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151.
Rafael Cubarsí 《Astrophysics and Space Science》1992,191(2):213-221
The superposition of two stellar velocity distribution functions of Gaussian type is applied in order to study the velocity distribution of a stellar sample in the galactic plane. Assuming that the Galaxy is symmetric respect to this plane, the relationships to be fulfilled by the central velocity moments of the total stellar sample are obtained. If the total velocity distribution is consistent with this type of superposition, the partial moments of the subsamples, which are associated with the Gaussian components, can be calculated and, also, the population percentage. Our results show that first component corresponds to young disk population and the second one to intermediate disk population.Paper presented at the 12th European Astronomical Meeting of theIAU on European Astronomers Look to the Future, held 8–11 October, 1990, Davos, Switzerland. 相似文献
152.
We develop a new method for analysis of meandering channels based on planform sinuosity. This analysis objectively identifies three channel‐reach lengths based on sinuosity measured at those lengths: the length of typical, simple bends; the length of long, often compound bends; and the length of several bends in sequence that often evolve from compound bends to form multibend loops. These lengths, when normalized by channel width, tend to fall into distinct and clustered ranges for different natural channels. Mean sinuosity at these lengths also falls into distinct ranges. That range is largest for the third and greatest length, indicating that, for some streams, multibend loops are important for planform sinuosity, whereas for other streams, multibend loops are less important. The role of multibend loops is seldom addressed in the literature, and they are not well predicted by previous modelling efforts. Also neglected by previous modelling efforts is bank–flow interaction and its role in meander evolution. We introduce a simple river meandering model based on topographic steering that has more in common with cellular approaches to channel braiding and landscape evolution modelling than to rigorous, physics‐based analyses of river meandering. The model is sufficient to produce reasonable meandering channel evolution and predicts compound bend and multibend loop formation similar to that observed in nature, in both mechanism and importance for planform sinuosity. In the model, the tendency to form compound bends is sensitive to the relative magnitudes of two lengths governing meander evolution: (i) the distance between the bend cross‐over and the zone of maximum bank shear stress, and (ii) the bank shear stress dissipation length related to bank roughness. In our simple model, the two lengths are independent. This sensitivity implies that the tendency for natural channels to form compound bends may be greater when the banks are smoother. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
153.
Claudio Codella Rafael Bachiller Milena Benedettini Paola Caselli 《Astrophysics and Space Science》2003,287(1-4):171-174
The preliminary results from a multiline mm-wave survey of H2S and SO2 emission towards the Cepheus A star forming region are presented. Large scale maps (? 5'×4') and high-resolution line profiles which clearly indicate the occurrence of multiple H2S and SO2 outflows are reported. In particular, a 0.6 pc long outflow flowing towards South from the CepA-East position has been discovered. In addition, besides the well known regions CepA-East and CepA-West other four clumps of shocked gas have been detected. These clumps exhibit distinct H2S/SO2 ratios, suggesting different chemical compositions for different regions and show that in Cepheus A the mass loss has strongly modified the chemistry of the quiescent gas. 相似文献
154.
Gutiérrez Pedro J. Ortiz José L. Rodrigo Rafael López-Moreno José J. Jorda Laurent 《Earth, Moon, and Planets》2002,90(1-4):239-247
A simplified thermal model has been used to calculate thenon-gravitational forces acting on small irregular nuclei in the
orbit ofComet 46P/Wirtanen. The torque of thenon-gravitational force has beencalculated and the Euler equations have been
solved in order toinvestigate the rotational evolution of several irregular nuclei duringa single orbital step. Several initial
spin axis orientations andactivity patterns on their surfaces have been considered. The nucleiconsidered have a mean radius
of 1 km and their inertia moments havebeen calculated assuming a homogeneous bulk density of 500 kg/m3. Inall the simulations, the initial spin period is 6h and the nucleiinitially rotate around their shortest axis. Under these
assumptions,significant changes in the angular momentum and in spin period have beenobtained in all the simulations, but the
nucleus is found to practicallyremain in its spin state of lowest energy during the entire orbitalperiod. 相似文献
155.
Ocean and atmosphere changes in the Caribbean Sea during the twenty-first century using CMIP5 models
Ocean Dynamics - Coastal communities around the Caribbean Sea are vulnerable to global warming impacts, partly because of constraints on their adaptive capacity. We use three climate models... 相似文献
156.
Elena Zubieta Juan C. Larrasoaña Alaitz Aldaz Javier Casalí Rafael Giménez 《地球表面变化过程与地形》2021,46(6):1103-1110
In gully erosion, the detached soil can be transported over long distances along the landscape. The eroded material can be redistributed and/or deposited on the soil surface along the landscape and then eventually be buried by newly eroded and deposited sediment. There can be significant variability of the soil conditions (e.g., texture and moisture content) over which the eroded material travels. The eroded material can be detected through the use of magnetic tracers attached to or mixed with the eroded soil. In this study we evaluated the degree to which the magnetic signal of the magnetite is conditioned by (i) burial depth of tracer, (ii) condition of soil covering the tracer and (iii) tracer concentration. In the laboratory containers were filled with a specific soil. In the filling process, a 0.5-cm layer of a soil–magnetite mixture was interspersed in the soil profile at a certain depth. Experiments encompassed three different soil–tracer concentrations (1000:1, 200:1, 100:1), four burial depths of tracer (0 cm, 3 cm, 5 cm and 10 cm from soil surface), and two different soils. In each case, the magnetic susceptibility was measured with a susceptometer. Experiments were repeated with different soil moisture contents. If the tracer is located under the soil surface, a minimum soil–tracer concentration of 200:1 is required for its correct detection. The intensity of the magnetic signal decreases dramatically with the vertical distance of the tracer from the soil surface. The maximum detection depth for the tracer's magnetic signal is strongly dependent on the natural magnetic susceptibility of the soil, which masks the tracer's signal. Variation in soil moisture content does not significantly affect the magnetic signal. For extensive field studies, the soil–tracer volume to be handled would be very high and therefore, it is necessary to explore new tracer application techniques. 相似文献
157.
Numerical analysis of an infinite pile group in a liquefiable soil was considered in order to investigate the influence of pile spacing on excess pore pressure distribution and liquefaction potential. It was found that an optimal pile spacing exists resulting in minimal excess pore pressure. It was also found that certain pile group configurations might reduce liquefaction potential, compared to free field conditions. It was observed that for closely spaced piles and low frequency of loading, pile spacing has little influence on the response of the superstructure. 相似文献
158.
The study of near-ground free convection conditions at Nam Co station on the Tibetan Plateau 总被引:1,自引:1,他引:0
Degang Zhou Rafael Eigenmann Wolfgang Babel Thomas Foken Yaoming Ma 《Theoretical and Applied Climatology》2011,105(1-2):217-228
This study investigates the near-ground free convection conditions (FCCs) based on eddy covariance (EC) measurements at Nam Co station near the Nam Co Lake on the Tibetan Plateau (TP). The spatial and temporal structure of EC measurements at this station is evaluated by using the comprehensive software package TK2 together with a footprint model. The obtained high-quality turbulent flux data are used to study the occurrence of FCCs, which can be detected with the EC system by calculating the stability parameter. Two types of generation of FCCs can be identified. (1) During the wind direction change of a diurnal thermally forced land-lake circulation system in the morning, strongly reduced wind speeds and simultaneously high buoyancy fluxes lead to a period of dominance of buoyancy over shear, and hence, to the occurrence of FCCs. (2) On days with the appearance of clouds, the land-lake circulation is weakened or reversed, dependent on the temperature gradients between the land and the Nam Co Lake. During the period of adaptation of the land-lake breeze to the alternating situation of heating differences, wind speeds decrease and buoyancy again dominates over shear near the ground. These are the situations where FCCs are also detected during the entire day at Nam Co station. The investigation of FCCs regarding the whole measurement period shows that FCCs can be mainly attributed to case (1) during the non-monsoon period, while FCCs are generated by both mechanisms (1 and 2) during the monsoon season. An impact of the FCCs on the near-ground profiles of air temperature and humidity is demonstrated. The FCCs are assumed to play an important role for the land surface-atmosphere exchange processes and the atmospheric boundary layer (ABL) conditions on the TP by providing an effective transport mechanism of near-ground air mass characteristics into upper parts of the ABL. 相似文献
159.
Rafael Bartolomé Juanjo Dañobeitia François Michaud Diego Córdoba Luis A. Delgado-Argote 《Pure and Applied Geophysics》2011,168(8-9):1373-1389
Three thousand kilometres of multichannel (MCS) and wide-angle seismic profiles, gravity and magnetic, multibeam bathymetry and backscatter data were recorded in the offshore area of the west coast of Mexico and the Gulf of California during the spring 1996 (CORTES survey). The seismic images obtained off Puerto Vallarta, Mexico, in the Jalisco subduction zone extend from the oceanic domain up to the continental shelf, and significantly improve the knowledge of the internal crustal structure of the subduction zone between the Rivera and North American (NA) Plates. Analyzing the crustal images, we differentiate: (1) An oceanic domain with an important variation in sediment thickness ranging from 2.5 to 1 km southwards; (2) an accretionary prism comprised of highly deformed sediments, extending for a maximum width of 15 km; (3) a deformed forearc basin domain which is 25 km wide in the northern section, and is not seen towards the south where the continental slope connects directly with the accretionary prism and trench, thus suggesting a different deformational process; and (4) a continental domain consisting of a continental slope and a mid slope terrace, with a bottom simulating reflector (BSR) identified in the first second of the MCS profiles. The existence of a developed accretionary prism suggests a subduction–accretion type tectonic regime. Detailed analysis of the seismic reflection data in the oceanic domain reveals high amplitude reflections at around 6 s [two way travel time (twtt)] that clearly define the subduction plane. At 2 s (twtt) depth we identify a strong reflection which we interpret as the Moho discontinuity. We have measured a mean dip angle of 7° ± 1° at the subduction zone where the Rivera Plate begins to subduct, with the dip angle gently increasing towards the south. The oceanic crust has a mean crustal thickness of 6.0–6.5 km. We also find evidence indicating that the Rivera Plate possibly subducts at very low angles beneath the Tres Marias Islands. 相似文献
160.
Rafael Maciel Peña Avto Goguitchaichvili Bernard Henry Leda Sánchez-Bettucci Juan Morales Bertha Aguilar Reyes Ana María Soler-Arechalde Manuel Calvo-Rathert 《Studia Geophysica et Geodaetica》2011,55(2):311-328
The paleosecular variation (PSV) and polarity transitions are two major features of the Earth’s magnetic field. Both PSV and
reversal studies are limited when age of studied units is poorly constrained. This is a case of Central and western Mexico
volcanics. Although many studies have been devoted to these crucial problems and more than 200 paleomagnetic directions are
available for the last 5 Ma, only few sites were dated directly. This paper presents new paleomagnetic results from seventeen
independent cooling units in the Michoacán-Guanajuato Volcanic Field (MGVF) in western Mexico. Twelve sites are directly dated
by 40Ar/39Ar or K-Ar methods and span from 2.78 to 0.56 Ma. The characteristic paleodirections are successfully isolated for 15 lava
flows. The mean paleodirection (inclination I and declination D) obtained in this study is I = 28.8°, D = 354.9°, and Fisherian
statistical parameters are k = 28, α95 = 7.3°, N=15, which corresponds to the mean paleomagnetic pole position Plat = 83.9°, Plong = 321.6°, K = 34, A95 = 6.6°. The paleodirections obtained in present study compiled with those, previously reported from the MGVF, are practically
undistinguishable from the expected Plio-Quaternary paleodirections. The paleosecular variation is estimated through the study
of the scatter of the virtual geomagnetic poles giving SF = 15.9 with SU =21.0 and SL = 12.7 (upper and lower limits respectively). These values agree reasonably well with the recent statistical Models. The
oldest sites analyzed (the Santa Teresa and Cerro Alto) yield normal polarity magnetizations as expected for the cooling units
belonging to the Gauss geomagnetic Chron. The interesting feature of the record comes from lava flows dated at about 2.35
Ma with clearly defined normal directions. This may point out the possible existence of a normal polarity magnetization in
the Matuyama reversed Chron older than the Reunion and may be correlated to Halawa event interpreted as the Cryptochron C2r.2r-1.
Another important feature of the geomagnetic record obtained from the MGVF is the evidence of fully reversed geomagnetic field
within Bruhnes Chron, at about 0.56 Ma corresponding to the relative paleointensity minimum of global extent found in marine
sediments at about 590 ka. 相似文献