全文获取类型
收费全文 | 98784篇 |
免费 | 1202篇 |
国内免费 | 1734篇 |
专业分类
测绘学 | 3474篇 |
大气科学 | 7372篇 |
地球物理 | 18865篇 |
地质学 | 37469篇 |
海洋学 | 7737篇 |
天文学 | 19575篇 |
综合类 | 2404篇 |
自然地理 | 4824篇 |
出版年
2022年 | 477篇 |
2021年 | 825篇 |
2020年 | 852篇 |
2019年 | 948篇 |
2018年 | 6457篇 |
2017年 | 5649篇 |
2016年 | 4919篇 |
2015年 | 1621篇 |
2014年 | 2496篇 |
2013年 | 4099篇 |
2012年 | 3385篇 |
2011年 | 5660篇 |
2010年 | 4608篇 |
2009年 | 5680篇 |
2008年 | 4840篇 |
2007年 | 5303篇 |
2006年 | 3072篇 |
2005年 | 2460篇 |
2004年 | 2615篇 |
2003年 | 2537篇 |
2002年 | 2311篇 |
2001年 | 1879篇 |
2000年 | 1787篇 |
1999年 | 1480篇 |
1998年 | 1506篇 |
1997年 | 1395篇 |
1996年 | 1238篇 |
1995年 | 1202篇 |
1994年 | 1073篇 |
1993年 | 924篇 |
1992年 | 878篇 |
1991年 | 890篇 |
1990年 | 939篇 |
1989年 | 790篇 |
1988年 | 774篇 |
1987年 | 892篇 |
1986年 | 764篇 |
1985年 | 956篇 |
1984年 | 1045篇 |
1983年 | 962篇 |
1982年 | 917篇 |
1981年 | 809篇 |
1980年 | 776篇 |
1979年 | 712篇 |
1978年 | 706篇 |
1977年 | 622篇 |
1976年 | 566篇 |
1975年 | 553篇 |
1974年 | 550篇 |
1973年 | 601篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
241.
Wiener optimal filtering of GRACE data 总被引:4,自引:0,他引:4
We present a spatial averaging method for Gravity Recovery and Climate Experiment (GRACE) gravity-field solutions based on
the Wiener optimal filtering.
The optimal filter is designed from the least-square minimization of the difference between the desired and filtered signals.
It requires information about the power spectra of the desired gravitational signal and the contaminating noise, which is
inferred from the average GRACE degree-power spectrum. We show that the signal decreases with increasing spherical harmonic
degree j with approximately j−b, where b = 1.5 for GRACE data investigations. This is termed the Second Kaula rule of thumb for temporal variations of the
Earth’s gravity field. The degree power of the noise increases, in the logarithmic scale, linearly with increasing j.
The Wiener optimal filter obtained for the signal model with b = 1.5 closely corresponds to a Gaussian filter with a spatial
half width of 4° (∼440 km). We find that the filtered GRACE gravity signal is relatively insensitive to the exponent b of
the signal model, which indicates the robustness of Wiener optimal filtering. This is demonstrated using the GFZ-GRACE gravity-field
solution for April 2004. 相似文献
242.
Oxygen and hydrogen isotope fractionation factors in the talc-water and serpentine-water systems have been determined by laboratory experiment from 250 to 450 °C at 50 MPa using the partial exchange technique. Talc was synthesized from brucite + quartz, resulting in nearly 100% exchange during reaction at 350 and 450 °C. For serpentine, D-H exchange was much more rapid than 18O-16O exchange when natural chrysotile fibers were employed in the initial charge. In experiments with lizardite as the starting charge, recrystallization to chrysotile enhanced the rate of 18O-16O exchange with the coexisting aqueous phase. Oxygen isotope fractionation factors in both the talc-water and serpentine-water systems decrease with increasing temperature and can be described from 250 to 450 °C by the relationships: 1000 ln = 11.70 × 106/T2 − 25.49 × 103/T + 12.48 and 1000 ln = 3.49 × 106/T2 − 9.48 where T is temperature in Kelvin. Over the same temperature interval at 50 MPa, talc-water D-H fractionation is only weakly dependent on temperature, similar to brucite and chlorite, and can be described by the equation: 1000 ln = 10.88 × 106/T2 − 41.52 × 103/T + 5.61 where T is temperature in Kelvin. Our D-H serpentine-water fractionation factors calibrated by experiment decrease with temperature and form a consistent trend with fractionation factors derived from lower temperature field calibrations. By regression of these data, we have refined and extended the D-H fractionation curve from 25 to 450 °C, 50 MPa as follows: 1000 ln = 3.436 × 106/T2 − 34.736 × 103/T + 21.67 where T is temperature in Kelvin. These new data should improve the application of D-H and 18O-16O isotopes to constrain the temperature and origin of hydrothermal fluids responsible for serpentine formation in a variety of geologic settings. 相似文献
243.
The circulation and salinity distribution in the Hooghly Estuary have been studied by developing a two‐dimensional depth‐averaged numerical model for the lower estuary, where the flow is vertically well mixed. This has been coupled with a one‐dimensional model for the upper estuary, where the flow is assumed to be unidirectional and well mixed over the depth and breadth. The Hooghly River receives high freshwater discharge during the monsoon season (June to September), which has significant effect on the salinity distribution in the estuary. The model‐simulated currents, elevations, and salinities are in good agreement with observations during the dry season. However, during the wet season the computed salinities seem to deviate slightly from the observed values. 相似文献
244.
245.
A critical examination of the components of the recent impulsive loop flare model of Takakura is made. It is found that his analysis of the stability of the electron distribution resulting from anomalous heat conduction is in error and electron plasma waves would not be excited. Rather, in the regions where the electron/proton temperature ratioT
e/T
i 10, electrostatic ion-cyclotron waves would be excited and in the regions whereT
e 10, ion-acoustic waves would be excited. Ratios ofT
e/T
i 10 occur only in the late time development behind the conduction fronts. Since the anomalous resistivity due to electrostatic ion-cyclotron waves is fortuitously about 70% of the one used by Takakura, the general development will follow closely the one calculated by him. Because the anomalous resistivity due to ion-acoustic waves is about 95 times the one used by Takakura, the development in the parts of the loop whereT
e/T
i 10 for late times would be altered considerably.Also Guest Worker at NOAA Space Environment Laboratory, Boulder, Colorado, U.S.A. 相似文献
246.
R. A. SHAKESBY C. J. CHAFER S. H. DOERR W. H. BLAKE P. WALLBRINK G. S. HUMPHREYS 《The Australian geographer》2003,34(2):147-175
Soil water repellency can enhance overland flow and erosion and may be altered by fire. The Christmas 2001 bushfires near Sydney allowed investigation of the relationship between fire severity, water repellency and hydrogeomorphological changes. For two sub-catchments with differences in fire severities in Nattai National Park, south-west of Sydney, this paper considers: (1) the links between fire severity based on SPOT image analysis and ground observation of fire severity and repellency; (2) the textural and organic/minerogenic characteristics of eroded sediment; and (3) erodibility, erosion and deposition of soils in both catchments. Ground surveys show that image analysis reflects well the degree of vegetation consumption by fire, but cannot adequately predict the degree of ground litter consumption, associated soil heating and repellency effects. Fire had varying effects on repellency, leaving it unchanged, destroying it or enhancing it, depending on the soil temperature reached. The main post-fire hydrogeomorphological changes have been widespread erosion and colluvial and alluvial deposition of topsoil in foot-slope locations and river systems, but only localised redistribution of the highly erodible, repellent sandy subsurface layer. The fire did not trigger major geomorphological change in the study area, but fires probably cause important topsoil and nutrient depletion and may also affect water quality. 相似文献
247.
This study analyzed the uncertainty of inversion and the resolution limit in the presence of noise by means of statistical experiments. The exhaustive method is adopted to obtain the global optimal solution in each experiment. We found that even with small level of noise, solutions fluctuate in a large range for the thin bed. The distribution of solutions in the presence of noise is closely related to the spread of the cost function in the absence of noise. As a result, the area of a certain neighborhood around the true solution on the spread of the cost function in the absence of noise is used to evaluate the uncertainty of inversion and the resolution limit in the presence of noise. In the case that the SNR (signal-to-noise ratio) is 5 in this study, solutions focus around the true solution with a very small uncertainty only when the bed thickness is greater than the reciprocal of the double predominant frequency of the convoluting wavelet. 相似文献
248.
249.
Three finite element codes, namely TELEMAC, ADCIRC and QUODDY, are used to compute the spatial distributions of the M2, M4 and M6 components of the tide in the sea region off the west coast of Britain. This region is chosen because there is an accurate
topographic dataset in the area and detailed open boundary M2 tidal forcing for driving the model. In addition, accurate solutions (based upon comparisons with extensive observations)
using uniform grid finite difference models forced with these open boundary data exist for comparison purposes. By using boundary
forcing, bottom topography and bottom drag coefficients identical to those used in an earlier finite difference model, there
is no danger of comparing finite element solutions for “untuned unoptimised solutions” with those from a “tuned optimised
solution”. In addition, by placing the open boundary in all finite element calculations at the same location as that used
in a previous finite difference model and using the same M2 tidal boundary forcing and water depths, a like with like comparison of solutions derived with the various finite element
models was possible. In addition, this open boundary was well removed from the shallow water region, namely the eastern Irish
Sea where the higher harmonics were generated. Since these are not included in the open boundary, forcing their generation
was determined by physical processes within the models. Consequently, an inter-comparison of these higher harmonics generated
by the various finite element codes gives some indication of the degree of variability in the solution particularly in coastal
regions from one finite element model to another. Initial calculations using high-resolution near-shore topography in the
eastern Irish Sea and including “wetting and drying” showed that M2 tidal amplitudes and phases in the region computed with TELEMAC were in good agreement with observations. The ADCIRC code
gave amplitudes about 30 cm lower and phases about 8° higher. For the M4 tide, in the eastern Irish Sea amplitudes computed with TELEMAC were about 4 cm higher than ADCIRC on average, with phase
differences of order 5°. For the M6 component, amplitudes and phases showed significant small-scale variability in the eastern Irish Sea, and no clear bias between
the models could be found. Although setting a minimum water depth of 5 m in the near-shore region, hence removing wetting
and drying, reduced the small-scale variability in the models, the differences in M2 and M4 tide between models remained. For M6, a significant reduction in variability occurred in the eastern Irish Sea when a minimum 5-m water depth was specified. In
this case, TELEMAC gave amplitudes that were 1 cm higher and phases 30° lower than ADCIRC on average. For QUODDY in the eastern
Irish Sea, average M2 tidal amplitudes were about 10 cm higher and phase 8° higher than those computed with TELEMAC. For M4, amplitudes were approximately 2 cm higher with phases of order 15° higher in the northern part of the region and 15° lower
in the southern part. For M6 in the north of the region, amplitudes were 2 cm higher and about 2 cm lower in the south. Very rapid M6 tidal-phase changes occurred in the near-shore regions. The lessons learned from this model inter-comparison study are summarised
in the final section of the paper. In addition, the problems of performing a detailed model–model inter-comparison are discussed,
as are the enormous difficulties of conducting a true model skill assessment that would require detailed measurements of tidal
boundary forcing, near-shore topography and precise knowledge of bed types and bed forms. Such data are at present not available. 相似文献
250.
Doklady Earth Sciences - Physical simulation of how a large shear zone forms in terms of an elastic-viscoplastic model of the lithosphere is conducted, and computer processing of the obtained photo... 相似文献