全文获取类型
收费全文 | 399篇 |
免费 | 5篇 |
国内免费 | 7篇 |
专业分类
测绘学 | 7篇 |
大气科学 | 38篇 |
地球物理 | 87篇 |
地质学 | 152篇 |
海洋学 | 48篇 |
天文学 | 43篇 |
自然地理 | 36篇 |
出版年
2021年 | 4篇 |
2019年 | 5篇 |
2018年 | 4篇 |
2016年 | 4篇 |
2015年 | 4篇 |
2014年 | 7篇 |
2013年 | 17篇 |
2012年 | 3篇 |
2011年 | 8篇 |
2010年 | 4篇 |
2009年 | 18篇 |
2008年 | 13篇 |
2007年 | 11篇 |
2006年 | 18篇 |
2005年 | 3篇 |
2004年 | 8篇 |
2003年 | 12篇 |
2002年 | 8篇 |
2001年 | 6篇 |
2000年 | 8篇 |
1999年 | 7篇 |
1998年 | 14篇 |
1997年 | 7篇 |
1996年 | 14篇 |
1995年 | 3篇 |
1994年 | 9篇 |
1993年 | 7篇 |
1992年 | 7篇 |
1991年 | 4篇 |
1990年 | 16篇 |
1989年 | 6篇 |
1988年 | 4篇 |
1987年 | 10篇 |
1986年 | 6篇 |
1985年 | 9篇 |
1984年 | 8篇 |
1983年 | 12篇 |
1982年 | 13篇 |
1981年 | 16篇 |
1980年 | 8篇 |
1979年 | 10篇 |
1978年 | 10篇 |
1977年 | 13篇 |
1976年 | 4篇 |
1975年 | 4篇 |
1974年 | 7篇 |
1973年 | 9篇 |
1971年 | 2篇 |
1969年 | 3篇 |
1934年 | 2篇 |
排序方式: 共有411条查询结果,搜索用时 218 毫秒
71.
72.
73.
Accepted practice dictates that waterflooding of gas reservoirs should commence, if ever, only when the reservoir pressure has declined to the minimum production pressure. Analytical proof of this hypothesis has yet to appear in the literature however. This paper considers a model for a gas-water reservoir with a variable production rate and enhanced recovery with waterflooding and, using an initial dynamic programming approach, confirms the above hypothesis. 相似文献
74.
During the time period of November 1968 to March 1970, 259 15.4 GHz impulsive microwave bursts have been identified of which 147 had associated 2–12 Å soft X-ray bursts. Average durations, rise times, and decay times for the microwave bursts are 2.9 ± 2.4 min, 0.9 ± 0.8 min, and 2.2 ± 2.1 min, respectively.Total durations and decay times for the X-ray events display a wide range of values from a few minutes to several hours. Rise times for 50 % of the events fell in the range of 2 to 7 min. A significant fraction (32 %) of the X-ray events may exhibit a flux enhancement prior to the main outburst.For 85 % of the flare cases, the X-ray event begins simultaneously with or before the microwave event. In 91 % of the cases the X-ray event peaks later than the microwave event. The average delay is 3.0 ± 1.9 min with 50 % of cases in the range of 0 to 4 min.The X-ray flux increases are significantly correlated with the microwave flux, increases, having a correlation coefficient of 0.43 (> 99.9 % confident).This work was supported in part by the Office of Naval Research under contract NOOO14-68-A-0196-0009 and the National Aeronautics and Space Administration through grant NGL-16-001-002. 相似文献
75.
We calculate the expected counting rate of a flat micrometeoroid detector of finite sensitivity passing in hyperbolic orbit near a planet. We assume that the distribution of particle sizes, s, can be expressed as a power law spectrum of index p, i.e. dN(s) = Cs?pds, and also that the particles encounter the sphere of influence of the planet with a certain speed v∞. The results of the calculations are then compared with the results returned by Pioneer 10 in its flyby of Jupiter. The observed increase in impact rate near Jupiter can be completely explained in terms of gravitational “focusing” of particles which are in heliocentric orbits; i.e., they are not in orbit about Jupiter. The absolute concentration of particles near the orbit of Jupiter is of the same order as at 1 AU: the exact ratio being a function of particle speed and spectral index. Data from one flyby are insufficient to determine a unique value for both the spectral index, p, and the particle velocity, v∞, but limits can be set. For reasonable encounter speeds (corresponding to eccentricities and inclinations of dust particles experienced near the Earth), the particles near Jupiter are characterized by a spectrum of index p ~ 3. The spectral index which best fits the data increases with increasing encounter speeds. 相似文献
76.
The residential preferences of Afro-American students in three black colleges in Pennsylvania, North Carolina, and Texas are factor analyzed to determine whether major differences exist. Mapping the factor scores for the students in each college reveals that state rankings vary markedly from one region to another, black spatial preferences differ markedly from those students attending predominately white universities, and preference patterns reflect contemporary interregional black migration trends. 相似文献
77.
The journey from safe yield to sustainability 总被引:35,自引:0,他引:35
Safe-yield concepts historically focused attention on the economic and legal aspects of ground water development. Sustainability concerns have brought environmental aspects more to the forefront and have resulted in a more integrated outlook. Water resources sustainability is not a purely scientific concept, but rather a perspective that can frame scientific analysis. The evolving concept of sustainability presents a challenge to hydrologists to translate complex, and sometimes vague, socioeconomic and political questions into technical questions that can be quantified systematically. Hydrologists can contribute to sustainable water resources management by presenting the longer-term implications of ground water development as an integral part of their analyses. 相似文献
78.
Stanley A. Changnon 《Climatic change》2009,94(3-4):473-482
High wind caused catastrophes, storms causing property losses >$1 million, during 1952–2006 averaged 3.1 events per year in the U.S. The average loss per event was $90 million, and the annual average loss was $354 million. High wind catastrophes were most frequent in the Northeast, Central, and West Coast areas. Storm losses on the West Coast were the nation’s highest, averaging $115 million per event. High wind losses are the nation’s only form of severe weather that maximizes on the West Coast. High wind catastrophes were most frequent in winter, and were infrequent in the late spring and early fall seasons. Loss areas were frequently confined to one state. Losses in the western U.S. and nationally have increased during the 1952–2006 period, both with statistically significant upward trends. 相似文献
79.
The Livelihood Vulnerability Index: A pragmatic approach to assessing risks from climate variability and change—A case study in Mozambique 总被引:3,自引:0,他引:3
We developed the Livelihood Vulnerability Index (LVI) to estimate climate change vulnerability in the Mabote and Moma Districts of Mozambique. We surveyed 200 households in each district to collect data on socio-demographics, livelihoods, social networks, health, food and water security, natural disasters and climate variability. Data were aggregated using a composite index and differential vulnerabilities were compared. Results suggest that Moma may be more vulnerable in terms of water resources while Mabote may be more vulnerable in terms of socio-demographic structure. This pragmatic approach may be used to monitor vulnerability, program resources for assistance, and/or evaluate potential program/policy effectiveness in data-scarce regions by introducing scenarios into the LVI model for baseline comparison. 相似文献
80.
The Valhalla uranium deposit, located 40 km north of Mount Isa, Queensland, Australia, is an albitite-hosted, Mesoproterozoic
U deposit similar to albitite-hosted uranium deposits in the Ukraine, Sweden, Brazil and Guyana. Uranium mineralisation is
hosted by a thick package of interbedded fine-grained sandstones, arkoses and gritty siltstones that are bound by metabasalts
belonging to the ca. 1,780 Ma Eastern Creek Volcanics in the Western Succession of the Mount Isa basin. Alteration associated
with U mineralisation can be divided into an early, main and late stage. The early stage is dominated by laminated and intensely
altered rock comprising albite, reibeckite, calcite, (titano)magnetite ± brannerite. The main stage of mineralisation is dominated
by brecciated and intensely altered rocks that comprise laminated and intensely altered rock cemented by brannerite, apatite,
(uranoan)-zircon, uraninite, anatase, albite, reibeckite, calcite and hematite. The late stage of mineralisation comprises
uraninite, red hematite, dolomite, calcite, chlorite, quartz and Pb-, Fe-, Cu-sulfides. Brannerite has U–Pb and Pb–Pb ages
that indicate formation between 1,555 and 1,510 Ma, with significant Pb loss evident at ca. 1,200 Ma, coincident with the
assemblage of Rodinia. The oldest ages of the brannerite overlap with 40Ar/39Ar ages of 1,533 ± 9 Ma and 1,551 ± 7 Ma from early and main-stage reibeckite and are interpreted to represent the timing
of formation of the deposit. These ages coincide with the timing of peak metamorphism in the Mount Isa area during the Isan
Orogeny. Lithogeochemical assessment of whole rock data that includes mineralised and unmineralised samples from the greater
Mount Isa district reveals that mineralisation involved the removal of K, Ba and Si and the addition of Na, Ca, U, V, Zr,
P, Sr, F and Y. U/Th ratios indicate that the ore-forming fluid was oxidised, whereas the crystal chemistry of apatite and
reibeckite within the ore zone suggests that F− and were important ore-transporting complexes. δ18O values of co-existing calcite and reibeckite indicate that mineralisation occurred between 340 and 380°C and involved a
fluid having δ18Ofluid values between 6.5 and 8.6‰. Reibeckite δD values reveal that the ore fluid had a δDfluid value between −98 and −54‰. The mineral assemblages associated with early and main stages of alteration, plus δ18Ofluid and δDfluid values, and timing of the U mineralisation are all very similar to those associated with Na–Ca alteration in the Eastern
Succession of the Mount Isa basin, where a magmatic fluid is favoured for this style of alteration. However, isotopic data
from Valhalla is also consistent with that from the nearby Mount Isa Cu deposit where a basinal brine is proposed for the
transport of metals to the deposit. Based on the evidence to hand, the source fluids could have been derived from either or
both the metasediments that underlie the Eastern Creek Volcanics or magmatism that is manifest in the Mount Isa area as small
pegmatite dykes that intruded during the Isan Orogeny. 相似文献