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241.
Georges Blaha 《Journal of Geodesy》1979,53(3):215-220
A key element in any attempt to achieve a0.1 m precision in geoid representation via satellite altimetry is obtaining an ephemeris of comparable precision, or circumventing
this requirement by adjusting the ephemeris in some way, together with the geoid. The first possibility requires extensive
satellite tracking and involves an enormous number of adjustable parameters in the long arc approach.
The second psssibility allows for a piece-wise treatment of short orbital arcs considered mutually independent, in which slight
adjustments of the state vector parameters can compensate for an inherent modeling error. The main question to be answered
when pondering the possibility of using the short arc adjustment model in SEASAT-A altimetry reductions is whether or not
this method is inherently capable of representing the detailed geoid with0.1 m precision. An analysis of computer simulations provides at least a partial answer to this question by pointing out the necessary
conditions in order to achieve the desired precision; under certain favorable circumstances, not described in detail, these
conditions could prove to be also sufficient. 相似文献
242.
243.
Georges Blaha 《Journal of Geodesy》1980,54(1):119-135
Second-order derivatives of a general scalar function of position (F) with respect to the length elements along a family of local Cartesian axes are developed in the spheroidal and spherical
coordinate systems. A link between the two kinds of formulations is established when the results in spherical coordinates
are confirmed also indirectly, through a transformation from spheroidal coordinates. IfF becomesW (earth's potential) the six distinct second-order derivatives—which include one vertical and two horizontal gradients of
gravity—relate the symmetric Marussi tensor to the curvature parameters of the field.
The general formulas for the second-order derivatives ofF are specialized to yield the second-order derivatives ofU (standard potential) and ofT (disturbing potential), which allows the latter to be modeled by a suitable set of parameters. The second-order derivatives
ofT in which the property ΔT=0 is explicitly incorporated are also given. According to the required precision, the spherical approximation may or may not
be desirable; both kinds of results are presented. The derived formulas can be used for modeling of the second-order derivatives
ofW orT at the ground level as well as at higher altitudes. They can be further applied in a rotating or a nonrotating field. The
development in this paper is based on the tensor approach to theoretical geodesy, introduced by Marussi [1951] and further
elaborated by Hotine [1969], which can lead to significantly shorter demonstrations when compared to conventional approaches. 相似文献
244.
Georges R. Darbre 《地震工程与结构动力学》1989,18(3):435-443
The free-vibration analysis of parabolic cables fixed at the lower end and free to move in the horizontal direction at the upper end is performed. The dynamic stiffness of such cables is zero at the natural frequencies and this property is used to obtain the governing frequency equation. Double-frequency points at which two consecutive natural frequencies are identical occur for specific values of stiffness parameters. Expressions for the natural modes of vibration and for the contribution of the extensional and inextensional cable resistances to the modal potential energy are given. 相似文献
245.
Eleonora Braschi Lorella Francalanci Georges E. Vougioukalakis 《Bulletin of Volcanology》2012,74(5):1083-1100
Based on detailed field, petrographic, chemical, and isotopic data, this paper shows that the youngest magmas of the active Nisyros volcano (South Aegean Arc, Greece) are an example of transition from rhyolitic to less evolved magmas by multiple refilling with mafic melts, triggering complex magma interaction processes. The final magmatic activity of Nisyros was characterized by sub-Plinian caldera-forming eruption (40?ka), emplacing the Upper Pumice (UP) rhyolitic deposits, followed by the extrusion of rhyodacitic post-caldera domes (about 31–10?ka). The latter are rich in magmatic enclaves with textural and compositional (basaltic–andesite to andesite) characteristics that reveal they are quenched portions of mafic magmas included in a cooler more evolved melt. Dome-lavas have different chemical, isotopic, and mineralogical characteristics from the enclaves. The latter have lower 87Sr/86Sr and higher 143Nd/144Nd values than dome-lavas. Silica contents and 87Sr/86Sr values decrease with time among dome-lavas and enclaves. Micro-scale mingling processes caused by enclave crumbling and by widespread mineral exchanges increase from the oldest to the youngest domes, together with enclave content. We demonstrate that the dome-lavas are multi-component magmas formed by progressive mingling/mixing processes between a rhyolitic component (post-UP) and the enclave-forming mafic magmas refilling the felsic reservoir (from 15?wt.% to 40?wt.% of mafic component with time). We recognize that only the more evolved enclave magmas contribute to this process, in which recycling of cumulate plagioclase crystals is also involved. The post-UP end-member derives by fractional crystallization from the magmas leftover after the previous UP eruptions. The enclave magma differentiation develops mainly by fractional crystallization associated with multiple mixing with mafic melts changing their composition with time. A time-related picture of the relationships between dome-lavas and relative enclaves is proposed, suggesting a delay between a mafic magma input and the relative dome outpouring. We also infer that the magma viscosity reduction by re-heating allows dome extrusion without explosive activity. 相似文献
246.
Georges Durry Alain Hauchecorne Joelle Ovarlez Henri Ovarlez Ivan Pouchet Virginie Zeninari Bertrand Parvitte 《Journal of Atmospheric Chemistry》2002,43(3):175-194
Telecommunication laser diodes emitting near 1.39 m and 1.65 m in combination with direct-differential absorption spectroscopy are efficient tools to monitor in situ stratospheric H2O andCH4 with a good precision error (a few percents), a high temporal resolution (ranging from 10 ms to 1 s), a large dynamic range in the concentration measurements (four orders of magnitude) and a high selectivity in the analyte species. To illustrate the capability of laser probing technique, we report balloonborne H2Oand CH4 simultaneous measurements obtained on October 2001 atmidlatitudes (43° N). The H2O vertical profile achieved with the lasersensor in the lower stratosphere is compared with the H2O data yielded by a balloonborne frost-point hygrometer. The total hydrogen mixing ratio in the lower stratosphere, 2[CH4] + [H2O], appears to beconstant at 7.5 ± 0.1 ppmv. Nevertheless, an unexpected largedehydration of 0.5 ppmv was detected by both the laser sensor and thehygrometer between 16 km and 23 km. We suspect the occurrence of a tropicalair intrusion into mid-latitudes. We support this interpretation using a high-resolution advection model for potential vorticity. 相似文献
247.
The method applied since 1996 for the analysis of the orbital residuals of the LAGEOS satellites in order to measure the Lense-Thirring effect has been the subject of the present work. This method, based on the difference between the orbital elements of consecutive arcs, is explained and analysed also from the analytical point of view. It is proved that this “difference method” works well for the determination of the secular effects, as in the case of the relativistic precession induced by the Earth's angular momentum, but also very useful for the determination and study of the long-term periodic effects. Indeed, the only limitation in the determination of the periodic effects is the possibility of the reduction of their amplitude by a factor which depends from the periodicity of the given perturbation and from the orbital arc length chosen for the satellite during the data analysis. In the case of the Yarkovsky-Schach effect, the main non-gravitational perturbation seen in the LAGEOS satellites orbital residuals, in particular in its perigee rate and eccentricity vector excitation residuals, we show that the “difference method” is quite good also for the determination of the long-period perturbations induced by this subtle non-conservative force. 相似文献
248.
The Côte d’Ivoire–Ghana marginal ridge, mainly formed by thick detrital sediments of Lower Cretaceous age, was surveyed by deep dives with the submersible Nautile (Equanaute survey). Samples retrieved during the dives, combined with observations of video and photos records, allowed us to document a compressional event that occurred along the continental margin off Côte d’Ivoire and Ghana, during Early Cretaceous times. Remarkable fracturing, folding, and cleavage characteristics in the sedimentary wedge are best explained by large-scale strike-slip transform activity between the two parting African and South American continents in Cretaceous times. 相似文献
249.
Maresca J. Jr. Georges T. Carlson C. Riley J. 《Oceanic Engineering, IEEE Journal of》1986,11(2):180-186
An estimate of the coverage efficiency of a high-frequency (HF) skywave (ionospheric) radar for mapping ocean wave height and surface wind direction at ranges between 1000 and 3000 km was made for two different but common operational ocean-monitoring applications. In the first test, three days in duration, wind direction and wave height were mapped over the entire coverage area. In the second test, four days in duration, wind direction was mapped over the entire coverage area and wave height was mapped at preselected small regions within the coverage area. On-line quality assessment and real-time ionospheric diagnostics helped select space-time-frequency windows with low ionospheric distortion. Wind direction over the 4 000 000-km2coverage area was mapped every day with the Wide Aperture Research Facility (WARF) skywave radar in about 1 h. For large-area wave-height mapping, 6 h was allowed, and for the coverage of a smaller grid, 3 h was allowed. Within these time windows, high-confidence wave-height measurements were obtained at 59 percent of the locations attempted, using on-line processing for both tests. When postexperiment processing was included, the coverage efficiency increased to 74 percent. Greater efficiencies would be possible operationally with more experience and more sophisticated radars. Eliminating the First day's results from the statistics shows the effects of increased operating experience. Then the overall on-line efficiency increases to 69 percent, and the off-line increases to 82 percent for the six days of the two tests. 相似文献
250.
Etienne Balan Thierry Allard Madeleine Sélo François Chabaux Georges Calas 《Geochimica et cosmochimica acta》2005,69(9):2193-2204
The content of radiation-induced defects (RIDs) in kaolinite samples originating from lateritic soils and continental detritic sediments of the middle Amazon Basin (Brazil) is investigated using electron paramagnetic resonance. The paleodose registered by kaolinites ranges from 80 to 900 kGy. Present-day dose rates of radiation, determined from the whole-rock U and Th content, range between 4000 and 40,000 mGy/ka. In most samples, U and Th concentrations are correlated, suggesting that U has not been remobilized by lateritization. This observation is consistent with the fact that ∼80% of the total U content is incorporated in resistant minerals, such as zircon and Ti oxides. The heterogeneous distribution of U, observed by induced fission tracks mapping, makes it possible to neglect the α-radiation contribution of the U decay chains in the dose-rate calculation. The interpretation of the measured content of RIDs in kaolinite is then performed using the calculated present-day dose rate and assuming equilibrium in the radioactive decay chains. For the sedimentary samples, the amount of RIDs is broadly correlated to the dose rate and provides apparent absolute ages older than 20 Ma. The RID contents in kaolinites from the lateritic soils provide apparent ages ranging from 10 to 6 Ma. The high RID content of these lateritic kaolinites shows that their chemical, isotopic, and crystallographic properties are not representative of present-day weathering conditions. Models assuming the “dynamical equilibrium” of kaolinites with local physical-chemical conditions prevailing in lateritic soils are thus questionable. Alternatively, our findings bring strong support for the use of the isotopic composition of kaolinites to decipher continental paleo-climates. 相似文献