全文获取类型
收费全文 | 833篇 |
免费 | 18篇 |
国内免费 | 12篇 |
专业分类
测绘学 | 18篇 |
大气科学 | 76篇 |
地球物理 | 179篇 |
地质学 | 295篇 |
海洋学 | 68篇 |
天文学 | 134篇 |
综合类 | 2篇 |
自然地理 | 91篇 |
出版年
2021年 | 5篇 |
2020年 | 13篇 |
2019年 | 8篇 |
2018年 | 7篇 |
2017年 | 21篇 |
2016年 | 22篇 |
2015年 | 13篇 |
2014年 | 17篇 |
2013年 | 41篇 |
2012年 | 23篇 |
2011年 | 37篇 |
2010年 | 32篇 |
2009年 | 36篇 |
2008年 | 34篇 |
2007年 | 34篇 |
2006年 | 27篇 |
2005年 | 18篇 |
2004年 | 21篇 |
2003年 | 25篇 |
2002年 | 27篇 |
2001年 | 17篇 |
2000年 | 10篇 |
1999年 | 5篇 |
1998年 | 12篇 |
1997年 | 12篇 |
1996年 | 14篇 |
1995年 | 15篇 |
1994年 | 11篇 |
1993年 | 18篇 |
1992年 | 11篇 |
1991年 | 8篇 |
1990年 | 9篇 |
1989年 | 14篇 |
1988年 | 10篇 |
1987年 | 18篇 |
1986年 | 11篇 |
1985年 | 15篇 |
1984年 | 14篇 |
1983年 | 18篇 |
1982年 | 25篇 |
1981年 | 17篇 |
1980年 | 14篇 |
1979年 | 18篇 |
1978年 | 16篇 |
1977年 | 9篇 |
1976年 | 9篇 |
1975年 | 12篇 |
1974年 | 8篇 |
1973年 | 5篇 |
1972年 | 6篇 |
排序方式: 共有863条查询结果,搜索用时 234 毫秒
31.
32.
33.
34.
35.
During the last 30 years, the methodology for assessment of undiscovered conventional oil and gas resources used by the Geological
Survey has undergone considerable change. This evolution has been based on five major principles. First, the U.S. Geological
Survey has responsibility for a wide range of U.S. and world assessments and requires a robust methodology suitable for immaturely
explored as well as maturely explored areas. Second, the assessments should be based on as comprehensive a set of geological
and exploration history data as possible. Third, the perils of methods that solely use statistical methods without geological
analysis are recognized. Fourth, the methodology and course of the assessment should be documented as transparently as possible,
within the limits imposed by the inevitable use of subjective judgement. Fifth, the multiple uses of the assessments require
a continuing effort to provide the documentation in such ways as to increase utility to the many types of users.
Undiscovered conventional oil and gas resources are those recoverable volumes in undiscovered, discrete, conventional structural
or stratigraphic traps. The USGS 2000 methodology for these resources is based on a framework of assessing numbers and sizes
of undiscovered oil and gas accumulations and the associated risks. The input is standardized on a form termed the Seventh
Approximation Data Form for Conventional Assessment Units. Volumes of resource are then calculated using a Monte Carlo program
named Emc2, but an alternative analytic (non-Monte Carlo) program named ASSESS also can be used.
The resource assessment methodology continues to change. Accumulation-size distributions are being examined to determine how
sensitive the results are to size-distribution assumptions. The resource assessment output is changing to provide better applicability
for economic analysis. The separate methodology for assessing continuous (unconventional) resources also has been evolving.
Further studies of the relationship between geologic models of conventional and continuous resources will likely impact the
respective resource assessment methodologies. 相似文献
36.
A granitic rock avalanche, one of the largest Quaternary landslides in Arizona outside the Grand Canyon with a volume of approximately 5.25 M m3 and a width a little under 0.5 km, ran 1 km from the eastern McDowell Mountains. With lateral levees and pressure ridges, the rock avalanche deposit displays many features found on classic sturzstroms. Failure occurred along a major joint plane paralleling the slope with a dip of 44°, when a major base level lowering event in the Salt River system would have undermined the base of the failed slope, and probably during a period of more moisture than normally available in the present-day arid climate. Failure at the subsurface weathering front highlights the importance of the dramatic permeability change between grussified regolith and relatively fresh bedrock. Rock varnish microlaminations (VMLs) dating, in concert with other geomorphic evidence, suggests that the rock avalanche deposit is slightly older than 500 ka. The rock vanish results also have important implications for sampling strategies designed to use cosmogenic nuclide to date Quaternary landslide deposits. Discovery of a large landslide in close proximity to the extending urban fringe of metropolitan Phoenix argues for a more careful analysis of landslide hazards in the region, especially where rapid development excavates bedrock at the base of steep mountain slopes and where the subsurface weathering front is near the surface. 相似文献
37.
38.
The South China Sea continental margin in the Qiongdongnan Basin (QDNB) area has incrementally prograded since 10.5 Ma generating a margin sediment prism more than 4km-thick and 150–200 km wide above the well-dated T40 stratigraphic surface. Core and well log data, as well as clinoform morphology and growth patterns along 28 2D seismic reflection lines, illustrate the evolving architecture and margin morphology; through five main seismic-stratigraphic surfaces (T40, T30, T27, T20 and T0) frame 15 clinothems in the southwest that reduce over some 200 km to 8 clinoforms in the northeast. The overall margin geometry shows a remarkable change from sigmoidal, strongly progradational and aggradational in the west to weakly progradational in the east. Vertical sediment accumulation rate increased significantly across the entire margin after 2.4 Ma, with a marked increase in mud content in the succession. Furthermore, an estimate of sediment flux across successive clinoforms on each of the three selected seismic cross sections indicate an overall decrease in sediment discharge west to east, away from the Red River depocenter, as well as a decrease in the percentage of total discharge crossing the shelf break in this same direction. The QDNB Late Cenozoic continental margin growth, with its overall increased sediment flux, responded to the climate-induced, gradual cooling and falling global sea level during this icehouse period. 相似文献
39.
40.
Modelling spatial variations in dissolved oxygen in fine‐grained streams under uncertainty 下载免费PDF全文
This paper presents a novel triple‐layer model, called VART DO‐3L, for simulation of spatial variations in dissolved oxygen (DO) in fine‐grained streams, characterized by a fluid mud (fluff or flocculent) layer (an advection‐dominated storage zone) as the interface between overlying stream water and relatively consolidated streambed sediment (a diffusion‐dominated storage zone). A global sensitivity analysis is conducted to investigate the sensitivity of VART DO‐3L model input parameters. Results of the sensitivity analysis indicate that the most sensitive parameter is the relative size of the advection‐dominated storage zones (As/A), followed by a lumped reaction term (R) for the flocculent layer, biological reaction rate (μo) in diffusive layer and biochemical oxygen demand concentration (L) in water column. In order to address uncertainty in model input parameters, Monte Carlo simulations are performed to sample parameter values and to produce various parameter combinations or cases. The VART DO‐3L model is applied to the Lower Amite River in Louisiana, USA, to simulate vertical and longitudinal variations in DO under the cases. In terms of longitudinal variation, the DO level decreases from 7.9 mg l at the Denham Springs station to about 2.89 mg l?1 at the Port Vincent station. In terms of vertical variation, the DO level drops rapidly from the overlying water column to the advection‐dominated storage zone and further to the diffusive layer. The DO level (CF) in the advective layer (flocculent layer) can reach as high as 40% of DO concentration (C) in the water column. The VART DO‐3L model may be applied to similar rivers for simulation of spatial variations in DO level. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献