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151.
R. C. Robbins 《Planetary and Space Science》1964,12(12):1143-1146
A theory concerning the origin and composition of the Venusian clouds is presented. Hydrogen from the solar wind reacts with components of the upper atmosphere, principally CO and N2. The initial reaction products are protected upon entering the dark hemisphere of Venus, and the long night (1400 hr) provides ample time for polymerization, condensation, and subsidence to lower levels of the atmosphere. A continuous source from above, coupled with regenerative processes from the hot regions below, fulfills the requirements for maintaining a perpetual cloud layer. 相似文献
152.
In numerical modeling of groundwater flow, the result of a given solution method is affected by the way in which transient flow conditions and geologic heterogeneity are simulated. An algorithm is demonstrated that simulates breakthrough curves at a pumping well by convolution-based particle tracking in a transient flow field for several synthetic basin-scale aquifers. In comparison to grid-based (Eulerian) methods, the particle (Lagrangian) method is better able to capture multimodal breakthrough caused by changes in pumping at the well, although the particle method may be apparently nonlinear because of the discrete nature of particle arrival times. Trial-and-error choice of number of particles and release times can perhaps overcome the apparent nonlinearity. Heterogeneous aquifer properties tend to smooth the effects of transient pumping, making it difficult to separate their effects in parameter estimation. Porosity, a new parameter added for advective transport, can be accurately estimated using both grid-based and particle-based methods, but predictions can be highly uncertain, even in the simple, nonreactive case. 相似文献
153.
Alicia M.Soderberg EnricoRamirez-Ruiz 《Monthly notices of the Royal Astronomical Society》2002,330(1):L24-L28
GRB 990123 was a long, complex gamma-ray burst accompanied by an extremely bright optical flash. We find different constraints on the bulk Lorentz of this burst to be consistent with the speculation that the optical light is emission from the reverse shock component of the external shock. Motivated by this currently favoured idea, we compute the prompt reverse shock emission to be expected for bursts in which multiwavelength observations allow the physical parameters to be constrained. We find that for reasonable assumptions about the velocity of source expansion, a strong optical flash mV ≈9 was expected from the reverse shocks, which were usually found to be mildly relativistic. The best observational prospects for detecting these prompt flashes are highlighted, along with the possible reasons for the absence of optical prompt detections in ongoing observations. 相似文献
154.
Aquifer vulnerability is frequently estimated through methodologies that don’t consider most physical and chemical soil parameters.
A soil attenuation index is proposed to estimate groundwater chemical pollution risk, that takes into account organic carbon
content, pH, cation exchange capacity, clay content, phreatic depth and landscape position. The attenuation index is constructed
by a methodology similar to that developed for water quality index. P, Mn and Zn concentrations in groundwater from selected
places were used to validate the proposed index. 相似文献
155.
David J. A. Evans David H. Roberts Mark D. Bateman Chris D. Clark Alicia Medialdea Louise Callard Elena Grimoldi Richard C. Chiverrell Jeremy Ely Dayton Dove Colm Ó Cofaigh Margot Saher Tom Bradwell Steven G. Moreton Derek Fabel Sarah L. Bradley 《第四纪科学杂志》2021,36(5):723-751
The findings of BRITICE-CHRONO Transect 2 through the North Sea Basin and eastern England are reported. We define ice-sheet marginal oscillation between ~31 and 16 ka, with seven distinctive former ice-sheet limits (L1–7) constrained by Bayesian statistical analysis. The southernmost limit of the North Sea Lobe is recorded by the Bolders Bank Formation (L1; 25.8–24.6 ka). L2 represents ice-sheet oscillation and early retreat to the northern edge of the Dogger Bank (23.5–22.2 ka), with the Garret Hill Moraine in north Norfolk recording a significant regional readvance to L3 at 21.5–20.8 ka. Ice-marginal oscillations at ~26–21 ka resulted in L1, L2 and L3 being partially to totally overprinted. Ice-dammed lakes related to L1–3, including Lake Humber, are dated at 24.1–22.3 ka. Ice-sheet oscillation and retreat from L4 to L5 occurred between 19.7 and 17.3 ka, with grounding zone wedges marking an important transition from terrestrial to marine tidewater conditions, triggered by the opening of the Dogger Lake spillway between 19.9 and 17.5 ka. L6 relates to ice retreat under glacimarine conditions and final ice retreat into the Firth of Forth by 15.8 ka. L7 (~15 ka) represents an ice retreat from Bosies Bank into the Moray Firth. 相似文献
156.
Anthropogenic hillslope terraces and swidden agriculture in Jiuzhaigou National Park, northern Sichuan, China 总被引:2,自引:0,他引:2
Amanda Henck James Taylor Yongxian Li Barbara Grub Alicia Robbins Tom Hinckley Lauren Urgenson Xinxin Hu Yaowu Yuan Xun Liao 《Quaternary Research》2010,73(2):201-207
Small, irregular terraces on hillslopes, or terracettes, are common landscape features throughout west central China. Despite their prevalence, there is limited understanding of the nature of these topographic features, the processes that form them, and the role humans played in their formation. We used an interdisciplinary approach to investigate the geology, ecology, and cultural history of terracette development within Jiuzhaigou National Park, Sichuan Province, China. Terracettes occur on south facing, 20° slopes at 2500 m elevation, which appears to coincide with places people historically preferred to build villages. Ethnographic interviews suggest that traditional swidden agricultural cycles removed tree roots, causing the loess sediments to lose cohesion, slump, and the terrace risers to retreat uphill over time. This evidence is supported by landslide debris at terracette faces. Archaeological analysis of terracette sites reveal remains of rammed spread soil structures, bones, stone tools, and ceramics dating from at least 2200 years before present within a distinct paleosol layer. Radiocarbon and optically stimulated luminescence dating of terracette sediments ranged in age from between 1500 and 2000 14C yr BP and between 16 and 0.30 ka, respectively. These multiple lines of evidence indicate a long history of human habitation within Jiuzhaigou National Park and, taken together, suggest strong links between terracette formation and human-landuse interactions. 相似文献
157.
Katherine B. Lininger Ann V. Rowan Bridget Livers Natalie Kramer Virginia Ruiz-Villanueva Alicia Sendrowski Sallie Burrough 《地球表面变化过程与地形》2021,46(14):2767-2772
Many geomorphologists who are mothers find it challenging to balance field research alongside pregnancy and caring for young children. We offer perspectives on the challenges to conducting fieldwork as mothers and possible solutions, as a means of promoting conversations and highlighting issues that are less commonly considered in field-based geomorphic research. Although every mother's experience and needs are different, we discuss strategies for conducting fieldwork, addressing childcare issues, and dealing with financial considerations. We call for our community to support geomorphologists who are pregnant or caring for young children in carrying out fieldwork to help enhance the diversity of voices and perspectives within our discipline. 相似文献
158.
Beth Ellen Clark Richard P. Binzel Ellen S. Howell Edward A. Cloutis Maureen Ockert-Bell Phil Christensen Maria Antonietta Barucci Francesca DeMeo Dante S. Lauretta Harold Connolly Jr. Alicia Soderberg Carl Hergenrother Lucy Lim Josh Emery Michael Mueller 《Icarus》2011,216(2):462-475
We present reflectance spectra from 0.4 to 2.4 μm of Asteroid (101955) 1999 RQ36, the target of the OSIRIS-REx spacecraft mission. The visible spectral data were obtained at the McDonald Observatory 2.1-m telescope with the ES2 spectrograph. The infrared spectral data were obtained at the NASA Infrared Telescope Facility using the SpeX instrument. The average visible spectrum is combined with the average near-infrared wavelength spectrum to form a composite spectrum. We use three methods to constrain the compositional information in the composite spectrum of Asteroid (101955) 1999 RQ36 (hereafter RQ36). First, we perform a least-squares search for meteorite spectral analogs using 15,000 spectra from the RELAB database. Three most likely meteorite analogs are proposed based on the least-squares search. Next, six spectral parameters are measured for RQ36 and their values are compared with the ranges in parameter values of the carbonaceous chondrite meteorite classes. A most likely meteorite analog group is proposed based on the depth of overlap in parameter values. The results of the least-squares search and the parametric comparisons point to CIs and/or CMs as the most likely meteorite analogs for RQ36, and COs and CHs as the least likely. RQ36 has a spectrally “blue” continuum slope that is also observed in carbonaceous chondrites containing magnetite. We speculate that RQ36 is composed of a “CM1”-like material. Finally, we compare RQ36 to other B-type asteroids measured by Clark et al. (Clark, B.E. et al. [2010]. J. Geophys. Res. 115, E06005). The results of this comparison are inconclusive. RQ36 is comparable to Themis spectral properties in terms of its albedo, visible spectrum, and near-infrared spectrum from 1.1 to 1.45 μm. However, RQ36 is more similar to Pallas in terms of its near-infrared spectrum from 1.6 to 2.3 μm. Thus it is possible that B-type asteroids form a spectral continuum and that RQ36 is a transitional object, spectrally intermediate between the two end-members. This is particularly interesting because Asteroid 24 Themis was recently discovered to have H2O ice on the surface (Rivkin, A., Emery, J. [2010]. Nature 464, 1322–1323; Campins, H. et al. [2010a]. Nature 464, 1320–1321). 相似文献
159.
The vertical distribution of stratospheric ozone has been simultaneously measured by means of five different instruments carried on the same balloon payload. The launches were performed from Gap during the intercomparison campaign conducted in June 1981 in southern France. Data obtained between altitudes of 20 and 40 km are compared and discussed. Vertical profiles deduced from Electrochemical Concentration Cell sondes launched from the same location by small balloons and from short Umkehr measurements made at Mt Chiran (France) are also included in this comparison. Systematic differences of the order of 20% between ozone profiles deduced from solar u.v. absorption and in situ techniques are found. 相似文献