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251.
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New gravity and magnetic data from the northern Red Sea reveal the extent of the large gravity anomaly (164 mgal) and the presence of significant magnetic anomalies over St. John's Island. Spectral transformation and three-dimensional potential-field modelling delineate the surface configuration and vertical extent of the causative body and the enormous density contrast required (1.2 g/cm3) suggests that it is composed of unserpentinised peridotite (density 3.4 g/cm3) to a depth of at least 8 km.St. John's Island is uniquely located, not only at a passive continental margin but also within a fracture zone at the transition from plate separation by seafloor spreading to extension by lithospheric attenuation. This precludes several suggested mechanisms for the emplacement of ultramafic bodies in fracture zones.Thermal contraction, serpentinite diapirism and changes in the poles of rotation do not seem possible mechanisms in this tectonic environment and the emplacement is most probably related to the spreading readjustment necessary to create a continent-to-continent fracture zone. A post-Mesozoic age of emplacement, associated with the onset of continental rifting and the rejuvenation of a pre-existing continental fracture, seems most probable. 相似文献
253.
Mark A.S. McMenamin David J. Blunt Keith A. Kvenvolden Scott E. Miller Leslie F. Marcus Richard R. Pardi 《Quaternary Research》1982,18(2):174-183
Low aspartic acid d:l ratios and modern collagenlike concentration values indicate that amino acids in bones from the Rancho La Brea asphalt deposit, Los Angeles, California are better preserved than amino acids in bones of equivalent age that have not been preserved in asphalt. Amino acids were recovered from 10 Rancho La Brea bone samples which range in age from less than 200 to greater than 36,000 yr. The calibrated rates of aspartic acid racemization range from 2.1 to 5.0 × 10?6yr?1. Although this wide range of rate constants decreases the level of confidence for age estimates, use of the larger rate constant of 5.0 × 10?6yr?1 provides minimum age estimates which fit the known stratigraphic and chronologic records of the Rancho La Brea deposits. 相似文献
254.
Keith Hoggart 《Geoforum》1978,9(6):415-423
Geographers have relied on somewhat simplistic stimulus-response models to explain shopping expenditure patterns. It is argued that this has resulted in an emphasis on the end-points of the shopping process (places of purchase), rather than on the process itself; so failing to recognise that the same end-points can be reached along a variety of paths. More attention should be given to process and hence to the strategies consumers adopt when engaging in purchasing activity. In this exploratory study, set in a North Yorkshire parish, it was found that the criteria consumers used to select amongst retail outlets were associated with particlar styles of shopping activity, which were in turn tied to particular spatial patterns of purchasing. 相似文献
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Michael F Hochella Keith D Keefer Bernard H.W.S deJong 《Geochimica et cosmochimica acta》1983,47(11):2053-2058
The crystal chemistry of a naturally occurring magnesium hydroxide sulfate hydrate, (designated MHSH()), and a number of existing and predicted compounds which are structurally related to MHSH() are discussed. An x-ray diffraction powder pattern for MHSH() is calculated from single crystal data to aid in the identification of this phase, recently found associated with active hydrothermal vents along the East Pacific Rise. It is suggested that this MHSH-type crystal structure can have a variable composition due to a novel chemical substitution, Mg2+ → 2 H+, which this particular structure may be able to accommodate. In relation to this, a method for determining the stoichiometry of MHSH crystals via density measurements is also proposed. Finally, the leaching properties of heated seawater, instrumental in the genesis of metalliferous deposits on the sea-floor, may be due in part to the growth of MHSH-type phases in certain seawater-rock systems. 相似文献
257.
We have observed (66652) 1999 RZ253 with the Hubble Space Telescope at seven separate epochs and have fit an orbit to the observed relative positions of this binary. Two orbital solutions have been identified that differ primarily in the inclination of the orbit plane. The best fit corresponds to an orbital period, days, semimajor axis a=4660±170 km and orbital eccentricity e=0.460±0.013 corresponding to a system mass m=3.7±0.4×1018 kg. For a density of the albedo at 477 nm is p477=0.12±0.01, significantly higher than has been commonly assumed for objects in the Kuiper belt. Multicolor, multiepoch photometry shows this pair to have colors typical for the Kuiper belt with a spectral gradient of 0.35 per 100 nm in the range between 475 and 775 nm. Photometric variations at the four epochs we observed were as large as 12±3% but the sampling is insufficient to confirm the existence of a lightcurve. 相似文献
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260.
Data use and information creation: challenges for marine scientists and for managers 总被引:1,自引:0,他引:1
In the coastal waters of European countries and in the offshore waters of the north-east Atlantic, there is an increasing need for scientists to meet challenging objectives, such as to identify meaningful measures of 'quality', and to recommend 'indicators' to underpin implementation of directives, conventions, statutes and other more informal national and international initiatives. Those indicators may relate to particular species or habitats, to changes in physical and chemical characteristics, and even to the use to which the system is put. The problems to be overcome are difficult, but new and developing approaches will make a significant contribution. The approaches include: criteria to identify 'sensitivity' and 'importance', structures to organise information and electronic information resources to access data.The real challenge is to make the results of the various scientific initiatives relevant to and understandable by a wide range of customers with similar overlapping requirements, and thus make a genuine contribution to protecting the marine environment. Above and beyond that is the need for scientists to drive the agenda to enable real and lasting progress to be made towards ecosystem-based management of our seas and a proper consideration of what 'sustainability' may mean in the marine environment and how we utilise its resources. 相似文献