全文获取类型
收费全文 | 433篇 |
免费 | 219篇 |
国内免费 | 132篇 |
专业分类
测绘学 | 4篇 |
大气科学 | 3篇 |
地球物理 | 45篇 |
地质学 | 526篇 |
海洋学 | 161篇 |
综合类 | 24篇 |
自然地理 | 21篇 |
出版年
2024年 | 3篇 |
2023年 | 2篇 |
2022年 | 22篇 |
2021年 | 29篇 |
2020年 | 31篇 |
2019年 | 25篇 |
2018年 | 33篇 |
2017年 | 20篇 |
2016年 | 25篇 |
2015年 | 36篇 |
2014年 | 42篇 |
2013年 | 38篇 |
2012年 | 46篇 |
2011年 | 45篇 |
2010年 | 39篇 |
2009年 | 43篇 |
2008年 | 35篇 |
2007年 | 30篇 |
2006年 | 42篇 |
2005年 | 26篇 |
2004年 | 24篇 |
2003年 | 11篇 |
2002年 | 21篇 |
2001年 | 13篇 |
2000年 | 12篇 |
1999年 | 11篇 |
1998年 | 18篇 |
1997年 | 9篇 |
1996年 | 8篇 |
1995年 | 9篇 |
1994年 | 11篇 |
1992年 | 5篇 |
1991年 | 5篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 1篇 |
排序方式: 共有784条查询结果,搜索用时 757 毫秒
761.
Laser vs. settling velocity differences in silt grainsize measurements: estimation of palaeocurrent vigour 总被引:1,自引:1,他引:1
The use of grain‐size distribution of muds for the reconstruction of past deep ocean currents is becoming established and applied in the palaeoceanographic community. The methods are also applicable to shallow marine and tidal flat muds with similar inferences concerning the energy of wave and current sorting being drawn. Fine sediment grain‐size distributions can be obtained using a variety of instruments based on fundamentally different theoretical principles. These machines may give varying, sometimes misleading, results giving divergent interpretations of flow speed history. The new evidence presented here, combined with earlier work, suggests that of the three most commonly used analytical methods for fine silts with clays, settling velocity (Sedigraph) should be the method and instrument of choice, with electrical resistance pulse counters (e.g. Coulter Counter) as a suitable alternative. The data also show that laser particle sizers should be avoided for palaeocurrent reconstructions because the measured size of platey minerals can be dominated by their large projected area. This causes them to be recorded as the same size as larger equant grains although they have much smaller settling velocity and were deposited in aggregates. This produces results with a weaker relationship to the dynamics of deposition. The central problem is thus that some coarse clay/fine silt is recorded as medium to coarse silt, the key size in the ‘sortable silt’ mean size method of inferring changes in flow speed. For coarse silts (e.g. loess) the laser gives more satisfactory results. 相似文献
762.
以东建大厦地下室基坑支护施工为例,介绍了新引进的钢板桩基坑支护工程施工工艺,即在基坑内采用一层内撑式钢板桩围护体系,应用SMW工法(钢板桩外侧加搅拌桩),钢板桩用H型钢,周边搅拌桩采用水泥土搅拌桩。搅拌桩采用“二次喷浆、二次搅拌”工艺,并制定了施工流程及相应的技术措施, 相似文献
763.
永城矿区煤矿排水钻孔施工技术 总被引:1,自引:0,他引:1
永城矿区某矿排水钻孔设计孔深584.1m,因钻孔与地下巷道平面位置相距仅15m,因此要求钻孔偏斜在表土层不大于0.3%,基岩段不大于0.5%。施工中选用稳定性好、扭矩力大的TSJ-2000型工程钻机,并配置850/5型泥浆泵进行钻井作业,钻进过程中严格控制钻进参数,并采取不同措施防止钻孔偏斜。井管加工严格执行技术标准,并在准备工作充分的情况下,一次安放到位。采用向管内连续注入泥浆,通过管底凡尔返到管外的方法进行井管壁外注浆。在注浆作业完成后72h.排除管内泥浆,完成排水钻孔施工。 相似文献
764.
765.
《国际泥沙研究》2020,35(6):563-575
Erosion of mixed cohesive and noncohesive sediments is studied using the erosion test instrument SEDFlume. The sediment mixtures are composed of well-sorted quartz sand (0.25–0.5 mm) and one of the three used muds: kaolinite, kaolinite-bentonite and Mississippi River muds. The mud contents cover from 0 to 100%. The measured data of erosion rate and bed shear stress are used to examine the segmented linear, nonlinear, and exponential erosion models. The parameters of each erosion model are related to the physical properties of sediment mixtures, including clay fraction, mud fraction, mixture dry density, and mud dry density. It is found that the three models can fit well with the data, and their parameters have strong relations with the mud fraction and mud dry density, to a less extent with the clay fraction, but not with the mixture dry density. 相似文献
766.
根据霄雲煤矿1313工作面突水情况,以及该突水点周围地层和构造,针对该突水点采用定向钻孔施工技术进行堵水施工,同时施工4个定向钻孔,其中施工2个突水通道注浆孔XY1、XY2封堵通道,定向封堵通道孔找准涌水通道是钻孔施工的关键;2个盖帽钻孔XY3、XY4,定向盖帽孔对准突水点封堵并提高注浆水头压力是钻孔施工的关键;钻孔钻穿突水点附近上方的采空区顶板,通过向突水点附近的采空区注入石子、沙子、水泥等封堵出水点,起到封盖出水点的作用。采用下行方式注浆,逢漏(漏失量大于10m3/h)注浆封堵。根据钻探揭示的实际地质及水文地质条件,对钻孔的裸孔段,采取下行、大小间歇、复扫、复注的方式,达到注浆终止条件为止,凝固24h后,继续钻进,遇漏重复注浆,直至设计终孔深度为止。 相似文献
767.
黑碳在渤海泥质区的百年沉积记录 总被引:2,自引:1,他引:1
在渤海泥质沉积区采集柱状沉积物样品,分析其黑碳含量和沉积通量,结合210Pb年龄测定,探讨百年尺度黑碳沉积通量的时间序列变化对区域人类活动的响应。结果表明:(1)研究区黑碳含量和沉积通量分别为0.24~0.49mg/g[平均为(0.32±0.07)mg/g]和0.026~0.053mg/(cm2·a)[平均为(0.040±0.008)mg/(cm2·a)],黑碳含量与我国东海内陆架等海区相当,但远低于美国港湾和瑞典大陆架等污染较严重地区;(2)黑碳通量在20世纪50年代-70年代中期和90年代初-90年代末出现两个明显的峰值,分别对应于两个阶段的生物质燃烧量和煤炭、石油等化石燃料燃烧量的急剧增加;而20世纪70年代末-80年代末较低的黑碳通量以及21世纪初黑碳通量下降的趋势则分别与黄河改道事件、我国能源利用技术提高及污染控制减排措施逐步实施等有关;(3)通过比较本文结果以及国内外其它海域和湖泊的研究资料,证明黑碳沉积记录可以作为一个良好的地球化学指标,用于评估人类活动对生态环境的影响历史。 相似文献
768.
Phytoplankton productivity and community structure in marginal seas have been altered significantly during the past three decades, but it is still a challenge to distinguish the forcing mechanisms between climate change and anthropogenic activities. High time-resolution biomarker records of two 210Pb-dated sediment cores(#34: 28.5°N, 122.272°E; CJ12-1269: 28.861 9°N, 122.515 3°E) from the Min-Zhe coastal mud area were compared to reveal changes of phytoplankton productivity and community structure over the past 100 years. Phytoplankton productivity started to increase gradually from the 1970 s and increased rapidly after the late 1990 s at Site #34; and it started to increase gradually from the middle 1960 s and increased rapidly after the late 1980 s at Site CJ12-1269. Productivity of Core CJ12-1269 was higher than that of Core #34. Phytoplankton community structure variations displayed opposite patterns in the two cores. The decreasing D/B(dinosterol/brassicasterol) ratio of Core #34 since the 1960 s revealed increased diatom contribution to total productivity. In contrast, the increasing D/B ratio of Core CJ12-1269 since the 1950 s indicated increased dinoflagellate contribution to total productivity. Both the productivity increase and the increased dinoflagellate contribution in Core CJ12-1269 since the 1950–1960s were mainly caused by anthropogenic activities, as the location was closer to the Changjiang River Estuary with higher nutrient concentration and decreasing Si/N ratios. However, increased diatom contribution in Core #34 is proposed to be caused by increased coastal upwelling, with higher nutrient concentration and higher Si/N ratios. 相似文献
769.
770.
为走出农民增收和农业增效困境,武威市确立了发展日光温室生产的主体生产模式,通过气象观测对引进和推广的新型日光温室进行有益探索,重点对比以土质墙体和草砖为后墙的2种温室冬季的保温性。结果表明:(1)与温室外温度相比,2种结构温室内的温度明显偏高,且土墙保温蓄热效果明显好于草砖。其中,土墙温室较草砖温室旬平均气温偏高1.8~1.9℃,旬极端最低气温偏高2.8~2.9℃,旬平均地温偏高0.9~4.1℃;(2)从不同时段看,土墙蓄热冷却速度明显低于草砖。冬季盖帘时段,土墙温室内气温较草砖偏高3.4~10.8℃,平均偏高3.1℃,地温偏高0.1~7.4℃,平均偏高3.5℃;揭帘时段,土墙温室内气温较草砖偏高4.1~14.7℃,平均偏高4.2℃,地温偏高0.3~7.7℃,平均偏高3.7℃;(3)从典型天气条件看,夜间及清晨土墙室内气温较草砖偏高幅度晴天阴天雪天;白天偏高幅度晴天的最大,雪天的次之,阴天相差不大。土墙温室内地温变化幅度较小,晴天和阴天草砖温室内地温变化幅度略大,且土墙温室内地温高于草砖,雪天相差最大;(4)2种温室结构对产量的影响效果显著,辣椒总产量土墙温室高于草砖温室18%。 相似文献