Wind velocity and sand transport on a barchan dune   总被引：2，自引：0，他引：2  

G. Sauermann  J. S. Andrade  Jr.  L. P. Maia  U. M. S. Costa  A. D. Araújo  H. J. Herrmann 《Geomorphology》2003,54(3-4):245-255

We present measurements of wind velocity and sand flux performed on the windward side of a large barchan dune in Jericoacoara, northeastern Brazil. From the measured profile, we calculate the air shear stress using an analytical approximation and treat the problem of flow separation by an heuristic model. We find that the results from this approach agree well with our field data. Moreover, using the calculated shear velocity, we predict the sand flux according to well-known equilibrium relations and with a phenomenological continuum saltation model that includes saturation transients and thus allows for nonequilibrium conditions. Based on the field data and theoretical predicted results, we indicate the principal differences between saturated and nonsaturated sand flux models. Finally, we show that the measured dune moves with invariant shape and predict its velocity from our data and calculations.  相似文献   

A critical state interpretation for the cyclic liquefaction resistance of silty sands   总被引：1，自引：0，他引：1  

George D. Bouckovalas  Konstantinos I. Andrianopoulos  Achilleas G. Papadimitriou 《Soil Dynamics and Earthquake Engineering》2003,23(2):115-125

Contrary to many laboratory investigations, common empirical correlations from in situ tests consider that the increase in the percentage of fines leads to an increase of the cyclic liquefaction resistance of sands. This paper draws upon the integrated Critical State Soil Mechanics framework in order to study this seemingly not univocal effect. Firstly the effect of fines on the Critical State Line (CSL) is studied through a statistical analysis of a large data set of published monotonic triaxial tests. The results show that increasing the content of non-plastic fines practically leads to a clockwise rotation of the CSL in (e–ln p) space. The implication of this effect on cyclic liquefaction resistance is subsequently evaluated with the aid of a properly calibrated critical state elasto-plastic constitutive model, as well as a large number of published experimental results and in situ empirical correlations. Both sets of data show clearly that a fines content, less than about 30% by weight, may prove beneficial at relatively small effective stresses (p₀<50–70 kPa), such as the in situ stresses prevailing in most liquefaction case studies, and detrimental at larger confining stresses, i.e. the stresses usually considered in laboratory tests. To the extent of these findings, a correction factor is proposed for the practical evaluation of liquefaction resistance in terms of the fines content and the mean effective confining stress.  相似文献   

Submarine hydrothermal mineralization in the Okinawa Trough, SW of Japan: an overview   总被引：3，自引：0，他引：3  

G. P. Glasby  K. Notsu   《Ore Geology Reviews》2003,23(3-4):299-339

The Okinawa Trough is a heavily sedimented, rifted back-arc basin formed in an intracontinental rift zone. Submarine hydrothermal activity is located within the six back-arc rifts located in the middle and southern Okinawa Trough and its distribution is controlled principally by tectonic factors. Subduction of the Daito and Gagua Ridges beneath the Ryukyu Arc has resulted in fracturing of the brittle lithosphere beneath the Okinawa Trough. Hydrothermal activity is strongest in the volcanic arc-rift migration phenomenon (VAMP) area plus the JADE site and Southernmost Part of the Okinawa Trough (SPOT) area which form the prolongation of these two ridges. These areas are characterized by extremely high heat flow locally. Submarine hydrothermal fluids from the Okinawa Trough tend to be strongly influenced by interaction of the hydrothermal fluids with organic matter in the sediment resulting in high alkalinity and NH₄⁺ concentrations of the fluids. The fluids also contain high concentrations of CO₂ of magmatic origin. Submarine hydrothermal mineralization in the trough is diverse. The CLAM site consists principally of carbonate chimneys. Interaction of the hydrothermal fluid with organic matter in the sediment is particularly strong at this site. This is most probably a sediment-hosted deposit in which sulphide minerals have deposited within the sediment column leaving ‘spent ore-fluids’ to emerge at the seafloor. The JADE site consists of active and inactive sulphide–sulphate chimneys and mounds. The Zn–Pb-rich sulphides at this site contain the highest concentrations of Pb, Ag and Au so far recorded in submarine hydrothermal sulphide deposits. At Minami-Ensei Knoll and Hatoma Knoll, active and inactive chimneys consist principally of anhydrite and barite as a result of phase separation of the hydrothermal fluids beneath the seafloor. An intense black smoker has recently been discovered at Yonaguni Knoll in the SPOT area. If it is confirmed that sulphide mineralization is dominant at this site, this could be a highly prospective area. The most prospective areas for economic-grade minerals in the Okinawa Trough appear to be the JADE site and the SPOT area.  相似文献   

中国西北部“4.5”沙尘暴过程中尺度低压的数值模拟   总被引：3，自引：9，他引：3  

陈伟民  王强  牛志敏  徐建芬  高风荣 《中国沙漠》1996,16(2):140-144

利用改进型ＰＳＵ／ＮＣＡＲ中尺度数值模式（ＭＭ４标准版），取模式水平格距４０ｋｍ，４６×６１网格，垂直方向ａ取１５层，即从地面到模式顶（１００ｈＰａ），σ＝０．０、０．１、０．２、０．３、０．４、０．５、０．６、０．７、０．７８、０．８４、０．８９、０．９３、０．９６、０．９８、０．９９、１．００，采用ＮＣＡＲ的３０＇×３０＇地形资料，以常规观测资料作为初始场，较好地模拟了此次沙尘暴过程的海平面气压的演变和分布，特别是张掖、柴达本盆地以及敦煌附近的三个中低压。同时，模拟了张掖中低压与蒙古冷高压之间的甘肃河西沙尘暴东大风。敏感性试验表明，沙尘暴中低压的形成发展主要是受于物理过程制约；沙尘暴中尺度系统的研究与暴雨中尺度系统的研究是有区别的，积云对流参数化并不是特别重要，在设计研究沙尘暴的数值模式中，应当合理地处理其他的热力、动力过程及大气外强迫源的作用。模式水平格距、地形真实程度对模拟中低压的位置、中心强度有重要贡献；下垫面变化中低压强度有一定影响。张掖热低压的形成发展主要是在有利的环境形势下，特殊地势起了重要作用，表现为直接动力强迫和间接热力强迫。  相似文献   

陕西直罗油田长_2油层组储层特征     

刘勇  孙冬胜 《陕西地质》1996,(2)

直罗油田主力产层为三叠系延长组长2油层组，储层以三角洲分支河道砂体为主，岩性主要为长石细砂岩，大量的自生矿物对原生孔隙产生了很大程度的破坏。较强的成岩后生作用使得储层以粒间溶孔及原生粒间余孔为主要储集空间。储层孔隙结构表现为以小孔隙、小喉道为主要特征，非均质性较强。物性分布主要与其在砂体中的分布部位有关。  相似文献   

建德铜矿床的海底喷流沉积成因   总被引：2，自引：0，他引：2  

刘家军  李元林 《矿产与地质》1996,10(3):145-154

建德铜矿床是浙江西部多金属成矿带中一个引人瞩目的重要矿床。它产于中石炭统底部伴有火山岩、硅质岩和碧玉岩的白云岩中。整合块状矿体之下有一筒状矿化蚀变带。其成分以Ｃｕ＞Ｚｎ＞Ｐｂ为特征。根据矿床地质、地球化学特征以及近年来研究所获得资料，认为该矿床属海底喷流沉积成因  相似文献   

Evolution of sand waves in the Messina Strait, Italy   总被引：1，自引：0，他引：1  

V. C. Santoro  E. Amore  L. Cavallaro  M. De Lauro 《Ocean Dynamics》2004,54(3-4):392-398

In the present paper the morphodynamics of sand waves in the Messina Strait, Italy, is analysed by comparing data gathered during two different surveys carried out in 1991 and 2001, respectively. In particular, a morphometric analysis on the most recent data and a qualitative analysis of the differences between bottom bedform patterns, are carried out. At locations characterised by greater depths, only minor changes to the planimetric configuration of the field, i.e. crest orientation (which is seen to be orthogonal to the direction of net gross sand transport) and wave length are observed, while differences in wave length and crest direction are more evident in more shallow areas. On the other hand, wave height has significantly increased in the whole field. A possible explanation of such a change, based on the previsions of a theory which relates sand-wave growth and migration to the main components of the tidal ellipse, is provided.Responsible Editor: Jens Kappenberg  相似文献   

沈阳地区沙尘天气分析   总被引：5，自引：1，他引：5       下载免费PDF全文

路爽  张菁  孙凤华 《气象科学》2004,24(1):112-119

本文总结了沈阳地区沙尘天气的时空分布特点，从天气条件、地表自然条件(沙源)两方面分析了沈阳地区沙尘天气的成因，总结了沈阳地区预报沙尘天气时应参考的5个因素。  相似文献   

塔河油田卡拉沙依组砂组沉积相与储层研究     

樊怀阳  陈文  刘百春 《新疆地质》2004,22(4):417-421

卡拉沙依组砂岩是塔河油田石炭系重要储层，从上到下分为4个砂组，即Ⅰ砂组、Ⅱ砂组、Ⅲ砂组及Ⅳ砂组，根据岩石组合特征、岩石粒度、分选性、以及沉积构造等特征，可得出Ⅰ砂组为较典型的扇三角洲沉积环境；Ⅱ-Ⅳ砂组为潮坪环境，其中Ⅲ砂组沉积时的古环境水动力相对最强，以潮间带下部为主，储层砂体最发育，单个砂体的厚度也最大，为纵向上最好储层段。  相似文献   

A parameterisation for the threshold shear velocity to initiate deflation of dry and wet sediment   总被引：1，自引：0，他引：1  

Wim M. Cornelis  Donald Gabriels  Roger Hartmann 《Geomorphology》2004,59(1-4):43

A new parameterisation for the threshold shear velocity to initiate deflation of dry and wet particles is presented. It is based on the balance of moments acting on particles at the instant of particle motion. The model hence includes a term for the aerodynamic forces, including the drag force, the lift force and the aerodynamic-moment force, and a term for the interparticle forces. The effect of gravitation is incorporated in both terms. Rather than using an implicit function for the effect of the aerodynamic forces as reported earlier in literature, a constant aerodynamic coefficient was introduced. From consideration of the van der Waals force between two particles, it was further shown that the effect of the interparticle cohesion force between two dry particles on the deflation threshold should be inversely proportional to the particle diameter squared. The interparticle force was further extended to include wet bonding forces. The latter were considered as the sum of capillary forces and adhesive forces. A model that expresses the capillary force as a function of particle diameter squared and the inverse of capillary potential was deduced from consideration of the well-known model of Fisher and the Young–Laplace equation. The adhesive force was assumed to be equal to tensile strength, and a function which is proportional to particle diameter squared and the inverse of the potential due to adhesive forces was derived. By combining the capillary-force model and the adhesive force model, the interparticle force due to wet bonding was simplified and written as a function of particle diameter squared and the inverse of matric potential. The latter was loglinearly related to the gravimetric moisture content, a relationship that is valid in the low-moisture content range that is important in the light of deflation of sediment by wind. By introducing a correction to force the relationship to converge to zero moisture content at oven dryness, the matric potential–moisture content relationship contained only one unknown model parameter, viz. moisture content at −1.5 MPa. Working out the model led to a rather simple parameterisation containing only three coefficients. Two parameters were incorporated in the term that applies to dry sediment and were determined by using experimental data as reported by Iversen and White [Sedimentology 29 (1982) 111]. The third parameter for the wet-sediment part of the model was determined from wind-tunnel experiments on prewetted sand and sandy loam aggregates. The model was validated using data from wind-tunnel experiments on the same but dry sediment, and on data obtained from simulations with the model of Chepil [Soil Sci. Soc. Am. Proc. 20 (1956) 288]. The experiments showed that soil aggregates should be treated as individual particles with a density equal to their bulk density. Furthermore, it was shown that the surface had to dry to a moisture content of about 75% of the moisture content at −1.5 MPa before deflation became sustained. The threshold shear velocities simulated with our model were found to be in good agreement with own observations and with simulations using Chepil's model.  相似文献