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991.
W. K. M. Rice Philip J. Armitage D. F. Hogg 《Monthly notices of the Royal Astronomical Society》2008,384(3):1242-1248
We use numerical simulations to model the migration of massive planets at small radii and compare the results with the known properties of 'hot Jupiters' (extrasolar planets with semimajor axes a < 0.1 au). For planet masses M pl sin i > 0.5 M J , the evidence for any 'pile-up' at small radii is weak (statistically insignificant), and although the mass function of hot Jupiters is deficient in high-mass planets as compared to a reference sample located further out, the small sample size precludes definitive conclusions. We suggest that these properties are consistent with disc migration followed by entry into a magnetospheric cavity close to the star. Entry into the cavity results in a slowing of migration, accompanied by a growth in orbital eccentricity. For planet masses in excess of 1 Jupiter mass we find eccentricity growth time-scales of a few ×105 yr, suggesting that these planets may often be rapidly destroyed. Eccentricity growth appears to be faster for more massive planets which may explain changes in the planetary mass function at small radii and may also predict a pile-up of lower mass planets, the sample of which is still incomplete. 相似文献
992.
993.
994.
Roger J. Sylvester Vincent Mannings 《Monthly notices of the Royal Astronomical Society》2000,313(1):73-86
Photometric observations at optical and near-infrared wavelengths are presented for members of a new sample of candidate Vega-like systems, or main sequence stars with excess infrared emission due to circumstellar dust. The observations are combined with IRAS fluxes to define the spectral energy distributions of the sources. Most of the sources show only photospheric emission at near-IR wavelengths, indicating a lack of hot (∼1000 K) dust. Mid-infrared spectra are presented for four sources from the sample. One of them, HD 150193, shows strong silicate emission, while another, HD 176363, was not detected. The spectra of two stars from our previous sample of Vega-like sources both show UIR-band emission, attributed to hydrocarbon materials. Detailed comparisons of the optical and IRAS positions suggest that in some cases the IRAS source is not physically associated with the visible star. Alternative associations are suggested for several of these sources. Fractional excess luminosities are derived from the observed spectral energy distributions. The values found are comparable to those measured previously for other Vega-like sources. 相似文献
995.
The Eocene La Meseta Formation is the youngest exposed unit of the back-arc James Ross Basin, Antarctic Peninsula, cropping out in Seymour (Marambio) Island. The formation comprises 720 m of clastic sedimentary rocks of deltaic, estuarine and shallow marine origin. It was subdivided into six unconformity-based units (Valle de Las Focas, Acantilados, Campamento, Cucullaea I, Cucullaea II and Submeseta Allomembers) grouped into three main facies associations. Facies association I represents valley-confined deposition in a progradational/aggradational tide-dominated and wave-influenced delta front/delta plain environment. Facies association II includes tidal channels, mixed tidal flats, tidal inlets and deltas, washover and beach environments. Facies association III represents nonconfined tide- and storm-influenced nearshore environments. La Meseta Formation sandstones are quartzofeldspathic with some hybrid arenites (glauconite and carbonate bioclasts-rich). Sandstone detrital modes are subdivided into two distinctive petrofacies: the low quartz petrofacies (petrofacies I, Q<55% and L>12%), interpreted to retain the original provenance signal, and the high quartz petrofacies (petrofacies II, Q>55% and L<12%), representing the reworking product of the former after selective elimination of the more labile components. Petrofacies I sandstone framework grains were mainly derived from a dissected magmatic arc and an associated metamorphic belt. Textural evidence for recycling of some grains (e.g. garnet) from older sedimentary units during valley incision is not conclusive. Changes in the relative participation of source areas during the evolution of the incised-valley system are evaluated from the relative proportions of lithic fragments and monomineralic clasts derived from each rock type. Two lithic assemblages were recognized. The mixed lithic assemblage (Rv/Rm+Rp<1.4) shows participation of all rock types; it represented valley-confined environments, either during the initial stage of valley development, or after main episodes of incision. The volcanic lithic assemblage (Rv/Rm+Rp>1.4) is clearly dominated by volcanic-derived clasts; it developed at times of high sea level and/or during later stages of the valley fill, when an “energy fence” at the shoreline prevented delivery of sediment from the Antarctic Peninsula, thus enhancing the relative participation of local volcanic sources. 相似文献
996.
In a previous paper (Gayon and Bois 2008a), we have shown the general efficiency of retrograde resonances for stabilizing
compact planetary systems. Such retrograde resonances can be found when two-planets of a three-body planetary system are both
in mean motion resonance and revolve in opposite directions. For a particular two-planet system, we have also obtained a new
orbital fit involving such a counter-revolving configuration and consistent with the observational data. In the present paper,
we analytically investigate the three-body problem in this particular case of retrograde resonances. We therefore define a
new set of canonical variables allowing to express correctly the resonance angles and obtain the Hamiltonian of a system harboring
planets revolving in opposite directions. The acquiring of an analytical “rail” may notably contribute to a deeper understanding
of our numerical investigation and provides the major structures related to the stability properties. A comparison between
our analytical and numerical results is also carried out. 相似文献
997.
The Sumatra Fault System provides a unique geologic setting to evaluate the influence of structural controls on geothermal activity. Whereas most of the geothermal systems in Indonesia are controlled by volcanic activity, geothermal systems at the Sumatra Fault System might be controlled by faults and fractures. Exploration strategies for these geothermal systems need to be verified because the typical pattern of heat source and alteration clays are missing so that conventional exploration with magnetotelluric surveys might not provide sufficient data to delineate favorable settings for drilling. We present field geological, structural and geomorphological evidence combined with mapping of geothermal manifestations to allow constraints between fault dynamics and geothermal activity in the Tarutung Basin in north central Sumatra. Our results indicate that the fault pattern in the Tarutung Basin is generated by a compressional stress direction acting at a high angle to the right-lateral Sumatra Fault System. NW–SE striking normal faults possibly related to negative flower structures and NNW-SSE to NNE-SSW oriented dilative Riedel shears are preferential fluid pathways whereas ENE–WSW striking faults act as barriers in this system. The dominant of geothermal manifestations at the eastern part of the basin indicates local extension due to clockwise block rotation in the Sumatra Fault System. Our results support the effort to integrate detailed field geological surveys to refined exploration strategies even in tropical areas where outcrops are limited. 相似文献
998.
George Voyatzis John D. Hadjidemetriou 《Celestial Mechanics and Dynamical Astronomy》2005,93(1-4):263-294
Families of asymmetric periodic orbits at the 2/1 resonance are computed for different mass ratios. The existence of the asymmetric
families depends on the ratio of the planetary (or satellite) masses. As models we used the Io-Europa system of the satellites
of Jupiter for the case m1>m2, the system HD82943 for the new masses, for the case m1=m2 and the same system HD82943 for the values of the masses m1<m2 given in previous work. In the case m1≥ m2 there is a family of asymmetric orbits that bifurcates from a family of symmetric periodic orbits, but there exist also an
asymmetric family that is independent of the symmetric families. In the case m1<m2 all the asymmetric families are independent from the symmetric families. In many cases the asymmetry, as measured by
and by the mean anomaly M of the outer planet when the inner planet is at perihelion, is very large. The stability of these asymmetric families has
been studied and it is found that there exist large regions in phase space where we have stable asymmetric librations. It
is also shown that the asymmetry is a stabilizing factor. A shift from asymmetry to symmetry, other elements being the same,
may destabilize the system. 相似文献
999.
天然沉积砂土力学特性受各向异性及结构性影响明显,实际工程中不能忽视。为探究二者的影响,首先在二维离散元程序NS2D中采用椭圆颗粒模拟了重力场中颗粒长轴主方向为水平的各向异性净砂样,随后基于结构性砂土胶结厚度分布规律及室内试验提出了一个新的微观胶结接触模型并将其引入各向异性净砂样以模拟天然各向异性结构性砂土,最后对该离散元试样进行了双轴试验模拟,将模拟结果与室内试验结果对比以验证该模型的适用性,并对其微观力学特性变化进行研 究。分析结果表明,随着剪切进行,各向异性结构性砂土呈明显应变软化及剪胀现象;胶结接触逐渐减少,且主方向始终为竖向方向;胶结破坏速率及胶结破坏率变化情况与宏观力学响应较一致,且胶结物多为拉剪破坏;土颗粒排列主方向始终为水平向,且水平向排列颗粒所占比例略微增大。 相似文献
1000.
This paper addresses the issue of system identification for linear structural systems using earthquake induced time histories of the structural response. The proposed methodology is based on the Eigensystem Realization Algorithm (ERA) and on the Observer/Kalman filter IDentification (OKID) approach to perform identification of structural systems using general input–output data via Markov parameters. The efficiency of the proposed technique is shown by numerical examples for the case of eight-storey building finite element models subjected to earthquake excitation and by the analysis of the data from the dynamic response of the Vincent-Thomas cable suspension bridge (Long Beach, CA) recorded during the Whittier and the Northridge earthquakes. The effects of noise in the measurements and of inadequate instrumentation are investigated. It is shown that the identified models show excellent agreement with the real systems in predicting the structural response time histories when subjected to earthquake-induced ground motion. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献