首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   931篇
  免费   223篇
  国内免费   160篇
测绘学   30篇
大气科学   17篇
地球物理   347篇
地质学   262篇
海洋学   523篇
天文学   1篇
综合类   44篇
自然地理   90篇
  2024年   1篇
  2023年   10篇
  2022年   20篇
  2021年   32篇
  2020年   37篇
  2019年   36篇
  2018年   40篇
  2017年   43篇
  2016年   42篇
  2015年   51篇
  2014年   56篇
  2013年   82篇
  2012年   54篇
  2011年   64篇
  2010年   74篇
  2009年   72篇
  2008年   78篇
  2007年   61篇
  2006年   70篇
  2005年   50篇
  2004年   47篇
  2003年   43篇
  2002年   34篇
  2001年   40篇
  2000年   19篇
  1999年   25篇
  1998年   13篇
  1997年   17篇
  1996年   6篇
  1995年   12篇
  1994年   11篇
  1993年   18篇
  1992年   5篇
  1991年   8篇
  1990年   4篇
  1989年   3篇
  1988年   3篇
  1987年   3篇
  1985年   5篇
  1984年   8篇
  1983年   5篇
  1982年   4篇
  1981年   6篇
  1978年   1篇
  1976年   1篇
排序方式: 共有1314条查询结果,搜索用时 15 毫秒
1.
本文首先提出了河流泥沙输移过程中泥沙中的钙镁矿物溶蚀消耗水体中的CO2并具有碳汇功能的观点。基于前人长江干流从源头到入海口和支流2003~2007年期间4次河流悬移质泥沙的化学元素组成和矿物组成资料,分析悬移质中CaO、MgO含量和方解石、白云石含量变化特征,定量计算了这些取样点悬移质泥沙的CO2总碳汇能力和非永久性、永久性碳汇能力,分析了不同碳汇能力沿程变化规律及其原因。碳汇计算结果表明:寸滩—大通河段1956~2000年期间泥沙输移过程中钙镁矿物溶蚀产生的总碳汇量、非永久性和永久性碳汇量分别为2572万t/a、1700万t/a和872万t/a。由于输沙量减少,寸滩站—大通站河段的总碳汇量、非永久性和永久性碳汇量2006~2019年期间较1956~2000年期间相应分别减少了1852万t、1224万t和872万t。三峡水库年均淤积量1. 145亿t,损失总碳汇量675. 6万t,相当于三峡电站减排二氧化碳8580万t的7. 9%。全球河流入海年输沙量126. 1亿t,以寸滩- 吴淞口河段碳汇功能0. 060 t/t计,总碳汇量7. 57亿t相当于全球岩石风化碳汇总量10. 56亿t CO2的71. 6%。河流泥沙输移过程中钙镁矿物溶蚀的碳汇量具有重要的作用,其溶蚀速率大于原地风化。  相似文献   
2.
邵晨晨 《探矿工程》2021,48(6):102-108
上海某国际金融大厦深大基坑项目北侧紧邻运营轨道交通8号线区间隧道,最近处仅7.87 m,南侧与现有世博共同管沟净距2.0~3.0 m,基坑变形控制要求高,周边环境敏感。同时,本项目位于黄浦江畔,场地浅层为典型的淤泥质软土地层,下部为富含承压水粉(砂)土层,且微承压水层与第一、第二承压水层互为连通,止水帷幕无法隔断承压水层,地质条件复杂。设计采用分坑施工、被动区加固、预应力伺服钢支撑系统、抽灌一体化降水方案、超深地下连续墙、跟踪注浆、型钢垫层等技术方案。实测结果表明,区间隧道的最大变形6.52 mm,共同管沟的最大变形15.3 mm,其最大变形均满足变形控制要求,确保了运营区间隧道和共同管沟的安全。  相似文献   
3.
Oyster reefs and their spatial patterns are deemed to change the local hydrodynamic condition and exert profound impacts on the grain size, concentration and transportation of suspended sediments. Meanwhile, high suspended sediment concentration often results in excess mortality among oysters. Oyster reefs are rare and vital ecosystem in Liyashan national marine park, Jiangsu Coast, China. However, urgent conservation efforts should be made on account of the drastic reduction in reef areas. To investigate the sediment dynamics and the geomorphology, two tripod observation systems were deployed and UAV aerial surveys with elevation measurement using Real Time Kinematic (RTK) were also carried out. High mud content (60%) was found in the bed sediment at the reef ridge, causing much lower drag coefficient than other recorded values of living oyster reefs, indicating the death of oysters and the degradation of reefs in Liyashan. Ridgelines of the string reefs at 45° to the current direction and high suspended sediment concentration in the water body (50–370 mg/L) that exceeds the threshold (200 mg/L), which would affect nutrient uptake efficiency and further result in gill saturation, decrease of clearance rate and associated deposition, were probably crucial causes of the death of oysters. The findings are useful for restoring natural oyster reefs and designing artificial reefs for nature-based coastal defense.  相似文献   
4.
为实现对用海项目施工期悬浮泥沙扩散的实时跟踪监测,尝试采用无人机携带可见光、多光谱传感器对古雷围填海项目的围堰区进行遥感探测。利用Pix4d软件对无人机携带的可见光及多光谱照片进行处理,结果表明,可见光及多光谱影像对水体中的悬浮泥沙分布均具有一定的识别能力。通过研究分析多光谱影像的光谱特征发现,近红外波段对悬浮泥沙含量的升高较为敏感,运用波段比值公式计算结果显示,围堰内含沙水体经溢流口排入海域后浓度明显降低,并可向南继续扩散约300 m。该方法可克服遥感卫星影像时空分辨率低的特点,为用海项目资源环境影响跟踪监测提供新的技术手段。  相似文献   
5.
油砂作为21世纪以来世界能源的重要组成部分,已经有大量的地质工作者对其形成的地质特征、成矿带分布特征等进行了详细研究,并取得一定成果,但是对于遥感在油砂勘探中的应用少有研究。遥感技术识别不同地物的理论基础是同一波长位置处的反射率差异及反射率随波长的变化规律,因此本文通过分析油砂的实测光谱数据,得到其反射光谱的特征及规律。为实现在Landsat 8 OLI卫星遥感影像上对油砂的识别,根据油砂实测光谱的研究,分别提出基于油砂光谱特征和相关性分析的Landsat 8遥感影像油砂识别模式,并将其应用在柴达木盆地冷湖和马海两个研究区。结果表明:利用遥感影像进行油砂找矿靶区圈定是可行的;本文总结出的基于相关性分析的油砂提取模式优于光谱特征分析模式;冷湖地区用比值方法进行油砂遥感信息增强效果较好,马海地区则差值方法较好。  相似文献   
6.
Part one of this paper reported results from experimental compaction measurements of unconsolidated natural sand samples with different mineralogical compositions and textures. The experimental setup was designed with several cycles of stress loading and unloading applied to the samples. The setup was aimed to simulate a stress condition where sediments underwent episodes of compaction, uplift and erosion. P-wave and S-wave velocities and corresponding petrophysical (porosity and density) properties were reported. In this second part of the paper, rock physics modelling utilizing existing rock physics models to evaluate the model validity for measured data from part one were presented. The results show that a friable sand model, which was established for normally compacted sediments is also capable of describing overconsolidated sediments. The velocity–porosity data plotted along the friable sand lines not only describe sorting deterioration, as has been traditionally explained by other studies, but also variations in pre-consolidation stress or degree of stress release. The deviation of the overconsolidated sands away from the normal compaction trend on the VP/VS and acoustic impedance space shows that various stress paths can be predicted on this domain when utilizing rock physics templates. Fluid saturation sensitivity is found to be lower in overconsolidated sands compared to normally consolidated sands. The sensitivity decreases with increasing pre-consolidation stress. This means detectability for four-dimensional fluid saturation changes can be affected if sediments were pre-stressed and unloaded. Well log data from the Barents Sea show similar patterns to the experimental sand data. The findings allow the development of better rock physics diagnostics of unloaded sediments, and the understanding of expected 4D seismic response during time-lapse seismic monitoring of uplifted basins. The studied outcomes also reveal an insight into the friable sand model that its diagnostic value is not only for describing sorting microtextures, but also pre-consolidation stress history. The outcome extends the model application for pre-consolidation stress estimation, for any unconsolidated sands experiencing similar unloading stress conditions to this study.  相似文献   
7.
砂土层在地面沉降过程中呈现一定的蠕变特性。文章采用一维压缩试验对4 MPa下砂土的变形特征进行了研究,并采用改进的试验装置对砂土的微观结构进行了提取。通过对8组不同级配砂土样的蠕变特性和微观结构分析,得到如下结论:粒径单一的砂土在蠕变过程中更容易出现突变,砂粒受力后更容易发生破碎,产生滑移错动;砂土蠕变的蠕变量与砂粒级配密切相关,砂土的粒径越单一且粒径越大时,砂土的蠕变量也越大;采用表征砂土颗粒形状复杂程度的形状系数,从微观上定量分析了砂土在蠕变过程中发生破碎的情况和蠕变量大小,且形状系数越小,表明该种砂土形状越复杂,在蠕变过程中砂土的破碎性越高,蠕变量越大。  相似文献   
8.
Laboratory experiments on the New Zealand freshwater mussel Echyridella menziesii were used to investigate the short-term effects (7–8 days) of food type on rates of biodeposition and benthic substrate respiration. Post-feeding biodeposition rates ranged from 0.34 to 1.52?mg?g?1?h?1 (mean?=?0.50?mg g?1?h?1) and were unaffected by the addition of toxin-producing Microcystis. Addition of suspended sediment (30?mg?L?1) visibly altered substrate composition, and increased total and inorganic biodeposit production rates by 24–33% compared to mussels fed commercial phytoplankton stock. Biodeposition rates of mussels in lake bed substrates were 38% higher than those in silica sand for identical feeding regimes, suggesting that a significant proportion of material produced in this experiment could have been derived from feeding on organic matter in the lake bed sediments. Respiration rates were higher in treatments with Microcystis but were unaffected by the presence of mussels. This laboratory study suggests that biodeposition by E. menziesii is resilient to short-term exposure to Microcystis, and highlights the ability of mussels to alter benthic substrate composition by incorporating suspended sediment into substrates.  相似文献   
9.
Slopes consisting of saturated sand have recently moved down-slope tens or hundreds of meters under the action of earthquakes. This paper presents a simplified but accurate method predicting the triggering and displacement of such landslides. For this purpose, a simplified constitutive model simulating soil response of saturated sands along slip surfaces is proposed and validated. Then, this constitutive model is coupled with the multi-block sliding system model to predict the triggering and displacement of such slides. The multi-block model considers a general mass sliding on a trajectory which consists of n linear segments. The steps needed to apply this method are described in detail. The method was applied successfully to predict the triggering, the motion and the final configuration of the well-documented (a) Higashi Takezawa, (b) Donghekou and (c) Nikawa earthquake-induced slides.  相似文献   
10.
The restoration of meadowland using the pond and plug technique of gully elimination was performed in a 9‐mile segment along Last Chance Creek, Feather River Basin, California, in order to rehabilitate floodplain functions such as mitigating floods, retaining groundwater, and reducing sediment yield associated with bank erosion and to significantly alter the hydrologic regime. However, because the atmospheric and hydrological conditions have evolved over the restoration period, it was difficult to obtain a comprehensible evaluation of the impact of restoration activities by means of field measurements. In this paper, a new use of physically based models for environmental assessment is described. The atmospheric conditions over the sparsely gauged Last Chance Creek watershed (which does not have any precipitation or weather stations) during the combined historical critical dry and wet period (1982–1993) were reconstructed over the whole watershed using the atmospheric fifth‐generation mesoscale model driven with the US National Center for Atmospheric Research and US National Center for Environmental Prediction reanalysis data. Using the downscaled atmospheric data as its input, the watershed environmental hydrology (WEHY) model was applied to this watershed. All physical parameters of the WEHY model were derived from the existing geographic information system and satellite‐driven data sets. By comparing the prerestoration and postrestoration simulation results under the identical atmospheric conditions, a more complete environmental assessment of the restoration project was made. Model results indicate that the flood peak may be reduced by 10–20% during the wet year and the baseflow may be enhanced by 10–20% during the following dry seasons (summer to fall) in the postrestoration condition. The model results also showed that the hydrologic impact of the land management associated with the restoration mitigates bank erosion and sediment discharge during winter storm events. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号