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   1140篇
  免费   201篇
  国内免费   171篇
测绘学   65篇
大气科学   247篇
地球物理   437篇
地质学   396篇
海洋学   158篇
天文学   75篇
综合类   38篇
自然地理   96篇
  2024年   1篇
  2023年   7篇
  2022年   23篇
  2021年   21篇
  2020年   21篇
  2019年   37篇
  2018年   27篇
  2017年   38篇
  2016年   35篇
  2015年   35篇
  2014年   64篇
  2013年   75篇
  2012年   33篇
  2011年   88篇
  2010年   58篇
  2009年   99篇
  2008年   81篇
  2007年   71篇
  2006年   88篇
  2005年   62篇
  2004年   59篇
  2003年   56篇
  2002年   63篇
  2001年   42篇
  2000年   44篇
  1999年   37篇
  1998年   29篇
  1997年   23篇
  1996年   19篇
  1995年   22篇
  1994年   19篇
  1993年   25篇
  1992年   14篇
  1991年   20篇
  1990年   18篇
  1989年   10篇
  1988年   13篇
  1987年   4篇
  1986年   3篇
  1985年   1篇
  1983年   2篇
  1982年   2篇
  1981年   1篇
  1980年   2篇
  1978年   17篇
  1977年   1篇
  1976年   1篇
  1954年   1篇
排序方式: 共有1512条查询结果,搜索用时 31 毫秒
991.
苏继泉 《云南地质》2009,28(3):254-258
矿床为典型的层控砂岩铜矿,重新估算铜矿床资源量,提出难选氧化矿石的“氨浸搅拌浸出→萃取→电积→浸渣浮选”加工处理流程,综合回收率提高至80%以上,并对矿床资源经济进行有效评价,为资源开发及回收利用提供可靠依据。  相似文献   
992.
以北京地区浅层地温能资源开发利用现状调查为基础,选取有代表性的昌平区某地源热泵供暖改造工程为实例,对地源热泵项目进行了技术.经济分析,并对地源热泵的设计方案提出了合理化建议。  相似文献   
993.
We calculate the heat flow from the depth of bottom-simulating seismic reflectors (BSRs) on a seismic profile in the Xisha Trough of the South China Sea, and compare them with the probe heat flow measurements. The BSR heat flow turn out to be 32–80 mW/m2, significantly lower than the measurements of 83–112 mW/m2. Such big disparity cannot be ascribed only to the errors from parameters (parameter errors) that traditionally believed to influence the BSR heat flow. Besides the parameter errors, we discuss emphatically the errors coming from the theoretical assumption for the BSR heat flow determination (theoretical errors), which occur when the BSR depth does not coincide with the base of the methane hydrate stability zone (MHSZ). If BSR stays bellow the base of MHSZ, lying at the top of free gas zone, the derived heat flow would be underestimated. Compared with the parameter errors, the theoretical errors would be relatively larger in some geological settings. The disparity between measured and BSR heat flow in the Xisha Trough might be mainly due to the theoretical error. Based on the theoretical model, assuming that the BSR lying at the top of the free gas zone, the methane flux along the Xisha seismic profile is estimated, and the thickness of the methane hydrate occurrence zone is predicted.  相似文献   
994.
利用北京市环境保护监测中心和美国大使馆的细颗粒物(PM2.5)逐时监测数据,中国科学院大气物理研究所325 m气象梯度塔资料以及实况天气图和探空资料,对2015年11月27日至12月1日北京的PM2.5重污染过程的边界层特征进行了分析。研究发现:这次重污染过程持续时间长、强度大,其中PM2.5浓度超过75 μg/m3的时次共计126 h,超过150 μg/m3共计116 h,小时最高PM2.5浓度为522 μg/m3。在高低空环流场配置的影响下,近地面静风和多层逆温结构抑制了污染物在水平和垂直方向上的输送,加上边界层内的深厚湿层,使得其中气溶胶不断吸湿增长,高PM2.5浓度得以维持。在重污染期间,湍流动能较低,不利于污染物的水平和垂直扩散。垂直方向的湍流动能一直占水平方向的15%~20%左右,水平湍流动能占主要贡献。摩擦速度与湍流动能呈现出相似的变化趋势,不同高度之间的摩擦速度差别不大。超出前后时次一个数量级的湍流强度尖峰的出现是湍流场发生调整的一个信号,是PM2.5浓度发生剧烈转变的前兆,预示着污染状况更加糟糕。重污染过程中感热通量的输送方向为从地面向大气输送,感热通量和潜热通量都大幅减少,并且表现出明显的日变化特征。对湍流功率谱计算和分析表明,在重污染过程期间,时间尺度为5 min至6 h的中尺度过程对从地面到大气方向的动量和热量通量输送做出了重要贡献。  相似文献   
995.
Aircraft-based vertical flux measurements fill a gap in the spatial domain for studies of biosphere–atmosphere exchange. To acquire valid flux data, a determination of the deviation from the mean vertical wind, w′, is essential. When using aircraft platforms, flux measurements are subject to systematic and random errors from airflow distortion caused by the lift-induced upwash ahead of the aircraft. Although upwash is typically considered to be a constant quantity over periods used for calculating fluxes, it can vary significantly over short (and longer) periods due to changes in aircraft lift. The characterization of such variations in upwash are of undeniable importance to flux measurements, especially when real-time computations of w′ are required. In this paper, the variability in upwash was compared to the calculated upwash from the model of Crawford et al. (Boundary-Layer Meteorol, 80:79–94, 1996) using data taken during a long-period (phugoid mode) free oscillation of the aircraft. The cyclic variation of lift during the free oscillation offers an ideal scenario in which to acquire in-flight data on the upwash that is present, as well as to test the capability of upwash correction models. Our results indicate that while this model corrects for much of the mean upwash, there can be significant variations in upwash on a time scale that is important to flux measurements. Our results suggest that use of the measured load factor could be an easily implemented operational constraint to minimize uncertainty in w′ due to changing upwash from changing aircraft lift. We estimate, using the phugoid data, and from variations in aircraft attitude and airspeed in flux-measurement configuration, that the uncertainty in w caused by variable upwash is approximately ± 0.05 m s−1.  相似文献   
996.
Model Simulations of the Boundary-Layer Evolution over an Arid Andes Valley   总被引:1,自引:1,他引:0  
The boundary layer of the Elqui valley in the arid north of Chile exhibits several interesting phenomena, such as a very shallow convective boundary layer (CBL) during the day. In the morning, warming is observed in and above the CBL, while the humidity decreases in the CBL. At midday, in and above the CBL of the valley, the temperature stagnates. In the afternoon in the CBL the temperature decreases and humidity increases, although the latent heat flux is very low. Because the characteristic features of the valley atmosphere are hard to interpret from observations alone, model simulations were applied. The simulations indicate that all components of the budget equations, i.e. the turbulent flux divergences, advection via the sea breeze, the upvalley and upslope wind systems, as well as subsidence, contribute to the evolution of the valley atmosphere.  相似文献   
997.
This paper presents the results of a study on the thermal conductivity of a soft saturated clay (Bangkok clay) carried out in relation to an investigation into using thermal treatment to enhance the consolidation process of soft soils. The thermal conductivity of clay specimens was measured in the laboratory using a steady state method (divided bar test) and a transient state method (needle probe test). In general, the laboratory test results show that the thermal conductivity increased with the increase in soil density. However, the needle probe test was found to yield greater thermal conductivities than those derived from the divided bar test. Furthermore, to assess the validity of the laboratory test results, the heat transfer results obtained from a full-scale embankment test that employed prefabricated vertical thermo-drains (PVTD) were simulated numerically using the laboratory determined thermal conductivity values. The numerical analysis indicates that the field thermal conductivity was close to the value obtained from the needle probe test. However, it was also found that the changes in thermal conductivity values obtained from the two laboratory methods did not impact significantly on heat flow behaviour, suggesting that the two methods are acceptable for characterizing the thermal conductivity of soils.  相似文献   
998.
This paper reviews the application of close-range terrestrial digital photogrammetry and terrestrial laser scanning for discontinuity characterization on rock cuts. Terrestrial remote sensing techniques are being increasingly used as a complement to traditional scanline and window mapping methods. They provide more comprehensive information on rock cuts, allow surveying of inaccessible outcrops, and increase user safety. Selected case studies are used to estimate the accuracy of several 3D model registration approaches and the most time-, effort- and cost-effective methods are highlighted. It is shown that simple registration networks are able to provide adequate measurement of discontinuity orientation for engineering purposes. The case studies presented also illustrate the effects of sampling bias and limitations related to discontinuity characterization using remote sensing techniques. Vertical orientation bias and occlusion can be of particular concern when persistent discontinuities dip at the same angle as the camera/scanner line-of-sight. Major advantages of the techniques are presented illustrating how terrestrial remote sensing techniques provide rapid spatial measurements of discontinuity location, orientation and curvature and are well suited to the quantification of persistence magnitudes greater than 3 m.  相似文献   
999.
Flow motion and deposition characteristics of debris flows are of concern regarding land use planning and management. A simple model for the prediction of mentioned characteristics has been developed, incorporating a friction–collision rheological model. It demonstrated to be able to satisfactorily simulate the two-dimensional behavior of laboratory results and the one-dimensional behavior of two real debris-flow events. The numerical results show that the topography of the channel bed, the yield stress level of the debris flows, and the inflow pattern have significant influence on the simulated flow motion and deposition characteristics of debris flows. In addition, the predicted run-out distance has been compared with analytical solutions and field observations. The model could be employed for the preliminary evaluation of one-dimensional run-out distance of granular debris flows provided that the volume of the debris involved in the initial mobilization is assumed.  相似文献   
1000.
The Late Miocene lacustrine Acıgöl Basin, SW Turkey, formed as an orogen-top, extensional half-graben, with the subaqueous accommodation controlled by the lake level and the bulk accommodation provided by active subsidence along a WSW-trending normal fault at the basin's southern margin. The basin-fill sedimentary succession consists of terminal alluvial-fan facies overlain by ephemeral lake-margin facies and perennial lake facies, with widespread fluvial facies at the top. The distal alluvial-fan facies include massive to stratified sandstones and massive mudstones with intervening nodular dolostones and incipient pedogenic horizons. The lake-margin facies are micritic magnesites passing laterally into peloidal, irregularly laminated magnesites towards the palaeolake margin and overlain by marlstones and dolostones, all with abundant evidence of episodic subaerial exposure (desiccation cracks, pedogenic features, and tepee structures). The perennial lake facies are micritic magnesites passing upwards into clayey dolostones and dolomitic or clayey marlstones. The fluvial facies capping the succession include planar cross-stratified conglomerates (channel-fill deposits), planar parallel-stratified, planar cross-stratified and rippled cross-laminated sandstones (crevasse-fill and crevasse splay deposits), and assemblages of mudstones intercalated with thin sandstone beds (overbank floodplain deposits).The sedimentological, mineralogical and geochemical data reveal large variations in the basin's hydrological regime, including short-term oscillations and bulk rise of the lake level, periodical changes in the Mg/Ca ratio and terrigenous mud supply, and a negative covariance of δ18O and δ13C fluctuations. The composition of terrigenous sediment and the chemistry of water supplied to the lake were controlled by the weathering, chemical leaching and erosion of the ultramafic–dolomitic bedrock in the catchment area. The bedrock yielded Mg-rich carbonate solutions that caused the deposition of Mg-carbonates in the lake.Despite short-term lake-level fluctuations, the lake's net water budget remained positive. It is suggested that the region's present-day climate and Mg-rich alkaline lakes can serve as an analogue for the climatic and hydrological conditions in the Late Miocene Acıgöl Basin.  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