全文获取类型
收费全文 | 661篇 |
免费 | 56篇 |
国内免费 | 36篇 |
专业分类
测绘学 | 25篇 |
大气科学 | 45篇 |
地球物理 | 172篇 |
地质学 | 375篇 |
海洋学 | 30篇 |
天文学 | 30篇 |
综合类 | 31篇 |
自然地理 | 45篇 |
出版年
2023年 | 8篇 |
2022年 | 24篇 |
2021年 | 57篇 |
2020年 | 35篇 |
2019年 | 33篇 |
2018年 | 96篇 |
2017年 | 60篇 |
2016年 | 86篇 |
2015年 | 49篇 |
2014年 | 46篇 |
2013年 | 62篇 |
2012年 | 28篇 |
2011年 | 29篇 |
2010年 | 12篇 |
2009年 | 24篇 |
2008年 | 11篇 |
2007年 | 12篇 |
2006年 | 11篇 |
2005年 | 7篇 |
2004年 | 4篇 |
2003年 | 6篇 |
2002年 | 2篇 |
2001年 | 6篇 |
2000年 | 3篇 |
1999年 | 2篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1996年 | 3篇 |
1995年 | 4篇 |
1993年 | 2篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1989年 | 4篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1981年 | 1篇 |
1976年 | 2篇 |
排序方式: 共有753条查询结果,搜索用时 296 毫秒
81.
Nisar Ahmed Perveiz Khalid Hafiz Muhammad Bilal Shafi Patrick Connolly 《Acta Geophysica》2017,65(5):991-1007
The use of seismic direct hydrocarbon indicators is very common in exploration and reservoir development to minimise exploration risk and to optimise the location of production wells. DHIs can be enhanced using AVO methods to calculate seismic attributes that approximate relative elastic properties. In this study, we analyse the sensitivity to pore fluid changes of a range of elastic properties by combining rock physics studies and statistical techniques and determine which provide the best basis for DHIs. Gassmann fluid substitution is applied to the well log data and various elastic properties are evaluated by measuring the degree of separation that they achieve between gas sands and wet sands. The method has been applied successfully to well log data from proven reservoirs in three different siliciclastic environments of Cambrian, Jurassic, and Cretaceous ages. We have quantified the sensitivity of various elastic properties such as acoustic and extended elastic (EEI) impedances, elastic moduli (K sat and K sat–μ), lambda–mu–rho method (λρ and μρ), P-to-S-wave velocity ratio (V P/V S), and Poisson’s ratio (σ) at fully gas/water saturation scenarios. The results are strongly dependent on the local geological settings and our modeling demonstrates that for Cambrian and Cretaceous reservoirs, K sat–μ, EEI, V P/V S, and σ are more sensitive to pore fluids (gas/water). For the Jurassic reservoir, the sensitivity of all elastic and seismic properties to pore fluid reduces due to high overburden pressure and the resultant low porosity. Fluid indicators are evaluated using two metrics: a fluid indicator coefficient based on a Gaussian model and an overlap coefficient which makes no assumptions about a distribution model. This study will provide a potential way to identify gas sand zones in future exploration. 相似文献
82.
Sediment transport in high‐speed flows over a fixed bed: 2. Particle impacts and abrasion prediction
下载免费PDF全文
![点击此处可从《地球表面变化过程与地形》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Single bed load particle impacts were experimentally investigated in supercritical open channel flow over a fixed planar bed of low relative roughness height simulating high‐gradient non‐alluvial mountain streams as well as hydraulic structures. Particle impact characteristics (impact velocity, impact angle, Stokes number, restitution and dynamic friction coefficients) were determined for a wide range of hydraulic parameters and particle properties. Particle impact velocity scaled with the particle velocity, and the vertical particle impact velocity increased with excess transport stage. Particle impact and rebound angles were low and decreased with transport stage. Analysis of the particle impacts with the bed revealed almost no viscous damping effects with high normal restitution coefficients exceeding unity. The normal and resultant Stokes numbers were high and above critical thresholds for viscous damping. These results are attributed to the coherent turbulent structures near the wall region, i.e. bursting motion with ejection and sweep events responsible for turbulence generation and particle transport. The tangential restitution coefficients were slightly below unity and the dynamic friction coefficients were lower than for alluvial bed data, revealing that only a small amount of horizontal energy was transferred to the bed. The abrasion prediction model formed by Sklar and Dietrich in 2004 was revised based on the new equations on vertical impact velocity and hop length covering various bed configurations. The abrasion coefficient kv was found to be vary around kv ~ 105 for hard materials (tensile strength ft > 1 MPa), one order of magnitude lower than the value assumed so far for Sklar and Dietrich's model. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
83.
Sarfraz Khan Muhammad Waseem Muhammad Asif Khan Waqas Ahmed 《Journal of Seismology》2018,22(4):841-861
A reliable and homogenized earthquake catalogue is essential for seismic hazard assessment in any area. This article describes the compilation and processing of an updated earthquake catalogue for Pakistan. The earthquake catalogue compiled in this study for the region (quadrangle bounded by the geographical limits 40–83° N and 20–40° E) includes 36,563 earthquake events, which are reported as 4.0–8.3 moment magnitude (MW) and span from 25 AD to 2016. Relationships are developed between the moment magnitude and body, and surface wave magnitude scales to unify the catalogue in terms of magnitude MW. The catalogue includes earthquakes from Pakistan and neighbouring countries to minimize the effects of geopolitical boundaries in seismic hazard assessment studies. Earthquakes reported by local and international agencies as well as individual catalogues are included. The proposed catalogue is further used to obtain magnitude of completeness after removal of dependent events by using four different algorithms. Finally, seismicity parameters of the seismic sources are reported, and recommendations are made for seismic hazard assessment studies in Pakistan. 相似文献
84.
Laurentiu Danciu Karin Şeşetyan Mine Demircioglu Levent Gülen Mehdi Zare Roberto Basili Ata Elias Shota Adamia Nino Tsereteli Hilal Yalçın Murat Utkucu Muhammad Asif Khan Mohammad Sayab Khaled Hessami Andrea N. Rovida Massimiliano Stucchi Jean-Pierre Burg Arkady Karakhanian Hektor Babayan Mher Avanesyan Tahir Mammadli Mahmood Al-Qaryouti Doğan Kalafat Otar Varazanashvili Mustafa Erdik Domenico Giardini 《Bulletin of Earthquake Engineering》2018,16(8):3465-3496
85.
Naveed Ahmad Asif Shahzad Qaisar Ali Muhammad Rizwan Akhtar Naeem Khan 《Bulletin of Earthquake Engineering》2018,16(10):4675-4703
Fragility functions are derived for low-rise code compliant & non-compliant special moment resisting frames (SMRFs). Non-compliant SMRFs those built in low strength concrete and lacking confining ties in joint panel zones, commonly found in developing countries. Shake table tests were performed on single-storey and two-storey 1:3 reduced scale representative frames to understand the damage mechanism and develop deformation-based damage scale. The non-compliant SMRF experienced column flexure cracking, longitudinal bar-slip in beam and observed with cover concrete spalling from the joint panels. The code compliant SMRF experienced flexure cracks in beam/column, and experienced joint cracking under extreme shaking. Numerical modeling technique is developed for inelastic modeling of reinforced concrete frame with beam bar-slip and joint damageability using SeismoStruct. Natural accelerograms were used to analyze the considered frames through incremental dynamic analyses in SeismoStruct. A probabilistic based approach was used to derive fragility functions for the considered frames. An example case study is presented for damageability evaluation of structures for earthquakes of various return periods (43, 72, 475, 2475 years). 相似文献
86.
Muhammad Adnan Paul A Longley Alex D Singleton Chris Brunsdon 《Transactions in GIS》2010,14(3):283-297
Geodemographic classifications provide discrete indicators of the social, economic and demographic characteristics of people living within small geographic areas. They have hitherto been regarded as products, which are the final “best” outcome that can be achieved using available data and algorithms. However, reduction in computational cost, increased network bandwidths and increasingly accessible spatial data infrastructures have together created the potential for the creation of classifications in near real time within distributed online environments. Yet paramount to the creation of truly real time geodemographic classifications is the ability for software to process and efficiency cluster large multidimensional spatial databases within a timescale that is consistent with online user interaction. To this end, this article evaluates the computational efficiency of a number of clustering algorithms with a view to creating geodemographic classifications “on the fly” at a range of different geographic scales. 相似文献
87.
SPATIAL VARIATION OF Zn GEOCHEMICAL FRACTIONS IN THE SURFACE SEDIMENTS OF THE STRAITS OF MALACCA: NATURAL OR ANTHROPOGENIC? 总被引:1,自引:1,他引:0
CheeKongYap AhmadIsmail SoonGuanTan 《湿地科学》2005,3(2):94-103
1INTRODUCTION Studiesonheavymetalcontaminationinsedi mentshavealwaysbeenfocusedonthecoastalandin tertidalareas(Sivalingam,etal.,1980;Ismail,1993;Ismail,etal.,1993;IsmailandRosniza,1997;Yap,etal.,2002a,2002b).Theheavymet alconcentrationsfoundinthesediments… 相似文献
88.
John Rivers Max Engel Robert Dalrymple Ruqaiya Yousif Christian J. Strohmenger Ismail Al-Shaikh 《Sedimentology》2020,67(1):534-558
Barrier islands are important landforms in many coastal systems around the globe. Studies of modern barrier island systems are mostly limited to those of siliciclastic realms, where the islands are recognized as mobile features that form on transgressive coastlines and migrate landward as sea-level rises. Barrier islands of the ‘Great Pearl Bank’ along the United Arab Emirates coast are the best-known carbonate examples. These Holocene islands, however, are interpreted to be anchored by older deposits and immobile. The mid-Holocene to late-Holocene depositional system at Al Ruwais, northern Qatar, provides an example of a mobile carbonate barrier island system, perhaps more similar to siliciclastic equivalents. Sedimentological and petrographic analyses, as well as 14C-dating of shells and biogenic remains from vibracored sediments and surface deposits, show that after 7000 years ago a barrier system with a narrow back-barrier lagoon formed along what is now an exposed coastal zone, while, contemporaneously, a laterally-extensive coral reef was forming immediately offshore. After 1400 years ago the barrier system was forced to step ca 3 km seaward in response to a sea-level fall of less than 2 m, where it re-established itself directly on the mid-Holocene reef. Since that time, the barrier has retreated landward as much as 1000 m to its current position, exposing previously-deposited back-barrier lagoonal sediment at the open-coast shoreline. In modern neritic warm-water carbonate settings mobile barrier island systems are rare. Their construction and migration may be inhibited by reef formation, early cementation, and the relative inefficiency of sourcing beach sediments from open carbonate shelves. Carbonate barrier island systems likely formed more commonly during geological periods when ramps and unrimmed shelves predominated and in calcite seas, when meteoric cementation was minimized as a result of initial calcitic allochem mineralogy. As with their siliciclastic analogues, however, recognition of the influence of these transient landforms in the rock record is challenging. 相似文献
89.
Natural Hazards - The Murree region in Pakistan is frequently affected by landslides and causes damages to lands, houses, life lines and even loss of livestock and human life. We have attempted in... 相似文献
90.
Natural Hazards - Highways in arid regions are highly exposed to damage by floods. This requires intensive analysis of flood quantity, time and direction that can be used to design the suitable... 相似文献