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61.
Radiocarbon-dated sediment cores from subalpine lakes were used to investigate post-glacial dust deposition in the Uinta Mountains (Utah, USA). Lake sediments were geochemically characterized with ICP-OES, ICP-MS and XRF core scanning. Collections from passive samplers constrain the properties of modern dust, and samples of regolith constrain properties of the local material within the watershed. Ca and Eu are more abundant in dust, whereas Ti and Zr are more abundant in local regolith. As a result, the Ca/Ti and Eu/Zr ratios are indices for the dust content of lake sediment. In all records, the dust index rises in the early Holocene as watersheds became stabilized with vegetation, reducing the influx of local material. After this point, values remained above average through the middle Holocene, consistent with an increased dust content in the sediment. Dust index values drop in the late Holocene in most lakes, suggesting a decrease in dust abundance. Generally synchronous shifts in dust index values in cores from lakes in different parts of this mountain range are evidence of enhanced dust deposition in this region during the middle Holocene, and are consistent with a variety of records for increased aridity in the south-western USA at this time. 相似文献
62.
Matthew Rioux Samuel Bowring Frank Dudás Richard Hanson 《Contributions to Mineralogy and Petrology》2010,160(5):777-801
U–Pb baddeleyite geochronology has become a major tool for dating mafic rocks, especially dikes associated with Large Igneous
Provinces. However, in many cases, post-crystallization Pb-loss and intergrowth of baddeleyite and zircon limit the precision
and/or accuracy of crystallization ages. We present results from multi-step digestion experiments designed to understand and
reduce these effects. Experiments were carried out on Neoproterozoic baddeleyites with zircon inter- and over-growths from
the Gannakouriep dike swarm, Namibia, and on fragments of a large Paleoproterozoic baddeleyite from the Phalaborwa carbonatite,
South Africa. Multi-step digestion experiments on annealed Phalaborwa baddeleyite were designed to test whether the recently
developed chemical abrasion technique for U–Pb zircon geochronology can be applied to baddeleyite. The experiments generated
complex results—individual digestion steps were both normally and reversely discordant suggesting that U and Pb were decoupled
in the multi-step digestions—and indicate that the current form of multi-step chemical abrasion is not an effective method
for reducing the impact of Pb-loss in baddeleyite. A separate set of experiments on the Gannakouriep baddeleyite focused on
isolating the zircon and baddeleyite components in composite grains. Conventional single-step digestion experiments for this
sample resulted in a discordant suite of analyses with significant scatter attributed to inter- and over-grown zircon and
highlight the difficulty of obtaining precise and accurate ages from composite grains. To isolate the baddeleyite and zircon
in these grains, a two-step HCl–HF chemical abrasion procedure for annealed grains was developed. This technique was successful
at selectively dissolving the baddeleyite and zircon components. Secondary zircon inter- and over-growths of baddeleyite can
occur in samples affected by low-temperature alteration to granulite facies metamorphism, and the new HCl–HF chemical abrasion
procedure provides a method for resolving both the igneous and metamorphic history of these composite grains. 相似文献
63.
The regional carbonate aquifer in the carbonate-rock province of the Great Basin, USA, covers thousands of square kilometers. It is a significant potential source of water for growth in this arid area. Few wells penetrate the carbonate aquifer, so information on water quantity and quality is derived in large part from ‘regional springs’ that discharge from regional interbasin flow systems. For this study, springs in the carbonate-rock province were sampled; their physical, chemical, and isotopic characteristics were compared to those of known regional springs to identify previously unrecognized regional waters using both examination of the data and multivariate statistical analysis. Criteria for comparison included temperature, discharge, 3H activity, carbon isotope values, and ratios of major and trace ions. Of the 18 springs selected for detailed chemical and isotopic sampling, five springs—Hot, Littlefield, Petrified, Saratoga, and Warm (a)—were identified as regional, and one (Monte Neva Hot) was identified as a possible regional spring. Regional springs provide an easy, low-cost means of investigating aquifer properties; identification of regional springs thus increases the ability to understand the regional carbonate aquifer. The techniques applied in this study can also be used in other regional aquifer systems with diverse and complex geology. 相似文献
64.
Solutions are given for intersection, lateration, and the direct problem by a simple method which finds the common point of
intersection of two planes and a sphere. The ways of determining a plane may vary, but the general procedure is the same for
all cases. An alternative solution is given for the direct problem. 相似文献
65.
66.
Multivariate statistics were used to characterize and test the effectiveness of grain-size frequencies as environmental discriminators. Sediment from the following two depositional systems along eastern Lake Michigan were studied: (1) a closed system with respect to available grain sizes (Little Sable Point), and (2) an open system (Sleeping Bear Point-Manitou Passage). Principal components analysis shows that grain-size distributions are composed of two or more subgroups that reflect surface creep bedload, mixed suspension bedload, and uniform suspension. Discriminant function and principal latent vector analyses of the Little Sable Point environments show that, when available sediment is limited with respect to grain size (0.5 φto 3.0 φ), similar size distributions can occur in environments supposedly characterized by different energy conditions. Sediment in the Sleeping Bear Point-Manitou Passage system is not restricted to available grain sizes and the environments discriminated very well (α < 0.001). The grain-size distributions are such that they reflect differences in energy conditions within the environments. It is apparent that the grain sizes available to a depositional system control to a great extent the effectiveness of environmental discrimination. 相似文献
67.
The nearshore sediments in the Bay of Naples show a buildup of organic carbon and increased levels of lead, copper and chromium as a result of discharge of domestic and industrial waste water. Twenty to twenty-five square kilometres of bottom area have been contaminated by the buildup of anoxic muds with elevated heavy metal concentrations. 相似文献
68.
The prevailing theory for the formation of trona [Na3(CO3)(HCO3) · 2(H2O)] relies on evaporative concentration of water produced by silicate hydrolysis of volcanic rock or volcaniclastic sediments. Given the abundance of closed drainage basins dominated by volcanics, it is puzzling that there are so few trona deposits and present-day lakes that would yield dominantly Na–CO3 minerals upon evaporation. Groundwater in the San Bernardino Basin (southeastern Arizona, USA and northeastern Sonora, Mexico) would yield mainly Na–CO3 minerals upon evaporation, but waters in the surrounding basins would not. Analysis of the chemical evolution of this groundwater shows that the critical difference from the surrounding basins is not lithology, but the injection of magmatic CO2. Many major deposits of trona and Na–CO3-type lakes appear to have had “excess” CO2 input, either from magmatic sources or from the decay of organic matter. It is proposed that, along with the presence of volcanics, addition of “excess” CO2 is an important pre-condition for the formation of trona deposits. 相似文献
69.
In order to improve the reliability of the ground penetrating radar (GPR) method in identifying subsurface sinkholes and karst cavities, laboratory investigations have been performed. The main objective of this work was to examine the relationship between horizontal/vertical voids dimensions and wavelengths of various antennas, and the corresponding GPR responses. Emphasis was given to the investigation of the factors that cause the appearance of reverberation phenomena in the signal pattern.The tests were conducted in 5 m × 10 m area by 2-m-deep trench filled with homogenous, dry sand. The voids models (empty fiberglass cylinders in diameters of 0.6 m, 1.0 m, 1.5 m and 2.4 m, with various heights) were buried vertically with their tops at depths of between 0.7 and 1.5 m. Investigations were performed for the various model conditions by towing 500, 300 and 100 MHz antennas along a pre-established grid, for the various model conditions.The GPR data collected using the 500 MHz bistatic antenna above the 1.0-m- and the 1.5-m-diameter cylinders, and using the 300 MHz bistatic antenna above the 1.5-m-diameter cylinder, confirmed the presence of a reverberation phenomenon, i.e. a strong convex signal pattern, containing a series of high amplitude extending oscillations with reduced frequency.Based on past practical GPR experience of void detection and presently obtained experimental data, two rules of thumbs may be adopted for the prediction of the appearance of resonant radar pictures:
- 1. The void diameter larger than the wavelength in air of the antenna used.
- 2. The vertical size of the empty void not significantly smaller than its horizontal dimension.
70.
Mathematical equations to transform geodetic to grid coordinates on the Lambert conical orthomorphic projection are derived
in a form stable enough to allow computations to be made on the Mercator and Polar Stereographic projections as special cases. 相似文献