全文获取类型
收费全文 | 40874篇 |
免费 | 693篇 |
国内免费 | 499篇 |
专业分类
测绘学 | 1388篇 |
大气科学 | 3155篇 |
地球物理 | 8276篇 |
地质学 | 14329篇 |
海洋学 | 3323篇 |
天文学 | 8966篇 |
综合类 | 176篇 |
自然地理 | 2453篇 |
出版年
2021年 | 385篇 |
2020年 | 428篇 |
2019年 | 480篇 |
2018年 | 1008篇 |
2017年 | 976篇 |
2016年 | 1260篇 |
2015年 | 787篇 |
2014年 | 1210篇 |
2013年 | 2170篇 |
2012年 | 1321篇 |
2011年 | 1686篇 |
2010年 | 1461篇 |
2009年 | 1870篇 |
2008年 | 1611篇 |
2007年 | 1581篇 |
2006年 | 1541篇 |
2005年 | 1286篇 |
2004年 | 1176篇 |
2003年 | 1178篇 |
2002年 | 1105篇 |
2001年 | 967篇 |
2000年 | 966篇 |
1999年 | 839篇 |
1998年 | 773篇 |
1997年 | 765篇 |
1996年 | 711篇 |
1995年 | 665篇 |
1994年 | 625篇 |
1993年 | 538篇 |
1992年 | 486篇 |
1991年 | 480篇 |
1990年 | 508篇 |
1989年 | 474篇 |
1988年 | 445篇 |
1987年 | 531篇 |
1986年 | 446篇 |
1985年 | 562篇 |
1984年 | 609篇 |
1983年 | 583篇 |
1982年 | 552篇 |
1981年 | 444篇 |
1980年 | 422篇 |
1979年 | 386篇 |
1978年 | 377篇 |
1977年 | 364篇 |
1976年 | 318篇 |
1975年 | 330篇 |
1974年 | 342篇 |
1973年 | 374篇 |
1972年 | 226篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
891.
Brett K Lord-Castillo Bruce R Mate Dawn J Wright Tomas Follett 《Transactions in GIS》2009,13(S1):63-83
The Arc Marine data model is a generalized template to guide the implementation of geographic information systems (GIS) projects in the marine environment. It developed out of a collaborative process involving research and industry shareholders in coastal and marine research. This template models and attempts to standardize common ocean and coastal data types to facilitate data sharing and analytical tool development. In this study, Arc Marine is extended from its core model to fit the research goals of the whale satellite-telemetry-tagging program of the Oregon State University Marine Mammal Institute (MMI). The study sought the best customization of the generic Arc Marine data model to enhance the key advantages of satellite telemetry for mapping the distribution and movement of endangered marine mammal species. It was found that three new groups of object classes were needed (Animal, Telemetry, and Operations). Further customization involved the development of a comprehensive framework for animal tracking with Argos satellite telemetry data. A new multidimensional data cube model was also devised, showing how this extension of Arc Marine serves as an appropriate target schema for the application of on-line analytical processing (OLAP) tools and spatial data mining of satellite telemetry tracking datasets. 相似文献
892.
Albert M. Fowler K. Michael Merrill William Ball Arne Henden Fred Vrba Craig McCreight 《Experimental Astronomy》2002,14(1):61-68
The Orion program is a project to develop a 2K × 2K infrared focal plane using InSb p-on-n diodes for detectors. It is the
natural follow-up to the successful Aladdin 1K × 1K program started in the early 90's. The work is being done at the Raytheon
Infrared Operations Division (RIO, previously known as the Santa Barbara Research Center) by many of the same people who created
the Aladdin focal plane. The design is very similar to the successful Aladdin design with the addition of reference pixels,
whole array readout (no quadrants), two-adjacent-side buttability, and a packaging design that includes going directly to
the ultimate focal plane size of 4K × 4K. So far we have successfully made a limited number of hybrid modules with InSb detectors.
In this paper we will describe the design features and test data taken from some of these devices.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
893.
894.
Sorption and Intraparticle Diffusion of Fluorescent Dyes with Consolidated Aquifer Media 总被引:2,自引:0,他引:2
David A. Sabatini 《Ground water》2000,38(5):651-656
895.
896.
897.
898.
A. I. Efimov L. N. Samoznaev V. E. Andreev I. V. Chashei M. K. Bird 《Astronomy Letters》2000,26(8):544-552
A statistical analysis of the Faraday-rotation fluctuations (FRFs) of linearly polarized radio signals from the Helios 1 and Helios 2 spacecraft shows that the FRF time power spectra can be of three types. Spectra of the first type are well fitted by a single power law in the range of fluctuation frequencies 1–10 mHz. Spectra of the second type are a superposition of a power law and two quasi-harmonic components with fluctuation frequencies of about v1=4 mHz (fundamental frequency) and v2=8 mHz (second harmonic). Spectra of the third type exhibit only one of the two quasi-harmonic components against the background of a power law. The spectral density of the quasi-harmonic components can be represented by a resonance curve with a fairly broad [Δυ ≈ (0.5–1.3)υ1,2] distribution relative to the v=v1, 2 peak. The intensity of the quasi-harmonic FRF has a radial dependence that roughly matches the radial dependence for the background FRF, while their period at the fundamental frequency is approximately equal to the period of the wellknown 5-min oscillations observed in the lower solar atmosphere. The fluctuations with 5-min periods in FRF records can be explained by the presence in the outer corona of isolated trains of Alfvén waves generated at the base of the chromosphere-corona transition layer and by acoustic waves coming from deeper layers. 相似文献
899.
The seasonality of physical, chemical, and biological water variables is a major characteristic of temperate, dimictic lakes.
Yet, few investigations have considered the potential information that is encoded in seasonal dynamics with respect to the
paleolimnological record. We used a one-year sequence of diatoms obtained from sediment traps and water samples, as well as
the sedimentary diatom record covering the past ca. 1000 years in Bates Pond, Connecticut (USA), to investigate which variables
influence the seasonal distribution of diatoms and how this can be used for the interpretation of the fossil record. The seasonal
patterns in diatom assemblages were related to stratification and, to a lesser extent, to nitrate, silica, and phosphorus.
During mixing periods in spring and autumn, both planktonic and benthic species were collected in the traps, while few lightly
silicified, spindle-shaped planktonic diatoms dominated during thermal stratification in summer. Changes in fossil diatom
assemblages reflected human activity in the watershed after European settlement and subsequent recovery in the 20th century.
A long-term trend in diatom assemblage change initiated before European settlement was probably related to increased length
of mixing periods during the Little Ice Age, indicated by the increase of taxa that presently grow during mixing periods and
by application of a preliminary seasonal temperature model. We argue that the analysis of seasonal diatom dynamics in temperate
lakes may provide important information for the refinement of paleolimnological interpretations. However, investigations of
several lakes and years would be desirable in order to establish a more robust seasonal data set for the enhancement of paleolimnological
interpretations. 相似文献
900.
A new spectral factorization method is presented for the estimation of a causal as well as a causally invertible ARMA operator from the correlation sequence of seismic traces. The method has been implemented for multichannel deconvolution of seismic traces with the aim of exploiting the trace-to-trace correlation that exists within seismograms. A layered earth model with a small reflectivity sequence has been considered, and the seismic traces have been considered as the output of a linear system driven by white noise reflection coefficient sequences. The present method is the concatenation of three algorithms, namely Kung's method for state variable ( F , G , H ) realization using a singular value decomposition (SVD) algorithm, Faurre's technique for computation of the strong spectral factor and Leverrier's algorithm for ARMA representation of the spectral factor. The inverted ARMA operator is used as a recursive filter for deconvolution of seismic traces. In the example shown, two traces with a covariance sequence of 160 ms length have been considered for multichannel deconvolution of stacked seismic traces. The results presented, when compared with those obtained from a conventional deconvolution algorithm, have shown encouraging prospects. 相似文献