全文获取类型
收费全文 | 1001篇 |
免费 | 38篇 |
国内免费 | 5篇 |
专业分类
测绘学 | 16篇 |
大气科学 | 74篇 |
地球物理 | 262篇 |
地质学 | 273篇 |
海洋学 | 86篇 |
天文学 | 198篇 |
综合类 | 13篇 |
自然地理 | 122篇 |
出版年
2021年 | 10篇 |
2020年 | 10篇 |
2019年 | 8篇 |
2018年 | 15篇 |
2017年 | 19篇 |
2016年 | 13篇 |
2015年 | 14篇 |
2014年 | 25篇 |
2013年 | 66篇 |
2012年 | 31篇 |
2011年 | 35篇 |
2010年 | 42篇 |
2009年 | 43篇 |
2008年 | 43篇 |
2007年 | 39篇 |
2006年 | 43篇 |
2005年 | 50篇 |
2004年 | 40篇 |
2003年 | 27篇 |
2002年 | 26篇 |
2001年 | 22篇 |
2000年 | 30篇 |
1999年 | 27篇 |
1998年 | 25篇 |
1997年 | 23篇 |
1996年 | 13篇 |
1995年 | 20篇 |
1994年 | 20篇 |
1993年 | 15篇 |
1992年 | 15篇 |
1991年 | 13篇 |
1990年 | 15篇 |
1989年 | 12篇 |
1988年 | 9篇 |
1987年 | 15篇 |
1986年 | 9篇 |
1985年 | 18篇 |
1984年 | 15篇 |
1983年 | 13篇 |
1982年 | 15篇 |
1981年 | 16篇 |
1980年 | 9篇 |
1979年 | 9篇 |
1978年 | 9篇 |
1977年 | 8篇 |
1976年 | 6篇 |
1975年 | 10篇 |
1974年 | 7篇 |
1973年 | 8篇 |
1972年 | 9篇 |
排序方式: 共有1044条查询结果,搜索用时 15 毫秒
81.
Fred I. Shimabukuro Gary A. Chapman Earle B. Mayfield Sidney Edelson 《Solar physics》1973,30(1):163-173
The general features of the slowly varying component at centimeter and millimeter wavelengths are explained by magneto-ionic thermal emission. A model of an active region is constructed in which the electron temperature and density profile is based on recent EUV measurements, and the current-free magnetic field configuration is derived from a longitudinal magnetogram and scalar potential theory. In the model, the contributions of the reflected component of the inward extraordinary wave is important in determining the characteristic features of the radio flux and polarization. Emission by the mechanism of resonance absorption does not appear to be a significant factor in this model. 相似文献
82.
Gary Latham Maurice Ewing James Dorman Yosio Nakamura Frank Press Nafi Toksőz George Sutton Fred Duennebier David Lammlein 《Earth, Moon, and Planets》1973,7(3-4):396-421
Analysis of seismic signals from man-made impacts, moonquakes, and meteoroid impacts has established the presence of a lunar crust, approximately 60 km thick in the region of the Apollo seismic network; an underlying zone of nearly constant seismic velocity extending to a depth of about 1000 km, referred to as the mantle; and a lunar core, beginning at a depth of about 1000 km, in which shear waves are highly attenuated suggesting the presence of appreciable melting. Seismic velocitites in the crust reach 7 km s–1 beneath the lower-velocity surface zone. This velocity corresponds to that expected for the gabbroic anorthosites found to predominate in the highlands, suggesting that rock of this composition is the major constituent of the lunar crust. The upper mantle velocity of about 8 km s–1 for compressional waves corresponds to those of terrestrial olivines, pyroxenites and peridotites. The deep zone of melting may simply represent the depth at which solidus temperatures are exceeded in the lower mantle. If a silicate interior is assumed, as seems most plausible, minimum temperatures of between 1450°C and 1600°C at a depth of 1000 km are implied. The generation of deep moonquakes, which appear to be concentrated in a zone between 600 km and 1000 km deep, may now be explained as a consequence of the presence of fluids which facilitate dislocation. The preliminary estimate of meteoroid flux, based upon the statistics of seismic signals recorded from lunar impacts, is between one and three orders of magnitude lower than previous estimates from Earth-based measurements.Paper dedicated to Professor Harold C. Urey on the occasion of his 80th birthday on 29 April, 1973. 相似文献
83.
Garrett?B.?Hazelton Gary?AxenEmail author Oscar?Lovera 《Contributions to Mineralogy and Petrology》2003,145(1):1-14
Sized aggregates of glasses (47–84 wt% SiO2) were fused from igneous-derived cohesive fault rock and igneous rock, and step-heated from ~400 to >1,200 °C to obtain their 39Ar diffusion properties (average E=33,400 cal mol?1; D o=4.63×10?3 cm2 s?1). At T<~1,000 °C, glasses containing <~69 wt% SiO2 and abundant network-forming cations (Ca, Fe, Mg) reveal moderate to strong non-linear increases in D and E, reflecting structural modifications as the solid transitions to melt. Extrapolation of these Arrhenius properties down to typical geologic T-t conditions could result in a 1.5 log10 unit underestimation in the diffusion rate of Ar in similar materials. Numerical simulations based upon the diffusion results caution that some common geologic glasses will likely yield 40Ar/39Ar cooling ages rather than formation ages. However, if cooling rates are sufficiently high, ambient temperatures are sufficiently low (e.g., <65–175 °C), and coarse particles (e.g., radius (r) >~1 mm) are analyzed, glasses with compositions similar to ours may preserve their formation ages. 相似文献
84.
Davis JA May MD Greenfield BK Fairey R Roberts C Ichikawa G Stoelting MS Becker JS Tjeerdema RS 《Marine pollution bulletin》2002,44(10):1117-1129
In 1997, seven sport fish species were sampled from seven popular fishing areas in San Francisco Bay. Mercury exceeded a human health screening value in 44 of 84 (52%) samples. All collected samples of leopard shark and striped bass exceeded the mercury screening value of 0.23 microg/g wet weight. PCBs exceeded the screening value in 51 of 72 (71%) samples. DDT, chlordane, and dieldrin, had lower numbers of samples above screening values: 16 of 72 (22%) for DDT, 11 of 72 (15%) for chlordanes, and 27 of 72 (37%) for dieldrin. Concentrations of PCBs and other trace organics were highest in white croaker and shiner surfperch, the two species with the highest fat content in their muscle tissue. Fish from one location, Oakland Harbor, had significantly elevated wet weight concentrations of mercury, PCBs, DDTs, and chlordanes compared to other locations. Removal of skin from white croaker fillets reduced lipid concentrations by 27-49% and concentrations of trace organics by 33-40%. 相似文献
85.
The capture zone or contributing area of a ground water extraction well can be defined as that portion of the aquifer from which the well draws its water. Accurate delineation of capture zones is important in many ground water remediation applications and in the definition of wellhead protection areas. Their mathematical delineation is often simplified by using quasi-steady-state models based on time-weighted average pumping rates and background hydraulic gradients. We present a new semianalytic approach for the definition of capture zones under transient-flow conditions. We then use this approach to evaluate the effects of time variations in the direction of the background hydraulic gradient on capture. Results are presented in the form of capture efficiency maps (CEMs). Although the area contributing to a given well is found to generally expand relative to the steady-state average capture zone when the gradient direction varies, the zone of 100% capture may expand or contract depending on site-specific conditions. We illustrate our CEM approach by applying it to the design of a plume containment system. 相似文献
86.
Following the spill of fuel oils from the New Carissa in February 1999, approximately 300 km of beaches on the Pacific coast of North America were surveyed. A long-term observation program focused on the documentation of stranded tar balls in the vicinity of the spill site. Systematic beach surveys which were conducted over the period March 1999 to April 2001 and semi-logarithmic scale, time-series plots proved the most useful format for identifying trends. Beach monitoring continued through to August 2001. by which time 212 tar balls had been analyzed by GC/MS for their chemical characteristics. The samples of tar balls collected between February 1999 and August 2001 were qualitatively compared with New Carissa source oils (NCSO) and 101 (48%) were not consistent with NSCO. The presence of tar balls that are not related to an incident can confound attempts to define cleanup or endpoint criteria and to assess possible injury to natural resources. 相似文献
87.
John Wilson Stewart Fotheringham Gary Hunter Jochen Albrecht 《Transactions in GIS》2006,10(2):153-153
88.
89.
90.
Infinite uncertainty,forgotten feedbacks,and cost-benefit analysis of climate policy 总被引:1,自引:1,他引:1
Tol (2003) questioned the applicability of expected cost-benefit analysis to global mitigation policy when he found evidence that the
uncertainty surrounding estimates of the marginal damage of climate change could be infinite even if total damages were finite.
Yohe (2003) suggested that this problem could be alleviated if international development aid were directed at eliminating the source
of the problem – climate induced negative growth rates in a few regions along a handful of troublesome scenarios. The hypothesis
about adding a second policy lever to the climate policy calculus is shown to hold, though perhaps not as robustly as originally
thought. A portfolio of international policies with at least two independent tools can avoid infinite uncertainty on the margins
and the associated implications for global mitigation policy at a reasonable price even in the relatively unlikely event that
climate change causes negative economic growth in a region or two.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献