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91.
This paper discusses some geochemical mapping techniques and their effectiveness in detecting of gold and its pathfinder's geochemical anomalies. The study area (Marrancos area) is situated in Northern Portugal. Gold occurs in a quartz breccia with pyrite and arsenopyrite hosted by metamorphic rocks (hornfels) and it is present in these two sulphide minerals as inclusions or disseminated in the crystal lattice. In this area, the geochemical anomalies obtained by isoconcentration mapping of single elements were difficult to interpret. The use of alternative geochemical mapping such as multielement halos (additive halos) proved to be effective in the definition of Au, As, Bi, Sb, Te and Se secondary geochemical signature. 相似文献
92.
N.B. Trivedi Elesbão G. de Souza Eurico R. de Paula J.M. da Costa Hugo Fournier 《Physics of the Earth and Planetary Interiors》1981,25(3):241-244
Magnetotelluric measurements in the period range 100–86400 s were conducted at a coastal station under the equatorial electrojet (Eusebio, 3.87°S, 321.58°E). The magnetotelluric data were hand-scaled and analysed to obtain a scalar apparent resistivity profile at Eusebio. The depth of the intermediate conducting layer was found to be in the range 45–75 km and the final conducting layer seems to begin at a depth of about 350 km. Possible effects of the source field equatorial electrojet are discussed and our results are compared with those of an African station under the equatorial electrojet. 相似文献
93.
Thirty crude oils were sampled in the South German Molasse Basin. The crude oils were chemically separated into five component groups and their stable carbon isotopic ratios were determined by mass spectrometry. A distinct differentiation of four regionally connected groups can be inferred from the results of the δ13C values and chemical analyses. This grouping is in accordance with the present geologic information on the origin of crude oils in the Molasse Basin.The chemistry of crude oils seems to have been influenced by secondary processes in the eastern part of the Molasse Basin. 相似文献
94.
Paula Schneiderhöhn 《Contributions to Mineralogy and Petrology》1954,4(1-2):172-191
Zusammenfassung Kurze Darlegung der zur Verwendung an Sandkornern geeigneten Methoden zur Ermittlung von Abrundung und Form. Vergleichende Betrachtung der an 25 Kernprojektionen mit den verschiedenen Methoden erzielten Untersuchungsergebnisse. Anwendung der gemachten Erfahrungen auf den Dünnschliff unter Erörterung der hierbei auftretendon besonderen Fragen und Schwierigkeiten.Herrn Professor Dr.Carl W. Correns zum 60. Geburtstag gewidmet. 相似文献
95.
María Paula Iglesia Llanos Jennifer Alice Tait Viktor Popov Alexandra Abalmassova 《Earth and Planetary Science Letters》2005,240(3-4):732-747
The study area is situated along the Zolotica river in NW Russia, located within the Kola–Dvyna Rift System in the Baltic Shield that developed during Meso and Neoproterozoic times. A 9-m thick section made up of shallow marine sediments of Upper Ediacaran age was sampled in this locality. Two volcaniclastic levels from the middle part of the section yielded an age of 556 Ma. (U/Pb SHRIMP-II on zircons). Two magnetic components were successfully isolated, component A (Decl = 157.1, Incl = 68.0, 95 = 1.9°, N = 575 in situ) carried by magnetite and component B (Decl = 120.3, Incl = − 31.7, 95 = 3.9°, N = 57, bedding corrected), carried by haematite. While component A is thought to represent a younger overprint direction, the in situ direction for component B on the other hand, is dissimilar to any expected younger direction and is considered to be primary magnetisation in origin, acquired during or soon after deposition of the sediments in the Late Ediacaran. The corresponding palaeomagnetic pole for component A in situ is located at Lon = 55.4°E, Lat = 31°N, A95 = 2.7° and for component B at Lon = 110°E, Lat = 28.3°S, A95 = 3.8°, N = 57. Combined with other palaeomagnetic poles of the same tectonostratigraphic unit an alternative apparent polar wander path for the Late Proterozoic–Early Palaeozoic of Baltica is proposed. Such an alternative path shows that after the mid Cryogenian (750 Ma), the poles that were situated over South Africa (p.d.c.) moved to the east until they reached Australia during the Late Ediacaran (555 Ma) where they remained approximately stationary until the beginning of the Cambrian (545 Ma). Finally, they moved to the northwest until they reached the Arabian Peninsula in the Early Ordovician. Palaeolatitudes indicate that Baltica situated near the equator from the Cryogenian through to the Ediacaran moving gradually to the south at c. 1 cm/yr. During the Late Early Ediacaran, the plate suddenly began to drift northward at c. 8 cm/yr and in the boundary with the Cambrian it was positioned in low to intermediate latitudes. Finally, Baltica began to move back to the south at c. 13 cm/yr until in the Early Ordovician, reaching intermediate to high southern latitudes. 相似文献
96.
Geochemical signatures and mechanisms of trace elements dispersion in the area of the Vale das Gatas mine (Northern Portugal) 总被引:2,自引:1,他引:2
P. Freire vila J.M. Santos Oliveira E. Ferreira da Silva E. Cardoso Fonseca 《Journal of Geochemical Exploration》2005,85(1):17-29
A geochemical survey involving the collection and analysis of 170 stream sediment samples was carried out in Vale das Gatas area in order to delineate geochemical signatures and to detect pollution hazards due to mining.Factor analysis applied to the data showed that the most important variables accounting for the first factor are Ag, Pb, Bi, As, W, Cd, Zn and Cu. These variables represent the most significant metalliferous elements of the original paragenesis, which still maintain a close relationship in the secondary geological environment. The spatial distribution of the factor scores referred to factor 1 indicates that positive scores occur predominantly in the Vale das Gatas and Sabrosa streams and in the main Pinhão river. The results also show that the Vale das Gatas mine is the most important contamination source in the Vale das Gatas district.In order to study the dispersion, transfer and uptake mechanisms of trace elements from the Vale das Gatas mine, additional sample media were collected, i.e., stream sediments, tailings, coatings, waters (surficial and mine waters) and specific biological species. Four sample stations were selected, namely two in connection with abandoned adits of the Vale das Gatas mine, one located in the Vale das Gatas stream and one located in the Pinhão River. The results show that chemical reactions of leaching and precipitation took place controlling the heavy metals in areas near the Vale das Gatas mine.At the same time, positive anomalous patterns are visible along the Pinhão river for about 20 km. The mineralogical composition of the stream sediments suggests that processes of mechanical dispersion also interfere in the secondary distribution of the metalliferous elements in that media. 相似文献
97.
Gloria Arancibia Stephen J. Matthews Paula Cornejo Carlos Pérez de Arce José I. Zuluaga Stabro Kasaneva 《Mineralium Deposita》2006,41(5):505-516
The epithermal El Peñon gold–silver deposit consists of quartz–adularia veins emplaced within a late Upper Paleocene rhyolitic dome complex, located in the Paleocene–Lower Eocene Au–Ag belt of northern Chile. Detailed K–Ar and 40Ar/39Ar geochronology on volcano–plutonic rocks and hydrothermal minerals were carried out to constrain magmatic and hydrothermal events. The Paleocene to Lower Eocene magmatism in the El Peñon area is confined to a rhomb-shaped basin, which was controlled by N–S trending normal faults and both NE- and NW-trending transtensional fault systems. The earliest products of the basin-filling sequences comprise of Middle to Upper Paleocene (~59–55 Ma) welded rhyolitic ignimbrites and andesitic to dacitic lavas, with occasional dacitic dome complexes. Later, rhyolitic and dacitic dome complexes (~55–52 Ma) represent the waning stages of volcanism during the latest Upper Paleocene and the earliest Eocene. Lower Eocene porphyry intrusives (~48–43 Ma) mark the end of the magmatism in the basin and a change to a compressive tectonomagmatic regime. 40Ar/39Ar geochronology of hydrothermal adularia from the El Peñon deposit yields ages between 51.0±0.6 and 53.1±0.5 Ma. These results suggest that mineralization occurred slightly after the emplacement of the El Peñon rhyolitic dome at 54.5±0.6 Ma (40Ar/39Ar age) and was closely tied to later dacitic–rhyodacitic bodies of 52 to 53 Ma (K–Ar ages), probably as short-lived pulses related to single volcanic events. 相似文献
98.
Thermal anomalies in tectonically active areas are often attributed to sub-seafloor fluid circulation and faulting mechanisms, particularly in subduction zones where the largest thrust earthquakes occur. Postseismic fluid flow is enabled by the poroelastic response of the fault system to the earthquake's strain field, as well as by the rupturing of permeability barriers in the vicinity of the fault zone. We investigated the relative importance of these mechanisms on postseismic pore-pressure diffusion and advective heat transport in the subduction zone setting. A two-dimensional numerical fluid flow and heat transport model was developed for the Costa Rica subduction zone offshore of the Nicoya Peninsula. The flow and transport model was coupled with an earthquake strain model to quantify the effects of coseismic strain and permeability enhancement on fluid pressures and temperatures within the Costa Rica margin. Coseismic changes in pore pressure and postseismic pore-pressure diffusion were found to be sensitive to the compressibility of the porous medium, and patterns of pore-pressure recovery were more complex than that predicted by theoretical faulting models. Coseismic contraction and extension of the crust produced high fluid pressures close to the fault, while the inflow of fluid from depth increased fluid pressures several years following the simulated fault slip. Crustal deformation alone was not observed to perturb the temperature field. Laterally extensive permeability increases of two orders of magnitude along the décollement were required to produce small changes in heat flow. Local permeability changes in the upper slope region of least five orders of magnitude were necessary to noticeably affect heat flow. The results of the numerical simulations may help to refine conceptual faulting models and provide guidance for locating long-term hydrologic monitoring sites at Costa Rica and other subduction zones. 相似文献
99.
Paula M. Davidson Dilip K. Mukhopadhyay 《Contributions to Mineralogy and Petrology》1984,86(3):256-263
Reversed phase equilibrium experiments in the system (Ca, Mg, Fe)2SiO4 provide four tielines at P?1 bar and 1 kbar and 800° C–1,100° C. These tielines have been used to model the solution properties of the olivine quadrilateral following the methods described by Davidson et al. (1981) for quadrilateral clinopyroxenes. The discrepancy between the calculated phase relations and the experimentally determined tielines is within the uncertainty of the experiments. The solution properties of quadrilateral olivines can be described by a non-convergent site-disorder model that allows for complete partitioning of Ca on the M2 site, highly disordered Fe-Mg cation distributions and limited miscibility between high-Ca and low-Ca olivines. The ternary data presented in this paper together with binary solution models for the joins Fo-Mo and Fa-Kst have been used to evaluate two solution parameters: $$\begin{gathered} F^0 \equiv 2(\mu _{{\rm M}o}^0 - \mu _{{\rm K}st}^0 ) + \mu _{Fa}^0 - \mu _{Fo}^0 = 12.660 (1.6) kJ, \hfill \\ \Delta G_*^0 \equiv \mu _{{\rm M}gFe}^0 + \mu _{FeMg}^0 - \mu _{Fo}^0 - \mu _{Fa}^0 = 7.030 (3.9) kJ. \hfill \\ \end{gathered} $$ . Ternary phase quilibrium data for olivines tightly constrain the value of F0, but not that for ΔG * 0 which describes nonideality in Fe-Mg mixing. From this analysis, we infer a function for the apparent standard state energy of Kst: $$\begin{gathered} \mu _{{\rm K}st}^0 = - 102.79 \pm 0.8 - (T - 298)(0.137026) \hfill \\ + (T - 298 - T1n(T/298))(0.155519) \hfill \\ + (T - 298)^2 (2.8242E - 05)/2 \hfill \\ + (T - 298)^2 (2.9665E + 03)/(2T(298)^2 ) kJ \hfill \\ \end{gathered} $$ where T is in Kelvins and the 298 K value is relative to oxides. 相似文献
100.
Application of geostatistical methods in gold geochemical anomalies identification (Montemor-O-Novo, Portugal) 总被引:1,自引:0,他引:1
The study described herein concerns the application of geostatistical methods to data soil from Montemor-O-Novo area (Southern Portugal). In the area, the gold mineralised zones (Banhos, Caeiras, Falés, Gamela, Malaca and Monfurado) are characterised by different geological settings and mineralogical assemblages. A total of 1211 soil samples were collected in Montemor-O-Novo area and analysed for Cu, Pb, Zn, As, Ba and Au by atomic absorption spectrometry.To account for spatial structure, simple and cross variograms were computed for the main directions of the grid sampling. From the experimental variograms a linear model of coregionalization composed of three structures, a nugget effect and two anisotropic spherical structures, was fitted to each of the six variables. The coregionalization matrices deduced from the theoretical model show the relationships between the variables at different scales. These matrices were compared with those obtained by principal component analysis (PCA).This methodology was the basis for estimating the corresponding spatial components (Y0, Y1 and Y2) using factorial kriging analysis (FKA). Maps of raw data, Y0, Y1 and Y2 were made for each variable.The use of multivariate analysis permit the study of the spatial structure intrinsic to geochemical data and the identification and refinement of significant anomalies related to Au-bearing mineral deposits. 相似文献