Effect of soil stiffness in the attenuation of Rayleigh-wave motions from field measurements     

G. A. Athanasopoulos  P. C. Pelekis  G. A. Anagnostopoulos 《Soil Dynamics and Earthquake Engineering》2000,19(4)

The amplitude of vertical ground surface vibrations generated by impact tests on the ground surface was measured at various radial distances from the point of impact at locations of Greece. The results of measurements were analyzed in the frequency domain (in the range from 0–100 Hz) and the attenuation characteristics of soil materials were studied in terms of a frequency-independent attenuation coefficient, a₀, of the empirical Bornitz equation. The aim of the study was to investigate the effect of soil stiffness (expressed by the value of low-amplitude shear wave velocity of soil, V_SO) on the value of attenuation coefficient, a₀. Values of V_SO for the tested soils were estimated by applying the methodology of Spectral Analysis of Surface Waves (SASW) technique and utilizing the surface vibration data. An empirical relationship between a₀ and V_SO1 (V_SO1 is the representative value of V_SO for the soil profile up to a depth of one wavelength) was established for values of V_SO1 ranging from 140 to 1000 m/s. A similar relationship in terms of the low-amplitude shear modulus of soil, G_O1, was also established by converting the V_SO1 values to G_O1 values. The experimental results were compared to values reported in the literature for comparable soil types and frequencies of vibration and a reasonable agreement was found to exist. The proposed empirical relationship can be utilized in many practical applications of soil dynamics requiring the knowledge of the attenuation rate of Rayleigh waves with distance in various types of soils.  相似文献   

The generation of non aspect sensitive plasma density irregularities by field aligned drifts in the lower ionosphere     

T. R. Robinson  K. Schlegel 《Annales Geophysicae》2000,18(7):799-806

A theory of the generation of plasma density irregularities with virtually no aspect sensitivity, in the lower ionosphere at high latitudes, by electron drifts aligned with the geomagnetic field, is presented. The theory is developed through fluid equations in which the destabilising mechanism involves positive feedback from electron collisional heating. When field aligned electron drift speeds exceed a few km s^–1, this effect destabilises waves with wavelengths in excess of a few tens of metres in the lower E-region, where collisional effects are sufficiently large. Furthermore, the threshold conditions are almost independent of the wave propagation direction and the unstable waves propagate at speeds well below the ion acoustic speed. The role that this new instability may play in recent radar backscatter observations of short scale irregularities propagating in directions close to that of the geomagnetic field, in the lower E-region is also considered.  相似文献   

Comment on “Concerning the generation of geomagnetic giant pulsations by drift-bounce resonance ring current instabilities” by K.-H. Glassmeier et al., Ann. Geophysicae, 17, 338/350, (1999)     

I. R. Mann  G. Chisham 《Annales Geophysicae》2000,18(2):161-166

  相似文献   

广义地震活动窗及其对震前异常过程的跟踪应用   总被引：1，自引：1，他引：0  

王琤琤  刘红桂  王炜  陈兴东  梅卫萍  张晓勇 《地震》2000,20(4):19-27

提出了广州地震窗的概念,并以此作为研究范围以突出局部构造特点。用多年地震频次等值线划分广义地震窗的方法,在中国东部得到４５个广义地震窗,它们具有明显的构造意义,应用原有地震活动参数提出一个相对指标ｆ,用以消除局部的地域特点和地震活动涨落的时段特点,提高原指标的异常信息提取能力。用地震活动指数Ａ（ｂ）值的相对指标ｆ（Ａ（ｂ）,提出了用多窗异常组合图综合平面的时间的三维信息以跟踪异常过程,结果显示主震  相似文献   

Determination of High Field Strength Elements, Rb, Sr, Mo, Sb, Cs, Tl and Bi at ng g⁻¹ Levels in Geological Reference Materials by Magnetic Sector ICP-MS after HF/HClO₄ High Pressure Digestion     

Zongshou Yu  Philip Robinson  Ashley T. Townsend  Carsten Mnker  Anthony J. Crawford 《Geostandards and Geoanalytical Research》2000,24(1):39-50

Six low abundance rock reference materials (basalt BIR-1, dunite DTS-1, dolerite DNC-1, peridotite PCC-1, serpentine UB-N and basalt TAFAHI) have been analysed for high field strength elements (Zr, Nb, Hf, Ta, Th and U), Rb, Sr, Mo, Sb, Cs, Tl and Bi at ng g⁻¹ levels (in rock) by magnetic sector inductively coupled plasma-mass spectrometry after HF/HClO₄ high pressure decomposition. The adopted method uses only indium as an internal standard. Detection limits were found to be in the range of 0.08 to 16.2 pg ml⁻¹ in solution (equivalent to 0.08 to 16.2 ng g⁻¹ in rock). Our data for high field strength elements, Rb, Sr, Mo, Sb, Cs, Tl and Bi for the six selected low abundance geological reference materials show general agreement with previously published data. Our Ta values in DTS-1 and PCC-1 (1.3 and 0.5 ng g⁻¹) are lower than in previously published studies, providing smooth primitive mantle distribution patterns. Lower values were also found for Tl in BIR-1, DTS-1 and PCC-1 (2, 0.4 and 0.8 ng g⁻¹). Compared with quadrupole ICP-MS studies, the proposed magnetic sector ICP-MS method can generally provide better detection limits, so that the measurement of high field strength elements, Rb, Sr, Mo, Sb, Cs, Tl and Bi at ng g⁻¹ levels can be achieved without pre-concentration, ion exchange separation or other specialised techniques.  相似文献   

饱和土半空间中圆柱形孔洞对平面P波的散射   总被引：1，自引：0，他引：1  

李伟华  赵成刚 《岩土力学》2004,25(12):1867-1872

在 Biot饱和多孔介质动力学理论的基础上,首次建立了求解饱和土半空间中圆柱形孔洞对平面 P 波散射问题的波函数展开法。首先。分析了具有圆柱形孔洞的饱和土半空间场地在平面 P 波入射下产生散射波系,并将入射波和散射波的波函数在圆柱坐标下展开。然后,引入边界条件,求出散射波函数的待定系数,从而,得到饱和土半空间中圆柱形孔洞对平面P波的散射问题的解析解。根据所得的波函数的解,可求解区域内的位移、应力的值,同时,分析了入射波频率、入射角对柱面上的应力集中因子的影响。  相似文献   

用瑞利面波资料反演中国大陆东部地壳上地幔横波速度的三维构造   总被引：33，自引：8，他引：25       下载免费PDF全文

徐果明  李光品  王善恩  陈虹  周虎顺 《地球物理学报》2000,43(3):366-376

利用中国大陆东部21个台站的43条面波大圆路径上瑞利面波记录的双台资料,计算出双台间地震面波相速度频散,采用Tarantola概率反演的方法求得相速度频散曲线的分布,并由各处相速度频散曲线反演得到地壳上地幔的三维横波波速图像,进而得到中国东部地壳上地幔的S波速度结构.结果表明:我国大陆东部地壳厚度总体上呈东薄西厚的趋势,以105°E为界向西地壳厚度逐渐加深到55 km以上,其中有一个北东向的h形地壳厚度的坡度带.豫西及晋南地区为相对薄地壳的地区.大别山地区和泰山附近地区地壳变厚,但秦岭地区地壳不变厚.上地幔低速层上界面的深度在华北地区较浅,为80-90km,在鄂尔多斯、四川东部以及黔湘地区为120-130km.扬子地块东部及华南褶皱系中、东部上地幔顶部速度偏低使低速层的速度反差不明显.滇黔褶皱系的西部在200 km以内的上地幔中未出现低速层.  相似文献   

Interpolation of wave heights   总被引：1，自引：0，他引：1  

M. C. Deo  N. Kiran Kumar 《Ocean Engineering》2000,27(9)

Remote sensing of waves often necessitates presentation of data in the form of wave height values grouped over large time intervals. This restricts their use to long-term applications only. This paper describes how such data can be made suitable for short-term usage in the field. Weekly mean significant wave heights were derived from their monthly mean observations with the help of different alternative techniques. These include model-free neural network schemes as well as model-based statistical and numerical methods. Superiority of neural networks was noted when the estimations were compared with corresponding observations. The network was trained using three different training algorithms, viz., error back propagation, conjugate gradient and cascade correlation. The technique of cascade correlation took minimum training time and showed better coefficient of correlation between observations and network output.  相似文献   

Relativistic anisotropic pair plasmas     

Estelle Asseo  Alain Riazuelo 《Monthly notices of the Royal Astronomical Society》2000,318(4):983-1004

The properties of waves able to propagate in a relativistic pair plasma are at the basis of the interpretation of several astrophysical observations. For instance, they are invoked in relation to radio emission processes in pulsar magnetospheres and to radiation mechanisms for relativistic radio jets. In such physical environments, pair plasma particles probably have relativistic, or even ultrarelativistic, temperatures. Besides, the presence of an extremely strong magnetic field in the emission region constrains the particles to one-dimensional motion: all the charged particles strictly move along magnetic field lines.
We take anisotropic effects and relativistic effects into account by choosing one-dimensional relativistic Jűttner–Synge distribution functions to characterize the distribution of electrons and/or positrons in a relativistic, anisotropic pair plasma. The dielectric tensor, from which the dispersion relation associated with plane wave perturbations of such a pair plasma is derived, involves specific coefficients that depend on the distribution function of particles. A precise determination of these coefficients, using the relativistic one-dimensional Jűttner–Synge distribution function, allows us to obtain the appropriate dispersion relation. The properties of waves able to propagate in anisotropic relativistic pair plasmas are deduced from this dispersion relation. The conditions in which a beam and a plasma, both ultrarelativistic, may interact and trigger off a two-stream instability are obtained from this same dispersion relation. Two astrophysical applications are discussed.  相似文献   

Linear and non-linear theory of a parametric instability of hydrodynamic warps in Keplerian discs     

Charles F. Gammie  Jeremy Goodman  Gordon I. Ogilvie 《Monthly notices of the Royal Astronomical Society》2000,318(4):1005-1016

We consider the stability of warping modes in Keplerian discs. We find them to be parametrically unstable using two lines of attack, one based on three-mode couplings and the other on Floquet theory. We confirm the existence of the instability and investigate its non-linear development in three dimensions, via numerical experiment. The most rapidly growing non-axisymmetric disturbances are the most nearly axisymmetric (low- m ) ones. Finally, we offer a simple, somewhat speculative model for the interaction of the parametric instability with the warp. We apply this model to the masing disc in NGC 4258 and show that, provided the warp is not forced too strongly, parametric instability can fix the amplitude of the warp.  相似文献