全文获取类型
收费全文 | 836篇 |
免费 | 17篇 |
国内免费 | 12篇 |
专业分类
测绘学 | 18篇 |
大气科学 | 76篇 |
地球物理 | 179篇 |
地质学 | 297篇 |
海洋学 | 68篇 |
天文学 | 134篇 |
综合类 | 2篇 |
自然地理 | 91篇 |
出版年
2021年 | 5篇 |
2020年 | 13篇 |
2019年 | 8篇 |
2018年 | 7篇 |
2017年 | 21篇 |
2016年 | 22篇 |
2015年 | 13篇 |
2014年 | 17篇 |
2013年 | 41篇 |
2012年 | 23篇 |
2011年 | 37篇 |
2010年 | 32篇 |
2009年 | 36篇 |
2008年 | 34篇 |
2007年 | 34篇 |
2006年 | 27篇 |
2005年 | 18篇 |
2004年 | 21篇 |
2003年 | 25篇 |
2002年 | 27篇 |
2001年 | 17篇 |
2000年 | 10篇 |
1999年 | 5篇 |
1998年 | 12篇 |
1997年 | 12篇 |
1996年 | 14篇 |
1995年 | 15篇 |
1994年 | 11篇 |
1993年 | 18篇 |
1992年 | 11篇 |
1991年 | 8篇 |
1990年 | 10篇 |
1989年 | 14篇 |
1988年 | 10篇 |
1987年 | 18篇 |
1986年 | 11篇 |
1985年 | 15篇 |
1984年 | 14篇 |
1983年 | 18篇 |
1982年 | 26篇 |
1981年 | 17篇 |
1980年 | 14篇 |
1979年 | 18篇 |
1978年 | 16篇 |
1977年 | 9篇 |
1976年 | 9篇 |
1975年 | 12篇 |
1974年 | 8篇 |
1973年 | 5篇 |
1972年 | 6篇 |
排序方式: 共有865条查询结果,搜索用时 15 毫秒
61.
Preferential leakage of H2O from fluid inclusions containing multiple gas components has been suspected in natural metamorphic rocks and has been demonstrated experimentally for synthetic H2O-CO2-rich inclusions in natural quartz. Knowledge of the physical and chemical characteristics of the leakage mechanism, which may be very complex, increases the value of natural fluid inclusions to metamorphic geology. It is proposed that crystal defects play a major role in nondecrepitative preferential H2O leakage through quartz, and remain effective during metamorphism. Inclusions with either an internal overpressure or underpressure produce strain in the adjacent quartz crystal via the nucleation of many dislocations and planar defects (like Dauphiné twin boundaries). These defects allow preferential loss of H2O from H2O-CO2-rich inclusions at supercritical conditions. The transport capacity of this leakage mechanism is enhanced by nucleation of small bubbles on defect structures. The nucleation of these bubbles seems to be a recovery process in strained crystals. Solubility gradients of quartz in water in a crystal with internally underpressurized inclusions may result in optical visible implosion halos in a three dimensional spatial arrangement, caused by the growth of small bubbles at the expense of the larger original fluid inclusion. Natural fluid inclusions from Naxos (Greece) are always associated with numerous interlinked dislocations. These dislocations may have been produced by plastic derormation or by crystal growth related processes (e.g. crack healing). The presence of small bubbles on these dislocations indicates that a similar leakage mechanism for H2O must have occurred in these rocks. 相似文献
62.
After discussing the changing nature of perceived risk problems, the status of risk assessment is described in relation to its origins and in particular to its roots in the environmental impact statement process. The nature of risk, its component elements and the manner in which existing concepts of risk have been reflected in risk assessment methods are described. The paper considers two emerging schools of thought in current risk assessment studies: one that calls for more accurate measures of risk and increasingly comprehensive event prediction models to determine risk acceptability and another which argues that the acceptance of risk is less dependent on the accuracy of risk analyses than it is on the nature of the decision-making process and in particular on whether compensation is provided for those bearing a disproportionate share of risk. 相似文献
63.
Richard M. Adams Ronald A. Fleming Ching-Chang Chang Bruce A. McCarl Cynthia Rosenzweig 《Climatic change》1995,30(2):147-167
This study uses recent GCM forecasts, improved plant science and water supply data and refined economic modeling capabilities to reassess the economic consequences of long-term climate change on U.S. agriculture. Changes in crop yields, crop water demand and irrigation water arising from climate change result in changes in economic welfare. Economic consequences of the three GCM scenarios are mixed; GISS and GFDL-QFlux result in aggregate economic gains, UKMO implies losses. As in previous studies, the yield enhancing effects of atmospheric CO2 are an important determinant of potential economic consequences. Inclusion of changes in world food production and associated export changes generally have a positive affect on U.S. agriculture. As with previous studies, the magnitude of economic effects estimated here are a small percentage of U.S. agricultural value. 相似文献
64.
65.
66.
Krystyna W. Semkow Ronald A. Rizzo Larry A. Haskin David J. Lindstrom 《Geochimica et cosmochimica acta》1982,46(10):1879-1889
Cyclic voltammetry has been done for Ni2+, Co2+, and Zn2+ in melts of diopside composition in the temperature range 1425 to 1575°C. Voltammetric curves for all three ions excellently match theoretical curves for uncomplicated, reversible charge transfer at the Pt electrode. This implies that the neutral metal atoms remain dissolved in the melt. The reference electrode is a form of oxygen electrode. Relative to that reference assigned a reduction potential of 0.00 volt, the values of standard reduction potential for the ions are , , and . The electrode reactions are rapid, with first order rate constants of the order of 10?2 cm/sec. Diffusion coefficients were found to be 2.6 × 10?6 cm2/sec for Ni2+, 3.4 × 10?6 cm2/sec for Co2+, and 3.8 × 10?6 cm2/sec for Zn2+ at 1500°C. The value of (, diopside) is a linear function of temperature over the range studied, with values of ?0.35 V at 1425°C and ?0.29 V at 1575°C. At constant temperature the value of ) was not observed to vary with composition over the range CaO · MgO · 2SiO2 to CaO·MgO·3SiO2 or from 1.67 CaO·0.33MgO·2SiO2 to 0.5 CaO·1.5MgO·2SiO2. The value for the diffusion coefficient for Ni2+ decreased by an order of magnitude at 1525°C over the compositional range CaO · MgO · 1.25SiO2 to CaO · MgO · 3SiO2. This is consistent with a mechanism by which Ni2+ ions diffuse by moving from one octahedral coordination site to another in the melt, with the same Ni2+ species discharging at the cathode regardless of the SiO2 concentration in the melt. 相似文献
67.
Barry G. Oliver Earl M. Thurman Ronald L. Malcolm 《Geochimica et cosmochimica acta》1983,47(11):2031-2035
An operationally defined carboxyl content of humic substances extracted from rivers, streams, lakes, wetlands, and groundwaters throughout the United States and Canada is reported. Despite the diversity of the samples, only small variations were observed in this humic carboxyl content. The dissociation behavior of two combined fulvic/humic acid extracts was studied and it was found that the dissociation of the humics varied in a predictable manner with pH. Using a carboxyl content of 10 μeq/ mg humic organic carbon, and mass action quotient calculated from sample pH, the ionic balances of three highly colored Nova Scotia rivers were estimated. 相似文献
68.
The weathering of a late Tertiary volcanic ash near Jackson, Wyoming, was studied by sampling water percolating through the ash with suction lysimeters, and by examination of the associated solid phases. Soluble organic compounds derived from vegetation control the release and transport of solutes by complexing Al and Fe, and by causing low pH values. The concentrations of Na, K, Ca, and Mg, as well as those of Al and Fe correlate with the dissolved organic carbon concentrations (DOC) and follow an annual cycle with a maximum in spring. DOC concentrations averaged 5O mg C per liter, and values as high as 260 mg/l were observed. Al and Fe concentrations ranged as high as 5 mg/l. The dissolved organic matter was largely in the form of humic acids, although minor amounts of oxalate, acetate, and formate were also present. The pH of the percolating waters ranged from 4.3 to 6.5 with a mean of 5.2.During laboratory weathering experiments with the same ash in the absence of dissolved organic compounds, pH values ranged from 7.3 to 9.5, dissolved Al and Fe concentrations were below the detection limit, and there was little resemblance between the compositions of the solutions and the compositions observed in the field. Any model attempting to describe weathering in a comparable setting must incorporate biological mechanisms as the dominant controls. 相似文献
69.
Interpolated grids of coal bed thickness are being considered for use in a proposed method for taxation of coal in the state of West Virginia (United States). To assess the origin and magnitude of possible inaccuracies in calculated coal tonnage, we used conditional simulation to generate equiprobable realizations of net coal thickness for two coals on a 7
min topographic quadrangle, and a third coal in a second quadrangle. Coals differed in average thickness and proportion of original coal that had been removed by erosion; all three coals crop out in the study area. Coal tonnage was calculated for each realization and for each interpolated grid for actual and artificial property parcels, and differences were summarized as graphs of percent difference between tonnage calculated from the grid and average tonnage from simulations. Coal in individual parcels was considered minable for valuation purposes if average thickness in each parcel exceeded 30 inches. Results of this study show that over 75% of the parcels are classified correctly as minable or unminable based on interpolation grids of coal bed thickness. Although between 80 and 90% of the tonnages differ by less than 20% between interpolated values and simulated values, a nonlinear conditional bias might exist in estimation of coal tonnage from interpolated thickness, such that tonnage is underestimated where coal is thin, and overestimated where coal is thick. The largest percent differences occur for parcels that are small in area, although because of the small quantities of coal in question, bias is small on an absolute scale for these parcels. For a given parcel size, maximum apparent overestimation of coal tonnage occurs in parcels with an average coal bed thickness near the minable cutoff of 30 in. Conditional bias in tonnage for parcels having a coal thickness exceeding the cutoff by 10 in. or more is constant for two of the three coals studied, and increases slightly with average thickness for the third coal. 相似文献
70.
W. David Rust Donald R. MacGorman Eric C. Bruning Stephanie A. Weiss Paul R. Krehbiel Ronald J. Thomas William Rison Timothy Hamlin Jeremiah Harlin 《Atmospheric Research》2005,76(1-4):247
Balloon-borne electric field soundings and lightning mapping data have been analyzed for three of the storms that occurred in the Severe Thunderstorm Electrification and Precipitation Study field program in 2000 to determine if the storms had inverted-polarity electrical structures. The polarities of all or some of the vertically stacked charge regions in such storms are opposite to the polarities observed at comparable heights in normal storms. Analyses compared the charge structures inferred from electric field soundings in the storms with charges inferred from three-dimensional lightning mapping data. Charge structures were inferred from electric field profiles by combining the one-dimensional approximation of Gauss's law with additional information from three-dimensional patterns in the electric field vectors. The three different ways of inferring the charge structure in the storms were found to complement each other and to be consistent overall. Charge deposition by lightning possibly occurred and increased the charge complexity of one of the storms.Many of the cloud flashes in each case were inverted-polarity flashes. Two storms produced ground flash activity comprised predominantly of positive ground flashes. One storm, which was an isolated thunderstorm, produced inverted-polarity cloud flashes, but no flashes to ground. The positive and negative thunderstorm charge regions were found at altitudes where, respectively, negative and positive charge would be found in normal-polarity storms. Thus, we conclude that these storms had anomalous and inverted-polarity electrical structures. Collectively, these three cases (along with the limited cases in the refereed literature) provide additional evidence that thunderstorms can have inverted-polarity electrical structures. 相似文献