全文获取类型
收费全文 | 611篇 |
免费 | 33篇 |
国内免费 | 5篇 |
专业分类
测绘学 | 16篇 |
大气科学 | 22篇 |
地球物理 | 163篇 |
地质学 | 188篇 |
海洋学 | 54篇 |
天文学 | 138篇 |
综合类 | 1篇 |
自然地理 | 67篇 |
出版年
2021年 | 6篇 |
2020年 | 8篇 |
2019年 | 10篇 |
2018年 | 17篇 |
2017年 | 23篇 |
2016年 | 23篇 |
2015年 | 17篇 |
2014年 | 15篇 |
2013年 | 24篇 |
2012年 | 14篇 |
2011年 | 24篇 |
2010年 | 30篇 |
2009年 | 27篇 |
2008年 | 19篇 |
2007年 | 14篇 |
2006年 | 16篇 |
2005年 | 19篇 |
2004年 | 14篇 |
2003年 | 14篇 |
2002年 | 14篇 |
2001年 | 12篇 |
2000年 | 13篇 |
1999年 | 10篇 |
1998年 | 9篇 |
1997年 | 10篇 |
1996年 | 7篇 |
1995年 | 5篇 |
1994年 | 6篇 |
1993年 | 9篇 |
1992年 | 6篇 |
1991年 | 6篇 |
1989年 | 4篇 |
1988年 | 9篇 |
1987年 | 9篇 |
1986年 | 6篇 |
1985年 | 10篇 |
1984年 | 12篇 |
1983年 | 4篇 |
1982年 | 10篇 |
1981年 | 7篇 |
1980年 | 11篇 |
1979年 | 9篇 |
1978年 | 6篇 |
1977年 | 9篇 |
1976年 | 12篇 |
1975年 | 11篇 |
1974年 | 14篇 |
1973年 | 8篇 |
1971年 | 6篇 |
1968年 | 6篇 |
排序方式: 共有649条查询结果,搜索用时 234 毫秒
481.
482.
Carl Heller 《Ocean Dynamics》1955,8(4):157-164
Zusammenfassung Zur Herstellung schwacher, homogener Magnetfelder wird die Verwendung quadratförmiger Rahmenspulen vorgeschlagen, die gegenüber gleich großen Kreisringspulen den Vorteil einer größeren Homogenität des Feldes bieten. Es wird die Feldverteilung auf der Achse und in der Ebene einer Quadratspule angegeben. Für die Kombination von zwei Quadratspulen wird die Ermittlung eines optimalen Spulenabstandes gezeigt für die Fälle, daß entweder größte lineare Ausdehnung des homogenen Bereichs auf der Feldachse oder größte flächenhafte Ausdehnung in der Mittelebene des Feldes gefordert wird. Im Helmholtzfall, für den der Abstand der Quadratspulen zum 0,5445fachen der Kantenlänge berechnet wird, erweist sich der räumliche Bereich mit 1 % Feldstärkeabweichung gegen den Feldmittelpunkt um rund 72 % größer als bei einer gleich großen Helmholtzspule der üblichen Form.
On the production of uniform magnetic fields of great volume for studying the behaviour of magnetic compasses and adjusting devices in different magnetic latitudes
Summary For the production of weak uniform magnetic fields it is proposed to use square frame coils which in contrast to circular coils of the same size offer the advantage of a greater field homogeneity. The field distribution on the axis and in the plane of a square coil is indicated. For the use of two combined square coils the determination of the optimum coil separation is given for the cases that there should either be required greatest linear extension of the uniform range on the axis or greatest homogeneous area in the median plane of the field. In the Helmholtz case, for which the separation between the square coils is calculated to the 0,5445th part of the edge's lenght the volume with 1 %deviation from the field intensity in the centre is by 72 % larger than that of a usual Helmholtz coil of the same size.
Sur la production des champs magnétiques homogènes de grand volume pour l'étude du comportement des compas magnétiques et des dispositifs de réglage par différentes latitudes magnétiques
Résumé Pour pouvoir produire des champs magnétiques faibles et homogènes on propose d'employer des bobines à cadres carrés qui en comparaison avec des bobines annulaires de la même grandeur offrent l'avantage d'une homogénéité supérieure du champ magnétique. La distribution du champ sur l'axe et dans le plan d'une bobine à cadre carré est indiquée. Pour l'emploi de deux bobines combinées à cadres carrés on montre de quelle manière on peut déterminer la distance optimale pour les cas où il est nécessaire de demander de la plus grande extension linéaire de la région homogène sur l'axe du champ ou de la plus grande aire homogène dans le plan central. En cas des bobines Helmholtz dont la distance pour des bobines à cadres carrés a été déterminé à la 0,5445e partie de la longueur de l'arête, le volume homogène avec une déviation de 1 % de l'intensité en comparaison avec l'intensité du centre du champ dépasse de 72 % le volume d'une bobine Helmholtz de la même grandeur et du type usuel.相似文献
483.
Nekton and macrocrustacean population levels and characteristics were studied in two similar tidal marsh creeks. Absolute and area-adjusted data were analyzed to determine variability resulting from daily population fluctuations. Two sampling schemes—intensive 3-d seasonal and periodic 1-d—were examined by constructing probability matrices to compare the accuracy of data comparisons resulting from each scheme. The probability of inaccuracies in comparisons of abundance using nonreplicated sampling schemes ranged from 0% to 100%. Significant differences between consecutive day data were observed for population characteristics such as blue crab (Callinectes sapidus) sex ratios, spot (Leiostomus xanthurus) length-weights, killifish: sciaenid ratios, and killifish: blue crab ratios. These data support the need to account for short-term variability when assessing mobile aquatic fauna abundance in estuarine wetlands. 相似文献
484.
In deltas, subsidence leads to a relative sea-level rise (RSLR) that is often much greater than eustatic rise alone. Because of high RSLR, deltaic wetlands will be affected early by an acceleration of eustatic sea-level rise. If there is sufficient vertical accretion, wetlands can continue to exist with RSLR; however, lack of sediment input eventually leads to excessive water logging and plant death. Areas with low tidal range, such as the Mediterranean and Gulf of Mexico, are especially vulnerable to rising water levels because the elevational growth range of coastal vegetation is related to tide range. Reduction of suspended sediments in rivers and prevention of wetland flooding by river dikes and impoundments have reduced sediment input to Mediterranean and Gulf of Mexico deltaic wetlands. This sediment deficit will become more important with an acceleration in sea-level rise from global warming. Most sediment input occurs during strong pulsing events such as river floods and storms, and management policies and decisions are especially designed to protect against such events. Management approaches must be reoriented to take advantage of pulsing events to nourish marsh surfaces with sediments. We hypothesize that deltas can be managed to withstand significant rates of sea-level rise by taking advantage of pulsing events leading to high sediment input, and that this type of management approach will enhance ecosystem functioning. 相似文献
485.
Elizabeth A. Allen Paul E. Fell Myron A. Peck Jennifer A. Gieg Carl R. Guthke Michael D. Newkirk 《Estuaries and Coasts》1994,17(2):462-471
We examined the gut contents of mummichogs, Fundulus heteroclitus L., entering and leaving ditches in three marsh regions within the Barn Island Wildlife Management Area in Connecticut: a restored impounded valley marsh, a marsh below the impoundment dike (Headquarters Marsh), and an unimpounded valley marsh (Davis Marsh). On the Headquarters Marsh and at certain times on the other two marshes, fish entered the ditches on the flooding tide with relatively little food in their guts and left them on the following ebbing tide with considerably more food in their guts. Since the high tides did not flood the surface of the high marsh, it appears that the ditches are important foraging areas. Major components of the gut contents were detritus, algae, amphipods, tanaids, copepods, and insects. During the summer, fish in the restored impounded marsh consumed less food per unit body weight than did fish inhabiting the other marsh regions. 相似文献
486.
487.
Ushijima-Mwesigwa Hayato Hyman Jeffrey D. Hagberg Aric Safro Ilya Karra Satish Gable Carl W. Sweeney Matthew R. Srinivasan Gowri 《Mathematical Geosciences》2021,53(8):1977-1978
Mathematical Geosciences - The publication of this article unfortunately contained a mistake. The assignment to the affiliations of author Satish Karra was not correct 相似文献
488.
Brent D. Turrin James T. Gutmann Carl C. Swisher III 《Journal of Volcanology and Geothermal Research》2008
Among the youngest lava flows of the Pinacate volcanic field, Sonora, Mexico, is a large outpouring of tholeiite, the Ives flow. This tube-fed pahoehoe flow contrasts sharply with other Pinacate lavas in its great volume, alkali-poor composition and morphologic features, which include novel small structures named “spatter tubes.” Despite its K-poor character, young age, and the presence of excess 40Ar, we determined a 40Ar/39Ar age on samples of this flow at 13 ± 3 ka. Such an age determination is made possible via careful monitoring of the mass discrimination of the mass spectrometer and by stacking results from multiple incremental-heating experiments into a single, composite isochron. This age is among the youngest ever to be determined with such precision by the 40Ar/39Ar method on a K-poor tholeiite. 相似文献
489.
Julian R. Thompson Honeyeh Iravani Hannah M. Clilverd Carl D. Sayer Catherine M. Heppell Jan C. Axmacher 《水文科学杂志》2017,62(15):2482-2510
Thirty UK Climate Projections 2009 (UKCP09) scenarios are simulated using a MIKE SHE/MIKE 11 model of a restored floodplain in eastern England. Annual precipitation exhibits uncertainty in direction of change. Extreme changes (10 and 90% probability) range between ?27 and +30%. The central probability projects small declines (4%). Wetter winters and drier summers predominate. Potential evapotranspiration increases for most scenarios (annual range of change: ?41 to +2%). Declines in mean discharge predominate (range: ?41 to +25%). Reductions of 11–17% are projected for the central probability. High and low flows, and the frequency of bankfull discharge exceedence reduce in most cases. Duration of winter high floodplain water tables declines. Summer water tables are on average at least 0.11 and 0.18 m lower for the 2050s and 2080s, respectively. Flood extent declines in most scenarios. Drier conditions will likely induce ecological responses including impacts on floodplain vegetation. 相似文献
490.