全文获取类型
收费全文 | 8914篇 |
免费 | 1662篇 |
国内免费 | 1925篇 |
专业分类
测绘学 | 947篇 |
大气科学 | 1136篇 |
地球物理 | 2829篇 |
地质学 | 4890篇 |
海洋学 | 1022篇 |
天文学 | 383篇 |
综合类 | 477篇 |
自然地理 | 817篇 |
出版年
2024年 | 49篇 |
2023年 | 118篇 |
2022年 | 269篇 |
2021年 | 333篇 |
2020年 | 275篇 |
2019年 | 323篇 |
2018年 | 729篇 |
2017年 | 629篇 |
2016年 | 492篇 |
2015年 | 450篇 |
2014年 | 411篇 |
2013年 | 449篇 |
2012年 | 1067篇 |
2011年 | 829篇 |
2010年 | 506篇 |
2009年 | 433篇 |
2008年 | 453篇 |
2007年 | 443篇 |
2006年 | 421篇 |
2005年 | 1115篇 |
2004年 | 1074篇 |
2003年 | 770篇 |
2002年 | 285篇 |
2001年 | 178篇 |
2000年 | 139篇 |
1999年 | 66篇 |
1998年 | 20篇 |
1997年 | 23篇 |
1996年 | 22篇 |
1995年 | 3篇 |
1992年 | 8篇 |
1991年 | 12篇 |
1990年 | 14篇 |
1989年 | 5篇 |
1987年 | 7篇 |
1985年 | 4篇 |
1980年 | 4篇 |
1979年 | 6篇 |
1976年 | 3篇 |
1975年 | 5篇 |
1966年 | 3篇 |
1965年 | 5篇 |
1957年 | 2篇 |
1955年 | 2篇 |
1954年 | 6篇 |
1951年 | 2篇 |
1948年 | 2篇 |
1934年 | 2篇 |
1928年 | 3篇 |
1925年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
海南文昌地区第四系沉积物特征及其地质意义 总被引:1,自引:0,他引:1
海南省文昌市东部沿海一带是华南地区第四系发育较为典型的地区之一,其分布受新生代晚期形成的新构造格局的控制。通过对第四系沉积物特征、粒度分析、重矿物锆石采用激光剥蚀等离子体质谱(LA-ICP-MS)技术进行年龄测定,并选用206Pb/238U年龄做谐和年龄频谱图。认为该区第四系松散沉积具有不完全相同的物源。八所组与烟墩组的沉积物源区具有一定的相似性,北海组的沉积物源区明显不同于八所组和烟墩组。这种物源的差异性可能直接导致了第四系成矿作用的不同。 相似文献
982.
This paper discusses a geostatistical approach to model a groundwater aquifer in 3-D. The study aims at utilizing geostatistics as a tool for characterizing zones of better-water quality in a brackish-saline aquifer. In particular, the geostatistical model was constructed to characterize the aquifer’s salinity, represented by total dissolved solids (TDS), using logs of porosity and resistivity. Quality-checked estimated TDS vertical profiles were employed to construct and model horizontal and vertical semivariograms. Parameters of semivariogram models were used to develop both the kriging plan and the generated model. Results of this modeling process are shown in the form of horizontal salinity distribution maps. The aquifer was sliced into 20 layers, each 20 m thick, to represent its overall thickness. Salinity layers maps reflect vertical stratification of TDS concentrations in the aquifer and show that water quality deteriorates with depth and toward the northern part of the aquifer. Relatively better-quality water (TDS ≤10,000 mg/l) can be found at depths between 100 and 250 m below the aquifer’s top in both eastern and southeastern parts. Water in the same interval to the western and southwestern parts reflects the presence of higher TDS concentration. From a planning point of view, it is more feasible to target the eastern part of the aquifer for pumping and desalination purposes. In addition, the generated model could be utilized as an initial condition for flow simulation. 相似文献
983.
Alunite minerals occur as white powdery lumps and laminated coloured deposits within cavity and solution channel infill of the palaeokarst zone of the Upper Eocene Dammam Formation. This formation is exposed in a quarry located on the Al Ahmadi ridge within the Greater Burgan oil field in southern Kuwait. Field occurrences and sedimentary structures of the alunite deposits were described. Collected samples were petrographically described, and their mineralogy and geochemistry were determined using X-ray diffraction and X-ray fluorescence, respectively. Microfabrics were investigated using SEM, revealing that they are primarily composed of fibrous alunogen (hydrous aluminium sulphate) and pseudo-cubical K-alunite (hydrous potassium aluminium sulphate). Their mode of occurrence suggests a hypogenetic origin, where sulphide gases associated with hydrocarbon gases reacted with an Al-rich solution leached from clay minerals and feldspars of the cavity-fill muddy sand sediments. The hydrocarbon gases may have seeped from sub-surface petroliferous formations within the Greater Burgan oil field along vertical fractures. This study suggests that these acidic seeps may have played a role in the development of the palaeokarst zone of the Dammam Formation. 相似文献
984.
Abdullah?Al-Khadouri Sultan?Al-YahyaiEmail author Yassine?Charabi 《Arabian Journal of Geosciences》2015,8(3):1623-1633
The impact of the air pollution generated by any industrial activities may be further aggravated if the location of the industrial area is exposed to certain atmospheric characteristics. Under such conditions, the likelihood of accumulation of local air pollution is high. This paper uses two approaches (statistical and numerical simulation) to investigate the contribution of atmospheric processes towards degradation of air quality. A case study of the two approaches was conducted over Sohar Industrial Area in the Sultanate of Oman. Measured wind data were used to account for specific atmospheric characteristics such as stagnation, ventilation, and recirculation using the statistical approach. In the second approach, numerical weather prediction model was used to simulate mesoscale circulation phenomena such as sea breeze and its contribution to the processes affecting the air quality. The study demonstrates that the atmospheric processes appear to contribute substantially to the degradation of air quality in the Sohar Industrial Area. The statistical analysis shows that the atmospheric dilution potential of Sohar Industrial Area is prone to stagnation and recirculation, rather than ventilation. Moreover, model simulation shows that there is a seasonal variation in the contribution of atmospheric processes to the degradation of the air quality at Sohar Industrial Area. 相似文献
985.
Roof separation is one of the major indications for coal mine roadway roof accidents. Therefore, it is important that efforts be made to detect roadway roof separation. Currently, the instrument used for detecting roadway roof separation is the roof-off-the-strata indicator, but its installation and use is complicated and the need is greater than can be managed. This thesis takes ground penetrating radar (GPR) as a means for roof separation detection and analyzes its feasibility for use in detecting roof separation both theoretically and experimentally. First, numerical simulation experiments on roof separation detection are carried out using GprMax2D software. The results show that it is feasible to detect roof separation by using GPR electromagnetic waves. Next, physical simulation experiment on roof separation detection is carried out using MALA GPR (RAMAC/GPR). Lastly, the traditional image segmentation algorithm of 2D maximum between a cluster variance is improved to interpret the GPR signal. Good results were achieved using the improved algorithm to interpret detection signals of roof separation by GPR in a physical simulation experiment. 相似文献
986.
Rohtash?Kumar Subhash?Chander?Gupta Arjun?KumarEmail author 《Arabian Journal of Geosciences》2015,8(8):5713-5726
The purposes of this seismological investigation are to understand and describe the effect of decrease in the azimuthal coverage of an earthquake on moment tensor solution estimated by waveform inversion. It will be very useful and worthwhile as mostly seismological networks are sparse and in case when only one or two station data are available. In this work, we have used two moderate earthquakes, 21 September 2009 (near Uttarakashi) and 3 May 2010 (near Ghansali). The waveform inversion has been carried using ISOLA software. The moment tensor is first estimated by using all station data and then by removing the stations so that the azimuthal coverage changes. The results show that strike of both nodal planes is varying with the change in azimuthal coverage. However, the slip and dip of both nodal planes remain quite stable against the variation in azimuthal coverage for these two earthquakes analyzed. The effect of decrease in the azimuthal coverage showed increase in double-couple percentage (DC %) and decrease in compensated linear vector decomposition (CLVD %). The other focal parameters such as T-axis azimuth, P-axis azimuth, T-axis plunge, and P-axis plunge have been found stable against the variation in azimuth coverage. The study also demonstrates that the moment tensor solutions obtained from waveform inversion using single station are almost similar to those estimated using maximum azimuthal coverage data and by polarity inversion. 相似文献
987.
形变监测是海塘安全运行的重要组成部分。本文利用杭州地区2006年—2010年获取的31景EnvisatASAR影像,根据MTInSAR(Multi-Temporal In SAR)方法,综合提取PS(Persistent Scatterer)和DS(Distributed Scatterer)点,得到密集的钱塘江海塘形变的In SAR测量结果。与28个点的水准数据对比表明,两者的平均误差为0.436 mm,最大误差为5.016 mm,验证了In SAR技术毫米级的测量精度和准确性。通过这两种数据的时间序列分析发现,海塘的沉降在空间上具有连续性,其空间分布呈现为单峰下沉曲线;在时间上,则具有明显的线性变化规律,并伴随有短时间范围的小幅度波动。 相似文献
988.
989.
利用超分辨率重建技术可以有效提高图像的空间分辨率,其中先验模型的选取尤为关键。在最大后验概率(MAP)的框架下引入双边结构张量测度,联合像素邻域4个方向的梯度算子提出一种局部自适应先验方法,构建图像超分辨率重建模型,并利用迭代重加权范数(iteratively reweighted norm)对其进行转换求解。基于标准的测试图像进行了实验,并将本方法与拉普拉斯先验、Huber-Markov先验以及BTV先验的重建结果进行对比,验证了该方法的有效性。 相似文献
990.
结合实际项目经验,浅谈警用地理信息系统建设中基础地理信息和公共地理信息数据的采集。采用高分辨率的卫星影像数据,结合外业采集的点状信息,进行警用基础地理信息数据和应急公共地理信息的采集,创建符合警用地理信息属性数据结构的空间矢量数据。 相似文献