全文获取类型
收费全文 | 70538篇 |
免费 | 3737篇 |
国内免费 | 4388篇 |
专业分类
测绘学 | 2395篇 |
大气科学 | 7614篇 |
地球物理 | 14601篇 |
地质学 | 30213篇 |
海洋学 | 6507篇 |
天文学 | 10852篇 |
综合类 | 1939篇 |
自然地理 | 4542篇 |
出版年
2023年 | 330篇 |
2022年 | 902篇 |
2021年 | 1100篇 |
2020年 | 983篇 |
2019年 | 1056篇 |
2018年 | 4539篇 |
2017年 | 4270篇 |
2016年 | 3366篇 |
2015年 | 1432篇 |
2014年 | 1838篇 |
2013年 | 2720篇 |
2012年 | 2955篇 |
2011年 | 4898篇 |
2010年 | 4608篇 |
2009年 | 4912篇 |
2008年 | 4012篇 |
2007年 | 4680篇 |
2006年 | 1931篇 |
2005年 | 2006篇 |
2004年 | 1706篇 |
2003年 | 1784篇 |
2002年 | 1544篇 |
2001年 | 1296篇 |
2000年 | 1393篇 |
1999年 | 1482篇 |
1998年 | 1314篇 |
1997年 | 1391篇 |
1996年 | 1231篇 |
1995年 | 1136篇 |
1994年 | 1067篇 |
1993年 | 878篇 |
1992年 | 814篇 |
1991年 | 643篇 |
1990年 | 589篇 |
1989年 | 519篇 |
1988年 | 524篇 |
1987年 | 464篇 |
1986年 | 429篇 |
1985年 | 488篇 |
1984年 | 472篇 |
1983年 | 501篇 |
1982年 | 471篇 |
1981年 | 411篇 |
1980年 | 459篇 |
1979年 | 354篇 |
1978年 | 315篇 |
1977年 | 303篇 |
1976年 | 272篇 |
1975年 | 267篇 |
1974年 | 277篇 |
排序方式: 共有10000条查询结果,搜索用时 484 毫秒
991.
992.
993.
A detailed survey of a 1°×1°-square of seafloor 100 miles south-east of the Azores shows a strong correlation between directions of regional topographic and magnetic lineations. The area is dissected by the East Azores Fracture Zone at 36°55N, identified as the active Eurasian-African plate boundary, and by another large, non-active fracture zone at 36°10N. Both fracture zones strike 265° and are accompanied by large amplitude magnetic anomalies. The general strike in the area in between is 000°–015°. The skewing effect at this magnetic latitude is very sensitive to variations in strike of the magnetic contrasts. This effect was eliminated by a non-linear transformation which also gives the positions of magnetic contrasts. Some N-S contrasts were identified as sea floor spreading polarity contrasts (anomalies 31 and 32). Weak contrasts could be identified as topographic effects and gave a magnetization intensity of 5 A m-1. The identified sea floor spreading anomalies to both sides of the fracture zone at 36°10N agree very well, also quantatively, with a three-dimensional model for the fracture zone anomalies. This model describes the non-linear anomalies as end effects of the magnetic layer which is divided in blocks of alternating polarity. 相似文献
994.
Measurements of 226Ra and 210Pb in barite samples separated from different depths of a piston core from the eastern equatorial Pacific are reported. 226Ra is enriched in the barite as would be expected from the chemistries of Ra and Ba; the ratio centers around 1.5 · 1014 atoms/mole. The results suggest that barite is concentrating Ra at a rate comparable to its decay rate, viz. 5.78 · 10?3 yr?1. The gross 210Pb depletion with respect to 226Ra is indicative of 222Rn loss due to the smallness of the barite crystals. 相似文献
995.
The distribution of the colour index is considered in the region bounded by 8–11°N and 13°30–18°30W based on the results of measurements made on board vessels of the Marine Hydrophysical Institute of the Ukrainian SSR Academy of Sciences (MHI) in 1977–1985. Mean values and statistical characteristics are calculated for the colour index variability over one-degree squares. A map of its multi-yearly average distribution is plotted.Translated by M. M. Trufanov. 相似文献
996.
渤海、黄海、东海沉积物中矿物组合的研究 总被引:12,自引:2,他引:12
本文对黄海、渤海、东海以及长江、黄河、辽河表层沉积物中的轻、重矿物进行了研究。分析资料表明,调查区内共有三种类型的矿物:陆源矿物、自生矿物和海底火山喷发形成的火山型矿物。根据轻、重矿物组合的差异,可把调查区分成8个矿物区。其物质来源主要为黄河与长江。黄河物质主要沉积在黄海、渤海;长江物质沉积在东海陆架区,而冲绳海槽则发现有海底火山喷发物质与浊流沉积。 相似文献
997.
998.
上元-坑园牡蛎礁位于高潮位以下3m,主要成分为长牡蛎和近江牡蛎,其~(14)C年龄为3043±82a,B.P.。据此推算出罗源湾一带海岸处于缓慢上升状态,速率为1.15mm/a。根据上元-坑园牡蛎礁的上述特征,并结合断块差异升降运动的表现,作者认为这一地区中、晚全新世以来以“陆动型”海平面变化占主导地位。 相似文献
999.
1000.
The NDRE-AUV flight control system 总被引:1,自引:0,他引:1
The flight control system of an autonomous underwater vehicle (AUV) developed at the Norwegian Defence Research Establishment (NDRE) is presented. A mathematical model of the vehicle is derived and discussed. The system is separated into lightly interacting subsystems, and three autopilots are designed for steering, diving, and speed control. The design of the separate controllers is based on PID techniques. Results from extensive sea testing show robust performance and stability for the autopilot 相似文献