全文获取类型
收费全文 | 7095篇 |
免费 | 1892篇 |
国内免费 | 1777篇 |
专业分类
测绘学 | 281篇 |
大气科学 | 2087篇 |
地球物理 | 2927篇 |
地质学 | 3074篇 |
海洋学 | 1189篇 |
天文学 | 156篇 |
综合类 | 470篇 |
自然地理 | 580篇 |
出版年
2024年 | 28篇 |
2023年 | 65篇 |
2022年 | 195篇 |
2021年 | 244篇 |
2020年 | 262篇 |
2019年 | 384篇 |
2018年 | 292篇 |
2017年 | 314篇 |
2016年 | 302篇 |
2015年 | 353篇 |
2014年 | 475篇 |
2013年 | 431篇 |
2012年 | 418篇 |
2011年 | 432篇 |
2010年 | 399篇 |
2009年 | 540篇 |
2008年 | 507篇 |
2007年 | 619篇 |
2006年 | 516篇 |
2005年 | 475篇 |
2004年 | 442篇 |
2003年 | 370篇 |
2002年 | 371篇 |
2001年 | 261篇 |
2000年 | 275篇 |
1999年 | 256篇 |
1998年 | 245篇 |
1997年 | 217篇 |
1996年 | 204篇 |
1995年 | 131篇 |
1994年 | 148篇 |
1993年 | 126篇 |
1992年 | 106篇 |
1991年 | 102篇 |
1990年 | 66篇 |
1989年 | 41篇 |
1988年 | 47篇 |
1987年 | 31篇 |
1986年 | 14篇 |
1985年 | 9篇 |
1984年 | 2篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 3篇 |
1980年 | 8篇 |
1979年 | 5篇 |
1978年 | 10篇 |
1977年 | 4篇 |
1971年 | 1篇 |
1954年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
211.
921型超低频有源伺服式速度,位移计 总被引:1,自引:2,他引:1
本介绍一种在很小体积的往复式摆上采用有源伺服技术实现超低频大幅值测量用的速度计、位移计,它可在多个频带测量速度和位移。最低频率可达0.04Hz,最大可测位移为±15m。它可用于海浪浪高、抽油机行程、高柔结构的超低频大位移测量。该仪器于1992年获得国家专利,并获得中国专利博览会银奖。 相似文献
212.
213.
214.
1976年唐山地震前,在距震源很远的地区内都观测到一些前兆趋势变化,如重力、重力位二次徽商W△、水氧、地电阻率、水位和油井出油量等变化。作者认为这些变化不是由震源体直接引起的,而是在区域应力场的作用下,在某些活动断层附近,浅层岩,尤其是含水砂岩层和含油层受挤压出现的一些与地震有关的异常现象。其特征是:(1)异常范围大,可能在距震源很远的地方发生,但就同一种方法的多个观测点来说却又是局部的,即只有其中部分测点才能观测到异常,不少测点观测不到异常。(2)异常发生的时间大致相同。有些异常有同步变化的特征,如同时上升或同时下降。(3)临震前多数异常有恢复的趋势。这些特征与引起异常的机理有关。作者还从理论上计算了这种趋势异常量级,重力变化100×10-6cm/s2左右,重力位二次微商变化(1~2)×10-9/s2,地电阻率变化2%~3%,Rn变化7.4~11.1Bq/L,这样的变化量在活动断层附近的一些台站可能观测到。作者还研究了干旱降雨对某些前兆的影响,其影响量级可以被一些方法观测到,因此在确定是否是地震异常时,必须注意利用综合分析的方法排除干旱降雨的影响,减少异常的多解性。 相似文献
215.
216.
遗传算法在上地幔速度结构研究中的应用 总被引:1,自引:0,他引:1
遗传算法是近年来发展较快的一种求解非线性优化问题的有效方法。本文通过对遗传算法基本原理的介绍,对该方法的特点进行了分析。我们采用WKBJ理论地震图作正演,遗传算法作反演,对用体波波形反演上地幔速度结构的方法进行了研究。在WKBJ理论地震图的计算中,通过计算主要射线的平均吸收特征时间,考虑了衰减随距离的变化,通过对不同震源引入不同的虚拟界面,同时对多个地震的波形记录进行反演。探讨了观测误差对反演结果的影响。对理论记录的反演表明,用遗传算法研究上地幔速度结构具有较好的效果。 相似文献
217.
L. Kristensen C. E. Andersen H. E. Jørgensen P. Kirkegaard K. Pilegaard 《Journal of Atmospheric Chemistry》1997,27(3):249-269
We have discussed the behavior of a non-conserved scalar in the stationary, horizontally homogeneous, neutral surface-flux layer and, on the basis of conventional second-order closure, derived analytic expressions for flux and for mean concentration of a gas, subjected to a first-order removal process. The analytic flux solution showed a clear deviation from the constant flux, characterizing a conserved scalar in the surface-flux layer. It decreases with height and is reduced by an order of magnitude of the surface flux at a height equal to about the typical mean distance a molecule can travel before destruction. The predicted mean concentration profile, however, shows only a small deviation from the logarithmic behavior of a conserved scalar. The solution is consistent with assuming a flux-gradient relationship with a turbulent diffusivity corrected by the Damköhler ratio, the ratio of a characteristic turbulent time scale and the scalar mean lifetime. We show that if we use only first-order closure and neglect the effect of the Damköhler ratio on the turbulent diffusivity we obtain another analytic solution for the profiles of the flux and the mean concentration which, from an experimental point of view, is indistinguishable from the first analytic solution. We have discussed two cases where the model should apply, namely NO which, by night, is irreversibly destroyed by interaction with mainly O3 and the radioactive 220Rn. Only in the last case was it possible to find data to shed light on the validity of our predictions. The agreement seemed such that a falsification of our model was impossible. It is shown how the model can be used to predict the surface flux of 220Rn from measured concentration profiles. 相似文献
218.
AN EVALUATION OF SEVERAL TURBULENCE SCHEMES FOR THE PREDICTION OF MEAN AND TURBULENT FIELDS IN COMPLEX TERRAIN 总被引:2,自引:0,他引:2
PETER J. HURLEY 《Boundary-Layer Meteorology》1997,83(1):43-73
A prognostic three-dimensional mesoscale model has been developed andused in one- and two-dimensional modes to evaluate ten local turbulenceclosure schemes. The schemes ranged from first-order to the two-equationprognostic schemes. Predictions by the models were compared for aone-dimensional convective boundary layer using mixed layer scaling andmeasurements to interpret the results. Two-dimensional simulations were alsoperformed for a sea-breeze flow and for flow over a hill. The results showedthat for all of the models considered, minor differences were produced in themean meteorological fields and in the vertical scalar fluxes, but majordifferences were apparent in the velocity variances and dissipation rate.Predicted tracer concentrations were very sensitive to the turbulence modelformulation for dispersion from a point source in the convective boundarylayer, particularly for the prediction of maximum concentrations. Predictedtracer concentrations from a surface volume source for the two-dimensionalsimulations were similar for all models, although the degree of mixing in themorning growth period produced some differences. Generally, good results forthe mean meteorological fields can be obtained with first-order schemes, evenif they underpredict the magnitude of turbulence in the convective boundarylayer, and reasonable tracer concentrations can also be obtained with thesemodels provided near-source effects are not important. The two-equationprognostic models performed best for the prediction of turbulence in theconvective boundary layer. 相似文献
219.
H. E. Scheel H. Areskoug H. Geiss B. Gomiscek K. Granby L. Haszpra L. Klasinc D. Kley T. Laurila A. Lindskog M. Roemer R. Schmitt P. Simmonds S. Solberg G. Toupance 《Journal of Atmospheric Chemistry》1997,28(1-3):11-28
Surface ozone data from 25 Europeanlow-altitude sites and mountain sites located between79°N and 28°N were studied. The analysiscovered the time period March 1989–February 1993.Average summer and winter O3 concentrations inthe boundary layer over the continent gave rise togradients that were strongest in the north-west tosouth-east direction and west-east direction, respectively. WintertimeO3 ranged from 19 to 27 ppbover the continent, compared to about 32 ppb at thewestern border, while for summer the continentalO3 values ranged between 39 and 56 ppb and theoceanic mixing ratios were around 37 ppb. In the lowerfree troposphere average wintertime O3 mixingratios were around 38 ppb, with only an 8 ppbdifference between 28°N and 79°N. For summerthe average O3 levels decreased from about 55 ppbover Central Europe to 32 ppb at 79°N. Inaddition, O3 and Ox(= O3 + NO2)in polluted and clean air were compared. Theamplitudes of the seasonal ozone variations increasedin the north-west to south-east direction, while thetime of the annual maximum was shifted from spring (atthe northerly sites) to late summer (at sites inAustria and Hungary), which reflected the contributionof photochemical ozone production in the lower partsof the troposphere. 相似文献
220.
SHUMING DU 《Boundary-Layer Meteorology》1997,83(2):207-219
In this paper, we evaluate the Lagrangian velocity structure function constant, C0, in the inertial subrange by comparing experimental diffusion data and simulation results obtained with applicable Lagrangian stochastic models. We find in several different flows (grid turbulence, laboratory boundary-layer flow and the atmospheric surface layer under neutral stratification) the value for C0 is 3.0 ± 0.5. We also identify the reasons responsible for earlier studies having not reached the present result. 相似文献