全文获取类型
收费全文 | 6486篇 |
免费 | 235篇 |
国内免费 | 65篇 |
专业分类
测绘学 | 130篇 |
大气科学 | 522篇 |
地球物理 | 1675篇 |
地质学 | 2152篇 |
海洋学 | 590篇 |
天文学 | 1057篇 |
综合类 | 19篇 |
自然地理 | 641篇 |
出版年
2022年 | 28篇 |
2021年 | 95篇 |
2020年 | 86篇 |
2019年 | 97篇 |
2018年 | 145篇 |
2017年 | 120篇 |
2016年 | 199篇 |
2015年 | 172篇 |
2014年 | 179篇 |
2013年 | 325篇 |
2012年 | 229篇 |
2011年 | 321篇 |
2010年 | 234篇 |
2009年 | 340篇 |
2008年 | 296篇 |
2007年 | 263篇 |
2006年 | 272篇 |
2005年 | 244篇 |
2004年 | 214篇 |
2003年 | 190篇 |
2002年 | 206篇 |
2001年 | 109篇 |
2000年 | 130篇 |
1999年 | 115篇 |
1998年 | 113篇 |
1997年 | 80篇 |
1996年 | 90篇 |
1995年 | 107篇 |
1994年 | 102篇 |
1993年 | 80篇 |
1992年 | 87篇 |
1991年 | 65篇 |
1990年 | 88篇 |
1989年 | 70篇 |
1988年 | 72篇 |
1987年 | 72篇 |
1986年 | 69篇 |
1985年 | 84篇 |
1984年 | 106篇 |
1983年 | 87篇 |
1982年 | 81篇 |
1981年 | 72篇 |
1980年 | 76篇 |
1979年 | 67篇 |
1978年 | 70篇 |
1977年 | 50篇 |
1976年 | 53篇 |
1975年 | 55篇 |
1974年 | 45篇 |
1973年 | 58篇 |
排序方式: 共有6786条查询结果,搜索用时 0 毫秒
561.
B. Scott P. G. Ranjtih S. K. Choi Manoj Khandelwal 《Environmental Earth Sciences》2010,60(5):1007-1019
About one quarter of the coal produced in Australia is by underground mining methods. The most commonly used underground coal
mining methods in Australia are longwall, and room and pillar. This paper provides a detailed review of the two methods, including
their advantages and disadvantages, the major geotechnical and operational issues, and the factors that need to be considered
regarding their choice, including the varying geological and geotechnical conditions suited to a particular method. Factors
and issues such as capital cost, productivity, recovery, versatility and mine safety associated with the two methods are discussed
and compared. The major advantages of the longwall mining method include its suitability for mining at greater depth, higher
recovery, and higher production rate compared to room and pillar. The main disadvantages of the room and pillar method are
the higher risks of roof and pillar collapse, higher capital costs incurred as well as lower recovery rate. 相似文献
562.
563.
The alluvial architecture of a suspended sediment dominated meandering river: the Río Bermejo,Argentina 下载免费PDF全文
Gregory H. Sambrook Smith James L. Best Jessica Z. Leroy Oscar Orfeo 《Sedimentology》2016,63(5):1187-1208
The alluvial architecture of fine‐grained (silt‐bed) meandering rivers remains poorly understood in comparison to the extensive study given to sand‐bed and gravel‐bed channels. This paucity of knowledge stems, in part, from the difficulty of studying such modern rivers and deriving analogue information from which to inform facies models for ancient sediments. This paper employs a new technique, the parametric echosounder, to quantify the subsurface structure of the Río Bermejo, Argentina, which is a predominantly silt‐bed river with a large suspended sediment load. These results show that the parametric echosounder can provide high‐resolution (decimetre) subsurface imaging from fine‐grained rivers that is equivalent to the more commonly used ground‐penetrating radar that has been shown to work well in coarser‐grained rivers. Analysis of the data reveals that the alluvial architecture of the Río Bermejo is characterized by large‐scale inclined heterolithic stratification generated by point‐bar evolution, and associated large‐scale scour surfaces that result from channel migration. The small‐scale and medium‐scale structure of the sedimentary architecture is generated by vertical accretion deposits, bed sets associated with small bars, dunes and climbing ripples and the cut and fill from small cross‐bar channels. This style of alluvial architecture is very different from other modern fine‐grained rivers reported in the literature that emphasize the presence of oblique accretion. The Río Bermejo differs from these other rivers because it is much more active, with very high rates of bank erosion and channel migration. Modern examples of this type of highly active fine‐grained river have been reported rarely in the literature, although ancient examples are more prevalent and show similarities with the alluvial architecture of the Río Bermejo, which thus represents a useful analogue for their identification and interpretation. Although the full spectrum of the sedimentology of fine‐grained rivers has yet to be revealed, meandering rivers dominated by lateral or oblique accretion probably represent end members of such channels, with the specific style of sedimentation being controlled by grain size and sediment load characteristics. 相似文献
564.
James P. Adams Robert Kirst Lance E. Kearns Mark P. S. Krekeler 《Environmental Geology》2009,58(6):1269-1280
The Chesapeake Bay is greatly impacted by numerous pollutants including heavy metals and understanding the controls on the
distribution of heavy metals in the watershed is critical to mitigation and remediation efforts in controlling this type of
pollution. Clasts from a stormwater catchment basin draining a subdivision near George Mason University, Fairfax VA (38°50.090°N
78°19.204°W) were investigated using X-ray diffraction (XRD), Scanning electron microcopy (SEM) and energy dispersive spectroscopy
(EDS) to determine the nature of Mn-oxide coatings and relationship to bound heavy metals. Mn-oxides are poorly crystalline
and occur as subhedral to anhedral platy particles and more rarely as euhedral plates. Micronodules are a commonly observed
texture. Chemical compositions of coatings are variable with average major constituent concentrations being Mn (33.38 wt%),
Fe (11.88 wt%), Si (7.33 wt%), Al (5.03 wt%), and Ba (0.90 wt%). Heavy metals are found in the coatings with Zn being most
prevalent, occurring in approximately 58% of analyses with an average concentration of (0.66 wt%). Minor amounts of Co, Ni,
Pb, and Cl are observed. Heavy metals and Cl are interpreted as being derived from road pollution. Mn-oxides can serve as
a sequestration mechanism for pollution but may also release heavy metals. Field and laboratory observations indicate Mn-oxides
occurring on the surface of the clasts can be mechanically mobilized. This is a mechanism for transporting heavy metals into
the Chesapeake Bay watershed. Deicing agents may serve as a mechanism to release heavy metals through cation exchange and
increased ionic strength. This is the first detailed mineralogical investigation of Mn-oxides and the roles they may play
in pollution in the Chesapeake Bay. 相似文献
565.
James C. White David Beamish 《Proceedings of the Geologists' Association. Geologists' Association》2014
A recent airborne geophysical survey has provided high resolution estimates of the electrical resistivity of the near-surface and deeper (bedrock) formations found across Anglesey and a portion of the coastal area of North Wales. This single small survey provides new geophysical information on both the complex configuration of the Neoproterozoic and Cambrian bedrock units and the shallow near-surface geology and glacial features. 相似文献
566.
Analysis of locational patterns and change within an industry may be assisted by division of industry participants into groups based on corporate and business strategies. These strategies reflect the assets of the companies and the characteristics of the industry. They affect the locational needs, behavior and impact of the companies. Hypothesized effects of strategy are explored in a brief study of U.S. semiconductor manufacturers. Of the possible strategic considerations, corporate diversification and integration, and business scale and cost/price position seem to be the most relevant for this industry. 相似文献
567.
568.
David Nettle Galen P. Halverson Grant M. Cox Alan S. Collins Mark Schmitz James Gehling Peter R. Johnson Khalid Kadi 《地学学报》2014,26(2):120-129
The Ediacaran Jibalah Group comprises volcano‐sedimentary successions that filled small fault‐bound basins along the NW–SE‐trending Najd fault system in the eastern Arabian‐Nubian Shield. Like several other Jibalah basins, the Antaq basin contains exquisitely preserved sedimentary structures and felsic tuffs, and hence is an excellent candidate for calibrating late Ediacaran Earth history. Shallow‐marine strata from the upper Jibalah Group (Muraykhah Formation) contain a diversity of load structures and intimately related textured organic (microbial) surfaces, along with a fragment of a structure closely resembling an Ediacaran frond fossil and a possible specimen of Aspidella. Interspersed carbonate beds through the Muraykhah Formation record a positive δ13C shift from ?6 to 0‰. U‐Pb zircon geochronology indicates a maximum depositional age of ~570 Ma for the upper Jibalah Group, consistent with previous age estimates. Although this age overlaps with that of the upper Huqf Supergroup in nearby Oman, these sequences were deposited in contrasting tectonic settings on opposite sides of the final suture of the East African Orogen. 相似文献
569.
The infrared spectrum of CaAl2Si2O7 · H2O-lawsonite, has been characterized to pressures of 20 GPa at 300 K. Our results constrain the response to compression of
the silicate tetrahedra, hydroxyl units, and water molecules in this material. The asymmetric and symmetric stretching and
bending vibrations of the Si2O7 groups (at zero pressure frequencies between 600 and 1000 cm−1) increase in frequency with pressure at rates between 3.6 and 5.9 cm−1/GPa. All silicate modes appear to shift continuously with pressure to 20 GPa, although the lowest frequency stretching vibration
becomes unresolvable above 18 GPa, and a splitting of the main bending vibration is observed near this pressure. The O-H stretches
of the hydroxyl units exhibit a discontinuity in their mode shifts at ∼8–9 GPa, which we interpret to be produced by a pressure-induced
change in hydrogen bonding. The stretching and bending vibrations of the water molecule are relatively unaffected by compression
to 20 GPa, thus demonstrating that the structural cavities in which water molecules reside are relatively rigid. Significant
changes in the amplitude of the O-H stretches of the hydroxyl and water units are observed at this pressure as well; nevertheless,
our results demonstrate that the dominant structural units in lawsonite persist metastably at 300 K with only modest structural
modifications well beyond the known stability field of this phase.
Received: 10 July 1998 / Revised, accepted: 23 October 1998 相似文献
570.
Gimli beach in Manitoba is one of the lowest elevation beaches in the southern Lake Agassiz basin, and is a distinct ridge composed of bedded sand and gravel that rises above the lake plain and extends for more than 40 km. Ten new optically stimulated luminescence (OSL) ages from Gimli beach yield ages mostly ranging from 9.7 ± 0.7 to 10.5 ± 0.8 ka (average 10.3 ± 0.5 ka), which is older by 0.6 to >1.0 ka than age estimates of previous researchers. Two of our new OSL ages are notably older than the others, dating to ~11.3 ± 0.8 and 13.9 ± 1.0 ka, which we attribute to poorly bleached sands. We ascribe an age of about 10 ka to Gimli beach, which is several centuries before overflow from Lake Agassiz and its vast drainage basin shifted from the western Great Lakes to glacial Lake Ojibway and the St. Lawrence Valley. 相似文献