全文获取类型
收费全文 | 1695篇 |
免费 | 115篇 |
国内免费 | 84篇 |
专业分类
测绘学 | 90篇 |
大气科学 | 100篇 |
地球物理 | 454篇 |
地质学 | 883篇 |
海洋学 | 81篇 |
天文学 | 130篇 |
综合类 | 29篇 |
自然地理 | 127篇 |
出版年
2024年 | 5篇 |
2023年 | 13篇 |
2022年 | 56篇 |
2021年 | 64篇 |
2020年 | 67篇 |
2019年 | 78篇 |
2018年 | 150篇 |
2017年 | 131篇 |
2016年 | 176篇 |
2015年 | 81篇 |
2014年 | 182篇 |
2013年 | 172篇 |
2012年 | 92篇 |
2011年 | 103篇 |
2010年 | 61篇 |
2009年 | 73篇 |
2008年 | 57篇 |
2007年 | 39篇 |
2006年 | 54篇 |
2005年 | 30篇 |
2004年 | 23篇 |
2003年 | 25篇 |
2002年 | 23篇 |
2001年 | 18篇 |
2000年 | 15篇 |
1999年 | 6篇 |
1998年 | 13篇 |
1997年 | 7篇 |
1996年 | 3篇 |
1995年 | 5篇 |
1994年 | 7篇 |
1993年 | 7篇 |
1992年 | 7篇 |
1991年 | 2篇 |
1990年 | 4篇 |
1989年 | 2篇 |
1988年 | 6篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1978年 | 2篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1972年 | 2篇 |
1971年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有1894条查询结果,搜索用时 11 毫秒
1.
Time–space fractional governing equations of transient groundwater flow in confined aquifers: Numerical investigation
下载免费PDF全文
![点击此处可从《水文研究》网站下载免费的PDF全文](/ch/ext_images/free.gif)
In order to model non‐Fickian transport behaviour in groundwater aquifers, various forms of the time–space fractional advection–dispersion equation have been developed and used by several researchers in the last decade. The solute transport in groundwater aquifers in fractional time–space takes place by means of an underlying groundwater flow field. However, the governing equations for such groundwater flow in fractional time–space are yet to be developed in a comprehensive framework. In this study, a finite difference numerical scheme based on Caputo fractional derivative is proposed to investigate the properties of a newly developed time–space fractional governing equations of transient groundwater flow in confined aquifers in terms of the time–space fractional mass conservation equation and the time–space fractional water flux equation. Here, we apply these time–space fractional governing equations numerically to transient groundwater flow in a confined aquifer for different boundary conditions to explore their behaviour in modelling groundwater flow in fractional time–space. The numerical results demonstrate that the proposed time–space fractional governing equation for groundwater flow in confined aquifers may provide a new perspective on modelling groundwater flow and on interpreting the dynamics of groundwater level fluctuations. Additionally, the numerical results may imply that the newly derived fractional groundwater governing equation may help explain the observed heavy‐tailed solute transport behaviour in groundwater flow by incorporating nonlocal or long‐range dependence of the underlying groundwater flow field. 相似文献
2.
A constitutive model that captures the material behavior under a wide range of loading conditions is essential for simulating complex boundary value problems. In recent years, some attempts have been made to develop constitutive models for finite element analysis using self‐learning simulation (SelfSim). Self‐learning simulation is an inverse analysis technique that extracts material behavior from some boundary measurements (eg, load and displacement). In the heart of the self‐learning framework is a neural network which is used to train and develop a constitutive model that represents the material behavior. It is generally known that neural networks suffer from a number of drawbacks. This paper utilizes evolutionary polynomial regression (EPR) in the framework of SelfSim within an automation process which is coded in Matlab environment. EPR is a hybrid data mining technique that uses a combination of a genetic algorithm and the least square method to search for mathematical equations to represent the behavior of a system. Two strategies of material modeling have been considered in the SelfSim‐based finite element analysis. These include a total stress‐strain strategy applied to analysis of a truss structure using synthetic measurement data and an incremental stress‐strain strategy applied to simulation of triaxial tests using experimental data. The results show that effective and accurate constitutive models can be developed from the proposed EPR‐based self‐learning finite element method. The EPR‐based self‐learning FEM can provide accurate predictions to engineering problems. The main advantages of using EPR over neural network are highlighted. 相似文献
3.
Karst aquifers represent important water resources in many parts of the world. Unfortunately, karst aquifers are characterised
by high contamination risks. This paper presents a travel time based method for the estimation of karst groundwater vulnerability.
It considers (1) physics-based lateral flow within the uppermost weathered zone (epikarst) in a limestone-dominated region
and (2) high velocities of vertical infiltration at discrete infiltration points (e.g. sinkholes) or lines (e.g. dry valleys,
faults). Consequently, the Transit Time Method honours the actual flow path within the unsaturated zone of a karst aquifer
system. A test site in Northern Jordan was chosen for the demonstration of the assessment technique, i.e. the catchment area
of the Qunayyah Spring north of the capital Amman. The results demonstrate that zones of highest vulnerability lie within
valleys and nearby main fault zones. It also reveals that regions, categorised as protected areas by other methods due to
thick unsaturated zones, contribute to a major degree to the total risk. 相似文献
4.
A fairing method is presented for giving a good representation of ship model resistance test data, which in general tend to exhibit a scattered trend and need to be smoothed appropriately. One component of the test data originates from the frictional resistance and can be formulated smoothly. The second component is the wave resistance which exhibits a specific harmonic character with peaks and troughs. In this method, this component is modelled using suitable harmonic cosine terms for obtaining the best representation of the corresponding resistance data. The performance of the method is then tested on two example cases, using the characteristic parameters of the sample data as input during the smoothing process. 相似文献
5.
A.E. Dodonov V.G. Trifonov T.P. Ivanova V.Yu. Kuznetsov F.E. Maksimov D.M. Bachmanov T.A. Sadchikova A.N. Simakova H. Minini A.-M. Al-Kafri O. Ali 《Quaternary International》2008,190(1):158
In order to provide new data on the neotectonics and geodynamic properties of western Syria, studies of marine terraces have been carried out. The most attention was paid to the lower terraces in the range of 3–5 to 30–35 m above sea level, because they have more complete distributions along the shore. The lower terraces were examined along the coastal area from Tartus to Latakia, and along the carbonate cliff on Arwad Island. Seven 230Th/U dates for these terraces are in the range of 85–130 ka, suggesting the age interval of the last interglacial (MIS 5). New dates on the lower terraces provide a basis for stratigraphical and geomorphological interpretation as well as neotectonic reconstruction. According to the geomorphological data and lithological composition of those terraces, two main uplifted blocks can be established. One coincides with the Latakia block, and another corresponds to the western margin of the Banias volcanic plateau. These blocks are divided by a subsided structure corresponding to the Nahr el Kebir graben. The amplitude of neotectonic uplifting in the Latakia and Banias blocks reaches 15–20 m for the Late Pleistocene. 相似文献
6.
印度板块和亚洲大陆在何时何地碰撞 总被引:1,自引:0,他引:1
印度板块和亚洲大陆的初始碰撞时间是所有相关的喜马拉雅-西藏造山体系演化模式的主控条件,并严重影响到对众多与青藏高原隆升和东亚大陆挤出相关的地质过程速率的解释,以及对新生代全球气候变化的理解。尽管印度板块和亚洲大陆汇聚的速率在55Ma突然减缓被广泛地认为是初始碰撞的标志,但这次碰撞所造成的主要构造效应直到20多个百万年以后才显现出来。对印度板块和亚洲大陆相对位置的重新估算,表明它们在55Ma时并没有达到可以彼此发生碰撞的距离。基于来自西藏新的野外证据和对已有数据的重新评估,认为初始碰撞发生在始新世—渐新世之交(约34Ma),并对55Ma时发生的地质事件提出了另一种解释 相似文献
7.
Ali Kahriman Umit Ozer Mehmet Aksoy Abdulkadir Karadogan Gungor Tuncer 《Environmental Geology》2006,50(7):1015-1023
This paper presents the results of ground vibration measurements carried out in Hisarcik Boron open pit mine located on the west side of central Anatolia near Kütahya province in Turkey. Within the scope of this study to predict peak particle velocity (PPV) level for this site, ground vibration components were measured for 304 shots during bench blasting. In blasting operations, ANFO (blasting agent), gelatin dynamite (priming), and delay electric detonators (firing) were used as explosives. Parameters of scaled distance (charge quantity per delay and the distance between the source and the station) were recorded carefully and the ground vibration components were measured for all blast events using two different types of vibration monitors (one White Mini-Seis and one Instantel Minimate Plus Model). The absolute distances between shot points and monitor stations were determined using GPS. The equation of square root scaled distance extensively used in the literature was taken into consideration for the prediction of PPV. Then, the data pairs of scaled distance and particle velocity obtained from the 565 event records were analyzed statistically. At the end of statistical evaluation of the data pairs, an empirical relation which gives 50% prediction line with a reasonable correlation coefficient was established between PPV and scaled distance. 相似文献
8.
EXPERIMENTAL AND FIELD STUDY ON MINING-PIT MIGRATION 总被引:5,自引:0,他引:5
1 INTRODUCTION Because of the large quantity of sand and gravel in their beds, rivers have always been considered as a major source of sand and gravel for civil works. Acceptable quality, ease of extraction and economy are some of the reasons could be mentioned. Unfortunately, specific laws and regulations regarding the safe in-stream mining have not been provided for users and officials. What should be taken into account are the effects of over-mining of sand and gravel, which can cause … 相似文献
9.
Contradictory results are reported for the behaviour of quaternary ammonium compounds (QACs) in sewage treatment plants (STPs). QACs may sorb onto activated sludge. Only little information is available with respect to effects of QACs against bacteria in STPs. Only 5 to 15 % of bacteria present in sewage sludge can be detected by means of culture dependent microbiological methods. The shift of the bacterial populations due to effects of test compounds have not been studied up to now with culture independent methods. The microbial populations shift was studied in situ using culture independent chemotaxonomy profiling ubiquinones and polyamines. Additionally, toxic effects of QACs against bacteria present in the test vessels of the Zahn‐Wellens test (OECD 302 B) were assessed with a toxicity control in the test. The ubiquinone profiles representing changes in Gram‐negative populations mainly showed that the activated sludge was affected only in test vessels containing benzalkonium chloride. According to chemotaxonomy Acinetobacter or/and some members of Pseudomonas spp. have been selected by benzalkonium chloride after some adaptation period (8 to 12 days). 相似文献
10.
本文研究的目的是确定软体动物对于铀在海洋生态系统中的分布所起的地球化学作用。对取自里海、日本海、马尔马拉海、爱琴海、黑海、地中海、波罗的海和印度洋的近代软体动物贝壳进行了生物地球化学研究,这些贝壳的物理、化学、地理和地球化学特征各不相同。本次研究中,分析了9个腹足类和54个双壳类不同种的贝壳以记录铀在不同季节层中的变化。季节层是由软体动物在整个生命过程中季节性的碳酸盐—有机相生成的。研究中使用的贝壳主要分3层:上(外)棱柱层,中棱柱层,内(珠母)层。此外,条件许可时,将贝壳性的头部、中部和下部分别进行分析,并且还计算出相对 相似文献