Aerodynamic dislodgement of multiple-size sand grains over time     

LONG LI  LAWRENCE W. MARTZ 《Sedimentology》1995,42(4):683-694

An experimental study was undertaken in a large-scale wind tunnel to investigate sand particle dislodgement by wind over time in the absence of grain-bed collisions. Aerodynamic dislodgement was measured for six groups of sand particles under two known wind velocity profiles. The results show that the dislodgement rate for both fine and coarse particles decreases rapidly during the transition of the particle surface from a non-wind-worked condition to a wind-worked condition, and that the dislodgement rate continues to decay under a wind-worked condition even though the mean grain size of surface particles remains nearly the same. A previously developed theoretical method for calculating the number of particles left on the bed by wind was developed further. The derived method was used to calculate the time-decay of the dislodgement rate and the length of time required for the dislodgement rate to reach an equilibrium. The length of time for dislodgement rate to reach an equilibrium in this study is of the order of 10–15 min. This not only provides further observation of the second, long stage of aeolian sediment transport system development reported previously but also indicates a potentially large variation in the time-decay of transport rate under different conditions. The results indicate that the time-decay of the particle dislodgement rate is related to sorting processes. Because of the artificial method of preparation of the grain surface and the wind velocity profiles, the results of this study should be applied with caution to natural conditions.  相似文献   

Convection in the earth's mantle     

Dan McKenzie 《Tectonophysics》1971

According to the hypothesis of global plate tectonics the surface motions of the earth are now known in considerable detail, but very little is known about the three-dimensional flow in the earth and about the forces which maintain the motions. The motions at depth are difficult to study because they produce few surface effects. For instance, there is now no reason to believe that ridges are the surface evidence for rising convection currents at depth. Only the plate motions themselves and the gravity field observed by satellites must be consequences of three-dimensional flow beneath the plates. Other observations, such as the high heat flow near ridges or deep earthquakes beneath trenches, now appear to be explained by the production and destruction of plates.  相似文献   

思茅盆地河流地貌特征及其对构造活动的响应          下载免费PDF全文

汝 鑫张世涛  董有浦段佳鑫 吴 可 王 丹 张东越 《地质科学》1958,55(4):1202-1213

河流地貌对构造活动具有极高的相关性、敏感性,是研究新构造活动最切实有效的地摘貌标要志之一。本文以30 m分辨率数字高程模型（DEM）为基础,利用ArcMap与Matlab软件提取的河流地貌因子（Horton定律比值和S-A双对数曲线）,先后分析了思茅盆地内部的李仙江、藤条江、勐戛河、威远江、南班河和南腊河等的河流地貌特征,揭示新生代晚期强烈构造活动对河流发育的影响。结果表明：1）思茅盆地的河流多沿或平行于走滑断裂发育,低级别河网较好的符合Horton定律中一般河网的发育规律,而沿活动断裂带发育的高级别河网则表现出了明显的偏差;2）各集水盆地 S-A双对数曲线均呈凹凸相间排列;3）各集水盆地河道的裂点分布特征与岩性变化不一致,这表明思茅盆地内部高级别河网的发育受岩性变化的影响较小,而主要受走滑断裂活动的影响。  相似文献   

Calculated trends of oh infrared stretching vibrations with composition and structure in aluminosilicate molecules     

J. D. Kubicki  Dan Sykes  George R. Rossman 《Physics and Chemistry of Minerals》1993,20(6):425-432

Ab initio, molecular orbital calculations have been performed on a variety of hypothetical aluminosilicate molecules to investigate relationships among composition, structure, and infrared spectra of OH. Vibrational analyses of the full-optimized molecular geometries at the 3–21G^** level were performed with Gaussian 92 to determine theoretical infrared spectra. Theoretical infrared OH frequencies, ν(OH), shift 10 to 100 cm^?1 with ionic substitutions. The inverse correlation of theoretical infrared OH intensities with OH stretching frequencies in these aluminosilicate molecules is similar to that observed for aluminosilicate glasses (Paterson 1982). O-H bond lengths, H-bond distances, and H?nd angles correlate with frequency. The dominant factor affecting ν(OH) is the H-bond distance, if this distance is less than 2 Å. Beyond H-bond distances of 2 Å, structural and compositional effects exert competitive influences on ν(OH).  相似文献   

The East Anatolian Fault: A major structure in Eastern Turkey     

Dan McKenzie 《Earth and Planetary Science Letters》1976,29(1):189-193

Satellite photographs of Eastern Turkey show a large strike slip fault about 550 km long extending from the Gulf of Iskenderun to the North Anatolian Fault with a strike of 60°E. This fault takes up most of the motion between the Turkish and Arabian plates.  相似文献   

The relationship between strain rates,crustal thickening,palaeomagnetism, finite strain and fault movements within a deforming zone   总被引：1，自引：0，他引：1  

Dan McKenzie  James Jackson 《Earth and Planetary Science Letters》1983,65(1):182-202

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Systematic compositional variations in the Cape York iron meteorite     

Kim H. Esbensen  Vagn F. Buchwald  Dan J. Malvin  John T. Wasson 《Geochimica et cosmochimica acta》1982,46(10):1913-1920

Concentrations of Re, Ir and Au are nearly constant within individual masses of the Cape York IIIAB iron meteorite, but differences between masses can be as large as a factor of 2, the extremes being Savik (5.1 μg/g Ir) and Agpalilik (2.7 μg/g Ir). The S concentration shows a still larger range from 13 mg/g in Agpalilik to 1.4 mg/g in Savik. A relatively large compositional hiatus between Dog and Agpalilik probably reflects inadequate sampling of the original material.Concentrations of Ir vary by ~10% and Au by ~3% between the ends of an 85-cm section from the Agpalilik mass of Cape York, but other sections through Agpalilik show smaller variations. These concentration ranges are much larger than expected from radial crystallization of a moderately large (radius 10 s of km) core. These variations in the Agpalilik mass may reflect dendritic crystallization, or they may have resulted from the process that produced the large concentration range among the Cape York masses.Large gradients in Re and Ir and small gradients in Ni and Au were also observed in samples within 2 cm of a large (100 cm³) troilite nodule. These gradients may reflect rapidly changing solid/liquid distribution coefficients during the final crystallization of S-rich liquid.The compositional trends among the various masses can either be explained by mixing of disparate end members followed by diffusive homogenization on a scale of m, or by dendritic crystallization on the ceiling of the IIIAB magma chamber. The mixing of a solid similar in composition to Savik with a liquid in equilibrium with this solid yields a good match to the observed trends, in which case Agpalilik consists of a mixture of 64% liquid and 36% solids. The bulk S content of the IIIAB core is calculated to be 14 mg/g on the basis of this model.  相似文献   

Matrix polynomial representation of the anisotropic magnetotelluric impedance tensor     

Yoram Shoham  Dan Loewenthal 《Physics of the Earth and Planetary Interiors》1975,11(2):128-138

A formalism based upon the equal penetration depth stratification assumption is extended to a layered anisotropic medium, yielding a recursive algorithm for the computation of the magnetotelluric impedance tensor elements. The development of this new procedure requires an appropriate layer-discretization as well as an extension of the reflection coefficient definition for the anisotropic layered model.This method transfers the differential problem into an algebraic one, and is independent of the electric and magnetic field vectors. The technique is a recursive process which gives a rational matrix polynomial representation for the magnetotelluric impedance tensor.The procedure involves the use of second-order matrices, rather than the fourth-order ones normally used for such a case. Using this representation, a separation of the contribution of the model parameters from that of the frequency is achieved. Consequently the elements of the magnetotelluric impedance tensor are computed, for each frequency, using coefficients which are evaluated just once for a given model.  相似文献   

湍流相干结构在中蒙中区一次沙尘暴起沙中的作用          下载免费PDF全文

赵建华  隆霄  张峰  杨永龙  刘世祥  梁芸 《地球物理学报》1954,63(11):3967-3980

起沙机制是沙尘暴天气研究中一个重要问题.本文基于中蒙中区一次沙尘暴天气过程,通过提取沙尘源地涡动相关数据中的湍流相干结构（Coherent structure,简称为CS）,研究了CS在起沙中的作用.结果发现：（1）起沙期间存在CS,其典型特征表现为上扬—下扫循环,即暖空气的辐合上升与冷空气的辐散下沉相伴;（2）起沙期间,CS具有频次高、持续时间短、间歇性适中、连发频繁、速度切变大、水平尺度远大于垂直尺度和尺度变化比较大的特点;（3）CS是重要的起沙机制,它可起动粒径在0.1~156 μm之间的各种沙粒,起动最多的是粒径低于8 μm的粉粒与粘粒,但CS不是唯一的起沙机制;（4）CS的上扬与下扫两过程均可引起起沙,区别在于前者通过暖空气的上升将沙粒向上空输送,后者则是将上空高速冷空气拖带下来引起地表沙粒的起动;（5）CS起沙分为单起式和连发配合式两种形式.单起式是单发CS产生的起沙形式,连发配合式是连发的CS产生的跃移—上扬的配合起沙形式.其中,连发配合式为主要形式;（6）下扫过程对起沙的贡献是上扬过程的1.8~15倍,上扬过程可将下扫过程中起动沙粒的1/3左右向上输送到空中;（7）一般情况下,CS对起沙具有稳定的贡献,其贡献率为51%,当临界起沙风速大于13 m·s^-1时,其贡献急剧降低.  相似文献   

Model anomalies for the earth from crustal data     

Dan Sharni 《Journal of Geodesy》1971,45(3):299-317

Crustal data of surface elevations and depth of Moho (and densities) can be utilized to form model-earth anomalies. These model-anomalies can closely approximate the free-air anomaly field of the earth, and could thus be used to predict the latter. A review of several such models is presented, with some elaboration on model developments, procedures, data analysis and accuracies. One of the models approaches a prediction accuracy of ±10 mgal for5°×5° mean free-air anomalies, whose r.m.s. value was about30% higher.  相似文献