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101.
Seafloor magnetotelluric (MT) data were collected at seven sites across the Hawaiian hot spot swell, spread approximately evenly between 120 and 800 km southwest of the Hawaiian-Emperor island chain. All data are consistent with an electrical strike direction of 300°, aligned along the seamount chain, and are well fit using two-dimensional (2D) inversion. The major features of the 2D electrical model are a resistive lithosphere underlain by a conductive lower mantle, and a narrow, conductive, ‘plume’ connecting the surface of the islands to the lower mantle. This plume is required; without it the swell bathymetry produces a large divergence of the along-strike and across-strike components of the MT fields, which is not seen in the data. The plume radius appears to be less than 100 km, and its resistivity of around 10 Ωm, extending to a depth of 150 km, is consistent with a bulk melt fraction of 5–10%.A seismic low velocity region (LVR) observed by Laske et al. [Laske, G., Phipp Morgan, J., Orcutt, J.A., 1999. First results from the Hawaiian SWELL experiment, Geophys. Res. Lett. 26, 3397–3400] at depths centered around 60 km and extending 300 km from the islands is not reflected in our inverse model, which extends high lithospheric resistivities to the edge of the conductive plume. Forward modeling shows that resistivities in the seismic LVR can be lowered at most to 30 Ωm, suggesting a maximum of 1% connected melt and probably less. However, a model of hot subsolidus lithosphere of 102 Ωm (1450–1500 °C) within the seismic LVR increasing to an off-swell resistivity of >103 Ωm (<1300 °C) fits the MT data adequately and is also consistent with the 5% drop in seismic velocities within the LVR. This suggests a ‘hot, dry lithosphere’ model of thermal rejuvination, or possibly underplated lithosphere depleted in volatiles due to melt extraction, either of which is derived from a relatively narrow mantle plume source of about 100 km radius. A simple thermal buoyancy calculation shows that the temperature structure implied by the electrical and seismic measurements is in quantitative agreement with the swell bathymetry. 相似文献
102.
Mayer R. Bucholtz F. Allman E. von Berg D.L. Kruer M. 《Geoscience and Remote Sensing Letters, IEEE》2005,2(2):113-117
Previous studies examined and tested a number of statistics-based registration-free transforms to find targets amid cluttered backgrounds. These transforms temporally evolve spectral target signatures under global, varying conditions using collected imagery of regions of similar objects and content distribution from datasets gathered at two different times. The transformed target signature is then inserted into the matched filter to search for targets. Although critical for transforming spectral target signatures, finding two suitable candidate regions is often difficult, computationally intensive, and may require the aid of an image analyst. This is the first study to examine a metric to help identify suitable areas for spectral target transformation. Specifically, this study examines and finds that the average correlation coefficient between the corrected histograms of the multispectral image cube collected at two times can help assess the similarity of the areas and indicate the target-to-clutter ratio, a metric shown to predict target detection performance in matched filter searches for targets. 相似文献
103.
LI Chengcai MAO Jietai Alexis K. H. Lau YUAN Zibing WANG Meihua LIU Xiaoyang 《中国科学D辑(英文版)》2005,48(Z2)
The direct correlation between NASA MODIS aerosol optical depth (AOD) products and the air pollution index (API) in Beijing was found relatively low based on the long-term comparison analysis. The correlation improved to some extent after taking account of the seasonal variation of scale height and the vertical distribution of aerosols. The correlation coefficient further improved significantly after considering the influencing factor of Relative Humidity (RH). This study concluded that satellite remote-sensing could serve as an efficient tool for monitoring the spatial distribution of particulate pollutants on the ground-level, as long as corrections have been made in the two aforementioned processes. Taking advantage of the MODIS information, we analyzed a pollution episode occurring in October 2004 in Beijing. It indicated that satellite remote-sensing could describe the formation process of the ground-level pollution episode in detail, and showed that regional transport and the topography were crucial factors to air quality in Beijing. The annual averaged distribution in the urban area of Beijing and its surroundings could be also obtained from the high-resolution retrieval results, implicating that high-resolution satellite remote-sensing might be potential in monitoring the source distribution of particulate pollutants. 相似文献
104.
Linear sandbanks are located globally in areas where there are strong currents and an abundance of sand. In the recent years,
these sandbanks have become of strategic interest as a potential source of marine aggregates (sand and gravel) and mineral
deposits. They form the seaward boundary of the nearshore zone and therefore are important for the stability of the coastal
system. They also commonly reach the sea surface and thus pose a threat to navigation. Headland-associated linear sandbanks
are a specific type of sandbanks which are located in the lee of coastal topographic features such as headlands and islands.
Interaction between tidal currents and topographic features generate complex three-dimensional circulation patterns that significantly
influence the distribution of sediments in the vicinity of the feature. Field and numerical model investigations of the three-dimensional
flow structure have been undertaken on the Levillain Shoal, a headland-associated linear sandbank present in the lee of Cape
Levillain (Shark Bay, Western Australia). The field data indicated the presence of secondary flows near the tip of the cape
and around the bank which were re-produced in the numerical simulations. Numerical results have shown that residual eddies
are not representative of the sediment transport and that secondary currents enhance the convergence of sediment towards the
sandbank. Maintenance processes have been investigated. Sediment transport paths near the cape and the bank indicate that
the sandbank is part of a sand circulation cell where the sand is circulating around the bank with exchanges between the sandbank
and the headland. 相似文献
105.
106.
P. Digranes R. Mjelde S. Kodaira H. Shimamura T. Kanazawa H. Shiobara E. W. Berg 《Pure and Applied Geophysics》1996,147(4):611-629
Three component recordings from an array of five ocean bottom seismographs in the northwestern part of the Vøring basin have been used to obtain a 2-D shear-wave (S-wave) velocity-depth model. The shear waves are identified by means of travel-time differences compared to the compressional (P) waves, and by analyzing their particle motions. The model has been obtained by kinematic (travel-time) ray-tracing modelling of the OBS horizontal components.The shear-wave modelling indicates that mode conversions occur at several high velocity interfaces (sills) in the 4–10 km depth range, previously defined by a compressional-wave velocity-depth model using the same data set.An averageV
p
/V
s
ratio of 2.1 is inferred for the layers above the uppermost sill, indicative of both poorly consolidated sediments and a low sand/shale ratio. A significant decrease in theV
p
/V
s
ratio (1.7) below the first sill may in part be atributed to well consolidated sediments, and to a change in lithology to more sandy sediments. This layer is interpreted to lie within the lower Cretaceous sequence. At 5–10 km depthV
p
/V
s
ratios of 1.85 indicate a lower sand/shale ratio consistent with the expected lithologies. The averageV
p
/V
s
ratio inferred for the crust is 1.75, which is consistent with values obtained north of Vøring, in the Lofoten area. An eastward thinning of the crystalline basement is supported by the shear-wave modelling. 相似文献
107.
R. Mjelde S. Kodaira P. Digranes H. Shimamura T. Kanazawa H. Shiobara E. W. Berg O. Riise 《Pure and Applied Geophysics》1997,149(4):641-665
—Semi-regional Ocean Bottom Seismograph (OBS) data acquired in the central and northern part of the Vøring Basin, mid-Norway margin, have been modeled by use of 2-D ray-tracing. The semi-regional model, derived from the study of twenty-five OBSs deployed along a 120-km long profile, is compared with a regional model consisting of five OBSs from the same profile. The semi-regional model is somewhat more detailed than the regional model, due to the considerably closer receiver spacing. The overall geometry and velocity distribution of the two models are remarkably similar, however, proving that the regional procedure with large OBS spacing provides a reliable regional model.¶Intrusions of sills, related to early Tertiary continental rifting and break up, are important at intermediate and deep sedimentary levels (2–10 km below sea floor) in most parts of the area. The semi-regional modeling suggests that one of the deepest sills extends much further east and is substantially thicker (locally more than 500 m) than indicated in the regional model. Another important difference is a high-velocity body within the upper crystalline crust at 11–12 km depth in the NW part of the area, indicating that the closer OBS spacing in the semi-regional modeling allows detection of local intra-crustal intrusions. Local differences are also inferred in the lower crust; at about 20 km depth a structure is inferred within the lower crust from wide-angle reflections. This might suggest that the high-velocity lower crustal layer, interpreted as magmatic underplating, consists of a mixture of underplated/intruded magmatic material and blocks of continental lower crust. 相似文献
108.
The large lead-zinc deposits of Middle Proterozoic age in northern Australia occur in sediments interpreted as having been deposited under saline, shallow-emergent conditions. Significant alkaline diagenesis is evident from the presence of abundant K-feldspar and locally albite. Alkali element compositions clearly distinguish the sediments of the mineralised shallow-emergent locations both from those of more permanent water cover (lacustrine or lagoonal), and from those close to basin margins. The key variables for identifying the geochemical contrasts are the levels of K, Rb and Na, and the ratios of K : Al and K : Rb. These geochemical parameters can be used in conjunction with the levels of the target and indicator elements to detect whether mineralisation may be present in a favourable geological environment. Two case studies of the application of whole rock geochemistry in base metal exploration are given for the Glyde River Sub-basin (McArthur Basin) and the Western Succession, Mt Isa Block. 相似文献
109.
Several gneissic xenoliths occur in granite which has been intruded along the contact between an anorthosite body and gneisses
in the Nain complex, Labrador. One of these xenoliths is a ferro-aluminous gneiss with ovoids, 1–3 cm in diameter, consisting
of an intimate mixture of fine, granular cordierite-olivine-spinel-orthopyroxene-plagioclase-ilmenite-pyrrhotite enclosed
by poikilitic grains of garnet and orthoamphibole. Biotite occurs as both granular and poikilitic grains. The textures indicate
a two-stage contact metamorphic history characterized by the simplified reaction: cordierite+olivine+spinel+plagioclase +orthopyroxene→garnet+orthoamphibole
Although, on the basis of texture, the reactants represent the early stage assemblage and the products represent the final
stage, the distribution coefficient data for Fe-Mg indicate that, except for orthopyroxene, all of the minerals were stable
and in equilibrium during the final stage. The coexistence of cordierite and olivine indicates that pressure was less than
3 kbar. The phase relationships agree well with the experimental work of Hsu and Burnham (1969) on the almandine-pyrope join
at 2 kbar and indicate a temperature of 800° C for the first stage and 615° C for the second stage of contact metamorphism.
This close agreement also suggests that the ovoids at one time may have been garnet porphyroblasts that were decomposed during
the first stage. These data correlate well with the field relations: the first-stage, high-temperature effects were likely
caused by the intrusion of the anhydrous anorthositic body and the second-stage, low-temperature effects by the invasion of
the volatile-rich granite. 相似文献
110.
Constant M.G. van den Berg 《Geochimica et cosmochimica acta》1984,48(12):2613-2617
The stability constants, , for borate complexes with the ions of Cu, Pb, Cd and Zn are determined in this work by DPASV in 0.7 M KNO3 at metal concentrations of 10?7 M. The acidity constants of the Cu2+ ion are determined by DPASV in the same conditions. The following values for log () have been obtained: CuB: 3.48, CuB2: 6.13, PbB: 2.20, PbB2: 4.41, ZnB: 0.9, ZnB2: 3.32, CdB: 1.42, and CdB2: 2.7, while the values for the acidity constants of Cu are and . At the low concentration of boron in 35%. S sea-water complexes with borate represent only about 0.2% Cu, 0.03% Pb, 0.02% Zn and 0.003% Cd. 相似文献