频率慢度域自适应迭代反演算法压制海上倾斜缆鬼波方法及其应用

海上倾斜缆采集技术具有多样的陷波特征,通过去鬼波处理可获得宽频数据.针对海水面波浪起伏及缆深误差引起的鬼波延迟时间估计误差以及崎岖海底和目的层深度变化使得鬼波和一次反射波的振幅差异系数随偏移距的变化而难以给定一个固定值的问题,本文推导出频率慢度域中鬼波滤波算子以及自适应迭代反演求解上行波算法,该鬼波滤波算子与不同水平慢度对应的鬼波和一次反射波的振幅差异系数以及鬼波延迟时间有关.并基于计算出的理论下行波与实际下行波之间的平方误差最小理论实现自适应反演迭代最优计算该振幅差异系数和鬼波延迟时间.合成的及某海上采集的倾斜缆数据去鬼波处理结果表明,频率慢度域自适应迭代反演算法能较好地去除海上变深度缆鬼波,能达到拓宽地震记录频带目的.     

The high‐frequency limit of usable response spectral ordinates from filtered analogue and digital strong‐motion accelerograms

The range of response frequencies for which spectral ordinates obtained from accelerograms may be considered reliable is limited by several factors, primary among them being the effects of filters that are routinely applied to remove noise from the records. Considerable attention has been focused on the low‐frequency limit of the usable spectral ordinates because of various engineering applications requiring long‐period spectral accelerations or displacements but only recently have rational approaches to selecting the high‐frequency limit been proposed. Since there are applications for which the high‐frequency spectral ordinates are important, the approaches to this issue presented in the recent studies are reviewed and their application to the ground‐motion database from Europe and the Middle East is explored. On the basis of the results of these analyses, it is concluded that a large proportion of this dataset can be used to provide reliable estimates of response spectral ordinates at much shorter periods than may have previously been considered feasible. Copyright © 2011 John Wiley & Sons, Ltd.     

Theory and experimental study for sliding isolators with variable curvature

Because a conventional isolation system with constant isolation frequency is usually a long‐period dynamic system, its seismic response is likely to be amplified in earthquakes with strong long‐period wave components, such as near‐fault ground motions. Seismic isolators with variable mechanical properties may provide a promising solution to alleviate this problem. To this end, in this work sliding isolators with variable curvature (SIVC) were studied experimentally. An SIVC isolator is similar to a friction pendulum system (FPS) isolator, except that its sliding surface has variable curvature rather being spherical. As a result, the SIVC's isolation stiffness that is proportional to the curvature becomes a function of the isolator displacement. By appropriately designing the geometry of the sliding surface, the SIVC is able to possess favorable hysteretic behavior. In order to prove the applicability of the SIVC concept, several prototype SIVC isolators, whose sliding surfaces are defined by a sixth‐order polynomial function, were fabricated and tested in this study. A cyclic element test on the prototype SIVC isolators and a shaking table test on an SIVC isolated steel frame were all conducted. The results of both tests have verified that the prototype SIVC isolators do indeed have the hysteretic property of variable stiffness as prescribed by the derived formulas in this study. Moreover, it is also demonstrated that the proposed SIVC is able to effectively reduce the isolator drift in a near‐fault earthquake with strong long‐period components, as compared with that of an FPS system with the same friction coefficient. Copyright © 2011 John Wiley & Sons, Ltd.     

套管井偶极弯曲波频散向高频偏移的特性

对套管井偶极弯曲模式波的频散特性进行了系统的数值考察、实例对比和分析.发现套管井弯曲波频散曲线随地层横波速度的降低,特别是地层横波速度小于2000 m·s^-1以下,会迅速移向高频区,偶极弯曲波基础模式主频散区（或截止频率）可出现在13 kHz以上,以致超出了现行低频偶极子声波测井仪的激发与接收频带,这是一过去没有被研究者注意到的现象,并进一步被现场实例所证实.研究表明控制套管井弯曲波频散曲线主频散区位置的主要是钢套管的厚度和地层横波速度.对地层横波速度大于井孔流体声速的快速地层,在钢套管壁厚一定（8 mm）的情况下,频散曲线主频散区可移至11 kHz以上,可能出现的最大可能频域位置是同一井孔内径,井外全钢时的频散曲线上等于、小于地层横波速度那一段,这对各种地层和套管参数都是适用的.对地层横波速度小于等于井孔流体声速（1500 m·s^-1）的慢速地层,弯曲波频散曲线随地层横波速度的降低移向高频区的特点更为明显,可能移至16 kHz以上;而套管厚度的影响,也比快速地层大的多,对地层横波速度小于1380 m·s^-1的慢速地层,无论用多高的频率激发,都不能在现行使用的各类套管井（壁厚6~12 mm）中用偶极声波测井仪测到弯曲模式波.     

一种优化的频率域三维声波有限差分模拟方法

为提高频率域有限差分（FD,finite-difference）正演模拟技术的计算精度和效率,基于旋转坐标系统的优化差分格式被广泛应用,但是只应用于正方形网格的情况.基于平均导数法（ADM）的优化差分格式,应用于正方形和长方形网格模拟.这些频率域有限差分算子,各自具有不同的差分格式和对应的优化系数求解表达式.本文基于三维声波方程发展了一种新的优化方法,只要给定FD模板形式,可直接构造频散方程,求取FD模板上各节点的优化系数.此方法的优点在于频率域FD算子的优化系数对应各个节点,可扩展优化其他格式.运用此优化方法,计算得到了不同空间采样间距比情况下27点和7点格式的优化系数.数值实验表明,优化27点格式与ADM 27点格式具有相同的精度,优化7点格式比经典的7点格式具有更小的数值频散.

     

基于广义旋转法双九点格式标量波方程数值模拟

前人虽然基于传统旋转法提出了四阶精度最优17点差分格式,用于提高频率域地震波场数值模拟精度,但其仅适用于等网格间距的情形.这大大限制了该格式的使用范围.为了进一步提高17点有限差分格式的数值精度并将其推广到矩形网格,本文基于二阶精度有限差分算子利用广义旋转法提出了双九点格式,由于其差分格点与前人17点格式在分布上一样,所以也可称为二阶精度最优17点格式,但由差分格式构造原理上来讲,称其为双九点格式更妥.前人四阶精度17点格式仅为本文等网格间距情形时的特殊情况,本文方法单位波长网格点数仅需要2.2个即可.本文格式在继承传统旋转法良好几何旋转性质的同时,拥有平均导数方法适用于矩形网格的特点,和平均导数法所得的广义17点格式相比,本文格式数值精度更高,数值频散抑制性能和差分算子对称性更好.同时,本文双九点格式方法和思想对于后人借助传统九点格式的构造方法将其扩展到17点格式求解各类波动方程具有十分重要的意义.

     

大功率高频电波加热电离层中人工沿场不均匀体散射特性的理论与数值模拟研究

大功率高频电波与电离层的相互作用会引起电子密度扰动,进而产生人工沿场不均匀体,其对无线电波特别是超短波信号有强的定向散射能力,可形成一种新型的超短波通信方式.基于各向异性介质的散射理论,首先通过求解电子密度扰动产生的附加极化势获得电子密度扰动散射方程,然后对电子密度扰动进行高斯自相关处理,并结合入射波/散射波与地磁场方向的空间几何关系,获得电子密度扰动的波数谱表达式,建立了人工沿场不均匀体的散射系数理论模型.利用模型对Platteville站实验中产生的人工不均匀体散射截面积进行了数值模拟,并通过与实测值对比验证了模型的正确性.根据人工沿场散射原理给出了利用其进行无线电信号传输的约束条件.重点仿真分析了人工沿场不均匀体的散射系数和散射覆盖区范围,结果表明：同等条件下,相对于高纬度地区,低纬度地区人工沿场不均匀体的散射系数小5~10 dB,但其散射覆盖区的地面范围大,东西向可达3000 km,南北向可达1500 km,完全可用于超短波信号的超远距离传输.本文结果为中低纬度地区开展相关实验研究提供了理论指导,对利用人工沿场不均匀体进行无线信号的超远距离传输应用研究具有重要意义.     

Erosional power in the Swiss Alps: characterization of slope failure in the Illgraben

Landslides and rockfalls are key geomorphic processes in mountain basins. Their quantification and characterization are critical for understanding the processes of slope failure and their contributions to erosion and landscape evolution. We used digital photogrammetry to produce a multi‐temporal record of erosion (1963–2005) of a rock slope at the head of the Illgraben, a very active catchment prone to debris flows in Switzerland. Slope failures affect 70% of the study slope and erode the slope at an average rate of 0.39 ± 0.03 m yr¯¹. The analysis of individual slope failures yielded an inventory of ~2500 failures ranging over 6 orders of magnitude in volume, despite the small slope area and short study period. The slope failures form a characteristic magnitude–frequency distribution with a rollover and a power‐law tail between ~200 m³ and 1.6 × 10⁶ m³ with an exponent of 1.65. Slope failure volume scales with area as a power law with an exponent of 1.1. Both values are low for studies of bedrock landslides and rockfall and result from the highly fractured and weathered state of the quartzitic bedrock. Our data suggest that the magnitude–frequency distribution is the result of two separate slope failure processes. Type (1) failures are frequent, small slides and slumps within the weathered layer of highly fractured rock and loose sediment, and make up the rollover. Type (2) failures are less frequent and larger rockslides and rockfalls within the internal bedded and fractured slope along pre‐determined potential failure surfaces, and make up the power‐law tail. Rockslides and rockfalls of high magnitude and relatively low frequency make up 99% of the total failure volume and are thus responsible for the high erosion rate. They are also significant in the context of landscape evolution as they occur on slopes above 45° and limit the relief of the slope. Copyright © 2012 John Wiley & Sons, Ltd.     

Inelastic displacement demands in steel structures and their relationship with earthquake frequency content parameters

This paper deals with the estimation of peak inelastic displacements of SDOF systems, representative of typical steel structures, under constant relative strength scenarios. Mean inelastic deformation demands on bilinear systems (simulating moment resisting frames) are considered as the basis for comparative purposes. Additional SDOF models representing partially‐restrained and concentrically‐braced (CB) frames are introduced and employed to assess the influence of different force‐displacement relationships on peak inelastic displacement ratios. The studies presented in this paper illustrate that the ratio between the overall yield strength and the strength during pinching intervals is the main factor governing the inelastic deformations of partially‐restrained models and leading to significant differences when compared with predictions based on bilinear structures, especially in the short‐period range. It is also shown that the response of CB systems can differ significantly from other pinching models when subjected to low or moderate levels of seismic demand, highlighting the necessity of employing dedicated models for studying the response of CB structures. Particular attention is also given to the influence of a number of scalar parameters that characterise the frequency content of the ground motion on the estimated peak displacement ratios. The relative merits of using the average spectral period T_aver, mean period T_m, predominant period T_g, characteristic period T_c and smoothed spectral predominant period T_o of the earthquake ground motion, are assessed. This paper demonstrates that the predominant period, defined as the period at which the input energy is maximum throughout the period range, is the most suitable frequency content scalar parameter for reducing the variability in displacement estimations. Finally, noniterative equivalent linearisation expressions based on the secant period and equivalent damping ratios are presented and verified for the prediction of peak deformation demands in steel structures. Copyright © 2011 John Wiley & Sons, Ltd.