全文获取类型
收费全文 | 743篇 |
免费 | 91篇 |
国内免费 | 68篇 |
专业分类
测绘学 | 60篇 |
大气科学 | 36篇 |
地球物理 | 277篇 |
地质学 | 356篇 |
海洋学 | 56篇 |
天文学 | 4篇 |
综合类 | 34篇 |
自然地理 | 79篇 |
出版年
2024年 | 1篇 |
2023年 | 8篇 |
2022年 | 29篇 |
2021年 | 31篇 |
2020年 | 31篇 |
2019年 | 36篇 |
2018年 | 15篇 |
2017年 | 32篇 |
2016年 | 30篇 |
2015年 | 26篇 |
2014年 | 51篇 |
2013年 | 37篇 |
2012年 | 28篇 |
2011年 | 44篇 |
2010年 | 31篇 |
2009年 | 46篇 |
2008年 | 53篇 |
2007年 | 57篇 |
2006年 | 44篇 |
2005年 | 50篇 |
2004年 | 36篇 |
2003年 | 23篇 |
2002年 | 28篇 |
2001年 | 22篇 |
2000年 | 15篇 |
1999年 | 12篇 |
1998年 | 14篇 |
1997年 | 9篇 |
1996年 | 9篇 |
1995年 | 6篇 |
1994年 | 8篇 |
1993年 | 2篇 |
1992年 | 5篇 |
1991年 | 7篇 |
1990年 | 5篇 |
1989年 | 1篇 |
1988年 | 5篇 |
1987年 | 2篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1976年 | 2篇 |
排序方式: 共有902条查询结果,搜索用时 78 毫秒
71.
Roughness control on hydraulic conductivity in fractured rocks 总被引:1,自引:0,他引:1
The influence of joint roughness on the typologies of fluid flow inside fractures is well known and, thanks to experiences
in the field of hydraulics, it has been studied from both a physical and mathematical point of view. Nevertheless, the formulations
adopted by traditional hydraulic models are hardly applicable in the geological field, because of the difficulty encountered
in the roughness parameter estimation. Normally this parameter can be estimated using the joint roughness coefficient (JRC),
which considers both the asperity height and its regularity and directional trend. The main advantage in using the JRC arises
from the fact that it can easily be obtained from geological-technical surveys and from comparison with the standard Barton
profiles. Some relationships have been built up that allow for the estimation of the hydraulic conductivity tensor (an essential
parameter for understanding water flow in fractured rock masses), not only as a function of traditional parameters like aperture,
spacing, dip and dip direction, etc., but also of joint roughness, precisely expressed in terms of the roughness coefficient.
These relationships have been studied initially from a theoretical point of view and then practically, through laboratory
investigations.
Resumen Se conoce muy bien la influencia de la rugosidad de las grietas en las tipologías del flujo de fluidos a lo interior de las fracturas y gracias a las experiencias en el campo de hidráulica ha sido posible estudiarla desde puntos de vista matemáticos y físicos. Sin embargo, las formulaciones adoptadas por los modelos hidráulicos tradicionales tienen poca aplicabilidad en el campo geológico debido a la dificultad relacionada con la estimación del parámetro de rugosidad. Normalmente este parámetro puede estimarse usando el coeficiente de rugosidad de grieta (JRC) el cual considera tanto la altura de la aspereza como su regularidad y tendencia direccional. La principal ventaja de utilizar el JRC se deriva del hecho que puede obtenerse fácilmente de levantamientos técnico-geológicos y de la comparación con los perfiles Standard Barton. Se han construido algunas relaciones que permiten la estimación del tensor de conductividad hidráulica (un parámetro esencial para el entendimiento del flujo de agua en masas de roca fracturadas), no solo en función de parámetros tradicionales como apertura, espaciado, buzamiento y dirección de buzamiento, etc., sino también en función de la rugosidad de la grieta estimada con precisión en términos del coeficiente de rugosidad. Estas relaciones se han estudiado inicialmente desde un punto de vista teórico y luego de modo práctico a través de investigaciones de laboratorio.
Résumé L’influence de la rugosité des joints sur les types d’écoulement de fluide dans les fractures est bien connue et a été étudiée aussi bien du point de vue physique que mathématique grace à des expériences menées dans le domaine de l’hydraulique. Cependant les formulations adoptées dans les modèles hydrauliques traditionnels sont difficilement applicables dans le domaine de la géologie à cause de la difficulté rencontrée pour estimer la rugosité. Ce paramètre peut normalement être apprécié grace au coefficient de rugosité du joint (JRC), lequel prend en compte à la fois la hauteur de l’aspérité ainsi que sa régularité et sa direction. Le principal avantage dans l’utilisation du JRC réside dans le fait qu’il peut facilement être obtenu à partir d’études techniques-géologiques et par comparaison avec la classification de Barton. Des relations qui permettent une estimation du tenseur de conductivité hydraulique (un paramètre essentiel pour comprendre l’écoulement de l’eau dans les masses rocheuses fracturées) ont été élaborées, pas seulement en fonction de paramètres traditionnels tels que l’ouverture, l’espacement, l’inclinaison et la direction d’inclinaison, etc , mais aussi en prenant en compte la rugosité des joints à travers le coefficient de rugosité. Ces relations ont initialement été étudiées d’un point de vue théorique puis expérimentalement à travers des recherches en laboratoire.相似文献
72.
73.
油气在盆地中产生和运移过程中,水动力因素起着重要的控制作用[1],如何确定这一作用的实际影响,是一个值得研究的问题。通过数学推导,得出储层中的稳态油层和非稳态油层两种情况下的水动力理论模型,并且对水动力圈闭、深度-压力系统和流体势等作出理论模拟,将其用于油气勘探中,结果与实际情况较相符。 相似文献
74.
William W. Haible 《地球表面变化过程与地形》1980,5(3):249-264
Walker Creek in Marin County, California is a coastal stream draining to Tomales Bay, which lies in the San Andreas Rift Zone. Its valley contains an alluvial fill with a basal gravel dated at 5000 years BP. In upstream parts of the watershed, channels are incised arroyo-like in the fill leaving the valley floor standing as a high terrace averaging 5·5 m (18 ft) high. Below this terrace is an inner terrace of historic age that stands 2·4 m (8 ft) above the streambed. The stratigraphy and morphology of this valley are seen in others nearby, and indicate that in the last half of Holocene time in this region a single episode of valley alluviation was followed by two episodes of valley cutting. The second episode of valley cutting is occurring in the present time. During the last 60 years the flow has become seasonal, the stream has incised 1·5 m (5 ft) below the inner terrace in upstream reaches, aggraded 1·2 m (4 ft) in downstream reaches, and extended its estuary. Incision upstream has begun to re-expose the bedrock valley floor and is associated with aggradation downstream that has caused the flood plain to overtop both terraces. This has decreased the stream's gradient. Using a stream that is currently effecting major changes in its valley and channel morphology, two aspects of hydraulic adjustment in fluvial systems are examined. The changes in the average slope of the longitudinal profile are small but measureable. Profile concavity has not changed measurably. The various profiles that have existed in Holocene time show that stream gradient can be, but is not necessarily, slightly adjusted during valley filling and cutting. Flow measurements at a high discharge show that the channel has begun to assume the hydraulic geometry of an ephemeral channel. Adjustments of depth, velocity, and roughness appear to be hydraulic adjustments in response to changing watershed conditions. 相似文献
75.
The run-up flow and related pressure of solitary waves breaking on a 1:20 plane beach were investigated experimentally in a super tank (300 m × 5 m × 5.2 m). Swash flow measurements of flow velocities are compared with an existing analytical solution. By incorporating an analytical solution, the hydrodynamic pressure for a quasi-steady flow state is determined and compared with laboratory data. Concerning the evident extra pressure exerted by the impact of swash flow, an empirical drag coefficient for a circular plate is also suggested in the present study. 相似文献
76.
Rock and flow parameters of three karstic-fissured-porous aquifers in the Krakow-Silesian Triassic formations were measured
using various methods and compared. Though cavern and fissure porosities are shown to be very low (cavern porosity below 0.5%
and fracture porosity below 0.2%), they contribute dominantly to the hydraulic conductivity (from about 1.3×10–6 to about 11×10–6 m/s). Matrix porosity (2–11%) is shown to be the main water reservoir for solute transport and the main or significant contributor
to the specific yield (<2%). Though the matrix porosity is shown to be much larger than the sum of the cavern and fissure
porosities, its contribution to the total hydraulic conductivity is practically negligible (hydraulic conductivity of the
matrix is from about 5×10–11 m/s to about 2×10–8 m/s). On the other hand, the matrix porosity (for neglected cavern and fissure porosities) when combined with tracer ages
(or mean travel times) is shown to yield proper values of the hydraulic conductivity (K) by applying the following formula:
K≅(matrix porosity×mean travel distance)/(mean hydraulic gradient×mean tracer age). Confirming earlier findings of the authors,
this equation is shown to be of great practical importance because matrix porosity is easily measured in the laboratory on
rock samples, whereas cavern and fracture porosities usually remain unmeasurable.
Received: 21 February 1997 · Accepted: 13 May 1997 相似文献
77.
基于多元要素流的珠三角城市群功能联系与空间格局分析 总被引:1,自引:0,他引:1
以珠三角城市群9个地级市为研究对象,结合当前热点网络开放大数据,对珠三角城市群内各城市的经济流、交通流、人口流和信息流的联系强度和作用方向进行测算与评价。借助赋值法分别对四种要素流的总量进行打分,以各城市的综合得分作为空间层级划分依据,提出关于“点—线—面”的空间互动格局,进而探索珠三角城市群未来的发展规划。研究结果表明:①从各要素流的联系强度上看,城市间的等级划分具有较高的重合度,说明四种要素流彼此之间相互联系、相互作用。从各要素流的作用方向上看,广州和深圳多作为区域内其它城市的首位空间联系城市,说明各要素流通常指向经济发展水平较高、通达性较好的城市。②从整体来看,珠三角城市群内部功能互动表现极度不平衡,一是中心城市占据了要素流强度的绝大部分,其余城市仅占据极小部分流量,两极分化严重;二是东西两岸发展不平衡,东岸发展水平明显要强于西岸发展水平。③广州、深圳、东莞和佛山等城市虽然在区域内起到了一定的辐射和带动作用,但是针对江门、肇庆等周边城市的辐射和带动强度仍有待提升,说明还需进一步优化珠三角城市群功能分工与产业布局,推进区域经济一体化。 相似文献
78.
In addition to reducing the incoming wave energy, submerged breakwaters also cause a setup of the sea level in the protected area, which is relevant to the whole shadow zone circulation, including alongshore currents and seaward flows through the gaps. This study examines such a leading hydraulic parameter under the simplified hypothesis of 2D motion and presents a prediction model that has been validated by a wide ensemble of experimental data. Starting from an approach originally proposed by Dalrymple and Dean [(1971). Piling-up behind low and submerged permeable breakwaters. Discussion note on Diskin et al. (1970). Journal of Waterways and Harbors Division WW2, 423–427], the model splits the rise of the mean water level into two contributions: one is due to the momentum flux release forced by wave breaking on the structure, and the other is associated with the mass transport process. For the first time, the case of random wave trains has been explicitly considered. 相似文献
79.
80.