首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   673篇
  免费   29篇
  国内免费   7篇
测绘学   28篇
大气科学   53篇
地球物理   186篇
地质学   206篇
海洋学   39篇
天文学   156篇
综合类   9篇
自然地理   32篇
  2021年   13篇
  2020年   13篇
  2019年   14篇
  2018年   24篇
  2017年   21篇
  2016年   27篇
  2015年   42篇
  2014年   35篇
  2013年   26篇
  2012年   28篇
  2011年   43篇
  2010年   31篇
  2009年   33篇
  2008年   29篇
  2007年   49篇
  2006年   26篇
  2005年   26篇
  2004年   21篇
  2003年   17篇
  2002年   19篇
  2001年   13篇
  2000年   10篇
  1999年   10篇
  1998年   10篇
  1997年   4篇
  1996年   3篇
  1995年   7篇
  1994年   7篇
  1993年   2篇
  1992年   3篇
  1991年   4篇
  1989年   11篇
  1988年   2篇
  1987年   12篇
  1986年   5篇
  1985年   8篇
  1984年   7篇
  1983年   4篇
  1982年   8篇
  1981年   9篇
  1979年   5篇
  1978年   4篇
  1976年   5篇
  1971年   4篇
  1967年   1篇
  1965年   1篇
  1963年   1篇
  1962年   1篇
  1953年   1篇
  1952年   1篇
排序方式: 共有709条查询结果,搜索用时 31 毫秒
141.
Detailed acoustic surveys of benthic sediments were conducted in July 1995 and September 1998 in the vicinity of Humboldt Bay, California. During these surveys, a band of enhanced acoustic backscatter was observed offshore from the bay entrance, approximately parallel to the isobaths, in water depths ranging from 16-24 m. In order to assess the cause of the increase in backscatter levels, a more comprehensive study was conducted in August and September 1999 using 100 kHz side-scan sonar, bottom grab sampling and underwater video recording. New observations indicated that a dense population of sand dollars ( Dendraster excentricus ) coincided with the enhanced backscatter band. Compared to the two previous acoustic studies, the central section of the band expanded westward by 180 m and the southern section of the band shifted eastward by 160 m, possibly resulting from a change in the biological or physical factors which influence the location and breadth of sand dollars. The relationship between high sand dollar abundance and enhanced acoustic backscatter was further verified in the nearshore region off Samoa Beach California, where a dense, banded population of sand dollars was previously observed. Video footage confirmed the presence of a band of sand dollars, also nominally parallel to the isobaths, in water depths of 8-15 m. A band of enhanced backscatter coincided with the dense sand dollar population. The identification of dense aggregations of sand dollars through enhanced acoustic backscatter could lead to the use of acoustic techniques to study sand dollar distributions and abundance.  相似文献   
142.
High-resolution Sea Beam bathymetry and Sea MARC I side scan sonar data have been obtained in the MARK area, a 100-km-long portion of the Mid-Atlantic Ridge rift valley south of the Kane Fracture Zone. These data reveal a surprisingly complex rift valley structure that is composed of two distinct spreading cells which overlap to create a small, zero-offset transform or discordant zone. The northern spreading cell consists of a magmatically robust, active ridge segment 40–50 km in length that extends from the eastern Kane ridge-transform intersection south to about 23°12′ N. The rift valley in this area is dominated by a large constructional volcanic ridge that creates 200–500 m of relief and is associated with high-temperature hydrothermal activity. The southern spreading cell is characterized by a NNE-trending band of small (50–200 m high), conical volcanos that are built upon relatively old, fissured and sediment-covered lavas, and which in some cases are themselves fissured and faulted. This cell appears to be in a predominantly extensional phase with only small, isolated eruptions. These two spreading cells overlap in an anomalous zone between 23°05′ N and 23°17′ N that lacks a well-developed rift valley or neovolcanic zone, and may represent a slow-spreading ridge analogue to the overlapping spreading centers found at the East Pacific Rise. Despite the complexity of the MARK area, volcanic and tectonic activity appears to be confined to the 10–17 km wide rift valley floor. Block faulting along near-vertical, small-offset normal faults, accompanied by minor amounts of back-tilting (generally less than 5°), begins within a few km of the ridge axis and is largely completed by the time the crust is transported up into the rift valley walls. Features that appear to be constructional volcanic ridges formed in the median valley are preserved largely intact in the rift mountains. Mass-wasting and gullying of scarp faces, and sedimentation which buries low-relief seafloor features, are the major geological processes occurring outside of the rift valley. The morphological and structural heterogeneity within the MARK rift valley and in the flanking rift mountains documented in this study are largely the product of two spreading cells that evolve independently to the interplay between extensional tectonism and episodic variations in magma production rates.  相似文献   
143.
Summary This paper presents the first results of stress determinations in the Bohemian Massif utilizing televiewer data from deeper boreholes. The televiewer survey was originally carried out by Geological Survey Ostrava for the purpose of studying fractures and bedding in coal basins to depths of 2 km. A reinterpretation of the amplitude logs reveal a large number of stress-induced borehole elongations (breakouts) which occur in a direction perpendicular to the maximum horizontal principal stress (SH). The breakout analysis in the Upper Silesian, Meno and Kladno Basins indicates a dominant NW-SE direction of SH which is consistent with the pattern of stress orientation in Western Europe. The NE-SW trend observed in the Plze Basin is less reliable. A model for stress magnitude determinations is formulated as a linear programming problem. If the a priori information on rock strength properties is limited, it reduces to explicit formulas for stress regime estimates. The model is applied to data from the Upper Silesian Basin for which a compressional stress regime is derived.  相似文献   
144.
A very bright and long bolide was observed over the eastern part of the Czech Republic during late local evening on December 9, 2014. This bolide was recorded by professional instruments in the Czech part of the European Fireball Network. Three meteorites weighing in total 87 g were found in the predicted area and were named ??ár nad Sázavou. The complete material composition of the meteorite was obtained from one cut‐off piece using petrography, mineralogy, and scanning electron microscopy (together with X‐ray energy dispersive spectroscopy and wavelength dispersive spectroscopy). X‐ray computed tomography (CT) was applied on all pieces for the determination of their grain and bulk density, digitization of shape, and examination of the structural homogeneity. CT has proved its important role for nondestructive exploration of brecciated meteorites formed by distinct lithologies or petrological types. In this article, we discuss its limits in terms of structural and material resolution based on the correlation of state‐of‐the‐art CT data and SEM images. Furthermore, we introduce a new way of air cavity quantification, which distinguishes the natural porosity of meteorite and cracks related to erosion processes. This helps to discuss the presence of weathering products based on comparison of the meteorite pieces found at different times after impact.  相似文献   
145.
Early-Middle Pleistocene deposits have been studied in Central and Northern Yakutia, the Magadan region, and Chukotka. The Brunhes-Matuyama boundary occurs in the Ozheleznenye Galechniki (=Ferruginated pebblestones) beds of Central Yakutia that belong to the Talagay horizon of the Early-Middle Pleistocene. These layers include classic Aldan mammal fauna. In Eastern Yakutia sediments of Early-Middle Pleistocene belong to the Akan horizon, and the Early Pleistocene sediments are from the Chukochya horizon. These sediments which belong to the Olyor Formation and its age-equivalents yielded numerous mammal remains, termed the Olyor faunal complex. The Brunhes-Matuyama boundary is located in Akan horizon. In the upper reaches of the Kolyma River, the Brunhes-Matuyama boundary occurs in sediments of the Middle Pleistocene Belichan horizon. The boundary of the Brunhes-Matuyama is characteristic within the Elhkakvun and Enmakay formations of Chukotka. In Kamchatka, the boundary of Matuyama and Brunhes is evident in volcanic sequences. In the eastern part of the peninsula, this boundary is located in the Tumrok or Iult volcanic complex and in Central Kamchatka in the Kreruk volcanic complex.  相似文献   
146.
One pre-requisite for the construction of a global chromium isotope mass balance is detailed understanding of Cr isotope systematics in the critical zone where redox-processes can modify the isotope signature of geogenic Cr input into the hydrosphere. A Cr isotope inventory of bedrock, soil, and runoff was performed in a Central European headwater catchment underlain by amphibolite, situated in the vicinity of two previously studied catchments underlain by different bedrock types (serpentinite and leucogranite). Fresh bedrock in the amphibolite catchment NAZ contained ~300 mg/kg Cr, serpentinite at PLB contained ~800 mg/kg Cr, and leucogranite at LYS contained ~2 mg/kg Cr. Monthly hydrochemical monitoring at all three sites revealed higher Cr(VI) export fluxes in winter than in summer. NAZ was characterized by a distinct seasonality in the δ53Cr values, with minima during winter/spring snowmelts (−0.35‰) and maxima during dry summers (0.40‰). Similar seasonality in δ53Cr values had been reported from PLB and LYS. Bedrock at all three sites had similar Cr isotope composition close to −0.10‰, a value indistinguishable from the δ53Cr value of bulk silicate Earth (BSE). Positive mean δ53Cr value of NAZ runoff indicated Cr-isotope fractionations during weathering of geogenic Cr(III), combined with adsorption of the resulting Cr(VI) on soil particles during pedogenesis. However, the mass-weighted mean δ53Cr of NAZ runoff was lower (−0.08‰), indistinguishable from the Cr isotope signature of bedrock. The same pattern of lower mass-weighted mean δ53Cr values of runoff, compared to arithmetic mean δ53Cr values of runoff, were observed also at PLB and LYS. We suggest that elevated Cr runoff fluxes in winter remove some of the residual isotopically light Cr that accumulated in the soil during summer. Seasonality in runoff δ53Cr values appears to be a relatively widespread phenomenon, de-coupled from Cr availability for chemical weathering.  相似文献   
147.
我国于2012年1月在南极Dome A区域正式开展实施了南极昆仑站深冰芯科学钻探工程,截至2021年,钻孔深度已达803.54 m。该工程是我国第一个深冰芯钻探工程,也是国际上第一个在Dome A地区开展的深冰芯钻探项目。本文介绍了昆仑站深冰芯科学钻探工程实施的整体情况,对过去近10年的钻探活动以及取得的成果和经验进行了总结,以期为后续的深冰芯钻探工作提供理论和经验指导。  相似文献   
148.
On February 15, 2013, after the observation of a brilliant fireball and a spectacular airburst over the southern Ural region (Russia), thousands of stones fell and were rapidly recovered, bringing some extremely fresh material for scientific investigations. We undertook a multidisciplinary study of a dozen stones of the Chelyabinsk meteorite, including petrographic and microprobe investigations to unravel intrinsic characteristics of this meteorite. We also study the short and long‐lived cosmogenic radionuclides to characterize the initial meteoroid size and exposure age. Petrographic observations, as well as the mineral compositions obtained by electron microprobe analyses, allow us to confirm the classification of the Chelyabinsk meteorite as an LL5 chondrite. The fragments studied, a few of which are impact melt rocks, contain abundant shock melt veins and melt pockets. It is likely that the catastrophic explosion and fragmentation of the Chelyabinsk meteoroid into thousands of stones was in part determined by the initial state of the meteoroid. The radionuclide results obtained show a wide range of concentrations of 14C, 22Na, 26Al, 54Mn, 57Co, 58Co, and 60Co, which indicate that the pre‐atmospheric object had a radius >5 m, consistent with other size estimates based on the magnitude of the airburst caused by the atmospheric entry and breakup of the Chelyabinsk meteoroid. Considering the observed 26Al activities of the investigated samples, Monte Carlo simulations, and taking into account the 26Al half‐life (0.717 Myr), the cosmic‐ray exposure age of the Chelyabinsk meteorite is estimated to be 1.2 ± 0.2 Myr. In contrast to the other radionuclides, 14C showed a very large range only consistent with most samples having been exposed to anthropogenic sources of 14C, which we associate with radioactive contamination of the Chelyabinsk region by past nuclear accidents and waste disposal, which has also been confirmed by elevated levels of anthropogenic 137Cs and primordial 40K in some of the Chelyabinsk fragments.  相似文献   
149.
We investigate the mean velocity dispersion and the velocity dispersion profile of stellar systems in modified Newtonian dynamics (MOND), using the N -body code n-mody , which is a particle-mesh-based code with a numerical MOND potential solver developed by Ciotti, Londrillo & Nipoti. We have calculated mean velocity dispersions for stellar systems following Plummer density distributions with masses in the range of 104 to  109 M  and which are either isolated or immersed in an external field. Our integrations reproduce previous analytic estimates for stellar velocities in systems in the deep MOND regime  ( a i, a e≪ a 0)  , where the motion of stars is either dominated by internal accelerations  ( a i≫ a e)  or constant external accelerations  ( a e≫ a i)  . In addition, we derive for the first time analytic formulae for the line-of-sight velocity dispersion in the intermediate regime  ( a i∼ a e∼ a 0)  . This allows for a much-improved comparison of MOND with observed velocity dispersions of stellar systems. We finally derive the velocity dispersion of the globular cluster Pal 14 as one of the outer Milky Way halo globular clusters that have recently been proposed as a differentiator between Newtonian and MONDian dynamics.  相似文献   
150.
Regional climate model (RCM) RegCM3 with 50 km horizontal resolution driven from the lateral boundaries by the data from NCEP/NCAR re-analysis is used in a series of ten climate downscaling experiments over the eastern Mediterranean (EM) region. Results of the experiments are characterized by seasonal precipitation patterns with notable offshore precipitation zones positioned ~50 km westward of a less intense precipitation zone over the coastal area. Atmospheric processes determining the distribution of seasonal precipitation patterns in the EM are analyzed based on results of the RCM experiments performed. Level of success of the model representation of the actual precipitation over the ECM appears to be depending on that of precipitation balance over different parts of the domain. Excessive moisture convergence over a sub-area usually takes place at the expense of moisture divergence from neighboring areas. Synoptic mechanism causing formation of the precipitation zone in the offshore zone appears to be associated with the role of meridionally oriented atmospheric trough systems extending from Scandinavia or Siberia to the EM during the period with rainy events. In such situations, air flows with strong northern components lead to intense transport of cold air masses to the EM. Meeting of the cold air masses the warm and humid air over the sea surface in the offshore zone causes formation of persistent squall lines and heavy rains there. Such processes may continue quite long as long as the troughs are stationary.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号