全文获取类型
收费全文 | 248篇 |
免费 | 4篇 |
国内免费 | 2篇 |
专业分类
测绘学 | 12篇 |
大气科学 | 11篇 |
地球物理 | 46篇 |
地质学 | 93篇 |
海洋学 | 45篇 |
天文学 | 22篇 |
综合类 | 1篇 |
自然地理 | 24篇 |
出版年
2022年 | 1篇 |
2021年 | 2篇 |
2020年 | 2篇 |
2019年 | 5篇 |
2018年 | 7篇 |
2017年 | 5篇 |
2016年 | 8篇 |
2015年 | 2篇 |
2014年 | 4篇 |
2013年 | 14篇 |
2012年 | 18篇 |
2011年 | 10篇 |
2010年 | 14篇 |
2009年 | 12篇 |
2008年 | 14篇 |
2007年 | 11篇 |
2006年 | 6篇 |
2005年 | 7篇 |
2004年 | 6篇 |
2003年 | 3篇 |
2002年 | 12篇 |
2001年 | 5篇 |
2000年 | 7篇 |
1999年 | 9篇 |
1998年 | 3篇 |
1997年 | 8篇 |
1996年 | 7篇 |
1995年 | 3篇 |
1994年 | 7篇 |
1993年 | 4篇 |
1992年 | 2篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1988年 | 2篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 3篇 |
1983年 | 5篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1971年 | 1篇 |
1956年 | 1篇 |
1935年 | 1篇 |
排序方式: 共有254条查询结果,搜索用时 10 毫秒
81.
In the 1920's the Swedish geologist Lennart von Post found evidence of irregular early Holocene isostatic uplift in the region northwest of Lake Vänern in western Sweden. von Post based his conclusions on levellings of ancient shore lines. These were indirectly dated by pollen stratigraphical investigations in basins situated at approximately the same altitudes as the shore lines. To test von Post's hypothesis, we have adopted a different methodological approach. The sediments in twelve small lake basins, within five minor areas in this region, have been investigated with respect to when they became isolated from ancient Lake Vänern. Initially all the lake thresholds were levelled. Altitudes range between 165 and 96 m above sea level. By combining the results of mineral magnetic and diatom stratigraphical analyses it has been possible to determine the level in the sediment when the basin became isolated. The 14C age of this isolation level has been inferred from AMS dates based on the NaOH-soluble fraction of bulk sediment, partly with very low organic content. Altogether, 36 radiocarbon datings have been carried out, ranging from 10080 to 8800 14C years BP. Pollen analyses, and subsequent correspondence analysis (CA) of all pollen spectra, were carried out around the isolation level in each basin as a complement to the AMS datings.If the region had been subject to irregular isostatic uplift, lakes at the same altitude in the five areas should have been isolated at different times. The AMS determined isolation ages, corrected according to CA of the pollen stratigraphy and compensated for the relatively higher rebound of the northerly situated sites support von Post's conclusion, from the beginning of this century, that the region northwest of Lake Vänern has been subjected to irregular isostatic uplift. 相似文献
82.
A reversible method is presented to transform waterlogged sediment cores into dry and stable specimens which can go on exhibition or into permanent storage, needing no special precautions. Wet sediment core segments are bath-impregnated with polyethylene glycol of molecular weight 3,350 (PEG 3,350) dissolved in water. The samples are then subjected to a freeze-drying process, during which the PEG forms a stabilising and bonding porous network structure in the capillary system of the sediment. Standard sediment cores at least 1 m long become strong enough to stand upright. Sediment types ranging from muddy deep-sea sediments to very coarse littoral Halimeda sand were successfully stabilised. For standard cores of 10-cm diameters split length-wise, impregnation times vary from 2 weeks for coarse sand to 6–10 weeks for more compacted sediments. With regard to the stability of the samples and the visual clarity of detail, best results were obtained by impregnation with 25 to 60% solutions of PEG 3,350, coarse sediments needing more PEG than finer ones. Colour changes are systematic – the processed samples are lighter in appearance and the contrasts are enhanced, but there is no serious shift in colour tones. The PEG can easily be re-dissolved and washed out of the sediments. Display specimens can thus be made available for research again. 相似文献
83.
Helena Alexanderson Christian Hjort Per Mller Oleg Antonov Maksim Pavlov 《Global and Planetary Change》2001,31(1-4)
The North Taymyr ice-marginal zone (NTZ) is a complex of glacial, glaciofluvial and glaciolacustrine deposits, laid down on the northwestern Taymyr Peninsula in northernmost Siberia, along the front of ice sheets primarily originating on the Kara Sea shelf. It was originally recognised from satellite radar images by Russian scientists; however, before the present study, it had not been investigated in any detail. The ice sheets have mainly inundated Taymyr from the northwest, and the NTZ can be followed for 700–750 km between 75°N and 77°N, mostly 80–100 km inland from the present Kara Sea coast.The ice-marginal zone is best developed in its central parts, ca. 100 km on each side of the Lower Taymyr River, and has there been studied by us in four areas. In two of these, the ice sheet ended on land, whereas in the two others, it mainly terminated into ice-dammed lakes. The base of the NTZ is a series of up to 100-m-high and 2-km-wide ridges, usually consisting of redeposited marine silts. These ridges are still to a large extent ice-cored; however, the present active layer rarely penetrates to the ice surface. Upon these main ridges, smaller ridges of till and glaciofluvial material are superimposed. Related to these are deltas corresponding to two generations of ice-dammed lakes, with shore levels at 120–140 m and ca. 80 m a.s.l. These glacial lakes drained southwards, opposite to the present-day pattern, via the Taymyr River valley into the Taymyr Lake basin and, from there, most probably westwards to the southern Kara Sea shelf.The basal parts of the NTZ have not been dated; however, OSL dates of glaciolacustrine deltas indicate an Early–Middle Weichselian age for at least the superimposed ridges. The youngest parts of the NTZ are derived from a thin ice sheet (less than 300 m thick near the present coast) inundating the lowlands adjacent to the lower reaches of the Taymyr River. The glacial ice from this youngest advance is buried under only ca. 0.5 m of melt-out till and is exposed by hundreds of shallow slides. This final glaciation is predated by glacially redeposited marine shells aged ca. 20,000 BP (14C) and postdated by terrestrial plant material from ca. 11,775 and 9500 BP (14C)–giving it a last global glacial maximum (LGM; Late Weichselian) age. 相似文献
84.
Warming permafrost in European mountains 总被引:3,自引:0,他引:3
Charles Harris Daniel Vonder Mühll Ketil Isaksen Wilfried Haeberli Johan Ludvig Sollid Lorenz King Per Holmlund Francesco Dramis Mauro Guglielmin David Palacios 《Global and Planetary Change》2003,39(3-4):215-225
Here we present the first systematic measurements of European mountain permafrost temperatures from a latitudinal transect of six boreholes extending from the Alps, through Scandinavia to Svalbard. Boreholes were drilled in bedrock to depths of at least 100 m between May 1998 and September 2000. Geothermal profiles provide evidence for regional-scale secular warming, since all are nonlinear, with near-surface warm-side temperature deviations from the deeper thermal gradient. Topographic effects lead to variability between Alpine sites. First approximation estimates, based on curvature within the borehole thermal profiles, indicate a maximum ground surface warming of +1 °C in Svalbard, considered to relate to thermal changes in the last 100 years. In addition, a 15-year time series of thermal data from the 58-m-deep Murtèl–Corvatsch permafrost borehole in Switzerland, drilled in creeping frozen ice-rich rock debris, shows an overall warming trend, but with high-amplitude interannual fluctuations that reflect early winter snow cover more strongly than air temperatures. Thus interpretation of the deeper borehole thermal histories must clearly take account of the potential effects of changing snow cover in addition to atmospheric temperatures. 相似文献
85.
Manfred Sthli Lars Nyberg Per‐Erik Mellander Per‐Erik Jansson Kevin H. Bishop 《水文研究》2001,15(6):927-941
A physically based SVAT‐model was tested with soil and snow physical measurements, as well as runoff data from an 8600 m2 catchment in northern Sweden in order to quantify the influence of soil frost on spring snowmelt runoff in a moderately sloped, boreal forest. The model was run as an array of connected profiles cascading to the brook. For three winter seasons (1995–98) it was able to predict the onset and total accumulation of the runoff with satisfactory accuracy. Surface runoff was identified as only a minor fraction of the total runoff occurring during short periods in connection with ice blocking of the water‐conducting pores. Little surface runoff, though, does not mean that soil frost is unimportant for spring runoff. Simulations without frost routines systematically underestimated the total accumulated runoff. The possibility of major frost effects appearing in response to specific combinations of weather conditions were also tested. Different scenarios of critical initial conditions for the winter, e.g. high water saturation and delayed snow accumulation leading to an increased frost penetration, were tested. These showed that under special circumstances there is potential for increased spring runoff due to soil frost. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
86.
James R. C. Garry Inge Loes ten Kate Zita Martins Per Nrnberg Pascale Ehrenfreund 《Meteoritics & planetary science》2006,41(3):391-405
Abstract— We have investigated the native amino acid composition of two analogs of Martian soil, JSC Mars‐1 and Salten Skov. A Mars simulation chamber has been built and used to expose samples of these analogs to temperature and lighting conditions similar to those found at low latitudes on the Martian surface. The effects of the simulated conditions have been examined using high‐performance liquid chromatography (HPLC). Exposure to energetic ultraviolet (UV) light in vacuum appears to cause a modest increase in the concentration of certain amino acids within the materials, which is interpreted as resulting from the degradation of microorganisms. The influence of low temperatures shows that the accretion of condensed water on the soils leads to the destruction of amino acids, supporting the idea that reactive chemical processes involving H2O are at work within the Martian soil. We discuss the influence of UV radiation, low temperatures, and gaseous CO2 on the intrinsic amino acid composition of Martian soil analogs and describe, with the help of a simple model, how these studies fit within the framework of life detection on Mars and the practical tasks of choosing and using Martian regolith analogs in planetary research. 相似文献
87.
Using an empirical study of a political protest, this paper examines a set of interdependent contracts symbolizing both the nodes of scalar interaction and the hegemonic nodes of resistance. The study focuses on a contentious political campaign aimed at the City of Cincinnati, a recycling facility, and a day labor temp agency initiated by contracted day laborers who hand-sorted garbage from recycling materials and suffered from adverse working conditions and alleged wage violations. A discussion of the rise of contingent labor through implementation of neoliberal deregulation policies is integrated with Jürgen Habermas' concepts of the lifeworld and the system providing an explanatory framework for understanding lifeworld colonization through flexible labor practices and the subsequent impetus behind the contentious political campaign. Using survey data, the shared lifeworld of day laborers is described. The study also examines how the position of protest participants affected the tactics and outcomes of the contestation. [Key words: day labor, contingent labor, living wage ordinances, Habermas.] 相似文献
88.
89.
Reconstructions of the Late Quaternary glacial history of the Kara Sea area show repeated build-up of ice-sheet domes over the shallow epicontinental Kara Sea. Inferred ice divides were situated over the central Kara Sea, and the ice sheet repeatedly inundated the surrounding coastal areas of western Siberia. Geological fingerprinting of the Kara Sea ice sheet include end moraine zones, raised beaches, tills, glaciotectonic deformations and coarsening-upward sediment sequences, reflecting isostatic rebound cycles. This paper reviews evidence from several areas along the perimeter of the Kara Sea, suggesting that peripheral sites were critical for the initiation of the large Kara Sea ice sheet. Ice-sheet inception progressed with the formation of local ice caps that later coalesced on the adjacent shelf with globally falling sea levels, eventually merging and growing into a large ice dome. 相似文献
90.
A major challenge in using GPS guidance for aircraft final approach and landing is to reject interference that can jam reception
of the GPS signals. Antenna arrays, which use space–time adaptive processing (STAP), significantly improve the signal to interference
plus noise ratio, but at the possible expense of distorting the received signals, leading to timing biases that may degrade
navigation performance. Rather than a sophisticated calibration approach to remove biases introduced by STAP, this paper demonstrates
that a relatively compact calibration strategy can substantially reduce navigation biases, even under elevated interference
conditions. Consequently, this paper develops an antenna bias calibration strategy for two classes of adaptive array algorithm
and validates this method using both simulated and experimental data with operational hardware in the loop. A proof-of-concept
system and an operational prototype are described, which implement the adaptive antenna algorithms and deterministic corrections.
This investigation demonstrates that systems with adaptive antenna arrays can approach the accuracy and integrity requirements
for automatic aircraft landing, and in particular for sea-based landing on board aircraft carriers, while simultaneously providing
significant attenuation of interference. Evidence suggests that achieving these goals is possible with minimal restrictions
on system hardware configuration—specifically, limitations on the permissible level of antenna anisotropy and the use of sufficient
analog-to-digital converter resolution. 相似文献