全文获取类型
收费全文 | 8339篇 |
免费 | 2225篇 |
国内免费 | 1744篇 |
专业分类
测绘学 | 831篇 |
大气科学 | 1235篇 |
地球物理 | 3514篇 |
地质学 | 2455篇 |
海洋学 | 3066篇 |
天文学 | 136篇 |
综合类 | 506篇 |
自然地理 | 565篇 |
出版年
2024年 | 19篇 |
2023年 | 107篇 |
2022年 | 212篇 |
2021年 | 295篇 |
2020年 | 340篇 |
2019年 | 388篇 |
2018年 | 295篇 |
2017年 | 353篇 |
2016年 | 339篇 |
2015年 | 430篇 |
2014年 | 495篇 |
2013年 | 507篇 |
2012年 | 584篇 |
2011年 | 579篇 |
2010年 | 428篇 |
2009年 | 522篇 |
2008年 | 490篇 |
2007年 | 680篇 |
2006年 | 564篇 |
2005年 | 524篇 |
2004年 | 502篇 |
2003年 | 440篇 |
2002年 | 385篇 |
2001年 | 339篇 |
2000年 | 312篇 |
1999年 | 297篇 |
1998年 | 283篇 |
1997年 | 242篇 |
1996年 | 247篇 |
1995年 | 236篇 |
1994年 | 211篇 |
1993年 | 166篇 |
1992年 | 128篇 |
1991年 | 103篇 |
1990年 | 57篇 |
1989年 | 56篇 |
1988年 | 53篇 |
1987年 | 32篇 |
1986年 | 15篇 |
1985年 | 14篇 |
1984年 | 7篇 |
1983年 | 7篇 |
1982年 | 1篇 |
1980年 | 4篇 |
1979年 | 5篇 |
1978年 | 3篇 |
1977年 | 1篇 |
1954年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Multiple ridges across prograding coasts may display variable geometries, commonly expressed through varying elevations. Changes in ridge elevation have been traditionally related to the occurrence of fluctuating progradation rates, which might, in turn, be driven by shifting environmental conditions. Here, we explore the geometry and growth mechanisms of multiple ridges, generated at Barreta Island (Ria Formosa, southern Portugal), as a consequence of the rapid progradation of the island over the last 70 years, following the artificial fixation of the downdrift Faro-Olhão inlet with jetties in 1955. The variability in the morphology of these features was analysed in combination with available wind and wave data, in order to better distinguish growth mechanisms and understand the main parameters determining the final geometry of the observed ridges. The results suggest that (1) most of the identified ridges fall in the beach ridge classification, as they have been mostly built by marine processes, and (2) the parameters derived from, or closely related to wave climate variability (e.g. progradation rates, storm occurrence) can jointly explain most of the observed morphological changes, while aeolian processes played a secondary role. Indeed, ridge geometry appears mainly controlled by progradation rates, with higher ridges associated with lower progradation rates. Progradation rate, in turn, is mostly related to longshore wave power, storminess, and the occurrence storm groups. Yet, the final configuration of ridges can also be affected by runup levels and onshore winds. Therefore, establishing the relation between ridge geometry and wave climate is not a straightforward task, because of the complex processes and interactions that control coastal morphodynamics. © 2019 John Wiley & Sons, Ltd. 相似文献
2.
随着科学技术的发展,测量工具和测量手段也发生了变化,其中测量型GNSS呈现价格降低、体积缩小的趋势。目前市场上一整套商用GNSS测量系统的价格从几万到几十万不等,且由于地面基站和移动站体积大、携带不方便等原因,使得商用GNSS测量系统的使用人群仅限于部分专业测量人员,难以在大众市场普及。当今众多领域如无人机、机器制导、精密农业等对高精度定位需求的增加,又促使GNSS测量系统向低成本、高精度、小型化、轻型化发展。本文对U-BLOX的NEO-M8P低成本OEM型GNSS接收机进行50 km基线下的静态观测,基于RTKLIB的测试结果和精度分析表明,低成本GNSS测量系统能够达到厘米级定位精度,能够满足测量工作和GIS应用的需求。试验证明,使用此低成本测量系统能降低测量任务的经济成本,是测量级GNSS接收机中经济适用的选择。 相似文献
3.
基于甘肃河西走廊15个国家基准基本站点提供的1966年1月1日~2018年12月31日春季(3~5月)逐日最低气温值数据,利用线性回归分析方法、Spline空间插值法和Mann-Kendall趋势和突变检验法,探讨了河西走廊地区1966—2018年春季(3~5月)寒潮频次变化及其影响因子。研究表明:(1) 1966—2018年河西走廊地区春季(3~5月)单站寒潮频次总体呈现下降趋势〔–0.098次·(10 a)–1〕,其中1980—2010年寒潮频次呈显著下降趋势,2010年之后下降趋势变缓,未通过显著性检验;区域寒潮53 a来频次总体呈缓慢下降趋势〔–0.015次·(10 a)–1〕。(2) 近53 a河西走廊地区春季三个月中,单站寒潮总量4月>3月>5月,其中4月、5月寒潮频次下降不明显,3月频次下降显著。(3) 空间上,大致以北大河和黑河干流为界,两河中间区域春季寒潮频次低,而北大河以北和黑河干流以东区域则是寒潮高值区,走廊外围地区寒潮频次较高,且大多呈显著下降趋势,寒潮频次与气温距平存在明显负相关关系,内部地区变化趋势不明显。(4) 河西地区春季寒潮频次受气候变暖、地形和大气环流的影响,寒潮频次变化趋势存在地区差异。研究可提高对甘肃河西走廊寒潮演化过程的认知,为河西走廊气候变化的进一步研究奠定基础。 相似文献
4.
喀斯特露石常常占据一定比例的地表面积,且具有许多已知或未知的水文—生态功能,对于喀斯特地区的植被恢复和环境改善发挥着重要的作用。本实验运用样线法和拍照法识别喀斯特断陷盆地不同位置(盆缘和盆地边坡)露石的数量特征和外部形态特征,同时比较了两种方法测量样地岩石裸露率的精准度。结果发现:盆地边坡与盆缘的露石数量和外部特征存在显著差异,盆缘露石分布密度(0.54个·m-2)>盆地边坡(0.39个·m-2);但盆地边坡露石的长宽比大于盆缘,单体占地面积为盆缘露石的两倍,接近40%的盆地边坡露石与山体等高线平行。样线法和拍照法测量样地岩石裸露率的结果存有差异,但并不显著,且这种差异呈现出随着岩石裸露率的增加而逐渐增加的趋势,拍照测量结果具有较小的变异系数。这些结果说明了研究区露石具有较强的空间异质性,对解释地表水土运移规律和植物分布提供重要参考。拍照法能准确地测量岩石裸露率,并提供露石外部形态特征的精准数据,可以作为喀斯特样地露石调查的新方法加以推广。 相似文献
5.
多点液压式波浪能海水淡化系统建模与仿真 总被引:1,自引:1,他引:0
为缓解淡水资源短缺及化石能源过度使用问题,提出多点液压式波浪能海水淡化系统,该系统主要由采能装置、液压传递系统与反渗透膜海水淡化设备组成。系统的采能装置采用振荡浮子式,可将波浪能转换为浮子振荡从而被液压系统吸收达到采集波浪能的目的。为了提高液压式波浪能海水淡化系统的采能效率及淡水率,利用AMEsim软件对液压传递系统进行建模与仿真,分析了蓄能器、浮子个数及波高对液压传递系统输出响应的影响。结果表明:蓄能器能够使液压马达的输出响应更加稳定;当浮子的数量增加时,液压系统达到稳定的运行状态所需的时间更短,从而有利于提高系统的效率;波高在2 m左右时,本系统的产水量达到最大。 相似文献
6.
采用“三小三高二绝招”的工作方法,通过静校正、多域多方法联合噪音衰减和精细的速度分析获得较高分辨率的地震时间剖面,揭示苏码头断裂的展布位置、构造形态及变形样式等。结果表明,区域构造以冲起式为主,在反冲断层与逆冲断层之间形成冲起构造或逆冲三角构造,在地震剖面上有明显的楔状特征。通过地震测线的断层走向以NE为主,倾向以SE为主,部分断层倾向为NW。 相似文献
7.
本文基于对南海东北部东沙海域近期采集的多道反射地震资料进行重新处理获得新的地震海洋学数据,分析了该海域内孤立波/内孤立波包、沙丘上方和陡坎附近特殊反射结构特征,从而提供了新的海水层与海底相互作用依据.研究结果表明,除之前已发表文章中地震海洋学资料显示存在的第一模态内孤立波/波包和沙丘上方常见的反射样式-披毛状发射外,地震海洋学资料上还发现了第二模态内孤立波、陡坎上方的上抬型波动反射结构样式.在新的地震海洋学数据中,第一模态内孤立波振幅均小于50 m,宽度上都小于5 km,单个内孤立波的最大振幅约为45 m.内孤立波包的内孤立波振幅都相对较小,均小于40 m,并且与之前不同的是,彼此之间振幅相差不大,没有明显的排列规律.此次地震海洋学数据记录到的第二模态内孤立波,形态较为完整,上层和下层反射的振幅相差不大,在30 m左右;中间层大约在水深130 m位置处,垂向结构的整体大小大于200 m.沙丘上方反射结构普遍存在弱反射层,可能是湍流边界层,并且存在特殊反射样式-披毛状反射.但并不是沙丘上方都存在披毛状反射样式,本文分析它出现在地震海洋学资料上可能是受测线与沙丘走向之间夹角的影响.陡坎区域的水层反射结构则表现为上抬型波动,并常常伴随着同相轴连续性的变化.该波动的大小及反射同相轴的连续性可能取决于陡坎的高度/坡度及水层动力的强度,新数据中出现的一个上抬型波动,高度达20~30 m,它的附近水层还存在一个形态不完整的内孤立波.陡坎附近的水层反射也常常出现弱反射带和小的波动. 相似文献
8.
Javier Lario Chris Spencer Teresa Bardají Ángel Marchante Victor H. Garduño-monroy Jorge Macias Sergio Ortega 《Sedimentology》2020,67(3):1481-1504
The Yucatán Peninsula, Mexico, has typically been considered a tectonically stable region with little significant seismic activity. The region though, is one that is regularly affected by hurricanes. A detailed survey of ca 100 km of the eastern Yucatán and Cozumel coast identified the presence of ridges containing individual boulders measuring >1 m in length. The boulder ridges reach 5 m in height and their origin is associated with extreme wave event activity. Previously modelled tsunami waves from known seismically active zones in the region (Muertos Thrust Belt and South Caribbean Deformed Belt) are not of sufficient scale in the area of the Yucatán Peninsula to have produced the boulder ridges recorded in this study. The occurrence of hurricanes in this region is more common, but two of the most destructive (Hurricane Gilbert 1988 and Hurricane Wilma 2005) produced coastal waves too small to have created the ridges recorded here. In this paper, a new tsunami model with a source area located on the Motagua/Swan Island Fault System has been generated that indicates a tsunami event may have caused the extreme wave events that resulted in the deposition of the boulder ridges. 相似文献
9.
Although the channel morphology of upland fluvial systems is known to be strongly controlled by sediment supply from hillslopes, it is still difficult to isolate this effect from the other controlling factors of channel forms, such as the sediment transport capacity (depending notably on the size of the catchment) and local conditions (e.g. confinement, riparian vegetation, valley-floor slope). The rivers in New Caledonia offer an interesting field laboratory to isolate the morphological effect of contrasted sediment supply conditions. Some of these rivers are known to be highly impacted by the coarse sediment waves induced by the mining of nickel deposits that started in the early 1870s, which was particularly intensive between the 1940s and 1970s. The propagation of the sediment pulses from the mining sites can be traced by the presence of wide and aggraded active channels along the stream network of nickel-rich peridotite massifs. A first set of 63 undisturbed catchments in peridotite massifs distributed across the Grande Terre was used to fit a classic scaling law between active channel width and drainage area. A second set of 86 impacted sites, where the presence of sediment waves was clearly attested by recent aerial imagery, showed systematically wider active channels, with a width ratio around 5 (established from the intercept ratio of width–area power laws). More importantly, this second set of disturbed sites confirmed that the residual of active channel widths, computed from the scaling law of undisturbed sites, is statistically positively related to the catchment-scale relative area of major mining sediment sources. It is therefore confirmed that the characterization of sediment supply conditions is crucial for the understanding of spatial patterns of active channel width, and this should be more thoroughly considered in morphological studies of rivers draining environments with contrasted geomorphic activities on hillslopes. © 2018 John Wiley & Sons, Ltd. 相似文献
10.
In this paper, we present and evaluate three long-term wave models for application in simulation-based design of ships and marine structures. Designers and researchers often rely on historical weather data as a source for ocean area characteristics based on hindcast datasets or in-situ measurements. The limited access and size of historical datasets reduces repeatability of simulations and analyses, making it difficult to assess the sampling variability of performance and loads on marine vessels and structures. Markov, VAR and VARMA wave models, producing independent long-term time series of significant wave height (Hs) and spectral peak period (Tp), is presented as possible solutions to this problem. The models are tested and compared by addressing how the models affect interpretation of design concepts and the ability to replicate statistical and physical characteristics of the wave process. Our results show that the VAR and VARMA models perform sufficiently in describing design performance, but does not capture the physical process fully. The Markov model is found to perform worst of the tested models in the applied tests, especially for measures covering several consecutive sea states. 相似文献