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591.
Ahmad Rami El-Nabulsi 《Astrophysics and Space Science》2009,324(1):71-75
The recent observational available data for an accelerated expansion state of the present universe, obtained from distant
SNeIa gave strong support to the search of alternative cosmologies. Recently, there have been a number of different attempts
to modify Einstein’s gravity to yield accelerated expansion at late times. Unfortunately, many of the theoretical models discussed
in the literature are plagued with theoretical problems, in particular the singularity problem at the origin of time. In the
present work we have analyzed a multidimensional spacetime Friedmann–Robertson–Walker (FRW) model with a decaying cosmological
constant and a varying gravitational constant. Many interesting consequences are revealed, in particular the behavior of the
scale factor and the shape of the universe in terms of the number of extra dimensions. 相似文献
592.
Gravitational instabilities with respect to warp modes of vertical oscillations are examined for nonlinearly nonequilibrium
disk models with isotropic and anisotropic velocity diagrams. Nonstationary analogs of the dispersion relations for vertical
oscillations in these models are derived in a general form. A detailed study is made of the major large scale oscillatory
modes, which correspond primarily to the most common type of warp in the form of an integral sign, as well as to dome-shaped,
U-shaped, and precessional warps. Critical diagrams showing the initial virial relation as a function of the rotation parameter
for the nonstationary model are constructed for each of these vertical oscillation modes. A comparative analysis is made of
the growth rates of the instabilities for these modes in order to determine the dependence of the characteristic times for
their appearance on the basic physical parameters of the two models. 相似文献
593.
594.
Application of an Improved Flow-Stress-Damage Model to the Criticality Assessment of Water Inrush in a Mine: a Case Study 总被引:1,自引:0,他引:1
H. Q. Zhang Y. N. He C. A. Tang Bashir Ahmad L. J. Han 《Rock Mechanics and Rock Engineering》2009,42(6):911-930
This paper presents a case study of water inrush on a mine working face from roof strata. The flow-rock failure process analysis
(F-RFPA2D) code with an improved flow-stress-damage (FSD) model was adopted to perform the failure and coupling analysis. The improved
FSD model was used to represent the permeability variation at the four stages (elastic, damaged, cracked, and crack closure)
of the rock failure process. The fracture initiation, propagation, and coalescence in the stressed strata and the seepage
field evolution in the stress field are represented visually during the whole process of water inrush. The failure zone with
high permeability induced by mining disturbance becomes the water-conducting zone after full excavation of the coal seam.
The height of the water-conducting zone obtained in this study is in reasonable accordance with that predicted by an empirical
formula. It is definitely clarified that the water inrush from the roof strata is induced by the failure zone full of vertical
coalesced mine fractures. It is unlikely that roof accidents resulting in water inrush would occur if the water-conducting
zone does not grow upwards into the aquifer. The main task for mine extraction under a confined aquifer is to locate the aquifer
and find the maximum height of the water-conducting zone, which is very important for mine construction and support design. 相似文献
595.
A finite element method for modeling coupled flow and deformation in porous fractured media 下载免费PDF全文
Ahmad Pouya 《国际地质力学数值与分析法杂志》2015,39(16):1836-1852
Modeling the flow in highly fractured porous media by finite element method (FEM) has met two difficulties: mesh generation for fractured domains and a rigorous formulation of the flow problem accounting for fracture/matrix, fracture/fracture, and fracture/boundary fluid mass exchanges. Based on the recent theoretical progress for mass balance conditions in multifractured porous bodies, the governing equations for coupled flow and deformation in these bodies are first established in this paper. A weak formulation for this problem is then established allowing to build a FEM. Taking benefit from recent development of mesh‐generating tools for fractured media, this weak formulation has been implemented in a numerical code and applied to some typical problems of hydromechanical coupling in fractured porous media. It is shown that in this way, the FEM that has proved its efficiency to model hydromechanical phenomena in porous media is extended with all its performances (calculation time, couplings, and nonlinearities) to fractured porous media. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
596.
597.
598.
Ajay Kalra William P. Miller Kenneth W. Lamb Sajjad Ahmad Thomas Piechota 《水文研究》2013,27(11):1543-1559
In a water‐stressed region, such as the western United States, it is essential to have long lead times for streamflow forecasts used in reservoir operations and water resources management. Current water supply forecasts provide a 3‐month to 6‐month lead time, depending on the time of year. However, there is a growing demand from stakeholders to have forecasts that run lead times of 1 year or more. In this study, a data‐driven model, the support vector machine (SVM) based on the statistical learning theory, was used to predict annual streamflow volume with a 1‐year lead time. Annual average oceanic–atmospheric indices consisting of the Pacific decadal oscillation, North Atlantic oscillation (NAO), Atlantic multidecadal oscillation, El Niño southern oscillation (ENSO), and a new sea surface temperature (SST) data set for the ‘Hondo’ region for the period of 1906–2006 were used to generate annual streamflow volumes for multiple sites in the Gunnison River Basin and San Juan River Basin, both located in the Upper Colorado River Basin. Based on the performance measures, the model showed very good forecasts, and the forecasts were in good agreement with measured streamflow volumes. Inclusion of SST information from the Hondo region improved the model's forecasting ability; in addition, the combination of NAO and Hondo region SST data resulted in the best streamflow forecasts for a 1‐year lead time. The results of the SVM model were found to be better than the feed‐forward, back propagation artificial neural network and multiple linear regression. The results from this study have the potential of providing useful information for the planning and management of water resources within these basins. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
599.
Abstract Based on 16 years of oceanographic and meteorological data the monthly variations of the net heat flux at the air‐sea interface in coastal waters near Jeddah show that the sea gains an average of about 14 ±2 W m?2 from April to October and loses about 79± 4W m?2from November to March. The loss of heat during the winter months is not compensated by the gain during the summer months and therefore leads to an annual average deficit of 25 ± 3 Wm?2. The gain during summer may not favour the formation of a strong seasonal thermocline. 相似文献
600.
Chin Yik Lin Mohd. Harun Abdullah Baba Musta Sarva Mangala Praveena Ahmad Zaharin Aris 《Natural Resources Research》2011,20(1):45-56
A total of 20 soil samples were collected from 10 boreholes constructed in the low lying area, which included ancillary samples
taken from the high elevation area. Redox processes were investigated in the soil as well as groundwater in the shallow groundwater
aquifer of Manukan Island, Sabah, Malaysia. Groundwater samples (n = 10) from each boreholes were also collected in the low lying area to understand the concentrations and behaviors of Fe
and Mn in the dissolved state. This study strives to obtain a general understanding of the stability behaviors on Fe and Mn
at the upper unsaturated and the lower-saturated soil horizons in the low lying area of Manukan Island as these elements usually
play a major role in the redox chemistry of the shallow groundwater. Thermodynamic calculations using PHREEQC showed that
the groundwater samples in the study area are oversaturated with respect to goethite, hematite, Fe(OH)3 and undersaturated with respect to manganite and pyrochroite. Low concentrations of Fe and Mn in the groundwater might be
probably due to the lack of minerals of iron and manganese oxides, which exist in the sandy aquifer. In fact, high organic
matters that present in the unsaturated horizon are believed to be responsible for the high Mn content in the soil. It was
observed that the soil samples collected from high elevation area (BK) comprises considerable amount of Fe in both unsaturated (6675.87 mg/kg) and saturated horizons (31440.49 mg/kg) compared
to the low Fe content in the low lying area. Based on the stability diagram, the groundwater composition lies within the stability
field for Mn2+ and Fe2+ under suboxic condition and very close to the FeS/Fe2+ stability boundary. This study also shows that both pH and Eh values comprise a strong negative value thus suggesting that
the redox potential is inversely dependent on the changes of pH. 相似文献