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21.
Robin S. Keir Oliver Schmale Maren Walter Jürgen Sültenfuß Richard Seifert Monika Rhein 《Earth and Planetary Science Letters》2008,270(3-4):338-348
Hydrothermal emission of mantle helium appears to be directly related to magma production rate, but other processes can generate methane and hydrogen on mid-ocean ridges. In an on-going effort to characterize these processes in the South Atlantic, the flux and distribution of these gases were investigated in the vicinity of a powerful black smoker recently discovered at 8°17.9' S, 13°30.4' W. The vent lies on the shoulder of an oblique offset in the Mid-Atlantic Ridge and discharges high concentrations of methane and hydrogen. Measurements during expeditions in 2004 and 2006 show that the ratio of CH4 to 3He in the neutrally buoyant plume is quite high, 4 × 108. The CTD stations were accompanied by velocity measurements with lowered acoustic Doppler current profilers (LADCP), and from these data we estimate the methane transport to have been 0.5 mol s− 1 in a WSW-trending plume that seems to develop during the ebb tidal phase. This transport is an order of magnitude greater than the source of CH4 calculated from its concentration in the vent fluid and the rise height of the plume. From this range of methane fluxes, the source of 3He is estimated to be between 0.14 and 1.2 nmol s− 1. In either case, the 3He source is significantly lower than expected from the spreading rate of the Mid-Atlantic Ridge. From the inventory of methane in the rift valley adjacent to the vent, it appears that the average specific rate of oxidation is 2.6 to 23 yr− 1, corresponding to a turnover time between 140 and 16 days. Vertical profiles of methane in the surrounding region often exhibited Gaussian-like distributions, and the variances appear to increase with distance from the vent. Using a Gaussian plume model, we obtained a range of vertical eddy diffusivities between 0.009 and 0.08 m2m2 s− 1. These high values may be due to tidally driven internal waves across the promontory on which the vent is located. 相似文献
22.
To assess differences between object and pixel-based reservoir modeling techniques, ten realizations of a UK Continental Shelf braided fluvial reservoir were produced using Boolean Simulation (BS) and Sequential Indicator Simulation (SIS). Various sensitivities associated with geological input data as well as with technique-specific modeling parameters were analyzed for both techniques. The resulting realizations from the object-based and pixel-based modeling efforts were assessed by visual inspection and by evaluation of the values and ranges of the single-phase effective permeability tensors, obtained through upscaling. The BS method performed well for the modeling of two types of fluvial channels, yielding well-confined channels, but failed to represent the complex interaction of these with sheetflood and other deposits present in the reservoir. SIS gave less confined channels and had great difficulty in representing the large-scale geometries of one type of channel while maintaining its appropriate proportions. Adding an SIS background to the Boolean channels, as opposed to a Boolean background, resulted in an improved distribution of sheetflood bodies. The permeability results indicated that the SIS method yielded models with much higher horizontal permeability values (20–100%) and lower horizontal anisotropy than the BS versions. By widening the channel distribution and increasing the range of azimuths, however, the BS-produced models gave results approaching the SIS behavior. For this reservoir, we chose to combine the two methods by using object-based channels and a pixel-based heterogeneous background, resulting in moderate permeability and anisotropy levels. 相似文献
23.
24.
W. Seifert 《Mineralogy and Petrology》2005,84(3-4):129-146
Summary The mineral chemistry of a Variscan lamprophyre (kersantite) from the Frankenwald, Germany, has been investigated by electron
microprobe. This potassic, Si-saturated, mafic rock contains an assemblage of different generations of titanite and allanite-(Ce),
Th-rich zircon, and metamict REE–Ti–Zr–Th silicates. The primary ferroan-ceroan titanite contains unusually high contents
of REE2O3 (max. (ΣLa to Sm)+Y = 36.8 oxide wt.%), ZrO2 (max. 5.4 wt.%), and ThO2 (max. 3.1 wt.%). Its empirical formula averages to (Ca0.31 La0.17 Ce0.30 Pr0.03 Nd0.08 Sm0.01 Y0.01 Fe2+0.06 Th0.02 Mn0.01)Σ1.00 (Ti0.60 Fe2+0.22 Al0.06 Zr0.07 Mg0.04 Nb0.01)Σ1.00 O1.00(Si0.93 Al0.07)Σ1.00 O4. Element correlations reveal operation of the complex substitution Ca2++Ti4++Th4+ ⇔ REE3++Al3++Zr4+. In comparison to allanite-(Ce), ferroan-ceroan titanite preferentially incorporated the LREE and Th. This finding is inconsistent
with previous experimental studies and suggests that both minerals are not cogenetic. High Zr contents in titanite, usually
known only from Si-undersaturated alkaline rocks, and the predominance of Fe2+ suggest that the ferroan-ceroan titanite crystallized from an alkali-rich, low-fO2 residual melt. 相似文献
25.
Use of alternative conceptual models to assess the impact of a buried valley on groundwater vulnerability 总被引:2,自引:1,他引:1
Dorte Seifert Torben O. Sonnenborg Peter Scharling Klaus Hinsby 《Hydrogeology Journal》2008,16(4):659-674
A buried valley incised into a sequence of pre-Quaternary sediments is shown to seriously affect the vulnerability of groundwater. Often the existence of buried valleys is not known or is not described explicitly in a hydrogeological model. In the present study, two numerical groundwater models, representing two alternative conceptual models, were produced to help quantify the effect of the valley on groundwater vulnerability. One model included the buried valley and the other did not. Both models were subjected to calibration and were found to describe hydraulic head and river discharge equally well. Even though the two models showed similar calibration statistics; fluxes, travel paths and travel times were affected by the inclusion of the buried valley. The recharge area and the groundwater age of potential abstraction wells placed in the pre-Quaternary deep aquifers surrounding the buried valley were different for the two models, with significantly higher vulnerability when the valley was included in the model. Based on the results of the present study, it is concluded that a buried valley may not always be detectable when calibrating a wrong conceptual model. If reliable results should be obtained a good geological model has to be constructed. 相似文献
26.
S. Heinemann T. G. Sharp F. Seifert D. C. Rubie 《Physics and Chemistry of Minerals》1997,24(3):206-221
Garnets along the join Mg4Si4O12 (majorite end member) – Mg3Al2Si3O12 (pyrope) synthesized at 2000 °C, 19 GPa are, after quench, tetragonal in the compositional range up to 20 mol% pyrope, but
cubic at higher Al contents. Lattice constants a
tet and a
tet in the tetragonal compositional range converge with increasing pyrope contents towards the lattice constant of the cubic
garnets. The elastic strain and the intensity of the (222) reflection as a function of composition indicate a second-order
phase transition near 20 mol% pyrope. From the wedge-like shape of pseudomerohedral twins and their interaction near 90° twin-boundary
corners, as well as from the absence of growth-induced dislocations, it is concluded that the Al-poor garnets are also cubic
at synthesis conditions but invert by (Mg,Si) ordering on the octahedral sites into tetragonal phases of space group I41/a upon quench. This implies that the cubic-to-tetragonal phase transition in Mg4Si4O12 garnet occurs below 2000 °C at 19 GPa and at even lower temperatures in more aluminous compositions. A composition-dependent
Landau model is consistent with a direct transformation from Ia3d to I41/a. Comparison of the T-X stability field of majorite-pyrope garnets with the chemistry of majorite-rich garnets expected to occur in the Earth's transition
zone shows that the latter will be cubic under all conditions. Softening of elastic constants, which commonly accompanies
ferroelastic phase transitions, may affect the seismic velocities of garnets in the deeper transition zone where majorite
contents are highest.
Received July 5, 1996 / Revised, accepted September 24, 1996 相似文献
27.
I. Davoli E. Paris S. Stizza M. Benfatto M. Fanfoni A. Gargano A. Bianconi F. Seifert 《Physics and Chemistry of Minerals》1992,19(3):171-175
The decrease of the mean Si-O-Si angle in vitreous silica upon densification from 2.20 to 2.36 gcm-3 has been followed by oxygen and silicon K-edge XANES spectroscopy. Multiple scattering calculations using clusters of two shells around the oxygen and silicon atoms, respectively, are in good agreement with experimental absorption spectra and confirm mean Si-O-Si angles between 130 and 144° for these samples, and a decrease of the mean angle with densification. The experimental spectra also exhibit features due to scattering at outer (>2) shells around the photoabsorbers. 相似文献
28.
Hans Seifert 《Contributions to Mineralogy and Petrology》1948,1(4):505-514
Ohne Zusammenfassung 相似文献
29.
30.
F. Seifert 《Contributions to Mineralogy and Petrology》1973,41(2):171-177
The assemblage Mg-cordierite — corundum is formed stably through the reaction chlorite+Al-silicate=cordierite+corundum+H2O at 535° C, 2kb; 615°, 5 kb; and 665° C, 7 kb water pressure. In the order of increasing pressure andalusite, sillimanite,
and kyanite participate as stable phases in this equilibrium. A spinel-Al-silicate tie-line is only stable at high temperatures
not likely to be attained in rocks. The natural assemblage spinel-Al-silicate is, however, to be explained by the additional
presence of FeO in these rocks. 相似文献