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   207篇
  免费   5篇
  国内免费   3篇
测绘学   13篇
大气科学   11篇
地球物理   57篇
地质学   51篇
海洋学   11篇
天文学   39篇
综合类   1篇
自然地理   32篇
  2021年   2篇
  2018年   1篇
  2017年   2篇
  2016年   3篇
  2015年   6篇
  2014年   9篇
  2013年   6篇
  2012年   3篇
  2011年   8篇
  2010年   6篇
  2009年   13篇
  2008年   7篇
  2007年   9篇
  2006年   14篇
  2005年   11篇
  2004年   9篇
  2003年   8篇
  2002年   11篇
  2001年   2篇
  2000年   3篇
  1999年   2篇
  1997年   2篇
  1996年   1篇
  1995年   1篇
  1994年   3篇
  1993年   4篇
  1992年   1篇
  1991年   3篇
  1990年   3篇
  1989年   3篇
  1988年   2篇
  1987年   4篇
  1986年   2篇
  1985年   3篇
  1984年   3篇
  1983年   7篇
  1982年   4篇
  1981年   3篇
  1980年   1篇
  1979年   4篇
  1978年   2篇
  1977年   4篇
  1976年   2篇
  1975年   4篇
  1974年   5篇
  1973年   3篇
  1972年   1篇
  1970年   1篇
  1969年   1篇
  1966年   1篇
排序方式: 共有215条查询结果,搜索用时 109 毫秒
51.
52.
A unified scheme for processing GPS dual-band phase observations   总被引:3,自引:4,他引:3  
A unified computational scheme is presented for sequential least-squares processing ofGPS dual-band carrier-beat-phase observations in network-mode positioning with orbit relaxation, and in orbit determination applications. This scheme is applicable to any spatial and temporal distribution of stations and satellites during a particularGPS experiment. Full covariance matrices can be specified for carrier-beat-phases and for weighted constraints on the ionosphere in order to improve phase ambiguity resolution. Physically meaningful choices for these covariance matrices are developed.  相似文献   
53.
By interferometric analysis ofGPS phase observations made at Owens Valley, Mojave, and Mammoth Lakes, California, we determined the coordinate components of the71–245–313 km triangle of baselines connecting these sites. A separate determination was made on each of four days, April 1–4, 1985. The satellite ephemerides used in these determinations had been derived from observations on other baselines. Therms scatters of the four daily determinations of baseline vector components about their respective means ranged from a minimum of6 mm for the north component of the71-km baseline to a maximum of34 mm for the vertical component of the245-km baseline. To test accuracy, we compared the mean of ourGPS determinations of the245-km baseline between Owens Valley and Mojave with independent determinations by others using very-long-baseline interferometry(VLBI) and satellite laser ranging(SLR). TheGPS-VLBI difference was within 2 parts in10 7 for every vector component. TheGPS-SLR difference was within6 parts in10 8 in the horizontal coordinates, but83 mm in height.  相似文献   
54.
Various methods are described for the analysis of Au and the Pt-group elements (PGE) using different geochemical media commonly collected by the exploration geochemist. The methods described utilize primarily the Instrumental Neutron Activation Analytical technique (INAA) in use at a commercial INAA laboratory.Rocks and soils are routinely analyzed by using a Pb fire-assay preconcentration on a 20-g sample and then analyzing the resultant Ag dore bead by INAA to a detection limit of 1 ppb. Growing rapidly in popularity is the analysis of a 30-g aliquot of sample for 34 elements including Au, solely by INAA. The elements included provide a wealth of information of a geological nature.Due to the very inhomogeneous nature of Au occurring in heavy-mineral concentrates it is imperative to analyze the whole concentrate. The only method which can provide a multielement analysis and still retain the sample intact is INAA.Biogeochemistry using either humus or vegetation as the sample media is rapidly becoming widely used. INAA provides a very sensitive means of determining Au and 34 other elements rapidly and at relatively low cost.These and other methods using the INAA technique for Au and the Pt-group will be described.  相似文献   
55.
The gold content of eleven French geochemical reference samples has been determined by a combined neutron-activation and fire assay method. These data represent the first reported low-level and high quality gold analyses for these reference samples.  相似文献   
56.
We use modern and novel techniques to study the problems associated with detection and analysis of multitudinous seismic events, which form the background for isolated great earthquakes. This new approach involves multivariate analysis of low and large magnitude events recorded in space over a couple of centuries in time. We propose here the deployment of the clustering scheme both for extracting small local structures and large-scale trends in synthetic data obtained from four numerically simulated models with: uniform properties (U), a Parkfield-type asperity (A), fractal brittle properties (F), and multi-size-heterogeneity fault zone (M). The mutual nearest neighbor (mnn) clustering scheme allows for extraction of multi-resolutional seismic anomalies in both the spatio-temporal and multi-dimensional feature space. We demonstrate that the large earthquakes are correlated with a certain pathway of smaller events. Visualization of the anomalies by using a recently introduced visualization package Amira reveals clearly the spatio-temporal relationships between clusters of small, medium and large earthquakes, indicating significant stress relaxation across the entire fault region. We demonstrate that this mnn scheme can extract distinct clusters of the smallest events, which precede and follow a singularly large magnitude earthquake. These clusters form larger spatio-temporal structures comprising a series of large earthquakes. The link between the large and medium magnitude events is not so clearly understood. Short-ranged correlations are dominated by strong spatio-temporal anomalies, thus reflecting the global seismic properties of the entire fault zone.Electronic Supplementary Material Supplementary material is available for this article if you access the article at . A link in the frame on the left on that page takes you directly to the supplementary material.  相似文献   
57.
Levy  Yehuda  Shalev  Eyal  Burg  Avihu  Yechieli  Yoseph  Gvirtzman  Haim 《Hydrogeology Journal》2021,29(5):1785-1795

A typical fresh–saline water interface in a coastal aquifer is characterized by saline-water circulation below the interface and freshwater flow above. Both flows are perpendicular to the shoreline. The flow pattern near two separated saline lakes is more complicated. For example, in the Middle East, the Dead Sea northern basin and the evaporation ponds of the Dead Sea Works are adjacent to each other but separated. The northern basin level is dropping by 1.1 m/year and the evaporation ponds’ levels are increasing by 0.2 m/year. The fresh–saline water interface in such situation is numerically simulated. Streamlines parallel or semiparallel to the shoreline are significant. Moreover, the fresh–saline water interface intrudes landward adjacent to the higher saline lake and is pushed lakeward adjacent to the lower saline lake. The simulation results support field observations showing that the interface migrates vertically at a faster rate relative to the changes in the water table and the lake levels.

  相似文献   
58.
The predominant period and corresponding equivalent viscous damping ratio, also known in various loading codes as effective period and effective damping coefficient, are two important parameters employed in the seismic design of base‐isolated and conventional building structures. Accurate determination of these two parameters can reduce the uncertainty in the computation of lateral displacement demands and interstory drifts for a given seismic design spectrum. This paper estimates these two parameters from data sets recorded from a full‐scale five‐story reinforced concrete building subjected to seismic base excitations of various intensities in base‐isolated and fixed‐base configurations on the outdoor shake table at the University of California, San Diego. The scope of this paper includes all test motions in which the yielding of the reinforcement has not occurred and the response can still be considered ‘elastic’. The data sets are used with three system identification methods to determine the predominant period of response for each of the test configurations. One of the methods also determines the equivalent viscous damping ratio corresponding to the predominant period. It was found that the predominant period of the fixed‐base building lengthened from 0.52 to 1.30 s. This corresponded to a significant reduction in effective system stiffness to about 16% of the original stiffness. The paper then establishes a correlation between predominant period and peak ground velocity. Finally, the predominant periods and equivalent viscous damping ratios recommended by the ASCE 7‐10 loading standard are compared with those determined from the test building. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   
59.
We describe and summarize the results of new interdisciplinary techniques for recovering and analyzing historical and paleoenvironmental data from mid-Holocene archaeological and geological deposits in the modern Nile floodplain at the site of Hierakonpolis in Upper Egypt. Application of these techniques has produced the first stratigraphic link between historic (Early Dynastic) and late prehistoric (Predynastic) periods in the Nile Valley at a site which played a central role in the evolution of Egyptian civilization.  相似文献   
60.
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