relative homogeneity of K within each facies
but may not be appropriate for the most conductive facies that often exhibits substantial variability. In this paper, we assessed
the impacts of assigning multiple random K, rather than a uniform K value, to the highly conductive facies on the results
of a flow and transport model. A set of fifty stochastic hydrofacies maps depicting an environment similar to the Snake River
Plain aquifer (SRPA) in south-east Idaho were generated. Simulations demonstrated that a uniform K value, if carefully chosen,
can reasonably reproduce the specific discharges and early particle arrival times produced by multiple K values. Yet, the
results obtained with a uniform K value are dramatically less variable than those obtained with multiple K values. It is therefore
concluded that stochastic simulations with uniform K assigned to the most conductive and variable facies do not necessarily
portray the entire uncertainty in the analyses. 相似文献
417.
Incoherent scatter radars are designed to detect scatter from thermalfluctuations in the ionosphere. These fluctuations contain, among other things,features associated with ion-acoustic waves driven by random motions within theplasma. The resulting spectra are generally broad and noisy, but neverthelessthe technique can, through a detailed analysis of spectra, be used to measure arange of physical parameters in the Earth's upper atmosphere, and provides apowerful diagnostic in studies of magnetosphere-ionosphere coupling,thermosphere dynamics and the geospace environment in general. In recent yearsthere has been much interest in naturally occurring (as opposed to artificiallystimulated) enhanced ion-acoustic spectra seen in the auroral zone andcusp/cleft region. A study of the plasma instability processes that lead tosuch spectra will help us to better understand auroral particle acceleration,wave-particle and wave-wave interactions in the ionosphere, and theirassociation with magnetospheric processes. There is now a substantial body ofliterature documenting observations of enhanced ion-acoustic spectra, but thereremains controversy over generation mechanisms. We present a review ofliterature documenting observations of naturally enhanced ion-acoustic spectra,observed mainly along the geomagnetic field direction, along with a discussionof the theories put forward to explain such phenomena. 相似文献
418.
The spatial distribution and fate of petrogenic and pyrogenic hydrocarbons in coastal sediments following the Aegean Sea oil spill (Galicia, NW Spain) was investigated through a detailed study of chemical markers. Alkanes and acyclic isoprenoid hydrocarbons were degraded within six months of the accident, but triterpane and sterane distributions were still detectable and were useful in monitoring, respectively, the oil source and weathering. Aromatic steranes were also useful source indicators, and oil degradation was clear from the decrease of certain alkyl dibenzothiophene and phenanthrene isomers. The pyrogenic polycyclic aromatic hydrocarbons produced in the tanker wreck were found more persistent than the petrogenic ones. However, the presence of the oil in the sediments was recognized even one year after the accident and was found particularly preserved in the subsurface layers. 相似文献
419.
M. Handy J. Braun M. Brown N. Kukowski M. Paterson S. Schmid B. Stöckhert K. Stüwe A. Thompson E. Wosnitza 《International Journal of Earth Sciences》2001,90(1):149-156
The application of continuum mechanics and microstructural analysis to geological studies over the past 30 years has spurred earth scientists to reassess fundamental tectonic processes such as subduction, collision and rifting in terms of dynamics. Armed with new analytical methods, geologists have returned to the field to look at rock structures with more mechanistic eyes. The advent of sophisticated computers, programs, and laboratory deformation equipment has facilitated the simulation of geodynamic processes that range in scale from the grain to the lithosphere. The result has been specialization, with the concomitant opening of communication gaps between geodynamicists, field geologists and rock mechanicists. Partly, these gaps reflect differences of perception and approach. In order to bridge these gaps, a workshop was organized after the DRM conference to debate how field and laboratory studies of deformed rocks can improve our understanding of lithospheric rheology, and in turn, how this understanding can be used to refine dynamic models of orogenesis. The workshop hosted participants with backgrounds in structural geology, experimental rock mechanics, metamorphic petrology and both numerical and analogue modelling. This paper summarizes the main controversies and conclusions reached during the workshop. For the sake of brevity, referencing in this summary is restricted to literature referred to during the oral presentations and to comments made by speakers themselves (names italicized). 相似文献
420.
Heavy metals pollution of aquatic ecosystems in the vicinity of a recently closed underground lead-zinc mine (Basque Country, Spain) 总被引:3,自引:0,他引:3
A presentation is made of the study of an underground polymetallic sulphide mine and the pollution caused by this in the adjoining aquatic ecosystems. Troya Mine is in the Basque Cantabrian region (northern Spain). The annual production of the ore deposit of over 3.7 million tons of Pb (0.9%), Zn (11.2%) and Cu (0.2%) was 300,000 t. It was open and producing from 1986-1993. The mineralization was made up of pyrite, marcasite, sphalerite, galena, chalcopyrite and arsenopyrite. Only the Zn and the Pb were mined. We studied the distribution and behaviour of the heavy metals Zn, Pb, Fe, Mn, Cu, Cr and Cd in the water column, dissolved and suspended fractions, and in the sediments of Estanda Stream and of Gezala Creek. Zn, Cd and Mn tend to be found in the water; Fe, Pb, Cu and Cr appear as an adsorbed fractionin the solid phases. Those of the second group are significantly linked to the fluvial sediments and present very high levels. The concentrations of the metals are conditioned by the waters from the mine galleries, by the leached waste, by the surface runoff, and by overflow from the spillway of the tailings pond. Our observations provide knowledge on the extent of the polluting power of the metals, the physico-chemical effects in play and the subsequent chances of recovering these highly affected environments. 相似文献