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141.
T. W. Thompson W. J. Roberts W. K. Hartmann R. W. Shorthill S. H. Zisk 《Earth, Moon, and Planets》1979,21(3):319-342
Radar, infrared, and photogeologic properties of lunar craters have been studied to determine whether there is a systematic difference in blocky craters between the maria and terrae and whether this difference may be due to a deep megaregolith of pulverized material forming the terra surface, as opposed to a layer of semi-coherent basalt flows forming the mare surface. Some 1310 craters from about 4 to 100 km diameter have been catalogued as radar and/or infrared anomalies. In addition, a study of Apollo Orbital Photography confirmed that the radar and infrared anomalies are correlated with blocky rubble around the crater.Analysis of the radar and infrared data indicated systematic terra—mare differences. Fresh terra craters smaller than 12 km were less likely to be infrared and radar anomalies than comparable mare craters: but terra and mare craters larger than 12 km had similar infrared and radar signatures. Also, there are many terra craters which are radar bright but not infrared anomalies.Our interpretation of these data is that while the maria are rock layers (basaltic flow units) where craters eject boulder fields, the terrae are covered by relatively pulverized megaregolith at least 2 km deep, where craters eject less rocky rubble. Blocky rubble, either in the form of actual rocks or partly consolidated blocks, contributes to the radar and infrared signatures of the crater. However, aging by impacts rapidly destroys these effects, possibly through burial by secondary debris or by disintegration of the blocks themselves, especially in terra regions.PSI Contribution No. 110. 相似文献
142.
143.
Summary. The frozen flux approximation of Roberts & Scott is a constraint on the core field that can be used to aid interpretation of the very sparse datasets that palaeomagnetism and archaeomagnetism provide. It gives bounds on the size of the components of the magnetic field at a point, of the Gauss coefficients, and, if valid over such long time periods, limits the shape of the field during transitions between normal and reversed polarities. The maximum intensity at a point, consistent with the present flux, is 281 μT or 4 times the maximum field observed today. The present dipole is about 50 per cent of its upper bound. Polarity reversal is impossible if the transition field is purely axisymmetric. None of the measurements we consider violate the frozen flux approximation. 相似文献
144.
145.
The long-period electromagnetic response of the Earth 总被引:1,自引:0,他引:1
R. G. Roberts 《Geophysical Journal International》1984,78(2):547-572
146.
The nature of oscillations in a magnetic cylinder embedded in a magnetic environment is investigated. It is shown that the standard slender flux tube analysis of a kink mode in a cylinder excludes the possibility of a second mode, which arises under photospheric conditions. Under coronal conditions, two widely separated classes of oscillation can be freely sustained, one on an acoustic time-scale and the other on an Alfvénic time-scale. The acoustic-type oscillations are always present, but the much shorter period, Alfvénic-type, oscillations arise only in high density (strictly, low Alfvén velocity) loops. An application to waves in fibrils is also given, and suggests (following Wentzel, 1979) that they are fast kink waves propagating in a density enhancement. 相似文献
147.
Since Berry argued in 1976 that counterurbanization had replaced urbanization in the U.S., a debate has ensued as to whether the phenomenon also exists in Western Europe. Proponents of a developmental theory of counterurbanization contend that the phenomenon should be evident in countries which have achieved a very high level of socioeconomic development, including the Federal Republic of Germany (FRG). Most of the German and English language literature on the topic suggests the contrary for the FRG. Here urbanization and counterurbanization are restricted to specific geographic meanings, i.e., spatial agglomeration and deglomeration. Results from four interrelated tests show a trend toward counterurbanization evident as early as the 1960s in the post-war regional demographic data of the FRG. 相似文献
148.
Plans for the deployment of a comprehensive set of instruments designed to study the high-energy emissions of solar flares have been developed as part of a series of studies for an Advanced Solar Observatory (ASO). These studies have identified the Space Station as the preferred mode for the deployment of the full ASO. The accident of the Space Shuttle Challenger in 1986, and the growth in the estimated costs for the Space Station during the fiscal year 1989 budget discussions, have created an uncertain climate within the United States Space Program, making it difficult to predict the schedule for major programs such as the ASO. Approaches to the deployment of instruments for the study of high-energy solar emissions alone, or in conjunction with other solar instruments are discussed. As an independent entity the High-Energy Facility could be on a co-orbiting platform. 相似文献
149.
Christine?MillerEmail author Roland?Mundil Martin?Th?ni Jürgen?Konzett 《Contributions to Mineralogy and Petrology》2005,150(1):70-84
High-pressure metamorphism in the Pohorje Mountains of Slovenia (Austroalpine unit, Eastern Alps) affected N-MORB type metabasic
and metapelitic lithologies. Thermodynamic calculations and equilibrium phase diagrams of kyanite–phengite-bearing eclogites
reveal PT conditions of >2.1 GPa at T<750°C, but within the stability field of quartz. Metapelitic eclogite country rocks contain the assemblage garnet + phengite
+ kyanite + quartz, for which calculated peak pressure conditions are in good agreement with results obtained from eclogite
samples. The eclogites contain a single population of spherical zircon with a low Th/U ratio. Combined constraints on the
age of metamorphism come from U/Pb zircon as well as garnet–whole rock and mineral–mineral Sm-Nd analyses from eclogites.
A coherent cluster of single zircon analyses yields a 206Pb/238U age of 90.7±1.0 Ma that is in good agreement with results from Sm-Nd garnet–whole rock regression of 90.7±3.9 and 90.1±2.0 Ma
(εNd: +8) for two eclogite samples. The agreement between U-Pb and Sm-Nd age data strongly suggests an age of approximately
90 Ma for the pressure peak of the eclogites in the Pohorje Mountains. The presence of garnet, omphacite and quartz inclusions
in unfractured zircon indicates high-pressure rather than ultrahigh pressure conditions. The analysed metapelite sample yields
a Sm-Nd garnet–whole rock scatterchron age of 97±15 Ma. These data probably support a single P-T loop for mafic and pelitic
lithologies of the Pohorje area and a late Cretaceous high-pressure event that affected the entire easternmost Austroalpine
basement including the Koralpe and Saualpe eclogite type locality in the course of the complex collision of the Apulian microplate
and Europe. 相似文献
150.
The concentration of selected trace metals (Cu, Pb and Zn) in salt-marsh sediments from within the Erme Estuary have been measured in order to assess possible historical sources of pollution. The Erme Estuary, south Devon, UK is an Area of Outstanding Natural Beauty and has remained largely unaffected by industrialisation, although a number of small silver–lead mines were in operation in the 1800s. Five cores reveal comparable geochemical profiles. An increase of lead at 40 cm depth is observed, reaching maximum values of 427 ppm. Less distinct trends are revealed by zinc and copper, probably reflecting the lack of widespread mining for ores of these elements within the catchment and possible post-depositional mobility rendering the metal concentrations non-contemporaneous with the chemostratigraphy of lead. The geochemical analysis of the salt-marsh sediments provides a fairly robust chemostratigraphic scheme and the likely sources of mine waste can be pinpointed within the catchment. Based upon reference to the historical mining record of these mines chemostratigraphic dating of the sediments can be achieved in order to provide an estimate of salt-marsh accretion rates and sea-level rise. 相似文献