全文获取类型
收费全文 | 283篇 |
免费 | 52篇 |
国内免费 | 64篇 |
专业分类
测绘学 | 13篇 |
大气科学 | 7篇 |
地球物理 | 64篇 |
地质学 | 192篇 |
海洋学 | 55篇 |
天文学 | 5篇 |
综合类 | 26篇 |
自然地理 | 37篇 |
出版年
2023年 | 5篇 |
2022年 | 8篇 |
2021年 | 11篇 |
2020年 | 16篇 |
2019年 | 15篇 |
2018年 | 7篇 |
2017年 | 8篇 |
2016年 | 8篇 |
2015年 | 12篇 |
2014年 | 16篇 |
2013年 | 32篇 |
2012年 | 19篇 |
2011年 | 13篇 |
2010年 | 7篇 |
2009年 | 22篇 |
2008年 | 14篇 |
2007年 | 22篇 |
2006年 | 16篇 |
2005年 | 16篇 |
2004年 | 16篇 |
2003年 | 17篇 |
2002年 | 14篇 |
2001年 | 10篇 |
2000年 | 13篇 |
1999年 | 10篇 |
1998年 | 10篇 |
1997年 | 15篇 |
1996年 | 6篇 |
1995年 | 4篇 |
1994年 | 3篇 |
1993年 | 4篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1985年 | 2篇 |
1982年 | 1篇 |
1980年 | 1篇 |
排序方式: 共有399条查询结果,搜索用时 15 毫秒
151.
通过二维离散元模拟斜坡地基静力触探试验,研究试验过程中锥尖阻力的变化以及斜坡地基的宏微观力学响应。结果表明:归一化锥尖阻力峰值在坡角较小时无明显提升,在坡角足够大时提升明显,稳定值随着坡角增大略微减小,说明斜坡边界对表层土力学性能影响明显,而对深层土影响较弱;锥尖贯入过程中,斜坡地基探杆两侧土体变形不对称,坡上一侧膨胀,坡下一侧压缩,不对称性随着坡角增大而增大;贯入到表层时,探杆两侧微观接触力链不对称,坡上一侧具有更大应力,但当贯入到深层时力链趋向于对称,其临界深度随着坡角增大而增大,与坡角呈近似线性关系。 相似文献
152.
排水粉喷桩(2D工法)是同时采用粉喷桩和塑料排水板加固软土地基的一种新工法。在某高速公路中进行了排水粉喷桩加固软土地基的现场试验段施工,分别采用静力触探试验、十字板剪切试验等方法测定加固前及其加固后不同龄期的粉喷桩桩周土强度,得到了施工后桩间土强度变化的规律。测试结果表明采用粉喷桩联合排水板法加固后,桩周土强度有一个先降低后提高的过程,28天龄期时恢复到原状土强度,随着龄期的增长桩周土强度可以进一步增加。最后对桩周土强度的变化机理进行了初步探讨。 相似文献
153.
Scoria cone eruptions are generally modeled as a simple succession from explosive eruption to form the cone to passive effusion of lava, generally from the base of the cone. Sector collapse of scoria cones, wherein parts of the cone are rafted on a lava flow, is increasingly recognized as common, but the reasons that a cone may not be rebuilt are poorly understood. 相似文献
154.
Shallow plumbing systems for small-volume basaltic volcanoes 总被引:3,自引:3,他引:0
Gordon N. Keating Greg A. Valentine Donathon J. Krier Frank V. Perry 《Bulletin of Volcanology》2008,70(5):563-582
Eruptive dynamics in basaltic volcanoes are controlled, in part, by the conduit geometry. However, uncertainties in conduit
shape and dike-to-conduit transition geometry have limited our predictive capability for hazards assessments. We characterize
the subvolcanic geometry of small-volume basaltic volcanoes (magmatic volatile-driven eruptions, 0.1 to 0.5 km3) based on a synthesis of field studies of five basaltic volcanoes exposed to varying degrees by erosion and exhibiting feeder
dikes, conduits, and vent areas ≤250 m depth. Study areas include East Grants Ridge (New Mexico, USA), Basalt Ridge, East
Basalt Ridge, Paiute Ridge, and Southeast Crater Flat (Nevada, USA). Basaltic feeder dikes 250 to 100 m deep have typical
widths of 4–12 m, with smooth host-rock contacts (rhyolite tuff). At depths less than 100 m, heterogeneities in the host rock
form preferential pathways for small dike splays and sills, resulting in a 30-m effective width at 50 m depth. The development
of a complex conduit at depths less than 70 m is reflected in bifurcating dikes and brecciation and incorporation of the country
rock. The overall zone of effect at depths less than 50 m is ≤110 m wide (220 m elongated along the feeder dike). Based on
comparisons with theoretical conduit flow models, the width of the feeder dike at depths from 250 to 500 m is expected to
range from 1 to 10 m and is expected to decrease to about 1–2 m at depths greater than 500 m. The flaring shape of the observed
feeder systems is similar to results of theoretical modeling using lithostatic pressure-balanced flow conditions. Sizes of
observed conduits differ from modeled dimensions by up to a factor of 10 in the shallow subsurface (<50 m depth), but at depths
greater than 100 m the difference is a factor of 2 to 4. This difference is primarily due to the fact that observed eroded
conduits record the superimposed effects of multiple eruptive events, while theoretical model results define dimensions necessary
for a single, steady eruption phase. The complex details of magma-host rock interactions observed at the study areas (contact
welding, brecciation, bifurcating dikes and sills, and stoping) represent the mechanisms by which the lithostatic pressure-balanced
geometry is attained. The similarity in the normalized shapes of theoretical and observed conduits demonstrates the appropriateness
of the pressure-balanced modeling approach, consistent with the conclusions of Wilson and Head (J Geophys Res 86:2971–3001,
1981) for this type of volcano. 相似文献
155.
三十烷醇对海带三个品系雌配子体克隆生长和发育的影响 总被引:4,自引:1,他引:3
以海带雌配子休克隆为实验材料,通过施用三十烷醇处理。结果表明,三个品系在供试条件下,LH36生长最好,LH30发育率最高。它们对TA的反应为浓度在0.5~1.0×10-5mg/L时,都有促生长作用,其中LH30反应最敏感,LH10次之。浓度在1.0×10-5mg/L时,发育率最高,其中LH30发育率为51.5%,LH10为18.0%。LH36对各浓度处理的发育反应,差异都不明显。 相似文献
156.
长白山地区是典型的近代火山活动地区之一。笔者等 应用遥感图像对白头山火山锥的形成过程、喷发期次、喷发特点进行了研究。还对白头山火山锥、望天鹅破火山口及朝鲜境内形成时代较早的南胞台山破火山口进行了 研究,为这一地区火山活动的深入研究提供了资料。 相似文献
157.
Hydrological exchange and subsurface water chemistry in streams varying in salinity in south-western Australia 总被引:1,自引:0,他引:1
Many streams in Westrn Australia are naturally saline. In others, especially in the south-western corner, land-clearance and
other human activities in the catchment have accelerated rates of salinisation of surface and groundwater. Trends in surface
water salinity are well-documented but the extent of penetration of saline stream water into the sediments has been little
studied. As many of these streams have porous sandy beds and their flows may derive from groundwater, hydrologic exchange
patterns between surface water and subsurface hyporheic water were hypothesised to govern the water chemistry of such rivers.
We predicted high rates of hydraulic conductivity, leading to a close relationship between surface and subsurface (to a depth
of 50 cm) salinity, and to a lesser extent, pH and dissolved oxygen. Where surface and hyporheic water differed in salinity,
other chemical differences were hypothesised to be similarly marked, perhaps resulting from disjunct shallow subsurface aquifers.
Triplicate wells were sampled from upwelling and downwelling zones of thirteen streams ranging in salinity from ca. 0.2 to
18 g L−1. Despite the seemingly-porous sandy beds at many sites, subsurface water chemistry only 20–40 cm below the bed sometimes
differed markedly from surface water. For example, hyporheic water was only one-fifth the salinity of surface water at some
saline sites (e.g., the Tone River) or 20 per cent more saline in streams with fresh surface water (e.g., the Weld River).
At some sites of intermediate salinity (e.g., the Warren River), subsurface water was up to three times fresher than surface
or downwelling water. Percentage saturation of dissolved oxygen in the hyporheic water was consistently low (<40%) whereas
pH was more acidic than surface water, presumably due to microbial activity. Vertical hydraulic conductivity may be limited
by layers of fine sediments and clays, implying that the meso-scale (1–100 cm) hydrological dynamics within the hyporheic
zones of these rivers are more complex than their sandy beds would indicate. Assumptions of ecosystem dynamics in saline streams
must be tempered by an understanding of hyporheic salinities as subsurface fresher water may support microbial and faunal
assemblages excluded from the surface benthos by high salinity. In saline streams, as in fresh ones, the hyporheic zone is
an important component of the stream ecosystem and equally prone to disruption by human activities. 相似文献
158.
159.
An Erratum has been published for this article in Earthquake Engineering & Structural Dynamics 33(6) 2004, 793. The dynamic stiffness of a foundation embedded in a multiple‐layered halfspace is calculated postulating one‐dimensional wave propagation in cone segments. In this strength‐of‐materials approach the sectional property of the cone segment increases in the direction of wave propagation. Reflections and refractions with waves propagating in corresponding cone segments occur at layer interfaces. Compared to rigorous procedures the novel method based on cone segments is easy to apply, provides conceptual clarity and physical insight in the wave propagation mechanisms. This method postulating one‐dimensional wave propagation in cone segments with reflections and refractions at layer interfaces is evaluated, calculating the dynamic stiffness of a foundation embedded in a multiple‐layered halfspace. For sites resting on a flexible halfspace and fixed at the base, engineering accuracy (deviation of ±20%) is achieved for all degrees of freedom with a vast parameter variation. The behaviour below the cut‐off frequency in an undamped site fixed at its base is also reliably predicted. The accuracy is, in general, better than for the method based on cone frustums, which can lead to negative damping. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
160.
【目的】比较雌、雄海马对小鼠抗疲劳和改善记忆障碍的不同作用。【方法】转棒实验检测小鼠抗疲劳能力,将小鼠分3组,1组喂食普通饲料,另外2组分别喂食含有质量分数0.22%雌、雄海马的饲料,共喂食6周。跳台实验检测小鼠记忆能力,将小鼠为4组,正常组与乙醇组喂食普通饲料,另外2组分别喂食含有质量分数0.22%雌、雄海马的饲料,共喂食6周。体积分数30%乙醇灌胃乙醇组和雌、雄海马组诱导小鼠记忆损伤模型。【结果】抗疲劳实验中,与对照组相比,喂食雄海马的小鼠在棒时间显著延长(P<0.05),其血清中乳酸含量显著降低(P<0.05);而喂食雌海马的小鼠在棒时间和血清乳酸含量均无明显改变。乙醇记忆损伤实验中,与对照组相比,模型组小鼠跳下平台的潜伏期显著下降(P<0.01),错误数(次)显著升高(P<0.01);与模型组相比,喂食雄海马的小鼠跳下平台的潜伏期显著升高(P<0.05),错误数(次)显著减少(P<0.05),脑组织中的乙酰胆碱酯酶活性显著降低(P<0.05);而喂食雌海马的小鼠上述指标均无改变。【结论】在增强小鼠抗疲劳能力和改善记忆方面,雄海马的功效显著而雌海马无效。 相似文献