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991.
992.
热敏电阻因温度系数大,尺寸小和价格低而得到广泛的应用。但其电阻和温度呈非线性关系 (1)式中T和T_0为绝对温度,R_(T_0)为参考温度T_0时的热敏电阻阻值。这给测量工作带来不便和限制。 不少作者研究过热敏电阻温度计的线性化问题。Beakley分析了负温度系数热敏电阻作为惠斯 相似文献
993.
潘乃礼 《华东地质学院学报》1987,(1)
能引起地下水质量或数量发生变化的因素很多。这些因素可能单独作用于地下水,也可能交互作于用地下水。通过方差分析可以定量评价某个变异因素的影响是否显著;在有重复试验的条件下,两个变异因素的方差分析还可以确定有无交互作用。此外,通过多重比较还可以进一步确定某个变异因素对地下水的影响程度。 相似文献
994.
995.
西起班公湖,向东大体沿黑(河)阿(里)公路,经改则、尼玛、东巧—那曲,再向东,大体与怒江上游相连接的一条超基性岩带逐渐引起人们的注意。经几次考察,已初步被认为是一条重要的构造界线,甚至可与南面的雅鲁藏布江带相对比。然而,由于过去工作的较少,对该构造带的研究尚很不够。 相似文献
996.
A two-layer model of soil hydrology 总被引:10,自引:1,他引:10
A two-layer model of soil hydrology is developed for applications where only limited computer time and complexity are allowed. Volumetric soil water is computed in a thin upper layer for use in calculation of surface evaporation. Storage of water is computed for an underlying deeper layer.In an effort to identify the influence of significant asymmetric truncation errors in the two-layer model, this model is compared with the 100-level model of Boersma et al. (1983). Comparisons are made for modelled soils with clay, loam and sand properties for various time dependencies of potential evaporation and precipitation. Truncation errors in the resulting two-layer model appear to be modest, at least compared to errors associated with difficulty in estimation of the hydraulic diffusivity and its strong dependence on soil water content.Minimization of the influence of truncation errors requires: (1) choosing the upper layer to be sufficiently thin, (2) allowing the soil water gradient to control surface evaporation directly and (3) using suitable numerical implementation of the evaluation of internal soil water flux.
Résumé On propose un modèle d'hydrologie du sol à deux couches, spécialement élaboré pour des applications où le temps de calcul et la complexité doivent être aussi réduits que possible. Le contenu en eau du sol dans la mince couche de surface est utilisé pour évaluer l'évaporation, tandis que la réserve en eau est calculée pour la couche profonde, beaucoup plus épaisse.Afin d'estimer les erreurs de troncature et leurs effets dans le modèle à deux couches, des comparaisons sont faites avec le modèle à 100 niveaux de Boersma et al. (1983). Ces comparaisons portent sur des sols de natures variées (argile, terres végétales et sable) et incluent divers taux d'évaporation potentielle et de précipitation. Les effets des erreurs de troncature dans le modèle à 2 couches semblent peu importants par rapport à ceux associés à la mauvaise connaissance de la diffusivité hydraulique et de sa dépendance à l'égard du contenu en eau du sol.La réduction de l'influence de ces erreurs de troncature nécessite: (1) le choix d'une couche supérieure suffisamment fine; (2) le contrôle direct de l'évaporation vers l'atmosphère à partir de la répartition verticale du contenu en eau du sol; (3) l'utilisation d'une procédure particulière pour évaluer le flux d'eau entre les deux couches du sol.相似文献
997.
Firstly, the impact of historical earthquakes on 34 China province-level capital cities is evaluated by using historical earthquake
catalog. The distribution of affected intensity shows, about 53% of cities have even not been affected by earthquake intensity
VI, and 44% of cities have been hit by earthquake intensity VII to IX. For most of the cities, occurrence frequency of affected
intensity VI is usually higher than that of affected intensity larger than VI, and the value of affected intensity with maximal
occurrence frequency may be very different among cities. So both the maximal affected intensity and the affected intensity
with maximal occurrence frequency should be taken into account when the prevention seismic intensity needs to be determined.
Secondly, considering the incompleteness of records of historical earthquakes, a method of earthquake catalog computer simulation
is introduced to study the features of affected intensity of big cities. 69 county-level cities of Fujian Province are selected
to be statistical objects. The statistical result shows, for different risk levels the seismic intensity changes greatly among
cities, the seismic intensity of 2% probability of exceedance in 50 years can be regarded as the characteristic affected intensity
of city, and can be the basis of determining the city special earthquake prevention level and a proper indicator of future
earthquake’s impact on cities.
Foundation item: A Public Benefit Foundation of the Ministry of Science and Technology of China. Contribution No. 04FE1005,
Institute of Geophysics, China Earthquake Administration. 相似文献
998.
999.
By the theories of potential flow and structural vibration, the formulae for evaluating the ‘wet’ (with water) frequencies and mode shapes of the beam‐supported aqueduct are derived through a simplified fluid‐structure interaction analysis. The time‐history formulae of structural responses to the vertical seismic excitation are obtained. Applying the response‐spectrum principle, the equivalent vertical earthquake load exerted on the beam and the corresponding effects are also derived. Several illustrative examples are conducted. The analytical results show that: (i) The ‘wet’ frequencies of the structure are lower than the corresponding ‘dry’ (without water) frequencies due to the participating water mass, but the ‘wet’ mode shapes are identical to the corresponding ‘dry’ ones. (ii) The water mass plays an important role in the vertical seismic response, which varies with the different geological sites. For the different seismic inputs, the deeper the water is, the greater are the structural responses. (iii) The vertical seismic effects on the beam are generally not too small to be neglected and should be considered in the structural designs of a beam‐supported aqueduct. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
1000.
Based on the Hilbert–Huang spectral analysis, a method is proposed to identify multi‐degree‐of‐freedom (MDOF) linear systems using measured free vibration time histories. For MDOF systems, the normal modes have been assumed to exist. In this method, the measured response data, which are polluted by noises, are first decomposed into modal responses using the empirical mode decomposition (EMD) approach with intermittency criteria. Then, the Hilbert transform is applied to each modal response to obtain the instantaneous amplitude and phase angle time histories. A linear least‐square fit procedure is proposed to identify the natural frequency and damping ratio from the instantaneous amplitude and phase angle for each modal response. Based on a single measurement of the free vibration time history at one appropriate location, natural frequencies and damping ratios can be identified. When the responses at all degrees of freedom are measured, the mode shapes and the physical mass, damping and stiffness matrices of the structure can be determined. The applications of the proposed method are illustrated using three linear systems with different dynamic characteristics. Numerical simulation results demonstrate that the proposed system identification method yields quite accurate results, and it offers a new and effective tool for the system identification of linear structures in which normal modes exist. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献