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291.
Vlastislav Červený Antoine B. Frangié Reviewer J. Vaněk 《Studia Geophysica et Geodaetica》1980,24(4):365-372
Summary Approximate expressions for elementary seismograms of seismic body waves propagating in media with small causal absorption are derived. Special attention is devoted to modulated signals with a smooth envelope, for which especially simple formulae were obtained. The derived expressions give a good picture of all important effects of causal absorption, viz., the frequency dependent exponential decrease of amplitudes, the velocity dispersion related to absorption, and the decrease of the prevailing frequency. 相似文献
292.
Summary The potential of a simple equipotential layer, defined by the equation of a surface, on which the layer is spread, and by the density function in the form of harmonic expansions, is expressed with the aid of the parameters of the surface and of the density function. The integral of the product of three Legendre associated functions is treated with the aid of Gaunt's integral [3, 4]. 相似文献
293.
Claude J. Allègre Francis Albarède Marc Grünenfelder Dr. Viktor Köppel 《Contributions to Mineralogy and Petrology》1974,43(3):163-194
Based on regular arrays in a U-Pb diagram of data points of zircons from polymetamorphic rocks of the Alps quantitative models are developed that include several episodic lead losses, continuous lead losses during discrete periods and combinations of both.The data from the Alps can be explained by assuming an initial age of the zircons of 2000 and/or 2500–2600 m.y. and three episodic lead losses 520–580 m.y., 300 m.y. and 30 m.y. ago with regionally varying intensities of the events as can be seen from the efficiency parameters which define the shape of the discordia. On the other hand, zircon data from North American basement rocks satisfy a model assuming a discrete interval of continuous lead loss that starts with the time of uplift of the rocks.The Th-U-Pb diagrams are valuable for recognising whether the history of the zircons and their host rock is a complex one or not which is not always noticeable in a U-Pb diagram. 相似文献
294.
295.
Second- and third-order fault-bounded Precambrian basins frequently host deposits of the sedimentary massive sulphide group.
Three-dimensional geometric modelling of the thickness of preserved basin-fill successions of the Transvaal Supergroup, using
DATAMINE software, and residual gravity modelling of the contemporary basement floor, help delineate areas of exploration
potential in this unit. Two main depositional axes are tentatively identified for the basal volcano-sedimentary protobasinal
Transvaal successions. A sheet-like geometry was indicated for the succeeding Black Reef sandstones and Chuniespoort Group
chemical sedimentary rocks. The uppermost Pretoria Group thickness model delineates eastern and western second-order basins
separated by a central submerged palaeohigh. A similar isopach pattern is noted for the thick shales of the Silverton Formation
in this group, with, in addition, a well-defined third-order basin in the northwest of the western second-order basin. The
residual gravity model indicates two linear palaeovalleys adjacent to this western basin, one coincident with one of the axes
inferred for the protobasinal rocks. The fault-bounded second- and third-order basins and depositional axes postulated here
are consistent with known geological data and suggested sedimentation models. Cumulative distortions implicit in the DATAMINE
computer modelling technique are reduced when the method is applied on the basin-wide scale, enabling identification of regional
exploration target areas rather than immediate prospecting targets.
Received: 14 August 1996 / Accepted: 13 March 1997 相似文献
296.
A chemical module describing the tropospheric photochemistry of ozone precursors in both gaseous and aqueous phases for a remote continental atmosphere has been developed within the framework of a two-dimensional cloud model. Dynamical, microphysical and chemical processes are fully interacting in order to study the influence of clouds on ozone chemistry and to quantify the relative importance of the different processes on the budget and evolution of 12 chemical species. Whereas the concentrations of highly soluble species are strongly affected by evaporation and sedimentation, less soluble species are affected primarily by accretion. The model reproduces previously observed chemical phenomena such as the enrichment of formic acid at the top of the cloud. 相似文献
297.
298.
M. Condomines K. Gr?nvold P.J. Hooker K. Muehlenbachs R.K. ONions N. óskarsson E.R. Oxburgh 《Earth and Planetary Science Letters》1983
3He/4He ratios have been obtained for basaltic, intermediate and acid volcanic glasses from Iceland. Basaltic glasses exhibit a wide range of 3He/4He ratios (4 < R/Ra < 24), which is consistent with the previously recorded range for Icelandic geothermal systems. In contrast the glasses with intermediate and acid compositions have 3He/4He values close to the atmospheric value (Ra) with the exception of a 13-Ma sample which has R/Ra= 0.07. 87Sr/86Sr, 143Nd/144Nd ratios and δ18O values are reported for the same samples.3He/4He does not correlate with either 87Sr/86Sr or 143Nd/144Nd ratio and radiogenic components of He, Sr and Nd have apparently been decoupled. Interaction of Icelandic magmas with hydrothermally altered and older Icelandic crust is the preferred explanation for variable and often low δ18O values. It is suggested that primary 3He/4He ratios may have been modified by incorporation of radiogenic helium developed within the Icelandic crust to impose a larger range of 3He/4He ratios on the erupted products than was actually inherited from the mantle beneath Iceland. Intermediate and acid samples have all been severely contaminated by atmospheric helium, most probably at very shallow levels within the crust. 相似文献
299.
Grégoire Pigeon Aude Lemonsu Nathalie Long Joël Barrié Valéry Masson Pierre Durand 《Boundary-Layer Meteorology》2006,120(2):315-351
Within the framework of ESCOMPTE, a French experiment performed in June and July 2001 in the south-east of France to study the photo-oxidant pollution at the regional scale, the urban boundary layer (UBL) program focused on the study of the urban atmosphere over the coastal city of Marseille. A methodology developed to optimize a network of 20 stations measuring air temperature and moisture over the city is presented. It is based on the analysis of a numerical simulation, performed with the non-hydrostatic, mesoscale Meso-NH model, run with four nested-grids down to a horizontal resolution of 250 m over the city and including a specific parametrization for the urban surface energy balance. A three-day period was modelled and evaluated against data collected during the preparatory phase for the project in summer 2000. The simulated thermodynamic surface fields were analysed using an empirical orthogonal function (EOF) decomposition in order to determine the optimal network configuration designed to capture the dominant characteristics of the fields. It is the first attempt of application of this kind of methodology to the field of urban meteorology. The network, of 20 temperature and moisture sensors, was implemented during the UBL-ESCOMPTE experiment and continuously recorded data from 12 June to 14 July 2001. The measurements were analysed in order to assess the urban thermodynamic island spatio-temporal structure, also using EOF decomposition. During nighttime, the influence of urbanization on temperature is clear the field is characterized by concentric thermo-pleths around the old core of the city, which is the warmest area of the domain. The moisture field is more influenced by proximity to the sea and airflow patterns. During the day, the sea breeze often moves from west or south-west and consequently the spatial pattern for both parameters is characterized by a gradient perpendicular to the shoreline. Finally, in order to assess the methodology adopted, the spatial structures extracted from the simulation of the 2000 preparatory campaign and observations gathered in 2001 have been compared. They are highly correlated, which is a relevant validation of the methodology proposed. The relations between these spatial structures and geographical characteristics of the site have also been studied. High correlations between temperature spatial structure during nighttime and urban cover fraction or street aspect ratio are observed and simulated. For temperature during daytime or moisture during both daytime and nighttime these geographical factors are not correlated with thermodynamic fields spatial structures. 相似文献
300.