全文获取类型
收费全文 | 1371篇 |
免费 | 211篇 |
国内免费 | 325篇 |
专业分类
测绘学 | 67篇 |
大气科学 | 99篇 |
地球物理 | 236篇 |
地质学 | 1068篇 |
海洋学 | 189篇 |
天文学 | 26篇 |
综合类 | 70篇 |
自然地理 | 152篇 |
出版年
2024年 | 5篇 |
2023年 | 14篇 |
2022年 | 27篇 |
2021年 | 53篇 |
2020年 | 39篇 |
2019年 | 60篇 |
2018年 | 47篇 |
2017年 | 53篇 |
2016年 | 58篇 |
2015年 | 73篇 |
2014年 | 94篇 |
2013年 | 113篇 |
2012年 | 116篇 |
2011年 | 109篇 |
2010年 | 95篇 |
2009年 | 77篇 |
2008年 | 96篇 |
2007年 | 100篇 |
2006年 | 100篇 |
2005年 | 74篇 |
2004年 | 56篇 |
2003年 | 61篇 |
2002年 | 52篇 |
2001年 | 40篇 |
2000年 | 28篇 |
1999年 | 34篇 |
1998年 | 38篇 |
1997年 | 23篇 |
1996年 | 35篇 |
1995年 | 16篇 |
1994年 | 36篇 |
1993年 | 17篇 |
1992年 | 17篇 |
1991年 | 11篇 |
1990年 | 8篇 |
1989年 | 6篇 |
1988年 | 4篇 |
1987年 | 7篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1979年 | 2篇 |
1954年 | 1篇 |
排序方式: 共有1907条查询结果,搜索用时 296 毫秒
1.
TANGYa XIEJiasui SUNHui 《山地科学学报》2004,1(1):38-45
Dry valleys are a striking geographic landscape in Hengduan Mountains Region and are characterized by low rainfall, desert type of vegetation and fragile environment. Past efforts and resources have been concentrated mainly on rehabilitation of degraded ecosystem and fragile environment, particularly reforestation, while socio-economic development has been largely overlooked. Despite successes in pocket areas, the overall trend of unsustainability and environmental deterioration are continuing. It is important to understand that uplift of the Tibetan Plateau is the root cause of development of dry valleys, and development and formation of dry valleys is a natural process. Human intervention has played a secondary role in development of dry valleys and degradation of dry valleys though human intervention in many cases has speeded up environmental degradation of the dry valleys. It is important to understand that dry valleys are climatic enclaves and an integrated approach that combines rehabilitation of degraded ecosystems and socio-economic development should be adopted if the overall goal of sustainable development of dry valleys is to be achieved. Promotion of niche-based cash crops, rural energy including hydropower, solar energy, biogas and fuelwood plantation is recommended as the priority activities. 相似文献
2.
对广西岩溶区三个铅锌硫化物矿床风化带观测结果表明,中低山岩溶区硫化物矿床具有更强烈的风化淋滤作用,但次生晕面积较小;岩溶平原区硫化物矿床风化与元素表生迁移特征较相似于非岩溶区的情况,表现在可产生较宽的pH偏低带和较大面积的次生晕 相似文献
3.
Sediments contained in the river bed do not necessarily contribute to morphological change. The finest part of the sediment mixture often fills the pores between the larger grains and can be removed without causing a drop in bed level. The discrimination between pore‐filling load and bed‐structure load, therefore, is of practical importance for morphological predictions. In this study, a new method is proposed to estimate the cut‐off grain size that forms the boundary between pore‐filling load and bed‐structure load. The method evaluates the pore structure of the river bed geometrically. Only detailed grain‐size distributions of the river bed are required as input to the method. A preliminary validation shows that the calculated porosity and cut‐off size values agree well with experimental data. Application of the new cut‐off size method to the river Rhine demonstrates that the estimated cut‐off size decreases in a downstream direction from about 2 to 0·05 mm, covariant with the downstream fining of bed sediments. Grain size fractions that are pore‐filling load in the upstream part of the river thus gradually become bed‐structure load in the downstream part. The estimated (mass) percentage of pore‐filling load in the river bed ranges from 0% in areas with a unimodal river bed, to about 22% in reaches with a bimodal sand‐gravel bed. The estimated bed porosity varies between 0·15 and 0·35, which is considerably less than the often‐used standard value of 0·40. The predicted cut‐off size between pore‐filling load and bed‐structure load (Dc,p) is fundamentally different from the cut‐off size between wash‐load and bed‐material load (Dc,w), irrespective of the method used to determine Dc,p or Dc,w. Dc,w values are in the order of 10?1 mm and mainly dependent on the flow characteristics, whereas Dc,p values are generally much larger (about 100 mm in gravel‐bed rivers) and dependent on the bed composition. Knowledge of Dc,w is important for the prediction of the total sediment transport in a river (including suspended fines that do not interact with the bed), whereas knowledge of Dc,p helps to improve morphological predictions, especially if spatial variations in Dc,p are taken into account. An alternative to using a spatially variable value of Dc,p in morphological models is to use a spatially variable bed porosity, which can also be predicted with the new method. In addition to the morphological benefits, the new method also has sedimentological applications. The possibility to determine quickly whether a sediment mixture is clast‐supported or matrix‐supported may help to better understand downstream fining trends, sediment entrainment thresholds and variations in hydraulic conductivity. 相似文献
4.
塔里木盆地库车坳陷大北地区白垩系巴什基奇克组储层以细一中粒岩屑砂岩为主夹少量长石岩屑砂岩,杂基为铁泥质和泥质(2%~10%).胶结物以方解石(3%~15%)为主.粘土矿物以伊利石、绿泥石和伊/蒙混层为主,不含高岭石,伊/蒙混层中的蒙皂石含量一般为(15~20)%;主要孔隙组合为残余原生粒问孔-溶蚀孔-微孔隙,占储集空间总量的(50~90)%,其次为构造缝-溶蚀孔,主要的孔隙类型为溶蚀孔和裂缝,总体属于低孔低渗-特低孔特低渗储层.综合分析认为:阶段性的前陆构造逆冲推覆作用对沉积旋回性、水体盐度变化及物源区距离变化的控制至关重要;构造活跃期扇三角洲前缘环境中形成的差分选中-粗砂岩、近源快速堆积造成的高岩屑含量、干旱成湖导致的高碳酸盐胶结和早成岩期相对深埋-后期持续快速深埋压实作用,是特低孔特低渗储层的主要成因;而较优质储层足构造平缓期辫状河三角洲前缘环境中形成的分选好的中-细砂岩、早期表生溶蚀及弱碳酸盐胶结作用、早期构造破裂的复合作用及其叠加效应的结果. 相似文献
5.
测定了2株球形棕囊藻Phaeocystis globosa P1、P2的psbA基因序列。发现得到的2个序列完全相同。以P2序列对比分析了P.globosa和P.antarctica的psbA基因在DNA序列、氮基酸序列和RNA二级结构上的差异性,发现2种棕囊藻psbA基因DNA序列和氨基酸序列非常保守,无插入/缺失,其核苷酸和氨基酸变异率分别为1.88%和1.13%。与核基因核苷酸的碱基替换不同,psbA基因核苷酸的碱基替换主要发生在密码子的第1位上。且不引起氨基酸的变化,引起氨基酸变化的碱基替换都发生在密码子的第2位和第3位上。在RNA二级结构上两序列的1~4茎环结构完全相同,表现出明显的棕囊藻属的特异性,其它结构区域差异较大。种间差异表现明显。由于psbA基因DNA序列和氨基酸序列非常保守,可能不适宜棕囊藻属的系统发育分析。但其RNA二级结构可能对于棕囊藻的分子分类有一定的参考价值。 相似文献
6.
Abstract. The temperate, symbiotic coral Cladocora caespitosa occurs naturally with variable numbers of zooxanthellae. This allows us to study differences in the physiology of symbiotic and "non-symbiotic" polyps and to correlate them with zooxanthellae activities.
Zooxanthellae density in "dark" (normal-colored) polyps was 50 times higher than in "white" (bleached) polyps.
The chlorophyll a content per zooxanthella was 3 times, and Chi c was 13 times higher in white polyps than in dark ones. O2 -flux experiments were conducted with colonies in situ and with single polyps - dark and white - under laboratory conditions. Two approaches were used to evaluate the contribution of zooxanthellae translocation products to animal respiration. Both revealed that the animal tissues derive a higher benefit from the zooxanthellae during periods of low water temperature than during warm periods. 相似文献
Zooxanthellae density in "dark" (normal-colored) polyps was 50 times higher than in "white" (bleached) polyps.
The chlorophyll a content per zooxanthella was 3 times, and Chi c was 13 times higher in white polyps than in dark ones. O
7.
深圳湾北岸泥滩大型底栖动物次级生产力研究 总被引:8,自引:2,他引:8
根据2005年3、6、9月和12月在深圳湾北岸泥滩9个取样站采集的大型底栖动物4个季度的定量样品,运用Brey(1990)的经验公式进行了大型底栖动物栖息丰度、生物量、次级生产力和P/B值的研究计算.整个研究区域大型底栖动物年次级生产力平均值(去灰干重)为54.55g/(m2.a),其中观鸟屋附近泥滩(A断面)较高,为68.85g/(m2.a),凤塘河口附近泥滩(H断面)为50.62g/(m2.a)次之,沙嘴码头附近泥滩(F断面)较低,为44.18g/(m2.a),深圳湾北岸泥滩大型底栖动物年平均P/B为2.95.可见,大型底栖动物次级生产力越靠近深圳河河口越低,这与越靠近深圳河河口,大型底栖动物个体越小、生活史更短的结果是一致的. 相似文献
8.
辽东湾S2冰侧限剪切强度的试验研究 总被引:3,自引:0,他引:3
利用试验方法系统地研究了辽东湾S2型海冰侧限剪切强度随剪切应变速率、加载方向、法向应力、温度和冰内孔隙率的变化规律。介绍了S2型海冰各向异性特点及剪切应变速率对剪切破坏方式的效应和韧脆破坏转变的条件。分析了不同加载方向海冰峰值剪切强度的差异 ,获得 - 6 2℃温度下S2冰内摩擦角和内聚力分别为 42°和 70 0kPa。利用统计分析给出峰值剪切强度与冰温和孔隙率的试验关系 相似文献
9.
李刚 《海洋地质与第四纪地质》2003,23(1):125-129
归一化层速度是对任意深度处的层速度进行归一化处理的一种速度形式,归一化处理的目的是把任意深度处的层速度转换为与深度无关的格式,归一化后层速度的横向变化主要取决于地层岩性,孔隙度等物性的变化,利用这一特点可以方便地利用已知钻井资料对其进行标定,比较简单地进行地层岩性,孔隙度等特性的预测。实际应用表明,其预测结果与钻井揭示吻合较好,预测效果令人满意。 相似文献
10.
Abstract. Respiration in Holothuria tubulosa was investigated in individuals from the Posidonia oceanica meadow off Lacco Ameno (Ischia Island, Italy). Respiratory rates increase with increasing body weight and increasing sea water temperature. Oxygen consumption of an average individual (7g dw body wall) ranges from 0.409 (14 °C) to 1.300 (26 °C) mg O2 · h-1 . Data on population density, mean size of individuals, and annual sea water temperature variations allow an assessment of holothuroid production. Values of 45.65 and 13.75 kJ · m-2 · y-1 were calculated for shallow (3 to 10 m) and deep (25 to 33 m) areas of the Posidonia meadow, respectively. Holothuroid production shows a bathymetric pattern similar to primary production of the Posidonia -epiphytes complex and the production of Posidonia litter. 相似文献