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   442篇
  免费   98篇
  国内免费   35篇
测绘学   64篇
大气科学   11篇
地球物理   24篇
地质学   64篇
海洋学   230篇
天文学   3篇
综合类   32篇
自然地理   147篇
  2024年   3篇
  2023年   14篇
  2022年   30篇
  2021年   35篇
  2020年   40篇
  2019年   31篇
  2018年   23篇
  2017年   16篇
  2016年   26篇
  2015年   20篇
  2014年   31篇
  2013年   37篇
  2012年   28篇
  2011年   40篇
  2010年   17篇
  2009年   24篇
  2008年   17篇
  2007年   27篇
  2006年   29篇
  2005年   9篇
  2004年   14篇
  2003年   15篇
  2002年   8篇
  2001年   4篇
  2000年   6篇
  1999年   5篇
  1998年   4篇
  1997年   2篇
  1996年   6篇
  1995年   5篇
  1994年   1篇
  1993年   2篇
  1992年   1篇
  1991年   2篇
  1990年   3篇
排序方式: 共有575条查询结果,搜索用时 15 毫秒
1.
目的:观察解郁安神法联合体外反搏(EECP)对经皮冠状动脉介入治疗(PCI)术后气滞血瘀型胸痹患者的临床疗效。方法:将80例冠心病PCI术后气滞血瘀型胸痹患者随机分为治疗组和对照组,每组各40例。对照组采用PCI术后规范化治疗,同时联合EECP治疗;治疗组在对照组基础上加服解郁安神方治疗,随访观察7周,比较2组临床证候疗效、Zung焦虑自评量表(SAS)评分、抑郁自评量表(SDS)评分、西雅图心绞痛调查量表(SAQ)评分。结果:总有效率治疗组为82.5%(33/40),高于对照组的60.0%(24/40),2组比较,差异有统计学意义(P<0.05);2组SAS、SDS、SAQ评分治疗前后组内比较及治疗后组间比较,差异均有统计学意义(P<0.05)。结论:解郁安神法联合EECP治疗可明显改善PCI术后气滞血瘀型胸痹患者的临床症状,缓解焦虑抑郁程度,提高患者生活质量,值得临床推广应用。  相似文献   
2.
Inhomogeneous substrate analysis using EM300 backscatter imagery   总被引:2,自引:0,他引:2  
Backscatter reflectivity from multibeam echo-sounders provides a powerful tool to efficiently characterize seafloor substrates. A comprehensive EM300 bathymetric and backscatter survey has been completed of Cook Strait, in central New Zealand. This paper presents a detailed analysis of the realtime corrections applied to the raw EM300 multibeam data and additional corrections required to compute angular variations of the backscatter strength. The corrections, including the local absorption coefficient, the influence of seafloor topography and sound refraction in the water column, are determined for different Cook Strait seafloor substrates. Modifying MB-System software code, we extracted the backscatter signal parameters in order to quantify the raw backscatter strength and apply additional processing. Profiles of backscatter strength versus incidence angle were computed for a variety of sites characterized by flat seafloor and homogeneous substrates, and for which ground-truth data were available. For each homogeneous site, different but characteristic backscatter profiles are observed that can be interpreted in terms of sediment facies. To analyze heterogeneous substrates, we present a statistical technique, based on a 3-dimensional distribution of (incidence angle, backscatter strength) couples that preserves geological information of the substrate components. This analysis, using backscatter data acquired on a submarine volcano, north of New Zealand, clearly differentiates soft sediments and lava flows within a heterogeneous substrate.  相似文献   
3.
多波束数据的海底数字地形模型构建   总被引:11,自引:0,他引:11  
提出大批量多格式原始多波束数据的DTM构建方法,以满足大区域、大比例尺海底地形制图的需要。在多种格式原始多波束数据接口和系统内部标准数据结构的基础上,通过对数据文件、数据种类和数据运算量的有效组织管理,实施边读入、边权重配赋的网格插值,分析了高斯、指数和距离平方反比权重函数的适用性,及最小值、最大值和平均值的实用性。在权重配赋网格插值基础上,提出分形fBM和张力样条配合使用的方程式插值方法,保证DTM数据的有效外延和地形分辨。整套算法效率高,并能有效保证DTM的精度,对存储在外部介质的数据遍历一次即可完成网格插值。  相似文献   
4.
Bathymetry of the Tonga Trench and Forearc: a map series   总被引:1,自引:0,他引:1  
Four new bathymetric maps of the Tonga Trench and forearc between 14 °S and 27 °S display the important morphologic and structural features of this dynamic convergent margin. The maps document a number of important geologic features of the margin. Major normal faults and fault lineaments on the Tonga platform can be traced along and across the upper trench slope. Numerous submarine canyons incised in the landward slope of the trench mark the pathways of sediment transport from the platform to mid- and lower-slope basins. Discontinuities in the trench axis and changes in the morphology of the landward slope can be clearly documented and may be associated with the passage and subduction of the Louisville Ridge and other structures on the subducting Pacific Plate. Changes in the morphology of the forearc as convergence changes from normal in the south to highly-oblique in the north are clearly documented. The bathymetric compilations, gridded at 500- and 200-m resolutions and extending along 500 km of the landward trench slope and axis, provide complete coverage of the outer forearc from the latitude of the Louisville Ridge-Tonga Trench collision to the northern terminus of the Tonga Ridge. These maps should serve as a valuable reference for other sea-going programs in the region, particularly the Ocean Drilling Program (ODP) and the National Science Foundation MARGINS initiative.  相似文献   
5.
针对国内在一定时期内既要生产传统纸质海图保证传统导航,又要生产电子海图满足电子导航的需求,构建了一个能够包容S-57电子海图和传统海图信息全集的一体化海图数据模型,基于此模型分析了两种数据到一体化数据模型的转换机制,并应用此模型开发原型系统,实现了二者之间的一体化组织和切换显示,为探索传统海图和电子海图的数据融合提供了思路和参考。  相似文献   
6.
In this paper we discuss the results of a swath bathymetric investigation of the Canary archipelago offshore area. These new data indicate that volcanism is pervasive throughout the seafloor in the region, much more that would be suggested by the islands. We have mapped tens of volcanic edifices between Fuerteventura and Gran Canaria and offshore Tenerife, La Gomera, El Hierro and La Palma. Volcanic flows are present between Tenerife and La Gomera and salic necks dominate the eastern insular slope of La Gomera. This bathymetry also supports land geologic studies that indicate that the oceanic archipelago has acquired its present morphology in part by mass wasting, a consequence of the collapse of the volcanic edifices. In the younger islands, Tenerife, La Palma and El Hierro, the Quaternary (1.2 to 0.15 Ma) debris avalanches are readily recognizable and can be traced offshore for distances measured in tens of km. Off the older islands, Lanzarote, Fuerteventura, Gran Canaria and La Gomera (<20 to 3.5 Ma), the avalanches have been obscured by subsequent turbidity current deposition and erosion as well as hemipelagic processes. The failure offshore western Lanzarote is in the form of a ramp at the base of the insular slope bound on the seaward side by a scarp. Its size and the lack of evidence of rotation along its landwards side precludes the possibility that it is a slump. It probably represents a slide whose outer scarp is caused by break-up of the slide. Mounds on the ramp’s surface may represent post-displacement volcanic structures or exotic blocks transported to their present locations by the slide. The failures offshore Fuerteventura are so large that, although they occurred in the Miocene-Pliocene, exotic blocks displaced from upslope are still recognizable in the insular margin morphology. The Canary Island insular margin appears to be a creation of Miocene-Pliocene mass wasting and more recent turbidity current deposition and erosion, and hemilepagic deposition. Failures offshore La Gomera are due to debris flows and/or turbidity currents. These events have obscured earlier mass wasting events. An erratum to this article is available at .  相似文献   
7.
Neil C. Mitchell   《Marine Geology》2005,220(1-4):131-151
Channels are relatively common on river-mouth deltas, but the process by which they arise from river sediment discharge is unclear because they can potentially be explained either by negatively buoyant (hyperpycnal) flows produced directly from the river outflow or by flows generated by repeated failure and mobilisation of sediment rapidly deposited at the delta front. Channels eroded through a dump site of dredge spoils are described here from multibeam and older sonar data collected in Commencement Bay, at the mouth of the Puyallup River. Shallow channels on the seaward upper surface of the dump site, away from any flows that could have been produced by delta front failures, suggest that at least some hyperpycnal flows were produced directly from the positively buoyant river outflow up to 200 m from the edge of the river mouth platform. The form of channel bed erosion is revealed by the longitudinal shape of the main eroded channel compared with the adjacent dump site profile. It suggests that the channel evolved by its steep front retreating, rather than by simple vertical entrenchment or diffusive-like evolution of the profile, a geometry interpreted as evidence that repeated failure of the bed occurred in response to shear stress imposed by bottom-travelling flows. Model calculations based on shear strengths back-calculated from the geometry of channel wall failures suggest that, if the main channel were eroded solely by hyperpycnal flows, their generation was remarkably efficient in order to create flows vigorous enough to cause channel bed failure. Besides the sediment concentration and discharge characteristics that have been considered to dictate the ability of rivers to produce hyperpycnal flows, it is suggested that the timing of floods with respect to the tidal cycle should also be important because extreme low tides may be needed to ensure that coarse sediment is transferred vigorously to the edge of river mouth platforms.  相似文献   
8.
A new bathymetric model for the central Fram Strait   总被引:1,自引:0,他引:1  
Based on data from R/V Polarstern multibeam sonar surveys between 1984 and 1997 high resolution bathymetry has been generated for the central Fram Strait. The area insonified covers approx. 36,500 km2 between 78–80°N and 0–7.5°E allowing the creation of a Digital Terrain Model (DTM) with 100 m grid spacing. The DTM was utilized for contouring and generation of a new series of bathymetric charts (AWI Bathymetric Charts of the Fram Strait, AWI BCFS) at a scale of 1:100,000. The paper starts with a brief introduction to the regional setting of the study area comprising information on the local links between bathymetry, sea ice transport and water mass exchange. The bathymetric feature names used in this article and how they were chosen is outlined. Next, the input data and processing applied are described. Thereafter the newly created grid and contour data are put into context with existing data sets. Finally the main bathymetric features of the area are characterized and the generated data products available for public disposal are specified.  相似文献   
9.
The aim of the Japanese-French Kaiyo 87 cruise was the study of the spreading axis in the North Fiji Basin (SW Pacific). A Seabeam and geophysical survey allowed us to define the detailed structure of the active NS spreading axis between 16° and 22° S and its relationships with the left lateral motion of the North Fiji Fracture Zone. Between 21° S and 18°10′ S, the spreading axis trends NS. From 18°10 S to 16°40 S the orientation of the spreading axis changes from NS to 015°. North of 16°40′ S the spreading axis trends 160°. These two 015° and 160° branches converge with the left lateral North Fiji fracture zone around 16°40′ S to define an RRFZ triple junction. Water sampling, dredging and photo TV deep towing give new information concerning the hydrothermal activity along the spreading axis. The discovery of hydrothermal deposits associated with living communities confirms this activity.  相似文献   
10.
Circular to elliptical mounds in the Canary Channel with reliefs of 75 to 375 m and diameters of 4 to 8 km partially surrounded by moats with reliefs of 25 to 75 m, were formed by piercement of the seafloor by Mesozoic evaporites. Several long gullies, < 1km wide, with abrupt terminations and pockmarks associated with these mounds were probably eroded by dense brine and hydrocarbon seeps. The salt brines that eroded the gullies were formed where salt diapirs intersect the seafloor, or in the subsurface by circulating ground water heated by igneous activity along the Canary Ridge. If the brines originated in the subsurface they reached the seafloor along faults. Displacement of the surficial sediments by sliding and creep is probably the result of the expulsion of hydrocarbons and/or vertical motion of the Mesozoic evaporites. Microtopographic features along or near the east flank of the Canary Ridge are the creation of uplift of the ridge, hydrothermal activity, mass wasting processes and turbidity currents.  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