全文获取类型
收费全文 | 716篇 |
免费 | 44篇 |
国内免费 | 8篇 |
专业分类
测绘学 | 21篇 |
大气科学 | 57篇 |
地球物理 | 255篇 |
地质学 | 245篇 |
海洋学 | 62篇 |
天文学 | 74篇 |
综合类 | 1篇 |
自然地理 | 53篇 |
出版年
2024年 | 2篇 |
2023年 | 6篇 |
2021年 | 18篇 |
2020年 | 10篇 |
2019年 | 15篇 |
2018年 | 32篇 |
2017年 | 23篇 |
2016年 | 23篇 |
2015年 | 24篇 |
2014年 | 29篇 |
2013年 | 51篇 |
2012年 | 32篇 |
2011年 | 36篇 |
2010年 | 48篇 |
2009年 | 42篇 |
2008年 | 39篇 |
2007年 | 40篇 |
2006年 | 30篇 |
2005年 | 30篇 |
2004年 | 29篇 |
2003年 | 32篇 |
2002年 | 23篇 |
2001年 | 14篇 |
2000年 | 12篇 |
1999年 | 12篇 |
1998年 | 11篇 |
1997年 | 7篇 |
1996年 | 8篇 |
1995年 | 4篇 |
1994年 | 3篇 |
1992年 | 4篇 |
1990年 | 3篇 |
1989年 | 5篇 |
1988年 | 2篇 |
1987年 | 10篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 5篇 |
1982年 | 2篇 |
1981年 | 5篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1974年 | 4篇 |
1973年 | 2篇 |
1971年 | 2篇 |
排序方式: 共有768条查询结果,搜索用时 15 毫秒
111.
We have studied the sublimation of ice and water vapor transport through various thicknesses of clay (<63 μm grain size). We experimentally demonstrate that both adsorption and diffusion strongly affect the transport of water, and that the processes of diffusion and adsorption can be separately quantified once the system comes to a steady state. At shallow depths of clay, water vapor transport is determined by diffusion through both the atmosphere and the clay layer, whereas at greater depth the rate of sublimation of the ice is governed only by diffusion through the clay. Using two different models, we determine the diffusion coefficient for water vapor through unconsolidated clay layer to be 1.08±0.04×10−4 and . We also determined the adsorption isotherms for the clay layer, which follow the Langmuir theory at low water vapor pressure (<100 Pa, where a monolayer of water molecules forms on the surface of the clay) and the BET theory at higher pressure (where multiple water layers form). From our analysis of both types of isotherms we determined the adsorption constants to be and c=30±10, respectively, and specific surface areas of 1.10±0.2×105 and , respectively. Finally, we report a theoretical kinetic model for the simultaneous diffusion and adsorption from which we determine adsorption kinetic constants according to the Langmuir theory of and . If the martian regolith possesses diffusive properties similar to those of the unconsolidated montmorillonite soil we investigated here, it would not represent a significant barrier to the sublimation of subsurface ice. However, at the low subsurface temperatures of high latitude (180 K on average), ice could survive from the last glaciation period (about 300 to 400,000 years ago). Higher subsurface temperatures in the equatorial regions would prevent long-timescale survival of ice in the shallow subsurface. In agreement with previous work, we show that adsorption of water by a clay regolith could provide a significant reservoir of subsurface water and it might account for the purported diurnal cycle in the water content of the atmosphere. 相似文献
112.
113.
Sources and history of heavy metal contamination and sediment deposition in Tivoli South Bay, Hudson River, New York 总被引:1,自引:0,他引:1
Gaboury Benoit Edward X. Wang William C. Nieder Michael Levandowsky Vincent T. Breslin 《Estuaries and Coasts》1999,22(2):167-178
Persistent inorganic constitutents preserved in sediments of aquatic ecosystems record temporal variability of biogeochemical
functioning and anthropogenic impacts.210Pb and137Cs dating techniques were used to study the past variations of heavy metals (Pb, Cu, and Zn) and accumulation rates of sediments
for Tivoli South Bay, in the Hudson River National Estuarine Research Reserve ecosystem. South Bay, a tidal freshwater embayment
of the Hudson, may play an important role in the sediment dynamics of this important river. The measured sedimentation rate
range of 0.59 to 2.92 cm yr−1 suggests that rapid accumulation occurred during the time period represented by the length of the cores (approximately the
past 50 yr). Direct measurements of sediment exchange with the Hudson River reveal high variability in the sediment flux from
one tidal cycle to the next. Net exchange does not seem to be adequate to explain sediment accumulation rates in the bay as
measured by210Pb and137Cs. The difference may be supplied from upland streams or the Hudson River during storm events. Concentrations of the metals
Pb, Cu and Zn were found to be well correlated with each other within individual cores at five of six sites tested. This suggests
a common proximate source for the three metals at a specific site. The evidence is consistent with mixing in some environmental
compartment before delivery to the bay. While metals self-correlate within individual cores, absolute concentrations, depth
distribution patterns, and ratios of the metals to each other vary among the cores collected at different locations within
the bay. Organic matter, Fe content, and particle size distribution of sediments do not account for the intercore variations
in metal concentration. It is likely that cores collected from different sites may have derived metals from different sources,
such as watershed streams and tidal exchange with the Hudson River. 相似文献
114.
Clara Lemos France Floc’h Marissa Yates Nicolas Le Dantec Vincent Marieu Klervi Hamon Véronique Cuq Serge Suanez Christophe Delacourt 《Ocean Dynamics》2018,68(9):1207-1220
Eleven-year long time series of monthly beach profile surveys and hourly incident wave conditions are analyzed for a macrotidal Low Tide Terrace beach. The lower intertidal zone of the beach has a pluriannual cycle, whereas the upper beach profile has a predominantly seasonal cycle. An equilibrium model is applied to study the variation of the contour elevation positions in the intertidal zone as a function of the wave energy, wave power, and water level. When forcing the model with wave energy, the predictive ability of the equilibrium model is around 60% in the upper intertidal zone but decreases to 40% in the lower intertidal zone. Using wave power increases the predictive ability up to 70% in both the upper and lower intertidal zones. However, changes around the inflection point are not well predicted. The equilibrium model is then extended to take into account the effects of the tide level. The initial results do not show an increase in the predictive capacity of the model, but do allow the model free parameters to represent more accurately the values expected in a macrotidal environment. This allows comparing the empirical model calibration in different tidal environment. The interpretation of the model free parameter variation across the intertidal zone highlights the behavior of the different zones along the intertidal beach profile. This contributes to a global interpretation of the four model parameters for beaches with different tidal ranges, and therefore to a global model applicable at a wide variety sites. 相似文献
115.
Analytic wave series solution of out-of-plane(SH) waves diffraction by an almost semi-circular shallow cylindrical hill 下载免费PDF全文
A closed-form wave equation analytic solution of two-dimensional scattering and diffraction of outof-plane(SH) waves by an almost semi-circular shallow cylindrical hill on a flat, elastic and homogeneous half space is proposed by applying the discrete Fourier series expansions of sine and cosine functions. The semi-circular hill problem is discussed as a special case for the new formulated equation.Compared with the previous semi-circular cases solutions, the present method can give surface displacement amplitudes which agrees well with previous results. Although the proposed equation can only solve the problem of SH-waves diffracted by almost semi-circular shallow hills, the stress and displacement residual amplitudes are numerical insignificantly everywhere. Moreover, the influences of the depth-towidth ratio(a parameter defined in this paper to evaluate the shallowness of the topography of hills) on ground motions are presented and summarized. The limitations and errors of truncation from Graf's addition theorem and Fourier series equations in the present paper are also discussed. 相似文献
116.
Jean‐Michel Brazier Anne‐Dsire Schmitt Eric Pelt Damien Lemarchand Sophie Gangloff Tho Tacail Vincent Balter 《Geostandards and Geoanalytical Research》2020,44(2):331-348
This work presents new 87Sr/86Sr and δ88/86SrSRM987 isotopic values of thirteen mineral, vegetal and animal reference materials. Except for UB‐N, all our results are consistent with previously published data. Our results highlight intermediate precisions among the best presently published and a non‐significant systematic shift with the calculated δ88/86SrSRM987 mean values for the three most analysed reference materials in the literature (i.e., IAPSO, BCR‐2 and JCp‐1). By comparison with the literature and between two distinct digestions, a significant bias of δ88/86SrSRM987 values was highlighted for two reference materials (UB‐N and GS‐N). It has also been shown that digestion protocols (nitric and multi‐acid) have a moderate impact on the δ88/86SrSRM987 isotopic values for the Jls‐1 reference materials suggesting that a nitric acid digestion of carbonate can be used without significant bias from partial digestion of non‐carbonate impurities. Different δ88/86SrSRM987 values were measured after two independent Sr/matrix separations, according to the same protocol, for a fat‐rich organic reference material (BCR‐380R) and have been related to a potential post‐digestion heterogeneity. Finally, the δ88/86SrSRM987 value differences measured between animal‐vegetal and between coral‐seawater reference materials agree with the previously published results, highlighting an Sr isotopic fractionation along the trophic chain and during carbonate precipitation. 相似文献
117.
Mustafa Al-Marzooq Mamadou S. Diallo Vincent Etienne Thierry Tonellot SanLinn I. Kaka 《Geophysical Prospecting》2019,67(7):1825-1837
The Rub Al Khali region in Saudi Arabia is characterized by the presence of sand dunes separated by salt flats, also called Sabkhas. In general, the elevation of dunes in this region varies between 90 and 250 m above sea level. The presence of these sand dunes, along with the rapidly changing surface topography poses challenges for seismic data acquisition and processing. The high contrast in acoustic impedance between the dune base and the underlying formation often results in amplification of seismic waves that are recorded at stations located on the surface of sand dunes. Attempts to address the issue using conventional surface-consistent amplitude scaling methods without reducing these amplification effects generally fail, thus compromising the suitability of the processed data for amplitude versus offset analysis. In this study, we propose a new reference site technique to reduce the effects of sand dune amplification, enabling the production of data sets that are suitable for amplitude versus offset processing. The proposed technique uses a deterministic approach to derive surface-consistent, frequency-dependent de-amplification functions for shots and stations located on the dunes only. Two-dimensional synthetic and field data examples show that the technique significantly reduces the effects of sand dune amplification. 相似文献
118.
Development of the ideas about the equilibrium between freshwater and saline water has received considerable attention in the literature, but little has been written so far about the earliest scientific works about well salinization. Based on a review of the literature from the second half of the 19th century and the early 20th century, this historical note explores how insights into groundwater abstraction and saltwater intrusion developed, and examples of the earliest field studies are provided. Fundamental research was driven by the need for increasing water supply, but the progress of science did not lead to sustainable management practices everywhere. Research outcomes were shared between scientists of different countries, marking the beginning of coastal hydrogeology as a scientific specialization in the first decade of the 20th century. 相似文献
119.
This paper aims to assess MODFLOW and MT3D capabilities for simulating the spread of contaminants from a river exhibiting an unusual relationship with an alluvial aquifer, with the groundwater head higher than the river head on one side and lower on the other (flow‐through stream). A series of simulation tests is conducted using a simple hypothetical model so as to characterize and quantify these limitations. Simulation results show that the expected contaminant spread could be achieved with a specific configuration composed of two sets of parameters: (1) modeled object parameters (hydraulic groundwater gradient, hydraulic conductivity values of aquifer and streambed), and (2) modeling parameters (vertical discretization of aquifer, horizontal refinement of stream modeled with River [RIV] package). The influence of these various parameters on simulation results is investigated, and potential complications and errors are identified. Contaminant spread from stream to aquifer is not always reproduced by MT3D due to the RIV package's inability to simulate lateral exchange fluxes between stream and aquifer. This paper identifies the need for a MODFLOW streamflow package allowing lateral stream‐aquifer interactions and streamflow routine calculations. Such developments could be of particular interest for modeling contaminated flow‐through streams. 相似文献
120.
Small‐scale hyporheic zone (HZ) models often use a spatial periodic boundary (SPB) pair to simulate an infinite repetition of bedforms. SPB's are common features of commercially available multiphysics modeling packages. MODFLOW's lack of this boundary type has precluded it from being effectively utilized in this area of HZ research. We present a method to implement the SPB in MODFLOW by development of the appropriate block‐centered finite‐difference expressions. The implementation is analogous to MODFLOW's general head boundary package. The difference is that the terms on the right hand side of the solution equations must be updated with each iteration. Consequently, models that implement the SPB converge best with solvers that perform both inner and outer iterations. The correct functioning of the SPB condition in MODFLOW is verified by two examples. This boundary condition allows users to build HZ‐bedform models in MODFLOW, facilitating further research using related codes such as MT3DMS and PHT3D. 相似文献