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971.
Jerry C. Ritchie Charles M. Cooper J. Roger McHenry Frank R. Schiebe 《Environmental Geology》1983,5(2):79-82
Lake Chicot is an oxbow lake located along the western side of the Mississippi River in southeastern Arkansas. A major flood
in 1927, levee construction, land use changes from bottomland hardwood to agriculture, a large increase in drainage area,
and stream channelization have altered the appearance of contributing watersheds and the lake. The lake often has high suspended
sediment concentrations making it undesirable for recreation and aquatic production. As part of a coordinated study to determine
changes in the lake, sediment accumulation patterns and rates were determined in Lake Chicot using the137Cs technique. Major sediment accumulation is occurring near the major inlet and along the thalweg of the oxbow. Average sediment
accumulation of 1.8 cm/yr since 1963 was measured for twelve samples. Calculations show sediment accumulation is decreasing.
This study has shown that the137Cs technique can provide useful planning information for the environmental geologist or reservoir manager. Results of the
current study will be used to develop better sediment and water management strategies for Lake Chicot.
Contribution of the U.S. Department of Agriculture, Agricultural Research Service, in cooperation with the Vicksburg District
of the U.S. Army Corps of Engineers. 相似文献
972.
Various lines of evidence point to the lower crust as the source of the long-wavelength magnetic anomaly field measured by the POGO and Magsat satellites. Using seismically determined lower crust thicknesses and equivalent source inversion of the satellite anomaly data, magnetization for the lower crust for much of the United States has been calculated. The average magnetization for two hundred sixty-six 150 × 150 km areas is 3.5 A/m with a standard deviation of 1.1 A/m. These values are consistent with laboratory measurements of mafic-ultramafic rocks expected in the lower crust, and in agreement with previous estimates of lower crust magnetization based on long-wavelength aeromagnetic data. Average lower crust thickness for the same areas is 18.2 km (σ = 6.4 km). Thus, over large regions, it appears that variation in magnetization and variation in magnetic layer thickness contribute almost equally in causing the anomaly field variation at satellite altitude. 相似文献
973.
Brett S. Ketter Aaron A. Velasco Charles J. Ammon George E. Randall 《Pure and Applied Geophysics》2006,163(7):1235-1255
We develop one-dimensional (1-D) path-specific velocity models in western China using new Rayleigh and Love wave group and
phase velocity dispersion measurements for 20 events in the region. The earthquakes were grouped into three geographic clusters
from which we compute the average phase and group velocity dispersion. We invert the average dispersion curves simultaneously
for 1-D shear-velocity models appropriate for the three central Asian paths, using three previous shear-velocity models as
initial models. The models are validated by forward modeling waveforms of recent events. The crustal thickness beneath western
China in the vicinity of the Lop Nor test site is 50–60 km and our velocity models are consistent with major geologic features
(e.g., basins and mountain ranges) and previous structural models for this region. 相似文献
974.
This article explores the dimensions of rapid urbanization in the Phoenix Standard Metropolitan Statistical Area (SMSA) over the past thirty years with respect to land use change. We devote primary attention to developing an understanding of what land transformations took place, the extent to which they occurred, and where they occurred. Our findings indicate that 32 percent of the Phoenix SMSA changed between 1970 and 2000. More than half of the overall change was from agriculture to some form of urban land use, and although a large percentage of the region remains open desert the parcels of desert are increasingly fragmented. This has significant implications for urban ecology and biodiversity. The growth indicates that rather than a pattern that reflects the agglomeration effects of a polycentric metropolis, the central business district of the City of Phoenix dominates the region. This has implications with respect to employment patterns, traffic congestion, and urban air quality and climate. 相似文献
975.
Colin E. Thorn Robert G. Darmody Sean W. Campbell Charles E. Allen John C. Dixon 《地球表面变化过程与地形》2007,32(14):2199-2211
The initial stages of cobble weathering, measured as increasing percentage porosity, were calculated for sets of cobbles taken from the foreland of Storbreen, a cirque glacier in the Jotunheimen, Norway. Cobbles were taken from in front of the 1998 glacier snout, from the 1928, 1870, 1810 and 1750 moraine crests and from the ~10 000‐year‐old land surface beyond the Neoglacial foreland limit. Three microenvironments were examined within each site: (1) lichen‐free surfaces from exposed cobbles, (2) lichen‐covered surfaces from exposed cobbles and (3) buried cobbles taken from a soil depth of ~40 cm. Percentage porosity within plagioclase minerals was calculated using backscatter electron images of prepared thin sections. Porosity was calculated from five depth profiles into each cobble. Depth profiles were subdivided into a sequence of 50 µm × 50 µm rectangles running to at least a nominal depth of 500 µm within each cobble. Three cobbles from each dated land surface were sampled, except for the 1750 surface, which had five cobbles. Statistical analysis was by analysis of variance of rank‐order transformed data. Findings indicate that cobbles close to the glacier snout are largely unweathered; also, weathering is generally weak in the 1928, 1870 and 1810 positions, but statistically significantly higher in the 1750‐ and 10 000‐year‐old positions. Weathering of buried cobbles always exceeded weathering of exposed cobbles and may possibly reach a value beyond which it cannot progress while retaining surface cohesion. The degree of weathering on lichen‐free and lichen‐covered cobble surfaces is not initially distinguishable, but diverges sharply after ~250 years, when lichen‐covered surfaces experience significantly higher totals. Overall, the weathering trend in cobbles matches that found in soils at the same sites. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
976.
977.
The decay of a plasmon into two neutrinos in the presence of an intense magnetic field has been studied by Canutoet al. (1970). They suggest that one of the principal longitudinal plasmon modes, which occurs only in magnetized plasmas, would cause certain magnetic stars to cool more rapidly than their unmagnetized counterparts. We show here that this mechanism is inoperative since the plasmon mode involved cannot be excited in the direction parallel to the magnetic field as considered by Canutoet al. Moreover, for ωc/ωp?1, we show that the other principal longitudinal plasmon mode considered earlier by Adamset al. (1963) (which is largely independent of the magnetic field) dominates the plasmon-neutrino decay cooling of magnetic stars. 相似文献
978.
979.
Xuyong Li Donald E. Weller Charles L. Gallegos Thomas E. Jordan Hae-Cheol Kim 《Estuaries and Coasts》2007,30(5):840-854
Watershed land use can affect submerged aquatic vegetation (SAV) by elevating nutrient and sediment loading to estuaries.
We analyzed the effects of watershed use and estuarine characteristics on the spatial variation of SAV abundance among 101
shallow subestuaries of Chesapeake Bay during 1984–2003. Areas of these subestuaries range from 0.1 to 101 km2, and their associated local watershed areas range from 6 to 1664 km2. Watershed land cover ranges from 6% to 81% forest, 1% to 64% cropland, 2% to 38% grassland, and 0.3% to 89% developed land.
Landscape analyses were applied to develop a number of subestuary metrics (such as subestuary area, mouth width, elongation
ratio, fractal dimension of shoreline, and the ratio of local watershed area to subestuary area) and watershed metrics (such
as watershed area). Using mapped data from aerial SAV surveys, we calculated SAV coverage for each subestuary in each year
during 1984–2003 as a proportion of potential SAV habitat (the area < 2 m deep). The variation in SAV abundance among subestuaries
was strongly linked with subestuary and watershed characteristics. A regression tree model indicated that 60% of the variance
in SAV abundance could be explained by subestuary fractal dimension, mean tidal range, local watershed dominant land cover,
watershed to subestuary area ratio, and mean wave height. Similar explanatory powers were found in wet and dry years, but
different independent variables were used. Repeated measures ANOVA with multiple-mean comparison showed that SAV abundance
declined with the dominant watershed land cover in the order: forested, mixed-undisturbed, or mixed-developed > mixed-agricultural
> agricultural > developed. Change-point analyses indicated strong threshold responses of SAV abundance to point source total
nitrogen and phosphorus inputs, the ratio of local watershed area to subestuary area, and septic system density in the local
watershed. 相似文献
980.
Fluid inclusions and geological relationships indicate that rodingite formation in the Asbestos ophiolite, Québec, occurred
in two, or possibly three, separate episodes during thrusting of the ophiolite onto the Laurentian margin, and that it involved
three fluids. The first episode of rodingitization, which affected diorite, occurred at temperatures of between 290 and 360°C
and pressures of 2.5 to 4.5 kbar, and the second episode, which affected granite and slate, occurred at temperatures of between
325 and 400°C and pressures less than 3 kbar. The fluids responsible for these episodes of alteration were moderately to strongly
saline (~1.5 to 6.3 m eq. NaCl), rich in divalent cations and contained appreciable methane. A possible third episode of alteration is suggested
by primary fluid inclusions in vesuvianite-rich bodies and secondary inclusions in other types of rodingite, with significantly
lower trapping temperatures, salinity and methane content. The association of the aqueous fluids with hydrocarbon-rich fluids
containing CH4 and higher order alkanes, but no CO2, suggests strongly that the former originated from the serpentinites. The similarities in the composition of the fluids in
all rock types indicate that the ophiolite had already been thrust onto the slates when rodingitization occurred. 相似文献