全文获取类型
收费全文 | 5694篇 |
免费 | 219篇 |
国内免费 | 54篇 |
专业分类
测绘学 | 117篇 |
大气科学 | 476篇 |
地球物理 | 1355篇 |
地质学 | 1904篇 |
海洋学 | 541篇 |
天文学 | 958篇 |
综合类 | 24篇 |
自然地理 | 592篇 |
出版年
2021年 | 71篇 |
2020年 | 68篇 |
2019年 | 85篇 |
2018年 | 105篇 |
2017年 | 101篇 |
2016年 | 150篇 |
2015年 | 154篇 |
2014年 | 147篇 |
2013年 | 264篇 |
2012年 | 167篇 |
2011年 | 267篇 |
2010年 | 199篇 |
2009年 | 294篇 |
2008年 | 245篇 |
2007年 | 222篇 |
2006年 | 233篇 |
2005年 | 211篇 |
2004年 | 186篇 |
2003年 | 171篇 |
2002年 | 171篇 |
2001年 | 92篇 |
2000年 | 120篇 |
1999年 | 92篇 |
1998年 | 100篇 |
1997年 | 72篇 |
1996年 | 69篇 |
1995年 | 97篇 |
1994年 | 92篇 |
1993年 | 70篇 |
1992年 | 77篇 |
1991年 | 55篇 |
1990年 | 82篇 |
1989年 | 76篇 |
1988年 | 70篇 |
1987年 | 76篇 |
1986年 | 65篇 |
1985年 | 86篇 |
1984年 | 107篇 |
1983年 | 82篇 |
1982年 | 82篇 |
1981年 | 68篇 |
1980年 | 83篇 |
1979年 | 75篇 |
1978年 | 70篇 |
1977年 | 51篇 |
1976年 | 59篇 |
1975年 | 61篇 |
1974年 | 45篇 |
1973年 | 65篇 |
1972年 | 27篇 |
排序方式: 共有5967条查询结果,搜索用时 912 毫秒
721.
722.
The topographically explicit distributed hydrology–soil–vegetation model (DHSVM) is used to simulate hydrological effects of changes in land cover for four catchments, ranging from 27 to 1033 km2, within the Columbia River basin. Surface fluxes (stream flow and evapotranspiration) and state variables (soil moisture and snow water equivalent) corresponding to historical (1900) and current (1990) vegetation are compared. In addition a sensitivity analysis, where the catchments are covered entirely by conifers at different maturity stages, was conducted. In general, lower leaf‐area index (LAI) resulted in higher snow water equivalent, more stream flow and less evapotranspiration. Comparisons with the macroscale variable infiltration capacity (VIC) model, which parameterizes, rather than explicitly represents, topographic effects, show that runoff predicted by DHSVM is more sensitive to land‐cover changes than is runoff predicted by VIC. This is explained by model differences in soil parameters and evapotranspiration calculations, and by the more explicit representation of saturation excess in DHSVM and its higher sensitivity to LAI changes in the calculation of evapotranspiration. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
723.
724.
Apparent polar wander in the mean-lithosphere (= no-net-rotation = no-net-torque uniform drag) reference frame is compared with apparent polar wander in the hotspot reference frame over the past 100 Myr. Palaeo-magnetic poles and plate rotations previously used to determine an apparent polar wander path for the hotspot reference frame are here used to determine an apparent polar wander path in the mean-lithosphere reference frame. We find that the two paths are similar, especially for Late Cretaceous time, when a 10°–20° shift of the pole occurred. To first-order the hotspots and lithosphere (as a whole) moved in unison relative to the palaeomagnetic axis during Late Cretaceous time. A non-dipole field explanation for the apparent shift can probably be excluded. However, either motion of the time-averaged geomagnetic axis relative to the spin axis or polar wandering could have caused this shift, the latter being the more likely explanation. 相似文献
725.
Abstract— Environmental conditions on Mars are conducive to the modification and erosion of impact craters, potentially revealing the nature of their substructure. On Earth, postimpact erosion of complex craters in a wide range of target rocks has revealed the nature and distribution of craterrelated fault structures and a complex array of breccia and pseudotachylyte dikes, which range up to tens of meters in width and tens of kilometers in length. We review the characteristics of fault structures, breccia dikes, and pseudotachylyte dikes on Earth, showing that they occur in complex network‐like patterns and are often offset along late‐stage crater‐related faults. Individual faults and dikes can undulate in width and can branch and bifurcate along strike. Detailed geological analyses of terrestrial craters show that faults and breccia dikes form during each of the major stages of the impact‐cratering process (compression, excavation, and modification). We report here on the discovery of prominent, lattice‐like ridge networks occurring on the floor of a highly modified impact crater 75 km in diameter near the dichotomy boundary of the northern lowland and southern upland. Interior fill and crater‐floor units have been exhumed by fluvial and eolian processes to reveal a unit below the crater floor containing a distinctive set of linear ridges of broadly similar width and forming a lattice‐like pattern. Ridge exposures range from ?1–4 km in length and ?65–120 m in width, are broadly parallel, straight to slightly curving, and are cross‐cut by near‐orthogonal ridges, forming a box or lattice‐like pattern. Ridges are exposed on the exhumed crater floor, extending from the base of the wall toward the center. On the basis of the strong similarities of these features to terrestrial crater‐related fault structures and breccia dikes, we interpret these ridges to be faults and breccia dikes formed below the floor of the crater during the excavation and modification stages of the impact event, and subsequently exhumed by erosion. The recognition of such features on Mars will help in documenting the nature of impact‐cratering processes and aid in assessment of crustal structure. Faults and breccia dikes can also be used as data for the assessment of post‐cratering depths and degrees of landform exhumation. 相似文献
726.
727.
James A. Klimchuk 《Solar physics》2006,234(1):41-77
The question of what heats the solar corona remains one of the most important problems in astrophysics. Finding a definitive
solution involves a number of challenging steps, beginning with an identification of the energy source and ending with a prediction
of observable quantities that can be compared directly with actual observations. Critical intermediate steps include realistic
modeling of both the energy release process (the conversion of magnetic stress energy or wave energy into heat) and the response
of the plasma to the heating. A variety of difficult issues must be addressed: highly disparate spatial scales, physical connections
between the corona and lower atmosphere, complex microphysics, and variability and dynamics. Nearly all of the coronal heating
mechanisms that have been proposed produce heating that is impulsive from the perspective of elemental magnetic flux strands.
It is this perspective that must be adopted to understand how the plasma responds and radiates. In our opinion, the most promising
explanation offered so far is Parker's idea of nanoflares occurring in magnetic fields that become tangled by turbulent convection.
Exciting new developments include the identification of the “secondary instability” as the likely mechanism of energy release
and the demonstration that impulsive heating in sub-resolution strands can explain certain observed properties of coronal
loops that are otherwise very difficult to understand. Whatever the detailed mechanism of energy release, it is clear that
some form of magnetic reconnection must be occurring at significant altitudes in the corona (above the magnetic carpet), so
that the tangling does not increase indefinitely. This article outlines the key elements of a comprehensive strategy for solving
the coronal heating problem and warns of obstacles that must be overcome along the way. 相似文献
728.
Amy M. Gaffney Lars E. Borg James N. Connelly 《Geochimica et cosmochimica acta》2007,71(20):5016-5031
Uranium-lead ratios (commonly represented as 238U/204Pb = μ) calculated for the sources of martian basalts preserve a record of petrogenetic processes that were active during early planetary differentiation and formation of martian geochemical reservoirs. To better define the range of μ values represented by the source regions of martian basalts, we completed U-Pb elemental and isotopic analyses on whole rock, mineral and leachate fractions from the martian meteorite Queen Alexandra Range 94201 (QUE 94201). The whole rock and silicate mineral fractions have unradiogenic Pb isotopic compositions that define a narrow range (206Pb/204Pb = 11.16-11.61). In contrast, the Pb isotopic compositions of weak HCl leachates are more variable and radiogenic. The intersection of the QUE 94201 data array with terrestrial Pb in 206Pb/204Pb-207Pb/204Pb-208Pb/204Pb compositional space is consistent with varying amounts of terrestrial contamination in these fractions. We calculate that only 1-7% contamination is present in the purified silicate mineral and whole rock fractions, whereas the HCl leachates contain up to 86% terrestrial Pb. This terrestrial Pb contamination generated a 206Pb-207Pb array in the QUE fractions that appears to represent an ancient age, which contrasts with a much younger crystallization age of 327 ± 10 Ma derived from Rb-Sr and Sm-Nd isochrons (Borg L. E., Nyquist L. E., Taylor L. A., Wiesmann H. and Shih C. -Y. (1997) Constraints on Martian differentiation processes from Rb-Sr and Sm-Nd isotopic analyses of the basaltic shergottite QUE 94201. Geochim. Cosmochim. Acta61, 4915-4931). Despite the contamination, and accepting 327 ± 10 Ma as the crystallization age, we use the U-Pb data to determine the initial 206Pb/204Pb of QUE 94201 to be 11.086 ± 0.008 and to calculate the μ value of its mantle source to be 1.82 ± 0.01. The μ value calculated for the QUE 94201 source is the lowest determined for any martian basalt source, and, when compared to the highest values determined for martian basalt sources, indicates that μ values in martian source reservoirs vary by at least a factor of two. Additionally, the range of source μ values indicates that the μ value of bulk silicate Mars is approximately three. The amount of variation in the μ values of the mantle sources (μ ∼ 2-4) is greater than can be explained by igneous processes involving silicate phases alone. We suggest the possibility that a small amount of sulfide crystallization may generate greater extents of U-Pb fractionation during formation of the mantle sources of martian basalts. 相似文献
729.
The added value of the joint pre-stack inversion of PP (incident P-wave and reflected P-wave) and PS (incident P-wave and reflected S-wave) seismic data for the time-lapse application is shown. We focus on the application of this technique to the time-lapse (four-dimensional) multicomponent Jubarte field permanent reservoir monitoring seismic data. The joint inversion results are less sensitive to noise in the input data and show a better match with the rock physics models calibrated for the field. Further, joint inversion improves S-impedance estimates and provides a more robust quantitative interpretation, allowing enhanced differentiation between pore pressure and fluid saturation changes, which will be extremely useful for reservoir management. Small changes in reservoir properties are expected in the short time between the time-lapse seismic acquisitions used in the Jubarte project (only 1 year apart). The attempt to recover subtle fourth-dimensional effects via elastic inversion is recurrent in reservoir characterization projects, either due to the small sensitivity of the reservoirs to fluid and pressure changes or the short interval between the acquisitions. Therefore, looking for methodologies that minimize the uncertainty of fourth-dimensional inversion outputs is of fundamental importance. Here, we also show the differences between PP only and joint PP–PS inversion workflows and parameterizations that can be applied in other projects. We show the impact of using multicomponent data as input for elastic seismic inversions in the analysis of the time-lapse differences of the elastic properties. The larger investment in the acquisition and processing of multicomponent seismic data is shown to be justified by the improved results from the fourth-dimensional joint inversion. 相似文献
730.
Roy James W. Bordeleau Genevive Rivard Christine Ryan M. Cathryn Malet Xavier Brown Susan J. Tremblay Vincent 《Hydrogeology Journal》2022,30(3):1005-1019
Hydrogeology Journal - Monitoring of dissolved methane concentrations in groundwater is required to identify impacts from oil and gas development and to understand temporal variability under... 相似文献