全文获取类型
收费全文 | 35138篇 |
免费 | 2438篇 |
国内免费 | 4229篇 |
专业分类
测绘学 | 2480篇 |
大气科学 | 4410篇 |
地球物理 | 6919篇 |
地质学 | 17372篇 |
海洋学 | 2840篇 |
天文学 | 2039篇 |
综合类 | 3087篇 |
自然地理 | 2658篇 |
出版年
2024年 | 109篇 |
2023年 | 259篇 |
2022年 | 588篇 |
2021年 | 705篇 |
2020年 | 596篇 |
2019年 | 647篇 |
2018年 | 5288篇 |
2017年 | 4536篇 |
2016年 | 3185篇 |
2015年 | 865篇 |
2014年 | 868篇 |
2013年 | 854篇 |
2012年 | 1806篇 |
2011年 | 3537篇 |
2010年 | 2802篇 |
2009年 | 3043篇 |
2008年 | 2618篇 |
2007年 | 2995篇 |
2006年 | 638篇 |
2005年 | 695篇 |
2004年 | 784篇 |
2003年 | 761篇 |
2002年 | 634篇 |
2001年 | 421篇 |
2000年 | 346篇 |
1999年 | 373篇 |
1998年 | 286篇 |
1997年 | 246篇 |
1996年 | 229篇 |
1995年 | 197篇 |
1994年 | 161篇 |
1993年 | 140篇 |
1992年 | 134篇 |
1991年 | 77篇 |
1990年 | 59篇 |
1989年 | 71篇 |
1988年 | 50篇 |
1987年 | 28篇 |
1986年 | 28篇 |
1985年 | 24篇 |
1984年 | 15篇 |
1983年 | 15篇 |
1982年 | 5篇 |
1981年 | 30篇 |
1980年 | 25篇 |
1979年 | 9篇 |
1978年 | 5篇 |
1976年 | 6篇 |
1958年 | 4篇 |
1957年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A hydro-dynamic model is established on basis of MIKE21 FM to simulate the hydro-dynamic characteristics of Xinghua Bay and investigate the influence of reclamation project on the tidal elevation and tidal currents. Tidal elevation data was obtained at the six tide gauge stations around the Xinghua Bay, and another six current stations were established to observe the tidal current velocity and direction. Validation shows that the model-simulated tidal elevation and tidal currents agree well with observations made at different stations. Predictions are made according to the reclamation project proposed in the regional marine planning of Hanjiang Industrial Park around the port in Putian City. The variations of hydro-dynamic factors, such as tide, current velocity and direction and tidal influx are obtained, and the adverse effect of reclamation on marine environment is discussed. It is shown that the tidal level inside the Xinghua Bay during high tide decreases after the reclamation project is completed. The tidal currents during flooding tide generally decrease in the southeast of the reclamation region, with the maximum decreasing amplitude reaching 0.44 m s-1. On the other hand, the tidal currents during flooding tide increase around the southeast and southwest corners of the reclamation region. The tidal currents during ebb tide increase around the southeast and southwest corners of the reclamation region, with the maximum increasing amplitude attaining 0.18 m s-1. The results in this paper can give some guidance for the marine environment management and the effective utilization of land in Putian. 相似文献
992.
Current data from a moored Acoustic Doppler Current Profiler(ADCP) deployed at 69?30.155′N,169?00.654′W in the central Chukchi Sea during 2012 summertime is analyzed in the present paper.Characteristics of tidal and residual currents are ob-tained with Cosine-Lanczos filter and cross-spectral analyses.The main achievements are as follows:1) Along with the local inertial frequency of 12.8 h,two other peaks at ~12-h and ~10-d dominate the time series of raw velocity;2) The M_2 dominates the 6 resolved tide constituents with significant amplitude variations over depth and the ratios of current speed of this constituent to that of the total tidal current are 54% and 47% for u and v components,respectively.All the resolved tidal constituents rotate clockwise at depth with the exception of MM and O1.The constituents of M_2 and S_2 with the largest major semi-axes are similar in eccentricity and orientation at deeper levels;3) The maximum of residual currents varies in a range of 20–30 cms~(-1) over depth and the current with lower velocities flow more true north with smaller magnitudes compared to the current in surface layer.The ~10 d fluctuation of residual current is found throughout the water column and attributed to the response of current to the local wind forcing,with an approximate 1.4 d lag-time at the surface level and occurring several hours later in the lower layer;4) Mean residual currents flow toward the north with the magnitudes smaller than 7 cms~(-1) in a general agreement with previous studies,which suggests a relatively weaker but stable northward flow indeed exists in the central Chukchi Sea. 相似文献
993.
Lei Xing Huaishan Liu Xilai Zheng Xueqin Liu Jin Zhang Linfei Wang Zhihui Zou Yiming Xu 《中国海洋大学学报(英文版)》2016,15(5):815-824
Air guns are important sources for marine seismic exploration. Far-field wavelet of air gun arrays, as a necessary parameter for pre-stack processing and source models, plays an important role during marine seismic data processing and interpretation. When an air gun fires, it generates a series of air bubbles. Similar to onshore seismic exploration, the water forms a plastic fluid near the bubble; the farther the air gun is located from the measurement, the more steady and more accurately represented the wavelet will be. In practice, hydrophones should be placed more than 100 m from the air gun; however, traditional seismic cables cannot meet this requirement. On the other hand, vertical cables provide a viable solution to this problem. This study uses a vertical cable to receive wavelets from 38 air guns and data are collected offshore Southeast Qiong, where the water depth is over 1000 m. In this study, the wavelets measured using this technique coincide very well with the simulated wavelets and can therefore represent the real shape of the wavelets. This experiment fills a technology gap in China. 相似文献
994.
995.
Jacques Sauriol 《Hydrogeology Journal》2016,24(1):123-139
An innovative mode of groundwater recharge to a buried esker aquifer is considered. The current conceptual model affords a natural safeguard to underlying aquifers from the overlying muds. A hypothesis of groundwater recharge to a buried esker aquifer via preferential pathways across its overlying muds is tested here by heuristic numerical one-dimensional and two-dimensional modeling simulations. The hypothesis has been tested against two other conventionally accepted scenarios involving: (1) distal esker outcrop areas and (2) remote shallow-bedrock recharge areas. The main evidence comes from documented recharge pressure pulses in the overlying mud aquitard and in the underlying esker hydraulic-head time series for the Vars-Winchester esker aquifer in Eastern Ontario, Canada. These perturbations to the potentiometric surface are believed to be the aquifer response to recharge events. The migration rate of these pressure pulses is directly related to the hydraulic diffusivity of the formation. The measured response time and response amplitude between singular radar precipitation events and well hydrographs constituted the heuristic model calibration targets. The main evidence also includes mud-layering deformation (water escape features) which was observed in seismic surveys of the over-esker muds. These disturbed stratigraphic elements provide a realistic mechanism for migrating water to transit through the muds. The effective hydraulic conductivities of these preferential pathways in the muds were estimated to be between 2?×?10?6 and 7?×?10?6 m/s. The implications of these findings relate to the alleged natural safeguard of these overlying muds. 相似文献
996.
997.
库车坳陷的构造变形及演化与南天山造山带的发育密切相关。库车坳陷新近纪以来的喜马拉雅晚期构造运动最为强烈,形成了天山山前大型冲断带,并造就了现今的构造格局。通过对库车坳陷北部构造样式的识别及各种应力场指示标志的测量、统计和构造解析,对野外获得的应力场指示标志划分了期次,认为喜马拉雅晚期应力场标志为近南北向挤压。结合库车坳陷区域构造要素,如地质体几何形态、边界条件、岩石力学参数等,运用弹性力学有限元数值模拟的方法获得了喜马拉雅晚期库车坳陷的区域应力场。模拟结果表明,库车坳陷喜马拉雅晚期最大主压应力方向为近南北向,与古应力场标志拟合较高,可以为库车坳陷裂缝预测和评价提供依据,对勘探开发有应用价值。 相似文献
998.
J. Bai Z. Shen T. Yan 《International Journal of Environmental Science and Technology》2016,13(7):1723-1730
Pathogenic bacteria are a serious public health concern. Exposure to these microorganisms can result in illnesses or even death. Therefore, it is important to control pathogenic bacteria sources, transport mechanisms and fate. Best management practices proved to be very effective in the control of non-point source pollution. In this study, the soil and water assessment tool (SWAT) was modified and used to simulate the fecal coliform in Chao River of Miyun Reservoir watershed, China. The model was then used to explore the effectiveness of vegetative filter strips (VFS) in reducing fecal coliform abundance throughout the watershed. The water temperature equation within the SWAT was modified to align the model more closely with the characteristics of the study area and generate a more accurate simulation. The DAFSratio (20, 50, 80, 120 and 150) and DFcon (0.25, 0.4, 0.6 and 0.75) parameters were considered for VFS to see their effects on removal efficiency. The results show that calibration and validation results for fecal coliform and flow can be accepted. Different values for DAFSratio and DFcon have great influence on VFS. Increasing values resulted in a decrease in the removal efficiency of VFS. 相似文献
999.
H. T. Boni D. de Oliveira A. A. Ulson de Souza S. M. A. G. Ulson de Souza 《International Journal of Environmental Science and Technology》2016,13(4):1169-1176
Several techniques have been used to reduce the oil and grease content in aqueous streams; one excellent is bioadsorption. With a view to provide a sustainable method for the treatment of oily effluents, the aim of this study was to evaluate sugarcane bagasse as bioadsorbent to reduce the oil and grease content of aqueous effluents. Analysis of the material was performed, and the adsorption experiments were in a batch system, using a synthetic effluent. The results showed that the kinetic equilibrium occurred in 0.08 h of contact. The study of influence of pH of the aqueous phase and the temperature showed that the adsorption capacity was proportional to the pH value, but inversely proportional to the temperature. The adsorption isotherm was obtained at a temperature of 298.15 K using the Langmuir and Langmuir–Freundlich models, and the maximum adsorption capacity obtained was 6.65 kg of hydrocarbon per kg of bioadsorbent. It was verified that sugarcane bagasse represents an excellent material for application in the treatment of oily aqueous effluents, since it is associated with low cost and a high adsorption capacity. The variation in the adsorption capacity observed as a function of pH of the aqueous phase was not found to be significant. This is an important characteristic of the material in terms of its industrial application, verifying the possibility for its implementation in any treatment process, demonstrating the great potential of sugarcane bagasse for use as a sustainable alternative in the reduction in oil and grease in aqueous effluents. 相似文献
1000.
Prediction of ground vibration due to quarry blasting based on gene expression programming: a new model for peak particle velocity prediction 总被引:1,自引:1,他引:0
R Shirani Faradonbeh D Jahed Armaghani M. Z. Abd Majid M. MD Tahir B. Ramesh Murlidhar M. Monjezi H. M. Wong 《International Journal of Environmental Science and Technology》2016,13(6):1453-1464
Blasting is a widely used technique for rock fragmentation in opencast mines and tunneling projects. Ground vibration is one of the most environmental effects produced by blasting operation. Therefore, the proper prediction of blast-induced ground vibrations is essential to identify safety area of blasting. This paper presents a predictive model based on gene expression programming (GEP) for estimating ground vibration produced by blasting operations conducted in a granite quarry, Malaysia. To achieve this aim, a total number of 102 blasting operations were investigated and relevant blasting parameters were measured. Furthermore, the most influential parameters on ground vibration, i.e., burden-to-spacing ratio, hole depth, stemming, powder factor, maximum charge per delay, and the distance from the blast face were considered and utilized to construct the GEP model. In order to show the capability of GEP model in estimating ground vibration, nonlinear multiple regression (NLMR) technique was also performed using the same datasets. The results demonstrated that the proposed model is able to predict blast-induced ground vibration more accurately than other developed technique. Coefficient of determination values of 0.914 and 0.874 for training and testing datasets of GEP model, respectively show superiority of this model in predicting ground vibration, while these values were obtained as 0.829 and 0.790 for NLMR model. 相似文献