全文获取类型
收费全文 | 3047篇 |
免费 | 509篇 |
国内免费 | 962篇 |
专业分类
测绘学 | 38篇 |
大气科学 | 281篇 |
地球物理 | 434篇 |
地质学 | 2656篇 |
海洋学 | 421篇 |
天文学 | 25篇 |
综合类 | 91篇 |
自然地理 | 572篇 |
出版年
2024年 | 29篇 |
2023年 | 56篇 |
2022年 | 120篇 |
2021年 | 123篇 |
2020年 | 165篇 |
2019年 | 180篇 |
2018年 | 131篇 |
2017年 | 173篇 |
2016年 | 149篇 |
2015年 | 168篇 |
2014年 | 209篇 |
2013年 | 240篇 |
2012年 | 228篇 |
2011年 | 178篇 |
2010年 | 164篇 |
2009年 | 194篇 |
2008年 | 232篇 |
2007年 | 230篇 |
2006年 | 229篇 |
2005年 | 200篇 |
2004年 | 156篇 |
2003年 | 142篇 |
2002年 | 111篇 |
2001年 | 72篇 |
2000年 | 96篇 |
1999年 | 103篇 |
1998年 | 69篇 |
1997年 | 59篇 |
1996年 | 51篇 |
1995年 | 39篇 |
1994年 | 50篇 |
1993年 | 48篇 |
1992年 | 21篇 |
1991年 | 17篇 |
1990年 | 20篇 |
1989年 | 12篇 |
1988年 | 10篇 |
1987年 | 7篇 |
1986年 | 8篇 |
1985年 | 7篇 |
1984年 | 7篇 |
1983年 | 5篇 |
1982年 | 3篇 |
1980年 | 1篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有4518条查询结果,搜索用时 15 毫秒
991.
992.
993.
美国海岸海洋管理的法律体系与实践 总被引:5,自引:0,他引:5
从四个阶段划分了美国海岸海洋管理的立法过程,阐述了美国海岸海洋管理法律体系的若干特征与存在的问题,并从体制、制度、政策等方面评述了海岸海洋的管理活动,最后分析了美国当前海岸海洋管理立法局长的新趋势。 相似文献
994.
Anne Briais 《Marine Geophysical Researches》1995,17(5):431-467
The morphological characteristics of the segmentation of the Central Indian Ridge (CIR) from the Indian Ocean Triple Junction (25°30S) to the Egeria Transform Fault system (20°30S) are analyzed. The compilation of Sea Beam data from R/VSonne cruises SO43 and SO52, and R/VCharcot cruises Rodriguez 1 and 2 provides an almost continuous bathymetric coverage of a 450-km-long section of the ridge axis. The bathymetric data are combined with a GLORIA side-scan sonar swath to visualize the fabric of the ridge and complement the coverage in some areas. This section of the CIR has a full spreading rate of about 50 mm yr–1, increasing slightly from north to south. The morphology of the CIR is generally similar to that of a slow-spreading center, despite an intermediate spreading rate at these latitudes. The axis is marked by an axial valley 5–35 km wide and 500–1800 m deep, sometimes exhibiting a 100–600 m-high neovolcanic ridge. It is offset by only one 40km offset transform fault (at 22°40S), and by nine second-order discontinuities, with offsets varying from 4 to 21 km, separating segments 28 to 85 km long. The bathymetry analysis and an empirical orthogonal function analysis performed on across-axis profiles reveal morphologic variations in the axis and the second-order discontinuities. The ridge axis deepens and the relief across the axial valley increases from north to south. The discontinuities observed south of 22°S all have morphologies similar to those of the slow-spreading Mid-Atlantic Ridge. North of 22°S, two discontinuities have map geometries that have not been observed previously on slow-spreading ridges. The axial valleys overlap, and their tips curve toward the adjacent segment. The overlap distance is 2 to 4 times greater than the offset. Based on these characteristics, these discontinuities resemble overlapping spreading centers (OSCs) described on the fast-spreading EPR. The evolution of one such discontinuity appears to decapitate a nearby segment, as observed for the evolution of some OSCs on the EPR. These morphological variations of the CIR axis may be explained by an increase in the crustal thickness in the north of the study area relative to the Triple Junction area. Variations in crustal thickness could be related to a broad bathymetric anomaly centered at 19°S, 65°E, which probably reflects the effect of the nearby Réunion hotspot, or an anomaly in the composition of the mantle beneath the ridge near 19°S. Other explanations for the morphological variations include the termination of the CIR at the Rodriguez Triple Junction or the kinematic evolution of the triple junction and its resultant lengthening of the CIR. These latter effects are more likely to account for the axial morphology near the Triple Junction than for the long-wavelength morphological variation. 相似文献
995.
Evidence has been found for the teleconnection of Indian Ocean Dipole mode (IOD) events in the southern high latitude sea
surface pressure field, although the mechanisms that might lead to such far-reaching links remain unresolved. Based on the
teleconnection pattern between IOD and the climate anomaly in the upper troposphere, we propose one such mechanism here: the
energy propagation theory of the atmospheric planetary wave. Ray traces of the atmospheric planetary waves suggest that the
energy propagation of the waves could be responsible for the teleconnection between IOD and tropospheric climate anomalies
in southern high latitudes. 相似文献
996.
997.
The Gulf of Nicoya, an estuary on the Pacific coast of Central America, contains a large population of the portunid crab Callinectes arcuatus. Results of a 12 month survey indicated that spawning activity occurs throughout the year, but with a distinct peak during the five-month dry season (December-April). Mature females were most prevalent in the upper regions of the gulf during the rainy season and appeared to migrate to the lower gulf to spawn during the dry season. Patterns of spawning and apparent migration differed from those reported in an earlier study of C. arcuatus along the Pacific coast of Mexico. 相似文献
998.
N. Angusamy J. Dajkumar Sahayam M. Suresh Gandhi G. Victor Rajamanickam 《Marine Georesources & Geotechnology》2005,23(3):137-174
Placer mineral exploration has been undertaken along the beach of Central Tamil Nadu coast from Pondicherry to Vedaranyam. On the basis of the drainage network, geomorphology, and the coastal environment, the study area has been grouped into three sectors, North, Central, and South. Heavy mineral by Wt% shows a slightly higher abundance in the Northern sector, an enrichment of opaques in the Central sector from stations between Poompuhar and Karaikal, and a total depletion in the Southern sector. An abundance of heavies in the Northern sector is considered to be the result of a higher wave energy, and the cymatogenic downwarping of the basin during the present transgression. An enrichment of heavies in the Central sector from Poompuhar to Karaikal is attributable to the reworking of the beach ridges, which were submerged during the present transgression, and to the role of density sorting. A depletion of heavies in the Southern sector is accounted for by the absence of a terrigenous supply and the prevalence of wave shadow conditions throughout the year. The nature of the heavy mineral assemblage reflects the derivation of sediments principally from khondalites, granites, metamorphic rocks and paleo-sediments. Factor analysis also supplements the predominant role of a density factor in the segregation of heavy minerals in the study area. A five-stage model is proposed for the formation of placer deposits in the study region. The present study has disclosed rich concentrations of ilmenites in the central sector between Poompuhar and Karaikal that can be commercially exploited. 相似文献
999.
T. M. Ward J. Staunton-Smith S. Hoyle I. A. Halliday 《Estuarine, Coastal and Shelf Science》2003,56(5-6):1125-1140
The diverse pelagic fish assemblage of sub-tropical southern Queensland includes fishes with predominantly temperate distributions, such as tailor Pomatomus saltatrix, sardine Sardinops sagax, round herring Etrumeus teres, and Australian anchovy Engraulis australis. The peak spawning seasons of P. saltatrix, S. sagax and E. teres occur during late winter and early spring (June–October). Eggs and larvae of these three species are widely distributed in shelf waters and comprise >50% of the ichthyoplankton assemblage during this period. Mean monthly sea surface temperatures (SSTs) during late winter and early spring range from 21 to 23 °C, and are thus similar to those recorded in southern Australia during summer and autumn, which is the spawning season of these three species in those temperate waters. E. australis eggs occur mainly in inshore waters, and comprise >50% of fish eggs collected during summer and autumn when mean monthly SSTs in southern Queensland exceed 27 °C. E. australis also spawns mainly during summer and autumn in temperate Australia. Hence, water temperature may be less important as a determinant of the spawning season of E. australis than it is for the other three species. The suitability of southern Queensland for spawning by predominantly temperate species during late winter and early spring may contribute to the high diversity of the region's pelagic fish assemblage. Adult P. saltatrix, S. sagax and E. teres appear to migrate northwards into southern Queensland during early winter to spawn, and larvae may be transported southwards into temperate waters by the East Australian Current. This dispersal-migration pattern is similar to those observed for several species, including P. saltatrix, in the western boundary current systems off the east coasts of North America and Africa. Hence, pelagic fishes in ecosystems off the east coast of three continents migrate into sub-tropical waters to spawn, and larvae are transported back into temperate nursery areas by the prevailing current. 相似文献
1000.