全文获取类型
收费全文 | 501篇 |
免费 | 21篇 |
专业分类
测绘学 | 6篇 |
大气科学 | 26篇 |
地球物理 | 116篇 |
地质学 | 152篇 |
海洋学 | 30篇 |
天文学 | 144篇 |
综合类 | 1篇 |
自然地理 | 47篇 |
出版年
2020年 | 5篇 |
2019年 | 3篇 |
2018年 | 11篇 |
2017年 | 16篇 |
2016年 | 15篇 |
2015年 | 11篇 |
2014年 | 13篇 |
2013年 | 37篇 |
2012年 | 13篇 |
2011年 | 17篇 |
2010年 | 12篇 |
2009年 | 30篇 |
2008年 | 24篇 |
2007年 | 22篇 |
2006年 | 20篇 |
2005年 | 24篇 |
2004年 | 20篇 |
2003年 | 16篇 |
2002年 | 19篇 |
2001年 | 23篇 |
2000年 | 16篇 |
1999年 | 7篇 |
1998年 | 9篇 |
1997年 | 10篇 |
1995年 | 11篇 |
1994年 | 14篇 |
1993年 | 7篇 |
1992年 | 7篇 |
1991年 | 7篇 |
1990年 | 7篇 |
1989年 | 12篇 |
1988年 | 3篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1977年 | 5篇 |
1976年 | 1篇 |
1975年 | 5篇 |
1974年 | 3篇 |
1973年 | 5篇 |
1972年 | 3篇 |
1971年 | 2篇 |
1970年 | 2篇 |
排序方式: 共有522条查询结果,搜索用时 15 毫秒
101.
de Jong Geoffrey Rotherham Jackie Phillips G. Neil Williams Patrick J. 《Geologie en Mijnbouw》1997,76(4):311-319
In Mid Proterozoic crystalline rocks of the Mount Isa Inlier, around Cloncurry, Australia, 2000 km2 of alteration and brecciation are the product of high-temperature (> 450 °C) concentrated saline solution activity. During retrogression, this fluid was locally responsible for mobility of V, Y, Nb and light rare-earth elements (15 × enrichment). Copper and S were leached during alteration and this may have been a significant source of components in nearby Cu-Au deposits. Similar rare-earth-element behaviour has been observed in the hematite breccias which host Cu-sulfides at the giant Olympic Dam Cu-Au deposit. 相似文献
102.
Using Pioneer Venus line-of-sight gravity data and orbit simulation procedures, we have estimated apparent depths of isostatic compensation (ADCs) for twelve Venusian highland features: Asteria, Atla, Bell, Beta, Ovda, Phoebe, Tellus, Thetis and Ulfrun Regiones, and Nokomis, Gula and Sappho Montes. ADCs range from 50 km to 270 km; half of the values are less than 100 km. Using these ADCs, we estimate geoid to topography ratios (GTRs) for each area to allow comparison with convection calculations and with terrestrial data for oceanic hot spots, swells and plateaus. The geoid is estimated in the wavenumber domain from the isostatic formula, using the topography and ADC for each region. In the space domain, the GTR is equal to the least squares slope of the linear fit of the geoid to the topography. The resulting GTR range is 7–31 m/km, which is much higher than terrestrial oceanic values (−1 to 5 m/km). The features fall into two distinct groups, one with a GTR range of 7–13 m/km, and one with a range of 19–25 m/km. The exception is Beta Regio, which has a GTR of 31 m/km. A model for thermal thinning of a 100 km thick lithosphere fits all values in the lower GTR group to within one standard deviation. Airy compensation could also be present, but cannot fully compensate these features. Partial dynamic compensation of the lower GTR group in combination with lithospheric mechanisms is also possible, but not required to fit the data. The upper GTR range, 19–25 m/km, can be fit with an upper mantle, constant viscosity convection model. The large GTR values are inconsistent with the presence of a low viscosity zone. If more than one compensation mechanism is present in the regions in the higher GTR group, the GTRs will be underestimated in terms of a dynamic interpretation. We thus fit the convection models to the upper end of the GTR range, 25 m/km. Rayleigh numbers in the range 104–106 will produce a GTR of 25 m/km when combined with conductive lid thicknesses of 85–150 km. The 6 m/km range in both of the GTR groups is probably due to varying degrees of crustal and thermal compensation, combined with dynamic compensation in the upper GTR group. The difference between terrestrial and Venusian GTR ranges can be explained largely by the lack of a low viscosity zone on Venus. 相似文献
103.
Cosmogenic chlorine-36 production rates in terrestrial rocks 总被引:2,自引:0,他引:2
Marek G. Zreda Fred M. Phillips David Elmore Peter W. Kubik Pankaj Sharma Ronald I. Dorn 《Earth and Planetary Science Letters》1991,105(1-3)
Chlorine-36 is produced in rocks exposed to cosmic rays at the earth surface through thermal neutron activation of 35Cl, spallation of 39K and 40Ca, and slow negative moun capture by 40Ca. We have measured the 36Cl content of 14C-dated glacial boulders from the White Mountains in eastern California and in a 14C-dated basalt flow from Utah. Effective, time-intergrated production parameters were calculated by simultaneous solution of the 36Cl production equations. The production rates due to spallation are 4160 ± 310 and 3050 ± 210 atoms 36Cl yr−1 mol−139K and 40Ca, respectively. The thermal neutron capture rate was calculated to be (3.07 ± 0.24) × 105 neutrons (kg of rock)−1 yr−1. The reported values are normalized to sea level and high geomagnetic latitudes. Production of 36Cl at different altitudes and latitudes can be estimated by appropriate scaling of the sea level rates. Chlorine-36 dating was performed on carbonate ejecta from Meteor Crater, Arizona, and late Pleistocene morainal boulders from the Sierra Nevada, California. Calculated 36Cl ages are in good agreement with previously reported ages obtained using independent methods. 相似文献
104.
Fred M. Phillips Stanley N. Davis Peter Kubik 《Ground Water Monitoring & Remediation》1990,10(3):106-113
Chlorine-36 has been produced in large amounts by hundreds of nuclear explosions on the Nevada Test Site as well as 12 off-site explosions at eight locations in five states. Continued monitoring of the redistribution of radionuclides by subsurface water is of concern in most of the areas affected by the detonations. Chlorine-36 has the following advantages as a built-in tracer for this redistribution: its mobility is equal to or greater than water, its long half-life assures its continued usefulness over long periods, collection and storage of samples is simple, it is not subject to vapor transport at ordinary temperatures, its natural background is very low, and it does not form insoluble precipitates. Chlorine-36 from the Gnome event near Carlsbad, New Mexico, illustrates how 36 C1 can be used to help study the redistribution of radionuclides in the soil profile. Chlorine-36 is also potentially useful as a tracer to study movement of contaminants around large nuclear reactor complexes and near respositories for radioactive waste. 相似文献
105.
Spectroscopic Observations of Comet C/1996 B2 (Hyakutake) with the Caltech Submillimeter Observatory
D.C. Lis J. Keene K. Young T.G. Phillips D. Bockelée-Morvan J. Crovisier P. Schilke P.F. Goldsmith E.A. Bergin 《Icarus》1997,130(2):355-372
The apparition of Comet C/1996 B2 (Hyakutake) offered an unexpected and rare opportunity to probe the inner atmosphere of a comet with high spatial resolution and to investigate with unprecedented sensitivity its chemical composition. We present observations of over 30 submillimeter transitions of HCN, H13CN, HNC, HNCO, CO, CH3OH, and H2CO in Comet Hyakutake carried out between 1996 March 18 and April 9 at the Caltech Submillimeter Observatory. Detections of the H13CN (4–3) and HNCO (160,16–150,15) transitions represent the first observations of these species in a comet. In addition, several other transitions, including HCN (8–7), CO (4–3), and CO (6–5) are detected for the first time in a comet as is the hyperfine structure of the HCN (4–3) line. The observed intensities of the HCN (4–3) hyperfine components indicate a line center optical depth of 0.9 ± 0.2 on March 22.5 UT. The HCN/HNC abundance ratio in Comet Hyakutake at a heliocentric distance of 1 AU is similar to that measured in the Orion extended ridge— a warm, quiescent molecular cloud. The HCN/H13CN abundance ratio implied by our observations is 34 ± 12, similar to that measured in giant molecular clouds in the galactic disk but significantly lower than the Solar System12C/13C ratio. The low HCN/H13CN abundance ratio may be in part due to contamination by an SO2line blended with the H13CN (4–3) line. In addition, chemical models suggest that the HCN/H13CN ratio can be affected by fractionation during the collapse phase of the protosolar nebula; hence a low HCN/H13CN ratio observed in a comet is not inconsistent with the solar system12C/13C isotopic ratio. The abundance of HNCO relative to water derived from our observations is (7 ± 3) × 10−4. The HCN/HNCO abundance ratio is similar to that measured in the core of Sagittarius B2 molecular cloud. Although a photo-dissociative channel of HNCO leads to CO, the CO produced by HNCO is a negligible component of cometary atmospheres. Production rates of HCN, CO, H2CO, and CH3OH are presented. Inferred molecular abundances relative to water are typical of those measured in comets at 1 AU from the Sun. The exception is CO, for which we derive a large relative abundance of 30%. The evolution of the HCN production rate between March 20 and March 30 suggests that the increased activity of the comet was the cause of the fragmentation of the nucleus. The time evolution of the H2CO emission suggests production of this species from dust grains. 相似文献
106.
107.
We evaluate the relationship between the hard X-ray photon spectrum and the flux of iron K emission in a thick-target electron bombardment model. Results are presented for various power-law hard X-ray spectra. We then apply these results to two events observed with the Hard X-Ray Burst Spectrometer and the K channel of the X-Ray Polychromator Bent Crystal Spectrometer on the Solar Maximum Mission satellite. For one of the events, on 29 March, 1980, at 09:18 UT, the K flux predicted for a thick-target non-thermal process is significant compared to the background fluorescent component, and the data are indeed consistent with an enhancement of the predicted amount. For the other event, on 14 October, 1980 at 06:09 UT, the hard X-ray spectrum is so steep that no significant Ka flux is predicted for this process, and no enhancement is seen. We conclude that the agreement between the predicted K flux and the observed magnitude of the K enhancement above the fluorescent background at the time of the large hard X-ray bursts lends support to a thick-target non-thermal interpretation of impulsive hard X-ray emission in solar flares. 相似文献
108.
L. W. Acton J. L. Culhane A. H. Gabriel R. D. Bentley J. A. Bowles J. G. Firth M. L. Finch C. W. Gilbreth P. Guttridge R. W. Hayes E. G. Joki B. B. Jones B. J. Kent J. W. Leibacher R. A. Nobles T. J. Patrick K. J. H. Phillips C. G. Rapley P. H. Sheather J. C. Sherman J. P. Stark L. A. Springer R. F. Turner C. J. Wolfson 《Solar physics》1980,65(1):53-71
The 1.4–22.4 Å range of the soft X-ray spectrum includes a multitude of emission lines which are important for the diagnosis of plasmas in the 1.5–50 million degree temperature range. In particular, the hydrogen and helium-like ions of all abundant solar elements with Z > 7 have their primary transitions in this region and these are especially useful for solar flare and active region studies. The soft X-ray polychromator (XRP) is a high resolution experiment working in this spectral region. The XRP consists of two instruments with a common control, data handling and power system. The bent crystal spectrometer is designed for high time resolution studies in lines of Fe i-Fe xxvi and Ca xix. The flat crystal scanning spectrometer provides for 7 channel polychromatic mapping of flares and active regions in the resonance lines of O viii, Ne ix, Mg xi, Si xiii, S xv, Ca xix, and Fe xxv with 14 spatial resolution. In its spectral scanning mode it covers essentially the entire 1.4–22.5 Å region.This paper summarizes the scientific objectives of the XRP experiment and describes the characteristics and capabilities of the two instruments. Sufficient technical information for experiment feasibility studies is included and the resources and procedures planned for the use of the XRP within the context of the Solar Maximum Mission is briefly discussed. 相似文献
109.
110.
Tjeerd H. Van Andel Richard P. Von Herzen J. D. Phillips 《Marine Geophysical Researches》1971,1(3):261-283
At 11°N latitude, the Mid-Atlantic ridge is offset 300 km by the Vema fracture zone. Between the ridge offset, the fracture consists of an elongate, parallelogram-shaped trough bordered on the north and south by narrow, high walls. The W-E trending valley floor is segmented by basement ridges and troughs which trend W10°N and are deeply buried by sediment. Uniform high heat flow characterizes the valley area. Seismically inactive valleys south of the Vema fracture, also trending W10°N, are interpreted as relict fracture zones. We explain the high heat flow and the shape of the Vema fracture as the results of secondary sea-floor spreading produced by a reorientation of the direction of sea-floor spreading from W10°N to west-east. This reorientation probably began approximately 10 million years ago. Rapid filling of the fracture valley by turbidites from the Demerara Abyssal plain took place during the last million years.The large amount of differential uplift in the Vema fracture is not explained by the reorientation model. Since the spreading rate across the valley is small compared to that across the ridge crest, we suggest that it takes place by intrusion of very thin dikes that cool rapidly and hence have high viscosity. Upwelling in the fracture valley will thus result in cosiderable loss of hydraulic head, according to models by Sleep and Biehler (1970), and recovery of the lost head could produce valley walls higher than the adjacent ridge crest. We further postulate that the spreading takes place along the edges of the fracture zone rather than in the center. This would account for the uniform distribution of heat flow along the fracture valley and for the lack of disturbance of the valley fill. As a consequence, a median ridge should form in the center, where head loss is compensated in the older crust; such a median ridge may be present. The width of the valley should be a function of the angle and time of reorientation, and of the spreading rate; the width so obtained for the Vema fracture is in accordance with the observed width. If this model is correct, the narrowness of the valley walls implies a thin lithosphere of very limited horizontal strength. 相似文献