全文获取类型
收费全文 | 413篇 |
免费 | 15篇 |
国内免费 | 9篇 |
专业分类
测绘学 | 15篇 |
大气科学 | 36篇 |
地球物理 | 87篇 |
地质学 | 125篇 |
海洋学 | 39篇 |
天文学 | 111篇 |
综合类 | 1篇 |
自然地理 | 23篇 |
出版年
2022年 | 4篇 |
2020年 | 5篇 |
2019年 | 8篇 |
2018年 | 10篇 |
2017年 | 5篇 |
2016年 | 10篇 |
2015年 | 12篇 |
2014年 | 14篇 |
2013年 | 22篇 |
2012年 | 19篇 |
2011年 | 20篇 |
2010年 | 20篇 |
2009年 | 26篇 |
2008年 | 10篇 |
2007年 | 12篇 |
2006年 | 12篇 |
2005年 | 11篇 |
2004年 | 8篇 |
2003年 | 12篇 |
2002年 | 15篇 |
2001年 | 8篇 |
2000年 | 7篇 |
1999年 | 8篇 |
1998年 | 8篇 |
1997年 | 7篇 |
1996年 | 8篇 |
1995年 | 7篇 |
1994年 | 8篇 |
1993年 | 4篇 |
1992年 | 3篇 |
1991年 | 5篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1988年 | 7篇 |
1987年 | 10篇 |
1986年 | 4篇 |
1985年 | 7篇 |
1984年 | 5篇 |
1983年 | 7篇 |
1981年 | 6篇 |
1980年 | 4篇 |
1979年 | 7篇 |
1978年 | 3篇 |
1977年 | 9篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1973年 | 4篇 |
1972年 | 3篇 |
1971年 | 4篇 |
1970年 | 2篇 |
排序方式: 共有437条查询结果,搜索用时 390 毫秒
121.
Harry Y. McSween 《Geochimica et cosmochimica acta》1977,41(4):477-491
C3(O) chondrites comprise a metamorphic sequence. The following order reflects increasing grade: Kainsaz. Felix. Ornans. Lance. Isna, Warrenton. Assignment of Karoonda to the Ornans subtype is uncertain, but it is certainly of higher petrologic type than C3. Average olivine and pyroxene compositions in the metamorphic sequence change progressively from Fo12. Fs3 to Fo34, Fs11, respectively, and per cent mean deviation decreases. Kamacite and taenite change composition with increasing grade, reflecting higher equilibration temperatures. Blurring of textural features and Fe/Mg exchange between matrix and inclusions are also evident. As in the ordinary chondrites. contents of rare gases and possibly volatiles correlate with degree of metamorphism, but the effects are small. The meteorite Ornans presents an intriguing paradox. Observed chemical enrichment and depletion patterns reflect a higher metamorphic grade than do petrographic properties. The data suggest that abundance patterns of volatile components were not generated by metamorphism, but may represent primary differences. Strong correlations present in other C3(O) chondrites indicate some genetic link between metamorphism and composition, although the relationship is probably not causal. The autometamorphism model of Larimer and Anders (1967) appears to be the most straightforward explanation, but an observed negative correlation between the amount of matrix and content of volatiles suggests a re-examination of the two-component model. The decoupling mechanism required for Ornans is uncertain. 相似文献
122.
123.
124.
This article reports on an empirical analysis of consumer information and usage fields in the city of Eindhoven. The main purposes of this study are to investigate the distance, sectoral and directional biases of these fields, to analyse whether the degree of biases is related to personal characteristics of consumers and to identify the factors of the basis on which a model which predicts the probability that a shopping centre will be known by consumers can be developed. The findings of the study suggest that strong distance, sectoral and directional biases are present in the information and usage fields of consumers and that no systematic relationships exist between these characteristics of information and usage fields and personal variables of consumers. Finally, the present studies suggests that a model which describes the formation of consumer choice sets should include spatial factors such as distance, size, intervening opportunities and direction towards the city centre. 相似文献
125.
Highly negative δC13 values, ?18 to ? 40%., for carbonate cements found in Recent barrier and beach sands of the Mississippi River Delta complex strongly suggest that considerable carbon is furnished to the CaCO3 cements by either chemical or biological oxidation of CH4. These cemented sands are commonly found on beaches of the Chandeleur barrier island chain and other sites along the Louisiana coast where Holocene sands are rapidly transgressing over highly organic marsh deposits. Generation of CH4 from underlying anoxic marsh sediments, followed by vertical migration and oxidation to CO2 in the porous overlying sand, appears to be the unique set of conditions regulating this process of carbonate cementation. 相似文献
126.
Harry E. LeGrand 《Ground water》1975,13(3):246-250
127.
MODFLOW 2000 head uncertainty,a first-order second moment method 总被引:1,自引:0,他引:1
A computationally efficient method to estimate the variance and covariance in piezometric head results computed through MODFLOW 2000 using a first-order second moment (FOSM) approach is presented. This methodology employs a first-order Taylor series expansion to combine model sensitivity with uncertainty in geologic data. MODFLOW 2000 is used to calculate both the ground water head and the sensitivity of head to changes in input data. From a limited number of samples, geologic data are extrapolated and their associated uncertainties are computed through a conditional probability calculation. Combining the spatially related sensitivity and input uncertainty produces the variance-covariance matrix, the diagonal of which is used to yield the standard deviation in MODFLOW 2000 head. The variance in piezometric head can be used for calibrating the model, estimating confidence intervals, directing exploration, and evaluating the reliability of a design. A case study illustrates the approach, where aquifer transmissivity is the spatially related uncertain geologic input data. The FOSM methodology is shown to be applicable for calculating output uncertainty for (1) spatially related input and output data, and (2) multiple input parameters (transmissivity and recharge). 相似文献
128.
Kiddon JA Paul JF Buffum HW Strobel CS Hale SS Cobb D Brown BS 《Marine pollution bulletin》2003,46(10):1224-1244
The Mid-Atlantic Integrated Assessment (MAIA-Estuaries) evaluated ecological conditions in US Mid-Atlantic estuaries during the summers of 1997 and 1998. Over 800 probability-based stations were monitored in four main estuarine systems--Chesapeake Bay, the Delaware Estuary, Maryland and Virginian coastal bays, and the Albemarle-Pamlico Estuarine System. Twelve smaller estuaries within the four main systems were also assessed to establish variance at the local scale. A subset of the MAIA-Estuaries data is used here to estimate the extent of eutrophication, sediment contamination, and benthic degradation in mid-Atlantic estuaries. An Environmental Report Card and Index of Environmental Integrity summarize conditions in individual estuaries, the four estuarine systems, and the entire MAIA region. Roughly 20-50% of the region showed signs of eutrophication (high nutrients, excessive production of organic matter, poor water clarity, or depleted dissolved oxygen), 30% had contaminated sediments, and 37% had degraded benthic communities. Compared with the Environmental Monitoring and Assessment Program (EMAP)-Virginian Province study in 1990-1993, larger fractions of Chesapeake Bay (17%) and Delaware River (32%) had increased metals or organics in sediments. 相似文献
129.
G. H. Goedecke Vladimir E. Ostashev D. Keith Wilson Harry J. Auvermann 《Boundary-Layer Meteorology》2004,112(1):33-56
The von Kármán spectra of turbulent temperature andvelocity fluctuations have been widely used in the literature on turbulenceand electromagnetic, seismic, and acoustic wave propagation in random media.In this paper we provide a phenomenological motivation for the vonKármán velocity spectrum in terms of the quasi-wavelet model ofturbulence developed recently. In this model, turbulence is represented as asuperposition of self-similar localized eddies of many different scales. Wefind a functional form for these eddies that yields the von Kármán velocity spectrum exactly. We also show that other eddy functions producevelocity spectra that have the same general form as the von Kármán spectrum, and we consider possible quasi-wavelet representations of the`Kansas' spectrum and the `-1' spectrum. We also present asystematic determination, based on turbulence similarity theories, of theparameters of the von Kármán spectra of temperature and velocityfluctuations in an unstable atmospheric boundary layer. 相似文献
130.
David D. Zhang C. Y. Jim George C-S Lin Yuan-Qing He James J. Wang Harry F. Lee 《Climatic change》2006,76(3-4):459-477
In recent years, the phenomenon of global warming and its implications for the future of the human race have been intensively studied. In contrast, few quantitative studies have been attempted on the notable effects of past climatic changes upon human societies. This study explored the relationship between climatic change and war in China by comparing high-resolution paleo-climatic reconstructions with known war incidences in China in the last millennium. War frequencies showed a cyclic pattern that closely followed the global paleo-temperature changes. Strong and significant correlations were found between climatic change, war occurrence, harvest level, population size and dynastic transition. During cold phases, China suffered more often from frequent wars, population decline and dynastic changes. The quantitative analyses suggested that the reduction of thermal energy input during a cold phase would lower the land carrying capacity in the traditional agrarian society, and the population size, with significant accretions accrued in the previous warm phase, could not be sustained by the shrinking resource base. The stressed human-nature relationship generated a ‘push force’, leading to more frequent wars between states, regions and tribes, which could lead to the collapse of dynasties and collapses of human population size. War frequencies varied according to geographical locations (North, Central and South China) due to spatial variations in the physical environment and hence differential response to climatic change. Moreover, war occurrences demonstrated an obvious time lag after an episode of temperature fall, and the three geographical regions experienced different length of time lags. This research also shows that human population increases and collapses were correlated with the climatic phases and the social instabilities that were induced by climate changes during the last millennium. The findings proposed a new interpretation of human-nature relationship in the past, with implications for the impacts of anomalous global warming on future human conflicts. 相似文献