全文获取类型
收费全文 | 807篇 |
免费 | 137篇 |
国内免费 | 196篇 |
专业分类
测绘学 | 11篇 |
大气科学 | 2篇 |
地球物理 | 92篇 |
地质学 | 519篇 |
海洋学 | 403篇 |
天文学 | 3篇 |
综合类 | 37篇 |
自然地理 | 73篇 |
出版年
2024年 | 2篇 |
2023年 | 14篇 |
2022年 | 37篇 |
2021年 | 37篇 |
2020年 | 57篇 |
2019年 | 45篇 |
2018年 | 36篇 |
2017年 | 29篇 |
2016年 | 46篇 |
2015年 | 41篇 |
2014年 | 44篇 |
2013年 | 53篇 |
2012年 | 39篇 |
2011年 | 52篇 |
2010年 | 43篇 |
2009年 | 55篇 |
2008年 | 61篇 |
2007年 | 38篇 |
2006年 | 64篇 |
2005年 | 25篇 |
2004年 | 39篇 |
2003年 | 42篇 |
2002年 | 30篇 |
2001年 | 22篇 |
2000年 | 21篇 |
1999年 | 30篇 |
1998年 | 25篇 |
1997年 | 19篇 |
1996年 | 16篇 |
1995年 | 21篇 |
1994年 | 10篇 |
1993年 | 14篇 |
1992年 | 4篇 |
1991年 | 4篇 |
1990年 | 4篇 |
1989年 | 2篇 |
1988年 | 6篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1976年 | 1篇 |
排序方式: 共有1140条查询结果,搜索用时 187 毫秒
761.
Assessment of the effectiveness of natural coral fragmentation as a dispersal mechanism for coral reef‐boring sponges 下载免费PDF全文
Eastern Pacific reefs are mostly made up of interlocking coral branches of Pocillopora, which are easily broken by physical forces associated with heavy swells and winds. In this study we investigated the potential of these coral fragments to enable propagation of boring sponges. For this, we quantified the frequency of occurrence and diversity of boring sponges in fragments of corals recently trapped among the branches of live colonies, and later tested the hypothesis that these sponges colonize new branches of corals. Nearly 80% of the coral colonies investigated had coral fragments among their branches, and 69% of these coral fragments contained boring sponges (11 species), some of these sponges in reproduction (23% of them carried oocytes). To test whether sponges inhabiting coral fragments could colonize new branching corals we transplanted them to healthy branches, and to branches whose living tissue was mechanically eliminated to simulate damage produced by grazing and death after bleaching and other causes of coral tissue mortality. All the transplanted coral fragments cemented to each new colony by means of calcification, and of the three sponge species tested (Cliona vermifera, Cliona tropicalis and Thoosa mismalolli) only C. vermifera was able to colonize both new living branches (26.9%) and cleaned branches (65.5%). The apparent capability of C. vermifera to colonize by direct contact may be another key ability of this species to maintain high frequency of occurrence in Pacific coral reefs. However, although C. tropicalis and T. mismalolli were not able to colonize new coral substrata by direct contact, coral fragments have the potential to contribute to local persistence of these sponges and to their dispersal, both by asexual (fragments) and sexual means (transport of sexual products). The present findings may partly explain the current increase of excavating sponges on deteriorating reefs with a large availability of dead branching corals. 相似文献
762.
The influence of petrophysical properties on the salt weathering of porous building rocks 总被引:1,自引:0,他引:1
D. Benavente N. Cueto J. Martínez-Martínez M. A. García del Cura J. C. Cañaveras 《Environmental Geology》2007,52(2):215-224
The influence of pore structure, water transport properties and rock strength on salt weathering is evaluated by means of
a thorough rock characterisation and a statistical analysis. The pore structure was described in terms of its porosity, pore
size distribution (quantified by mean pore radius) and specific surface area, density and water transport was characterised
by means of water permeability (saturated flow) and capillary imbibition (unsaturated flow); whilst the rock strength test
was carried out using uniaxial compressive strength, compressional and shear wave velocities, dynamic elastic constants and
waveform energy and attenuation were obtained from the digital analysis of the transmitted signal. A principal component analysis
and a stepwise multiple regression model was carried out in order to examine the direct relationships between salt weathering
and petrophysical properties. From the principal component analysis, two main components were obtained and assigned a petrophysical
meaning. The first component is mostly linked to mechanical properties, porosity and density whereas the second component
is associated with the water transport and pore structure. Salt weathering, quantified by the percentage of weight loss after
salt crystallisation, was included in both principal components, showing its dependence on their petrophysical properties.
The stepwise multiple regression analysis found that rock strength has a predominant statistical weight in the prediction
of salt weathering, with a minor contribution of water transport and pore structure parameters. 相似文献
763.
764.
With ports and other near shore structures expanding, and regions previously not considered prone to earthquakes being re-classified
after recent earthquake events, ground improvement by stone columns is increasingly considered to improve loose or soft in situ soils. The Dry Bottom Feed Stone Column construction technique can be used under water if a double-lock stone delivery system
is attached to the vibroprobe. The use of pneumatic stone transport from the barge into the vibroprobe receiver tank makes
it possible to reach large water and treatment depths. Digital recording of all relevant operation parameters provides a very
high level of quality assurance, including a diameter-over-depth profile for each stone column. An introduction to earthquake-resistant
design of stone columns and guidelines for specifying Marine Stone Columns are given. 相似文献
765.
Microbial biofilms are ubiquitous in aquatic and terrestric ecosystems as well as on man-made material. They are initial colonizers on all surfaces and take part in biogenic weathering on natural rocks as well as on building stone. The structure and function of the biofilm matrix, mainly extracellular polysaccharides (EP), is documented for biofilms on stone surfaces: the hydrated gel acts as glue between the organisms and the material surface. Besides EP, living cells, cell debris and mineral particles are embedded in the matrix. These particles appear to be deposited on the surfaces of cell walls and interfaces in the biofilm matrix. As an important function of the matrix, EP stabilizes the biological activity against periodic desiccation. It was demonstrated that in several polymers (alginate, dextran, levan and others) a typical extracellular enzyme, the alpha-amylase, develops enhanced resistance against desiccation stress. Consequences of EP production and biofilm development on surfaces are discussed.Special issue: Stone decay hazards 相似文献
766.
粘性泥石流稳定运动的力学分析 总被引:1,自引:0,他引:1
重点论讨的是,泥石流形成之后的流通段粘性泥石流稳定运动特征。以野外观察为基础,提出了沙石体整体滑动的运动模式和膨胀流体的运动模式,并对这两种稳定运动模式提出了各自的计算方法及其参数的选用意见。 相似文献
767.
本文根据古植被、古动物的间接标志以及冰缘现象等直接标志为依据,认为把我国晚更新世多年冻土南界划到北纬39—40°最为合适。进入全新世中国北方的气候出现过明显的波动和干湿变化,在高温期时气温较现在高1—2℃,在小冰期气温较现在低2.0—2.5℃。这些变化对植被和动物的生存、迁移以及人类活动产生深刻的影响。此外,多年冻土下限和雪线高度与现代相比有明显的变化。 相似文献
768.
769.
770.
在班公湖—怒江缝合带西段铁杂—日雍构造混杂岩带物玛地区,原划为上石炭统拉嘎组地层中新发现了中生代孢粉化石14种及未定种,这些化石的发现为将该地层修订为下白垩统多尼组提供了古生物学依据。上石炭统拉嘎组被修订后,研究区班公湖—怒江缝合带不再存在拉嘎组混杂在中生代地层中的现象。同时本次研究在铁杂—日雍构造混杂岩带以南的冈底斯—腾冲地层区物玛分区原划为上石炭统拉嘎组中,发现中生代孢粉化石10种及未定种,并在其灰岩夹层中发现晚侏罗世珊瑚化石6种及未定种,同时在原划分为中二叠统下拉组中也发现晚侏罗世珊瑚化石。根据化石时代和区域地层对比,拉嘎组和下拉组分别被修定为上侏罗统萨波直不勒组和吐卡日组。至此,物玛分区的上侏罗统以萨波直不勒组和吐卡日组为代表,与其北侧的班公湖—怒江地层区上侏罗统可进行对比。本文以班公湖—怒江缝合带西段铁杂—日雍构造混杂岩带为例,证实在混杂岩带开展古生物学研究非常必要。 相似文献