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281.
Synergistic effect of calcium and bicarbonate in enhancing arsenate release from ferrihydrite 总被引:1,自引:0,他引:1
Many groundwater systems contain anomalously high arsenic concentrations, associated with less than expected retention of As by adsorption to iron (hydr)oxides. Although carbonates are ubiquitous in aquifers, their relationship to arsenate mobilization is not well characterized. This research examines arsenate release from poorly crystalline iron hydroxides in abiotic systems containing calcium and magnesium with bicarbonate under conditions of static and dynamic flow (pH 7.5-8). Aqueous arsenic levels remained low when arsenate-bearing ferrihydrite was equilibrated with artificial groundwater solution containing Ca, Mg, and HCO3−. In batch titrations in which a solution of Ca and HCO3− was added repeatedly, the ferrihydrite surface became saturated with adsorbed Ca and HCO3−, and aqueous As levels increased by 1-2 orders of magnitude. In columns containing Ca or Mg and HCO3−, As solubility initially mimicked titrations, but then rapidly increased by an additional order of magnitude (reaching 12 μM As). Separately, calcium chloride and other simple salts did not induce As release, although sodium bicarbonate and lactate facilitated minor As release under flow. Results indicate that adsorption of calcium or magnesium with bicarbonate leads to As desorption from ferrihydrite, to a degree greater than expected from competitive effects alone, especially under dynamic flow. This desorption may be an important mechanism of As mobilization in As-impacted, circumneutral aquifers, especially those undergoing rapid mineralization of organic matter, which induces calcite dissolution and the production of dissolved calcium and bicarbonate. 相似文献
282.
Two unrelated data sources (quantitative secondary data and qualitative primary data) and mixed methodologies (statistical analysis and ethnography) are used to define the concept of, and develop indicators for, fishing dependence. Techniques for integrating differing data sources are developed. Comparisons of the qualitative rankings with the quantitative rankings were, overall, positive and statistically significant. The process used thus confirmed that the indicators were reliable measures for fishing dependence. In terms of external validity and triangulation, the process used was more rigorous than using ethnography “after-the-fact” to ground-truth the quantitative indicators. 相似文献
283.
The mass redistribution in the earth as a result of an earthquake faulting changes the earth's inertia tensor, and hence its rotation. Using the complete formulae developed byChao andGross (1987) based on the normal mode theory, we calculated the earthquake-induced polar motion excitation for the largest 11,015 earthquakes that occurred during 1977.0–1993.6. The seismic excitations in this period are found to be two orders of magnitude below the detection threshold even with today's high precision earth rotation measurements. However, it was calculated that an earthquake of only one tenth the size of the great 1960 Chile event, if happened today, could be comfortably detected in polar motion observations. Furthermore, collectively these seismic excitations have a strong statistical tendency to nudge the pole towards 140°E, away from the actually observed polar drift direction. This non-random behavior, similarly found in other earthquake-induced changes in earth rotation and low-degree gravitational field byChao andGross (1987), manifests some geodynamic behavior yet to be explored. 相似文献
284.
Burkhard O. Dressler Virgil L. Sharpton Benjamin C. Schuraytz 《Contributions to Mineralogy and Petrology》1998,130(3-4):275-287
The Slate Islands archipelago is believed to represent the central uplifted portion of a complex impact structure. Planar
microstructures in quartz and feldspars and shock vitrification of rocks are the most common shock metamorphic features encountered.
No diaplectic quartz was identified in the exposed rocks, but minor maskelynite is present. Shatter cones occur on all islands
of the archipelago suggesting minimum pressures of 4 ± 2 GPa. The relative frequency of low index planar microstructures of
specific, optically determined crystallographic orientations in quartz are correlated with results from shock barometric experiments
to estimate peak shock pressures experienced by the exposed target rocks. In general, there is a decrease in shock pressure
recorded in the target rocks from about 20–25 GPa in east-central Patterson Island to about 5–10 GPa at the western shore
of this island and on Mortimer Island. The shock attenuation gradient is ∼4.5 GPa/km across this section of the island group.
However, the shock attenuation has a roughly concentric plan only over the western part of the archipelago. There is no distinct
shock center and there are other deviations from circularity. This is probably the result of: (1) the shock wave not having
expanded from a point or spherical source because of the ∼1. 0 to 1.5 km size of the impactor; (2) differential movement of
large target rock blocks during the central uplift and crater modification phases of the impact process. The orientation of
planar deformation features in quartz appears to be independent of the shock wave direction suggesting that crystal structure
exerts the primary control on microstructure development. Based on the results of XRD analyses, residual, post-impact temperatures
were high enough to cause annealing of submicroscopic damage in shocked quartz.
Received: 15 July 1997 / Accepted: 6 October 1997 相似文献
285.
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287.
Computing satellite velocity using the broadcast ephemeris 总被引:1,自引:0,他引:1
288.
289.
Operational Altimeter Data Processing for Mesoscale Monitoring 总被引:1,自引:0,他引:1
Since 1996, global, near-real-time maps of mesoscale anomalies derived from tandem sampling provided by altimeters aboard the TOPEX/Poseidon and ERS-2 satellites have been posted on web pages hosted at the Colorado Center for Astrodynamics Research. The original, near-real-time processing system was based on a quick-look analysis that referenced the data to a high-resolution gridded mean sea surface available at the time. Recently, state-of-the-art mean sea surfaces have been derived that are based on a more complete record of altimeter observations. An updated mesoscale monitoring system based on a new mean surface is described and shown to provide improved mesoscale monitoring to the successful system implemented in 1996. 相似文献
290.
Rationalizing tax increment financing in Chicago 总被引:1,自引:1,他引:0
Benjamin F. Teresa 《Urban geography》2017,38(2):199-220
A central problem in planning is how planners can be both technical experts and political actors sensitive to the moral consequences of planning. Rationality refers to the reasons for choosing a means to achieve an end; a rationality that considers the morality of means and ends is value rationality, and one that does not is instrumental rationality. Through the case of using Tax Increment Financing (TIF) to subsidize corporate headquarters relocation in Chicago, I follow City follows planners’ struggle with TIF policy and their engagement with instrumental and value rationality within a state that exercised an entrepreneurial planning strategy. This position meant that planners were constrained from acting value-rationally to consider and then take action on questions about the moral content of TIF projects. Nonetheless, planning staff developed an instrumentally-rational planning and policy exercise, which they performed as a way to channel their value-rational concerns about the assumption that every economic development project is an unambiguously valuable goal. 相似文献