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381.
Bruce R. Julian 《Geophysical Journal International》1986,84(2):431-443
Summary. Linear-programming methods are powerful and efficient tools for objectively analysing seismic focal mechanisms and are applicable to a wide range of problems, including tsunami warning and nuclear explosion identification. The source mechanism is represented as a point in the six-dimensional space of moment-tensor components. Each observed polarity provides an inequality constraint, linear with respect to the moment tensor components, that restricts the solution to a half-space bounded by a hyperplane passing through the origin. The intersection of these half-spaces is the convex set of all acceptable solutions. Using linear programming, a solution consistent with the polarity constraints can be obtained that maximizes or minimizes any desired linear function of the moment tensor components; the dilatation, the thrust-like nature, and the strike-slip-like nature of an event are examples of such functions. The present method can easily be extended to fit observed seismic-wave amplitudes (either signed or absolute) subject to polarity constraints, and to assess the range of mechanisms consistent with a set of measured amplitudes. 相似文献
382.
Michael J. Hambrey Jeremy R. Davies Neil F. Glasser Richard A. Waters Julian A. Dowdeswell Philip R. Wilby David Wilson James L. Etienne 《第四纪科学杂志》2001,16(5):455-482
The depositional processes associated with late Devensian ice in areas bordering the Irish Sea basin have been the subject of considerable debate. Among the key areas around the Irish Sea, southwest Wales occupies a particularly crucial position because it is here that ice flowing from the north impinged upon the coast orthogonally and encroached inland. Two main hypotheses have emerged concerning deglaciation of the Irish Sea basin. The traditional hypothesis holds that sedimentation was ice‐marginal or subglacial, whereas an alternative hypothesis that emerged in the 1980s argued that sedimentation was glaciomarine. Southwest Wales is well‐placed to contribute to this debate. However, few detailed sedimentological studies, linked to topography, have been made previously in order to reconstruct glacial environments in this area. In this paper, evidence is presented from four boreholes drilled recently in the Cardigan area, combined with data from coastal and inland exposures in the lower Teifi valley and adjacent areas. A complex history of glaciation has emerged: (i) subglacial drainage channel formation in pre‐Devensian time, (ii) deposition of iron‐cemented breccias and conglomerates possibly during the last interglacial (or in the early/mid‐Devensian interstadial), (iii) late Devensian ice advance across the region, during which a glaciolacustrine sequence over 75 m thick accumulated, within a glacial lake known as Llyn Teifi, (iv) a second high‐level glaciolacustrine succession formed near Llandudoch, (v) outside the Teifi valley, ice‐marginal, subglacial and glaciofluvial sediments were also laid down, providing a near‐continuous cover of drift throughout the area. Glacial advance was characterized by reworking, deformation and sometimes erosion of the underlying sediments. The glaciomarine hypothesis is thus rejected for southwest Wales. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
383.
Impact of the Late Glacial Eruption of the Laacher See Volcano, Central Rhineland, Germany 总被引:1,自引:0,他引:1
Michael Baales Olaf Jris Martin Street Felix Bittmann Bernhard Weninger Julian Wiethold 《Quaternary Research》2002,58(3):273
Within a period of a few weeks toward the end of the Allerød Interstadial, the major Plinian eruption of the Laacher See volcano produced some 20 km3 of eruptiva, covering and preserving the late-glacial landscape in the German Central Rhineland over an area of more than 1000 km2. Correlation of terrestrial archives with the Greenland ice-core records and improved calibration of the radiocarbon timescale permit a precise, accurate age determination of the Laacher See event some 200 yr before the onset of the Younger Dryas cold episode. Carbonized trees and botanical macrofossils preserved by Laacher See Tephra permit detailed regional paleoenvironmental reconstruction and show that open woodland were typical for the cool and humid hemiboreal climatic conditions during the late Allerød. This woodland provided the habitat for a large variety of animal species, documented at both paleontological and Final Paleolithic archeological sites preserved below Laacher See deposits. Of special interest are numerous animal tracks intercalated in Middle Laacher See deposits at the south of the Neuwied Basin. This knowledge may help to evaluate possible supraregional impacts of this volcanic event on northern hemispheric environment and climate during the late Allerød. 相似文献
384.
This paper reviews involuntary resettlement resulting from dam-building, which has been ignored relative to the dominant focus of migration research in China, rural to urban migration. Reservoir resettlement in China has a long history, often of misery and hardship for those displaced. Relocatees affected by the Three Gorges Project (1994–2009) on the Yangtze River face a similar situation. In China priority has been given to building the dam to provide electricity, flood control and navigation. Less attention has been paid to the problems of the people affected by the reservoir inundation. The rural population forced to relocate and rural-urban migrants in general have been discriminated against by national policies. 相似文献
385.
Julian Tejera De Leon 《Journal of African Earth Sciences》1997,24(4):455-472
The terranes which outcrop to the north and south of the westward extension of the Jebha Fault show considerable differences in their nature and structural organisation. The pre-Oligocene basement is composed of synorogenic synclines filled with detrital deposits. In the North, the synclinal troughs are oriented North-South and filled with siliciclastic material. The synclinal structures on top of the western extension of the Jebha Fault are orientated N 140° and are also filled with the same material. South of this westward extension of the Jebha Fault, the orientation of the synclinal structures is north-south; they are filled with bioclastic calcareous material.The synorogenic synclines, situated on top of the Jebha-Arbaoua limit show characteristics of the detached sedimentary cover by their position and orientation. This cover formed the Arbaoua-Jebha lateral ramp during its movement towards the west. Developement of this ramp is explained by the crustal discontinuity inherited from the period of rifting.The resulting palaeomorphological relief allows the difference between the zones situated at each part of the limit to be understood, in particular, it permits the understanding of the differences in the detrital material, in its role as a barrier opposing the arrival of bioclastic calcareous material from the African margin (situated southwards of the limit).The Arbaoua-Jebha lateral ramp appears as a major and original feature of the Rift chain. 相似文献
386.
The possibility that chlorine may deplete stratospheric chlorine has received considerable attention recently. The only termination steps considered up to now involve HCl formation by chlorine atom attack on hydrogen-bearing molecules. We propose that an important removal mechanism for chlorine in the stratosphere may be the formation of HClO4 via the sequence of steps Cl + O2 + O3 → ClO3 + O2 ClO3 + OH → HClO4. In addition to being produced as often as HCl, HClO4 may be more stable to radical attack and thus a more efficient sink than HCl for stratospheric chlorine. 相似文献
387.
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390.
The clam Scrobicularia plana and the polychaete worm Nereis diversicolor were collected in several sites from a littoral enclosure in SW Spain. The aim of our study was to relate various biomarker responses in these species to a pollution gradient caused by untreated domestic discharges and to verify the adequacy of the selected species as sentinels in this habitat. The biomarkers selected were the antioxidant enzymes catalase (CAT), glutathione peroxidase (GPX) and DT-diaphorase (DT-D). In addition, the activities of cytochrome P450-dependent ethoxyresorufin O-deethylase (EROD) activity, the phase II detoxifying enzyme glutathione S-transferase (GST) and the neurotoxicity marker acetylcholinesterase (AChE) were measured. Metallothionein levels were selected as biomarkers of heavy metals exposure in both species. The results suggest a different response in the water filtering organism (clam) and the sediment eater (polychaete), probably as a consequent of different pollution exposure and that samples from the “Caño Sancti-Petri” were exposed to biologically active compounds that altered some of their biochemical responses. AChE was the most sensitive biomarker in both species and N. diversicolor proved to be a more robust sentinel in this ecosystem. 相似文献